The mechanisms of gallium-catalysed skeletal rearrangement of 1,6-enynes - Insights from quantum mechanical computations

Abstract

The transition metal-catalysed skeletal reorganization of 1,6-enynes can lead to three types of products - a type I product occurring via the cleavage of the alkene C-C bonds and the migration of the terminal alkene carbon to the terminus of the alkyne; a type II product arising from cleavage of both the double and the triple bonds followed by insertion of the terminal alkene carbon into the alkyne C-C triple bond; and a type III product which is obtained when there is a cleavage of the olefinic bond followed by formation of two new bonds from each carbon to each of the acetylenic carbons. The course of these reactions is highly dependent on the metal catalyst used and type of substitution at the alkene and alkyne moieties of the enyne. In this mechanistic study of the re-organization of 1,6-enynes catalysed by GaCl₃, performed at the DFT M06/6-311G(d,p) level of theory, the parent reaction selectively leads to the formation of the type I product through the formation of the open cyclopropane ring. The presence of substituents at the acetylenic moiety governs the preferred position of the metal along the alkyne bond within the pi-complex: with electron-withdrawing groups (EWGs), the metal prefers the terminal carbon while electron-donating groups (EDGs) lead to the metal preferring the internal carbon. EWGs at the alkyne moiety efficiently favour the formation of the type I product. Substituents at the olefin moiety alter the mechanism of the reaction which may favour the selective formation of the type I or III product depending on the type of substituent. EWGs at the olefinic moiety favour formation of the type III product when the alkyne moiety is unsubstituted whiles EDGs forms the type I product selectively. Solvent and temperature have no substantial effects on the energetic trends and product distribution. Hence, gas-phase calculations are deemed adequate for the problem at hand.

Keywords: 1,6-Enynes; Cycloisomerization; Gallium catalysis; Mechanistic study; Post-transition metal; Skeletal reorganization.