The formyl C–H⋯O hydrogen bond as a critical factor in enantioselective Lewis-acid catalyzed reactions of aldehydes
Abstract
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 CO group and Diels–Alder
reactions of α,β-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.