The formyl C-H center dot center dot center dot O hydrogen bond as a critical factor in enantioselective Lewis-acid catalyzed reactions of aldehydes

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 hydr ogen bond can be regarded as 'induced' or 'cooperative' in the sense that i ts strength can be expected to increase as the bonding between the formyl o xygen 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 effect ively restricts rotation about the donor-acceptor bond between the formyl o xygen and the metal center of the Lewis acid, thus creating an additional o rganizing element in these complexes. This organizing element provides a si mple and logical basis for understanding the mechanistic basis for enantios electivity in many reactions of achiral aldehydes which are catalyzed by ch iral Lewis acids. These reactions include aldol, allylation and ene additio n to the formyl C=O group and Diels-Alder reactions of alpha,beta -unsatura ted 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 a s well as a mechanistic principle for understanding preferred transition st ate assemblies.