BX₃-Mediated Intermolecular Formation of Functionalized 3-Halo-1H-indenes via Cascade Halo-Nazarov-Type Cyclization

Abstract

A BX₃-promoted, intermolecular regioselective synthesis of 3-halo-functionalized 1H-indenes from 4-oxo-4H-chromene-3-carb­aldehydes and alkynes has been developed. BX₃ displays a dual role of Lewis acid catalyst and halide source for haloallyl cation formation for the intended halo-Nazarov-type cyclization. The overall transformation represents an efficient cascade annulation that employs readily available starting materials, inexpensive reagents and a convenient and mild reaction procedure to generate halo-functionalized indenes (45 examples). The reaction was also extended to 8-formylcoumarins to deliver coumarin-based 3-halo-1H-indenes in 79–95% yield (6 examples). The reaction involves conversion of the aldehyde into an sp³ carbon with two new C–C bonds and additionally a C–X bond is formed (X = halide).

Key words

Lewis acids - Nazarov cyclization - hybrid structures - C–C bond formation - electrocyclic reactions

Indenes[1] and chromones[2] are very promising targets and central structural units ubiquitous in many biologically active molecules. A hybrid displaying both these units might hold promise for resourceful biological activities. Combination of two different bioactive pharmacophores in a single molecule has been shown to possess better activity.[3] The Nazarov cyclization has been well explored since its discovery in the middle of the last century.[4] Traditionally, the reaction is based on cross-conjugated divinyl ketones undergoing ring closure to give the cyclopentenone or indanone products. Several natural products and valuable compounds have been synthesized employing this reaction.[4] [5] However, it is also associated with some limitations, like the difficulty to synthesize and handle dienone precursors, especially when terminal disubstituted vinyl groups are needed. Recently, there have been interesting reports on halo-Nazarov-type cyclizations to provide the haloindene and halocyclopentene products.[6] However, most of these reactions involved the generation of vinylhalo compounds that were then subjected to halo-Nazarov cyclization, thus involving two steps in the protocol.

The traditional 4π-electrocyclization of an oxy- or aza-arylallyl cation or -dienyl cation 1a (OH or NR₂) would be slower, affecting the equilibrium between 1a and 1b (Scheme [1]). This can be attributed to the electron-releasing ability of oxygen or nitrogen to stabilize the cation, thereby decreasing its reactivity. Frontier and co-workers[6] have recently demonstrated that generation of a halo cation 2a (X = halide) would speed up the reaction delivering the useful vinylhalo compounds. By computational studies, it has been observed that the arylallyl cation cyclizations are thermodynamically disfavoured and endothermic by >15 kcal/mol, due to aromaticity loss.[6c] [7] However, the corresponding halo cation cyclizations were thermoneutral, despite aromaticity loss, and were predicted to be exothermic. Experimentally, the reactions occurred efficiently at 0 °C validating the computational data. Over the past decade, isolated examples of similar reactions have emerged, although these were considered as intramolecular Friedel–Crafts arylations.[8] The reaction now termed as ‘halo-Nazarov cyclization’ is underdeveloped,[6] [8] probably due to a lack of direct methods to prepare the intermediates of type 2a from suitable precursors. While the work of Frontier and co-workers[6] marks an excellent beginning, there exists immense potential for further development in this area, as the vinylhalo compounds are useful intermediates in various transition-metal-catalyzed­ coupling reactions.[6a] [b] [8b] [d] [9]

Scheme 1 Oxy- and halo-Nazarov cyclization processes (LA = Lewis acid­)

Our venture into the halo-Nazarov cyclization came serendipitously. Lee and co-workers[10] recently reported the BF₃·OEt₂-promoted cyclization of chromenone-3-carbaldehydes 3 with terminal arylacetylenes 4 in CH₃CN for the construction of 2,5-disubstituted 2-arylpyridines 5 (Scheme [2]). The nitrogen source for the pyridine moiety is derived from the solvent, CH₃CN. We visualized to explore the outcome with internal alkynes, to result in the expected 2,3,5-trisubstituted pyridine 5′ that can be further subjected to an intramolecular oxa-Michael addition resulting in the fused chromenone–pyridine hybrid. The reaction of 3a with diphenylacetylene (4a) in CH₃CN failed to deliver 2,3,5-trisubstituted pyridine 5′, even with excess BF₃·OEt₂ or under heating conditions. A change to the solvent mixture CH₃CN/CH₂Cl₂ (1:1) furnished a new product, characterized to be a chromenone–3-indene incorporating the aryl ring, the alkyne and the aldehyde group of the chromenone with a further interesting incorporation of fluoride from BF₃·OEt₂ leading to the functionalized five-membered-ring compound 6aa (12%, Scheme [2]). The reaction in only CH₂Cl₂ provided 6aa in an improved yield of 21% and can be visualized as an intermolecular cascade halo-Nazarov-type cyclization with the incorporation of the vinyl halide group and differs in the substrates used by Frontier and co-workers[6] for similar cyclizations. Often the aryl vinyl ketone substrates for such reactions need to be synthesized with a heavy bias toward reactivity and the intended Nazarov cyclization becomes an intramolecular process. The present reaction being intermolecular and interesting in having the indene cyclopentene ring functionalized, the added vinylfluoro group, and the 4-chromenone moiety, we considered studying the scope and limitations of this cyclization reaction. It also holds the promise that a change of Lewis acid BF₃·OEt₂ could incorporate other halide groups in the products.

Scheme 2 Strategies for 2-arylpyridines and haloindenes

Table 1 Optimization of the Halo-Nazarov Reaction Conditions^a

Entry	Acid (equiv)	Solvent	Temp	Time (h)	X	Yield (%)b
1	BF3·OEt2 (1.0)	CH2Cl2	rt	4	F	21
2	BF3·OEt2 (1.0)	THF	rt	36	F	NR
3	BF3·OEt2 (1.0)	DMF	rt	36	F	NR
4	BF3·OEt2 (1.0)	DMSO	rt	22	F	NR
5	BF3·OEt2 (1.0)	CH3CN	rt	1	F	CM
6	BF3·OEt2 (1.0)	DCE	rt	24	F	NR
7	BF3·OEt2 (1.0)	benzene	rt	16	F	8c
8	BF3·OEt2 (2.5)	CH2Cl2	rt	3.5	F	58
9	BF3·OEt2 (3.0)	CH2Cl2	rt	3.5	F	60
10	BF3·OEt2 (3.0)	CH2Cl2	reflux	3.0	F	57
11	BF3·THF (2.5)	CH2Cl2	rt	8	F	CM
12	HF–pyridine (2.5)	CH2Cl2	rt	6	F	CM
13	BCl3 (2.5)	CH2Cl2	rt	5	Cl	87
14	BBr3 (2.5)	CH2Cl2	rt	5.5	Br	85
15	BF3·OEt2 (2.5)/PhI(OAc)2 (1.0)	CH2Cl2	rt	8	I	CM
16	BCl3 (2.5)/I2 (1.0)	CH2Cl2	rt	24	I	NR
17	BCl3 (2.5)/KI (1.0)	CH2Cl2	rt	24	I	NR

^a Reaction conditions: 3a (0.172 mmol), 4a (0.172 mmol), acid (1.0–3.0 equiv), solvent (2 mL), rt or reflux, 1–36 h.

^b Isolated yield. NR = no reaction, CM = complex mixture.

^c In place of halogen, solvent addition observed (X = Ph).

We screened various halide sources and available Lewis acids containing halides for the reaction of 3a and diphenylacetylene (4a) as model substrates with variation of solvent and temperature, as shown in Table [1]. Various solvents were initially screened with BF₃·OEt₂ (1.0 equiv) at room temperature (for full details, see the Supporting Information). Except CH₂Cl₂ (entry 1), other solvents were not compatible for this reaction (entries 2–6). In benzene, the reaction occurred with addition of a phenyl group (6, X = Ph, entry 7), in 8% yield. We considered raising the amount of BF₃·OEt₂ used (1.5–3.0 equiv), the yield of 6aa being increased with 2.5–3.0 equivalents (entries 8 and 9). There was no change in yield in carrying out the reaction at reflux in CH₂Cl₂ (entry 10). Other sources of fluoride like BF₃·THF or HF–pyridine did not yield product 6aa (entries 11 and 12). We considered 2.5 equivalents of BF₃·OEt₂ to be optimal. Further, the reaction was mimicked for chlorine incorporation using various chloride sources (AlCl₃, FeCl₃, TiCl₄ and BCl₃). Except BCl₃ (entry 13), other chloride sources were not successful (see the Supporting Information). The optimum requirement was 2.5 equivalents. Similarly, the use of BBr₃ in 2.5 equivalents was optimum for bromide incorporation (entry 14). The reactions for iodine incorporation using various iodide sources in combination with BF₃·OEt₂ or BCl₃ (entries 15–17) failed to deliver the iodine-incorporated products. This study revealed that BF₃·OEt₂, BCl₃ or BBr₃ (each 2.5 equiv) among the various halide sources in CH₂Cl₂ at room temperature are the optimum requirements. Among the three, the fluoride incorporation occurred in moderate yield in comparison to bromide and chloride.

Scheme 3 Substrate scope for 3-fluoro-functionalized 1H-indene synthesis. Reagents and conditions: 3 (0.172 mmol), 4 (0.172 mmol), BF₃·OEt₂ (2.5 equiv), CH₂Cl₂ (2 mL), rt, 1.5–6.5 h. Isolated yields were recorded.

The scope and limitations of this method were next investigated on a series of substituted 4-oxo-4H-chromene-3-carbaldehydes 3 with diarylacetylenes 4 (Scheme [3]) using BF₃·OEt₂ for fluoride incorporation. Substrates 3 with a Me, F, NO₂ or Br group reacted well with 4a giving the 3-fluoroindenes 6ab–6ae in moderate yields. Compound 6ae gave suitable crystals for X-ray analysis,[11] unambiguously confirming the structure. The naphthalene-based substrates also reacted well to provide 6af and 6ag in 50% and 48% yield, respectively. The use of the unsymmetrical alkyne 1-(4-bromophenyl)-2-phenyl­acetylene gave 3-fluoroindene 6ah in 59% yield, with the unsubstituted benzene chemoselectively involved in indene formation.

We continued the scope of substrates for chloride incorporation using BCl₃ (2.5 equiv), as shown in Scheme [4]. Substrates 3 with a NO₂, Me, F or Br group reacted well with 4a giving the 3-chloroindenes 6bb–6be in good yields (68–76%). Similarly, di-p-tolylacetylene reacted with various 4-oxo-4H-chromene-3-carbaldehydes 3 to deliver the 3-chloroindene compounds 6bf–6bj in good to excellent yields (61–94%). The naphthalene-based substrates also reacted well to provide 6bk and 6bl in 96% and 76% yield, respectively. The unsymmetrical diarylalkyne 1-(4-methoxyphenyl)-2-phenylacetylene also worked well to regio- and chemoselectively provide the 3-chloroindenes 6bm–6bo in up to 95% yield, in which the electron-rich methoxybenzene ring is chemoselectively involved in indene formation. Arylalkylalkynes (1-butyn-1-ylbenzene and 1-hexyn-1-ylbenzene) also reacted to give the 3-chloroindenes 6bp–6bs in 80–93% yield. In these cases also, the benzene ring is involved in the annulation reaction. Compounds 6bb, 6bi and 6bm provided suitable crystals for X-ray analysis.[11]

Scheme 4 Substrate scope for 3-chloro-functionalized 1H-indene synthesis. Reagents and conditions: 3 (0.172 mmol), 4 (0.172 mmol), BCl₃ (2.5 equiv), CH₂Cl₂ (2 mL), rt, 1–7 h. Isolated yields were recorded.

We next examined BBr₃, for bromine incorporation in the intermolecular cascade bromo-Nazarov indene formation (Scheme [5]). With 4a and with di-p-tolylacetylene, the reaction with various 4-oxo-4H-chromene-3-carbaldehydes 3 in the presence of BBr₃ furnished the 3-bromo­indenes 6ca–6cl in good yields (72–88%). The unsymmetrical alkyne 1-(4-bromophenyl)-2-phenylacetylene provided 3-bromoindene 6cm selectively, with the unsubstituted benzene ring undergoing the indene ring formation. This was further confirmed by X-ray analysis of 6cm.[11] Arylalkylalkynes (1-butyn-1-ylbenzene, 1-heptyn-1-ylbenzene and 1-hexyn-1-ylbenzene) also reacted to give 3-bromoindenes 6cn–6cq in 80–85% yield. In the case of 4-phenylbut-3-yn-1-ol, reaction with 3a gave product 6cr (80% yield), with the hydroxyl group converted into a bromide with the additional 1.0 equivalent of BBr₃. A reaction with 6.1 mmol of 3a (R = H, 1.1 g) delivered 6ca in 74% yield, indicating possible scale-up of the reaction (Scheme [5]).

Scheme 5 Substrate scope for 3-bromo-functionalized 1H-indene synthesis­ and a gram-scale reaction. Reagents and conditions: 3 (0.172 mmol), 4 (0.172 mmol), BBr₃ (2.5 equiv), CH₂Cl₂ (2 mL), rt, 2.5–8 h. Isolated yields were recorded. ^a BBr₃ (3.5 equiv) used.

The present method was extended to 8-formylcoumarins 7, as shown in Scheme [6]. These, on reaction with diphenylacetylene (4a) or di-p-tolylacetylene, delivered the 3-chloro- or 3-bromo-functionalized 1H-indenes 8a–8f in good to excellent yields (79–95%). Compound 8e gave suitable crystals for X-ray analysis, confirming the structure.[11]

Scheme 6 Substrate scope for coumarin-based 3-halo-functionalized 1H-indene synthesis. Reagents and conditions: 7 (0.172 mmol), 4 (0.172 mmol), BX₃ (2.5 equiv), CH₂Cl₂ (2 mL), rt, 4–11 h. Isolated yields were recorded.

We also considered exploring alkynes with an electron-withdrawing group and various other aldehydes to gain insight into the mechanism of the reaction (Scheme [7]). The reaction of 4-oxo-4H-chromene-3-carbaldehydes 3a, 3b or 3d with 1-(2-methoxyphenyl)-2-(4-nitrophenyl)acetylene (4f) in the presence of BCl₃ or BBr₃ failed to deliver the chloro- or bromoindene compounds. Similarly, the reaction of aldehyde 3a with phenylacetylene (9a) (terminal alkyne) in the presence of BF₃·OEt₂ did not yield the corresponding indene product. Most of the starting materials were recovered in all these cases. This indicated that the alkyne should be nucleophilic enough for reaction with the aldehyde. The reaction of various other aldehydes, like p-chlorobenzaldehyde (10a), cinnamaldehyde (10b), o-anisaldehyde (10c), salicylaldehyde (10d) and 3-oxocyclohex-1-ene-2-carb­aldehyde (10e), with diphenylacetylene (4a) in the presence of BF₃·OEt₂ or BBr₃ gave either a complex mixture or no reaction, with the substrates being recovered unreacted. Comparison of ¹³C NMR and IR spectral values for the aldehyde carbonyl of 10e with that of chromonecarbaldehyde 3a was not conclusive for the reactivity difference. The reaction studied is substrate specific and at this stage only the chromone- and coumarincarbaldehydes react. It appears clear that simple aryl aldehydes do not react probably due to lack of a β-keto group that is required for complexation with the Lewis acid. Also, the chromone oxygen stabilizes the benzylic carbocation, as shown by intermediate 14′ in Scheme [8]. This oxygen is absent in 10e. The reaction of benzoylacetone (11) and 4a with BF₃·OEt₂ delivered the BF₂ complex 12 in 60% yield, the structure of which was established by X-ray analysis.[11] Thus, the β-keto group in 4-oxo-4H-chromene-3-carbaldehydes 3 is necessary for the reaction apart from the chromone oxygen. It has also been observed in this work that chloride and bromide incorporation gave superior yields in comparison to fluoride (Schemes 4 and 5 vs Scheme [3]).

Scheme 7 Reactivity of alkyne with an electron-withdrawing group on the aryl ring and scope of other aldehydes

Considering the selective reactivity of 4-oxo-4H-chromene-3-carbaldehydes and not other aldehydes in this reaction, we considered the plausible mechanism as shown in Scheme [8], with probable involvement of the chromone oxygen. Suitably placed neighbouring groups accelerating the reaction has been developed earlier.[12] Thus, coordination of Lewis acid BF₃·OEt₂ (or BCl₃ or BBr₃) with the aldehyde carbonyl initiates alkyne attack on the boron-complexed aldehyde 3a′, with fluoride attack on the alkyne, giving intermediate 13. Further generation of the fluoroallylic cation 14 triggers the halo-Nazarov-type cyclization. The chromone oxygen probably stabilizes the carbocation, generating the extended oxocarbenium ion 14′. Intramolecular aryl ring closure produces the vinylfluoroindene product 6aa. Terminal arylacetylenes may not be reactive as the intermediate cation before halide attack may not be stable or formed. This substrate bias is evident in the Nazarov cyclization being mostly explored with substituted vinyl bonds.

Scheme 8 Plausible mechanism (F incorporation in 6aa is representative)

In summary, we have developed an efficient intermolecular halo-Nazarov-type cyclization of 4-oxo-4H-chromene-3-carbaldehydes and 1,2-disubstituted acetylenes in the presence of a halide source, BF₃·OEt₂, BCl₃ or BBr₃, which also acts as a Lewis acid to mediate the reaction, giving vinylhaloindenes (45 examples).[13] The aldehyde carbon of 4-oxo-4H-chromene-3-carbaldehydes is converted from sp² into the sp³ carbon in the product by formation of two new C–C bonds. An additional C–X bond formed by incorporation of a halide into the cyclopentene ring is remarkable, generating the functionalized indene moiety. The reaction was also extended to 8-formylcoumarins to deliver coumarin-based 3-halo-1H-indenes in good to excellent yields (6 examples). The overall transformation represents an efficient intermolecular cascade halo-Nazarov-type cyclization strategy that employs readily available starting materials, inexpensive reagents and a convenient and mild reaction procedure to generate halo-functionalized indenes.

TLC was performed on EM 250 Kieselgel 60 F254 silica gel plates. The spots were visualized by staining with KMnO₄ or by using a UV lamp. ¹H and ¹³C NMR spectra were recorded with spectrometers operating at 400 or 500 and at 100 or 125 MHz for proton and carbon nuclei, respectively. The chemical shifts are based on the TMS peak at δ = 0.00 pm for proton NMR and the CDCl₃ peak at δ = 77.00 ppm (t) for carbon NMR. IR spectra were obtained on a FT-IR spectrophotometer. HRMS (ESI-TOF) spectra were recorded using positive electrospray ionization by the TOF method. CH₂Cl₂ was dried by refluxing over P₂O₅ and distillation on calcium hydride. THF and benzene were dried over sodium. All chromenone-3-carbaldehydes (except 3-formylbenzo[h]chromone that was prepared[14]) were obtained from Aldrich Chemical Co. All Lewis acids, phenylacetylene and 1-butyn-1-ylbenzene are commercial reagents and were used as such without further purification. Other alkynes were prepared using established literature protocols.[15] Room temperature = 32–35 °C.

3-(2-Aryl-3-fluoro-1H-inden-1-yl)-4H-chromen-4-ones 6aa–6ah; General Procedure

To a mixture of carbaldehyde 3 (0.172 mmol, 1.0 equiv) and alkyne 4 (0.172 mmol, 1.0 equiv) in CH₂Cl₂ (2 mL) was added BF₃·OEt₂ (0.43 mmol, 2.5 equiv) at room temperature. The mixture was stirred for the required time (monitored by TLC). After completion of the reaction, it was quenched with a few drops of sat. Na₂S₂O₃ solution. The solvent was removed through vacuum and the residue was purified by silica gel column chromatography (petroleum ether/EtOAc, 4:1) to afford 6aa–6ah.

3-(3-Fluoro-2-phenyl-1H-inden-1-yl)-4H-chromen-4-one (6aa)

Yield: 35.4 mg (58%, reaction time = 3.5 h); white solid; mp 182–184 °C.

IR (CHCl₃): 3058, 2850, 1648, 1617, 1465, 1350, 1220, 1176, 696, 594 cm^–1.

¹H NMR (500 MHz, CDCl₃): δ = 8.38 (t, J = 6.1 Hz, 1 H), 7.66–7.61 (m, 3 H), 7.53 (t, J = 5.7 Hz, 1 H), 7.47–7.43 (m, 2 H), 7.39–7.32 (m, 5 H), 7.27–7.21 (m, 2 H), 5.63 (s, 1 H).

¹³C{¹H} NMR (125 MHz, CDCl₃): δ = 177.8, 157.3 (d, ¹ J _C-F = 248.4 Hz), 156.0, 153.0, 144.5, 135.5, 135.3, 133.7, 131.3 (d, ⁴ J _C-F = 4.1 Hz), 128.8, 128.6, 127.7 (d, ³ J _C-F = 6.2 Hz), 127.4, 127.2, 125.9, 125.2, 124.0, 123.8, 122.7, 119.8, 118.1 (d, ² J _C-F = 18.9 Hz), 40.7.

¹⁹F NMR (470 MHz, CDCl₃): δ = –128.5.

HRMS (ESI-TOF): m/z [M + H]⁺ calcd for C₂₄H₁₆FO₂: 355.1129; found: 355.1130.

3-(3-Fluoro-2-phenyl-1H-inden-1-yl)-6-methyl-4H-chromen-4-one (6ab)

Yield: 33 mg (52%, reaction time = 2 h); white solid; mp 162–164 °C.

IR (CHCl₃): 3056, 2924, 1642, 1618, 1484, 1376, 1319, 1161, 911, 812, 693, 544 cm^–1.

¹H NMR (400 MHz, CDCl₃): δ = 8.14 (s, 1 H), 7.60 (d, J = 8.1 Hz, 2 H), 7.51 (d, J = 7.3 Hz, 1 H), 7.45 (d, J = 7.5 Hz, 2 H), 7.36–7.32 (m, 4 H), 7.24–7.19 (m, 3 H), 5.63 (d, J = 5.4 Hz, 1 H), 2.49 (s, 3 H).

¹³C{¹H} NMR (100 MHz, CDCl₃): δ = 177.8, 157.1 (d, ¹ J _C-F = 280.2 Hz), 154.6, 152.9, 144.6 (d, ³ J _C-F = 6.9 Hz), 135.5, 135.3, 134.9, 131.4, 131.38, 128.8, 127.5 (d, ³ J _C-F = 6.5 Hz), 127.4, 127.2, 125.2, 124.1, 123.6, 122.4, 119.9, 117.98 (d, ⁴ J _C-F = 2.5 Hz), 117.9, 40.7, 21.0.

¹⁹F NMR (470 MHz, CDCl₃): δ = –128.7.

HRMS (ESI-TOF): m/z [M + Na]⁺ calcd for C₂₅H₁₇FNaO₂: 391.1105; found: 391.1106.

6-Fluoro-3-(3-fluoro-2-phenyl-1H-inden-1-yl)-4H-chromen-4-one (6ac)

Yield: 34 mg (53%, reaction time = 3 h); white solid; mp 220–222 °C.

IR (CHCl₃): 2932, 2857, 1637, 1625, 1476, 1457, 1049, 910, 610 cm^–1.

¹H NMR (400 MHz, CDCl₃): δ = 7.98 (dd, J = 7.9, 2.6 Hz, 1 H), 7.60–7.58 (m, 2 H), 7.51–7.45 (m, 2 H), 7.39 (s, 1 H), 7.38–7.33 (m, 5 H), 7.23 (t, J = 7.3 Hz, 2 H), 5.58 (d, J = 5.5 Hz, 1 H).

¹³C{¹H} NMR (100 MHz, CDCl₃): δ = 177.1, 159.7 (d, ¹ J _C-F = 248.6 Hz), 155.8, 153.2, 152.6, 144.4 (d, ³ J _C-F = 7.3 Hz), 135.5, 135.3, 131.3 (d, ⁴ J _C-F = 4.9 Hz), 128.8, 127.7 (d, ³ J _C-F = 6.4 Hz), 127.6, 127.3, 124.9 (d, ³ J _C-F = 7.5 Hz), 124.0 (d, ⁴ J _C-F = 2.5 Hz), 122.2 (d, ⁴ J _C-F = 4.6 Hz), 121.9, 120.3 (d, ³ J _C-F = 8.1 Hz), 119.7, 118.1 (d, ⁴ J _C-F = 2.3 Hz), 110.8 (d, ² J _C-F = 23.9 Hz), 40.7.

¹⁹F NMR (470 MHz, CDCl₃): δ = –114.8, –128.5.

HRMS (ESI-TOF): m/z [M + H]⁺ calcd for C₂₄H₁₅F₂O₂: 373.1035; found: 373.1042.

3-(3-Fluoro-2-phenyl-1H-inden-1-yl)-6-nitro-4H-chromen-4-one (6ad)

Yield: 35 mg (51%, reaction time = 6.5 h); white solid; mp 189–191 °C.

IR (CHCl₃): 1652, 1632, 1531, 1467, 1345, 826, 695 cm^–1.

¹H NMR (500 MHz, CDCl₃): δ = 9.23 (d, J = 2.4 Hz, 1 H), 8.46 (dd, J = 9.2, 2.3 Hz, 1 H), 7.58 (d, J = 8.3 Hz, 2 H), 7.50–7.45 (m, 4 H), 7.39–7.34 (m, 3 H), 7.26–7.23 (m, 2 H), 5.58 (d, J = 5.3 Hz, 1 H).

¹³C{¹H} NMR (125 MHz, CDCl₃): δ = 176.5, 159.0, 157.3 (d, ¹ J _C-F = 280.4 Hz), 153.3, 144.8, 143.7, 135.4 (d, ² J _C-F = 24.8 Hz), 131.1, 128.9, 128.0, 127.8, 127.7, 127.6, 127.5, 124.0, 123.9, 123.8, 122.8, 120.0, 119.3, 118.2, 40.6.

¹⁹F NMR (470 MHz, CDCl₃): δ = –127.9.

HRMS (ESI-TOF): m/z [M + Na]⁺ calcd for C₂₄H₁₄FNNaO₄: 422.0799; found: 422.0788.

6-Bromo-3-(3-fluoro-2-phenyl-1H-inden-1-yl)-4H-chromen-4-one (6ae)

Yield: 37.2 mg (50%, reaction time = 1.5 h); white solid; mp 156–158 °C.

IR (CHCl₃): 2923, 2857, 1676, 1641, 1605, 1461, 1376, 910, 815, 734, 693 cm^–1.

¹H NMR (500 MHz, CDCl₃): δ = 8.48 (d, J = 2.3 Hz, 1 H), 7.71 (dd, J = 8.8, 2.5 Hz, 1 H), 7.58 (d, J = 7.3 Hz, 2 H), 7.49–7.45 (m, 2 H), 7.38–7.33 (m, 4 H), 7.24–7.21 (m, 3 H), 5.58 (d, J = 5.7 Hz, 1 H).

¹³C{¹H} NMR (125 MHz, CDCl₃): δ = 176.5, 157.2 (d, ¹ J _C-F = 279.2 Hz), 155.1, 153.1, 144.3 (d, ³ J _C-F = 7.2 Hz), 136.7, 135.5, 135.3, 131.3 (d, ⁴ J _C-F = 4.9 Hz), 128.8, 128.6, 127.7 (d, ³ J _C-F = 6.4 Hz), 127.6, 127.4, 125.2, 124.0, 123.1, 120.1, 119.7, 118.7, 118.1, 40.7.

¹⁹F NMR (470 MHz, CDCl₃): δ = –128.3.

HRMS (ESI-TOF): m/z [M + Na]⁺ calcd for C₂₄H₁₄BrFNaO₂: 457.0036; found: 457.0035.

2-(3-Fluoro-2-phenyl-1H-inden-1-yl)-1H-benzo[f]chromen-1-one (6af)

Yield: 34.8 mg (50%, reaction time = 6 h); white solid; mp 185–187 °C.

IR (CHCl₃): 2926, 2855, 1642, 1609, 1516, 1441, 1375, 910, 820, 732, 693, 607 cm^–1.

¹H NMR (400 MHz, CDCl₃): δ = 10.26 (d, J = 8.4 Hz, 1 H), 8.04 (d, J = 8.9 Hz, 1 H), 7.92 (d, J = 8.0 Hz, 1 H), 7.85 (t, J = 7.8 Hz, 1 H), 7.69–7.60 (m, 4 H), 7.48–7.44 (m, 2 H), 7.38–7.33 (m, 4 H), 7.21 (t, J = 7.5 Hz, 2 H), 5.78 (s, 1 H).

¹³C{¹H} NMR (125 MHz, CDCl₃): δ = 179.2, 157.5, 157.2 (d, ¹ J _C-F = 279.3 Hz), 150.4, 144.6, 135.5, 135.3, 131.4, 130.6, 130.5, 129.3, 128.8, 128.3, 127.7 (d, ³ J _C-F = 6.3 Hz), 127.4, 127.2, 127.1, 126.7, 125.3, 124.1, 119.9, 118.0, 117.5, 117.1, 40.8.

¹⁹F NMR (470 MHz, CDCl₃): δ = –128.4.

HRMS (ESI-TOF): m/z [M + Na]⁺ calcd for C₂₈H₁₇FNaO₂: 427.1105; found: 427.1098.

3-(3-Fluoro-2-phenyl-1H-inden-1-yl)-4H-benzo[h]chromen-4-one (6ag)

Yield: 33.4 mg (48%, reaction time = 6 h); white solid; mp 138–140 °C.

IR (CHCl₃): 1683, 1642, 1601, 1445, 1380, 910, 694, 668 cm^–1.

¹H NMR (500 MHz, CDCl₃): δ = 8.30–8.26 (m, 2 H), 7.90 (d, J = 8.1 Hz, 1 H), 7.80 (d, J = 8.7 Hz, 1 H), 7.68–7.64 (m, 3 H), 7.60–7.57 (m, 3 H), 7.47 (d, J = 7.4 Hz, 1 H), 7.37–7.34 (m, 3 H), 7.24–7.20 (m, 2 H), 5.70 (d, J = 5.3 Hz, 1 H).

¹³C{¹H} NMR (125 MHz, CDCl₃): δ = 177.5, 157.3 (d, ¹ J _C-F = 279.9 Hz), 153.9, 152.1, 144.5, 135.7, 135.4 (d, ² J _C-F = 25.7 Hz), 131.4, 129.4, 128.8, 128.7, 128.0, 127.7 (d, ³ J _C-F = 6.2 Hz), 127.5, 127.3, 127.2, 126.7, 125.5, 124.2 (d, ² J _C-F = 13.5 Hz), 123.9, 122.1, 120.8, 120.2, 119.8, 118.0, 40.8.

¹⁹F NMR (470 MHz, CDCl₃): δ = –128.0.

HRMS (ESI-TOF): m/z [M + Na]⁺ calcd for C₂₈H₁₇FNaO₂: 427.1105; found: 427.1100.

3-(2-(4-Bromophenyl)-3-fluoro-1H-inden-1-yl)-4H-chromen-4-one (6ah)

Yield: 44 mg (59%, reaction time = 2 h); white solid; mp 103–105 °C.

IR (CHCl₃): 2945, 2833, 1686, 1640, 1608, 1465, 1395, 1028, 824, 690 cm^–1.

¹H NMR (500 MHz, CDCl₃): δ = 8.15 (d, J = 7.1 Hz, 1 H), 7.45 (dt, J = 7.7, 1.7 Hz, 1 H), 7.29–7.24 (m, 6 H), 7.16–7.13 (m, 3 H), 7.06 (s, 1 H), 7.03 (dt, J = 7.5, 1.0 Hz, 1 H), 5.38 (s, 1 H).

¹³C{¹H} NMR (100 MHz, CDCl₃): δ = 177.7, 157.6 (d, ¹ J _C-F = 262.5 Hz), 156.2, 153.0, 144.5, 135.3, 135.0, 133.8, 132.0, 131.6, 130.3, 129.2 (d, ³ J _C-F = 6.3 Hz), 127.6, 126.0, 125.4, 124.1, 123.8, 122.4, 121.2, 119.0, 118.2, 40.6.

¹⁹F NMR (470 MHz, CDCl₃): δ = –127.5.

HRMS (ESI-TOF): m/z [M + Na]⁺ calcd for C₂₄H₁₄BrFNaO₂: 455.0053; found: 455.0056.

3-(2-Aryl-3-chloro-1H-inden-1-yl)-4H-chromen-4-ones 6ba–6bs; General Procedure

Compounds 6ba–6bs were prepared by following a similar procedure as described for compounds 6aa–6ah, using BCl₃ (2.5 equiv) instead of BF₃·OEt₂ and 0.172 mmol each of 3 and 4.

3-(3-Chloro-2-phenyl-1H-inden-1-yl)-4H-chromen-4-one (6ba)

Yield: 55.5 mg (87%, reaction time = 5 h); white semisolid.

IR (CHCl₃): 3057, 2923, 2850, 1650, 1617, 1465, 1349, 1220, 912, 736, 701, 592 cm^–1.

¹H NMR (500 MHz, CDCl₃): δ = 8.32 (d, J = 7.7 Hz, 1 H), 7.78 (d, J = 7.5 Hz, 2 H), 7.60 (dt, J = 6.9, 1.6 Hz, 1 H), 7.55 (d, J = 7.6 Hz, 1 H), 7.50 (d, J = 7.4 Hz, 1 H), 7.40–7.34 (m, 4 H), 7.32 (s, 1 H), 7.30–7.27 (m, 2 H), 7.24–7.22 (m, 1 H), 5.80 (s, 1 H).

¹³C{¹H} NMR (100 MHz, CDCl₃): δ = 177.4, 156.2, 153.2, 144.9, 141.4, 139.9, 133.6, 132.7, 128.9, 128.6, 128.0, 127.6, 127.0, 125.9, 125.2, 123.8, 123.5, 122.6, 119.6, 118.1, 45.2.

HRMS (ESI-TOF): m/z [M + Na]⁺ calcd for C₂₄H₁₅ClNaO₂: 393.0653; found: 393.0650.

3-(3-Chloro-2-phenyl-1H-inden-1-yl)-6-nitro-4H-chromen-4-one (6bb)

Yield: 51.5 mg (72%, reaction time = 6 h); white solid; mp 73–74 °C.

IR (CHCl₃): 2923, 2853, 1651, 1629, 1465, 1345, 911, 746, 593 cm^–1.

¹H NMR (500 MHz, CDCl₃): δ = 9.18 (d, J = 2.5 Hz, 1 H), 8.44 (dd, J = 9.2, 2.7 Hz, 1 H), 7.74 (d, J = 7.6 Hz, 2 H), 7.56 (d, J = 7.6 Hz, 1 H), 7.46 (d, J = 8.9 Hz, 2 H), 7.42–7.37 (m, 4 H), 7.30–7.22 (m, 2 H), 5.74 (s, 1 H).

¹³C{¹H} NMR (125 MHz, CDCl₃): δ = 176.1, 158.9, 153.5, 144.8, 144.1, 141.5, 139.3, 132.5, 129.5, 128.7, 128.5, 128.2, 127.93, 127.9, 127.2, 123.8, 123.7, 123.4, 122.8, 120.0, 119.9, 45.1.

HRMS (ESI-TOF): m/z [M + Na]⁺ calcd for C₂₄H₁₄ClNNaO₄: 438.0504; found: 438.0501.

3-(3-Chloro-2-phenyl-1H-inden-1-yl)-6-methyl-4H-chromen-4-one (6bc)

Yield: 49.6 mg (75%, reaction time = 2 h); white solid; mp 140–142 °C.

IR (CHCl₃): 3057, 2924, 2853, 1641, 1622, 1483, 1319, 1160, 910, 748, 695 cm^–1.

¹H NMR (500 MHz, CDCl₃): δ = 8.10 (s, 1 H), 7.76 (d, J = 6.9 Hz, 2 H), 7.54 (d, J = 7.5 Hz, 1 H), 7.49–7.48 (m, 1 H), 7.42–7.35 (m, 4 H), 7.29 (s, 1 H), 7.27–7.23 (m, 2 H), 7.22–7.19 (m, 1 H), 5.81 (s, 1 H), 2.46 (s, 3 H).

¹³C{¹H} NMR (125 MHz, CDCl₃): δ = 177.5, 154.5, 153.1, 145.1, 141.4, 140.0, 135.2, 134.9, 132.8, 128.9, 128.61, 128.6, 128.0, 127.5, 127.0, 125.2, 123.52, 123.5, 122.4, 119.6, 117.9, 45.3, 20.9.

HRMS (ESI-TOF): m/z [M + Na]⁺ calcd for C₂₅H₁₇ClNaO₂: 407.0809; found: 407.0811.

3-(3-Chloro-2-phenyl-1H-inden-1-yl)-6-fluoro-4H-chromen-4-one (6bd)

Yield: 50.7 mg (76%, reaction time = 1 h); white solid; mp 183–186 °C.

IR (CHCl₃): 3057, 2922, 2850, 1635, 1617, 1476, 1265, 1129, 817, 725, 509 cm^–1.

¹H NMR (500 MHz, CDCl₃): δ = 7.94 (dd, J = 7.9, 2.4 Hz, 1 H), 7.74 (d, J = 7.4 Hz, 2 H), 7.55 (d, J = 7.6 Hz, 1 H), 7.47 (d, J = 7.4 Hz, 1 H), 7.41–7.35 (m, 3 H), 7.34–7.31 (m, 3 H), 7.28–7.22 (m, 2 H), 5.76 (s, 1 H).

¹³C{¹H} NMR (125 MHz, CDCl₃): δ = 176.8, 159.6 (d, ¹ J _C-F = 247.3 Hz), 153.4, 152.5, 144.8, 141.4, 139.7, 132.7, 129.1, 128.6 (d, ³ J _C-F = 6.3 Hz), 128.1, 127.7, 127.1, 124.91, 124.9, 123.5, 122.1 (d, ³ J _C-F = 5.5 Hz), 121.9, 120.3 (d, ³ J _C-F = 8.1 Hz), 119.7, 110.7 (d, ² J _C-F = 23.6 Hz), 45.2.

¹⁹F NMR (470 MHz, CDCl₃): δ = –114.8.

HRMS (ESI-TOF): m/z [M + Na]⁺ calcd for C₂₄H₁₄ClFNaO₂: 411.0559; found: 411.0551.

6-Bromo-3-(3-chloro-2-phenyl-1H-inden-1-yl)-4H-chromen-4-one (6be)

Yield: 52.6 mg (68%, reaction time = 1.5 h); white solid; mp 88–90 °C.

IR (CHCl₃): 3066, 2925, 1641, 1604, 1461, 1312, 1266, 1146, 908, 732, 697, 595 cm^–1.

¹H NMR (500 MHz, CDCl₃): δ = 8.43 (s, 1 H), 7.75 (d, J = 7.7 Hz, 2 H), 7.67 (dd, J = 8.7, 1.7 Hz, 1 H), 7.55 (d, J = 7.5 Hz, 1 H), 7.47 (d, J = 7.4 Hz, 1 H), 7.41–7.36 (m, 3 H), 7.31–7.28 (m, 2 H), 7.24–7.23 (m, 1 H), 7.18 (d, J = 8.8 Hz, 1 H), 5.76 (s, 1 H).

¹³C{¹H} NMR (125 MHz, CDCl₃): δ = 176.1, 154.9, 153.3, 144.6, 141.4, 139.6, 136.6, 132.6, 129.1, 128.6, 128.5, 128.1, 127.7, 127.1, 125.0, 123.4, 122.9, 120.0, 119.7, 118.6, 45.2.

HRMS (ESI-TOF): m/z [M + H]⁺ calcd for C₂₄H₁₅BrClO₂: 448.9938; found: 448.9936.

3-(3-Chloro-6-methyl-2-(p-tolyl)-1H-inden-1-yl)-4H-chromen-4-one (6bf)

Yield: 64.5 mg (94%, reaction time = 1.5 h); white solid; mp 221–224 °C.

IR (CHCl₃): 2921, 2856, 1633, 1610, 1466, 1354, 1164, 817, 601 cm^–1.

¹H NMR (500 MHz, CDCl₃): δ = 8.34 (d, J = 7.8 Hz, 1 H), 7.66–7.60 (m, 3 H), 7.44–7.39 (m, 2 H), 7.32–7.29 (m, 3 H), 7.19–7.16 (m, 3 H), 5.74 (s, 1 H), 2.34 (s, 3 H), 2.32 (s, 3 H).

¹³C{¹H} NMR (125 MHz, CDCl₃): δ = 177.6, 156.2, 153.3, 145.1, 139.0, 138.8, 137.8, 137.0, 133.6, 130.0, 129.3, 128.4, 128.3, 126.0, 125.2, 124.3, 123.8, 123.0, 119.2, 118.1, 44.9, 21.5, 21.2.

HRMS (ESI-TOF): m/z [M + H]⁺ calcd for C₂₆H₂₀ClO₂: 399.1146; found: 399.1145.

3-(3-Chloro-6-methyl-2-(p-tolyl)-1H-inden-1-yl)-6-methyl-4H-chromen-4-one (6bg)

Yield: 65.3 mg (92%, reaction time = 1 h); white solid; mp 233–236 °C.

IR (CHCl₃): 2920, 2881, 1642, 1624, 1509, 1483, 1320, 1167, 818, 726, 508 cm^–1.

¹H NMR (500 MHz, CDCl₃): δ = 8.12 (s, 1 H), 7.66 (d, J = 8.2 Hz, 2 H), 7.43–7.39 (m, 2 H), 7.31–7.29 (m, 2 H), 7.18 (dd, J = 14.9, 8.4 Hz, 4 H), 5.75 (s, 1 H), 2.47 (s, 3 H), 2.34 (s, 3 H), 2.32 (s, 3 H).

¹³C{¹H} NMR (125 MHz, CDCl₃): δ = 177.6, 154.5, 153.1, 145.2, 138.9, 138.8, 137.7, 136.9, 135.1, 134.8, 130.0, 129.2, 128.4, 128.2, 128.1, 125.2, 124.2, 123.5, 122.7, 119.1, 117.8, 44.9, 21.5, 21.2, 20.9.

HRMS (ESI-TOF): m/z [M + H]⁺ calcd for C₂₇H₂₂ClO₂: 413.1303; found: 413.1308.

3-(3-Chloro-6-methyl-2-(p-tolyl)-1H-inden-1-yl)-6-fluoro-4H-chromen-4-one (6bh)

Yield: 43.7 mg (61%, reaction time = 2.5 h); white solid; mp 155–158 °C.

IR (CHCl₃): 2912, 1636, 1626, 1613, 1478, 1326, 964, 816, 725, 603, 508 cm^–1.

¹H NMR (400 MHz, CDCl₃): δ = 7.95 (dd, J = 8.1, 2.6 Hz, 1 H), 7.63 (d, J = 7.1 Hz, 2 H), 7.40 (d, J = 7.3 Hz, 1 H), 7.37–7.29 (m, 3 H), 7.28 (s, 1 H), 7.19–7.16 (m, 3 H), 5.70 (s, 1 H), 2.34 (s, 3 H), 2.32 (s, 3 H).

¹³C{¹H} NMR (100 MHz, CDCl₃): δ = 176.9, 159.5 (d, ¹ J _C-F = 247.5 Hz), 153.5, 152.5, 144.9, 139.0, 138.6, 137.9, 137.1, 129.9, 129.3, 128.4, 128.36, 124.9 (d, ³ J _C-F = 7.6 Hz), 124.2, 122.5, 122.0, 121.8, 120.3 (d, ³ J _C-F = 8.1 Hz), 119.3, 110.8 (d, ² J _C-F = 23.8 Hz), 44.9, 21.5, 21.2.

¹⁹F NMR (470 MHz, CDCl₃): δ = –114.9.

HRMS (ESI-TOF): m/z [M + H]⁺ calcd for C₂₆H₁₉ClFO₂: 417.1052; found: 417.1054.

6-Bromo-3-(3-chloro-6-methyl-2-(p-tolyl)-1H-inden-1-yl)-4H-chromen-4-one (6bi)

Yield: 62.5 mg (76%, reaction time = 1.5 h); white solid; mp 218–220 °C.

IR (CHCl₃): 2920, 1642, 1604, 1462, 1437, 1335, 1313, 1267, 819, 733, 598 cm^–1.

¹H NMR (400 MHz, CDCl₃): δ = 8.45 (d, J = 2.5 Hz, 1 H), 7.69 (dd, J = 8.9, 2.4 Hz, 1 H), 7.62 (d, J = 8.2 Hz, 2 H), 7.40 (d, J = 7.7 Hz, 1 H), 7.31 (s, 1 H), 7.26 (s, 1 H), 7.22–7.16 (m, 4 H), 5.69 (s, 1 H), 2.34 (s, 3 H), 2.32 (s, 3 H).

¹³C{¹H} NMR (100 MHz, CDCl₃): δ = 176.3, 155.0, 153.4, 144.8, 139.0, 138.5, 137.9, 137.1, 136.6, 129.9, 129.4, 128.6, 128.43, 128.4, 125.1, 124.2, 123.3, 120.1, 119.3, 118.6, 44.9, 21.5, 21.2.

HRMS (ESI-TOF): m/z [M + H]⁺ calcd for C₂₆H₁₉BrClO₂: 477.0251; found: 477.0251.

3-(3-Chloro-6-methyl-2-(p-tolyl)-1H-inden-1-yl)-6-nitro-4H-chromen-4-one (6bj)

Yield: 50.4 mg (66%, reaction time = 1.5 h); white solid; mp 230–232 °C.

IR (CHCl₃): 2925, 2857, 1641, 1629, 1529, 1464, 1341, 1263, 814, 690, 593 cm^–1.

¹H NMR (400 MHz, CDCl₃): δ = 9.19 (d, J = 2.6 Hz, 1 H), 8.43 (dd, J = 6.5, 2.9 Hz, 1 H), 7.62 (d, J = 8.1 Hz, 2 H), 7.46 (d, J = 9.2 Hz, 1 H), 7.40 (d, J = 7.7 Hz, 1 H), 7.37 (s, 1 H), 7.27 (s, 1 H), 7.21–7.17 (m, 3 H), 5.68 (s, 1 H), 2.35 (s, 3 H), 2.33 (s, 3 H).

¹³C{¹H} NMR (100 MHz, CDCl₃): δ = 176.3, 159.0, 153.5, 144.7, 144.3, 140.0, 138.12, 138.1, 137.3, 129.7, 129.4, 128.8, 128.6, 128.3, 127.9, 124.2, 123.8, 122.9, 119.9, 119.4, 44.8, 21.5, 21.3.

HRMS (ESI-TOF): m/z [M + H]⁺ calcd for C₂₆H₁₉ClNO₄: 444.0997; found: 444.0995.

3-(3-Chloro-2-phenyl-1H-inden-1-yl)-4H-benzo[h]chromen-4-one (6bk)

Yield: 69.5 mg (96%, reaction time = 2.5 h); white solid; mp 80–84 °C.

IR (CHCl₃): 3061, 2925, 2850, 1642, 1462, 1400, 1265, 1156, 698, 583 cm^–1.

¹H NMR (500 MHz, CDCl₃): δ = 8.25 (dd, J = 8.5, 4.5 Hz, 2 H), 7.88 (d, J = 8.1 Hz, 1 H), 7.81 (d, J = 7.6 Hz, 2 H), 7.77 (d, J = 8.7 Hz, 1 H), 7.65 (t, J = 7.6 Hz, 1 H), 7.58–7.52 (m, 4 H), 7.41–7.36 (m, 3 H), 7.27–7.26 (m, 1 H), 7.24–7.22 (m, 1 H), 5.87 (s, 1 H).

¹³C{¹H} NMR (125 MHz, CDCl₃): δ = 177.2, 153.8, 152.3, 144.9, 141.5, 139.9, 135.7, 134.9, 132.8, 129.4, 129.0, 128.6, 128.03, 128.0, 127.6, 127.13, 127.1, 125.4, 124.2, 123.9, 123.6, 122.1, 120.8, 120.1, 119.7, 45.3.

HRMS (ESI-TOF): m/z [M + H]⁺ calcd for C₂₈H₁₈ClO₂: 421.0990; found: 421.0985.

3-(3-Chloro-6-methyl-2-(p-tolyl)-1H-inden-1-yl)-4H-benzo[h]-chromen-4-one (6bl)

Yield: 58.7 mg (76%, reaction time = 1 h); white solid; mp 223–226 °C.

IR (CHCl₃): 2983, 2928, 2850, 1637, 1625, 1444, 1415, 1374, 1241, 1047, 814, 639 cm^–1.

¹H NMR (400 MHz, CDCl₃): δ = 8.25 (t, J = 8.5 Hz, 2 H), 7.88 (d, J = 8.1 Hz, 1 H), 7.77 (d, J = 78.8 Hz, 1 H), 7.69 (d, J = 8.2 Hz, 2 H), 7.67–7.62 (m, 1 H), 7.56 (dt, J = 7.1, 0.9 Hz, 1 H), 7.50 (s, 1 H), 7.42 (d, J = 7.7 Hz, 1 H), 7.35 (s, 1 H), 7.20–7.17 (m, 3 H), 5.81 (s, 1 H), 2.34 (s, 3 H), 2.30 (s, 3 H).

¹³C{¹H} NMR (100 MHz, CDCl₃): δ = 177.3, 153.8, 152.4, 145.1, 139.0, 138.7, 137.8, 137.0, 135.7, 130.0, 129.33, 129.3, 128.4, 128.3, 128.0, 127.1, 125.4, 124.5, 124.3, 123.9, 122.1, 120.9, 120.1, 119.2, 45.0, 21.5, 21.2.

HRMS (ESI-TOF): m/z [M + H]⁺ calcd for C₃₀H₂₂ClO₂: 449.1303; found: 449.1297.

3-(3-Chloro-6-methoxy-2-phenyl-1H-inden-1-yl)-4H-chromen-4-one (6bm)

Yield: 65.5 mg (95%, reaction time = 7 h); white solid; mp 83–86 °C.

IR (CHCl₃): 3003, 2955, 2833, 1638, 1622, 1484, 1320, 1285, 1031, 817, 695, 609 cm^–1.

¹H NMR (400 MHz, CDCl₃): δ = 8.32 (d, J = 6.7 Hz, 1 H), 7.74 (d, J = 7.3 Hz, 2 H), 7.61 (dt, J = 7.7, 1.6 Hz, 1 H), 7.44–7.41 (m, 2 H), 7.39–7.34 (m, 3 H), 7.30 (d, J = 8.4 Hz, 1 H), 7.25–7.22 (m, 1 H), 7.08 (d, J = 1.6 Hz, 1 H), 6.93 (dd, J = 8.4, 2.2 Hz, 1 H), 5.76 (s, 1 H), 3.78 (s, 3 H).

¹³C{¹H} NMR (100 MHz, CDCl₃): δ = 177.7, 159.8, 156.4, 153.6, 147.0, 137.8, 134.6, 133.8, 133.2, 128.8, 127.8, 126.1, 125.4, 124.0, 123.0, 120.5, 118.3, 113.7, 109.8, 55.8, 45.3.

HRMS (ESI-TOF): m/z [M + Na]⁺ calcd for C₂₅H₁₇ClNaO₃: 423.0758; found: 423.0754.

3-(3-Chloro-6-methoxy-2-phenyl-1H-inden-1-yl)-6-methyl-4H-chromen-4-one (6bn)

Yield: 59.2 mg (83%, reaction time = 6 h); white solid; mp 93–95 °C.

IR (CHCl₃): 3006, 2925, 2833, 1639, 1618, 1444, 1285, 1032, 817, 695, 543 cm^–1.

¹H NMR (400 MHz, CDCl₃): δ = 8.09 (s, 1 H), 7.73 (d, J = 7.5 Hz, 2 H), 7.42 (d, J = 8.4 Hz, 2 H), 7.35 (t, J = 7.9 Hz, 2 H), 7.32 (s, 1 H), 7.24 (d, J = 7.4 Hz, 1 H), 7.21 (d, J = 8.5 Hz, 1 H), 7.07 (d, J = 1.6 Hz, 1 H), 6.92 (dd, J = 8.4, 2.3 Hz, 1 H), 5.76 (s, 1 H), 3.78 (s, 3 H), 2.46 (s, 3 H).

¹³C{¹H} NMR (125 MHz, CDCl₃): δ = 177.5, 159.6, 154.5, 153.3, 147.0, 137.7, 135.2, 134.9, 134.4, 133.0, 128.5, 128.3, 127.6, 125.2, 123.5, 122.6, 120.3, 117.9, 113.6, 109.5, 55.6, 45.1, 20.9.

HRMS (ESI-TOF): m/z [M + Na]⁺ calcd for C₂₆H₁₉ClNaO₃: 437.0915; found: 437.0902.

3-(3-Chloro-6-methoxy-2-phenyl-1H-inden-1-yl)-6-fluoro-4H-chromen-4-one (6bo)

Yield: 68.4 mg (95%, reaction time = 6 h); white solid; mp 165–168 °C.

IR (CHCl₃): 3067, 2925, 2844, 1634, 1612, 1479, 1454, 1286, 1029, 829, 688, 558 cm^–1.

¹H NMR (400 MHz, CDCl₃): δ = 7.94 (dd, J = 8.4, 2.2 Hz, 1 H), 7.72 (d, J = 7.5 Hz, 2 H), 7.43 (d, J = 8.4 Hz, 1 H), 7.38–7.32 (m, 5 H), 7.24–7.22 (m, 1 H), 7.06 (s, 1 H), 6.93 (dd, J = 8.4, 2.0 Hz, 1 H), 5.72 (s, 1 H), 3.78 (s, 3 H).

¹³C{¹H} NMR (100 MHz, CDCl₃): δ = 176.8, 159.6, 159.5 (d, ¹ J _C-F = 247.3 Hz), 153.6, 152.5, 146.6, 137.4, 134.4, 132.9, 128.8, 128.5 (d, ² J _C-F = 27.8 Hz), 127.7, 124.9, 122.3, 122.1, 121.8, 120.4, 120.3 (d, ³ J _C-F = 8.3 Hz), 113.6, 110.7 (d, ² J _C-F = 23.9 Hz), 109.6, 55.6, 45.0.

¹⁹F NMR (470 MHz, CDCl₃): δ = –114.8.

HRMS (ESI-TOF): m/z [M + Na]⁺ calcd for C₂₅H₁₆ClFNaO₃: 441.0664; found: 441.0657.

3-(3-Chloro-2-ethyl-1H-inden-1-yl)-4H-chromen-4-one (6bp)

Yield: 51.6 mg (93%, reaction time = 4 h); white semisolid.

IR (CHCl₃): 2964, 2927, 2856, 1643, 1611, 1466, 1349, 1143, 724, 670 cm^–1.

¹H NMR (400 MHz, CDCl₃): δ = 8.32 (d, J = 7.8 Hz, 1 H), 7.67 (dt, J = 7.7, 1.7 Hz, 1 H), 7.47–7.39 (m, 3 H), 7.36–7.31 (m, 3 H), 7.18 (dt, J = 6.9, 1.2 Hz, 1 H), 5.18 (s, 1 H), 2.74 (sextet, J = 7.5 Hz, 1 H), 2.20 (sextet, J = 7.4 Hz, 1 H), 1.14 (t, J = 7.5 Hz, 3 H).

¹³C{¹H} NMR (100 MHz, CDCl₃): δ = 177.3, 156.4, 152.9, 145.8, 144.8, 141.7, 133.7, 127.8, 127.3, 126.0, 126.0, 125.3, 123.8, 123.3, 122.3, 118.8, 118.1, 44.5, 20.0, 13.2.

HRMS (ESI-TOF): m/z [M + Na]⁺ calcd for C₂₀H₁₅ClNaO₂: 345.0653; found: 345.0655.

3-(3-Chloro-2-ethyl-1H-inden-1-yl)-6-fluoro-4H-chromen-4-one (6bq)

Yield: 48.7 mg (83%, reaction time = 3 h); white solid; mp 121–123 °C.

IR (CHCl₃): 3071, 2970, 2934, 1646, 1479, 1319, 1264, 1175, 960, 824, 722, 600, 555 cm^–1.

¹H NMR (400 MHz, CDCl₃): δ = 7.94 (d, J = 6.0 Hz, 1 H), 7.45–7.37 (m, 3 H), 7.36–7.31 (m, 3 H), 7.18 (dt, J = 7.5, 1.0 Hz, 1 H), 5.15 (s, 1 H), 2.74 (sextet, J = 7.1 Hz, 1 H), 2.19 (sextet, J = 7.4 Hz, 1 H), 1.14 (t, J = 7.6 Hz, 3 H).

¹³C{¹H} NMR (100 MHz, CDCl₃): δ = 176.6, 159.7 (d, ¹ J _C-F = 247.2 Hz), 153.1, 152.6, 145.5, 144.5, 141.6, 128.0, 127.4, 126.0, 124.9 (d, ³ J _C-F = 7.3 Hz), 123.2, 122.1, 121.8 (d, ³ J _C-F = 6.9 Hz), 120.3 (d, ³ J _C-F = 8.1 Hz), 118.8, 110.8 (d, ² J _C-F = 23.7 Hz), 44.6, 20.0, 13.2.

¹⁹F NMR (470 MHz, CDCl₃): δ = –114.7.

HRMS (ESI-TOF): m/z [M + Na]⁺ calcd for C₂₀H₁₄ClFNaO₂: 363.0559; found: 363.0556.

3-(2-Butyl-3-chloro-1H-inden-1-yl)-4H-chromen-4-one (6br)

Yield: 50.7 mg (84%, reaction time = 5 h); pale yellow oil.

IR (CHCl₃): 3068, 2929, 2858, 1647, 1480, 1318, 1157, 822, 723, 600 cm^–1.

¹H NMR (500 MHz, CDCl₃): δ = 8.31 (d, J = 7.5 Hz, 1 H), 7.69–7.66 (m, 1 H), 7.46–7.39 (m, 4 H), 7.35–7.33 (m, 2 H), 7.17 (t, J = 7.5 Hz, 1 H), 5.16 (s, 1 H), 2.71 (sextet, J = 7.9 Hz, 1 H), 2.17 (septet, J = 5.5 Hz, 1 H), 1.56–1.46 (m, 2 H), 1.37–1.28 (m, 2 H), 0.89 (t, J = 7.3 Hz, 3 H).

¹³C{¹H} NMR (125 MHz, CDCl₃): δ = 177.3, 156.4, 152.9, 145.0, 144.5, 141.6, 133.7, 128.5, 127.3, 126.1, 126.0, 125.3, 123.9, 123.3, 122.4, 118.8, 118.2, 44.9, 30.8, 26.4, 22.5, 13.8.

HRMS (ESI-TOF): m/z [M + Na]⁺ calcd for C₂₂H₁₉ClNaO₂: 373.0966; found: 373.0970.

3-(2-Butyl-3-chloro-1H-inden-1-yl)-6-fluoro-4H-chromen-4-one (6bs)

Yield: 50.8 mg (80%, reaction time = 6 h); pale yellow oil.

IR (CHCl₃): 3067, 2932, 2857, 1645, 1481, 1459, 1315, 1159, 817, 722, 600 cm^–1.

¹H NMR (400 MHz, CDCl₃): δ = 7.94 (d, J = 7.2 Hz, 1 H), 7.44–7.38 (m, 3 H), 7.36–7.31 (m, 3 H), 7.17 (t, J = 7.2 Hz, 1 H), 5.12 (s, 1 H), 2.71 (quint, J = 7.9 Hz, 1 H), 2.15 (septet, J = 5.6 Hz, 1 H), 1.58–1.45 (m, 2 H), 1.39–1.29 (m, 2 H), 0.89 (t, J = 7.3 Hz, 3 H).

¹³C{¹H} NMR (100 MHz, CDCl₃): δ = 176.6, 159.7 (d, ¹ J _C-F = 247.0 Hz), 153.1, 152.6, 144.7, 144.3, 141.6, 128.7, 127.4, 126.1, 124.9 (d, ³ J _C-F = 7.6 Hz), 123.3, 122.1, 121.8 (d, ⁴ J _C-F = 4.6 Hz), 120.3 (d, ³ J _C-F = 8.1 Hz), 118.8, 110.8 (d, ² J _C-F = 23.7 Hz), 44.9, 30.8, 26.4, 22.5, 13.8.

¹⁹F NMR (470 MHz, CDCl₃): δ = –114.7.

HRMS (ESI-TOF): m/z [M + Na]⁺ calcd for C₂₂H₁₈ClFNaO₂: 391.0872; found: 391.0868.

3-(2-Aryl-3-bromo-1H-inden-1-yl)-4H-chromen-4-ones 6ca–6cr; General Procedure

Compounds 6ca–6cr were prepared by following a similar procedure as described for compounds 6aa–6ah, using BBr₃ (2.5 equiv) instead of BF₃·OEt₂ and 0.172 mmol each of 3 and 4. In the case of 6cr, 3.5 equivalents of BBr₃ were used.

3-(3-Bromo-2-phenyl-1H-inden-1-yl)-4H-chromen-4-one (6ca)

Yield: 60.7 mg (85%, reaction time = 5.5 h); white solid; mp 150–153 °C.

IR (CHCl₃): 2915, 2857, 1631, 1613, 1465, 1395, 1351, 909, 816, 592, 534 cm^–1.

¹H NMR (500 MHz, CDCl₃): δ = 8.29 (dd, J = 8.5, 1.9 Hz, 1 H), 7.77–7.75 (m, 2 H), 7.61 (ddd, J = 7.0, 5.1, 1.8 Hz, 1 H), 7.54 (d, J = 7.6 Hz, 1 H), 7.47 (d, J = 7.5 Hz, 1 H), 7.42–7.36 (m, 4 H), 7.33 (s, 1 H), 7.31–7.28 (m, 2 H), 7.23 (dt, J = 7.4, 1.1 Hz, 1 H), 5.76 (s, 1 H).

¹³C{¹H} NMR (125 MHz, CDCl₃): δ = 177.3, 156.2, 153.2, 145.2, 143.7, 142.7, 133.6, 133.5, 128.7, 128.5, 128.1, 127.6, 127.0, 125.9, 125.2, 123.8, 123.4, 122.4, 120.9, 118.8, 118.1, 46.6.

HRMS (ESI-TOF): m/z [M + Na]⁺ calcd for C₂₄H₁₅BrNaO₂: 437.0148; found: 437.0152.

3-(3-Bromo-2-phenyl-1H-inden-1-yl)-6-methyl-4H-chromen-4-one (6cb)

Yield: 59 mg (80%, reaction time = 5 h); white solid; mp 172–175 °C.

IR (CHCl₃): 3057, 2926, 1641, 1622, 1574, 1483, 1243, 1046, 807, 696, 604 cm^–1.

¹H NMR (500 MHz, CDCl₃): δ = 8.08 (s, 1 H), 7.76 (d, J = 7.4 Hz, 2 H), 7.53 (d, J = 7.6 Hz, 1 H), 7.46 (d, J = 7.5 Hz, 1 H), 7.42–7.35 (m, 4 H), 7.30–7.27 (m, 2 H), 7.24–7.18 (m, 2 H), 5.77 (s, 1 H), 2.45 (s, 3 H).

¹³C{¹H} NMR (125 MHz, CDCl₃): δ = 177.4, 154.5, 153.1, 145.2, 143.7, 142.6, 135.2, 134.9, 133.5, 128.7, 128.5, 128.1, 127.6, 127.0, 125.2, 123.44, 123.4, 122.2, 120.9, 118.7, 117.8, 46.6, 20.9.

HRMS (ESI-TOF): m/z [M + Na]⁺ calcd for C₂₅H₁₇BrNaO₂: 451.0304; found: 451.0304.

6-Bromo-3-(3-bromo-2-phenyl-1H-inden-1-yl)-4H-chromen-4-one (6cc)

Yield: 72.2 mg (85%, reaction time = 5.5 h); white solid; mp 110–112 °C.

IR (CHCl₃): 3064, 2925, 1641, 1620, 1483, 1320, 1160, 935, 746, 696, 604 cm^–1.

¹H NMR (400 MHz, CDCl₃): δ = 8.41 (d, J = 2.3 Hz, 1 H), 7.75 (s, 1 H), 7.73 (s, 1 H), 7.69 (dd, J = 8.9, 2.4 Hz, 1 H), 7.53 (d, J = 7.6 Hz, 1 H), 7.44–7.42 (m, 1 H), 7.40–7.36 (m, 3 H), 7.33–7.29 (m, 2 H), 7.23–7.19 (m, 2 H), 5.72 (s, 1 H).

¹³C{¹H} NMR (125 MHz, CDCl₃): δ = 176.0, 154.9, 153.3, 144.8, 143.4, 142.6, 136.6, 133.3, 128.7, 128.5, 128.2, 127.7, 127.0, 125.0, 123.3, 122.7, 120.9, 120.0, 119.0, 118.6, 46.5.

HRMS (ESI-TOF): m/z [M + Na]⁺ calcd for C₂₄H₁₄Br₂NaO₂: 514.9253; found: 514.9248.

3-(3-Bromo-2-phenyl-1H-inden-1-yl)-6-fluoro-4H-chromen-4-one (6cd)

Yield: 61.1 mg (82%, reaction time = 3.5 h); white solid; mp 190–193 °C.

IR (CHCl₃): 3064, 2923, 2850, 1637, 1580, 1478, 1458, 1264, 1129, 939, 828, 731, 693, 594 cm^–1.

¹H NMR (400 MHz, CDCl₃): δ = 7.92 (dd, J = 7.9, 2.5 Hz, 1 H), 7.75 (d, J = 7.6 Hz, 2 H), 7.53 (d, J = 7.6 Hz, 1 H), 7.46–7.42 (m, 1 H), 7.40–7.36 (m, 3 H), 7.34–7.29 (m, 4 H), 7.26–7.22 (m, 1 H), 5.73 (s, 1 H).

¹³C{¹H} NMR (125 MHz, CDCl₃): δ = 176.6, 159.5 (d, ¹ J _C-F = 247.3 Hz), 153.4, 152.4, 144.9, 143.5, 142.7, 133.4, 128.6 (d, ² J _C-F = 15.4 Hz), 128.2, 127.7, 127.1, 124.9 (d, ³ J _C-F = 7.1 Hz), 123.4, 122.1, 121.9, 121.85, 121.0, 120.3 (d, ³ J _C-F = 8.0 Hz), 119.0, 110.7 (d, ² J _C-F = 23.8 Hz), 46.5.

¹⁹F NMR (470 MHz, CDCl₃): δ = –114.8.

HRMS (ESI-TOF): m/z [M + H]⁺ calcd for C₂₄H₁₅BrFO₂: 435.0216; found: 435.0218.

3-(3-Bromo-2-phenyl-1H-inden-1-yl)-6-nitro-4H-chromen-4-one (6ce)

Yield: 64.1 mg (81%, reaction time = 3.5 h); white solid; mp 180–184 °C.

IR (CHCl₃): 3080, 2924, 2857, 1649, 1628, 1529, 1465, 1345, 1321, 1262, 746, 696, 592 cm^–1.

¹H NMR (400 MHz, CDCl₃): δ = 9.16 (d, J = 2.6 Hz, 1 H), 8.44 (dd, J = 9.2, 2.7 Hz, 1 H), 7.74 (d, J = 7.5 Hz, 2 H), 7.54 (d, J = 7.5 Hz, 1 H), 7.48–7.44 (m, 2 H), 7.42–7.37 (m, 4 H), 7.32–7.27 (d, J = 7.4 Hz, 1 H), 7.25–7.23 (m, 1 H), 5.71 (s, 1 H).

¹³C{¹H} NMR (125 MHz, CDCl₃): δ = 176.0, 158.9, 153.5, 144.8, 144.4, 143.0, 142.7, 133.2, 128.7, 128.6, 128.3, 128.0, 127.9, 127.2, 123.74, 123.7, 123.3, 122.8, 121.2, 119.9, 119.4, 46.4.

HRMS (ESI-TOF): m/z [M + K]⁺ calcd for C₂₄H₁₄BrKNO₄: 497.9738; found: 497.9740.

3-(3-Bromo-2-phenyl-1H-inden-1-yl)-4H-benzo[h]chromen-4-one (6cf)

Yield: 65.6 mg (82%, reaction time = 8 h); white solid; mp 68–70 °C.

IR (CHCl₃): 3065, 2979, 2928, 1642, 1462, 1442, 1379, 1254, 1046, 914, 698, 570 cm^–1.

¹H NMR (400 MHz, CDCl₃): δ = 8.23 (t, J = 8.8 Hz, 2 H), 7.87 (d, J = 8.2 Hz, 1 H), 7.81 (d, J = 7.7 Hz, 2 H), 7.75 (d, J = 8.8 Hz, 1 H), 7.64 (t, J = 7.8 Hz, 1 H), 7.57–7.52 (m, 4 H), 7.42–7.36 (m, 3 H), 7.29–7.27 (m, 1 H), 7.23–7.22 (m, 1 H), 5.84 (s, 1 H).

¹³C{¹H} NMR (125 MHz, CDCl₃): δ = 177.1, 153.7, 152.3, 145.1, 143.6, 142.7, 135.7, 133.5, 129.3, 128.7, 128.6, 128.2, 128.0, 127.7, 127.1, 127.0, 125.4, 124.0, 123.8, 123.5, 122.1, 121.0, 120.8, 120.1, 118.9, 46.7.

HRMS (ESI-TOF): m/z [M + Na]⁺ calcd for C₂₈H₁₇BrNaO₂: 489.0287; found: 489.0286.

3-(3-Bromo-6-methyl-2-(p-tolyl)-1H-inden-1-yl)-4H-benzo[h]-chromen-4-one (6cg)

Yield: 71.3 mg (84%, reaction time = 6.5 h); white solid; mp 172–174 °C.

IR (CHCl₃): 2985, 2938, 1642, 1447, 1374, 1243, 1048, 914, 738, 609 cm^–1.

¹H NMR (500 MHz, CDCl₃): δ = 8.24 (dd, J = 8.1, 2.3 Hz, 2 H), 7.87 (d, J = 8.1 Hz, 1 H), 7.76 (d, J = 8.8 Hz, 1 H), 7.69 (d, J = 8.3 Hz, 2 H), 7.64 (dt, J = 7.3, 1.3 Hz, 1 H), 7.56 (dt, J = 7.6, 1.1 Hz, 1 H), 7.51 (s, 1 H), 7.40 (d, J = 7.7 Hz, 1 H), 7.32 (s, 1 H), 7.20–7.16 (m, 3 H), 5.79 (s, 1 H), 2.35 (s, 3 H), 2.29 (s, 3 H).

¹³C{¹H} NMR (125 MHz, CDCl₃): δ = 177.3, 153.8, 152.4, 145.3, 142.4, 140.2, 138.0, 137.0, 135.7, 130.7, 129.3, 129.27, 128.6, 128.3, 128.0, 127.1, 125.4, 124.3, 124.2, 123.9, 122.1, 120.9, 120.5, 120.1, 118.1, 46.3, 21.4, 21.2.

HRMS (ESI-TOF): m/z [M + K]⁺ calcd for C₃₀H₂₁BrKO₂: 531.0357; found: 531.0353.

2-(3-Bromo-2-phenyl-1H-inden-1-yl)-1H-benzo[f]chromen-1-one (6ch)

Yield: 70.4 mg (88%, reaction time = 2.5 h); white solid; mp 88–90 °C.

IR (CHCl₃): 3064, 2981, 2925, 1642, 1461, 1442, 1400, 1244, 1046, 792, 698, 575 cm^–1.

¹H NMR (500 MHz, CDCl₃): δ = 8.21 (dd, J = 8.1, 2.5 Hz, 2 H), 7.85–7.81 (m, 3 H), 7.72 (d, J = 8.8 Hz, 1 H), 7.61 (dt, J = 7.5, 1.0 Hz, 1 H), 7.56–7.50 (m, 4 H), 7.42–7.37 (m, 3 H), 7.28–7.27 (m, 1 H), 7.23 (dt, J = 7.5, 0.8 Hz, 1 H), 5.84 (s, 1 H).

¹³C{¹H} NMR (125 MHz, CDCl₃): δ = 177.1, 153.7, 152.3, 145.1, 143.6, 142.6, 135.6, 133.5, 129.3, 128.7, 128.5, 128.1, 127.9, 127.6, 127.1, 127.0, 125.4, 123.9, 123.8, 123.4, 122.0, 120.9, 120.7, 120.0, 118.9, 46.6.

HRMS (ESI-TOF): m/z [M + K]⁺ calcd for C₂₈H₁₇BrKO₂: 503.0043; found: 503.0038.

3-(3-Bromo-6-methyl-2-(p-tolyl)-1H-inden-1-yl)-4H-chromen-4-one (6ci)

Yield: 61 mg (80%, reaction time = 5.5 h); white solid; mp 184–186 °C.

IR (CHCl₃): 3054, 2918, 2854, 1631, 1465, 1393, 1351, 907, 816, 710, 590 cm^–1.

¹H NMR (400 MHz, CDCl₃): δ = 8.31 (d, J = 7.0 Hz, 1 H), 7.65–7.62 (m, 3 H), 7.43–7.38 (m, 2 H), 7.32–7.27 (m, 3 H), 7.19–7.16 (m, 3 H), 5.70 (s, 1 H), 2.34 (s, 3 H), 2.31 (s, 3 H).

¹³C{¹H} NMR (100 MHz, CDCl₃): δ = 177.5, 156.2, 153.3, 145.3, 142.5, 140.2, 137.9, 137.0, 133.6, 130.7, 129.2, 128.6, 128.3, 126.0, 125.2, 124.1, 123.9, 122.8, 120.5, 118.12, 118.1, 46.3, 21.5, 21.3.

HRMS (ESI-TOF): m/z [M + Na]⁺ calcd for C₂₆H₁₉BrNaO₂: 467.0443; found: 467.0444.

3-(3-Bromo-6-methyl-2-(p-tolyl)-1H-inden-1-yl)-6-methyl-4H-chromen-4-one (6cj)

Yield: 65.3 mg (83%, reaction time = 6 h); white solid; mp 228–230 °C.

IR (CHCl₃): 3027, 2921, 2854, 1641, 1621, 1484, 1320, 1166, 909, 816, 787, 594 cm^–1.

¹H NMR (500 MHz, CDCl₃): δ = 8.10 (s, 1 H), 7.64 (d, J = 8.2 Hz, 2 H), 7.43–7.37 (m, 2 H), 7.29–7.27 (m, 2 H), 7.21–7.15 (m, 4 H), 5.71 (s, 1 H), 2.46 (s, 3 H), 2.34 (s, 3 H), 2.31 (s, 3 H).

¹³C{¹H} NMR (125 MHz, CDCl₃): δ = 177.5, 154.5, 153.2, 145.4, 142.5, 140.2, 137.9, 136.9, 135.1, 134.8, 130.7, 129.2, 128.5, 128.2, 125.2, 124.1, 123.5, 122.5, 120.4, 118.0, 117.9, 46.3, 21.4, 21.2, 20.9.

HRMS (ESI-TOF): m/z [M + H]⁺ calcd for C₂₇H₂₂BrO₂: 457.0798; found: 457.0799.

3-(3-Bromo-6-methyl-2-(p-tolyl)-1H-inden-1-yl)-6-fluoro-4H-chromen-4-one (6ck)

Yield: 68.2 mg (86%, reaction time = 4 h); white solid; mp 250–252 °C.

IR (CHCl₃): 3061, 2921, 2854, 1635, 1613, 1476, 1454, 1390, 1265, 1129, 817, 593 cm^–1.

¹H NMR (400 MHz, CDCl₃): δ = 7.95 (t, J = 7.5 Hz, 1 H), 7.64 (t, J = 7.7 Hz, 2 H), 7.42–7.33 (m, 4 H), 7.27–7.26 (m, 1 H), 7.22–7.16 (m, 3 H), 5.68 (s, 1 H), 2.36–2.31 (m, 6 H).

¹³C{¹H} NMR (125 MHz, CDCl₃): δ = 176.8, 159.6 (d, ¹ J _C-F = 248.6 Hz), 153.5, 152.5, 145.1, 142.3, 140.2, 138.0, 137.0, 130.6, 129.3, 128.5, 128.4, 124.9 (d, ³ J _C-F = 7.3 Hz), 124.1, 122.3, 121.9 (d, ³ J _C-F = 10.2 Hz), 120.6, 120.3 (d, ³ J _C-F = 8.1 Hz), 118.2, 110.7 (d, ² J _C-F = 23.7 Hz), 46.2, 21.5, 21.3.

¹⁹F NMR (470 MHz, CDCl₃): δ = –114.9.

HRMS (ESI-TOF): m/z [M + H]⁺ calcd for C₂₆H₁₉BrFO₂: 461.0547; found: 461.0552.

6-Bromo-3-(3-bromo-6-methyl-2-(p-tolyl)-1H-inden-1-yl)-4H-chromen-4-one (6cl)

Yield: 64.7 mg (72%, reaction time = 6 h); white solid; mp 184–186 °C.

IR (CHCl₃): 2921, 2854, 1641, 1621, 1483, 1432, 1320, 1166, 909, 817, 787, 594 cm^–1.

¹H NMR (400 MHz, CDCl₃): δ = 8.43 (d, J = 2.2 Hz, 1 H), 7.68 (dd, J = 8.9, 2.4 Hz, 1 H), 7.62 (d, J = 8.2 Hz, 2 H), 7.38 (d, J = 7.9 Hz, 1 H), 7.31 (s, 1 H), 7.24 (s, 1 H), 7.21–7.16 (m, 4 H), 5.66 (s, 1 H), 2.35 (s, 3 H), 2.32 (s, 3 H).

¹³C{¹H} NMR (100 MHz, CDCl₃): δ = 176.2, 155.0, 153.4, 145.0, 142.2, 140.2, 138.0, 137.1, 136.6, 130.6, 129.3, 128.6, 128.5, 128.4, 125.1, 124.1, 123.1, 120.6, 120.1, 118.6, 118.3, 46.2, 21.5, 21.3.

HRMS (ESI-TOF): m/z [M + H]⁺ calcd for C₂₆H₁₉Br₂O₂: 520.9746; found: 520.9739.

3-(3-Bromo-2-(4-bromophenyl)-1H-inden-1-yl)-6-methyl-4H-chromen-4-one (6cm)

Yield: 63.8 mg (73%, reaction time = 2.5 h); white solid; mp 160–164 °C.

IR (CHCl₃): 1640, 1622, 1483, 1458, 1319, 1158, 910, 819, 732, 606 cm^–1.

¹H NMR (500 MHz, CDCl₃): δ = 8.07 (s, 1 H), 7.73 (d, J = 7.7 Hz, 1 H), 7.64–7.61 (m, 1 H), 7.53–7.50 (m, 2 H), 7.48 (d, J = 8.5 Hz, 1 H), 7.44–7.41 (m, 2 H), 7.39–7.35 (m, 2 H), 7.31–7.28 (m, 1 H), 7.25–7.19 (m, 1 H), 5.72 (s, 1 H), 2.46 (s, 3 H).

¹³C{¹H} NMR (125 MHz, CDCl₃): δ = 177.2, 154.5, 153.2, 142.8, 142.6, 135.4, 135.0, 132.4, 131.7, 130.7, 130.2, 128.7, 127.7, 127.2, 125.2, 123.4, 122.3, 121.9, 121.0, 119.3, 117.9, 46.6, 20.9.

HRMS (ESI-TOF): m/z [M + Na]⁺ calcd for C₂₅H₁₆Br₂NaO₂: 528.9409; found: 528.9401.

3-(3-Bromo-2-ethyl-1H-inden-1-yl)-4H-chromen-4-one (6cn)

Yield: 53.7 mg (85%, reaction time = 5 h); pale yellow oil.

IR (CHCl₃): 2966, 2923, 1644, 1621, 1483, 1459, 1339, 1318, 1160, 1139, 935, 817, 727, 597 cm^–1.

¹H NMR (400 MHz, CDCl₃): δ = 8.32 (d, J = 7.1 Hz, 1 H), 7.68 (t, J = 7.6 Hz, 1 H), 7.47–7.41 (m, 2 H), 7.39–7.31 (m, 4 H), 7.17 (t, J = 7.1 Hz, 1 H), 5.18 (s, 1 H), 2.72 (sextet, J = 7.4 Hz, 1 H), 2.21 (sextet, J = 6.9 Hz, 1 H), 1.14 (t, J = 7.5 Hz, 3 H).

¹³C{¹H} NMR (100 MHz, CDCl₃): δ = 177.3, 156.3, 153.0, 149.3, 145.0, 142.8, 133.7, 128.0, 127.4, 126.0, 125.3, 123.8, 123.2, 122.2, 119.9, 118.2, 118.0, 45.5, 21.7, 13.3.

HRMS (ESI-TOF): m/z [M + Na]⁺ calcd for C₂₀H₁₅BrNaO₂: 389.0147; found: 389.0143.

3-(3-Bromo-2-ethyl-1H-inden-1-yl)-6-methyl-4H-chromen-4-one (6co)

Yield: 54.4 mg (83%, reaction time = 6 h); pale yellow oil.

IR (CHCl₃): 2965, 2927, 2871, 1644, 1621, 1483, 1459, 1318, 1160, 1045, 816, 727, 542 cm^–1.

¹H NMR (400 MHz, CDCl₃): δ = 8.09 (s, 1 H), 7.48 (d, J = 7.7 Hz, 1 H), 7.38–7.30 (m, 5 H), 7.16 (t, J = 7.2 Hz, 1 H), 5.18 (s, 1 H), 2.71 (sextet, J = 7.3 Hz, 1 H), 2.48 (s, 3 H), 2.21 (sextet, J = 6.8 Hz, 1 H), 1.13 (t, J = 7.5 Hz, 3 H).

¹³C{¹H} NMR (100 MHz, CDCl₃): δ = 177.4, 154.7, 152.9, 149.5, 145.1, 142.8, 135.3, 135.0, 127.4, 126.0, 125.3, 123.5, 123.2, 121.9, 119.9, 117.9, 117.87, 45.5, 21.7, 21.0, 13.2.

HRMS (ESI-TOF): m/z [M + Na]⁺ calcd for C₂₁H₁₇BrNaO₂: 403.0304; found: 403.0305.

6-Bromo-3-(3-bromo-2-pentyl-1H-inden-1-yl)-4H-chromen-4-one (6cp)

Yield: 67.2 mg (80%, reaction time = 5.5 h); pale yellow oil.

IR (CHCl₃): 2959, 2928, 2856, 1647, 1622, 1605, 1461, 1435, 1335, 1149, 933, 818, 725, 600 cm^–1.

¹H NMR (500 MHz, CDCl₃): δ = 8.44 (s, 1 H), 7.75 (dd, J = 8.9, 2.5 Hz, 1 H), 7.39–7.29 (m, 5 H), 7.17 (dt, J = 7.2, 1.2 Hz, 1 H), 5.12 (s, 1 H), 2.70–2.64 (m, 1 H), 2.16 (septet, J = 5.2 Hz, 1 H), 1.58–1.48 (m, 2 H), 1.34–1.27 (m, 4 H), 0.85 (t, J = 6.7 Hz, 3 H).

¹³C{¹H} NMR (100 MHz, CDCl₃): δ = 176.0, 155.1, 153.1, 147.8, 144.8, 142.7, 136.7, 128.7, 127.5, 126.1, 125.1, 123.2, 122.6, 120.13, 120.0, 119.0, 118.8, 45.7, 31.5, 28.4, 28.2, 22.4, 13.9.

HRMS (ESI-TOF): m/z [M + H]⁺ calcd for C₂₃H₂₁Br₂O₂: 488.9883; found: 488.9883.

3-(3-Bromo-2-butyl-1H-inden-1-yl)-6-fluoro-4H-chromen-4-one (6cq)

Yield: 59.7 mg (84%, reaction time = 6 h); pale yellow oil.

IR (CHCl₃): 2959, 2928, 2856, 1647, 1622, 1606, 1461, 1435, 1335, 1149, 933, 725, 600 cm^–1.

¹H NMR (400 MHz, CDCl₃): δ = 7.94 (d, J = 6.7 Hz, 1 H), 7.44–7.42 (m, 1 H), 7.41–7.39 (m, 1 H), 7.37–7.29 (m, 4 H), 7.17 (dt, J = 7.3, 1.4 Hz, 1 H), 5.12 (s, 1 H), 2.73–2.65 (m, 1 H), 2.16 (septet, J = 5.2 Hz, 1 H), 1.54–1.45 (m, 2 H), 1.39–1.30 (m, 2 H), 0.89 (t, J = 7.3 Hz, 3 H).

¹³C{¹H} NMR (125 MHz, CDCl₃): δ = 176.6, 159.7 (d, ¹ J _C-F = 248.1 Hz), 153.1, 152.6, 147.8, 145.0, 142.7, 127.5, 126.1, 124.9 (d, ³ J _C-F = 7.4 Hz), 123.2, 122.0 (d, ² J _C-F = 25.8 Hz), 121.7, 120.3 (d, ³ J _C-F = 8.0 Hz), 120.0, 118.9, 110.8 (d, ² J _C-F = 23.7 Hz), 45.7, 30.9, 27.9, 22.5, 13.8.

¹⁹F NMR (470 MHz, CDCl₃): δ = –114.7.

HRMS (ESI-TOF): m/z [M + K]⁺ calcd for C₂₂H₁₈BrFKO₂: 451.0106; found: 451.0106.

3-(3-Bromo-2-(2-bromoethyl)-1H-inden-1-yl)-4H-chromen-4-one (6cr)

Yield: 61.3 mg (80%, reaction time = 5.5 h); white semisolid.

IR (CHCl₃): 3077, 2925, 2857, 1639, 1610, 1479, 1463, 1317, 909, 824, 668 cm^–1.

¹H NMR (400 MHz, CDCl₃): δ = 8.30 (d, J = 7.9 Hz, 1 H), 7.72–7.67 (m, 1 H), 7.48–7.43 (m, 3 H), 7.40–7.36 (m, 2 H), 7.31 (d, J = 7.2 Hz, 1 H), 7.23 (dt, J = 7.4, 0.9 Hz, 1 H), 5.17 (s, 1 H), 3.65–3.55 (m, 2 H), 3.25 (quint, J = 6.7 Hz, 1 H), 2.76 (septet, J = 7.3 Hz, 1 H).

¹³C{¹H} NMR (100 MHz, CDCl₃): δ = 177.1, 156.3, 153.4, 144.6, 144.3, 142.5, 133.9, 127.7, 126.7, 126.1, 125.5, 123.7, 123.3, 121.7, 121.3, 120.5, 118.2, 46.1, 31.7, 30.1.

HRMS (ESI-TOF): m/z [M + Na]⁺ calcd for C₂₀H₁₄Br₂NaO₂: 468.9233; found: 468.9238.

8-(2-Aryl-3-halo-1H-inden-1-yl)-2H-chromen-2-ones 8a–8f; General Procedure

Compounds 8a–8f were prepared by following a similar procedure as described for compounds 6aa–6ah, using BBr₃ or BCl₃ (2.5 equiv) instead of BF₃·OEt₂ and 0.172 mmol each of 7 and 4.

8-(3-Chloro-2-phenyl-1H-inden-1-yl)-2H-chromen-2-one (8a)

Yield: 60.5 mg (95%, reaction time = 4 h); white solid; mp 126–130 °C.

IR (CHCl₃): 2923, 2850, 1735, 1602, 1490, 1400, 1267, 1076, 918, 868, 665, 534 cm^–1.

¹H NMR (500 MHz, CDCl₃): δ = 7.72–7.70 (m, 3 H), 7.56 (d, J = 7.5 Hz, 1 H), 7.39 (t, J = 7.5 Hz, 1 H), 7.32–7.29 (m, 2 H), 7.27–7.25 (m, 2 H), 7.22–7.19 (m, 2 H), 6.96 (t, J = 7.0 Hz, 1 H), 6.84 (s, 1 H), 6.50 (d, J = 8.9 Hz, 1 H), 6.04 (s, 1 H).

¹³C{¹H} NMR (125 MHz, CDCl₃): δ = 160.5, 152.1, 144.0, 141.9, 141.5, 133.0, 130.4, 129.1, 128.5, 128.4, 127.8, 127.6, 127.3, 127.0, 126.8, 124.6, 123.6, 119.7, 119.1, 116.5, 47.8.

HRMS (ESI-TOF): m/z [M + Na]⁺ calcd for C₂₄H₁₅ClNaO₂: 393.0653; found: 393.0647.

8-(3-Chloro-6-methyl-2-(p-tolyl)-1H-inden-1-yl)-2H-chromen-2-one (8b)

Yield: 63.8 mg (93%, reaction time = 4.5 h); white solid; mp 139–142 °C.

IR (CHCl₃): 2854, 1734, 1602, 1490, 1400, 1266, 1076, 914, 699, 578, 538 cm^–1.

¹H NMR (500 MHz, CDCl₃): δ = 7.69 (d, J = 8.8 Hz, 1 H), 7.62 (d, J = 7.8 Hz, 2 H), 7.43 (d, J = 7.7 Hz, 1 H), 7.24 (d, J = 7.5 Hz, 1 H), 7.18 (d, J = 7.7 Hz, 1 H), 7.11 (d, J = 8 Hz, 2 H), 7.06 (s, 1 H), 6.95 (t, J = 6.8 Hz, 1 H), 6.86 (s, 1 H), 6.50 (d, J = 8.7 Hz, 1 H), 5.99 (s, 1 H), 2.32 (s, 3 H), 2.27 (s, 3 H).

¹³C{¹H} NMR (125 MHz, CDCl₃): δ = 160.5, 152.0, 145.2, 143.9, 140.4, 139.4, 137.5, 136.8, 130.4, 130.2, 129.0, 128.3, 128.27, 128.25, 127.7, 126.7, 124.6, 124.3, 119.2, 119.1, 116.3, 47.3, 21.4, 21.1.

HRMS (ESI-TOF): m/z [M + H]⁺ calcd for C₂₆H₂₀ClO₂: 399.1146; found: 399.1138.

8-(3-Bromo-2-phenyl-1H-inden-1-yl)-2H-chromen-2-one (8c)

Yield: 64.3 mg (90%, reaction time = 10 h); white solid; mp 110–112 °C.

IR (CHCl₃): 3067, 1734, 1602, 1574, 1488, 1402, 1175, 918, 621, 578 cm^–1.

¹H NMR (500 MHz, CDCl₃): δ = 7.70–7.69 (m, 3 H), 7.55 (d, J = 7.6 Hz, 1 H), 7.40 (t, J = 7.0 Hz, 1 H), 7.32–7.29 (m, 2 H), 7.25–7.19 (m, 4 H), 6.95 (t, J = 6.8 Hz, 1 H), 6.82 (s, 1 H), 6.47 (d, J = 10.4 Hz, 1 H), 6.01 (s, 1 H).

¹³C{¹H} NMR (125 MHz, CDCl₃): δ = 160.4, 152.1, 145.3, 143.9, 143.1, 133.7, 129.3, 128.7, 128.2, 127.9, 127.7, 127.0, 126.9, 126.8, 124.6, 123.5, 120.9, 119.1, 118.9, 117.0, 116.4, 49.0.

HRMS (ESI-TOF): m/z [M + Na]⁺ calcd for C₂₄H₁₅BrNaO₂: 437.0148; found: 437.0140.

8-(3-Bromo-6-methyl-2-(p-tolyl)-1H-inden-1-yl)-2H-chromen-2-one (8d)

Yield: 67.1 mg (88%, reaction time = 11 h); white solid; mp 145–148 °C.

IR (CHCl₃): 2922, 1735, 1602, 1491, 1258, 1075, 733, 618 cm^–1.

¹H NMR (500 MHz, CDCl₃): δ = 7.68 (d, J = 6.7 Hz, 1 H), 7.60 (d, J = 8 Hz, 2 H), 7.41 (d, J = 7.7 Hz, 1 H), 7.24 (d, J = 7.5 Hz, 1 H), 7.19 (d, J = 7.7 Hz, 1 H), 7.10 (d, J = 7.9 Hz, 2 H), 7.03 (s, 1 H), 6.96 (t, J = 7.0 Hz, 1 H), 6.84 (s, 1 H), 6.49 (d, J = 8.8 Hz, 1 H), 5.96 (s, 1 H), 2.32 (s, 3 H), 2.27 (s, 3 H).

¹³C{¹H} NMR (125 MHz, CDCl₃): δ = 160.5, 152.0, 144.2, 143.9, 143.2, 140.6, 137.6, 136.8, 130.9, 130.5, 129.0, 128.4, 128.3, 127.5, 126.7, 124.6, 124.2, 120.5, 119.0, 118.2, 116.3, 48.7, 21.4, 21.2.

HRMS (ESI-TOF): m/z [M + H]⁺ calcd for C₂₆H₂₀BrO₂: 443.0641; found: 443.0634.

8-(3-Chloro-2-phenyl-1H-inden-1-yl)-6-methyl-2H-chromen-2-one (8e)

Yield: 53.1 mg (79%, reaction time = 5 h); yellow solid; mp 105–108 °C.

IR (CHCl₃): 2924, 1732, 1625, 1606, 1583, 1429, 1295, 1110, 878, 655, 593 cm^–1.

¹H NMR (500 MHz, CDCl₃): δ = 7.96 (d, J = 7.6 Hz, 2 H), 7.87 (d, J = 8.8 Hz, 1 H), 7.81 (d, J = 7.6 Hz, 1 H), 7.63 (t, J = 7.3 Hz, 1 H), 7.57–7.54 (m, 2 H), 7.50–7.44 (m, 4 H), 6.86 (s, 1 H), 6.70 (d, J = 8.1 Hz, 1 H), 6.26 (s, 1 H), 2.34 (s, 3 H).

¹³C{¹H} NMR (125 MHz, CDCl₃): δ = 160.7, 150.2, 145.2, 143.9, 141.8, 141.6, 134.3, 133.1, 131.0, 128.9, 128.5, 128.3, 127.7, 127.5, 126.9, 126.8, 123.6, 119.6, 118.9, 116.3, 47.7, 20.5.

HRMS (ESI-TOF): m/z [M + Na]⁺ calcd for C₂₅H₁₇ClNaO₂: 407.0809; found: 407.0802.

8-(3-Bromo-6-methyl-2-(p-tolyl)-1H-inden-1-yl)-6-methyl-2H-chromen-2-one (8f)

Yield: 69.1 mg (88%, reaction time = 6 h); yellow solid; mp 78–80 °C.

IR (CHCl₃): 2922, 1732, 1623, 1606, 1459, 1395, 1286, 1156, 1106, 914, 790, 666, 614 cm^–1.

¹H NMR (400 MHz, CDCl₃): δ = 7.59 (d, J = 8.1 Hz, 2 H), 7.40 (d, J = 7.7 Hz, 1 H), 7.30–7.25 (m, 1 H), 7.18 (d, J = 7.7 Hz, 1 H), 7.09 (d, J = 7.9 Hz, 2 H), 7.01 (d, J = 5.5 Hz, 2 H), 6.60 (s, 1 H), 6.48–6.42 (m, 1 H), 5.91 (s, 1 H), 2.32 (s, 3 H), 2.27 (s, 3 H), 2.11 (s, 3 H).

¹³C{¹H} NMR (125 MHz, CDCl₃): δ = 160.9, 150.3, 144.4, 143.9, 140.7, 137.6, 136.8, 134.3, 131.2, 131.1, 129.0, 128.5, 128.3, 127.1, 126.9, 124.3, 120.5, 118.9, 118.2, 116.4, 48.7, 21.5, 21.2, 20.6.

HRMS (ESI-TOF): m/z [M + Na]⁺ calcd for C₂₇H₂₁BrNaO₂: 479.0617; found: 479.0609.

2-(Hydroxymethyl)cyclohex-2-en-1-one (15)[16]

To a stirred mixture of 36% aq HCHO (0.45 mL, 5.20 mmol, 5.0 equiv) and cyclohex-2-enone (100 mg, 1.040 mmol, 1.0 equiv) was added DMAP (38 mg, 0.312 mmol, 0.3 equiv). The resulting mixture was stirred at room temperature. After consumption of the starting material (2 d), the reaction was quenched with 1.5 N HCl until pH 4. The mixture was extracted with CH₂Cl₂ (3 × 3 mL) and the combined organic layers were washed successively with sat. aq NaHCO₃ solution and brine. Solvents were removed through vacuum and the residue was purified by silica gel column chromatography (petroleum ether/EtOAc, 3:2) to afford 15 (92 mg, 70%) as a pale yellow oil.

IR (CHCl₃): 2929, 2869, 1670, 1391, 1173, 1068, 910, 543 cm^–1.

¹H NMR (400 MHz, CDCl₃): δ = 6.92 (t, J = 3.8 Hz, 1 H), 4.21 (s, 2 H), 2.59 (s, 1 H), 2.44–2.36 (m, 4 H), 1.99 (quint, J = 6.6 Hz, 2 H).

¹³C{¹H} NMR (100 MHz, CDCl₃): δ = 200.6, 146.9, 138.2, 61.8, 38.1, 25.6, 22.6.

6-Oxocyclohex-1-ene-1-carbaldehyde (10e)[17]

To a stirred solution of 2-(hydroxymethyl)cyclohex-2-en-1-one (15; 50 mg, 0.396 mmol, 1.0 equiv) in EtOAc (3 mL) was added IBX (166 mg, 0.594 mmol, 1.5 equiv) under N₂ atmosphere. The resulting suspension was refluxed for 5 h (TLC monitoring). The reaction mixture was cooled to room temperature and filtered through a sintered glass funnel. The filter cake was washed with EtOAc (3 × 2 mL). The combined filtrate was concentrated and the residue was purified by silica gel column chromatography (petroleum ether/EtOAc, 7:3) to afford 10e (39.3 mg, 80%) as a green oil.

IR (CHCl₃): 2924, 2854, 1714, 1460, 1267, 1068, 822, 669 cm^–1.

¹H NMR (400 MHz, CDCl₃): δ = 10.07 (s, 1 H), 7.81 (t, J = 4.1 Hz, 1 H), 2.63–2.58 (m, 2 H), 2.55–2.51 (m, 2 H), 2.07 (quint, J = 7.1 Hz, 2 H).

¹³C{¹H} NMR (100 MHz, CDCl₃): δ = 197.9, 189.2, 157.8, 135.0, 38.0, 26.4, 21.8.

LRMS (ESI-TOF): m/z [M + K]⁺ calcd for C₇H₈KO₂: 163.0155; found: 163.0735.

2,2-Difluoro-6-methyl-4-phenyl-2H-1,3,2-dioxaborinin-1-ium-2-uide (12)

To a mixture of benzoylacetone (11; 50 mg, 0.308 mmol, 1.0 equiv) and alkyne 4a (55 mg, 0.308 mmol, 1.0 equiv) in CH₂Cl₂ (2 mL) was added BF₃·OEt₂ (0.215 mL, 0.77 mmol, 2.5 equiv) at room temperature. The mixture was stirred for 7 h at which time TLC indicated consumption of 11. The reaction was then quenched with a few drops of sat. aq Na₂S₂O₃ solution. The solvent was removed through vacuum and the residue was purified by silica gel column chromatography (petroleum ether/EtOAc, 4:1) to afford 12 (38.4 mg, 60%) as a cream-coloured solid; mp 120–123 °C.[11]

IR (CHCl₃): 1542, 1440, 1357, 1116, 1069, 707, 680, 575 cm^–1.

¹H NMR (400 MHz, CDCl₃): δ = 8.05 (d, J = 7.6 Hz, 2 H), 7.68 (t, J = 7.4 Hz, 1 H), 7.52 (t, J = 7.7 Hz, 2 H), 6.60 (s, 1 H), 2.41 (s, 3 H).

¹³C{¹H} NMR (100 MHz, CDCl₃): δ = 192.6, 182.8, 135.4, 131.1, 129.1, 129.0, 97.5, 24.7.

¹⁹F NMR (470 MHz, CDCl₃): δ = –138.6.

HRMS (ESI-TOF): m/z [M + Na]⁺ calcd for C₁₀H₉BF₂NaO₂: 233.0558; found: 233.0554.

Acknowledgment

A.K. thanks the Indian Institute of Technology Bombay for a research fellowship.

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