DESIGN AND STUDY OF RH(III) CATALYSTS FOR THE SELECTIVE TAIL-TO-TAIL DIMERIZATION OF METHYL ACRYLATE

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)l(Cp* = C(5)Me(5)) to yield Cp*Rh(C2H4) (CH2CH2-mu-H)(+) (7) followed by reaction with methylacrylate. The catalyst resting state has been generated by low-temperature protonation of Cp*Rh(CH2CHCO2-CH3)(2)l (15) and identified as Cp*Rd(CH(2)CH(2)COOMe)(eta(2)-CH(2)CHCO(2)Me)(+) (8) by H-1 and C-13 NMR spectroscopy. Investigation of iridium analogs has led to the isolation and X-ray structural characterization of Cp*Ir(CH(2)CH(2)COOMe)(eta(2)-CH(2)CHCO(2)Me)(+) (23a), in which the orientation of the acrylate ligands is that required for tail-to-tail coupling. At -23 degrees C, complex 8 undergoes beta-migratory insertion to give Cp*RhCH(CH(2)COOMe)(CH2-CH(2)COOMe)(+) (10). Complex 10 was independently synthesized by treatment of complex 7 with trans-MeO(2)CCH=CHCH(2)CH(2)CO(2)Me 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 degrees C, Delta G* = 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 Hz from complex 10 to produce Cp*Rh(eta(3)-CH3OCOCH2CHCHCHCO2CH3)(+) (9). The structure of complex 9 was verified by an X-ray crystallographic study. Exposure of 9 to an atmosphere of H-2 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 C(5)Me(5) 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 OC under N-2. 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 H-1 and C-13 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* = C-5(CH3)(4)CF3) results in slow dimerization. Low-temperature protonation of CpRh(CH2CHCO2CH3)(2) (27) with H(Et(2)O)(2)BAr'(4) yields a mixture of rhodium species which upon warming to 23 degrees C converge to the bis-chelate complex CpRhCH(CH(2)COOMe)(CH(2)CH(2)COOMe)(+) (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(Et(2)O)(2)BAr'(4) yields Cp*Rh(C2H4)(CH2CH2-mu-H)(+) (31), which upon reaction with MA clearly produces Cp*RhCH(CH(2)COOMe)(CH(2)CH(2)COOMe)(+) (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.