Palladium-catalyzed oxidations of alcohols and olefins under aerobic conditions
The development of metal-catalysts for the oxidation of various functional groups is an ongoing endeavor in the field of organometallic chemistry. A constant challenge is employing practical oxidants, of which, molecular oxygen is advantageous because is it readily available, inexpensive, and produces benign byproducts. Herein, research performed by Mitchell John Schultz under the guidance of Professor Matthew S. Sigman on the Pd-catalyzed aerobic oxidation of alcohols and dialkoxylation of olefins is discussed. Three Pd-catalysts developed for the aerobic oxidation of alcohols are presented and the effectiveness of each catalyst system is evaluated for a broad scope of alcohols. The application of these oxidation catalysts to chiral nonracemic, chemo-, and diastereoselective substrate oxidations are explored. Additionally, the experimental and computational details of the Pd(OAc)2/TEA oxidation system are presented. Contrary to mechanistic studies on related systems, which propose rate limiting 0-hydride elimination, the details for the Pd(OAc)2/TEA oxidation system support rate determining deprotonation of the Pd-bound alcohol. TEA as a ligand is proposed to enhance formation of an active catalyst with only one ligand bound to the Pd. The result is a significantly lower activation energy for f3-hydride elimination, and an active catalyst at room temperature. In addition to Pd-catalyzed aerobic alcohol oxidations, the development of the first Pd-catalyzed aerobic dialkoxylation of ortho-propenyl phenols is discussed. Initial scope of this reaction is presented, and based on early experiments, a unique mechanism involving formation of an ortho-quinone methide intermediate is proposed.