Mechanism of silver(I)-catalyzed Mukaiyama aldol reaction: active species in solution in AgPF 6 -(S)-BINAP versus AgOAc-(S)-BINAP systems

Abstract Silver(I)-diphosphine complex is an effective catalyst for Mukaiyama Aldol reaction in polar solvents. AgPF 6 -( S )-BINAP cationic chiral complex indicated a good activity and could afford fairly high enantioselectivity in the reaction of aromatic aldehydes and silyl enol ethers. On the other hand, AgOAc-( S )-BINAP system afforded the aldol product of the absolute configuration opposite to that by AgPF 6 -( S )-BINAP and very high catalytic activity was shown. The structure and equilibrium state of Ag(I)-BINAP complexes in solution were examined to understand the reaction mechanism. In AgPF 6 system [Ag(( S )-BINAP) 2 ]PF 6 ( 1a ), [Ag(( S )-BINAP)]PF 6 ( 1b ), [Ag 2 (( S )-BINAP)](PF 6 ) 2 ( 1c ) and AgPF 6 are present in solution. The active species of the aldol reaction in DMF is [Ag(( S )-BINAP)]PF 6 ( 1b ), which exists as a minor species in solution. For this cationic Ag(I) catalyst, cyclic transition state containing substrate and silyl enol ether is assumed. In AgOAc-( S )-BINAP system, active species is also monomeric AgOAc(( S )-BINAP) ( 2b ) species which exists as a major component in solution and strong interaction was observed with 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 species and is activated. This complex attacks aldehydes to afford aldol adduct via acyclic transition state.