Palladium-catalyzed oxidation of bicyclic monoterpenes by hydrogen peroxide

Abstract

The activity of the PdCl 2–CuCl 2 combination in the oxidation of camphene, α-pinene, and β-pinene by dioxygen in acetic acid solutions has been studied. The reactions of α-pinene and β-pinene yield a mixture of carveyl acetate (up to 25% on reacted olefin), α-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 α-pinene. No oxidation products are detected. In an attempt to develop a CuCl 2-free catalytic system for the selective oxidation of bicyclic monoterpenes, the oxidation of β-pinene and camphene by hydrogen peroxide catalyzed by Pd(OAc) 2 in acetic acid solutions has been studied. β-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–H 2O 2 system, more than 200 turnover numbers could be achieved in the acetoxylation of camphene.