Cycloisomerization of Enynes via Rhodium Vinylidene-Mediated Catalysis

摘要

The transition metal-catalyzed Cycloisomerization of enynes is a powerful method for accessing cyclic structures from acyclic precursors of substantially less molecular complexity.In this inherently atom-economical process,the unsaturated substrates are mobilized to undergo cyclizations typically infeasible under thermal conditions.From a mechanistic perspective,the majority of the reported examples can be classified into a few modalities based on key intermediates,such as metallacycle (A),metallo-eta-alkyne (B),and metal alkylidene (C) species (Scheme 1).An intriguing mechanistic motif is the potential intermediacy of a metal vinylidene (D).This strategy is attractive since a variety of transition metals are capable of generating vinylidene complexes from alkynes under mild conditions.However,the involvement of a metal vinylidene in the Cycloisomerization process has been noted only in dienyne systems,particularly in the context of aromatic substrates.A potentially more useful prototype with a simple unactivated alkene,such as D,has remained unreported.We reasoned that the archetypal reaction pathways making use of intermediates A-C could be diverted by facile formation of a metal vinylidene and its subsequent C-C bond-forming process induced by a reaction between the two remote pi-systems of the C=M and C=C bonds.Described here are the verification of this proposition and consequent discovery of a new mode of enyne Cycloisomerization mediated by rhodium-(I) catalysis.