Selective Palladium‐Catalyzed Dehydrogenation of Limonene to Dimethylstyrene

Abstract

Abstract Conditions for the selective dehydrogenation of (+)‐limonene 1 to the polymer building block dimethylstyrene 2 are described. The reaction occurs smoothly in the presence of Pd(OTFA) 2 as catalyst and CuCl 2 as oxidant. High selectivity for retaining the exocyclic double bond during aromatization is achieved (>14:1). Initially, variable‐temperature NMR experiments, under stoichiometric conditions, showed a stepwise formation of a π‐allyl intermediate and subsequent reaction to give a mixture of products. However, kinetic experiments showed a sigmoidal curve, pointing to the heterogeneous nature of the catalytically active species. A number of experiments were undertaken to differentiate between homogeneous, molecularly defined catalysis and heterogeneous, nanocluster‐based catalysis. Based on the results, it is proposed that the true catalytic system is heterogeneous in nature.