The aim of this tutorial review is to highlight the potential application of N-heterocyclic carbenes (NHC) in heterogeneous catalysis. The unique combination of the high reactivity of NHCs together with the ease of separation, purification and recyclability of the solid catalyst makes the heterogeneous system one of the most promising strategies for the synthesis of fine chemicals on an industrial scale. This tutorial review focuses on the most representative examples of this nascent research area and highlights recent achievements of NHCs in heterogeneous catalysis (other than metathesis reactions).We hope this serves as inspiration for further progress in this field.
Chiral (nitrosyl)ruthenium(salen) complexes were found to be efficient catalysts for aerobic oxidative desymmetrization of meso-diols under photoirradiation to give optically active lactols. The scope of the applicability of this reaction ranges widely from acyclic diols to mono-cyclic diols, although fine ligand-tuning of the ruthenium(salen) complexes was required to attain high enantioselectivity (up to 93% ee). In particular, the nature of the apical ligand was found to affect not only enantioselectivity but also kinetics of the desymmetrization reaction. Spectroscopic analysis of the oxidation disclosed that irradiation of visible light is indispensable not only for dissociation of the nitrosyl ligand but also for single electron transfer from the alcohol-bound ruthenium ion to dioxygen.
Reactions of 2-(trimethylsilyloxy)furan 1 and aldehydes catalyzed by a chiral cationic Cr(salen) complex proceeded with remarkably improved enantioselectivity, when a secondary alcohol was added. The role of the alcohol is acceleration of the conversion of an intermediary 1-aldehyde adduct into the final product.
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