Highly Chemoselective Palladium-Catalyzed Conjugate Reduction of α,β-Unsaturated Carbonyl Compounds with Silicon Hydrides and Zinc Chloride Cocatalyst

Article abstract of DOI:10.1021/ja00283a029

A three-component system comparised of a soluble palladium catalyst, hydridosilane, and zinc chloride is capable of efficient conjugate reduction of α,β-unsaturated ketones and aldehydes.The optimal set of condition includes diphenylsilane as the most effective hydride donor, any soluble palladium complex in either the 0 or II oxidation state, when it is stabilized by phosphine ligands, and ZnCl2 as the best Lewis acid cocatalyst.The reaction is very general with respect to a broad range of unsaturated ketones and aldehydes, and it is highly selective for these Michael acceptors, as reduction of α,β-unsaturated carboxylic acid derivatives is very sluggish under these conditions.When dideuteriodiphenylsilane is used to reduce unsaturated ketones, deuterium is stereoselectively introduced at the less-hindered face of the substrate and regioselectively at the β-position.Conversely, when reductions are carried out in the presence of traces of D2O, deuterium incorporation occurs at the α-position.On the basis of deuterium-incorporation experiments and 1H NMR studies, a catalytic cycle is postulated in which the first step involves reversible coordination of the palladium complex to the electron-deficient olefin and oxidative addition of silicon hydride to form a hydropalladium olefin complex.Migratory insertion of hydride into the coordinated olefin produces an intermediate palladium enolate which, via reductive elimination, collapses back to the Pd(0) complex and a silyl enol ether, which is then hydrolyzed to the saturated ketone.In addition to catalyzing that hydrolysis, ZnCl2 facilitates the hydrosilation process.