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Mechanistic studies of copper(I)-catalyzed 1,3-halogen migration.

ABSTRACT: 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.

SUBMITTER: Van Hoveln R

PROVIDER: S-EPMC4701576 | biostudies-literature | 2015 Apr

REPOSITORIES: biostudies-literature

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Mechanistic studies of copper(I)-catalyzed 1,3-halogen migration.

Van Hoveln Ryan R Hudson Brandi M BM Wedler Henry B HB Bates Desiree M DM Le Gros Gabriel G Tantillo Dean J DJ Schomaker Jennifer M JM

Journal of the American Chemical Society 20150416 16

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 ox ...[more]

PMID: 25828031

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Project description:Mechanistic studies of the copper-catalyzed asymmetric hydroboration of vinylarenes and internal alkenes are reported. Catalytic systems with both DTBM-SEGPHOS and SEGPHOS as the ligands have been investigated. With DTBM-SEGPHOS as the ligand, the resting state of the catalyst, which is also a catalytic intermediate, for hydroboration of 4-fluorostyrene is a phenethylcopper(I) complex ligated by the bisphosphine. This complex was fully characterized by NMR spectroscopy and X-ray crystallography. The turnover-limiting step in the catalytic cycle for the reaction of vinylarenes is the borylation of this phenethylcopper complex with pinacolborane (HBpin) to form the boronate ester product and a copper hydride. Experiments showed that the borylation occurs with retention of configuration at the benzylic position. β-Hydrogen elimination and insertion of the alkene to reform this phenethylcopper complex is reversible in the absence of HBpin but is irreversible during the catalytic process because reaction with HBpin is faster than β-hydrogen elimination of the phenethylcopper complex. Studies on the hydroboration of a representative internal alkene, trans-3-hexenyl 2,4,6-trichlorobenzoate, which undergoes enantio- and regioselective addition of HBpin catalyzed by DTBM-SEGPHOS, KOtBu, and CuCl, also was conducted, and these studies revealed that a DTBM-SEGPHOS-ligated copper(I) dihydridoborate complex is the resting state of the catalyst in this case. The turnover-limiting step in the catalytic cycle for hydroboration of the internal alkene is insertion of the alkene into a copper(I) hydride formed by reversible dissociation of HBpin from the copper dihydridoborate species. With SEGPHOS as the ligand, a dimeric copper hydride was observed as the dominant species during the hydroboration of 4-fluorostyrene, and this complex is not catalytically competent. DFT calculations provide a view into the origins of regio- and enantioselectivity of the catalytic process and indicate that the charge on the copper-bound carbon and delocalization of charge onto the aryl ring control the rate of the alkene insertion and the regioselectivity of the catalytic reactions of vinylarenes.

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Mechanistic studies of copper(I)-catalyzed 1,3-halogen migration.

Publications

Mechanistic studies of copper(I)-catalyzed 1,3-halogen migration.

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