The mechanisms of gallium-catalysed skeletal rearrangement of1,6-enyneseInsights from quantum mechanical computations

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Elsevier Inc
The transition metal-catalysed skeletal reorganization of 1,6-enynes can lead to three types of productsea typeIproduct occurring via the cleavage of the alkene CeC bonds and the migration of the terminalalkene carbon to the terminus of the alkyne; a typeIIproduct arising from cleavage of both the doubleand the triple bonds followed by insertion of the terminal alkene carbon into the alkyne CeC triple bond;and a typeIIIproduct which is obtained when there is a cleavage of the olefinic bond followed byformation of two new bonds from each carbon to each of the acetylenic carbons. The course of thesereactions is highly dependent on the metal catalyst used and type of substitution at the alkene andalkyne moieties of the enyne. In this mechanistic study of the re-organization of 1,6-enynes catalysed byGaCl3, performed at the DFT M06/6-311G(d,p) level of theory, the parent reaction selectively leads to theformation of the typeIproduct through the formation of the open cyclopropane ring. The presence ofsubstituents at the acetylenic moiety governs the preferred position of the metal along the alkyne bondwithin the pi-complex: with electron-withdrawing groups (EWGs), the metal prefers the terminal carbonwhile electron-donating groups (EDGs) lead to the metal preferring the internal carbon. EWGs at thealkyne moiety efficiently favour the formation of the typeIproduct. Substituents at the olefin moietyalter the mechanism of the reaction which may favour the selective formation of the typeIorIIIproductdepending on the type of substituent. EWGs at the olefinic moiety favour formation of the typeIIIproduct when the alkyne moiety is unsubstituted whiles EDGs forms the typeIproduct 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.
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Skeletal reorganization, 1,6-Enynes, Gallium catalysis, Cycloisomerization, Post-transition metal, Mechanistic study
J.B. Borketey et al. / Journal of Molecular Graphics and Modelling 94 (2020) 107476,