A Cu(I)-catalyzed 1,3-halogen migration reaction effectively recycles an activating group by transferring bromine or iodine from a sp2 to a benzylic carbon with concomitant borylation of the Ar-X bond. The resulting benzyl halide can be reacted in the same vessel under a variety of conditions to form an additional carbon-heteroatom bond. Cross-over experiments using an isotopically enriched bromide source support intramolecular transfer of Br. The reaction is postulated to proceed via a Markovnikov hydrocupration of the o-halostyrene, oxidative addition of the resulting Cu(I) complex into the Ar-X bond, reductive elimination of the new sp3 C-X bond, and final borylation of an Ar-Cu(I) species to turn over the catalytic cycle.

An ongoing challenge in modern catalysis is to identify and understand new modes of reactivity promoted by earth-abundant and inexpensive first-row transition metals. Herein, we report a mechanistic study of an unusual copper(I)-catalyzed 1,3-migration of 2-bromostyrenes that reincorporates the bromine activating group into the final product with concomitant borylation of the aryl halide bond. A combination of experimental and computational studies indicated this reaction does not involve any oxidation state changes at copper; rather, migration occurs through a series of formal sigmatropic shifts. Insight provided from these studies will be used to expand the utility of aryl copper species in synthesis and develop new ligands for enantioselective copper-catalyzed halogenation.

First-row transition metal catalysis offers a cheaper, more environmentally sustainable alternative to second- and third-row transition metal catalysts. Nickel has shown great promise as a tool for the borylation of unsaturated compounds to yield boronic esters, but Markovnikov-selective hydroborations of simple styrenes have not been well-explored. Herein, we report the synthesis of benzyl boronic esters via nickel-catalyzed hydroboration of styrenes using a heteroleptic N-heterocyclic carbene (NHC)–phosphine nickel complex, IMes­(Cy3P)­NiCl2. The IMes­(Cy3P)­NiCl2 complex displays a broad substrate scope and maintains the integrity of yield and regioselectivity when challenged with substrates bearing increased steric hindrance. The heteroleptic complexes also tolerate both electron-withdrawing and -donating groups, in contrast to traditional bis-phosphine and Ni(0) complexes.

A series of NHC−copper complexes was synthesized and their potential to catalyze 1,3-halogen migration explored. Increasing the steric bulk around the metal drastically improves the lifetime of NHC−CuH species and promotes 1,3-halogen migration of both 2-bromo-and 2-chlorostyrenes through transfer of an aryl halogen to a benzylic carbon with concomitant arene borylation. The NHC-based system displays a broad substrate scope with notable advantages over previously reported phosphine-based catalysts, including complete selectivity for migration versus competing benzylic borylation, increased steric tolerance, efficient aryl chloride migration, and facile formation and characterization of organocopper catalytic intermediates. Experimental evidence and DFT calculations support a mechanism proceeding through dearomatization of a benzyl copper species, followed by a 1,4-halogen shift and borylation of the resulting ArCu(I) intermediate.

Organosilanes have become a mainstay in organic synthesis as they can participate in a wide number of reactions where they act as a soft carbon nucleophile. They can engage in coupling reactions where C–C, C–N, C–O, or a variety of other bonds are formed. Furthermore, organosilanes are effective coupling partners in C–H activations. Despite their utility, synthesis of organosilanes typically relies on precious metals such as platinum and palladium. In the past several years, there has been considerable effort to develop new procedures, which rely on inexpensive catalysts such as first-row transition metals. More specifically, copper-catalyzed silylations have undergone significant development in the past decade. Copper-catalyzed silylations generally rely on either a silylborane, silylzinc, or disilane as the silicon source. However, a number of different transformations can be performed with this small set of reagents including conjugate addition, addition to alkynes, allenes, and carbonyls, coupling reactions, and substitutions. Nearly all of these transformations exhibit high levels of diastereoselectivity, regioselectivity, or enantioselectivity, depending on the transformation. Moreover, there is still plenty of room for further development of copper-catalyzed silylations, which should provide an inexpensive alternative to more traditional silylations.