A Hydrometalation Initiation Mechanism via a Discrete Cobalt-Hydride for a Rapid and Controlled Radical Polymerization

Markovnikov-Selective Cobalt-Catalyzed Wacker-Type Oxidation of Styrenes into Ketones under Ambient Conditions Enabled by Hydrogen Bonding

The replacement of palladium catalysts for Wacker-type oxidation of olefins into ketones by first-row transition metals is a relevant approach for searching more sustainable protocols. Besides highly sophisticated iron catalysts, all the other first-row transition metal complexes have only led to poor activities and selectivities. Herein, we show that the cobalt-tetraphenylporphyrin complex is a competent catalyst for the aerobic oxidation of styrenes into ketones with silanes as the hydrogen sources. Remarkably, under room temperature and air atmosphere, the reactions were exceedingly fast (up to 10 minutes) with a low catalyst loading (1 mol %) while keeping an excellent chemo- and Markovnikov-selectivity (up to 99 % of ketone). Unprecedently high TOF (864 h-1 ) and TON (5,800) were reached for the oxidation of aromatic olefins under these benign conditions. Mechanistic studies suggest a reaction mechanism similar to the Mukaiyama-type hydration of olefins with a change in the last fundamental step, which controls the chemoselectivity, thanks to a unique hydrogen bonding network between the ethanol solvent and the cobalt peroxo intermediate.

Controlled Radical Polymerization of Acrylates and Isocyanides Installs Degradable Functionality into Novel Copolymers

Installing ketones into a polymer backbone is a known method for introducing photodegradability into polymers; however, most current methods are limited to ethylene-carbon monoxide copolymerization. Here we use isocyanides in place of carbon monoxide in a copolymerization strategy to access degradable nonalternating poly(ketones) that either maintain or enhance the thermal properties. A cobalt-mediated radical polymerization of acrylates and isocyanides synthesizes nonalternating poly(acrylate-co-isocyanide) copolymers with tunable incorporation using monomer feed ratios. The kinetic product of the polymerization is a dynamic β-imine ester that tautomerizes to the β-enamine ester. Hydrolysis of this copolymer affords a third copolymer microstructure─the elusive nonalternating poly(ketone)─from a single copolymerization strategy. Analysis of the copolymer properties demonstrates tunable thermal properties with the degree of incorporation. Finally, we show that poly(acrylate-co-isocyanide) and poly(acrylate-co-ketone) are photodegradable with 390 nm light, enabling chain cleavage.

Vitamin B12 Derivative Enables Cobalt-Catalyzed Atom Transfer Radical Polymerization

Advances in controlled radical polymerizations by cobalt complexes have primarily taken advantage of the reactivity of cobalt as a persistent radical to reversibly deactivate propagating chains by forming a carbon-cobalt bond. However, cobalt-mediated radical polymerizations require stoichiometric ratios of a cobalt complex, deterring its utility in synthesizing well-defined polymers. Here, we developed a strategy to use cobalt as a catalyst to control radical polymerizations via halogen atom transfer with alkyl halide initiators. Using a modified, hydrophobic analogue of vitamin B12 (heptamethyl ester cobyrinate) as a cobalt precatalyst, we controlled the polymerization of acrylate monomers. The polymerization efficiency of the cobalt catalyst was significantly improved by additional bromide anions, which enhanced the deactivation of propagating radicals yielding polymers with dispersity values <1.2 using catalyst concentrations as low as 5 mol %. We anticipate that the development of cobalt catalysis in atom transfer radical polymerization will enable new opportunities in designing catalytic systems for the controlled synthesis of polymers.

Unraveling Hydrogen Atom Transfer Mechanisms with Voltammetry: Oxidative Formation and Reactivity of Cobalt Hydride

The utility of transition metal hydride catalyzed hydrogen atom transfer (MHAT) has been widely demonstrated in organic transformations such as alkene isomerization and hydrofunctionalization reactions. However, the highly reactive nature of the hydride and radical intermediates has hindered mechanistic insight into this pivotal reaction. Recent advances in electrochemical MHAT have opened up the possibility for new analytical approaches for mechanistic diagnosis. Here, we report a voltammetric interrogation of Co-based MHAT reactivity, describing in detail the oxidative formation and reactivity of the key Co-H intermediate and its reaction with aryl alkenes. Insights from cyclic voltammetry and finite element simulations help elucidate the rate-limiting step as metal hydride formation, which we show to be widely tunable based on ligand design. Voltammetry is also suggestive of the formation of Co-alkyl intermediates and a dynamic equilibrium with the reactive neutral radical. These mechanistic studies provide information for the design of future hydrofunctionalization reactions, such as catalyst and silane choice, the relative stability of metal-alkyl species, and how hydrofunctionalization reactions utilize Co-alkyl intermediates. In summary, these studies establish an important template for studying MHAT reactions from the perspective of electrochemical kinetic frameworks.

Hydrogen-Atom-Transfer-Initiated Radical/Polar Crossover Annulation Cascade for Expedient Access to Complex Tetralins

A radical/polar crossover annulation between allyl-substituted arenes and electron-deficient alkenes is described. Cobalt-catalyzed hydrogen atom transfer (HAT) facilitates tandem radical C-C bond formation that generates functionalized tetralin products in the face of potentially problematic hydrofluorination, hydroalkoxylation, hydrogenation, alkene isomerization, and radical polymerization reactions. The reactions proceed under mild conditions that tolerate many functional groups, leading to a broad substrate scope. This powerful ring-forming reaction very quickly assembles complex tetralins that are the formal products of the largely infeasible Diels-Alder cycloadditions of styrenes.

The recent advances in cobalt-catalyzed C(sp3)-H functionalization reactions

Over the past decades, reactions involving C-H functionalization have become a hot theme in organic transformations because they have a lot of potential for the streamlined synthesis of complex molecules. C(sp³)-H bonds are present in most organic species. Since organic molecules have massive significance in various aspects of life, the exploitation and functionalization of C(sp³)-H bonds hold enormous importance. In recent years, the first-row transition metal-catalyzed direct and selective functionalization of C-H bonds has emerged as a simple and environmentally friendly synthetic method due to its low cost, unique reactivity profiles and easy availability. Therefore, research advancements are being made to conceive catalytic systems that foster direct C(sp³)-H functionalization under benign reaction conditions. Cobalt-based catalysts offer mild and convenient reaction conditions at a reasonable expense compared to conventional 2^nd and 3^rd-row transition metal catalysts. Consequently, the probing of Co-based catalysts for C(sp³)-H functionalization is one of the hot topics from the outlook of an organic chemist. This review primarily focuses on the literature from 2018 to 2022 and sheds light on the substrate scope, selectivity, benefits and limitations of cobalt catalysts for organic transformations.