Asymmetric carbon coupling at boron The Matteson reaction produces carbon–carbon bonds by coupling halocarbons such as widely available dichloromethane with an alkyl substituent on boron. Sharma et al . report asymmetric catalysis of this reaction. Their catalyst, derived from a chiral thiourea, a boronic ester, and an alkyl lithium base, appears to accelerate a chloride abstraction step through its lithium center. The product, still bearing a chloride, can be further modified through stereospecific displacement to generate a wide variety of trisubstituted chiral centers. —JSY

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 β-substituted styrenes are catalyzed by the C 2-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. File list (4) download file view on ChemRxiv HAS Wagner Meerwein submission vs.pdf (859.05 KiB) download file view on ChemRxiv WM SI final.pdf (13.61 MiB) download file view on ChemRxiv CIF for compound 3e.cif (1.06 MiB) download file view on ChemRxiv CIF for compound 6.cif (674.28 KiB)

Our group’s discovery of lithium-isothiourea-boronate–catalyzed Matteson homologations is chronicled. Chiral thiourea dual–hydrogen bond donors were initially found to promote enantioselective dichloromethyl boronate rearrangements, albeit with poor reproducibility. Systematic investigations of the fate of the thiourea led to the discovery that lithium-isothiourea-boronate derivatives were being generated in situ as highly enantioselective catalytically active species. The optimal lithium-isothiourea-boronate catalyst displays significant generality in the rearrangement of primary alkyl migrating groups, affording synthetically valuable α-chloro boronic ester products with consistently high enantioselectivities. The catalyst is proposed to act as a structurally rigid chiral framework that precisely positions two lithium cations to enable a dual-lithium–mediated chloride abstraction.

A convergent, transition-metal-free synthesis of 2-aryla-zaindoles 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.