Kinetics, mechanism, and applications of hydrogen atom transfer from transition-metal hydrides

Part 1. Kinetics and mechanism of hydrogen atom transfer from transition-metal hydrides. A value of 59.6 kcal/mol has been measured for the Cr-H bond dissociation energy of C₅Ph₅Cr(CO) ₃H, and the rates and activation parameters for hydrogen atom transfer from C₅Ph₅Cr(CO)₃H and C₅Me ₅Cr(CO)₃H to tris(p-t-butylphenyl)methyl radical in toluene have been measured. These rates are compared to the previously measured rate of the parent chromium compound and other transition-metal hydrides. Knowledge of M-H bond dissociation energies and Arrhenius activation energies for the transfer of a hydrogen atom from the metal to the tris(p- t-butylphenyl)methyl radical allows for an Evans-Polanyi analysis which indicates that sterics controls the rate of such reactions. The reaction of C₅Ph₅Cr(CO)₃H with the stable radical 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) was investigated, and the possibility of using TEMPO as a hydrogen atom transfer catalyst was explored.; Part II. Kinetics and mechanism of catalytic chain transfer in the free radical polymerization of methyl methacrylate mediated by C ₅Ph₅Cr(CO)₃·. The stable metalloradical C₅Ph₅Cr(CO)₃· was shown to be an effective chain transfer catalyst in the free radical polymerization of styrene and methyl methacrylate (MMA). Detailed NMR analysis of the product polymer from the catalytic chain transfer (CCT) polymerization of MMA supports the previously proposed mechanism. Kinetic modeling has allowed for the measurement of the rates of hydrogen atom transfer from a methylisobutyryl radical to C₅Ph₅Cr(CO)₃· and from C₅Ph ₅Cr(CO)₃H back to MMA. The kinetic data allows for an estimation of both the bond dissociation energy of the C-H bond α to the radical center in a methylisobutyryl radical and the heat of formation of the methylisobutyryl radical. The regulation of such CCT processes by the “persistent radical effect” is also discussed.