Palladium-catalyzed oxidation of bicyclic monoterpenes by hydrogen peroxide

The activity of the PdCl2-CuCl2 combination in the oxidation of camphene, alpha-pinene, and beta-pinene by dioxygen in acetic acid solutions has been studied. The reactions of alpha-pinene and beta-pinene yield a mixture of carveyl acetate (up to 25% on reacted olefin), alpha-terpenyl acetate, bornyl chloride, and fenchyl chloride. Camphene undergoes a skeletal rearrangement and an acetic acid/water addition resulting in bornyl acetate as a major product, along with borneol and alpha-pinene. No oxidation products are detected. In an attempt to develop a CuCl2-free catalytic system for the selective oxidation of bicyclic monoterpenes, the oxidation of beta-pinene and camphene by hydrogen peroxide catalyzed by Pd(OAc)(2) in acetic acid solutions has been studied. beta-Pinene gave the allylic oxidation products, i.e., pinocarveol, pinocarveyl acetate and myrtenyl acetate, with selectivity up to 75% at virtually complete conversion, and camphene gave camphene glycol monoacetate with a 90% selectivity at 80% conversion. The oxidation reaction competes with the skeletal rearrangement of monoterpenes accompanied by a nucleophilic addition of hydroxy and acetoxy groups. The introduction of benzoquinone (BQ) in catalytic amounts exerts a beneficial effect on the catalyst stability and selectivity for glycol monoacetate formation. For the Pd(OAc)(2)-BQ-H2O2 system, more than 200 turnover numbers could be achieved in the acetoxylation of camphene. (C) 1998 Elsevier Science B.V. All rights reserved.