Mechanistic studies on the catalytic cycle of rhodium-catalyzed asymmetric 1,4-addition of aryltitanate reagents to α,β-unsaturated ketones

Addition of lithium aryl(tetraisopropoxy)titanates [ArTi(OPr-i)4(-)Li(+)] to alpha,beta-unsaturated ketones proceeded with high enantioselectivity (up to 99% ee) in the presence of an excess amount of chlorotrimethylsilane and a rhodium catalyst (3 mol % Rh), generated from [RhCl(C2H4)(2)](2) and (S)-binap, in tetrahydrofuran at 20degreesC to give high yields of the corresponding silyl enolates as 1,4-addition products. The presence of chlorotrimethylsilane is essential for the 1,4-addition to take place. P-31 NMR spectroscopic studies revealed that the catalytic cycle consists of three transformations, that is, (i) insertion of an enone into arylrhodium species forming (oxa-pi-allyl)rhodium intermediate, (ii) silylation of the (oxa-pi-allyl)rhodium with chlorotrimethylsilane giving silyl enolate and a chloro-rhodium complex, and (iii) transmetalation of aryl group from aryltitanate to the chloro-rhodium regenerating the aryl-rhodium.