Palladium catalyzed oxidation of monoterpenes: multistep electron transfer catalytic systems Pd(OAc) 2/benzoquinone/M(OAc) 2 (M=Cu, Co or Mn) for the allylic oxidation of limonene with dioxygen

Abstract

Oxidation of limonene ( 1) with dioxygen in acetic acid solutions containing catalytic amounts of Pd(OAc) 2, benzoquinone and M(OAc) 2 (M=Cu, Co or Mn) has been developed. Main reaction products are three isomeric allylic acetates, i.e., trans-2-acetoxy- p-mentha-1(7),8-diene ( 2), perilla acetate ( 3) and trans-carveyl acetate ( 4). A good control of chemo- and regioselectivities has been achieved through the addition of p-toluenesulfonic acid. The systems with Cu(OAc) 2 have shown the most promising results. Acetate 2, which has a mild scent of flower or fruit and can be useful as a component of synthetic fragrances, has been obtained in higher than 90% chemoselectivity at 90–95% limonene conversions.