Abstract
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.
Highlights
Base-catalyzed reactions such as the isomerization of alkenes/alkynes, C-C bond forming reactions, the Tishchenko reaction, hydrogenation,esterification, and oxidation are important for the production of bulk and specialty chemicals [1,2,3,4,5,6]
POMs have the following advantages as catalysts: (i) the redox and acid-base properties can be fine-tuned by changing the chemical structures and compositions; (ii) POMs are less susceptible to oxidative and thermal degradation than organometallic complexes; and (iii) the catalytically active sites of POMs can be precisely controlled with an appropriate combination of transition metals and lacunary POMs as inorganic ligands
The α-Dawson-type silicotungstate, TBA8 [α-Si2 W18 O62 ], which was synthesized by dimerization of a Lacunary trivacant lacunary α-Keggin-type
Summary
Base-catalyzed reactions such as the isomerization of alkenes/alkynes, C-C bond forming reactions (aldol condensation, Michael addition, Henry reaction, etc.), the Tishchenko reaction, hydrogenation, (trans)esterification, and oxidation are important for the production of bulk and specialty chemicals [1,2,3,4,5,6]. POMs have the following advantages as catalysts: (i) the redox and acid-base properties can be fine-tuned by changing the chemical structures and compositions; (ii) POMs are less susceptible to oxidative and thermal degradation than organometallic complexes; and (iii) the catalytically active sites of POMs can be precisely controlled with an appropriate combination of transition metals and lacunary POMs as inorganic ligands. These superior properties allow the POMs-based catalysts to be designed at the atomic and molecular levels [20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35]. Structures, properties, and applications of POMs have been summarized in many excellent books and review articles [7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35]
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