Enantioselective catalytic 1,2-boronate rearrangements.

A strategy that facilitates the construction of a wide variety of trisubstituted stereocenters through a catalytically accessed common chiral intermediate could prove highly enabling for the field of synthetic chemistry. We report the discovery of enantioselective, catalytic 1,2-boronate rearrangements for the synthesis of α-chloro pinacol boronic esters from readily available boronic esters and dichloromethane. The chiral building blocks produced in these reactions can undergo two sequential stereospecific elaborations to generate a wide assortment of trisubstituted stereocenters. The enantioselective reaction is catalyzed by a lithium-isothiourea-boronate complex, which is proposed to promote rearrangement through a dual­lithium-induced chloride abstraction orchestrated by Lewis basic functionality on the catalyst scaffold.

Enantioselective Aryl-Iodide-Catalyzed Wagner-Meerwein Rearrangements.

We report a strategy for effecting catalytic, enantioselective carbocationic rearrangements through the intermediacy of alkyl iodanes as stereodefined carbocation equivalents. Asymmetric Wagner-Meerwein rearrangements of ß-substituted styrenes are catalyzed by the C2-symmetric aryl iodide 1 to provide access to enantioenriched 1,3-difluorinated molecules possessing interesting and well-defined conformational properties. Hammett and kinetic isotope effect studies, in combination with computational investigations, reveal that two different mechanisms are operative in these rearrangement reactions, with the pathway depending on the identity of the migrating group. In reactions involving alkyl-group migration, intermolecular fluoride attack is product- and enantio-determining. In contrast, reactions in which aryl rearrangement occurs proceed through an enantiodetermining intramolecular 1,2-migration prior to fluorination. The fact that both pathways are promoted by the same chiral aryl iodide catalyst with high enantioselectivity provides a compelling illustration of generality across reaction mechanisms in asymmetric catalysis.

Azaindole synthesis through dual activation catalysis with N-heterocyclic carbenes.

A convergent, transition-metal-free synthesis of 2-aryl-azaindoles has been developed. The interception of a reactive aza-ortho-azaquinone methide intermediate by an acyl anion equivalent generated through carbene catalysis provides high yields, a wide substrate scope, and the synthesis of previously inaccessible azaindoles.