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 (TBA(6)RE-POM, RE = Y(3+), Nd(3+), Eu(3+), Gd(3+), Tb(3+), or Dy(3+)) were designed and synthesized by reactions of TBA(4)H(4)[γ-SiW(10)O(36)] (TBA = tetra-n-butylammonium) with RE(acac)(3) (acac = acetylacetonato). TBA(6)RE-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.
Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Tokyo 113-8656, Japan.
Kosuke
K
Suzuki
Midori
M
Sugawa
Yuji
Y
Kikukawa
Keigo
K
Kamata
Kazuya
K
Yamaguchi
Noritaka
N
Mizuno
D002384:Catalysis; D058116:Lewis Acids / Q000737:chemistry*; D058115:Lewis Bases / Q000737:chemistry*; D008674:Metals, Rare Earth / Q000737:chemistry*; D008958:Models, Molecular; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000138:chemical synthesis* / Q000737:chemistry; D017973:Tungsten Compounds / Q000737:chemistry*; D000099294:Polyoxometalates
D058116:Lewis Acids; D058115:Lewis Bases; D008674:Metals, Rare Earth; D009942:Organometallic Compounds; D017973:Tungsten Compounds; C102296:polyoxometalate I; D000099294:Polyoxometalates