Exchange of organic radicals with organo-cobalt complexes formed in the living radical polymerization of vinyl acetate

Abstract

Exchange of organic radicals between solution and organo-cobalt complexes is experimentally observed and the reaction pathway is probed through DFT calculations. Cyanoisopropyl radicals from AIBN (2,2′- azobisisobutyronitrile) enter solutions of cobalt(II) tetramesityl porphyrin ((TMP)CoII,• 1) and vinyl acetate (VAc) in benzene and react to produce transient hydride (TMP)Co-H and radicals (•CH(OAc)CH 2C(CH3)2CN (R1•)) that proceed on to form organo-cobalt complexes (TMP)CO-CH(OAC)CH3 (4, Co-R 2) and (TMP)Co-CH(OAc)CH2C(CH3)2CN (3, Co-R1), respectively. Rate constants for cyanoisopropyl radical addition with vinyl acetate and hydrogen atom transfer to (TMP)Co II• are reported through kinetic studies for the formation and transformation of organo-cobalt species in this system. Rate constants for near-degenerate exchanges of radicals in solution with organo-cobalt complexes are deduced from 1H NMR studies and kinetic modeling. DFT computations revealed formation of an unsymmetrical adduct of (TMP)CO-CH(OAC)CH3 (4) with •CH(OAc)CH3 (R 2•) and support an associative pathway for radical interchange through a three-centered three-electron transition state [R⋯CO⋯R]. Associative radical interchange of the latent radical groups in organo-cobalt porphyrin complexes with freely diffusing radicals in solution that is observed in this system provides a pathway for mediation of living radical polymerization of vinyl acetate. © 2008 American Chemical Society.