On the mechanism of the puzzling “endocyclic” skeletal rearrangement of 1,6-enynes

Skeletal rearrangements of simple 1,6-enynes have been studied in order to determine the factors that control the formation of five- or six-membered rings. Simple 1,6-enynes substituted only at C-4 preferentially give six-membered rings on skeletal rearrangement in the presence of gold(I) catalysts, whereas increasing electron-withdrawing character of substituents at C-4 leads to five-membered rings. Reactions of these simple enynes in the presence of PtCl₄ as catalyst give exclusively exo-double skeletal rearrangements. Enynes substituted at the alkyne also react with AuI catalysts to give exclusively products of exo-double rearrangement. The different mechanisms have been analyzed by DFT calculations. Although a pathway for the formation of six-membered rings involving two steps in a ring-expansion/ring-contraction process was found, the activation energy of the first step is too high. Instead, this skeletal rearrangement appears to follow an exo-single skeletal rearrangement in which the initial cyclopropyl gold carbene opens to form a six-membered ring.