Pyridines as bifunctional co-catalysts in the CrO₃-catalyzed oxygenation of olefins by t-butyl hydroperoxide

t-Butyl hydroperoxide (TBHP) oxidizes olefins to epoxides and allylic oxidation products in the presence of a Cr(VI) catalyst. A concurrent decomposition of the oxidant occurs. Pyridine-derived additives alter the behavior of this catalytic system: monodentate pyridines and trans-chelated bidentate bipyridines retard the decomposition of TBHP, and arrest the epoxidation reaction, shifting the product selectivity towards allylic oxidation. Adversely, cis-chelated bipyridines accelerate the decomposition of TBHP. Depending on ligand nature and concentration, the initial decomposition rate can be slowed down to 1/8th, or accelerated up to two orders of magnitude, (relative to CrO₃ catalysis). The allylic oxidation and the TBHP decomposition are free-radical reactions, but the epoxidation is evidently not. A reaction mechanism is proposed, where the diverse role of the pyridine ligands is attributed to specific complex formations.