Rhodium-Catalyzed Regioselective Silylation of Alkyl C-H Bonds for the Synthesis of 1,4-Diols.

A rhodium-catalyzed intramolecular silylation of alkyl C-H bonds has been developed that occurs with unusual selectivity for the C-H bonds located δ to the oxygen atom of an alcohol-derived silyl ether over typically more reactive C-H bonds more proximal to the same oxygen atom. (Hydrido)silyl ethers, generated in situ by dehydrogenative coupling of tertiary alcohols with diethylsilane, undergo regioselective silylation at a primary C-H bond δ to the hydroxyl group in the presence of [(Xantphos)Rh(Cl)] as catalyst. Oxidation of the resulting 6-membered oxasilolanes generates 1,4-diols. This silylation and oxidation sequence provides an efficient method to synthesize 1,4-diols by a hydroxyl-directed, aliphatic C-H bond functionalization reaction and is distinct from the synthesis of 1,3-diols from alcohols catalyzed by iridium. Mechanistic studies show that the rhodium-catalyzed silylation of alkyl C-H bonds occurs with a resting state and relative rates for elementary steps that are significantly different from those for the rhodium-catalyzed silylation of aryl C-H bonds. The resting state of the catalyst is a (Xantphos)Rh(I)(SiR3)(norbornene) complex, and an analogue was synthesized and characterized crystallographically. The rate-limiting step of the process is oxidative addition of the δ C-H bond to Rh. Computational studies elucidated the origin of high selectivity for silylation of the δ C-H bond when Xantphos-ligated rhodium is the catalyst. A high barrier for reductive elimination from the six-membered metalacyclic, secondary alkyl intermediate formed by cleavage of the γ C-H bond and low barrier for reductive elimination from the seven-membered metalacyclic, primary alkyl intermediate formed by cleavage of the δ C-H accounts for the selective functionalization of the δ C-H bond.