Mechanistic studies on the catalytic cycle of rhodium-catalyzed asymmetric 1,4-addition of aryltitanate reagents to , -unsaturated ketones
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چکیده
Addition of lithium aryl(tetraisopropoxy)titanates [ArTi(OPri)4 Li ] to , -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 20°C 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. 31P NMR spectroscopic studies revealed that the catalytic cycle consists of three transformations, that is, (i) insertion of an enone into arylrhodium species forming (oxa-allyl)rhodium intermediate, (ii) silylation of the (oxa-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.
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