Theoretical investigation on the mechanisms of the PtCl2-mediated cycloisomerization of polyfunctionalized 1,6-enynes.: 1.: Role of the propargylic substituents

The PtCl2-mediated intramolecular cycloisomerization of 1,6-enynes functionalized at propargylic positions shows a high versatility and can give rise to a variety of products with potential synthetic applications. A broad computational study at the DFT level is reported to provide a better mechanistic understanding of these reactions and of the factors that control the course of these processes. For every type of cycloisomerization, several reasonable pathways have been proposed and evaluated. The results suggest that these reactions probably share common steps and could proceed through the formation of cyclopropyl platinum carbenes by endo- or by exo-cyclization routes. The role of the propargylic substituent and the additional alkene chain for dienyne precursors is also discussed. The metathesis process involves an initial 5-exo cyclopropanation path, in common with the formation of polycyclic adducts, but it is retarded or inhibited by the presence of an additional alkene chain.