Alkyne–Aldehyde Reductive CC Coupling through Ruthenium-Catalyzed Transfer Hydrogenation: Direct Regio- and Stereoselective Carbonyl Vinylation to Form Trisubstituted Allylic Alcohols in the Absence of Premetallated Reagents

Abstract

Nonsymmetric 1,2-disubstituted alkynes engage in reductive coupling to a variety of aldehydes under the conditions of ruthenium-catalyzed transfer hydrogenation by employing formic acid as the terminal reductant and delivering the products of carbonyl vinylation with good to excellent levels of regioselectivity and with complete control of olefin stereochemistry. As revealed in an assessment of the ruthenium counterion, iodide plays an essential role in directing the regioselectivity of CC bond formation. Isotopic labeling studies corroborate reversible catalytic propargyl CH oxidative addition in advance of the CC coupling, and demonstrate that the CC coupling products do not experience reversible dehydrogenation by way of enone intermediates. This transfer hydrogenation protocol enables carbonyl vinylation in the absence of stoichiometric metallic reagents.