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. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)