Asymmetric oxidative α-fluorination of 2-alkylphenylacetaldehydes with AgHF2 and ruthenium/PNNP catalysts
作者:Martin Althaus、Antonio Togni、Antonio Mezzetti
DOI:10.1016/j.jfluchem.2009.05.008
日期:2009.8
During attempts directed at epoxide ring opening with different HF sources, we discovered that the Lewis acidic [RuCl(PNNP)](+) (1) or [Ru(OEt2)(2)(PNNP)](2+) (2) catalysts promote the [1,2]-phenyl shift (Meinwald rearrangement) in phenyl-substituted epoxides to give the corresponding 2-alkylphenylacetaldehydes, which are fluorinated at the alpha-position in the presence of silver bifluoride (AgHF2) (PNNP is (1S,2S)-N,N'-bis[o-(diphenylphosphino)benzylidene]cyclohexane-1,2-diamine). The optimization of aldehyde fluorination with PhCH(R)CHO (R = Me, Et, Pr-i, Bu-t) as substrates showed that catalyst [1]SbF6 gives a moderate degree of enantioselectivity (up to 27% ee) and 35% yield. The substrate scope is limited to benzylic aldehydes. The reaction is unprecedented for transition metal catalysts. Circumstantial evidence suggests that the mechanism involves chemical oxidation followed by enantioselective fluorination with F-. (C) 2009 Elsevier B.V. All rights reserved.