Nickel-Carbon Bond Oxygenation with Green Oxidants via High-Valent Nickel Species

Abstract

Describedherein is the synthesis of the Ni-II complex(( t )BuMe(2)tacn)-Ni-II(cycloneophyl) (( t )BuMe(2)tacn= 1-tert-butyl-4,7-dimethyl-1,4,7-triazacyclononane,cycloneophyl = -CH2CMe2-o-C6H4-) and its reactivity with dioxygenand peroxides. The new ( t )BuMe(2)tacn ligand is designed to enhance the oxidatively induced bond-formingreactivity of high-valent Ni intermediates. Tunable chemoselectivityfor Csp(2)-O vs Csp(2)-Csp(3) bond formation was achieved by selecting the appropriate solventand reaction conditions. Importantly, the use of cumene hydroperoxideand meta-chloroperbenzoic acid suggests a heterolyticO-O bond cleavage upon reaction with (( t )BuMe(2)tacn)-Ni-II(cycloneophyl). Mechanisticstudies using isotopically labeled H2O2 supportthe generation of a high-valent Ni-oxygen species via an inner-spheremechanism and subsequent reductive elimination to form the Csp(2)-O bond. Kinetic studies of the exceptionally fastCsp(2)-O bond-forming reaction reveal a first-orderdependence on both (( t )BuMe(2)tacn)-Ni-II(cycloneophyl) and H2O2, and thus anoverall second-order reaction. Eyring analysis further suggests thatthe oxidation of the Ni-II complex by H2O2 is the rate-determining step, which can be modulated by thepresence of coordinating solvents. Moreover, computational studiesfully support the conclusions drawn from experimental results. Overall,this study reveals for the first time the ability to control the oxidativelyinduced C-C vs C-O bond formation reactions at a Nicenter. Importantly, the described system merges the known organometallicreactivity of Ni with the biomimetic oxidative transformations resemblingoxygenases and peroxidases, and involving high-valent metal-oxygenintermediates, which is a novel approach that should lead to unprecedentedoxidative catalytic transformations.