Ja. Franz et al., Absolute rate expressions for hydrogen atom abstraction from molybdenum hydrides by carbon-centered radicals, J AM CHEM S, 121(42), 1999, pp. 9824-9830
A new family of basis rate expressions for hydrogen atom abstraction by pri
mary, 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 expres
sions 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-met
hylhept-6-en-2-yl, log(k/M-1 s(-1)) = (9.36 +/- 0.18) - (3.19 +/- 0.30)/the
ta (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 ra
dicals 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, PhS
eH, PhSH) with alkyl radicals, together with the very fast benzyl hydrogen-
transfer rate, suggest the relative unimportance of simple enthalpic effect
s and the dominance of steric effects for the early transition-state hydrog
en 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.