Palladium-catalyzed sabinene oxidation with hydrogen peroxide: Smart fragrance production and DFT insights

Abstract

Fragrance substances derived from natural sources are essential to the fine chemical industry, being an interesting task to pursue. Herein, we present a smart catalytic oxidation route for bio-renewable (1R,5R)-sabinene obtainment in acetonitrile or acetone, just using Pd(OAc)2 as catalyst and hydrogen peroxide as an final oxidant. The products obtained from (1R,5R)-sabinene and their mixtures have a distinct and pleasant smell and therefore are candidates for fragrance and pharmacological applications. Three oxyfunctionalized products were obtained with ca. 80% combined selectivity at high substrate conversions. The major products, two novel isomeric carane-type ketones with a bicyclo[4.1.0] skeleton, were formed by expanding the five-membered ring in the sabinene molecule, while (-)-sabina ketone was produced as a minor product. An uncommon mechanistic route was suggested and supported by density functions theory (DFT), which could suitably explain the formation of these ketones. DFT results supported a proposed concerted mechanism for the CC activation, like a semipinacol rearrangement, which includes a heterolysis of the Pd-C bond and synchronous cleavage of the CC bond in an intermediate organopalladium peroxo complex. We highly believe that such findings allow a step forward a deep understanding of such organic transformation, helping in future new routes for value-added products.