Hydrogen atom transfer reactions of transition-metal hydrides. Kinetics and mechanism of the hydrogenation of .alpha.-cyclopropylstyrene by metal carbonyl hydrides

Abstract

The hydrogenation of {alpha}-cyclopropylstyrene (CPS) by a series of metal carbonyl hydrides (MH) gives a mixture of the unrearranged hydrogenation product Ph(CH{sub 3})(c-C{sub 3}H{sub 5})CH (UN) and the rearranged hydrogenation product (E)-Ph(CH{sub 3})C{double bond}CHCH{sub 2}CH{sub 3} (RE). With the exception of HCr(CO){sub 3}Cp, second-order kinetics are found, conforming to the rate law {minus}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){sub 3}Cp at 22{degree}C, the equilibrium constant for this step is K {approximately} 10{sup {minus}12}. The effect of the significant amount of 17-electron {sup {sm bullet}}Cr(CO){sub 3}Cp radical formed in the hydrogenation of CPS by HCr(CO){sub 3}Cp 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.