Design and study of Rh(III) catalysts for the selective tail-to-tail dimerization of methyl acrylate

Article abstract of DOI:10.1021/ja00097a011

The development of an efficient, highly selective Rh(III) catalyst system for the tail-to-tail dimerization of methyl acrylate (MA) to dimethyl hexenedioates, precursors to adipic acid, is described. The catalytic cycle is entered by protonation of Cp*Rh(C₂H₄)₂ (Cp* = C₅Me₅) to yield Cp*Rh(C₂H₄)(CH₂CH ₂-μ-H)⁺ (7) followed by reaction with methyl acrylate. The catalyst resting state has been generated by low-temperature protonation of Cp*Rh(CH₂CHCO₂-CH₃)₂ (15) and identified as Cp*Rh(CH₂CH₂COOMe)(η²-CH ₂CHCO₂Me)⁺ (8) by ¹H and ¹³C NMR spectroscopy. Investigation of iridium analogs has led to the isolation and X-ray structural characterization of Cp*Ir(CH₂-CH₂COOMe)(η²-CH ₂CHCO₂Me)⁺ (23a), in which the orientation of the acrylate ligands is that required for tail-to-tail coupling. At -23°C, complex 8 undergoes β-migratory insertion to give Cp*RhCH(CH₂COOMe)(CH₂ CH₂COOMe)⁺ (10). Complex 10 was independently synthesized by treatment of complex 7 with trans-MeO₂CCH=CHCH₂CH₂CO₂Me and was characterized by X-ray crystallography. The free energy of activation for the migration reaction is 18.7 kcal/mol and matches that based on the catalytic turnover (TO) frequency (6.6 TO/min at 25°C, ΔG^? = 19 kcal/mol). This observation confirms 8 as the resting state and the C-C coupling reaction as the turnover-limiting step. The catalyst deactivates by formal loss of H₂ from complex 10 to produce Cp*Rh(η³-CH₃-OCOCH₂CHCHCHCO ₂CH₃)⁺ (9). The structure of complex 9 was verified by an X-ray crystallographic study. Exposure of 9 to an atmosphere of H₂ in the presence of MA regenerates the resting state 8, and dimerization proceeds. Second generation catalysts with increased activity and lifetimes have been developed by replacing the C₅Me₅ ligand by methylated indenyl ligands. Using the catalytic system derived from (1,2,3-trimethylindenyl)Rh(C₂H₄)₂ (11), conversion of 54 000 equiv of methyl acrylate to dimethyl hexenedioates could be achieved after 68 h at 55°C under N₂. Details of the mechanism have been elucidated and resemble closely those of the Cp* system. Similar intermediates to 8 and 10 have been characterized by ¹H and ¹³C NMR spectroscopy. In contrast, treatment with methyl acrylate of the more electrophilic systems derived from CpRh(C₂H₄)₂ (25) (Cp = C₅H₅) and Cp*Rh(C₂H₄)₂ (30) (Cp* = C₅(CH₃)₄CF₃) results in slow dimerization. Low-temperature protonation of CpRh(CH₂CHCO₂CH₃)₂ (27) with H(Et₂O)₂BAr′₄ yields a mixture of rhodium species which upon warming to 23°C converge to the bis-chelate complex CpRhCH(CH₂COOMe)(CH₂CH₂COOMe)⁺ (28). Exposure of complex 28 to MA generates the unusual bridged species CpRh(CH₂CHCOOCH₃)H(CH₂CHCOOCH₃) ⁺ (29), which serves as the resting state during dimerization. Treatment of complex 30 with H(Et₂O)₂BAr′₄ yields Cp^?Rh(C₂H₄)(CH₂CH ₂-μ-H)⁺ (31), which upon reaction with MA clearly produces Cp^?RhCH(CH₂COOMe)(CH₂CH ₂COOMe)⁺ (33), and dimerization proceeds. Finally, attempts to catalyze the dimerization of other functionalized olefins including methyl vinyl ketone, methyl crotonate, 2-vinylpyridine, and 1-vinyl-2-pyrrolidinone are presented.