Kikukawa Yuji, Yamaguchi Syuhei, Nakagawa Yoshinao, Uehara Kazuhiro, Uchida Sayaka, Yamaguchi Kazuya, Mizuno Noritaka
Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.
J Am Chem Soc. 2008 Nov 26;130(47):15872-8. doi: 10.1021/ja8014154.
A novel dialuminum-substituted silicotungstate TBA(3)H[gamma-SiW(10)O(36){Al(OH(2))}(2)(mu-OH)(2)] x 4 H(2)O (1, TBA = tetra-n-butylammonium) was synthesized by the reaction of the potassium salt of gamma-SiW(10)O(36) (SiW10) with 2 equiv of Al(NO(3))(3) in an acidic aqueous medium. It was confirmed by the X-ray crystallographic analysis that compound 1 was a monomer of the gamma-Keggin dialuminum-substituted silicotungstate with the {Al(2)(mu-OH)(2)} diamond core. The cluster framework of 1 maintained the gamma-Keggin structure in the solution states. The reaction of 1 with pyridine yielded TBA(3)[(C(5)H(5)N)H][gamma-SiW(10)O(36){Al(C(5)H(5)N)}(2)(mu-OH)(2)] x 2 H(2)O (2), and the molecular structure was successfully determined by the X-ray crystallographic analysis. In compound 2, two of three pyridine molecules coordinated to the axial positions of aluminum centers and one of them existed as a pyridinium cation, showing that compound 1 has two Lewis acid sites and one Brønsted acid site. Compound 1 showed high catalytic activity for the intramolecular cyclization of citronellal derivatives such as (+)-citronellal (3) and 3-methylcitronellal (4) without formation of byproduct resulting from etherification and dehydration. For the 1-catalyzed cyclization of 3, the diastereoselectivity toward (-)-isopulegol (3a) reached ca. 90% and the value was the highest level among those with reported systems so far. The reaction rate for the 1-catalyzed cyclization of 3 decreased by the addition of pyridine, and the cyclization hardly proceeded in the presence of 2 equiv of pyridine with respect to 1. On the other hand, the reaction rate and diastereoselectivity to 3a in the presence of 2,6-lutidine were almost the same as those in the absence. Therefore, the present cyclization is mainly promoted by the Lewis acid sites (aluminum centers) in 1. DFT calculations showed that the formation of the transition state to produce 3a is sterically and electronically more favorable than the other three transition states for the present 1-catalyzed cyclization of 3.
通过在酸性水介质中使[γ-SiW₁₀O₃₆]⁸⁻（SiW10）的钾盐与2当量的Al(NO₃)₃反应，合成了一种新型的二铝取代硅钨酸盐TBA₃H[γ-SiW₁₀O₃₆{Al(OH₂)}(₂)(μ-OH)(₂)]·4H₂O（1，TBA = 四正丁基铵）。X射线晶体学分析证实化合物1是具有{Al₂(μ-OH)(₂)}菱形核心的γ-凯吉二铝取代硅钨酸盐的单体。1的簇框架在溶液状态下保持γ-凯吉结构。1与吡啶反应生成TBA₃[(C₅H₅N)H][γ-SiW₁₀O₃₆{Al(C₅H₅N)}(₂)(μ-OH)(₂)]·2H₂O（2），并且通过X射线晶体学分析成功确定了其分子结构。在化合物2中，三个吡啶分子中的两个与铝中心的轴向位置配位，其中一个以吡啶鎓阳离子形式存在，表明化合物1具有两个路易斯酸位点和一个布朗斯特酸位点。化合物1对香茅醛衍生物如(+)-香茅醛（3）和3-甲基香茅醛（4）的分子内环化显示出高催化活性，且不形成由醚化和脱水产生的副产物。对于1催化的3的环化反应，对(-)-异蒲勒醇（3a）的非对映选择性达到约90%，该值是迄今为止报道的体系中最高的。加入吡啶会降低1催化的3的环化反应速率，相对于1存在2当量吡啶时环化反应几乎不进行。另一方面，在2,6-二甲基吡啶存在下对3a的反应速率和非对映选择性与不存在时几乎相同。因此，当前的环化反应主要由1中的路易斯酸位点（铝中心）促进。密度泛函理论计算表明，对于当前1催化的3的环化反应，生成3a的过渡态的形成在空间和电子方面比其他三个过渡态更有利。
