Cobalt(III)-Catalyzed Redox-Neutral Coupling of Acrylamides with Activated Alkenes via C–H Bond Activation

We would like to dedicate this work to Prof. Chien-Hong Cheng on the occasion of his 70^th birthday.

Key words

acrylamides - maleimides - C–H activation - alkylation - succinimides - acrylates - cobalt

The transition-metal-catalyzed C–H functionalization reaction has evolved as an indispensable tool in synthetic organic chemistry to construct complex organic motifs in a highly atom- and step-economical manner.[1] [2] [3] After the seminal findings from Murai and co-workers,[1] transition-metal complexes such as those of Pd, Ru, Rh and Ir have been widely employed to attain carbon–carbon and carbon–heteroatom bond formation via the C–H bond activation reaction.[2] However, these transition-metal salts are less abundant in nature and consequently these transition metals are expensive. Recently, the C–H functionalization of organic molecules catalyzed by first-row transition metals, such as cobalt, nickel, copper and iron, has gained much attention from synthetic organic chemists due to the high abundance and biocompatibility of such metals.[3] In particular, the Cp*Co-catalyzed C–H bond activation reaction has emerged as a promising alternative to second- and third-row metal catalysts for the functionalization of organic molecules.[4] The functionalization of aromatics and heteroaromatics has been extensively studied by employing the Cp*Co(III) catalyst. However, only few examples are reported for functionalization of vinylic C–H bonds, due to the ineffective formation of the key cobaltocycle intermediate.[5]

Scheme 1 Cp*Co(III)-catalyzed functionalization of vinylic C–H bonds (DG = directing group)

In 2014, the Glorius group reported the Cp*Co-catalyzed C–H iodination and allylation of acrylamide derivatives.[5a] [b] Subsequently, Zhang’s group and Zhong’s group have explored the vinylic C–H bond activation of enamide derivatives (Scheme [1]).[5`] [d] [e] On the other hand, succinimide is one of the important cores often found in various natural products and biologically active molecules.[6] The succinimide moiety can be incorporated into the C–H bond of organic molecules via 1,4-addition to maleimides. It is important to note that the maleimide unit is also often found in a range of natural products and biologically active molecules.[7] [8] In addition, succinimide derivatives can be readily converted into pyrrolidines, also one of the important cores found in alkaloids.[9] Due to these facts, maleimides have evolved as one of the widely employed coupling partners in C–H bond activation reactions.[10] Herein, we report a cobalt(III)-catalyzed coupling of acrylamides with maleimides via C–H activation. The present protocol provides substituted succinimide derivatives under redox-neutral conditions. The catalytic reaction is compatible with substituted acrylamides as well as maleimides. The scope of the C–H alkylation reaction was also examined with substituted acrylates. A possible reaction mechanism involving a cobaltocycle intermediate is proposed.

Table 1 Optimization Studies^a

Entry	Solvent	Acetate source	Additive	Yield (%)b
1	CH3CN	NaOPiv	AgSbF6	NR
2	1,2-DCE	NaOPiv	AgSbF6	29
3	C6H5Cl	NaOPiv	AgSbF6	NR
4	TFE	NaOPiv	AgSbF6	63
5	C6H5CF3	NaOPiv	AgSbF6	NR
6	HFIP	NaOPiv	AgSbF6	NR
7	1,4-dioxane	NaOPiv	AgSbF6	NR
8	TFE	AgOAc	AgSbF6	47
9	TFE	Cu(OAc)2·H2O	AgSbF6	NR
10	TFE	CsOAc	AgSbF6	NR
11	TFE	NaOAc	AgSbF6	NR
12	TFE	AcOH	AgSbF6	NR
13	TFE	PivOH	AgSbF6	79
14	TFE	Adm-1-COOH	AgSbF6	NR
15	TFE	mesitylenic acid	AgSbF6	NR
16	TFE	PivOH	AgOTf	NR
17	TFE	PivOH	AgBF4	45
18	TFE	PivOH	KPF6	NR
19	TFE	PivOH	–	59c
20	TFE	PivOH	AgSbF6	NR
21	TFE	PivOH	AgSbF6	39d
22	TFE	PivOH	AgSbF6	63e

^a Reaction conditions, unless indicated otherwise: 1a (50 mg), 2a (1.2 equiv), Cp*Co(CO)I₂ (10 mol%), AgSbF₆ (20 mol%), acetate source (30 mol%), solvent (3.0 mL), 120 °C, 16 h.

^b Isolated yield.

^c [Cp*Co(CH₃CN)₃][SbF₆]₂ (5 mol%) was used.

^d 60 °C, 16 h.

^e 100 °C, 16 h.

Treatment of N-isobutylmethacrylamide (1a) with N-benzylmaleimide (2a) (1.2 equiv) in the presence of Cp*Co(CO)I₂ (10 mol%), AgSbF₆ (20 mol%) and pivalic acid (PivOH) (30 mol%) in CF₃CH₂OH (TFE) at 120 °C for 16 hours under nitrogen atmosphere yielded product 3aa in 79% yield (Table [1], entry 13). Initially, the alkylation reaction using NaOPiv was examined with various solvents, such as CH₃CN, 1,2-dichloroethane (1,2-DCE), chlorobenzene, TFE, (trifluoromethyl)benzene, hexafluoro-2-propanol (HFIP) and 1,4-dioxane (entries 1–7). Among them, TFE was very effective, giving product 3aa in 63% yield (entry 4). 1,2-DCE was partially effective, providing 3aa in 29% yield (entry 2), while the remaining solvents were totally ineffective. Then, the reaction in TFE was examined with various acetate sources, such as NaOPiv, AgOAc, Cu(OAc)₂·H₂O, CsOAc and NaOAc. NaOPiv and AgOAc provided product 3aa in 63% and 47% yield, respectively (entries 4 and 8). Other acetate sources were ineffective. The reaction was also examined in the presence of organic acids, such as acetic acid, PivOH, mesitylenic acid and adamantane-1-carboxylic acid (Adm-1-COOH) (entries 12–15). Among them, PivOH was very effective, yielding 3aa in 79% yield (entry 13). In addition, the present reaction was examined with various additives, such as AgSbF₆, AgOTf, AgBF₄ and KPF₆. Among them, AgSbF₆ was very effective (79% yield of 3aa, entry 13), AgBF₄ was partially effective (45% yield of 3aa, entry 17), and AgOTf and KPF₆ were totally ineffective (entries 16 and 18). The efficiency of the cationic cobalt complex [Cp*Co(CH₃CN)₃][SbF₆]₂ was also examined. In this reaction, product 3aa was obtained in 59% yield (entry 19). This result clearly revealed that the combination of Cp*Co(CO)I₂ and AgSbF₆ is more effective than the cationic complex. The reaction condition in entry 20 was done under room temperature, 16h. It is important to note that the yield of product 3aa decreased drastically at lower temperatures (entries 21 and 22). These optimization studies clearly revealed that Cp*Co(CO)I₂ (10 mol%), AgSbF₆ (20 mol%) and PivOH (30 mol%) in TFE at 120 °C for 16 hours are the best conditions for the present reaction.

The scope of the present alkylation reaction was examined with acrylamides having various N-protecting groups under the optimized reaction conditions (Scheme [2]). N-Methyl-, N-ethyl- and N-butylacrylamides 1b, 1c and 1d reacted with 2a providing the expected alkylation products 3ba, 3ca and 3da in 64%, 69% and 72% yield, respectively. N-Allylacrylamide 1e yielded alkylation product 3ea without affecting the allyl group, in 75% yield. N-Cyclohexylacrylamide 1f reacted with N-benzylmaleimide (2a), as well as with N-methylmaleimide (2b), to produce products 3fa and 3fb in 71% and 67% yield, respectively.

Scheme 2 Scope of acrylamides

N-Benzylacrylamide 1g reacted with 2a under the optimized reaction conditions providing product 3ga in 79% yield. Sterically hindered N-tert-butylacrylamide 1h also produced the expected product 3ha in 59% yield. The reaction was also examined with α-arylacrylamides such as N-isobutyl-2-(p-tolyl)acrylamide (1i) which resulted in alkylated product 3ia in 79% yield. However, the alkylation reaction is not compatible with β-alkyl- or β-aryl-substituted acrylamides due to steric hindrance at the β-position which seems to hinder formation of the key metalacycle.

The present reaction was also further examined with various N-substituted maleimides 2b–i (Scheme [3]). Maleimides 2b–e having an alkyl substituent such as methyl, ethyl, cyclohexyl and phenethyl reacted with 1a to give the corresponding alkylated products 3ab–ae in 64%, 69%, 73% and 69% yield, respectively. Then, the scope of maleimides was extended to N-aryl-substituted maleimides 2f–i. N-Phenylmaleimide (2f) produced the expected product 3af in 79% yield, while N-arylmaleimides 2g–i having a 4-Me, 4-OMe and 4-Br substituent, respectively, gave the corresponding alkylated products 3ag–ai in 74%, 81% and 86% yield. N-Allylmaleimide (2j) produced product 3aj in 35% yield in a 10:3 E/Z ratio. However, N–H free maleimide and maleimide 2l derived from the amino acid l-alanine were found to be ineffective. Similarly, internal olefins such as maleate and fumarate were also ineffective in this reaction.

Scheme 3 Scope of maleimides

The present alkylation reaction is compatible with substituted acrylates. In these reactions, the linear-selective alkylated product was observed (Scheme [4]).[11] It is important to note that in this type of transformation only vinylic C–H alkenylation was observed with acrylates. In the present reaction, interestingly, C–H alkylated product was observed which is not known in the literature. Treatment of N-benzylmethacrylamide (1g) with methyl acrylate (4a) (1.2 equiv) in the presence of Cp*Co(CO)I₂ (10 mol%), AgSbF₆ (20 mol%) and PivOH (30 mol%) in TFE at 120 °C for 16 hours provided C–H alkylated product 5ga in 72% yield (Scheme [4]). Ethyl acrylate (4b) and n-butyl acrylate (4c) reacted with 1g giving the linear-selective alkylated products 5gb and 5gc in 68% and 57% yield, respectively. It is interesting to note that in the reaction of 1g with cyclohexyl acrylate (4d), a seven-membered azepine derivative 5gd was observed in 55% yield under the optimized reaction conditions. However, phenyl acrylate (4e) reacted with 1g to give a 1:1 mixture of alkylated product 5ge and cyclized product 5ge′ in 54% yield.

Scheme 4 Scope of activated alkenes

Based on known cobalt-catalyzed C–H bond cleavage reactions,[3] [4] [5] a possible reaction mechanism is proposed in Scheme [5]. The reaction likely commences with the formation of active catalyst 6 from Cp*Co(CO)I₂, AgSbF₆ and PivOH. Then, coordination of the oxygen atom of the amide group of 1 followed by C–H bond cleavage leads to the formation of cobaltocycle intermediate 7. Further, coordination of maleimide 2a into complex 7 followed by migratory insertion produces intermediate 9. Protonation of intermediate 9 by PivOH leads to the formation of alkylated product 10 and regenerates the active catalyst 6 for the next cycle.

Scheme 5 Proposed mechanism

Olefin migration in intermediate 10 furnishes product 3aa, similar to Kim’s observation in the presence of a rhodium catalyst.[10e] It is important to note that the rhodium complex effectively catalyzed the C–H alkylation of β-substituted acrylamides with maleimides; however, it could not catalyze the C–H alkylation of α-substituted acrylamides with maleimides. Interestingly, in the present reaction, a less expensive cobalt complex effectively catalyzed the alkylation of α-substituted acrylamides with maleimides. It is also important to note that the whole catalytic cycle proceeds in the Co(III) state and thus an oxidation step is not needed. The reaction proceeds in a redox-neutral version and organic acid plays the dual role of protonation of the C–Co bond in intermediate 9 and deprotonation of the vinylic C–H bond by the corresponding acetate anion.

In conclusion, we have demonstrated a cobalt-catalyzed C–H alkylation of acrylamides with maleimides to provide olefin-migrated alkylated products in good to excellent yields. The reaction is compatible with various substituted acrylamides and maleimides. The scope of the C–H alkylation reaction was also examined with substituted acrylates. A possible reaction mechanism involving a five-membered cobaltocycle intermediate has been proposed. The reaction proceeds in a redox-neutral version and organic acid plays the dual role of protonation of C–Co in a seven-membered cobalt intermediate and deprotonation of the C–H bond of the acrylamide by the corresponding acetate anion.

All reactions were carried out under N₂ atmosphere in flame-dried glassware. Syringes which were used to transfer anhydrous solvents or reagents were purged with nitrogen prior to use (three times). Anhydrous solvents were used for the reactions. Column chromatographic purifications were performed using silica gel (120–200 mesh ASTM) from Merck, if not indicated otherwise. Standard abbreviations are used for NMR signal coupling. Commercially available metal salts were purchased from Sigma-Aldrich and used without further purification.

Substituted Succinimide Derivatives 3; General Procedure

A 15 mL Schlenk tube with septum containing Cp*Co(CO)I₂ (10 mol%), acrylamide 1 (50 mg, 1 equiv), maleimide 2 (1.2 equiv) and AgSbF₆ (20 mol%) was evacuated and purged with nitrogen gas three times (AgSbF₆ was transferred inside a glovebox). To the tube were then added PivOH (30 mol%) and TFE (3.0 mL) via syringe; after that, the reaction mixture was evacuated and purged with nitrogen gas three times. Then, the septum was taken out immediately and a screw cap was used to cover the tube under the nitrogen atmosphere, and the reaction mixture was stirred at room temperature for 5 min. Then, the mixture was stirred at 120 °C for 16 h. After cooling to ambient temperature, the mixture was diluted with CH₂Cl₂ and filtered through Celite, and the filtrate was concentrated. The crude residue was purified through a silica gel column (40% ethyl acetate in hexanes as eluent) to give pure product 3.

(E)-3-(1-Benzyl-2,5-dioxopyrrolidin-3-ylidene)-N-isobutyl-2-methylpropanamide (3aa)

Colorless semisolid; yield: 92.0 mg (79%).

¹H NMR (400 MHz, CDCl₃): δ = 7.41–7.37 (m, 2 H), 7.33–7.27 (m, 3 H), 6.84 (d, J = 9.8 Hz, 1 H), 5.62 (bs, 1 H), 4.72 (s, 2 H), 3.31 (dd, J = 21.4, 2.2 Hz, 1 H), 3.23 (dd, J = 21.4, 2.2 Hz, 1 H), 3.16–3.09 (m, 1 H), 3.06 (t, J = 6.6 Hz, 2 H), 1.80–1.72 (m, 1 H), 1.36 (d, J = 7.2 Hz, 3 H), 0.88 (d, J = 6.8 Hz, 6 H).

¹³C NMR (100 MHz, CDCl₃): δ = 173.2, 171.5, 164.1, 136.9, 135.7, 128.9, 128.7, 128.4, 127.2, 126.4, 47.1, 42.5, 42.4, 31.8, 28.4, 20.0, 17.5.

HRMS (ESI): m/z [M + H]⁺ calcd for [(C₁₉H₂₄N₂O₃)H]: 329.1865; found: 329.1859.

(E)-3-(1-Benzyl-2,5-dioxopyrrolidin-3-ylidene)-N,2-dimethyl­propanamide (3ba)

Colorless semisolid; yield: 92.0 mg (64%).

¹H NMR (400 MHz, CDCl₃): δ = 7.40–7.38 (m, 2 H), 7.32–7.27 (m, 3 H), 6.80 (d, J = 9.8 Hz, 1 H), 5.75 (bs, 1 H), 4.71 (s, 2 H), 3.30 (dd, J = 21.4, 1.6 Hz, 1 H), 3.22 (dd, J = 21.4, 1.6 Hz, 1 H), 3.11 (dq, J = 14.2, 7.2 Hz, 1 H), 2.78 (d, J = 4.8 Hz, 3 H), 1.34 (d, J = 7.2 Hz, 3 H).

¹³C NMR (100 MHz, CDCl₃): δ = 173.3, 172.1, 169.2, 136.9, 135.6, 128.9, 128.7, 128.1, 127.2, 42.5, 42.2, 31.8, 26.6, 17.2.

HRMS (ESI): m/z [M + H]⁺ calcd for [(C₁₆H₁₈N₂O₃)H]: 287.1396; found: 287.1339.

(E)-3-(1-Benzyl-2,5-dioxopyrrolidin-3-ylidene)-N-ethyl-2-methylpropanamide (3ca)

colorless semisolid; yield: 91.0 mg (69%).

¹H NMR (400 MHz, CDCl₃): δ = 7.40–7.36 (m, 2 H), 7.33–7.23 (m, 3 H), 6.82 (dt, J = 9.6, 2.0 Hz, 1 H), 5.69 (s, 1 H), 4.71 (s, 2 H), 3.37–3.18 (m, 4 H), 3.09 (dq, J = 14.2, 7.2 Hz, 1 H), 1.34 (d, J = 7.2 Hz, 3 H), 1.12 (t, J = 7.2 Hz, 3 H).

¹³C NMR (100 MHz, CDCl₃): δ = 173.3, 171.4, 169.2, 136.9, 135.6, 128.9, 128.7, 128.0, 127.1, 42.4, 42.3, 34.8, 31.9, 17.3, 14.7.

HRMS (ESI): m/z [M + H]⁺ calcd for [(C₁₇H₂₀N₂O₃)H]: 301.1552; found: 301.1552.

(E)-3-(1-Benzyl-2,5-dioxopyrrolidin-3-ylidene)-N-butyl-2-methylpropanamide (3da)

Colorless semisolid; yield: 84.0 mg (72%).

¹H NMR (400 MHz, CDCl₃): δ = 7.42–7.37 (m, 2 H), 7.35–7.29 (m, 3 H), 6.82 (dt, J = 9.6, 2.2 Hz, 1 H), 5.52 (bs, 1 H), 4.72 (s, 2 H), 3.27 (dd, J = 10.8, 2.2 Hz, 2 H), 3.24–3.19 (m, 2 H), 3.09 (dq, J = 14.6, 7.2 Hz, 1 H), 1.48–1.43 (m, 2 H), 1.35 (d, J = 7.2 Hz, 3 H), 1.32–1.28 (m, 2 H), 0.91 (t, J = 7.2 Hz, 3 H).

¹³C NMR (100 MHz, CDCl₃): δ = 173.2, 172.2, 171.4, 169.6, 169.2, 136.9, 135.7, 129.0, 128.7, 128.1, 127.2, 42.5, 42.4, 39.6, 31.9, 31.6, 20.0, 17.4, 13.7.

HRMS (ESI): m/z [M + H]⁺ calcd for [(C₁₉H₂₄N₂O₃)H]: 329.1865; found: 329.1858.

(E)-N-Allyl-3-(1-benzyl-2,5-dioxopyrrolidin-3-ylidene)-2-methylpropanamide (3ea)

Colorless semisolid; yield: 94.0 mg (75%).

¹H NMR (400 MHz, CDCl₃): δ = 7.42–7.40 (m, 2 H), 7.34–7.27 (m, 3 H), 6.83 (dt, J = 9.8, 2.2 Hz, 1 H), 5.80 (ddd, J = 16.2, 10.8, 5.8 Hz, 1 H), 5.63 (bs, 1 H), 5.19–5.15 (m, 1 H), 5.14–5.13 (m, 1 H), 4.72 (s, 2 H), 3.86 (t, J = 5.8 Hz, 2 H), 3.32 (dd, J = 21.2, 2.4 Hz, 1 H), 3.24 (dd, J = 21.2, 2.4 Hz, 1 H), 3.13 (dq, J = 14.2, 7.2 Hz, 1 H), 1.37 (d, J = 7.2 Hz, 3 H).

¹³C NMR (101 MHz, CDCl₃): δ = 173.2, 172.0, 169.21, 136.8, 135.6, 128.9, 128.9, 128.7, 128.6,128.0, 127.2, 42.4, 42.2, 31.8, 26.5, 17.2.

HRMS (ESI): m/z [M + H]⁺ calcd for [(C₁₈H₂₀N₂O₃)H]: 313.1552; found: 313.1580.

(E)-3-(1-Benzyl-2,5-dioxopyrrolidin-3-ylidene)-N-cyclohexyl-2-methylpropanamide (3fa)

Colorless semisolid; yield: 75.0 mg (71%).

¹H NMR (400 MHz, CDCl₃): δ = 7.41–7.40 (m, 2 H), 7.33–7.27 (m, 3 H), 6.83 (d, J = 9.6 Hz, 1 H), 5.38 (d, J = 7.4 Hz, 1 H), 4.72 (s, 2 H), 3.80–3.63 (m, 1 H), 3.31 (dd, J = 21.2, 2.2 Hz, 1 H), 3.22 (dd, J = 21.2, 2.1 Hz, 1 H), 3.06 (dq, J = 14.2, 7.2 Hz, 1 H), 1.91–1.87 (m, 2 H), 1.72–1.60 (m, 4 H), 1.33 (d, J = 7.2 Hz, 3 H), 1.18–1.04 (m, 4 H).

¹³C NMR (101 MHz, CDCl₃): δ = 184.0, 173.5, 170.9, 169.3, 137.2, 135.6, 128.9, 128.9, 128.7, 128.7, 128.0, 126.9, 48.7, 42.4, 38.5, 32.9, 31.8, 27.0, 25.4, 24.8, 17.5.

HRMS (ESI): m/z [M + H]⁺ calcd for [(C₂₁H₂₆N₂O₃)H]: 355.2022; found: 355.2019.

(E)-N-Cyclohexyl-2-methyl-3-(1-methyl-2,5-dioxopyrrolidin-3-ylidene)propanamide (3fb)

Colorless semisolid; yield: 56.0 mg (67%).

¹H NMR (400 MHz, CDCl₃): δ = 6.83 (d, J = 9.4 Hz, 1 H), 5.84 (d, J = 7.2 Hz, 1 H), 3.84–3.68 (m, 1 H), 3.59 (bs, 1 H), 3.34 (d, J = 21.4 Hz, 1 H), 3.25 (d, J = 21.4 Hz, 1 H), 3.20–3.14 (m, 1 H), 3.06 (s, 3 H), 1.91–1.89 (m, 2 H), 1.74–1.70 (m, 2 H), 1.64–1.61 (m, 1 H), 1.38–1.30 (m, 4 H), 1.19–1.10 (m, 3 H).

¹³C NMR (100 MHz, CDCl₃): δ = 173.7, 171.2, 169.7, 136.4, 127.3, 48.9, 42.2, 32.9, 31.7, 25.4, 24.8, 17.5.

HRMS (ESI): m/z [M + H]⁺ calcd for [(C₁₅H₂₂N₂O₃)H]: 279.1709; found: 279.1702.

(E)-N-Benzyl-3-(1-benzyl-2,5-dioxopyrrolidin-3-ylidene)-2-methylpropanamide (3ga)

Colorless semisolid; yield: 82.0 mg (79%).

¹H NMR (400 MHz, CDCl₃): δ = 7.39–7.37 (m, 2 H), 7.33–7.30 (m, 2 H), 7.29–7.26 (m, 4 H), 7.21 (d, J = 6.8 Hz, 2 H), 6.83 (dt, J = 9.6, 2.0 Hz, 1 H), 5.99 (bs, 1 H), 4.69 (s, 2 H), 4.39 (t, J = 5.6 Hz, 2 H), 3.25 (dd, J = 21.2, 2.2 Hz, 1 H), 3.21–3.08 (m, 2 H), 1.36 (d, J = 7.0 Hz, 3 H).

¹³C NMR (100 MHz, CDCl₃): δ = 189.4, 173.1, 171.3, 169.1, 137.7, 136.6, 135.6, 128.9, 128.8, 128.7, 128.1, 127.9, 127.8, 127.4, 43.9, 42.5, 42.3, 31.8, 29.7, 17.3.

HRMS (ESI): m/z [M + H]⁺ calcd for [(C₂₂H₂₂N₂O₃)H]: 363.1709; found: 363.1721.

(E)-3-(1-Benzyl-2,5-dioxopyrrolidin-3-ylidene)-N-(tert-butyl)-2-methylpropanamide (3ha)

Off-white semisolid; yield: 69.0 mg (59%).

¹H NMR (400 MHz, CDCl₃): δ = 7.42–7.40 (m, 2 H), 7.33–7.29 (m, 3 H), 6.83 (d, J = 9.6 Hz, 1 H), 5.37 (s, 1 H), 4.72 (s, 2 H), 3.31 (dd, J = 21.4, 1.8 Hz, 1 H), 3.22 (dd, J = 21.4, 1.8 Hz, 1 H), 3.09–2.94 (m, 1 H), 1.97 (s, 3 H), 1.33 (s, 9 H).

¹³C NMR (101 MHz, CDCl₃): δ = 173.5, 171.1, 171.00, 137.6, 128.8, 128.6, 127.9, 121.3, 51.6, 43.0, 42.3, 31.8, 28.6, 27.1, 18.5, 17.5.

HRMS (ESI): m/z [M + H]⁺ calcd for [(C₁₉H₂₄N₂O₃)H]: 329.1865; found: 329.1883.

(E)-3-(1-Benzyl-2,5-dioxopyrrolidin-3-ylidene)-N-isobutyl-2-(p-tolyl)propanamide (3ia)

Colorless semisolid; yield: 74.0 mg (79%).

¹H NMR (400 MHz, CDCl₃): δ = 7.31–7.30 (m, 2 H), 7.24–7.22 (m, 4 H), 7.13–7.07 (m, 4 H), 5.35 (bs, 1 H), 4.63 (s, 2 H), 4.15 (d, J = 8.6 Hz, 1 H), 3.21 (d, J = 21.2 Hz, 1 H), 2.99–2.87 (m, 3 H), 2.27 (s, 3 H), 1.65–1.57 (m, 1 H), 0.72 (d, J = 6.8 Hz, 6 H).

¹³C NMR (100 MHz, CDCl₃): δ = 173.4, 170.2, 169.2, 138.3, 135.7, 135.4, 133.7, 133.1, 130.3, 129.9, 129.7, 128.9, 128.8, 128.7, 128.6, 128.2, 128.1, 127.9, 53.3, 47.2, 42.4, 32.1, 28.4, 21.1, 19.9.

HRMS (ESI): m/z [M + H]⁺ calcd for [(C₂₅H₂₈N₂O₃)H]: 405.2178; found: 405.2198.

(E)-N-Isobutyl-2-methyl-3-(1-methyl-2,5-dioxopyrrolidin-3-ylidene)propanamide (3ab)

Colorless semisolid; yield: 57.0 mg (64%).

¹H NMR (400 MHz, CDCl₃): δ = 6.84 (d, J = 9.6 Hz, 1 H), 5.72 (s, 1 H), 3.33 (d, J = 21.2 Hz, 1 H), 3.25 (d, J = 21.2 Hz, 1 H), 3.20–3.11 (m, 1 H), 3.12–3.08 (m, 2 H), 3.06 (s, 3 H), 1.86–1.69 (m, 1 H), 1.38 (d, J = 7.2 Hz, 3 H), 0.90 (d, J = 6.6 Hz, 6 H).

¹³C NMR (100 MHz, CDCl₃): δ = 173.7, 171.6, 169.6, 136.5, 127.2, 47.1, 42.4, 31.8, 28.4, 24.8, 20.0, 17.5.

HRMS (ESI): m/z [M + H]⁺ calcd for [(C₁₃H₂₀N₂O₃)H]: 253.1552; found: 253.1553.

(E)-3-(1-Ethyl-2,5-dioxopyrrolidin-3-ylidene)-N-isobutyl-2-methylpropanamide (3ac)

Colorless semisolid; yield: 65.0 mg (69%).

¹H NMR (400 MHz, CDCl₃): δ = 6.83 (d, J = 9.6 Hz, 1 H), 5.70 (s, 1 H), 3.63 (q, J = 7.2 Hz, 2 H), 3.31 (d, J = 21.2 Hz, 1 H), 3.23 (d, J = 21.2 Hz, 1 H), 3.18–3.12 (m, 1 H), 3.08 (t, J = 6.4 Hz, 2 H), 1.76 (dt, J = 20.2, 6.8 Hz, 1 H), 1.38 (d, J = 7.0 Hz, 3 H), 1.20 (t, J = 7.2 Hz, 3 H), 0.90 (d, J = 6.8 Hz, 6 H).

¹³C NMR (100 MHz, CDCl₃): δ = 173.4, 171.7, 136.4, 127.4, 47.1, 42.4, 33.8, 31.8, 28.4, 20.0, 17.5, 13.1.

HRMS (ESI): m/z [M + H]⁺ calcd for [(C₁₄H₂₂N₂O₃)H]: 267.1709; found: 267.1716.

(E)-3-(1-Cyclohexyl-2,5-dioxopyrrolidin-3-ylidene)-N-isobutyl-2-methylpropanamide (3ad)

Colorless semisolid; yield: 83.0 mg (73%).

¹H NMR (400 MHz, CDCl₃): δ = 6.78 (d, J = 9.6 Hz, 1 H), 5.59 (bs, 1 H), 4.05 (tt, J = 12.2, 3.6 Hz, 1 H), 3.26 (dd, J = 21.2, 2.0 Hz, 1 H), 3.22–3.15 (m, 1 H), 3.15–3.11 (m, 1 H), 3.08 (t, J = 6.5 Hz, 2 H), 2.16 (q, J = 12.6 Hz, 2 H), 1.83 (d, J = 12.8 Hz, 2 H), 1.76 (dd, J = 13.6, 6.9 Hz, 2 H), 1.68–1.55 (m, 2 H), 1.37 (d, J = 7.2 Hz, 3 H), 1.34–1.31 (dd, J = 26.6, 13.2 Hz, 3 H), 0.90 (d, J = 6.8 Hz, 6 H).

¹³C NMR (100 MHz, CDCl₃): δ = 173.6, 171.6, 169.6, 135.9, 127.3, 51.8, 47.1, 42.4, 31.8, 28.8, 28.5, 25.8, 25.0, 20.1, 17.5.

HRMS (ESI): m/z [M + H]⁺ calcd for [(C₁₈H₂₈N₂O₃)H]: 321.2178; found: 321.2235.

(E)-3-(2,5-Dioxo-1-phenethylpyrrolidin-3-ylidene)-N-isobutyl-2-methylpropanamide (3ae)

Colorless semisolid; yield: 84.0 mg (69%).

¹H NMR (400 MHz, CDCl₃): δ = 7.36–7.26 (m, 2 H), 7.24–7.17 (m, 3 H), 6.82 (d, J = 9.7 Hz, 1 H), 5.57 (s, 1 H), 3.83–3.79 (m, 2 H), 3.28 (d, J = 21.2 Hz, 1 H), 3.25–3.13 (m, 2 H), 3.08 (t, J = 6.6 Hz, 2 H), 2.94–2.87 (m, 2 H), 1.81–1.73 (m, 1 H), 1.38 (d, J = 7.2 Hz, 3 H), 0.90 (d, J = 6.6 Hz, 6 H).

¹³C NMR (100 MHz, CDCl₃): δ = 176.6, 172.1, 171.6, 137.7, 137.2, 136.5, 135.9,128.9, 128.8, 128.6, 127.2, 126.7, 47.1, 42.4, 40.0, 33.7, 31.7, 28.5, 20.1, 17.5.

HRMS (ESI): m/z [M + H]⁺ calcd for [(C₂₀H₂₆N₂O₃)H]: 343.2022; found: 343.2038.

(E)-3-(2,5-Dioxo-1-phenylpyrrolidin-3-ylidene)-N-isobutyl-2-methylpropanamide (3af)

Colorless semisolid; yield: 88.0 mg (79%).

¹H NMR (400 MHz, CDCl₃): δ = 7.48 (t, J = 7.6 Hz, 2 H), 7.40 (t, J = 7.6 Hz, 1 H), 7.31 (d, J = 7.6 Hz, 2 H), 6.96 (d, J = 9.6 Hz, 1 H), 5.82 (bs, 1 H), 3.52 (dd, J = 21.6, 2.2 Hz, 1 H), 3.42 (dd, J = 21.6, 2.2 Hz, 1 H), 3.22 (dq, J = 14.2, 7.2 Hz, 1 H), 3.09 (t, J = 6.6 Hz, 2 H), 1.85–1.70 (m, 1 H), 1.41 (d, J = 7.2 Hz, 3 H), 0.91 (d, J = 6.7 Hz, 6 H).

¹³C NMR (100 MHz, CDCl₃): δ = 172.7, 171.7, 168.6, 137.8, 131.8, 129.2, 128.7, 126.9, 126.4, 47.2, 42.5, 32.0, 28.4, 20.1, 17.5.

HRMS (ESI): m/z [M + H]⁺ calcd for [(C₁₈H₂₂N₂O₃)H]: 315.1709; found: 315.1701.

(E)-3-(2,5-Dioxo-1-(p-tolyl)pyrrolidin-3-ylidene)-N-isobutyl-2-methylpropanamide (3ag)

Colorless semisolid; yield: 86.0 mg (74%).

¹H NMR (400 MHz, CDCl₃): δ = 7.28 (d, J = 8.6 Hz, 2 H), 7.23–7.13 (m, 2 H), 6.95 (dt, J = 9.6, 2.4 Hz, 1 H), 5.82 (s, 1 H), 3.50 (dd, J = 21.4, 2.4 Hz, 1 H), 3.41 (dd, J = 21.4, 2.4 Hz, 1 H), 3.21 (dq, J = 9.6, 7.2 Hz, 1 H), 3.10–3.04 (m, 2 H), 2.38 (s, 3 H), 1.81–1.73 (m, 1 H), 1.40 (d, J = 7.2 Hz, 3 H), 0.90 (d, J = 6.6 Hz, 6 H).

¹³C NMR (100 MHz, CDCl₃): δ = 172.8, 171.6, 168.8, 138.8, 137.8, 129.9, 129.1, 126.9, 126.2, 47.1, 42.5, 32.0, 28.5, 21.2, 20.1, 17.5.

HRMS (ESI): m/z [M + H]⁺ calcd for [(C₁₉H₂₄N₂O₃)H]: 329.1865; found: 329.1857.

(E)-N-Isobutyl-3-(1-(4-methoxyphenyl)-2,5-dioxopyrrolidin-3-ylidene)-2-methylpropanamide (3ah)

Colorless semisolid; yield: 99.0 mg (81%).

¹H NMR (400 MHz, CDCl₃): δ = 7.15 (d, J = 8.8 Hz, 2 H), 6.91 (d, J = 8.8 Hz, 2 H), 6.87 (d, J = 9.6 Hz, 1 H), 5.64 (s, 1 H), 3.75 (s, 3 H), 3.45–3.39 (m, 1 H), 3.37–3.28 (m, 1 H), 3.16–3.10 (m, 1 H), 3.01 (t, J = 6.6 Hz, 2 H), 1.69 (dd, J = 13.4, 6.6 Hz, 1 H), 1.33 (d, J = 7.2 Hz, 3 H), 0.84 (d, J = 6.3 Hz, 6 H).

¹³C NMR (100 MHz, CDCl₃): δ = 172.9, 171.5, 168.8, 159.6, 137.7, 127.6, 126.9, 124.4, 114.5, 55.5, 47.1, 42.5, 31.9, 28.5, 20.1, 17.5.

HRMS (ESI): m/z [M + H]⁺ calcd for [(C₁₉H₂₄N₂O₄)H]: 345.1814; found: 345.1808.

(E)-3-(1-(4-Bromophenyl)-2,5-dioxopyrrolidin-3-ylidene)-N-isobutyl-2-methylpropanamide (3ai)

Colorless semisolid; yield: 120.0 mg (86%).

¹H NMR (400 MHz, CDCl₃): δ = 7.60 (d, J = 8.6 Hz, 2 H), 7.23 (d, J = 8.6 Hz, 2 H), 6.97 (d, J = 9.6 Hz, 1 H), 5.71 (bs, 1 H), 3.51 (dd, J = 21.6, 1.8 Hz, 1 H), 3.42 (dd, J = 21.6, 1.8 Hz, 1 H), 3.24–3.15 (m, 2 H), 3.09 (t, J = 6.6 Hz, 1 H), 1.83–1.73 (m, 1 H), 1.41 (d, J = 7.2 Hz, 3 H), 0.91 (d, J = 6.6 Hz, 6 H).

¹³C NMR (100 MHz, CDCl₃): δ = 172.3, 171.4, 168.2, 138.4, 132.3, 130.8, 127.8, 126.5, 122.5, 47.1, 42.6, 32.0, 28.5, 20.0, 17.5.

HRMS (ESI): m/z [M + H]⁺ calcd for [(C₁₈H₂₁BrN₂O₃)H]: 393.0814; found: 393.0870.

(E)-3-(1-Allyl-2,5-dioxopyrrolidin-3-ylidene)-N-isobutyl-2-methylpropanamide (3aj)

Orange liquid; yield: 66 mg (35% yield).

¹H NMR (500 MHz, CDCl₃): δ = 6.78 (dt, J = 9.6, 2.4 Hz, 1 H), 5.77–5.68 (m, 1 H), 5.56 (s, 1 H), 5.15 (m, 1 H), 4.11 (d, J = 5.9 Hz, 2 H), 3.23 (m, 2 H), 3.01 (td, J = 6.9, 2.6 Hz, 2 H), 2.08 (dd, J = 14.4, 7.1 Hz, 1 H), 1.72–1.65 (m, 2 H), 1.31 (d, J = 7.0 Hz, 3 H), 0.83 (d, J = 6.7 Hz, 6 H).

¹³C NMR (126 MHz, CDCl₃): δ = 173.09, 171.50, 170.78, 169.03, 136.83, 134.16, 134.11, 133.77, 130.55, 129.54, 128.72, 127.13, 118.53, 115.32, 60.39, 47.06, 42.41, 40.94, 37.34, 31.81, 29.67, 28.51, 28.43, 27.51, 25.45, 20.12, 20.02, 17.45, 14.17, 12.87.

HRMS (ESI): m/z [M + Na]⁺ calcd for [(C₁₅H₂₂N₂O₃)Na]: 301.1528; found: 301.1523.

Linear-Selective Alkylated Acrylamide Derivatives 5; General Procedure

A 15 mL Schlenk tube with septum containing Cp*Co(CO)I₂ (10 mol%), N-benzylacrylamide 1g (100 mg, 1 equiv) and AgSbF₆ (20 mol%) was evacuated and purged with nitrogen gas three times (AgSbF₆ was transferred inside a glovebox). To the tube were then added acrylate 4 (3.0 equiv), PivOH (30 mol%) and TFE (3.0 mL) via syringe; after that, the reaction mixture was evacuated and purged with nitrogen gas three times. Then, the septum was taken out immediately and a screw cap was used to cover the tube under the nitrogen atmosphere, and the reaction mixture was stirred at room temperature for 5 min. Then, the mixture was stirred at 120 °C for 16 h. After cooling to ambient temperature, the mixture was diluted with CH₂Cl₂ and filtered through Celite, and the filtrate was concentrated. The crude residue was purified through a silica gel column (40% ethyl acetate in hexanes as eluent) to give pure product 5.

Methyl (Z)-6-(Benzylamino)-5-methyl-6-oxohex-4-enoate (5ga)

Colorless liquid; yield: 96.0 mg (72%).

¹H NMR (400 MHz, CDCl₃): δ = 7.39–7.32 (m, 2 H), 7.32–7.25 (m, 3 H), 6.33–6.31 (m, 1 H), 6.07 (bs, 1 H), 4.50 (d, J = 5.6 Hz, 2 H), 3.68 (s, 3 H), 2.53–2.34 (m, 4 H), 1.89 (s, 3 H).

¹³C NMR (100 MHz, CDCl₃): δ = 173.2, 168.9, 138.3, 133.9, 132.1, 128.8, 127.9, 127.6, 51.8, 43.9, 32.9, 23.7, 12.8.

HRMS (ESI): m/z [M + Na]⁺ calcd for [(C₁₅H₁₉NO₃)Na]: 284.1263; found: 284.1259.

Ethyl (Z)-6-(Benzylamino)-5-methyl-6-oxohex-4-enoate (5gb)

Colorless liquid; yield: 90 mg (68%).

¹H NMR (400 MHz, CDCl₃): δ = 7.37–7.26 (m, 5 H), 6.33 (t, J = 6.2 Hz, 1 H), 6.09 (s, 1 H), 4.49 (d, J = 5.6 Hz, 2 H), 4.13 (q, J = 7.0 Hz, 2 H), 2.49–2.38 (m, 4 H), 1.89 (s, 3 H), 1.25 (t, J = 7.1 Hz, 3 H).

¹³C NMR (100 MHz, CDCl₃): δ = 172.7, 168.9, 138.3, 133.9, 131.9, 128.7, 127.8, 127.5, 60.5, 43.8, 33.2, 23.6, 14.1, 12.8.

HRMS (ESI): m/z [M + Na]⁺ calcd for [(C₁₆H₂₁NO₃)Na]: 298.1419; found: 298.1412.

Butyl (Z)-6-(Benzylamino)-5-methyl-6-oxohex-4-enoate (5gc)

Colorless liquid; yield: 75.0 mg (57%).

¹H NMR (500 MHz, CDCl₃): δ = 7.39–7.22 (m, 5 H), 6.32 (t, J = 6.5 Hz, 1 H), 6.01 (s, 1 H), 4.50 (d, J = 5.6 Hz, 2 H), 4.07 (t, J = 6.7 Hz, 2 H), 2.44 (m, 4 H), 1.89 (s, 3 H), 1.59 (dd, J = 14.7, 7.1 Hz, 2 H), 1.37 (m, 2 H), 0.93 (t, J = 7.4 Hz, 3 H).

¹³C NMR (126 MHz, CDCl₃): δ = 172.8, 168.9, 138.3, 133.9, 132.1, 128.7, 127.9, 127.5, 64.5, 43.9, 33.2, 30.6, 23.7, 19.1, 13.7, 12.8.

HRMS (ESI): m/z [M + Na]⁺ calcd for [(C₁₈H₂₅NO₃)Na]: 303.181; found: 303.187.

1-Benzyl-6-methyl-3,4-dihydro-1H-azepine-2,7-dione (5gd)

Colorless liquid; yield: 36.0 mg (55%).

¹H NMR (400 MHz, CDCl₃): δ = 7.34 (dd, J = 7.8, 6.8 Hz, 3 H), 7.31–7.26 (m, 2 H), 6.31 (td, J = 7.2, 1.2 Hz, 1 H), 4.50–4.49 (m, 2 H), 2.57–2.52 (m, 2 H), 2.53–2.48 (m, 2 H), 1.89 (d, J = 1.2 Hz, 3 H).

¹³C NMR (100 MHz, CDCl₃): δ = 171.1, 168.8, 138.3, 133.0, 132.6, 128.8, 128.7, 127.8, 127.6, 43.9, 32.6, 23.3, 12.8.

HRMS (ESI): m/z [M + Na]⁺ calcd for [(C₁₄H₁₅NO₂)Na]: 252.0995; found: 252.0999.

Phenyl (Z)-6-(Benzylamino)-5-methyl-6-oxohex-4-enoate (5ge) + 1-Benzyl-6-methyl-3,4-dihydro-1H-azepine-2,7-dione (5ge′)

Colorless liquid; yield: 86.0 mg (54%).

¹H NMR (400 MHz, CDCl₃): δ = 7.43–7.18 (m, 13 H), 7.06 (d, J = 7.7 Hz, 2 H), 6.42 (t, J = 7.1 Hz, 1 H), 6.32 (t, J = 6.7 Hz, 1 H), 6.09 (s, 1 H), 4.49 (dd, J = 9.4, 4.8 Hz, 4 H), 2.70 (t, J = 7.3 Hz, 2 H), 2.55 (dq, J = 12.8, 6.9 Hz, 6 H), 1.92 (s, 3 H), 1.89 (s, 3 H).

¹³C NMR (100 MHz, CDCl₃): δ = 171.22, 171.00, 168.83, 168.71, 150.48, 138.26, 133.44, 133.01, 132.45, 132.30, 129.41, 128.69, 127.81, 127.49, 125.86, 121.42, 43.82, 33.23, 32.54, 23.58, 23.31, 12.85, 12.78.

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Review:
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