Photocatalytic Oxidation of Secondary Alcohols Through Cooperative Hydrogen Atom Transfer Using Decatungstate and Cobaloxime

Abstract

Due to its unique photocatalytic properties, the decatungstate anion (W\(_{10}\)O\(_{32}\)\(^{4-}\)) is emerging as a useful tool in organic chemistry. A photoactivated decatungstate anion has the ability to abstract hydrogen atoms from high energy (greater than 100 kcal/mol) C-H bonds, generating a carbon-centered radical and a photoreduced decatungstate species. On the other hand, cobaloxime complexes are able to abstract H atoms from weak C-H bonds (less than 50 kcal/mol), such as the C-H bond β from a radical center,\(^{1}\) and generate the corresponding double bond and oxidized cobaloxime. Thus, these two complexes can be used to sequentially abstract two H atoms from an alcohol to generate the corresponding carbonyl through a cooperative hydrogen atom transfer process. The reduced decatungstate and oxidized cobaloxime can then turn over one another and release H\(_{2}\) gas in the process. The results of this research show that a combination of 2 mol % TBADT (tetrabutylammonium decatungstate) and 1 mol% COPC (pyridine (chloride) cobaloxime) in acetonitrile is the best system to use for oxidizing various secondary alcohols to their corresponding ketones. These reaction conditions, however, fail to oxidize primary alcohols to their corresponding aldehydes. Thus, a novel dual catalyst system for the oxidation of secondary alcohols to ketones under extremely mild conditions has been developed and optimized. Major advantages of this system include the use of inexpensive, earth-abundant metals, easily synthesized catalysts, a simple reaction set up, the release of H\(_{2}\) gas as a byproduct, and the use of light as the only energy source.