Strategic Design and Refinement of Lewis Acid–Base Catalysis by Rare-Earth-Metal-Containing Polyoxometalates

Efficient polyoxometalate (POM)-based Lewis acid–base catalysts
of the rare-earth-metal-containing POMs (TBA6RE-POM, RE
= Y3+, Nd3+, Eu3+, Gd3+, Tb3+, or Dy3+) were designed and synthesized
by reactions of TBA4H4[γ-SiW10O36] (TBA = tetra-n-butylammonium) with
RE­(acac)3 (acac = acetylacetonato). TBA6RE-POM
consisted of two silicotungstate units pillared by two rare-earth-metal
cations. Nucleophilic oxygen-enriched surfaces of negatively charged
POMs and the incorporated rare-earth-metal cations could work as Lewis
bases and Lewis acids, respectively. Consequently, cyanosilylation
of carbonyl compounds with trimethylsilyl cyanide ((TMS)­CN) was efficiently
promoted in the presence of the rare-earth-metal-containing POMs via
the simultaneous activation of coupling partners on the same POM molecules.
POMs with larger metal cations showed higher catalytic activities
for cyanosilylation because of the higher activation ability of CO
bonds (higher Lewis acidities) and sterically less hindered Lewis
acid sites. Among the POM catalysts examined, the neodymium-containing
POM showed remarkable catalytic performance for cyanosilylation of
various kinds of structurally diverse ketones and aldehydes, giving
the corresponding cyanohydrin trimethylsilyl ethers in high yields
(13 substrates, 94–99%). In particular, the turnover frequency
(714 000 h–1) and the turnover number (23 800)
for the cyanosilylation of n-hexanal were of the
highest level among those of previously reported catalysts.