Thomas W. Lee

The formyl CH--O hydrogen bond as a key to transition-state organization in enantioselective allylation, aldol and Diels-Alder reactions catalyzed by chiral lewis acids

Abstract The occurrence of formyl CH--O hydrogen bonding in enantioselective allylation, aldol and Diels-Alder reactions of aldehydes with chiral Lewis acid catalysts is a key to the understanding of absolute stereochemistry.

The formyl CH--O hydrogen bond as a critical factor in enantioselective reactions of aldehydes, part 4. Aldol, ethylation, hydrocyanation and Diels-Alder reactions catalyzed by chiral B, Ti and Al lewis acids

Abstract Formyl CH--O hydrogen bonding provides a rational explanation of several diverse catalytic enantioselective addition reactions of aldehydes, as outlined in Figures 1–5.

The formyl C–H⋯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 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.

Enantioselective Diels-Alder reactions between cyclopentadiene and α,β-acetylenic aldehydes catalyzed by a chiral super Lewis acid

Abstract The first examples of catalytic enantioselective Diels-Alder reactions of cyclopentadiene and α,β-acetylenic aldehydes such as Bu3SnCCCHO are described.

A transition-state model for the mikami enantioselective ene reaction

Abstract A logically derived mechanistic model is presented for the Mikami enantioselective ene reaction which correctly describes the absolute and relative stereochemistry resulting from transition-state structures such as 3 and 5.

The mechanistic basis for diastereoselectivity in the Matteson rearrangement

Abstract The diastereoselective rearrangement of α,α-dichloromethylboronate derivatives of chiral 1,2-diols (Matteson rearrangement) can be understood readily in terms of a bidentate interaction between the catalytic Lewis acid (ZnCl 2 ) and the substrate, leading to favored transition states such as 4 and 5 .