Palladium-Catalyzed Germylation of Aryl Bromides and Aryl Triflates Using Hexamethyldigermane

Abstract

Palladium-catalyzed germylation of aryl bromides and aryl triflates using commercially available hexamethyldigermane is described. Optimized reaction conditions afforded various functionalized aryltrimethylgermanes, including drug-like molecules, in moderate to good yields, demonstrating the versatility of the presented protocols.

Key words

palladium catalysis - germylation - digermane - aryl bromide - aryl triflate

Organosilicon[1] and organotin[2] compounds are useful reagents in organic synthesis and can be applied to a variety of synthetic transformations, including transition-metal-catalyzed cross-coupling reactions. Organogermanium compounds, however, have attracted much less attention. Germanium is located between silicon and tin in the periodic table, and the properties of a C–Ge bond are intermediate between a C–Si bond and a C–Sn bond.[3] Arylgermanes are expected to be more reactive toward electrophiles[4] than arylsilanes due to the stronger β-effect from the C–Ge bond compared with the C–Si bond.[5] Organotin compounds are more reactive, but highly toxic.[6] Therefore, arylgermanes are potentially attractive synthetic intermediates, but only a limited number of synthetic reactions using arylgermanes are reported.[3] ^, [4b] [c] [d] ^, [7] [8] This is in part due to the high cost of germanium, but the lack of the general methods to prepare arylgermanes is also an important issue. Nucleophilic substitution of halogermanes by aryllithium or Grignard reagents is the most reliable method for accessing arylgermanes.[3b] These highly reactive organometallic reagents are, however, incompatible with sensitive functional groups.

Scheme 1 Germylation of aryl (pseudo)halides

Transition-metal-catalyzed silylation of aryl (pseudo)halides using disilanes[9] or hydrosilanes[10] has been extensively investigated over the last several decades for synthesizing arylsilanes without using aryllithium or Grignard reagents. On the other hand, studies of transition-metal-catalyzed germylation of aryl halides are scarce,[8c] [9b] [10h] [k] [11] although such reactions enable the direct synthesis of functionalized arylgermanes. Oshima achieved Pd-catalyzed germylation of aryl iodides using tri(2-furyl)germane (Scheme [1a]),[8c] but electron-deficient aryl iodides were not investigated, and an aryl bromide was unreactive. Yamanoi and Nishihara reported general conditions for Pd-catalyzed coupling reactions of various aryl iodides and hydrogermanes (Scheme [1a]).[10k] In contrast to aryl iodides, less reactive aryl bromides are still difficult substrates for germylation (Scheme [1b]). Eaborn reported Pd-catalyzed germylation of aryl bromides­ with hexaethyldigermane, but the reactions required harsh conditions (140–180 °C) and resulted in a low yield.[9b] Tanaka reported Pd-catalyzed germylation of bromobenzene and 2,5-dibromothiophene, but moisture sensitive dichlorotetramethyldigermane was required, and the results were rather complicated due to halogen exchange.[11] To date, general and practical conditions for transition-metal-catalyzed germylation of aryl bromides and aryl triflates have not been reported.

Table 1 Optimization of Reaction Conditions for Germylation of 1a and 2a ^a

Entry	Substrate	Pd cat.	Solvent	Base	PTC	Additive	Temp (°C)	Yield of 4a (%)b	Ratio of 1a/2a:4a:6:7:8:9 c
1	1a	[PdCl(allyl)]2	THF/H2O	NaOH	Bu4NBr	–	100	44	2:75:4:18:0:0
2	1a	[PdCl(allyl)]2	toluene/H2O	NaOH	Bu4NBr	–	100	60	3:80:13:3:1:0
3	1a	Pd2(dba)3	toluene/H2O	NaOH	Bu4NBr	–	100	39	0:65:12:21:2:0
4	1a	Pd(OAc)2	toluene/H2O	NaOH	Bu4NBr	–	100	62	0:73:3:23:1:0
5	1a	Pd(OAc)2	toluene/H2O	KOt-Bu	Bu4NBr	–	100	48	0:66:2:32:1:0
6	1a	Pd(OAc)2	toluene/H2O	KOAc	Bu4NBr	–	100	47	0:67:25:6:2:0
7	1a	Pd(OAc)2	toluene/H2O	Cs2CO3	Bu4NBr	–	100	77	0:89:6:4:1:0
8	1a	Pd(OAc)2	toluene/H2O	Cs2CO3	Bu4NOAc	–	100	56	20:59:0:21:0:0
9	1a	Pd(OAc)2	toluene/H2O	Cs2CO3	Et4NHCO3	–	100	93 (86)d	0:92:8:0:0:0
10e	1a	Pd(OAc)2	toluene/H2O	Cs2CO3	Et4NHCO3	–	100	63	0:71:3:0:26:0
11	2a	Pd(OAc)2	toluene/H2O	Cs2CO3	Et4NHCO3	–	100	0	99:0:0:0:0:1
12	2a	Pd(OAc)2	toluene	Cs2CO3	Et4NHCO3	–	100	11	25:16:0:0:16:43
13	2a	Pd(OAc)2	toluene	Cs2CO3	Et4NHCO3	LiCl	100	13	61:11:0:0:5:23
14	2a	Pd(OAc)2	toluene	Cs2CO3	Et4NHCO3	LiBr	100	9	94:5:1:0:0:0
15	2a	Pd(OAc)2	toluene	Cs2CO3	Et4NHCO3	Bu4NCl	100	2	94:2:0:0:0:4
16	2a	Pd(OAc)2	toluene	Cs2CO3	Et4NHCO3	Bu4NBr	100	8	88:7:0:0:0:5
17	2a	Pd(OAc)2	toluene	Cs2CO3	Et4NHCO3	Bu4NI	100	15	57:22:0:4:1:16
18	2a	Pd(OAc)2	toluene	Cs2CO3	Et4NHCO3	Et4NCl	100	35	7:54:1:0:9
19	2a	Pd(OAc)2	toluene	Cs2CO3	Et4NHCO3	Et4NBr	100	49	30:42:0:2:14:12
20	2a	Pd(OAc)2	toluene	Cs2CO3	Et4NHCO3	Et4NBr	120	77	2:59:0:0:27:12
21	2a	Pd(OAc)2	toluene	Cs2CO3	–	Et4NBr	120	85 (83%)d	1:66:2:0:24:7

^a The reactions were performed using 1a or 2a (0.10 mmol), 3 (0.12 mmol), Pd cat. (10 mol% of Pd), ligand 5 (0.02 mmol, 20 mol%), PTC (0.01 mmol, 10 mol%), and base (0.12 mmol) in THF/H₂O (1:1) or toluene/H₂O (1:1), or toluene (0.5 mL) for 24 h, unless otherwise noted.

^b Determined by GC/MS analysis using pentadecane as an internal standard.

^c The radio of the TIC area in GC/MS analysis.

^d Isolated yield in 0.50 mmol scale.

^e PPh₃ (0.02 mmol, 20 mol%) was used as a ligand instead of 5.

In this article, we report Pd-catalyzed germylation of aryl bromides 1 and aryl triflates 2 using commercially available hexamethyldigermane (3) (Scheme [1c]).

Scheme 2 Scope and limitations of the germylation of aryl bromides 1 and aryl triflates 2. Reagents and conditions A: 1 (1.0 equiv), 3 (1.2 equiv), Pd(OAc)₂ (10 mol%), 5 (20 mol%), Et₄NHCO₃ (10 mol%), and Cs₂CO₃ (1.2 equiv) in toluene/H₂O at 100 °C for 24 h. Reagents and conditions B: 1 or 2 (1.0 equiv), 3 (1.2 equiv), Pd(OAc)₂ (10 mol%), ligand 5 (20 mol%), Cs₂CO₃ (1.2 equiv), and Et₄NBr (1.0 equiv) in toluene at 120 °C for 24 h. Isolated yields are shown. See experimental section for detailed conditions and scale of each substrate. ^a Reaction was carried out at 120 °C. ^b Only a trace amount of the desired product was observed in GC/MS analysis. ^c Pd(OAc)₂ (20 mol%) and 5 (40 mol%) were used. ^d KOAc instead of Cs₂CO₃ was used. ^e Reaction was carried out at 130 °C. ^f Digermane 3 (2.0 equiv) was used, 62 h.

We initiated our investigation by optimizing the reaction conditions for germylation of aryl bromide 1a using hexamethyldigermane (3) (Table [1], entries 1–10). The reaction conditions of Pd-catalyzed silylation of aryl halides with hexamethyldisilane reported by Shirakawa and Hiyama[9g] were selected as the initial conditions (entries 1,2). These conditions afforded the desired germylated arene 4a in moderate yield along with several by-products. GC/MS analysis revealed the formation of reduced product 6, biaryl 7, and unidentified products bearing an allyl or propenyl group. Other palladium sources using toluene/H₂O as the solvent were next investigated to circumvent incorporation of the C3 units derived from [PdCl(allyl)]₂. While Pd₂(dba)₃ exhibited inferior catalytic activity (entry 3), the use of Pd(OAc)₂ resulted in 62% yield (entry 4). In these cases, incorporation of the C3 unit was avoided, but a significant amount of 7 was produced. Several bases and phase transfer catalysts (PTC) were then screened to improve the selectivity (entries 5–9), and Cs₂CO₃ and Et₄NHCO₃ were the most effective for providing 4a in 93% yield, with only a tiny amount of 6 (entry 9). A control experiment using PPh₃ as a ligand instead of 5 (entry 10) was performed. The reaction unexpectedly proceeded with moderate yield, but a relatively large amount of phenyl(trimethyl)germane (8) was observed. This by-product would be formed by reduction of 4a or an aryl–aryl exchange between an arylpalladium intermediate­ and PPh₃ during the catalytic process.[12] Germylation of aryl triflate 2a (entries 11–21) was investigated next. The optimized conditions for aryl bromide 1a in entry 9 afforded no products, and 2a remained intact (entry 11). The use of toluene as the sole solvent provided the desired product 3, but the yield was low and a significant amount of 9 was observed (entry 12). We speculated the lower reactivity of 2a compared with 1a might be due to the absence of a halide ion in the reaction, and several halide sources were screened as potential additives (Table [1], entries 13–19). The addition of one equivalent of Et₄NBr was found to be effective, and the yield was improved to 49% (entry 19). Raising the reaction temperature to 120 °C further improved the yield (entry 20). Finally, the conditions without Et₄NHCO₃ resulted in a slightly better yield (entry 21), and were determined to be the optimal conditions for 2a. Although a significant amount of 8 was observed at 120 °C, the yield of 4a based on an internal standard was high (entries 20 and 21), indicating that 8 was derived from ligand 5 rather than 2a or 4a in these cases.

The scope and limitations of the optimized conditions for germylation are summarized in Scheme [2]. Both electron-deficient and electron-rich aryl bromides afforded the desired products in good yields (4a, 4b). Various functional groups were well tolerated, providing germylated building blocks that are useful for further transformation (4c–f). Heteroaryl­ bromides were reactive to give the corresponding germylated products (4g, 4h, 4i). The conditions optimized for aryl triflates (Conditions B) were more effective for less reactive aryl bromides to afford 4f, 4h and 4i. 4-Bromo­phenol (1j), however, failed to give the germylated product 4j, and phenol was detected as a major product under both reaction conditions. Germylated drug-like structures (4k,[13] 4l [14]) were accessible from the corresponding aryl bromides under Conditions B. In addition to aryl bromides, various types of aryl triflates afforded the desired germylated arenes. Electron-rich arenes such as 2n and 2s exhibited low reactivity (4n, 4s), while highly electron-deficient substrates resulted in low yields due to fast hydrolysis of the sulfonate group (4o, 4p). In some cases, the use of KOAc instead of Cs₂CO₃ was beneficial for avoiding the hydrolysis (4o, 4u). Moderately electron-deficient aryl triflates were good substrates, and the products were obtained in good yield (4a, 4q, 4r, 4h) under the standard conditions. The triflate of estrone 2t exhibited low reactivity, but moderate yield was obtained using 20 mol% of Pd(OAc)₂ (4t).

We also investigated germylation of aryl iodide 10 under the optimized conditions for aryl bromides (Scheme [3]). The conditions for aryl bromides were effective to provide 4a in 95% yield at 80 °C, but the lower reaction temperature resulted in lower conversion.

Scheme 3 Germylation of aryl iodide 10

When β-bromostyrene was used as a substrate, both Conditions A and Conditions B in Scheme [2] afforded a dimerized product as a major product (see Supporting Information). Thus, the developed conditions were not suitable for germylation of alkenyl bromides.

The exact catalytic cycle was not elucidated, but a plausible cycle comprises oxidative addition of 1 or 2 to Pd(0), transmetalation with digermane 3, and reductive elimination to release arylgermane 4. The use of PPh₃ as a ligand also afforded the product in moderate yield (Table [1], entry 10), and therefore a hydroxy group of 5 would only have minor effects for the desired catalytic cycle, in contrast to the dramatic ligand effects observed in silylation.[9g] The role of Et₄NBr in germylation of aryl triflates was unclear. A tetraethylammonium ion might be important rather than a bromide ion (entries 13–19).

In summary, we have developed general conditions for the germylation of aryl bromides 1 and aryl triflates 2 using hexamethyldigermane (3) under palladium catalysis. Various functionalized substrates, including drug-like molecules, afforded the germylated products in moderate to good yields, demonstrating the versatility of the presented protocols. These methods enable easy access to functionalized arylgermanes, and may encourage further investigation of the properties and reactivity of arylgermane derivatives.

Reported melting points are uncorrected. IR spectra were recorded on a JASCO FT/IR-5300 spectrophotometer and absorbance bands are reported in wave numbers (cm^–1). NMR spectra were recorded on JEOL JNM-ECS400 spectrometers operating at 391.78 MHz for ¹H NMR and 98.52 MHz for ¹³C NMR, JEOL JNM-ECX400 spectrometers operating at 395.88 MHz for ¹H NMR and 99.55 MHz for ¹³C NMR, and JNM-ECA500 spectrometers operating at 500.16 MHz for ¹H NMR and 125.77 MHz for ¹³C NMR. Chemical shifts were reported in the scale relative to TMS (0.00 ppm for ¹H NMR), CHCl₃ (7.26 ppm for ¹H NMR), CDCl₃ (77.0 ppm for ¹³C NMR), C₆HD₅ (7.15 ppm for ¹H NMR), and C₆D₆ (128.06 ppm for ¹³C NMR) as an internal reference, respectively. ESI mass spectra were recorded on JEOL JMS-T100LCP spectrometer. Silica gel column chromatography was performed with Kanto Silica gel 60 N (40–50 mesh). Gel permeation chromatography was performed with YMC LC-forte/R using CHCl₃ as an eluent. Commercially available THF, toluene (Wako Ltd., deoxidized grade) were used without further manipulation unless otherwise stated. All aryl triflates 2 were prepared from the corresponding commercially available phenol. Aryl bromides 1a, 1b, 1c, 1d, 1h, 1i were commercially available and distilled under reduced pressure or recrystallized before use. 1-(4-Bromophenethyl)piperidine (1f),[15] 5-bromo-1-tosyl-1H-indole (1g),[16] 2-bromobenzo[b]thiophene (1i),[17] and ethyl 8-bromo-5-methyl-6-oxo-5,6-dihydro-4H-benzo[f]imidazo[1,5-a][1,4]diazepine-3-carboxylate (1l)[14] were synthesized according to the literature. Structures of bromides 1 and aryl triflates 2 are listed in Figure S1 in Supporting Information. Hexamethyldigermane (3) was purchased from Sigma-Aldrich and used as received. All other reagents were commercially available and used as received.

Germylation of Aryl Bromides 1 and Triflates 2; General Procedure

Conditions A: To a screw vial with a septum cap were added aryl bromide 1 (0.50 mmol), Pd(OAc)₂(11.2 mg, 0.05 mmol, 10 mol%), ligand 5 (27.8 mg, 0.10 mmol, 20 mol%), Cs₂CO₃ (195.5 mg, 0.60 mmol, 1.2 equiv), Et₄NHCO₃ (9.5 mg, 0.05 mmol, 10 mol%), hexamethyldigermane (3; 120 μL, 0.60 mmol, 1.2 equiv), and toluene (1.25 mL) under argon atmosphere in a glove box. The vial was capped and removed from the glove box, and then H₂O (1.25 mL) was injected via syringe. The vial was heated at 100 °C for 24 h with stirring. After cooling to r.t., the organic layer was separated, and the aqueous layer was extracted with EtOAc (3 × 3 mL). The combined organic layers were washed with brine (3 mL) and dried (Na₂SO₄). After filtration and evaporation, purification of the crude product by silica gel column chromatography afforded the corresponding product 4.

Conditions B: To a dried screw capped vial were added aryl bromide 1 or aryl triflate 2 (0.50 mmol), Pd(OAc)₂(11.2 mg, 0.05 mmol, 10 mol%), ligand 5 (27.8 mg, 0.10 mmol, 20 mol%), Cs₂CO₃ (195.5 mg, 0.60 mmol, 1.2 equiv), Et₄NBr (105.1 mg, 0.5 mmol, 1.0 equiv), hexamethyldigermane (3; 120 μL, 0.60 mmol, 1.2 equiv), and toluene (2.5 mL) under argon atmosphere in a glove box. The vial was capped and heated at 120 °C for 24 h with stirring. After cooling to r.t., H₂O (2 mL) was added. After dilution with EtOAc, the organic layer was separated, and the aqueous layer was extracted with EtOAc (3 × 3 mL). The combined organic layers were washed with brine (3 mL) and dried (Na₂SO₄). After filtration and evaporation, purification of the crude product by silica gel column chromatography afforded the corresponding product 4.

(4-Chlorophenyl)trimethylgermane (4a)

Conditions A using 1-bromo-4-chlorobenzene (1a; 0.50 mmol) and purification of the crude product by silica gel column chromatography (hexane) followed by gel permeation chromatography afforded 4a as a colorless oil (98.6 mg, 86%). Conditions B using 4-chlorophenyl trifluoromethanesulfonate (2a; 0.50 mmol) and purification of the crude product by silica gel column chromatography (hexane) afforded 4a as a colorless oil (95.1 mg, 83%); R_f = 0.75 (hexane).

IR (neat): 2972, 2907,1481, 1381, 1238, 1077, 1015, 825, 602, 571 cm^–1.

¹H NMR (400 MHz, CDCl₃): δ = 7.41–7.38 (m, 2 H), 7.33–7.30 (m, 2 H), 0.38 (s, 9 H).

¹³C NMR (100 MHz, CDCl₃): δ = 140.8, 134.5, 134.3, 128.1, –1.83.

HRMS (EI): m/z (M⁺) calcd for C₉H₁₃Cl⁷⁰Ge: 225.9948; found: 225.9945.

(4-Methoxyphenyl)trimethylgermane (4b)

Conditions A using 1-bromo-4-methoxybenzene (1b) at 120 °C and purification of the crude product by silica gel column chromatography (hexane/EtOAc 20:1) afforded 4b as a colorless oil (82.0 mg, 73%); R_f = 0.59 (hexane/EtOAc 8:1).

IR (neat): 2969, 2905, 1592, 1568, 1499, 1461, 1279, 1246, 1180, 1094, 1032, 823, 599, 567 cm^–1.

¹H NMR (400 MHz, CDCl₃): δ = 7.42–7.37 (m, 2 H), 6.94–6.90 (m, 2 H), 3.81 (s, 3 H), 0.36 (s, 9 H).

¹³C NMR (100 MHz, CDCl₃): δ = 159.8, 134.1, 133.2, 113.7, 55.0, –1.66.

HRMS (EI): m/z (M⁺) calcd for C₁₀H₁₆ ⁷⁰GeO: 222.0444; found: 222.0444.

4-(Trimethylgermyl)benzaldehyde (4c)

Conditions A using 4-bromobenzaldehyde (1c) and purification of the crude product by silica gel column chromatography (hexane/EtOAc 30:1 to 20:1) followed by gel permeation chromatography afforded 4c as a colorless oil (91.2 mg, 82%); R_f = 0.65 (hexane/EtOAc 5:1).

IR (neat): 2979, 2911, 2825, 1703, 1594, 1211, 1172, 825, 679, 603, 571 cm^–1.

¹H NMR (400 MHz, CDCl₃): δ = 10.01 (s, 1 H), 7.83 (d, J = 8.1 Hz, 2 H), 7.65 (d, J = 8.1 Hz, 2 H), 0.43 (s, 9 H).

¹³C NMR (100 MHz, CDCl₃): δ = 192.7, 151.9, 136.1, 133.5, 128.8, –1.93.

HRMS (EI): m/z (M⁺) calcd for C₁₀H₁₄ ⁷⁰GeO: 220.0287; found: 222.0286.

Trimethyl[4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl]germane (4d)

Conditions A using 2-(4-bromophenyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (1d) and purification of the crude product by silica gel column chromatography (hexane/EtOAc 20:1) afforded 4d as a colorless solid (128.3 mg, 80%); mp 119.3–120.2 °C; R_f = 0.73 (hexane/EtOAc 5:1).

IR (KBr): 2977, 1598, 1327, 1296, 1108, 859, 656, 602 cm^–1.

¹H NMR (400 MHz, CDCl₃): δ = 7.78 (d, J = 8.2 Hz, 2 H), 7.49 (d, J = 8.2 Hz, 2 H), 1.34 (s, 12 H), 0.38 (s, 9 H).

¹³C NMR (125 MHz, CDCl₃): δ = 146.6, 134.0, 132.3, 83.7, 24.8, –1.91; the carbon directly attached to the boron atom was not detected.

HRMS (ESI): m/z (M + Na⁺) calcd for C₁₅H₂₅B⁷⁰GeO₂Na: 340.1124; found: 340.1134.

N-[4-(Trimethylgermyl)phenyl]benzamide (4e)

Conditions B using N-(4-bromophenyl)benzamide (1e) and purification of the crude product by silica gel column chromatography (hexane/EtOAc 10:1 to 5:1) afforded 4e as a colorless solid (70.1 mg, 45%); mp 114.6–115.3 °C; R_f = 0.65 (hexane/EtOAc 2:1).

IR (KBr): 3311, 2972, 1578, 1525, 1504, 1388, 1322, 1285, 819, 720, 694, 593 cm^–1.

¹H NMR (400 MHz, CDCl₃): δ = 7.90–7.85 (m, 2 H), 7.76 (br s, 1 H), 7.63 (d, J = 8.2 Hz, 2 H), 7.58–7.47 (m, 5 H), 0.39 (s, 9 H).

¹³C NMR (125 MHz, CDCl₃): δ = 165.8, 138.4, 138.0, 134.9, 133.7, 131.8, 128.7, 127.0, 119.7, –1.78.

HRMS (ESI): m/z (M + Na⁺) calcd for C₁₆H₁₉ ⁷⁰GeNONa: 334.0601; found: 334.0603.

1-[4-(Trimethylgermyl)phenethyl]piperidine (4f)

Conditions B using 1-(4-bromophenethyl)piperidine (1f; 268 mg, 1.0 mmol) and purification of the crude product by silica gel column chromatography (hexane/EtOAc 6:1, 3% Et₃N) afforded 4f as a yellow oil (255 mg, 83%); R_f = 0.38 (hexane/EtOAc 6:1, 3% Et₃N).

IR (neat): 2969, 2934, 2853, 2798, 1758, 1235, 1155, 1120, 1090, 823, 757, 600, 572 cm^–1.

¹H NMR (400 MHz, CDCl₃): δ = 7.40 (d, J = 8.2 Hz, 2 H), 7.20 (d, J = 8.2 Hz, 2 H), 2.83–2.77 (m, 2 H), 2.59–2.53 (m, 2 H), 2.51–2.44 (m, 4 H), 1.67–1.59 (m, 4 H), 1.50–1.42 (m, 2 H), 0.36 (s, 9 H).

¹³C NMR (125 MHz, CDCl₃): δ = 140.6, 139.5, 132.9, 128.3, 61.3, 54.5, 33.5, 25.9, 24.4, –1.84.

HRMS (ESI): m/z (M + H⁺) calcd for C₁₆H₂₈ ⁷⁰GeN: 304.1459; found: 304.1461.

Tosyl-5-(trimethylgermyl)-1H-indole (4g)

Conditions A using 5-bromo-1-tosyl-1H-indole (1g) and purification of the crude product by silica gel column chromatography (hexane/EtOAc 10:1 to 8:1) afforded 4g as a colorless solid (165 mg, 85%); mp 110.2–110.8 °C; R_f = 0.50 (hexane/EtOAc 5:1)

IR (KBr): 3140, 3111, 2968, 2916, 1447, 1371, 1257, 1188, 1172, 1131, 1095, 996, 585, 576 cm^–1.

¹H NMR (500 MHz, CDCl₃): δ = 7.97 (d, J = 8.3 Hz, 1 H), 7.77 (d, J = 8.3 Hz, 2 H), 7.63 (s, 1 H), 7.54 (d, J = 3.7 Hz, 1 H), 7.39 (dd, J = 8.3, 4.2 Hz, 1 H), 7.22 (d, J = 8.3 Hz, 2 H), 6.64 (d, J = 3.7 Hz, 1 H), 2.34 (s, 3 H), 0.38 (s, 9 H).

¹³C NMR (125 MHz, CDCl₃): δ = 144.9, 136.6, 135.4, 135.0, 130.6, 129.9, 128.9, 126.8, 126.1, 126.0, 113.0, 108.7, 21.5, –1.62.

HRMS (ESI): m/z (M + Na⁺) calcd for C₁₈H₂₁ ⁷⁰GeNO₂SNa: 408.0428; found: 408.0442.

6-(Trimethylgermyl)quinoline (4h)

Conditions A using 6-bromoquinoline (1h) and purification of the crude product by silica gel column chromatography (hexane/EtOAc 5:1) afforded 4h as a colorless oil (107 mg, 87%). Conditions B using quinolin-6-yl trifluoromethanesulfonate (2h) and purification of the crude product by silica gel column chromatography (hexane/EtOAc = 5:1) afforded 4h as a colorless oil (83.5 mg, 68%); R_f = 0.30 (hexane/EtOAc 4:1).

IR (neat): 2970, 2906, 1564, 1491, 1341, 1237, 1071, 856, 831, 799, 771, 623, 601, 587, 567 cm^–1.

¹H NMR (500 MHz, CDCl₃): δ = 8.91 (dd, J = 4.0, 1.7 Hz, 1 H), 8.15 (d, J = 8.0 Hz, 1 H), 8.08 (d, J = 8.0 Hz, 1 H), 7.92 (s, 1 H), 7.82 (dd, J = 8.0, 1.1 Hz, 1 H), 7.40 (dd, J = 8.0, 4.0 Hz, 1 H), 0.48 (s, 9 H).

¹³C NMR (125 MHz, CDCl₃): δ = 150.4, 148.3, 141.4, 135.8, 133.5, 132.8, 128.5, 127.9, 121.1, –1.77.

HRMS (ESI): m/z (M + H⁺) calcd for C₁₂H₁₆ ⁷⁰GeN: 244.0520; found: 244.0523.

Benzo[b]thiophen-2-yltrimethylgermane (4i)

Conditions B using 2-bromobenzo[b]thiophene (1i) and purification of the crude product by silica gel column chromatography (hexane) followed by gel permeation chromatography afforded 4i as a colorless oil (87.1 mg, 69%); R_f = 0.67 (hexane/EtOAc 20:1).

IR (neat): 3056, 2973, 2907, 1453, 1240, 945, 826, 761, 744, 726, 605, 574, 561 cm^–1.

¹H NMR (400 MHz, CDCl₃): δ = 7.87 (d, J = 7.7 Hz, 1 H), 7.79 (d, J = 7.7 Hz, 1 H), 7.38 (s, 1 H), 7.35–7.26 (m, 2 H), 0.51 (s, 9 H).

¹³C NMR (100 MHz, CDCl₃): δ = 143.7, 143.4, 141.1, 129.6, 124.1, 123.9, 123.2, 122.2, –0.53.

HRMS (EI): m/z (M⁺) calcd for C₁₁H₁₄ ⁷⁰GeS: 248.0059; found: 248.0057.

Ethyl 2-(4-Bromophenoxy)-2-methylpropanoate (1k)

4-Bromophenol (519 mg, 3.0 mmol) and Cs₂CO₃ (2.44 g, 7.5 mmol, 2.5 equiv) were dissolved in anhyd DMF (10 mL). The solution was stirred for 10 min, and then ethyl 2-bromoisobutyrate (1.17 g, 6.0 mmol, 2.0 equiv) was added. The resulting reaction mixture was stirred at 100 °C for 23 h. After cooling to r.t., the residue was taken up in EtOAc (50 mL). The solution was successively washed with H₂O (2 × 20 mL) and brine (20 mL), and dried (Na₂SO₄). After filtration and evaporation, the crude product was purified by silica gel column chromatography to give 1k as a colorless oil (818 mg, 95%); R_f = 0.50 (hexane/EtOAc 5:1).

IR (neat): 2987, 2938, 1734, 1587, 1486, 1468, 1383, 1284, 1238, 1177, 1140, 1073, 1023, 1007, 825, 647 cm^–1.

¹H NMR (400 MHz, CDCl₃): δ = 7.36–7.31 (m, 2 H), 6.75–6.71 (m, 2 H), 4.23 (q, J = 7.2 Hz, 2 H), 1.58 (s, 6 H), 1.25 (t, J = 7.2 Hz, 3 H).

¹³C NMR (125 MHz, CDCl₃): δ = 173.9, 154.5, 132.0, 120.8, 114.5, 79.4, 61.5, 25.2, 14.1.

HRMS (ESI): m/z (M + Na⁺) calcd for C₁₂H₁₅BrO₃Na: 24 309.0097; found: 309.0100.

Ethyl 2-Methyl-2-[4-(trimethylgermyl)phenoxy]propanoate (4k)

Conditions B using ethyl 2-(4-bromophenoxy)-2-methylpropanoate (1k; 114.9 mg, 0.40 mmol), Pd(OAc)₂(20 mol%), and ligand 5 (40 mol%), and purification of the crude product by silica gel column chromatography (hexane/EtOAc = 10:1) followed by gel permeation chromatography afforded 4k as a colorless oil (84.4 mg, 65%); R_f = 0.63 (hexane/EtOAc 5:1).

IR (neat): 2977, 2906, 1733, 1590, 1498, 1382, 1272, 1237, 1178, 1142, 1093, 1024, 824, 761, 599, 569 cm^–1.

¹H NMR (400 MHz, CDCl₃): δ = 7.34–7.30 (m, 2 H), 6.84–6.80 (m, 2 H), 4.24 (q, J = 7.1 Hz, 2 H), 1.60 (s, 6 H), 1.25 (t, J = 7.1 Hz, 3 H), 0.34 (s, 9 H).

¹³C NMR (125 MHz, CDCl₃): δ = 174.3, 155.7, 134.8, 133.8, 118.4, 78.8, 61.4, 25.4, 14.0, –1.69.

HRMS (ESI): m/z (M + Na⁺) calcd for C₁₅H₂₄ ⁷⁰GeO₃Na: 345.0860; found: 345.0862.

Ethyl 5-Methyl-6-oxo-8-(trimethylgermyl)-5,6-dihydro-4H-benzo­[f]imidazo[1,5-a][1,4]diazepine-3-carboxylate (4l)

Conditions B using ethyl 8-bromo-5-methyl-6-oxo-5,6-dihydro-4H-benzo[f]imidazo[1,5-a][1,4]diazepine-3-carboxylate (1l; 109.3 mg, 0.30 mmol) and purification of the crude product by silica gel column chromatography (hexane/EtOAc 1:2 to 1:3) afforded 4l as a pale yellow solid (82 mg, 68%); mp 164.5–165.2 °C; R_f = 0.36 (toluene/EtOAc 1:3).

IR (KBr): 3112, 2975, 2905, 1728, 1704, 1647, 1503, 1296, 1260, 1189, 1109, 1065, 833, 605 cm^–1.

¹H NMR (400 MHz, CDCl₃): δ = 8.15 (d, J = 1.4 Hz, 1 H), 7.90 (s, 1 H), 7.72 (dd, J = 7.7, 1.4 Hz, 1 H), 7.38 (d, J = 7.7 Hz, 1 H), 5.25–5.14 (m, 1 H), 4.55–4.29 (m, 3 H), 3.26 (s, 3 H), 1.46 (t, J = 7.2 Hz, 3 H), 0.45 (s, 9 H).

¹³C NMR (125 MHz, CDCl₃): δ = 166.8, 162.9, 144.1, 137.1, 136.8, 135.5, 134.8, 131.8, 128.5, 128.0, 120.9, 60.8, 42.2, 35.7, 14.3, –1.9.

HRMS (ESI): m/z (M + Na⁺) calcd for C₁₈H₂₃ ⁷⁰GeN₃O₃Na: 422.0874; found: 422.0877.

[4-(tert-Butyl)phenyl]trimethylgermane (4m)

Conditions B using 4-(tert-butyl)phenyl trifluoromethanesulfonate (2m) and purification of the crude product by silica gel column chromatography (hexane/EtOAc 30:1 to 10:1) afforded 4m as a colorless oil (63.2 mg, 50%); mp 69.2–70.0 °C; R_f = 0.72 (hexane/EtOAc 8:1).

IR (KBr): 3421, 2961, 2905, 2865, 1383, 1267, 1235, 1078, 818, 760, 602, 576, 552 cm^–1.

¹H NMR (400 MHz, CDCl₃): δ = 7.44–7.41 (m, 2 H), 7.40–7.37 (m, 2 H), 1.32 (s, 9 H), 0.37 (s, 9 H).

¹³C NMR (100 MHz, CDCl₃): δ = 151.1, 138.9, 132.8, 124.9, 34.6, 31.3, –1.79.

HRMS (EI): m/z (M⁺) calcd for C₁₃H₂₂ ⁷⁰Ge: 248.0964; found: 248.0960.

{4-[4-(Trimethylgermyl)phenyl]piperazin-1-yl}ethan-1-one (4n)

Conditions B using 4-(4-acetylpiperazin-1-yl)phenyl trifluoromethanesulfonate (2n) and purification of the crude product by silica gel column chromatography (EtOAc/MeOH 19:1) followed by gel permeation chromatography afforded 4n as a pink solid (47.2 mg, 29%); mp 66.7–67.8 °C; R_f = 0.65 (EtOAc/MeOH 9:1).

IR (KBr): 3438, 2979, 2899, 2830, 1625, 1592, 1455, 1432, 1234, 998 cm^–1.

¹H NMR (500 MHz, CDCl₃): δ = 7.39 (d, J = 8.6 Hz, 2 H), 6.93 (d, J = 8.6 Hz, 2 H), 3.77 (t, J = 5.2 Hz, 2 H), 3.62 (t, J = 5.2 Hz, 2 H), 3.20 (t, J = 5.2 Hz, 2 H), 3.16 (t, J = 5.2 Hz, 2 H), 2.14 (s, 3 H), 0.35 (s, 9 H).

¹³C NMR (125 MHz, CDCl₃): δ = 169.0, 150.9, 133.9, 133.0, 116.2, 49.4, 49.1, 46.2, 41.3, 21.4, –1.72.

HRMS (ESI): m/z (M + Na⁺) calcd for C₁₅H₂₄ ⁷⁰GeN₂ONa: 341.1023; found: 341.1025.

Trimethyl(4-nitrophenyl)germane (4o)

Conditions B using 4-nitrophenyl trifluoromethanesulfonate (2o) and KOAc instead of Cs₂CO₃, and purification of the crude product by silica gel column chromatography (hexane/EtOAc 8:1) and gel permeation chromatography afforded 4o as a pale yellow solid (39.2 mg, 33%); mp 40.2–41.7 °C; R_f = 0.59 (hexane/EtOAc 8:1).

IR (KBr): 3432, 3037, 2979, 2905, 1594, 1513, 1386, 1351, 1240, 835, 730, 711, 603 cm^–1.

¹H NMR (400 MHz, CDCl₃): δ = 8.17 (d, J = 8.1 Hz, 2 H), 7.64 (d, J = 8.1 Hz, 2 H), 0.44 (s, 9 H).

¹³C NMR (125 MHz, CDCl₃): δ = 152.5, 148.2, 133.8, 122.3, –1.91.

HRMS (EI): m/z (M⁺) calcd for C₉H₁₃ ⁷⁰GeNO₂: 237.0189; found: 237.0194.

Ethyl 4-(Trimethylgermyl)benzoate (4p)

Conditions B using ethyl 4-{[(trifluoromethyl)sulfonyl]oxy}benzoate (2p) and purification of the crude product by silica gel column chromatography (hexane/EtOAc 20:1) followed by reverse phase column chromatography (C18, H₂O/MeCN 15:85) afforded 4p as a colorless oil (20 mg, 15%); R_f = 0.61 (hexane/EtOAc 8:1).

IR (neat): 2975, 2927, 2908, 1720, 1277, 1266, 1081, 602, 570 cm^–1.

¹H NMR (500 MHz, CDCl₃): δ = 7.99 (d, J = 8.0 Hz, 2 H), 7.55 (d, J = 8.0 Hz, 2 H), 4.38 (q, J = 7.2 Hz, 2 H), 1.39 (t, J = 7.2 Hz, 3 H), 0.41 (s, 9 H).

¹³C NMR (125 MHz, CDCl₃): δ = 166.9, 149.2, 132.9, 130.2, 128.6, 60.9, 14.3, –1.92.

HRMS (APCI): m/z (M + H⁺) calcd for C₁₂H₁₉ ⁷⁰GeO₂: 265.0622; found: 265.0623.

Ethyl 3-(Trimethylgermyl)benzoate (4q)

Conditions B using ethyl 3-{[(trifluoromethyl)sulfonyl]oxy}benzoate (2q) and purification of the crude product by silica gel column chromatography (hexane/EtOAc 20:1) afforded 4q as a colorless oil (81.4 mg, 61%); R_f = 0.54 (hexane/EtOAc 8:1).

IR (neat): 2976, 2906, 1719, 1410, 1365, 1261, 1172, 1116, 1071, 826, 742, 602, 570 cm^–1.

¹H NMR (500 MHz, CDCl₃): δ = 8.14 (s, 1 H), 7.99 (d, J = 7.4 Hz, 1 H), 7.66 (d, J = 7.4 Hz, 1 H), 7.41 (dd, J = 7.4, 7.4 Hz, 1 H), 4.39 (q, J = 7.3 Hz, 2 H), 1.40 (t, J = 7.3 Hz, 3 H), 0.41 (s, 9 H).

¹³C NMR (125 MHz, CDCl₃): δ = 167.0, 143.0, 137.4, 133.9, 129.8, 129.4, 127.8, 60.9, 14.4, –1.83.

HRMS (EI): m/z (M⁺) calcd for C₁₂H₁₈ ⁷⁰GeO₂: 264.0549; found: 264.0551.

Trimethyl(naphthalen-2-yl)germane (4r)

Conditions B using 2-naphthyl trifluoromethanesulfonate (2r) and purification of the crude product by silica gel column chromatography (hexane/EtOAc 20:1) afforded 4r as a colorless oil (97 mg, 79%); R_f = 0.77 (hexane/EtOAc 8:1).

IR (neat): 3051, 2971, 2906, 1236, 1073, 815, 757, 738, 630, 600, 579, 564 cm^–1.

¹H NMR (500 MHz, CDCl₃): δ = 7.94 (s, 1 H), 7.84–7.80 (m, 3 H), 7.57 (d, J = 7.4 Hz, 1 H), 7.49–7.44 (m, 2 H), 0.46 (s, 9 H).

¹³C NMR (100 MHz, C₆D₆): δ = 133.7, 127.8, 127.5, 127.1, 123.8, 121.8, 121.8, 121.3, 119.9, 119.8, –8.13.

HRMS (EI): m/z (M⁺) calcd for C₁₃H₁₆ ⁷⁰Ge: 242.0495; found: 242.0496.

Benzo[d][1,3]dioxol-5-yltrimethylgermane (4s)

Conditions B using benzo[d][1,3]dioxol-5-yl trifluoromethanesulfonate (2s), Pd(OAc)₂(20 mol%), and ligand 5 (40 mol%) and purification of the crude product by silica gel column chromatography (hexane/EtOAc 20:1) afforded 4s as a colorless oil (50.8 mg, 43%); R_f = 0.64 (hexane/EtOAc 8:1).

IR (KBr): 2974, 2905, 1482, 1414, 1232, 1050, 1040, 937, 881, 825, 590 cm^–1.

¹H NMR (500 MHz, CDCl₃): δ = 6.94–6.92 (m, 2 H), 6.86–6.83 (m, 1 H), 5.93 (s, 2 H), 0.35 (s, 9 H).

¹³C NMR (100 MHz, CDCl₃): δ = 147.7, 147.4, 135.4, 126.3, 112.5, 108.6, 100.4, –1.61.

HRMS (EI): m/z (M⁺) calcd for C₁₀H₁₄ ⁷⁰GeO₂: 236.0236; found: 236.0232.

(8R,9S,13S)-13-Methyl-3-(trimethylgermyl)-6,7,8,9,11,12,13,14,15,16-decahydro-17H-cyclopenta[a]phenanthren-17-one (4t)

Conditions B using (8R,9S,13S)-13-methyl-17-oxo-7,8,9,11,12,13,14, 15,16,17-decahydro-6H-cyclopenta[a]phenanthren-3-yl trifluoromethanesulfonate (2t; 402.4 mg, 1.0 mmol), Pd(OAc)₂ (20 mol%), and ligand 5 (40 mol%) at 130 °C and purification of the crude product by silica gel column chromatography (hexane/EtOAc 20:1 to 10:1) afforded 4t as a colorless solid (174.0 mg, 47%); mp 119.3–120.2 °C; R_f = 0.75 (hexane/EtOAc 7:3).

IR (KBr): 3454, 2969, 2928, 2866, 2837, 1739, 1452, 1235, 1081, 828, 603 cm^–1.

¹H NMR (CDCl₃, 500 MHz): δ = 7.32–7.27 (m, 2 H), 7.22 (s, 1 H), 2.94 (dd, J = 8.9, 4.3 Hz, 2 H), 2.51 (dd, J = 18.9, 8.6 Hz, 1 H), 2.47–2.41 (m, 1 H), 2.36–2.29 (m, 1 H), 2.19–1.94 (m, 4 H), 1.68–1.59 (m, 2 H), 0.90 (s, 3 H), 0.37 (s, 9 H).

¹³C NMR (125 MHz, CDCl₃): δ = 220.7, 139.8, 139.4, 135.9, 133.7, 130.4, 124.9, 50.4, 47.9, 44.4, 38.0, 35.8, 31.5, 29.3, 26.5, 25.5, 21.5, 13.7, –1.84.

HRMS (ESI): m/z (M + Na⁺) calcd for C₂₁H₃₀ ⁷⁰GeONa: 391.1431; found: 391.1431.

Methyl (S)-2-[(tert-Butoxycarbonyl)amino]-3-[4-(trimethylgermyl)phenyl]propanoate (4u)

Conditions B using methyl (S)-2-[(tert-butoxycarbonyl)amino]-3-(4-{[(trifluoromethyl)sulfonyl]oxy}phenyl)propanoate (2u) and KOAc instead of Cs₂CO₃ for 62 h, and purification of the crude product by silica gel column chromatography (hexane/EtOAc 10:1) afforded 4u as a colorless solid (132.6 mg, 67%); mp 55.5–56.2 °C; R_f = 0.50 (EtOAc/ MeOH 9:1).

IR (KBr): 3370, 2975, 1747, 1717, 1500, 1437, 1366, 1248, 1214, 1168, 825, 757, 600, 568 cm^–1.

¹H NMR (500 MHz, CDCl_3,50 °C): δ = 7.39 (d, J = 8.0 Hz, 2 H), 7.10 (d, J = 8.0 Hz, 2 H), 4.96–4.87 (m, 1 H), 4.62–4.52 (m, 1 H), 3.71 (s, 3 H), 3.13–3.06 (m, 1 H), 3.05–2.93 (m, 1 H), 1.40 (s, 9 H), 0.36 (s, 9 H).

¹³C NMR (125 MHz, CDCl₃): δ = 172.3, 155.0, 140.9, 135.9, 133.1, 128.9, 79.8, 54.3, 52.2, 38.2, 28.3, –1.85.

HRMS (ESI): m/z (M + Na⁺) calcd for C₁₈H₂₉ ⁷⁰GeNO₄Na: 416.1231; found: 416.1234.

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