Kinetics of Hydrogen Atom Transfer from (η5-C5H5)Cr(CO)3H to Various Olefins: Influence of Olefin Structure

Treating (η5-C5H5)Cr(CO)3H (1) or (η5-C5H5)Cr(CO)3D (1-d1) 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 kH and kD, the rate constants for H• (D•) transfer from (η5-C5H5)Cr(CO)3H (D) to various olefins. The values of kH and kD vary appreciably with the substituents on the double bond. Phenyl-substituted olefins accept H• more readily than do carbomethoxy-substituted olefins, although the latter accept H• 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• is being transferred decreases kH substantially. On the whole, the rate constants for H• transfer reflect steric effects as well as the stability of the resulting carbon-centered radicals.