The formyl C-H•••O hydrogen bond as a critical factor in enantioselective Lewis-acid catalyzed reactions of aldehydes
The formyl C-H•••O hydrogen bond as a critical factor in enantioselective Lewis-acid catalyzed reactions of aldehydes
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DOI：
10.1039/b104800g
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发表时间：
2001-01-01
影响因子：
4.9
通讯作者：
Lee, TW
中科院分区：
文献类型：
--
作者：
Corey, EJ;Lee, TW
X射线晶体学研究为醛和硼刘易斯酸的配合物中存在与氧或氟配体的分子内甲酰基C-H氢键提供了实验证据。这种类型的氢键可以被认为是“诱导的”或“协同的”，因为当甲酰基氧和刘易斯酸之间的键合变得更强时，其强度可以预期增加。甲酰基和金属-X亚基的共面性有效地限制了围绕甲酰基氧和刘易斯酸的金属中心之间的供体-受体键的旋转，从而在这些络合物中产生了额外的有机元素。这种有机元素为理解手性刘易斯酸催化的非手性醛的许多反应中对映体选择性的机理基础提供了一个简单而合乎逻辑的基础。这些反应包括羟醛缩合、烯丙基化和烯加成到甲酰基C=O基团上以及α，β-不饱和醛与1，3-二烯的Diels-Alder反应。诱导的甲酰基C-H氢键的想法可以作为指导在新的对映选择性催化剂的设计，以及理解优选的过渡态组装的机械原理。
X-Ray crystallographic studies have provided experimental evidence for the existence of intramolecular formyl C-H hydrogen bonds to oxygen or fluorine ligands in complexes of aldehydes and boron Lewis acids. This type of hydrogen bond can be regarded as 'induced' or 'cooperative' in the sense that its strength can be expected to increase as the bonding between the formyl oxygen and the Lewis acid becomes stronger. Coplanarity of the formyl group and the metal-X subunit to which it is bound in a five-membered ring effectively restricts rotation about the donor-acceptor bond between the formyl oxygen and the metal center of the Lewis acid, thus creating an additional organizing element in these complexes. This organizing element provides a simple and logical basis for understanding the mechanistic basis for enantioselectivity in many reactions of achiral aldehydes which are catalyzed by chiral Lewis acids. These reactions include aldol, allylation and ene addition to the formyl C=O group and Diels-Alder reactions of alpha,beta -unsaturated aldehydes with 1,3-dienes. The idea of the induced formyl C-H hydrogen bond can serve as a guide in the design of new enantioselective catalysts as well as a mechanistic principle for understanding preferred transition state assemblies.