Palladium-catalyzed oxidation of monoterpenes: Highly selective syntheses of allylic ethers from limonene

Selective oxidation of limonene (1) with benzoquinone in the solutions of various alcohols (methanol, ethanol and 2-propanol) containing catalytic amounts of palladium(II) complexes and p-toluenesulfonic acid has been carried out. Two isomeric allylic ethers have been characterized as major products in each alcohol, showing that they arise from either exo- or endocyclic η3-allyl intermediates formed by the reaction of palladium catalysts with the endocyclic CC bond of limonene. A good control of regioselectivity was achieved through the appropriate choice of the ligand on palladium and reaction conditions. Allylic ethers 2a–c with two exocyclic double bonds are main reaction products in chloride-free catalytic systems, while carveol derivatives 3a–c are formed when palladium chloride is used as the catalyst. p-Toluenesulfonic acid exerts a great accelerative and catalyst stabilizing effect in these reactions. Monoterpenic ethers 2a–c and 3a–c have mild scents of flower or fruit and can be useful as components of synthetic fragrances. These compounds were obtained in 80–97% chemoselectivity each at a limonene conversion of 95–98% under appropriate conditions.

Graphical abstractSelective syntheses of a series of allylic ethers 2a–c and 3a–c, which are potentially useful as fragrance ingredients, by the oxidation of limonene (1) with benzoquinone employing a Pd(II)/p-toluenesulfonic acid (PTSA) catalytic system were developed. A good control of regioselectivity was achieved through the appropriate choice of the ligand on palladium and reaction conditions. Figure optionsDownload full-size imageDownload high-quality image (90 K)Download as PowerPoint slide