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

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