Toward Total Spontaneous Resolution of <i>sec-</i>Butylzinc Complexes

Di-sec-butylzinc has been used to synthesize [Zn(s-Bu)2L2] complexes, where L denotes amine ligands: e.g., pyridine (py), 2-picoline (2-pic), and 3-picoline (3-pic). The crystal structures of [Zn(s-Bu)2(py)2] (1), [Zn(s-Bu)2(2-pic)2] (2), and [Zn(s-Bu)2(3-pic)2] (3) were determined. Reaction of di-sec-butylzinc with the bidentate amine ligands N,N,N′,N’-tetramethylethylenediamine (TMEDA) and N,N,N′,N’-tetraethylethylenediamine (TEEDA) afforded the chiral complexes [Zn(s-Bu)2(tmeda)] (4) and [Zn(s-Bu)2(teeda)] (5); their crystal structures were determined. Since they coordinate two s-Bu groups each, complexes 1−5 may exist both as chiral and meso molecules in solution, but the crystalline complexes were found to be exclusively chiral. The enantiomerization rate of 1−5 in benzene was found to be slow on the NMR time scale. Chiral crystalline complexes containing the Zn(s-Bu)2 moiety are of particular interest, since they display reactive chirogenic α-carbon atoms. The identification of a Zn(s-Bu)2 complex such as 1−5, which crystallized as a conglomerate of chiral crystals, would afford a simple and cheap way to optical resolution of a chiral organometallic reagent, without the need to use enantiopure ligands. Using preferential crystallization, it should be possible to perform total spontaneous resolution, which may be regarded as a form of absolute asymmetric synthesis. However, all complexes isolated so far, including the dinuclear alkylzinc amide [Zn(s-Bu)(Me2N(CH2)2NH)]2 (6), crystallize in centrosymmetric space groups. As a remedy, recrystallization of Zn(s-Bu)2 complexes from different solvents was studied: in at least two cases, viz. [Zn(s-Bu)2(3-pic)2]·C7H8 (7) and [Zn(s-Bu)2(3,5-lut)2]·C7H8 (8), it was found that recrystallization from toluene gave rise to clathrates.二仲丁基锌（di-sec-butylzinc）已被用于合成[Zn(s-Bu)₂L₂]型配合物，其中L代表胺配体（amine ligands）：例如吡啶（pyridine）、2-甲基吡啶（2-picoline）与3-甲基吡啶（3-picoline）。本文已解析了[Zn(s-Bu)₂(py)₂]（1）、[Zn(s-Bu)₂(2-pic)₂]（2）与[Zn(s-Bu)₂(3-pic)₂]（3）的晶体结构。二仲丁基锌与双齿胺配体N,N,N′,N'-四甲基乙二胺（TMEDA）、N,N,N′,N'-四乙基乙二胺（TEEDA）发生反应，得到手性配合物[Zn(s-Bu)₂(tmeda)]（4）与[Zn(s-Bu)₂(teeda)]（5），并解析了二者的晶体结构。由于每个配合物均配位两个s-Bu基团，配合物1~5在溶液中可同时以手性与内消旋（meso）分子形式存在，但结晶得到的配合物仅为手性形式。研究发现，配合物1~5在苯溶液中的对映异构化速率在核磁共振（NMR）时间尺度上较为缓慢。含Zn(s-Bu)₂结构单元的手性晶体配合物备受关注，因其带有具有反应活性的手性α-碳原子。若能获得以手性晶体聚集体形式结晶的Zn(s-Bu)₂类配合物（如1~5），即可无需使用对映纯配体，实现一种简便廉价的手性有机金属试剂光学拆分（optical resolution）方法。采用优先结晶法（preferential crystallization）可实现完全自发拆分，这一过程可被视为绝对不对称合成（absolute asymmetric synthesis）的一种形式。然而目前已分离得到的所有配合物，包括双核烷基锌酰胺配合物[Zn(s-Bu)(Me₂N(CH₂)₂NH)]₂（6），均结晶于中心对称空间群（centrosymmetric space groups）中。为解决这一问题，本文研究了不同溶剂中Zn(s-Bu)₂类配合物的重结晶（recrystallization）行为：结果发现，至少在两种情况下，即[Zn(s-Bu)₂(3-pic)₂]·C₇H₈（7）与[Zn(s-Bu)₂(3,5-lut)₂]·C₇H₈（8），从甲苯中重结晶可得到笼形包合物（clathrates）。