KINETICS AND MECHANISMS OF HALOGEN ABSTRACTION REACTIONS OF THE 17-ELECTRON, METAL-CENTERED RADICAL CPCR(CO)(3) WITH ORGANIC HALIDES

This paper describes detailed kinetics investigations of the reactions of the 17-electron compound CpCr(CO)(3), formed by thermal homolysis of the metal-metal-bonded dimer [CpCr(CO)(3)](2), with the non-beta-hydrogen-containing organic halides B1.CH2CN, BrCH(2)CO(2)Me, CF3CH2I, and p-NO2C6H4CH2Br, in addition to less detailed kinetics investigations of the reactions of CpCr(CO)(3) with the homologous series of alkyl iodides MeI, EtI, i-PrI, and t-BuI, with the benzylic bromides p-XC(6)H(4)CH(2)Br (X = H, NO2, CN, CF3, OMe, Me, F, t-Bu), and with the beta-hydrogen-containing substrate PhCHMeBr. In most cases, the rate law followed and the observed relative rates are consistent with rate-determining steps involving halogen atom transfer from carbon to chromium. The resulting organic radicals can then in most cases couple with a second molecule of CpCr(CO)(3) to form the corresponding 18-electron, unstable alkylchromium compounds CpCr(CO)(3)R; in cases of organic radicals containing beta-hydrogen atoms, however, CpCr(CO)(3) can abstract a hydrogen atom to form an olefin plus the metal hydride CpCr(CO)(3)H. The latter in turn can transfer the hydridic hydrogen atom to the organic radicals to form the corresponding aliphatic products.