Absolute Rate Expressions for Hydrogen Atom Abstraction from Molybdenum Hydrides by Carbon-Centered Radicals

Abstract

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)3H and for reactions of a primary alkyl radical with CpMo(CO)3H in dodecane are reported (Cp* = η5-pentamethylcyclopentadienyl, Cp = η5-cyclopentadienyl). Rate expressions for reaction of primary, secondary, and tertiary radical clocks with Cp*Mo(CO)3H were as follow: for hex-5-enyl, log(k/M-1 s-1) = (9.27 ± 0.13) − (1.36 ± 0.22)/θ, θ = 2.303RT kcal/mol; for hept-6-en-2-yl, log(k/M-1 s-1) = (9.12 ± 0.42) − (1.91 ± 0.74)/θ; and for 2-methylhept-6-en-2-yl, log(k/M-1 s-1) = (9.36 ± 0.18) − (3.19 ± 0.30)/θ (errors are 2σ). Hydrogen atom abstraction from CpMo(CO)3H by hex-5-enyl is described by log(k/M-1 s-1) = (9.53 ± 0.34) − (1.24 ± 0.62)/θ. Relative rate constants for 1°:2°:3° 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)3H 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)3H by benzyl radicals is described by log(k/M-1 s-1) = (8.89 ± 0.22) − (2.31 ± 0.33)/θ.