Pd/C-Catalyzed Hydrosilylation of Enals and Enones with Triethylsilane: Conformer Populations Control the Stereoselectivity

Abstract

The palladium-on-charcoal-catalyzed chemo-, regio-, and stereoselective 1,4-hydrosilylation and transfer hydrogenation reactions of α,β-unsaturated aldehydes and ketones with triethylsilane have been investigated with a combination of experimental and theoretical methods. The reaction mechanism has been studied experimentally by monitoring the reactions by 1H NMR from aliquots withdrawn from the stirred reaction mixtures, labeling experiments, and control experiments. Our density functional theory results indicate that both aforementioned reactions are initiated with a dissociative adsorption of the triethylsilane on the palladium catalyst. In the hydrosilylation reaction, the α,β-unsaturated aldehyde or cyclic ketone is adsorbed on the silyl-covered palladium catalyst, where it is first irreversibly O-silylated and then reduced by a hydride addition to the 4-position. The O-silylation step has a low barrier, and it essentially freezes the solution-phase s-trans:s-cis conformer ratio in the form of enol silane stereoisomers. In support of the stereoselectivity model, the experimental Z:E stereoselectivities of enol silane products are in very close agreement with the computed s-trans:s-cis conformer population ratios. The transfer hydrogenation reaction is carried out in the presence of water, and there the product is a saturated aldehyde or ketone. In this reaction regime, the silyl species on the palladium surface rapidly reacts with water to give triethylsilanol as a side product. The α,β-unsaturated carbonyl is then adsorbed on the palladium surface, where it undergoes a conjugate reduction in a 1,4-fashion: the hydride addition to the 4-position occurs on the palladium surface, giving an enolate-type intermediate which is then protonated by the protic solution phase. Finally, we present a revised protocol for the stereoselective hydrosilylation reaction with Pd/C that improves the product stereoselectivity by a proper choice of solvent and reaction time.