In sharp contrast with acid-, photo-, and oxidation-catalysis by polyoxometalates, base catalysis by polyoxometalates has scarcely been investigated. The use of polyoxometalates as base catalysts have very recently received much attention and has been extensively investigated. Numerous mono- and polyoxometalate base catalyst systems effective for the chemical fixation of CO2
, cyanosilylation of carbonyl compounds, and C–C bond forming reactions have been developed. Mono- and polyoxometalate base catalysts are classified into four main groups with respect to their structures: (a) monomeric metalates; (b) isopolyoxometalates; (c) heteropolyoxometalates; and (d) transition-metal-substituted polyoxometalates. This review article focuses on the relationship among the molecular structures, the basic properties, and the unique base catalysis of polyoxometalates on the basis of groups (a)–(d). In addition, reaction mechanisms including the specific activation of substrates and/or reagents such as the abstraction of protons, nucleophilic action toward substrates, and bifunctional action in combination with metal catalysts are comprehensively summarized.
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