Gold-Catalyzed Oxidative Cyclizations on 1,4-Enynes: Evidence for a γ-Substituent Effect on Wagner-Meerwein Rearrangements

摘要

The Wagner-Meerwein rearrangement refers to a 1,2-shift of an alkyl, aryl, and alkenyl group to an adjacent carbocationic center; formation of this carbon-carbon bond has found widespread applications in many Lewis acid or Br0nsted acid mediated reactions [Eq. (1)] For this well-known reaction, the 1,2-shift of R~1 versus R~2 is determined primarily by the relative stability of carbocation B (or B'), as well as the intrinsic properties of the migrating group (Scheme 1). No instance of a γ-substituent to stereospecifically direct a 1,2-shift of the R group in β-position was reported to date; those carbocations bearing a γ-silyl group are no examples either. Although the γ-effect on this arrangement was claimed in an early report, the 1,2-shift actually occurred within a benzene skeleton comprising the C_β and C_γ carbon atoms. According to carbocation chemistry, we envisage that a metal substituent in the y-position might facilitate an anti activation through hyperconjugation to induce a 1,2-R~1-shift (C→D) in an antiperiplanar conformation (C) [Eq. (2)]. Alternatively, this metal might exert steric interaction to induce a syn activation to enable a 1,2-R~2-shift (C→D') in a synperiplanar conformation C [Eq. (3)] . The realization of such an unprecedented effect of a metal in y-position relies on the availability of suitable carbocations applicable for a study. Herein, we report our experimental and theoretical work to support an anti'-activation route [Eq. (2)] for M = Au (path C→D) even in a synperiplanar conformation. This work represents an atypical Wagner-Meerwein rearrangement, because the intrinsic properties of the migrating group (R~1, R~2) are no longer decisive.