Silylformylation of Carbonyl Compounds: A Study of Substrate, Catalyst, and Reaction Conditions

This paper presents a study of the rhodium-catalyzed silylformylation [Rh(I) precatalyst, PhMe 2 SiH, THF, carbon monoxide (15−1500 psig), 23 °C] of aldehydes. This new catalytic homologation reaction produces α-(silyloxy) aldehydes in a highly efficient manner. Rhodium(I) appears to be the optimum transition metal for the catalytic process. The reaction is optimized at carbon monoxide pressures over 50 psig. It appears that PhMe 2 SiH is the silane reagent of choice. The silylformylation of carbonyl compounds is very general for aldehyde substrates (aromatic, heterocyclic, alkyl, and ferrocenyl: 16 examples presented) and can tolerate the presence of internal alkene and alkyne, ester, and acyclic ketone functional groups. Aldehydes with α-substituents show moderately good diastereoselectivity, producing the syn -α-(silyloxy) aldehyde (10 to 20:1, syn: anti ) as the major product. Ketone substrates possessing β-hydrogens yield only silyl enol ether without concomitant hydrosilylation coproducts. Imine substrates are found unreactive under normal silylformylation conditions. The rhodium-catalyzed silylformylation is a concentration- and solvent-dependent catalytic process. THF is the optimum solvent. Performing the reaction in acetonitrile or neat leads to reduced yields, and dichloromethane and benzene afford no silylformylation product. Dioxane can be used only if employed in conjunction with an auxiliary ligand (pyridine or N -methylpyrazole). Phosphine and phosphite ligands, both mono and bidentate, inhibit the rhodium-catalyzed silylformylation, whereas nitrogen-based ligands like 2,2‘-bipyridine can be used at high ligand to metal ratios ( e . g . ligand/rhodium, 10/1, respectively).