Gold-catalyzed dehydro-Diels-Alder reactions of ynamide derivatives allow efficient access to a variety of N-containing aromatic heterocycles. A dual gold catalysis mechanism was postulated for transformations involving the formation of C-C bonds by reaction between a terminal alkyne and an enynamide fragment. In this article, complete experimental and computational investigations into the mechanism of such a transformation are reported. Support for a dual gold catalysis was found and it was shown that the concerted or stepwise nature of the cyclization event depends on the substitution of the ynamide moiety. The reaction was found to proceed in three stages: 1) formation of a σ,π-digold complex from the terminal alkyne, 2) cyclization to produce a gem-diaurated aryl complex, and 3) catalyst transfer to free the product and regenerate the σ,π-digold complex.
Keywords: Diels-Alder reactions; density functional theory; gold catalysis; gold vinylidene species; mechanisms.
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