The hydrogenation of α-cyclopropylstyrene (CPS) by a series of metal carbonyl hydrides (MH) gives a mixture of the unrearranged hydrogenation product Ph(CH3)(c-C3H5)CH (UN) and the rearranged hydrogenation product (E)Ph(CH3)C=CHCH2CH3 (RE). With the exception of HCr(CO)3Cp, second-order kinetics are found, conforming to the rate law-d[CPS]/dt = k[CPS][MH], The proposed mechanism involves hydrogenation by sequential hydrogen atom transfers from the metal hydride to the organic substrate. The rate-determining step is the first hydrogen atom transfer in which a carbon-centered radical and a metal-centered radical are formed. In the case of HCr(CO)3Cp at 22 °C, the equilibrium constant for this step is K ∼ 10-12. The effect of the significant amount of 17-electron *Cr(CO)3Cp radical formed in the hydrogenation of CPS by HCr(CO)3Cp is accommodated by the kinetic analysis. Since the initially formed carbon-centered radical undergoes first-order ring-opening rearrangement in competition with second-order trapping by MH, analysis of the product ratio as a function of [MH] concentration provides relative rates of hydrogen atom transfer from metal hydrides to a carbon-centered radical. Relative rates of hydrogen atom transfer at 60 °C from MH to 1 are as follows: krel = 1 for HMn(CO)4PPh3, krel = 4 for HMo(CO)3(C5Me5), krel = 93 for HMo(CO)3Cp, krel = 94 for HFe(CO)2(C5Me5). Comparison of the hydrogenation of CPS by HW(CO)3Cp and DW(CO)3Cp indicates that the kinetic isotope effect is inverse (kHW/kDW = 0.55) for the first hydrogen atom transfer but normal (kHW/kDW = 1.8-2.2) for the second hydrogen atom transfer. The first hydrogen atom transfer is endothermic, and its rate is largely influenced by the strength of the M-H bond. Steric effects appear to exert a dominant influence on the rate of the second hydrogen atom transfer, which is exothermic. Kinetic and mechanistic experiments indicate that hydrogenation of 2-cyclopropylpropene by HCr(CO)3Cp also occurs by a radical pathway.
ASJC Scopus subject areas
- Colloid and Surface Chemistry