Organorhenium dioxides as oxygen transfer systems: Synthesis, reactivity, and applications

Organorhenium complexes are attracting interest because of their possible uses as deoxygenation catalysts in the refinement of lignocellulosic biomass such as lignin, carbohydrates, and sugar alcohols in the homogeneous phase. Furthermore, organorhenium compounds are known for their promising catalytic applications in the olefination of aldehydes, deoxygenation of epoxides, deoxydehydration of diols, hydrosilylation, and hydrogenation, where organorhenium dioxides (ORDs) are the key compounds in several cases. The wide substrate range of ORDs in mild reaction conditions is unrivaled by other catalysts. In this review, we summarize the roles of ORDs in molecular catalysis and the fundamental coordination chemistry of these compounds to facilitate a better understanding of the catalytic reaction steps. We provide comprehensive descriptions and visualizations of the generation, synthesis, and coordination chemistry of ORDs. ORDs are formed either by the reduction of common Re(VII) trioxides or by the oxidation of Re(III) compounds. Thus, we explain the strategies employed to allow the stabilization and isolation of very reactive dioxo-compounds. Their fundamental chemistry is based mainly on oxygen atom transfer reactions, thus we discuss possible reaction partners and conditions. In the second part of this review, we summarize all the known ORD-catalyzed reactions, thereby providing an overview and a mechanistic discussion of their catalytic applications. In particular, we review the roles and applications of ORDs in the refinement of biomass-derived compounds. The deoxydehydration of carbohydrates and sugar alcohols yields very useful synthetic building blocks, and thus we consider each carbon chain length from C₃ to C₆. In addition, we discuss catalytic CO bond cleavage in lignin model compounds.