Formation of S-shaped disilicoicosatungstate and efficient Baeyer-Villiger oxidation with hydrogen peroxide.
Formation of S-shaped disilicoicosatungstate and efficient Baeyer-Villiger oxidation with hydrogen peroxide.
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DOI：
10.1002/anie.200502543
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发表时间：
2006-03
影响因子：
--
通讯作者：
A. Yoshida;Masayuki Yoshimura;Kazuhiro Uehara;S. Hikichi;N. Mizuno
中科院分区：
文献类型：
--
作者：
A. Yoshida;Masayuki Yoshimura;Kazuhiro Uehara;S. Hikichi;N. Mizuno
Metal–oxygen clusters, namely, polyoxometalates (POMs), have attracted much attention because of their tunability of composition, size, shape, acid–base properties, and redox potential.[1] The molecular properties lead to the use of POMs in catalysis, medicine, and materials science. Structural variations of POMs may arise from self-assembly of MÀO/MÀOH components and from isomerization by repetitive hydrolysis and condensation of metal fragments in aqueous media, and these behaviors complicate POM chemistry.[2] Therefore, synthesis, structural characterization, and chemical properties of novel POMs are still attractive in modern chemistry. Lacunary POMs are useful precursors for mixedmetal–oxide clusters and organic–inorganic hybrid compounds.[1] Especially the g isomer of a divacant Keggin-type silicodecatungstate, namely,[SiW10O36] 8À (1’),[3] has been used as a molecular replica to synthesize dimetal-substituted POMs with reactive dinuclear M2 (mO) 2 sites.[4] We have also synthesized aqua-ligated divacant silicodecatungstate [g-SiW10O34 (H2O) 2] 4À (1) by treatment of 1’with H+ in aqueous solution at pH 2 (Scheme 1).[5] Single-crystal X-ray structure analysis of 1 indicates that two of four oxo ligands located at the vacant sites are selectively protonated to give the aquatungsten moieties.Baeyer–Villiger oxidation is well known and widely used in organic synthesis because of its versatility, although it is carried out with expensive and/or hazardous peracids and leads to formation of one equivalent of the corresponding carboxylic acid. Therefore, efforts have been made to develop homogeneous and heterogeneous catalytic systems with hydrogen peroxide or organic peracids generated by the reaction of either the corresponding aldehyde with dioxygen or the carboxylic acid with hydrogen peroxide.[6] Herein we report novel disilicoicosatungstates [{g-SiW10O32 (H2O) 2} 2 (m-O) 2] 4À (2) and [(g-SiW10O32) 2 (mO) 4] 8À (3), formed by dimerization of 1 by dehydrative condensation in an organic solvent. Interestingly, 2 can catalyze the Baeyer–Villiger oxidation of cycloalkanones with H2O2 and shows high selectivity (! 90%) for the corresponding lactones and high turnover numbers (! 1900), whereas 1 and 3 with the common [g-SiW10O32] fragment are almost inactive. The tetra-n-butylammonium salt of 1 ((TBA) 4 [1]) was transformed to the TBA salt of 2 (Scheme 1 and Experimental Section). The IR spectrum of 2 was clearly different from that of 1, and this suggested a structural change of the cluster framework. The molecular structure of 2 was determined by X-ray analysis on a colorless single crystal. As shown in Figure 1 a, 2 has an S-shaped molecular structure composed of
