Origins of stereoselectivity in optically pure phenylethaniminopyridine tris-chelates M(NN′)3n+ (M = Mn, Fe, Co, Ni and Zn)
作者:Suzanne E. Howson、Laura E. N. Allan、Nikola P. Chmel、Guy J. Clarkson、Robert J. Deeth、Alan D. Faulkner、Daniel H. Simpson、Peter Scott
DOI:10.1039/c1dt10588d
日期:——
One-pot reactions of 2-pyridinecarboxaldehyde, chiral phenylethanamines and Fe(II) give single diastereomer facdiimine complexes at thermodynamic equilibrium so that no chiral separations are required (d.r. > 200â:â1). The origins of this stereoselectivity are partly steric and partly a result of the presence of three sets of inter-ligand parallel-offset Ï-stacking interactions. Mn(II), Co(II), Co(III), Ni(II) and Zn(II) give similar fac structures, alongside the imidazole analogues for Fe(II). While most of the complexes are paramagnetic, the series of molecular structures allows us to assess the influence of the Ï-stacking present, and there is a strong correlation between this and the MâN bond length. Fe(II) is close to optimal. For the larger Zn(II) ion, very weak Ï-stacking leads to poorer measured stereoselectivity (NMR) but this is improved with increased solvent polarity. The mechanism of stereoselection is further investigated via DFT calculations, chiroptical spectroscopy and the use of synthetic probes.
2-吡啶甲醛、手性苯乙胺和Fe(II)的一锅反应在热力学平衡下生成单一的非对映异构体fac二亚胺复合物,因此不需要进行手性分离(d.r. > 200:1)。这种立体选择性的来源部分是立体因素,部分是由于存在三组配体间的平行偏移π-堆叠相互作用。Mn(II)、Co(II)、Co(III)、Ni(II)和Zn(II)也形成类似的fac结构,此外还有Fe(II)的咪唑类类似物。虽然大多数复合物是顺磁性的,但这系列分子结构使我们能够评估π-堆叠的影响,并且这种影响与M–N键长之间有很强的相关性。Fe(II)接近最佳状态。对于较大的Zn(II)离子,极弱的π-堆叠导致测得的立体选择性较差(NMR),但随着溶剂极性的增加,这种情况得到改善。立体选择机制还通过DFT计算、手性光谱分析以及合成探针的使用进一步研究。