Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Tokyo 113-8656, Japan.
Inorg Chem. 2012 Jun 18;51(12):6953-61. doi: 10.1021/ic3008365. Epub 2012 Jun 6.
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.
通过 TBA(4)H(4)[γ-SiW(10)O(36)]（TBA=四正丁基铵）与 RE(acac)(3)（acac=乙酰丙酮）的反应，设计并合成了高效的多金属氧酸盐（POM）基 Lewis 酸碱催化剂，其中含有稀土金属的 POM（TBA(6)RE-POM，RE=Y(3+)、Nd(3+)、Eu(3+)、Gd(3+)、Tb(3+)或 Dy(3+)）。TBA(6)RE-POM 由两个硅钨酸盐单元通过两个稀土金属阳离子支撑而成。带负电荷的 POM 的亲核富氧表面和掺入的稀土金属阳离子分别可以作为 Lewis 碱和 Lewis 酸。因此，在含有稀土金属的 POM 的存在下，通过在同一 POM 分子上同时激活偶联伙伴，可以有效地促进羰基化合物与三甲基硅氰化物（(TMS)CN）的氰硅烷化反应。由于 C═O 键（更高的路易斯酸度）的更高活化能力和空间位阻较小的路易斯酸位，具有较大金属阳离子的 POM 对氰硅烷化反应表现出更高的催化活性。在所研究的 POM 催化剂中，含钕的 POM 对各种结构多样的酮和醛的氰硅烷化反应表现出显著的催化性能，以高收率（13 种底物，94-99%）得到相应的氰醇三甲基硅醚。特别是，对于正己醛的氰硅烷化反应，其周转率（714,000 h(-1)）和周转数（23,800）在以前报道的催化剂中处于最高水平。
