Theoretical Investigation on the Mechanisms of the PtCl₂-Mediated Cycloisomerization of Polyfunctionalized 1,6-Enynes. 2. Propargylic Carboxylates

The PtCl 2 -mediated cycloisomerization of hept-1-en-6-ynes functionalized at propargylic positions shows a high versatility and may afford different kinds of products. On the basis of a broad DFT computational study, mechanistic understandings of these processes are provided. The results suggest that these reactions could proceed through cyclopropyl platinum carbenes formed by endo or by exo cyclization routes. The role of the propargylic substituent and the additional alkene chain for dienyne precursors in the course of each type of cycloisomerization is discussed. Thus, although the metathesis process and formation of polycyclic adducts involves an initial 5-exo cyclopropanation path, a propargylic acyl group promotes the formation of bicyclic enol esters by an initial endo cyclopropanation, followed by [1,2]-acyl migration. This bulky propargylic group also inhibits other cycloisomerization reactions due to steric interactions, such as the Alder-ene process, and simultaneously allows an easy [1,2]-acyl migration by anchimeric assistance to yield bicyclic [ n .1.0] enol esters.