Cobalt-Catalyzed Tandem Radical Cyclization/C–C Coupling Initiated by Directed C–H Activation

Iridium-catalyzed tandem olefination/aza-Michael reaction: rapid access to N-N functionalized hydrazides

An Ir-catalyzed tandem olefination/aza-Michael reaction of protected benzoylhydrazine derivatives with olefins under mild conditions has been developed. This method can be successfully applied to the construction of various structurally N-N-functionalized hydrazide derivatives bearing the α,β-unsaturated side chain in good to excellent yields. In particular, the deaminoprotected products can be used as potential precursors for the construction of N-N axially chiral compounds.

Radical Cyclization-Initiated Difunctionalization Reactions of Alkenes and Alkynes

Radical reactions are powerful in the synthesis of diverse molecular scaffolds bearing functional groups. In previous review articles, we have presented 1,2-difunctionalizations, remote 1,3-, 1,4-, 1,5-, 1,6- and 1,7-difunctionalizations, and addition followed by cyclization reactions. Presented in this paper is radical cyclization followed by the second functionalization reaction. The second functionalization could be realized by atom transfer reactions, radical or transition metal-assisted coupling reactions, and reactions with neutral molecules, cationic and anionic species.

Cobalt-catalyzed cascade hydroalkenylation of 1,6-enynes with chalcones

An efficient stereoselective cobalt-catalyzed hydrovinylative cyclization of 1,6-enynes with chalcones to obtain functionalized pyrrolidines has been developed.

Cationic Co(I) Catalysts for Regiodivergent Hydroalkenylation of 1,6-Enynes. An Uncommon cis-β-C-H Activation Leads to Z-Selective Coupling of Acrylates

Two intermolecular hydroalkenylation reactions of 1,6-enynes are presented which yield substituted 5-membered carbo- and -heterocycles. This reactivity is enabled by a cationic bis-diphenylphosphinopropane (DPPP)Co^I species which forms a cobaltacyclopentene intermediate by oxidative cyclization of the enyne. This key species interacts with alkenes in distinct fashion, depending on the identity of the coupling partner to give regiodivergent products. Simple alkenes undergo insertion reactions to furnish 1,3-dienes whereby one of the alkenes is tetrasubstituted. When acrylates are employed as coupling partners, the site of intermolecular C-C formation shifts from the alkyne to the alkene motif of the enyne, yielding Z-substituted-acrylate derivatives. Computational studies provide support for our experimental observations and show that the turnover-limiting steps in both reactions are the interactions of the alkenes with the cobaltacyclopentene intermediate via either a 1,2-insertion in the case of ethylene, or an unexpected β-C-H activation in the case of most acrylates. Thus, the H syn to the ester is activated through the coordination of the acrylate carbonyl to the cobaltacycle intermediate, which explains the uncommon Z-selectivity and regiodivergence. Variable time normalization analysis (VTNA) of the kinetic data reveals a dependance upon the concentration of cobalt, acrylate, and activator. A KIE of 2.1 was observed with methyl methacrylate in separate flask experiments, indicating that C-H cleavage is the turnover-limiting step in the catalytic cycle. Lastly, a Hammett study of aryl-substituted enynes yields a ρ value of -0.4, indicating that more electron-rich substituents accelerate the rate of the reaction.

Vitamin B12-Catalyzed Dicarbofunctionalization of Bromoalkenes Under Visible Light Irradiation

Vitamin B12 plays a crucial role in enzymatic transformations. This natural compound proved also useful as a catalyst in numerous organic reactions. Commercial availability and lower cost than precious metal complexes, make cobalamin an attractive candidate for a broader use as a benign Co-catalyst. Herein, the vitamin B12-catalyzed dicarbofuntionalization of bromoalkenes with electrophilic olefins is reported leading to substituted pyrrolidines and piperidines in decent yields after only 15 minutes under light irradiation.

Nickel-Catalyzed α-Carbonylalkylarylation of Vinylarenes: Expedient Access to γ,γ-Diarylcarbonyl and Aryltetralone Derivatives

We report a Ni-catalyzed regioselective α-carbonylalkylarylation of vinylarenes with α-halocarbonyl compounds and arylzinc reagents. The reaction works with primary, secondary, and tertiary α-halocarbonyl molecules, and electronically varied arylzinc reagents. The reaction generates γ,γ-diarylcarbonyl derivatives with α-secondary, tertiary, and quaternary carbon centers. The products can be readily converted to aryltetralones, including a precursor to Zoloft, an antidepressant drug.

N-Heterocyclic Carbene Complexes in C-H Activation Reactions

In this contribution, we provide a comprehensive overview of C-H activation methods promoted by NHC-transition metal complexes, covering the literature since 2002 (the year of the first report on metal-NHC-catalyzed C-H activation) through June 2019, focusing on both NHC ligands and C-H activation methods. This review covers C-H activation reactions catalyzed by group 8 to 11 NHC-metal complexes. Through discussing the role of NHC ligands in promoting challenging C-H activation methods, the reader is provided with an overview of this important area and its crucial role in forging carbon-carbon and carbon-heteroatom bonds by directly engaging ubiquitous C-H bonds.

Iron and cobalt catalysis: new perspectives in synthetic radical chemistry

Iron and cobalt complexes are at the origin of high valuable synthetic pathways involving radical intemediates.

Indium-mediated difunctionalization of iodoalkyl-tethered unactivated alkenes via an intramolecular cyclization and an ensuing palladium-catalyzed cross-coupling reaction with aryl halides

A cobalt-catalyzed, indium-mediated difunctionalization of iodoalkyl-tethered unactivated alkenes for accessing five-membered cyclic compounds via a cyclization/cross-coupling sequence was developed.