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), 2 3 degrees C] of aldehydes. This new catalytic homologation reaction pr oduces alpha-(silyloxy) aldehydes in a highly efficient manner. Rhodiu m(I) appears to be the optimum transition metal for the catalytic proc ess. The reaction is optimized at carbon monoxide pressures over 50 ps ig. It appears that PhMe(2)SiH is the silane reagent of choice. The si lylformylation of carbonyl compounds is very general for aldehyde subs trates-(aromatic, heterocyclic, alkyl, and ferrocenyl: 16 examples pre sented) and can tolerate the presence of-internal alkene and alkyne, e ster, and acyclic ketone functional groups. Aldehydes with alpha-subst ituents show moderately good diastereoselectivity, producing the syn-a lpha-(silyloxy) aldehyde (10 to 20:1, syn:anti) as the major product. Ketone substrates possessing beta-hydrogens yield only silyl enol ethe r without concomitant hydrosilylation coproducts. Imine substrates are found unreactive under normal silylformylation conditions. The rhodiu m-catalyzed silylformylation is a concentration- and solvent-dependent catalytic process. THF is the optimum solvent. Performing the reactio n in acetonitrile or neat leads to reduced yields, and dichloromethane and benzene afford no silylformylation product. Dioxane can be used o nly if employed in conjunction, with an auxiliary ligand (pyridine or N-methylpyrazole). Phosphine and phosphite ligands, both mono and bide ntate, inhibit the rhodium-catalyzed silylformylation, whereas nitroge n-based ligands like 2,2'-bipyridine can be used at high ligand to met al ratios (e.g. ligand/rhodium, 10/1, respectively).