Unexpected Skeletal Rearrangement in the Gold(I)/Silver(I)-Catalyzed Conversion of 7-Aryl-1,6-enynes to Bicyclo[3.2.0]hept-6-enes via Hidden Brønsted Acid Catalysis

Abstract

Cycloaddition of an isotopically labeled 7-phenyl-1,6-enyne catalyzed by a mixture of LAuCl [L = P(t-Bu)2o-biphenyl)] and AgSbF6 forms the corresponding 6-phenylbicyclo[3.2.0]hept-6-ene with concomitant scrambling of the olefinic CPh (C6) and CH (C7) groups of the product. The extent of C6/C7 scrambling was sensitive to the nature of the silver salt and was likewise observed in the cycloaddition of the 7-phenyl-1,6-enyne catalyzed by mixtures of [LAuNCMe]+SbF6– and Bronsted acids such as HOTf. These observations and low-temperature NMR analysis of organic and organometallic intermediates in the cycloaddition process were in accord with a mechanism for C6/C7 scrambling initiated by protonation of free bicyclo[3.2.0]hept-7-ene to generate a bicyclo[3.2.0]heptyl cation that undergoes reversible alkyl migration, presumably involving bicyclo[4.1.0]- and bicyclo[3.1.1]heptyl cations prior to elimination of the C7 proton.