Synthesis of a Dialuminum-Substituted Silicotungstate and the Diastereoselective Cyclization of Citronellal Derivatives

A novel dialuminum-substituted silicotungstate TBA3H[γ-SiW10O36{Al(OH2)}2(μ-OH)2]·4H2O (1, TBA = tetra-n-butylammonium) was synthesized by the reaction of the potassium salt of [γ-SiW10O36]8− (SiW10) with 2 equiv of Al(NO3)3 in an acidic aqueous medium. It was confirmed by the X-ray crystallographic analysis that compound 1 was a monomer of the γ-Keggin dialuminum-substituted silicotungstate with the {Al2(μ-OH)2} diamond core. The cluster framework of 1 maintained the γ-Keggin structure in the solution states. The reaction of 1 with pyridine yielded TBA3[(C5H5N)H][γ-SiW10O36{Al(C5H5N)}2(μ-OH)2]·2H2O (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.

一种新型二铝取代硅钨酸盐TBA₃H[γ-SiW₁₀O₃₆{Al(OH₂)}₂(μ-OH)₂]·4H₂O（1，其中TBA为四正丁基铵（tetra-n-butylammonium）），通过[γ-SiW₁₀O₃₆]⁸⁻（SiW₁₀）的钾盐与2当量硝酸铝在酸性水介质中反应合成。经X射线晶体衍射分析证实，化合物1为带有{Al₂(μ-OH)₂}金刚石型核的γ-凯金（Keggin）结构二铝取代硅钨酸盐单体。该化合物的团簇骨架在溶液状态下仍保持γ-凯金（Keggin）结构。化合物1与吡啶反应得到产物TBA₃[(C₅H₅N)H][γ-SiW₁₀O₃₆{Al(C₅H₅N)}₂(μ-OH)₂]·2H₂O（2），其分子结构经X射线晶体衍射分析成功解析。在化合物2中，3个吡啶分子中的2个与铝中心的轴向位点配位，剩余1个以吡啶鎓阳离子形式存在，这表明化合物1具备2个路易斯酸位点与1个布朗斯特酸位点。化合物1对香茅醛衍生物（如(+)-香茅醛（3）与3-甲基香茅醛（4））的分子内环化反应展现出优异的催化活性，且未生成醚化与脱水副产物。在化合物1催化的3的环化反应中，对(-)-异胡薄荷醇（3a）的非对映选择性可达约90%，该数值为目前已报道同类催化体系中的最高水平。向该催化体系中加入吡啶后，反应速率会下降；当吡啶用量为化合物1的2当量时，环化反应几乎无法进行。与之相反，当体系中加入2,6-二甲基吡啶时，反应速率与对3a的非对映选择性均与未添加助剂时基本一致。因此，本次研究中的环化反应主要由化合物1中的路易斯酸位点（铝中心）催化促进。密度泛函理论（DFT）计算表明，在化合物1催化的3的环化反应中，生成3a的过渡态在空间位阻与电子效应上均优于其余三种过渡态。