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

Article abstract of DOI:10.1021/ja991412e

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(kl/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.

Full text of DOI:10.1021/ja991412e

9824
J. Am. Chem. Soc. 1999, 121, 9824-9830
Absolute Rate Expressions for Hydrogen Atom Abstraction from
Molybdenum Hydrides by Carbon-Centered Radicals
James A. Franz,*,† John C. Linehan,*,† Jerome C. Birnbaum,† Kenneth W. Hicks,‡ and
M. S. Alnajjar†
Contribution from the Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352,
and Norfolk State UniVersity, 2401 Corprew AVenue, Norfolk, Virginia 23504
ReceiVed April 29, 1999
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.
Introduction
individual rate constants for reaction of alkyl radicals with
several transition metal hydrides have been reported,7,8,11
Arrhenius expressions have been reported only for abstraction
reactions of the trityl radical with organometallic hydrides.9,10
Whereas main group tin and silicon hydrides have been
widely used in kinetic measurements and organic synthesis,20-22
transition metal hydrides have received relatively little attention
as reagents for kinetic measurements and in free-radical-based
synthetic transformations. Rate expressions for hydrogen ab-
straction from the metal hydride by organic free radicals and
for the corresponding reactions of the resulting metal-centered
radicals with halides and other groups provide basis rates for
design and control of carbon-carbon and carbon-heteroatom
bond formation in radical-based synthetic reactions.23,24 Thus,
we present rate expressions for reactions of primary, secondary,
and tertiary radical clocks and benzyl radical with molybdenum
hydrides. We show that, unlike the less exothermic, but
completely unselective, reactions of thiophenol and phenyl-
selenol with primary, secondary, and tertiary alkyl radicals,25,26
Transition metal hydrides are particularly important chemical
species because of their role in catalytic and stoichiometric
hydrogenation reactions.1-4 Metal-hydrogen bond strengths,5,6
hydrogen atom7-11 and hydride-transfer reactions,12-14 and
radical-mediated reduction of olefins via transition metal
hydrides have been the subject of many studies.15-19 Although
† Pacific Northwest National Laboratory.
‡ Norfolk State University.
(1) Dedieu, A. Transition Metal Hydrides; VCH: New York, 1992.
(2) Moore, D. S.; Robinson, S. D. Chem. Soc. ReV. 1983, 12, 415.
(3) Hlatky, G. G.; Crabtree, R. H. Coord. Chem. ReV. 1985, 65, 1.
(4) Pearson, R. G. Chem. ReV. 1985, 85, 41.
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2529-2537.
(6) Tilset, M.; Parker, V. D. J. Am. Chem. Soc. 1989, 111, 6711-6717.
(7) Bakac, A. Inorg. Chem. 1998, 37, 3548-3552.
(8) Ash, E. A.; Hurd, P. W.; Darensbourg, M. Y.; Newcomb, M. J. J.
Am. Chem. Soc. 1987, 109, 3313-3317.
(9) Eisenberg, D. C.; Lawrie, C. J. C.; Moody, A. E.; Norton, J. R. J.
Am. Chem. Soc. 1991, 113, 4888-4895.
(10) Eisenberg, D. C.; Norton, J. R. Isr. J. Chem. 1991, 31, 55-66.
(11) Kinney, R. J.; Jones, W. D.; Bergman, R. G. J. Am. Chem. Soc.
1978, 100, 7902-7915.
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(17) Connolly, J. W. Organometallics 1984, 3, 1333-1337.
(18) Garst, J. F.; Bockman, T. M.; Batlaw, R. J. Am. Chem. Soc. 1986,
108, 1689-1691.
(19) Ungvary, F.; Marko, L. Organometallics 1982, 1, 1120-1125.
(20) Curran, D. P. Synthesis 1988, 6, 417-439.
(21) Baguley, P. A.; Walton, J. C. Angew. Chem., Int. Ed. Engl. 1998,
37, 7, 3072-3082.
(12) Cheng, T.-Y.; Brunschwig, B. S.; Bullock, R. M. J. Am. Chem. Soc.
1998.
(13) Darensbourg, M. Y.; Ash, C. E. AdV. Organomet. Chem. 1987, 27,
1-50.
(14) Kao, S. C.; Spillett, C. T.; Ash, C. L. R.; Park, Y. K.; Darensbourg,
M. Y. Organometallics 1985, 4, 83-91.
(15) Bullock, R. M.; Samsel, E. G. J. Am. Chem. Soc. 1990, 112, 6886-
6898. See also the related preliminary communication: Masnovi, J.; Samsel,
E. G.; Bullock, R. M. J. Chem. Soc., Chem. Commun. 1989, 1044. The
reversibility of the rearrangement of cyclopropylbenzyl and related radicals
studied by Bullock et al. has been pointed out (Bowry, V. W.; Lusztyk, J.;
Ingold, K. U. J. Chem. Soc., Chem. Commun. 1990, 923). This reversibility
complicates quantitative interpretation of rearrangement/competition kinetic
measurements in the reference cited above.
(22) Davies, A. G. Organotin Chemistry; Wiley-VCH: Weinheim, 1997.
(23) Curran, D. P. AdV. Free Radical Chem. 1990, 1, 121-157.
(24) Curran, D. P. Synthesis 1988, 7, 489-513.
(25) Franz, J. A.; Bushaw, B. A.; Alnajjar, M. S. J. Am. Chem. Soc 1989,
111, 268-275.
(26) Newcomb, M.; Choi, S.-Y.; Horner, J. H. J. Org. Chem. 1999, 64,
1225.
10.1021/ja991412e CCC: $18.00 © 1999 American Chemical Society
Published on Web 10/13/1999

Rate Expressions for H Abstraction from Mo Hydrides
J. Am. Chem. Soc., Vol. 121, No. 42, 1999 9825
Scheme 1
Figure 1. Plot of un-rearranged (1-heptene) to rearranged reduced
hydrocarbon (cis-1,2-dimethylcyclopentane) as a function of hydride
donor concentration at 373 K.
Scheme 2
Figure 2. Arrhenius plots for the abstraction of a hydrogen atom from
molybdenum hydrides by primary, secondary, and tertiary alkyl radicals
in dodecane. The abstraction from Cp*Mo(CO)3H by hex-5-enyl (9),
by hept-6-en-2-yl (2), and by 2-methylhept-6-en-2-yl (() and the
abstraction from CpMo(CO)3H by hex-5-enyl (4) are shown.
Cp*Mo(CO)3H exhibits appreciable selectivity in abstraction
reactions, reflecting a trend, primary > secondary > benzyl >
tertiary, that is predominately steric, rather than enthalpic, in
origin. The present kinetic results offer a new family of basis
rate expressions for hydrogen-transfer reactions of alkyl and
benzyl radicals for use in competition kinetics and the design
of radical-based synthetic strategies.
unrearranged product (1-heptene, E in Scheme 1) to the major
rearranged product (cis-1,2-dimethylcyclopentane, D1 in Scheme
1) vs molybdenum hydride concentrations at short (<10%)
extent of conversion of hydride. The values of kabs at each
temperature were calculated from the product of the slope of
Results
Alkyl Radical Clock Kinetics. The rates of hydrogen atom
abstraction from Cp*Mo(CO)3H and CpMo(CO)3H by alkyl
radicals were determined using the appropriate alkenyl radical
clocks (Scheme 1).27 For primary and tertiary clock radicals,
an integrated rate expression which accounts for changing
hydride concentration was employed to determine the relative
rate constants for radical clock rearrangement vs abstraction,
the plot of E/D1 vs Cp*Mo(CO)3H concentrations, slope ) kabs
/
kre, with values of kre available from the literature Arrhenius
values.28,29 An example of a plot of the ratio of unrearranged
to rearranged product (E/D1, Scheme 1) as a function of
molybdenum hydride concentration at short extent of hydride
conversion for the reaction of hept-6-en-2-yl with Cp*Mo-
(CO)3H at 373 K is shown in Figure 1. This treatment corrects
for the presence of secondary hydrogen-donating processes,
represented by k′, where k′ ) kabs′[DH′]. Here, DH′ is an
adventitious hydrogen donor and kabs′ is the associated rate
constant for abstraction. A near-zero intercept k′ indicates the
lack of significant hydrogen sources in the solvent, or other
alternate sources of the unrearranged hydrocarbon not propor-
tional to the primary donor concentration.
krel, where krel ) kre/kabs. The abstraction rate constants, kabs
,
were determined from ratios of concentrations of unrearranged,
reduced hydrocarbon to rearranged hydrocarbon using values
of kre from Arrhenius rate expressions for the clock reactions
available in the literature.28,29 Both alkenyl bromides and tert-
butyl peresters of alkenoic acids were used to generate the
desired primary and tertiary radicals in reactions with Cp*Mo-
(CO)3H (Scheme 2). The ring-unsubstituted hydride CpMo-
(CO)3H was found to react rapidly at room temperature with
peresters to produce exclusively alkenoic acids and tert-butyl
alcohol, and thus bromide radical clock precursors rather than
perester radical precursors were employed with this hydride.
The rates of abstraction by a secondary radical clock from
Cp*Mo(CO)3H were determined using the alkenyl bromide
secondary clock precursor by a linear fit of the ratio of
The Arrhenius plots of data for the reaction of Cp*Mo(CO)3H
with primary, secondary, and tertiary radicals are presented in
Figure 2. These plots include results from both alkenyl bromide
and perester radical precursors for the primary and tertiary
radicals, while the secondary radical rate was determined using
the bromide clock precursor. The Arrhenius and Eyring
parameters for hydrogen abstraction from Cp*Mo(CO)3H by
alkyl radicals are presented in Table 1.
(27) Griller, D.; Ingold, K., U. Acc. Chem. Res. 1980, 13, 317-323.
(28) Chatgilialoglu, C.; Ingold, K. U.; Scaiano, J. C. J. Am. Chem. Soc.
1981, 103, 7739-7742.
(29) Lusztyk, J.; Maillard, B.; Deycard, S.; Lindsay, D. A.; Ingold, K.
U. J. Org. Chem. 1987, 52, 3509-3514.
Both perester and bromide precursors were used over the
entire temperature range and gave identical results within
statistical error limits. Unlike the rapid iodine atom-exchange
reactions between radicals and organic iodides, which may

9826 J. Am. Chem. Soc., Vol. 121, No. 42, 1999
Franz et al.
Table 1. Arrheniusa and Eyring Parameters for the Reaction of Primary, Secondary, and Tertiary Radical Clocksb and Benzyl Radicals with
Cp*Mo(CO)3H and Primary Radicals with CpMo(CO)3H in Dodecanec
log A
∆Sq (eu)
(Tm, K)
Ea
∆Hq
(kcal/mol)
k (M-1 s-1), temp range mean temp
radical
hex-5-enyl
hept-6-en-2-yl
hydride
(M-1 s-1
)
(kcal/mol)
298 K
(K)
(Tm, K)
Cp*Mo(CO)3H 9.27 ( 0.13 -19.8 ( 1.7 1.36 ( 0.22 0.61 ( 0.20
Cp*Mo(CO)3H 9.12 ( 0.42 -19.3 ( 2.3 1.91 ( 0.74 1.15 ( 0.71
1.9 × 108
5.2 × 107
1.1 × 107
4.2 × 108
1.5 × 107
319-400
318-458
343-448
318-458
297-373
384
387
390
399
325
2-methylhept-6-en-2-yl Cp*Mo(CO)3H 9.36 ( 0.18 -18.2 ( 1.7 3.19 ( 0.30 2.42 ( 0.30
hex-5-enyl
benzyl
CpMo(CO)3H
9.53 ( 0.34 -17.5 ( 1.2 1.24 ( 0.62 0.47 ( 0.63
Cp*Mo(CO)3H 8.89 ( 0.22 -20.0 ( 2.0 2.31 ( 0.33 1.65 ( 0.39
a Eyring and Arrhenius parameters are related by ∆Hq ) -R(∂ ln(kabs/T)/∂(1/T)) ) Ea - RTm, where Tm is the mean temperature of the kinetics,
and ∆Sq ) R(ln(hkT /kbTm)) + ∆H/Tm ) R ln(hA/ekbTm), where kb is Boltzmann’s constant, h is Planck’s constant, kTm is the rate constant from the
Arrhenius expressiomn at mean temperature, and e is the base of natural logarithms. b The radical cyclization rates used in this work are (ref 29): for
hex-5-enyl, log(kc,5/s-1) ) 10.42 - 6.85/θ, θ ) 2.303RT kcal/mol; for hept-6-en-2-yl, cyclization to cis-2-methylcyclopentylmethyl is given by
log(k6,cis/s-1) ) 9.79 - 6.50/θ and to trans-2-methylcyclopentylmethyl is given by log(k5,trans/s-1) ) 9.92 - 7.44/θ; for 2-methyl-hept-en-2-yl,
log(ke,5/s-1) ) 10.0 - 6.1/θ. c The reaction of benzyl radical with Cp*Mo(CO)3H was carried out in benzene. Errors are 2σ.
compromise unrearranged/rearranged hydrocarbon ratios30 and
affect their use in synthetic transformations,31 bromine atom
exchange between the alkenyl bromides and the cyclized radical
is unimportant, reflected in the present results, that show close
agreement between perester and bromide precursors.
The Arrhenius plot for hydrogen abstraction from CpMo-
(CO)3H by hex-5-enyl is also shown in Figure 2, and the rate
expression for abstraction of hydrogen by the primary clock
from CpMo(CO)3H is presented in Table 1.
Benzyl Radical Kinetics. Rate constants for hydrogen
abstraction by the benzyl radical from Cp*Mo(CO)3H were
Figure 3. Product evolution of toluene (9) and bibenzyl ([) at 373
K at initial concentrations of [Cp*Mo(CO)3H], 0.000105 M, and
[dibenzyl ketone], 0.02 M. Rate constants are determined using data
at 0.5, 1.0, and 1.5 s and eq 1c. The solid lines are the predicted
concentrations of toluene and bibenzyl from minimizing the sum of
the squares of residuals for toluene and bibenzyl in a numerical
integration of Scheme 3 using a rate constant for photolysis of DBK,
determined by a competition of abstraction to form toluene with
benzyl radical self-termination to produce bibenzyl during
photolysis of dibenzyl ketone (DBK) in benzene solutions. The
benzyl radical was produced by broadband visible UV irradiation
(>330 nm) of optically dilute solutions of the ketone and
hydride, as shown in Scheme 3. Figure 3 shows the time-
dependent evolution of products during photolysis of a 0.02 M
solution of DBK in benzene at 373 K.
Under conditions of constant rate of photolysis of dibenzyl
ketone, the absence of photolysis of starting hydride, and no
significant absorption of light by photoproducts, the relationship
between toluene, bibenzyl (BB), and the initial hydride con-
centration, [MoH]0, that accounts for consumption of MoH is
given by eq 1a. At low conversion of hydride, the exponent
term, kabs[BB]1/2∆t1/2/kt1/2 in eq 1a is small, and simplifies to
eqs 1b and 1c, where [MoH]av represents the average concentra-
tion of the molybdenum hydride over a period of photolysis
time, ∆t:
8.7 × 10-6 M/s, kt ) 0.5kct ) kt′ ) 6.2 × 109 M-1 s-1, giving kabs
)
3.44 × 107 M-1 s-1. The agreement of the two approaches is due to
the near linearity of the product curves through about 50% conversion
of hydride. All of the points of Figure 3 can be alternatively fit to eq
1a, allowing kabs and the initial hydride concentration (MoH) to vary
in a nonlinear least-squares optimization. This results in a predicted
initial concentration of hydride of 9.13 × 10-5 M and a rate constant
kabs ) 3.46 × 107 M-1 s-1. Alternatively, the first three points (0.5,
1.0, and 1.5 s) can be fit to eq 1a. This results in values of kabs identical
to those resulting from an average of 0.5, 1.0, and 1.5 s points using
eq 1c.
Scheme 3
abs[BB]1/2∆t1/2/kt1/2
[toluene] ) [MoH]0(1 - e-k
)
(1a)
(1b)
kabs[BB]1/2∆t1/2
[toluene] ) [MoH]0
kt1/2
(
)
[toluene]kt1/2
[BB]1/2[MoH]av∆t1/2
kabs
)
(1c)
In practice, conversion of hydride up to 35% leads to less than
about 10% error in rate constant kabs using eq 1c. In the present
experiments, the yield of toluene was used to calculate an
average hydride concentration at each photolysis time. Com-
bining experimental values of bibenzyl and toluene at 0.5, 1.0,
and 1.5 s photolysis time with an independent value of the rate
constant for self-termination of bibenzyl, kt (see below), provided
values of kabs at each temperature.
The rate of photolysis of DBK employed in the kinetic
measurements was in the range [DBK] × (7-9) × 10-6 s-1
,
corresponding to a steady-state concentration of benzyl radical
in the range (2-5) × 10-8 M. The rate of formation of toluene
and bibenzyl was found to be approximately linear during the
first 2 s of photolysis. The evolution of toluene and bibenzyl
with time was examined by numerical integration of the
(30) Newcomb, M.; Curran, D. P. Acc. Chem. Res. 1988, 21, 206-214.
(31) Curran, D. P. NATO ASI Ser., Ser. C 1989, 260 (Free Radicals
Synth. Biol.), 37-51.

Rate Expressions for H Abstraction from Mo Hydrides
J. Am. Chem. Soc., Vol. 121, No. 42, 1999 9827
equations of Scheme 3. In general, it was found that toluene
and bibenzyl formation was accurately predicted through about
50% conversion of hydride using the experimentally measured
initial concentration of hydride in the numerical integration. At
longer reaction times, toluene production is less than that
predicted by typically 10% (see Figure 3). This may be due to
several circumstances, including (1) the presence of less hydride
initially in the photolysis, (2) partial photolysis of the hydride
to a non-hydrogen-donating species during the course of the
reaction, (3) a decrease in the rate of photolysis due to light
absorption by the products, or (4) more complex photochemistry
of the system at high conversion. The photochemistry of CpMo-
(CO)3H involves CO loss and subsequent reactions of the 16-
electron CpMo(CO)2H.32 Photolysis of the dimer [CpMo(CO)3]2
results in CO loss, giving Cp2Mo2(CO)5 or metal-metal bond
homolysis to form the radical CpMo(CO)3.33 In the benzyl
radical experiments, a 1-kW high-pressure (Pyrex-filtered) xenon
arc lamp was used. Those conditions led to a small extent of
photolysis (<10%) of the hydride during the kinetic experi-
ments, whereas in the experiments utilizing halide radical clock
precursors, the Pyrex-filtered, diffuse light of a 100-W long-
wavelength sunlamp (λmax ) 365 nm) was employed, which
led to significant photolysis of the hydride only after prolonged
photolysis. (In the halide radical clock precursor experiments,
radical initiation most probably occurred via photolysis of small
amounts of the dimer [Cp*Mo(CO)3]2 present in the hydride
Cp*Mo(CO)3H.) Kinetic modeling of case 1, by reducing the
initial hydride concentration, or of case 2, by adding an
additional decay pathway in the numerical integration of the
equations of Scheme 3 (khν in Scheme 3), produced an overall
better fit of the data through complete consumption of hydride
but poorer agreement with the experimental concentrations of
hydride used and poorer agreement with initial rates of reaction
calculated using eq 1c. Careful analysis of material balances
revealed no more than 6-7% decrease in the rate of photolysis
of DBK over 5 s; thus, case 3 was found insignificant. Because
of the small discrepancy at longer photolysis times, we preferred
to use eq 1c for data obtained through typically 1.5 s, or less
than one half-life of hydride conversion, to avoid ambiguities
of modeling the system at high conversion. For times between
0.5 and 1.5 s, values of kabs found by numerical integration
(Scheme 3, neglecting khν) were found to be in excellent
agreement with the rates calculated with eq 1c from initial
product ratios ([PhCH3]/[PhCH2CH2Ph]1/2) when the kinetic
simulation employs kabs as the only variable and values of kt,
rate of photolysis of DBK, and hydride concentration are
experimental constants. The Arrhenius plot for reaction of benzyl
radical with Cp*Mo(CO)3H is shown in Figure 4, and the rate
expression is presented in Table 1.
Figure 4. Arrhenius data for abstraction of hydrogen by benzyl radical
from Cp*Mo(CO)3H, calculated using eq 1c from toluene and bibenzyl
from photolysis times of 0.5-1.5 s in benzene.
tertiary, secondary, and primary alkyl radicals. Relative rates
for tertiary, secondary, and primary and benzyl radicals are
shown in Table 2 for thiophenol,25 phenylselenol,26 and tribu-
tylstannane. The S-H and Se-H bond strengths of thiophenol
and phenylselenol are 80 ( 2 kcal/mol34-36 and 78 ( 2 kcal/
mol,37 respectively. The M-H bond strengths of Cp*Mo-
(CO)3-H and CpMo(CO)3-H are 69 and 70 kcal/mol, respec-
tively.5,6,38 Table 2 illustrates that the relative magnitudes of
rate constants and activation barriers for highly exothermic
hydrogen abstraction reactions are not related by simple enthalpy
considerations. For main group hydrides, stabilized radicals react
more slowly than simple alkyl radicals. Thus, benzyl radical
abstracts hydrogen atom about 2-300 times more slowly from
PhSH and Bu3SnH than alkyl radicals, reflecting the trend in
C-H bond dissociation energies for primary (1-propyl C-H),
secondary (2-propyl C-H), tertiary (tert-butyl C-H), and
benzyl (toluene R C-H) radicals: 101, 99, 95, and 89 kcal/
mol, respectively.39-41 Thus, the selectivity of alkyl radicals in
the much more highly exothermic reactions of Cp*Mo(CO)3H
with alkyl radicals is quite remarkable compared to the
selectivity in reactions of primary, secondary, and tertiary alkyl
radicals with PhSH, Bu3SnH, and PhSeH. The relative reactivity
of benzyl radical with Cp*Mo(CO)3H is also surprising by
comparison with the main-group hydrides, exhibiting a rate
constant higher than that of tertiary alkyl radical. Since the
benzyl radical is slower than the secondary alkyl radical,
enthalpic effects are almost, but not completely, removed. The
results of Table 2 suggest that the selectivity of the hydrogen
abstraction reactions of Cp*Mo(CO)3H can be attributed in
significant extent to metal ligand/radical steric effects. In
comparing the results of thiophenol and Cp*Mo(CO)3H in Table
2, it will be noted that the reaction of benzyl radical with
Cp*Mo(CO)3H (∆H° ≈ -20 kcal/mol) is similar in exother-
micity to the reaction of secondary (∆H° ≈ -21 kcal/mol) or
tertiary radical (∆H° ≈ -19 kcal/mol)39-41 with PhSH.
Several examples of the steric effects of pentamethyl substitu-
tion of the cyclopentadiene group in homolytic displacement
and atom-transfer chemistry can be cited. In the present work,
Discussion
Alkyl and Benzyl Radical Selectivities in Hydrogen Atom
Abstractions of Molybdenum Hydrides. The rate constant for
hydrogen atom abstraction by a primary radical from Cp*Mo-
(CO)3H at 298 K (1.9 × 108 M-1 s-1) is similar in magnitude
to rates of abstraction of alkyl radicals from thiophenol: e.g.,
for the reaction of n-butyl radical with thiophenol, kabs ) 1.4
× 108 M-1 s-1 and is about an order of magnitude less than
that for phenylselenol.25,26 Thiophenol and phenylselenol have
the useful properties (as kinetic radical trapping agents) of
exhibiting, within experimental error, no selectivity between
(34) Colussi, A. J.; Benson, S. W. Int. J. Chem. Kinet. 1977, 9, 925.
(35) McMillen, D. F.; Golden, D. M. Annu. ReV. Phys. Chem. 1982, 33,
493.
(36) Bordwell, F. G.; Xian-Man, Z.; Satish, A. V.; Cheng, J.-P. J. Am.
Chem. Soc. 1994, 116, 6605-6610.
(37) Leeck, D. T.; Li, R.; Chyall, L. J.; Kenttaemaa, H. I. J. Phys. Chem.
1996, 100, 6608-6611.
(38) Tilset, M. P.; Vernon B. J. Am. Chem. Soc. 1990, 112, 2843.
(39) Tsang, W. In Heats of Formation of Organic Free Radicals by
Kinetic Methods; Simo˜es, J. A. M., Greenberg, A., Liebman, J. F., Eds.;
Blackie Academic & Professional: New York, 1996; Vol. 4, pp 22-58.
(40) Lias, S. G.; Bartness, J. E.; Liebman, J. F.; Holmes, J. L.; Levin,
R. D.; Mallard, W. G. J. Phys. Chem. Ref. Data 1988, 17, Suppl. 1.
(41) Seakins, P. W.; Pilling, M. J.; Niiranen, J. T.; Gutman, D.;
Krasnaperov, L. N. J. Phys. Chem. 1992, 96, 9847.
(32) Mahmoud, K. A. R.; Anthony J.; Alt, H. G. J. Chem. Soc., Dalton
Trans. 1984, 187-197.
(33) Peters, J. G.; Michael W.; Turner, J. J. Organometallics 1995, 14,
1503-1506.

9828 J. Am. Chem. Soc., Vol. 121, No. 42, 1999
Franz et al.
Table 2. Absolute Rates and Selectivities for Reaction of Alkyl and Benzyl Radicals with Hydrides
absolute rates of hydrogen abstraction at 298 K (M-1 s-1
)
relative rates of hydrogen abstraction at 298 K
1° radical 2° radical 3° radical benzyl
X-H BDE
kcal/mol)
hydrogen donor
1° radical
2° radical
3° radical
benzyl
Cp*Mo(CO)3Ha
Bu3SnHb
PhSHc
70e
1.9 × 108
2.4 × 106
1.1 × 108
1.3 × 109
5.2 × 107
1.8 × 106
1.05 × 108
1.1 × 109
1.1 × 107
1.8 × 106
1.14 × 108
1.3 × 109
1.5 × 107
3.6 × 104
3.1 × 105
-
25.5
1.3
1.0
7.0
1.0
0.9
(1)
(1)
(1)
(1)
1.4
74,f 78i
80g
2.0 × 10-12
2.7 × 10-2
-
PhSeHd
78h
1.0
0.85
a This work. Alkyl radical rates were measured in alkane solvent, and benzyl radical rates were determined in benzene; see Experimental Section.
b Rate determined in alkane-trialkylphosphine/di-tert-butylperoxide media; see ref 28. c Rate measurements in hydrocarbons; see ref 25. d Rate
measurements in tetrahydrofuran: Newcomb, M.; Choi, S.-Y.; Horner, J. H. J. Org. Chem. 1999, 64, 1225. e References 5 and 6. f Burkey, T. J.;
Majewski, M.; Griller, D. J. Am. Chem. Soc. 1986, 108, 2218-2221. Jackson, R. A. J. Organomet. Chem. 1979, 166, 17. g References 34-36.
h Reference 37. i Laarhaven, L. J. J.; Mulder, P.; Wayner, D. D. M. Acc. Chem. Res. 1999, 32, 342-349.
Scheme 4
radicals on the peresters to produce the molybdenum carboxylate
complexes. CpMo(CO)3H reacted completely with all three
peresters at room temperature before the samples could be sealed
for kinetic experiments, whereas no significant reduction of the
peresters by Cp*Mo(CO)3H occurred at room temperature. At
higher temperatures, both the induced decomposition pathway
and the desired unimolecular decomposition of the peresters
occur with Cp*Mo(CO)3H. In the kinetic studies employing the
peresters, a correction for hydride consumption through induced
decomposition was made by careful analysis of the excess yield
of tert-butyl alcohol formed over reduced radical products. The
trimethylsilyl derivative of the tertiary acid, 2-methylhept-6-
enoic acid, was identified after treatment of the reaction mixture
with bis(trimethylsilyl)acetamide (BSA) and pyridine. The
amount of the silylated acid corresponded closely to the
difference between the alcohol concentration and the total
hydrocarbon concentration. It is assumed that the product
alkenoic acid was present in the solution as a molybdenum
carboxylate complex.47,48
CpMo(CO)3H reacts an additional factor of 1.9 faster than
Cp*Mo(CO)3H with primary radicals. Pentamethyl substitution
at Cp has been shown to attenuate the rate of reaction of hydrides
in reactions with tris(p-tert-butylphenyl)methyl radical.9 Ligand-
based steric effects were noted on the rates of hydrogen atom
abstraction from macrocyclic rhodium hydrides by methyl
radicals.7 Hydrogen abstraction from PhSH and PhSeH is
unselective, because the single ligand of the divalent sulfides
and selenides presents minimal steric interaction with the
attacking radical centers. Lacking appreciable steric interaction
of the selenide with substituents about the alkyl radical, alkyl
radicals react with PhSeH at partially diffusion-controlled rates,
independent of the exothermicity of the reaction for primary,
secondary, and tertiary alkyl radicals.42-46 In general, more
pronounced steric effects are typically observed for higher
coordination number transition metal complexes in homolytic
displacement reactions than for main-group organometallic
radicals. Appropriate choice of ligand on tin hydrides and
reduced substrate leads to appreciable stereoselectivity in tin
hydride-based reductions,21 and the replacement of a carbonyl
ligand by a bulkier ligand decreased the rate of hydrogen atom
transfer significantly (by 200-fold) for HMn(CO)5 (Mn-H BDE
) 68 kcal/mol) vs HMn(PEtPh2)(CO)4 (Mn-H BDE ) 71 kcal/
mol) at 298 K.9
The efficiency of the induced decomposition pathway of the
peresters in reaction with Cp*Mo(CO)3H decreased with R-alkyl
substitution of the peresters. The perester precursor of hex-5-
enyl radical underwent 99% reduction at 97 °C, the secondary
96% at 72 °C, and the tertiary 70% at 70 °C. At higher
temperatures (>120 °C), the rate of unimolecular decomposition
of the tertiary and secondary peresters is sufficiently fast to
provide ample radical production compared to the induced
decomposition pathway of reduction by Cp*Mo(CO)3H. Note
that the site of the reaction of molybdenum radicals, carbonyl
or peroxidic oxygen, is not specified in Figure 4. Although
•
CpMo(CO)3 radical is known to react at the carbonyl groups
of quinones,49,50 reactions of alkyl and other radicals with
peresters have been established to occur at the peroxidic
oxygen.51 Whether attack occurs at carbonyl or peroxidic
oxygen, the efficiency of the induced decomposition pathway
suggests that pyridinethiocarboxylic acid esters, employed as
convenient radical precursors,26 will also be useful as radical
precursors in conjunction with the molybdenum hydrides.
Induced Decomposition of Peresters by Molybdenum
Hydrides. The peresters used in this study, tert-butyl 2,2-
dimethylperhept-6-enoate, tert-butyl 2-methylperhept-6-enoate,
and tert-butyl perhept-6-enoate, were found to undergo reaction
with Cp*Mo(CO)3H to give the corresponding molybdenum
alkenoate acid complexes in competition with unimolecular
thermal decomposition of the perester to give alkyl radicals
(Scheme 4). The induced decomposition is most probably a
chain reaction, involving attack of the molybdenum-centered
Conclusions
A new family of rate expressions using transition metal
hydrides has been developed for use in competition kinetics
and radical-based synthetic transformations, and for estimation
(42) Newcomb, M.; Manek, M. B. J. Am. Chem. Soc. 1990, 112, 9662-
9663.
(47) Cotton, F. A.; Darensbourg, D. J.; Kolthammer, B. W. S. J. Am.
Chem. Soc. 1981, 103, 398-405.
(43) Newcomb, M.; Johnson, C. C.; Manek, M. B.; Varick, T. R. J. Am.
Chem. Soc. 1992, 114, 10915-10921.
(48) Garner, C. D.; Hughes, B. AdV. Inorg. Chem. Radiochem. 1975,
17, 1.
(44) Martin-Esker, A. A.; Johnson, C. C.; Horner, J. H.; Newcomb, M.
J. Am. Chem. Soc. 1994, 116, 9174-9181.
(49) Hanaya, M.; Tero-Kubota, S.; Iwaizumi, M. Organometallics 1988,
7, 1500-1504.
(50) Hanaya, M.; Kwaizumi, M. Organometallics 1989, 8, 672-676.
(51) Ingold, K. U.; Roberts, B. P. Free-Radical Substitution Reactions;
Bimolecular Homolytic Substitutions (SH2 Reactions) at Saturated Multi-
Valent Atoms; Wiley-Interscience: New York, 1971.
(45) Le Tadic-Biadatti, M. H.; Newcomb, M. J. Chem. Soc., Perkin
Trans. 2 1996, 1467-1473.
(46) Newcomb, M.; Varick, T. R.; Ha, C.; Manek, M. B.; Yue, X. J.
Am. Chem. Soc. 1992, 114, 8158-8163.

Rate Expressions for H Abstraction from Mo Hydrides
J. Am. Chem. Soc., Vol. 121, No. 42, 1999 9829
pathway of the peresters forming tert-butyl alcohol. For halide radical
precursors, f ) 1. For peresters, 2 equiv of hydride is consumed for
each event of unimolecular perester decomposition (eq 4), or 1 equiv
of hydride per chain-reaction-induced decomposition (eqs 5a,b, etc.).
of fundamental steps in hydrogen transfer involving the
molybdenum hydride group in the context of catalysis. The rate
constants for hydrogen abstraction from Cp*Mo(CO)3H and
CpMo(CO)3H by alkyl radicals, and the efficiency of induced
decomposition reactions of the corresponding molybdenum
radicals Cp*Mo(CO)3 and CpMo(CO)3 in reactions with
peresters, have been shown to be influenced by the steric bulk
of the metal ligands and the R-alkyl substituents on the radical
center, or by the effects of R-alkyl substitution of the perester.
The influence of steric interactions on the rates of highly
exothermic reactions of the hydrides with carbon-centered
radicals is generally recognized.10
•
•
RCO3Bu-t + 2Cp*Mo(CO)3H f
R-H + t-BuOH + CO2 + [Cp*Mo(CO)3]2 (4)
RCO3Bu-t + Cp*Mo(CO)3• f Cp*Mo(CO)3O2CR + t-BuO• (5a)
t-BuO• + Cp*Mo(CO)3H f t-BuOH + Cp*Mo(CO)3 (5b)
•
Product concentrations are corrected for temperature-dependent density
changes.59
Experimental Section
Rate Constants for Reaction of Benzyl Radical with Cp*Mo-
(CO)3H. Pyrex reaction tubes containing solutions of 0.02 M DBK,
10-4 M Cp*Mo(CO)3H, and 10-3 M tert-butylbenzene were freeze-
thaw degassed, sealed, and photolyzed with a water-filtered 1-kW high-
pressure xenon arc lamp for controlled periods of time (0.50-5.00 (
0.005 s) using a Uniblitz model 225L0AOT522952 computer-controlled
electronic shutter. Reagent concentrations were corrected for solvent
vapor pressure and density.59,60 Rate constants for hydrogen abstraction
were calculated using eq 1c at 0.5-1.5 s photolysis times, using the
measured product concentrations, the calculated average molybdenum
hydride donor concentration, and the benzyl self-radical termination
rate, 2kt, calculated from the expression ln(2kt/M-1 s-1) ) 27.23 -
2952.5/RT. This expression was calculated using the von Smoluchowski
equation modified, eq 6, using the Spernol-Wirtz modification of the
Debye-Einstein equation, eq 7, where f in eq 7 is the SW microfriction
factor.61
CpMo(CO)3H,52 Cp*Mo(CO)3H,52 6-bromoheptene,53 2,2-dimeth-
ylhept-6-enoic acid,54 2-chloro-2-methylhept-6-ene,55 and 2-bromo-2-
methylhept-6-ene56 were synthesized using modified literature proce-
dures. 2-Methylhept-6-enoic acid and hept-6-enoic acid were synthesized
in a manner similar to that used for 2,2-dimethylhept-6-enoic acid.54
tert-Butyl 2,2-Dimethylperhept-6-enoate. The acid chloride was
produced by refluxing 2,2-dimethylhept-6-enoic acid with excess SOCl2
in CH2Cl2, to give the acid chloride, >95% purity by NMR. 1H NMR,
CDCl3: δ 1.31 (6H, s), 1.41 (2H, m), 1.68 (2H, m), 2.10 (2H, m),
5.03 (2H, m), 5.81 (1H, m). 13C NMR: δ 23.85 (1C), 25.17 (2C), 33.82
(1C), 39.74 (1C), 52.79 (1C), 115.10 (1C), 137.95 (1C), 180.23 (1C).
The acid chloride was converted to the perester by reaction with tert-
butyl hydroperoxide in CH2Cl2 at 0 °C. Chromatography of the crude
perester on silica gel using 2.5% methyl tert-butyl ether in pentane
gave a fraction containing pure liquid perester, 0.48 g (17% yield).
tert-Butyl perhept-6-enoate and tert-butyl 2-methylhept-6-enoate
were similarly prepared as clear liquids in 25-55% isolated yields.
tert-Butyl 6-perheptenoate: 13C NMR (CDCl3) δ 24.20 (1C), 25.93
(3C), 28.01 (1C), 30.88 (1C), 33.05 (1C), 83.06 (1C), 114.71 (1C),
137.93 (1C), 170.81 (1C).
2kt ) (8π/1000)σFDAB
DAB ) kT/6πrAηf
N
(6)
(7)
tert-Butyl 2-methylperhept-6-enoate: 13C NMR (CDCl3) δ 173.89
(C(O)O), 138.17 (CH2dCHs), 114.81 (CH2dCHs), 83.22 (Os
C(CH3)), 37.21 (sC(CH3)Hs), 33.38 (sCH2s), 33.16 (sCH2s),
26.39 (sCH2s), 26.12 (C(CH3)3), 17.40 (CH(CH3)).
Fischer and co-workers62-64 have measured self-termination rate
expressions for benzyl radical in cyclohexane, ln(2kt/M-1 s-1) ) 26.2
- 2497.6/RT, and in toluene, ln(2kt/M-1 s-1) ) 27.05 - 2797.4/RT.
For self-termination of benzyl in hexane, we developed the expression
ln(2kt/M-1 s-1) ) 26.0 - 1803.6/RT.65 Parameters for estimation of
the rate expression employed in these kinetics are presented.66 The
Kinetic Procedure for Alkyl Radical Clock Kinetics. Solutions
of halide or perester radical precursor, hydride, and internal GC standard
were freeze-thaw degassed, sealed in Pyrex tubes, and heated through
four half-lives of the peresters or heated and photoinitiated (halide
(59) Reid, R. C.; Prausnitz, J. M.; Sherwood, T. K. The Properties of
Liquids and Gases; McGraw-Hill: New York, 1977.
(60) Weast, R. C.; Astle, M. J. CRC Handbook of Chemistry and Physics;
CRC Press: Boca Raton, FL, 1982-83.
(61) Fischer, H.; Paul, H. Acc. Chem. Res. 1987, 20, 200-266.
(62) Lehni, M.; Schuh, H.; Fischer, H. Int. J. Chem. Kinet. 1979, 11,
705-713.
(63) Claridge, R. F. C.; Fischer, H. J. Phys. Chem. 1983, 87, 1960-
1967.
(64) Huggenberger, C.; Fischer, H. HelV. Chim. Acta 1981, 64, 338-
353.
(65) Franz, J. A.; Suleman, N. K.; Alnajjar, M. S. J. Org. Chem. 1986,
51, 19.
precursors) with a tungsten filament lamp. The usual integrated rate
57,58
expression,
eq 2, with a correction factor for consumption of
hydride, f, was used to determine the relative rate constants for the
radical clock experiments:
E + D ) 1/f(B0 + r)(1 - e-fD/r
)
(2)
(3)
[t-BuOH]
f ) 1 +
(E + D)
(66) Diffusion coefficients DAB, where DAB ) kT/6πrAηf, were calculated
for estimation of the self-termination rate of benzyl radical in benzene using
the following data. For the radical model, toluene: mp 178 K, bp 383.8 K,
MW 92.14, F ) 6.02 × 10-8 cm (average of van der Waals (Edward, J. T.
J. Chem. Educ. 1970, 47 (4), 261), LeBas (Ghai, R. L.; Dullien, F. A. L.
J. Phys Chem. 1974, 78, 2283), and other (Spernol, A.; Wirtz, K. Z.
Naturforsch. 1953, 8a, 522) diameters, σ ) 1/4, density of toluene, dens(T,
K) ) 1.1787 - 1.048 × 10-3(T,K). For the solvent benzene: mp, 278.7,
bp 353.3, MW 78.1, Andrade viscosity, ln(η, cP) ) -4.117 + 2.0973 ×
B0 is the initial hydride concentration, D is the sum of rearrangement
products, and E is the concentration of unrearranged reduced hydro-
carbon. Equation 2 is solved by computer for the relative rate, r )
kre/kabs, where kre represents the sum of rearrangement products, e.g.,
for 2-heptenyl radical, kre ) k5,cis + k5,trans + k6. The factor f (eq 3)
corrects for consumption of hydride from the induced decomposition
(52) Rakowski DuBois, M.; DuBois, D. L.; Vanderveer, M. L.; Halti-
wangen, R. C. J. Inorg. Chem. 1981, 20, 3064.
(53) Ashby, E. C.; DePriest, R. N.; Goel, A. B.; Wenderoth, B.; Pham,
T. N. J. Org. Chem. 1984, 49, 3545-3556.
(54) Coates, R. M.; Johnson, M. W. J. Org. Chem. 1980, 45, 2685-
2697.
(55) Ashby, E. C.; Bowers, J. R. J. J. Am. Chem. Soc. 1981, 103, 2242-
2250.
(56) Alnajjar, M. S.; Kuivila, H. G. J. Am. Chem. Soc. 1985, 107, 416-
423.
(57) Ru¨chardt, C. Chem. Ber. 1961, 94, 1599.
(58) Ru¨chardt, C.; Hecht, R. Chem. Ber. 1965, 98, 2460.
103, dens(T, K) ) 1.192-1.059 × 10-3(T, K). The microfriction factor f
r
of Spernol and Wirtz is given by f ) (0.16 + 0.4rA/rB)(0.9 + 0.4TA
-
-
r
r
b
0.25TB ), and reduced temperatures are given by TX ) (T - TXf)/(TX
f
b
T
Xf), where TX and TX are freezing and boiling points of species X )
benzyl (A) or benzene (B). The radii in the microfriction factor term are
given by rX ) (3VX(ø)/4πN)1/3, where ø ) 0.74, the volume fraction for
cubic closest packed spheres, VX are density-based molecular volumes, and
N is Avogadro’s number. The resulting Smoluchowski expression for self-
termination of the benzyl radical in benzene is ln(2kt/M-1 s-1) ) 27.23 -
2952.47/RT. A similar treatment for benzyl self-termination in toluene is
in near-perfect agreement with experimental values (refs 61-64).

9830 J. Am. Chem. Soc., Vol. 121, No. 42, 1999
Franz et al.
absolute error in estimation of self-termination rate constants by this
method is estimated to be less than ca. 25%.61
driven optical shutter, and Dr. Tom Autrey for help in
preparation of 2,2-dimethylhept-6-enoic acid.
Acknowledgment. This work supported by the Office of
Science, Office of Basic Energy Sciences, Chemical Sciences
Division of the U.S. Department of Energy under contract DE-
ACO6-76RLO 1830. The authors thank Dr. Donald M. Ca-
maioni for useful discussions and for assembling the computer-
Supporting Information Available: Tabulations of kinetic
data for the entries in Table 1 (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
JA991412E