Dramatic Acceleration of the Pd-Catalyzed [4+2] Benzannulation Reaction of Enynes and Diynes in the Presence of Lewis Acids and Bases: Expanded Scope and New Mechanistic Insights

Abstract

Dramatic acceleration of the palladium catalyzed [4+2] benzannulation and sequential [2+2+2] trimerization reactions in the presence of a Lewis acid/phosphine combination or in the presence of Bronsted bases has been explored. These novel sets of conditions allowed for remarkable enhancement of reaction rates and broadening of the substrate scope and for a significant improvement of reaction yields, particularly for problematic pentasubstituted benzenes. It was found that the real nature of Lewis acid acceleration does not only lie in the isomerization of conjugated enynes but also in the direct acceleration of the [4+2] benzannulation reaction. These experimental findings, combined with the deuterium-labeling studies and DFT calculations, led to a mechanistic rationale, which (a) reasonably accounts for the observed acceleration of the reaction by Lewis acid and bases; (b) provides a viable alternative route for the ring-closing step in the mechanism of benzannulation; (c) clarifies the mechanism of hydrogen migration; and (d) for the first time provides a rationale for the origins of the remarkable stereoselectivity of the hydrogen migration during the benzannulation reaction.