The default explanation for good to high diastereomeric excess when reducing N-chiral imines possessing only mediocre cis/trans-imine ratios (>15% cis-imine) has invariably been in situ cis-to-trans isomerization before reduction; but until now no study unequivocally supported this conclusion. The present study co-examines an alternative hypothesis, namely that some classes of cis-imines may hold conformations that erode the inherent facial bias of the chiral auxiliary, providing more of the trans-imine reduction product than would otherwise be expected. The ensuing experimental and computational (DFT) results favor the former, pre-existing, explanation.
将默认解释为在还原仅具有中等
cis/
trans-
亚胺比率(>15%
cis-
亚胺)的
N-手性
亚胺时获得良好至高对映体过量的解释一直是在还原之前发生原位
cis到
trans异构化;但直到现在,没有研究明确支持这一结论。本研究同时考虑了另一种假设,即一些类别的
cis-
亚胺可能具有侵蚀手性辅助体固有面部偏好的构象,提供比预期更多的
trans-
亚胺还原产物。随后的实验和计算(DFT)结果支持了前者,即已存在的解释。