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Gold-Catalyzed Access to 1H‑Isochromenes: Reaction Development and Mechanistic Insight

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posted on 28.11.2016 by Eder Tomás-Mendivil, Clément F. Heinrich, Jean-Claude Ortuno, Jérôme Starck, Véronique Michelet
The gold-catalyzed domino cyclization/nucleophilic reaction of ortho-carbonylalkynylaryls has been studied. Thus, 2-(pyridin-2-ylethynyl)­benzaldehyde has been chosen to isolate key intermediates that may take part in the reaction mechanism. Employing Hantzsch ester (HEH) as nucleophile, it has been impossible to isolate the corresponding gold–alkenyl specie; however, when methanol was used as solvent (and nucleophile), the expected chelate gold–vinyl complex was isolated and unambiguously characterized by X-ray analysis. When HEH is present in the alcoholic reaction mixture, isotopic studies show that the cleavage of the Au–C bond of gold–vinyl complex proceeds through a protodemetalation pathway, rather than a plausible metal–hydride reductive elimination mechanism. Finally, with the aim of broadening the scope of the cyclization/reduction reaction previously reported, we present that the catalytic system is robust and applicable for a diverse family of challenging substrates presenting ester, aldehyde, ether, alkene, and alkyne functionalities.

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