Group 6 carbonyl complexes of N,O,P-ligands as precursors of high-valent metal-oxo catalysts for olefin epoxidation

The metal-catalyzed epoxidation of olefins is an important reaction in organic synthesis, as epoxides are useful intermediates for the production of bulk and fine chemicals. Molybdenum complexes are known as effective catalysts for selective epoxidation with alkyl hydroperoxides as oxidants. Very early on, it was recognized that molybdenum hexacarbonyl could be used directly as a catalyst precursor, being converted in situ to a soluble MoVI active species. In recent years this field has expanded greatly with the study of a wide range of group 6 carbonyl complexes as precursors to catalysts and/or functional metal oxides. In this review, pioneering work on the use of group 6 hexacarbonyls, mainly Mo(CO)6, as pre-catalysts is summarized, followed by a comprehensive analysis of recent research in which heteroleptic complexes of the type [Mo(CO)x(L)n] (x = 3 or 4; n = 1 or 3) and [MXm(CO)3(L)n] (M = Mo, W; X = halide; m = 1 or 2; n = 1 or 2) were employed, where L is a N-, O- or P-donor ligand. The topics covered include precursor synthesis and structural chemistry, solvent, oxidant and ligand effects, catalytic applications for the valorization of functionalized and nonfunctionalized olefins, catalyst activation and mechanistic studies. Current understanding about the oxidation of the heteroleptic Mo0/II complexes to oxomolybdenum complexes or hybrid materials is discussed, with a review of the behavior of these compounds as (pre)catalysts for homogeneous or heterogeneous olefin epoxidation.