Mechanism of silver(I)-catalyzed Mukaiyama aldol reaction: active species in solution in AgPF6-(S)-BINAP versus AgOAc-(S)-BINAP systems

Silver(I)-diphosphine complex is an effective catalyst for Mukaiyama Aldol reaction in polar solvents. AgPF6-(S)-BINAP cationic chiral complex indicat ed a good activity and could afford fairly high enantioselectivity in the r eaction of aromatic aldehydes and silyl enol ethers. On the other hand, AgO Ac-(S)-BINAP system afforded the aldol product of the absolute configuratio n opposite to that by AgPF6-(S)-BINAP and very high catalytic activity was shown. The structure and equilibrium state of Ag(I)-BINAP complexes in solu tion were examined to understand the reaction mechanism. In AgPF6 system [A g((S)-BINAP)(2)]PF6 (1a), [Ag((S)-BINAP)]PF6 (1b), [Ag-2((S)-BINAP)](PF6)(2 ) (1c) and AgPF6 are present in solution. The active species of the aldol r eaction in DMF is [Ag((S)-BINAP)]PF6 (1b), which exists as a minor species in solution. For this cationic Ag(I) catalyst, cyclic transition state cont aining substrate and silyl enol ether is assumed. In AgOAc-(S)-BINAP system , active species is also monomeric AgOAc((S)-BINAP) (2b) species which exis ts as a major component in solution and strong interaction was observed wit h a silyl enol ether. The reaction by AgOAc-(S)-BINAP catalyst is concluded to proceed as follows: nucleophile forms a complex with AgOAc-(S)-BINAP sp ecies and is activated. This complex attacks aldehydes to afford aldol addu ct via acyclic transition state. (C) 2001 Elsevier Science B.V. Ah rights r eserved.