Mechanistic Exploration of the Competition Relationship between a Ketone and C═C, C═N, or C═S Bond in the Rh(III)-Catalyzed Carbocyclization Reactions.

Abstract

The introduction of a C═O, C═C, C═S, or C═N bond has emerged as an effective strategy for carbocycle synthesis. A computational mechanistic study of Rh(III)-catalyzed coupling of alkynes with enaminones, sulfoxonium ylides, or α-carbonyl-nitrones was carried out. Our results uncover the roles of dual directing groups in the three substrates and confirm that the ketone acts as the role of the directing group while the C═C, C═N, or C═S bond serves as the cyclization site. By comparing the coordination of the ketone versus the C═C, C═N, or C═S bond, as well as the chemoselectivity concerning the six- versus five-membered formation, a competition relationship is revealed within the dual directing groups. Furthermore, after the alkyne insertion, instead of the originally proposed direct reductive elimination mechanism, the ketone enolization is found to be essential prior to the reductive elimination. The following C(sp2)-C(sp2) reductive elimination is more favorable than the C(sp3)-C(sp2) formation, which can be explained by the aromaticity difference in the corresponding transition states. The substituent effect on controlling the selectivity was also discussed.