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

Abstract

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 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.

Cobalt‐Catalyzed Wagner–Meerwein Rearrangements with Concomitant Nucleophilic Hydrofluorination

“…The combination of cobalt(II)-salen catalyst A, silane, and Me 3 NFPY • BF 4 is proposed to generate a transient Co III À H species B and a cationic Co III complex C. [51] Hydrogen atom transfer from the metal hydride (MHAT) to the olefin 1 b would generate a carbon-centred radical solvent-caged with the Co II catalyst A. [52] At this stage cage collapse would form organocobalt(III) complex D, which appears to resist rearrangement. Subsequent, single electron oxidation (SEO) of the alkylCo(III) species D could be effected by the cationic Co III complex C or by the excess of oxidant.…”

Cobalt‐Catalyzed Wagner–Meerwein Rearrangements with Concomitant Nucleophilic Hydrofluorination

“…The combination of cobalt(II)-salen catalyst A, silane, and Me 3 NFPY • BF 4 is proposed to generate a transient Co III À H species B and a cationic Co III complex C. [51] Hydrogen atom transfer from the metal hydride (MHAT) to the olefin 1 b would generate a carbon-centred radical solvent-caged with the Co II catalyst A. [52] At this stage cage collapse would form organocobalt(III) complex D, which appears to resist rearrangement. Subsequent, single electron oxidation (SEO) of the alkylCo(III) species D could be effected by the cationic Co III complex C or by excess of oxidant.…”