Absolute rate expressions for hydrogen atom abstraction from molybdenum hydrides by carbon-centered radicals

A new family of basis rate expressions for hydrogen atom abstraction by primary, secondary, and tertiary alkyl radicals in dodecane and benzyl radical in benzene from the molybdenum hydride Cp*Mo(CO)(3)H and for reactions of a primary alkyl radical with CpMo(CO)3H in dodecane are reported (Cp* = eta(5)-pentamethylcyclopentadienyl, Cp = eta(5)-cyclopentadienyl). Rate expressions for reaction of primary, secondary, and tertiary radical clocks with Cp*Mo(CO)(3)H were as follow: for hex-5-enyl, log(k/M-1 s(-1)) = (9.27 +/- 0.13) - (1.36 +/- 0.22)/theta, theta = 2.303RT kcal/mol; for hept-6-en-2-yl, log(k/M-1 s(-1)) = (9.12 +/- 0.42) - (1.91 +/- 0.74)/theta; and for 2-methylhept-6-en-2-yl, log(k/M-1 s(-1)) = (9.36 +/- 0.18) - (3.19 +/- 0.30)/theta (errors are 2 sigma). Hydrogen atom abstraction from CpMo(CO)(3)H by hex-5-enyl is described by log(k/M-1 s(-1)) = (9.53 +/- 0.34) - (1.24 +/- 0.62)/theta. Relative rate constants for 1 degrees:2 degrees:3 degrees alkyl radicals were found to be 26:7:1 at 298 K. Benzyl radical was found to react 1.4 times faster than tertiary alkyl radical. The much higher selectivities for Cp*Mo(CO)(3)H than those observed for main group hydrides (Bu3SnH, PhSeH, PhSH) with alkyl radicals, together with the very fast benzyl hydrogen-transfer rate, suggest the relative unimportance of simple enthalpic effects and the dominance of steric effects for the early transition-state hydrogen transfers. Hydrogen abstraction from Cp*MO(CO)(3)H by benzyl radicals is described by log(k/M-1 s(-1)) = (8.89 +/- 0.22) - (2.31 +/- 0.33)/theta.