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 sys tem for the tail-to-tail dimerization of methyl acrylate (MA) to dimet hyl hexenedioates, precursors to adipic acid, is described. The cataly tic cycle is entered by protonation of CpRh(C2H4)(2)l(Cp* = C(5)Me(5) ) to yield CpRh(C2H4) (CH2CH2-mu-H)(+) (7) followed by reaction with methylacrylate. The catalyst resting state has been generated by low-t emperature protonation of CpRh(CH2CHCO2-CH3)(2)l (15) and identified as CpRd(CH(2)CH(2)COOMe)(eta(2)-CH(2)CHCO(2)Me)(+) (8) by H-1 and C-1 3 NMR spectroscopy. Investigation of iridium analogs has led to the is olation and X-ray structural characterization of CpIr(CH(2)CH(2)COOMe )(eta(2)-CH(2)CHCO(2)Me)(+) (23a), in which the orientation of the acr ylate ligands is that required for tail-to-tail coupling. At -23 degre es C, complex 8 undergoes beta-migratory insertion to give CpRhCH(CH( 2)COOMe)(CH2-CH(2)COOMe)(+) (10). Complex 10 was independently synthes ized by treatment of complex 7 with trans-MeO(2)CCH=CHCH(2)CH(2)CO(2)M e and was characterized by X-ray crystallography. The free energy of a ctivation 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 degr ees C, Delta G = 19 kcal/mol). This observation confirms 8 as the res ting state and the C-C coupling reaction as the turnover-limiting step . The catalyst deactivates by formal loss of Hz from complex 10 to pro duce CpRh(eta(3)-CH3OCOCH2CHCHCHCO2CH3)(+) (9). The structure of comp lex 9 was verified by an X-ray crystallographic study. Exposure of 9 t o an atmosphere of H-2 in the presence of MA regenerates the resting s tate 8, and dimerization proceeds. Second generation catalysts with in creased activity and lifetimes have been developed by replacing the C( 5)Me(5) ligand by methylated indenyl ligands. Using the catalytic syst em derived from (1,2,3-trimethylindenyl)Rh(C2H4>(2) (11), conversion o f 54 000 equiv of methyl acrylate to dimethyl hexenedioates could be a chieved after 68 h at 55 OC under N-2. Details of the mechanism have b een elucidated and resemble closely those of the Cp system. Similar i ntermediates to 8 and 10 have been characterized by H-1 and C-13 NMR s pectroscopy. In contrast, treatment with methyl acrylate of the more e lectrophilic systems derived from CpRh(C2H4)(2) (25) (Cp = C5H5) and C pRh(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 t o 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 u nusual 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 CpRh(C2H4)(CH2CH2-mu-H)(+) (31), w hich upon reaction with MA clearly produces CpRhCH(CH(2)COOMe)(CH(2)C H(2)COOMe)(+) (33), and dimerization proceeds. Finally, attempts to ca talyze the dimerization of other functionalized olefins including meth yl vinyl ketone, methyl crotonate, 2-vinylpyridine, and 1-vinyl-2-pyrr olidinone are presented.