1,3-Cationic Alkylidene Migration of Nonclassical Carbocation: A Density Functional Theory Study on Gold(I)-Catalyzed Cycloisomerization of 1,5-Enynes Containing Cyclopropene Moiety

Abstract

For quite a long time, nonclassical carbocations have only been regarded as special intermediates with limited cases in solvolysis reactions. However, the present work shows that in common reaction, typical nonclassical carbocations may be involved in and have significant effects on the reaction mechanisms. In this work, DFT studies have been performed on the mechanism of gold(I)-catalyzed cycloisomerization of 1,5-enynes containing cyclopropene moiety at PBE1PBE/6-31+G**/SDD level. An unprecedented pathway containing two consecutive 1,3-cationic alkylidene migrations of nonclassical carbocation intermediates derived from norbornenyl cation, rather than the generally considered Wagner-Meerwein 1,2-alkyl migrations, was found. Detailed structural analysis shows the nature of this 1,3-cationic alkylidene migration: it is promoted by strong cation-π interaction between the cationic center and the double bond. Topological analysis shows that for certain nonclassical carbocation intermediates (1c'-A and 1c'-F), there do exist bond critical point and bond path between the cationic center and the double bond. On the basis of the mechanisms proposed, the product selectivity controlled by the substituent effects was also rationalized.