Kinetics of hydrogen atom transfer from (η5-C5H5)Cr(CO)3H to various olefins:: Influence of olefin structure

Treating (eta(5)-(CH5)-H-5)Cr(CO)(3)H (1) or (eta(5)-C5H5) Cr(CO)(3)D (1-d(1)) with an excess of olefin containing the opposite isotope generally leads to H/D exchange, although hydrogenation is also observed in some cases. Application of an appropriate statistical correction to the observed exchange rate gives k(H) and k(D), the rate constants for H-center dot (D-center dot) transfer from (eta(5)-C5H5) Cr(CO)(3)H (D) to various olefins. The values of k(H) and k(D) vary appreciably with the substituents on the double bond. Phenyl-substituted olefins accept H-center dot more readily than do carbomethoxy-substituted olefins, although the latter accept H-center dot more readily than do alkyl-substituted olefins. A methyl substituent on the incipient radical site increases kH at 323 K by a factor between 5 and 50. A methyl substituent on the carbon to which the H-center dot is being transferred decreases k(H) substantially. On the whole, the rate constants for H-center dot transfer reflect steric effects as well as the stability of the resulting carbon-centered radicals.