Laser flash photolysis studies of alkoxyl radical kinetics using 4-nitrobenzenesulfenate esters as radical precursors

Article abstract of DOI:10.1021/ol006469g

(matrix presented) 4-Nitrobenzenesulfenate esters were used as precursors for the generation of alkoxyl radicals under laser flash photolysis conditions. The esters were efficiently cleaved using the Nd:YAG third harmonic (355 nm) to produce alkoxyl radic

Full text of DOI:10.1021/ol006469g

ORGANIC
LETTERS
2000
Vol. 2, No. 21
3369-3372
Laser Flash Photolysis Studies of
Alkoxyl Radical Kinetics Using
4-Nitrobenzenesulfenate Esters as
Radical Precursors
John H. Horner,* Seung-Yong Choi, and Martin Newcomb*
Department of Chemistry, Wayne State UniVersity, Detroit, Michigan 48202
men@chem.wayne.edu
Received August 16, 2000
ABSTRACT
4-Nitrobenzenesulfenate esters were used as precursors for the generation of alkoxyl radicals under laser flash photolysis conditions. The
esters were efficiently cleaved using the Nd:YAG third harmonic (355 nm) to produce alkoxyl radicals and the 4-nitrobenzenethiyl radical. Rate
constants for â-scission and 1,5-hydrogen abstraction reactions of alkoxyl radicals were measured.
Alkoxyl radicals are important, highly reactive intermediates.
Under conditions where intermolecular hydrogen abstraction
can be minimized (in the gas phase or in an inert solvent),
alkoxyl radicals are well-known to undergo intramolecular
reaction either by â-scission or by 1,5-hydrogen abstraction.1
Although both of these reactions are used synthetically,2 there
have been relatively few solution phase studies of the
absolute kinetics of either reaction. Rates of â-scission of
tert-butoxyl radical and cumyloxyl radicals were measured
by laser flash photolysis and kinetic ESR methods.3 Com-
petition kinetics also have been used to study â-scissions
and 1,5-hydrogen abstractions of alkoxyl radicals.4
We report here the generation of alkoxyl radicals from
4-nitrobenzenesulfenate esters using the Nd:YAG third
harmonic (355 nm) and the subsequent direct observation
of â-scission and 1,5-abstraction reactions of the initially
generated alkoxyl radicals. 4-Nitrobenzenesulfenate esters
have an intense long wavelength chromophore suitable for
irradiation at 355 nm and have been used previously in steady
state photolyses to generate alkoxyl radicals.5 The required
esters were easily prepared by the reaction of the appropriate
alcohols with 4-nitrobenzenesulfenyl chloride. The alcohols
1
used were characterized by H NMR and 13C NMR spec-
troscopy and high-resolution mass spectrometry. The 4-ni-
(2) Recent synthetic applications and methods: (a) Hartung, J.; Schwarz,
M.; Svoboda, I.; Fuess, H.; Duarte, M. T. Eur. J. Org. Chem. 1999, 1275-
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Thang, S. H.; Suyama, S. J. Org. Chem. 2000, 65, 16-23. (c) Platz, J.;
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10.1021/ol006469g CCC: $19.00 © 2000 American Chemical Society
Published on Web 09/26/2000

trobenzenesulfenates, purified by chromatography, were
unobserved absorbance of the 4-nitrobenzenethiyl radical.
The tert-butoxyl radical is known to exhibit only weak
absorbances at λ > 300 nm.3a The spectrum generated by
cleavage of 2 was subtracted from the spectrum generated
from 1 to give the corrected spectrum in Figure 1c, which
clearly shows only the intense absorbance due to the
diphenylmethyl radical centered at 330 nm.
The rapid â-scission of 1‚ to give the highly stabilized
diphenylmethyl radical is consistent with the known tendency
for the rate of â-scission to increase with increasing stability
of the radical being eliminated. Recent calculations for
cleavage of simple alkoxyl radicals show a linear relationship
between calculated reaction enthalpy and calculated transition
state energies.7 This relationship prompted us to investigate
the kinetics of reactions of radicals 3‚ and 4‚ (Scheme 2).
1
greater than 95% pure by H NMR and 13C NMR.
Preliminary experiments were carried out with precursors
1 and 2 to generate the 2,2-diphenylethoxyl (1‚) and tert-
butoxyl radicals (2‚), respectively. When subjected to LFP
in (trifluoromethyl)benzene using the Nd:YAG third har-
monic (355 nm), precursor 1 gave an intense signal (λmax
330 nm) consistent with formation of the diphenylmethyl
radical within the 7 ns duration of the laser pulse (Scheme
1). The estimated yield of radicals in the laser path is 50%;
Scheme 1
Scheme 2
it is likely that 100% cleavage occurred with 50% cage
recombination. Cooling to -20 °C led to no noticeable
change in the observed kinetic traces. On the basis of these
observations, it can be concluded that 1‚ fragments at >2 ×
108 s-1 at -20 °C. The UV-vis spectrum generated 41 ns
after the laser pulse is given in Figure 1a. In addition to the
Because of the stabilization imparted by a cyclopropyl ring
to an adjacent radical center, 3‚ was expected to undergo
â-scission faster than the cyclobutyl analogue 4‚.8,9 Upon
photolysis, 3 and 4 should yield the alkoxyl radicals (3‚ and
4‚), which were expected to cleave to the 2,2-diphenylcyclo-
propylcarbinyl radical (5‚) and the 2,2-diphenylcyclobutyl-
carbinyl radical (6‚), respectively. Radicals 5‚ and 6‚ have
been shown to open to 7‚ and 8‚ with rate constants of 4 ×
1011 s-1 and 2.5 × 108 s-1, respectively, at 20 °C.10 Thus,
the kinetic slow steps in these sequences were expected to
be the â-scission processes. Previous experiments showed
Figure 1. Uv-vis spectra (41 ns) generated by LFP (355 nm) of
4-nitrobenzenesulfenate esters 1 and 2: (a) compound 1, (b)
compound 2, (c) spectrum b subtracted from spectrum a.
diphenylmethyl radical signal at 330 nm, bleaching of the
precursor is readily apparent in the range 340-380 nm. A
weak broad signal was also observed between 390 and 500
nm, consistent with formation of the 4-nitrobenzenethiyl
radical, for which a visible spectrum (>400 nm) has been
reported.6
To correct for bleaching and the presence of the 4-nitro-
benzenethiyl radical, precursor 2 was cleaved under condi-
tions matched for sample absorbance with precursor 1. The
resulting spectrum (Figure 1b) exhibits bleaching (340-370
nm) due to destruction of the precursor in addition to a weak
absorbance centered at 320 nm apparently due to a previously
(5) (a) Pasto D. J.; Cottard, F. J. Am. Chem. Soc. 1994, 116, 8973-
8977. (b) Pasto, D. J.; Cottard, F.; Picconatto, C. J. Org. Chem. 1994, 59,
7172-7177.
(6) Alam, M. M.; Ito, O. J. Org. Chem. 1999, 64, 1285-1290.
(7) Wilsey, S.; Dowd, P.; Houk, K. N. J. Org. Chem. 1999, 64, 8801-
8811.
(8) Walton, J. C. Magn. Reson. Chem. 1987, 25, 998-1000.
3370
Org. Lett., Vol. 2, No. 21, 2000

that 7‚ and 8‚ absorb strongly near 330 nm; therefore LFP
of 3 and 4 were expected to produce changes in absorbance
at 330 nm proportional to the rate of â-scission.
temperature is reduced, the rate of â-scission decreases more
rapidly than the rate of 1,5-abstraction. At any given
temperature, the observed rate constant will be the sum of
the two processes, but since only â-scission gives rise to an
observable signal, at lower temperature the signal intensity
for the formation of 8‚ diminishes.
LFP of compound 3 in (trifluoromethyl)benzene led to the
formation of an intense signal (λmax ) 334 nm) that was
observed to grow in with a rate constant of 2.2 × 107 s-1 at
20 °C. The time-resolved spectrum is shown in Figure 2a.
Variable temperature data (52.5 to -30.6 °C) gave the
Arrhenius function in eq 1 where θ ) 2.303/RT kcal/mol.
The observed log A value is typical of that expected for a
fragmentation in solution and is similar to results for the
â-scission of the tert-butoxyl radical in aromatic solvents of
Fischer and Tsentalovitch who found values ranging from
12.8 to 13.4.1e,3a
log (k/s-1) ) (12.43 ( 0.09) - (6.78 ( 0.11)/θ (1)
Figure 3. (a) Arrhenius plot for the disappearance of 3‚ (O) and
4‚ (b). (b) Arrhenius plot for the disappearance of 4‚ separated
into components for â-scission (O) and 1,5-abstraction ([).
LFP of compound 4 in (trifluoromethyl)benzene led to the
formation of a signal (λmax ) 334 nm) that was observed to
grow in at 1.5 × 107 s-1 at 20.1 °C. The time-resolved
spectrum is given in Figure 2b. When a comparison is made
between the signal intensities resulting from the photolysis
of solutions of 3 and 4 matched for absorbance at 355 nm,
it is clear that the signal resulting from the cleavage of 4‚ is
less than one-half the intensity of that from 3‚ (note
absorbance scales in Figure 2).
To estimate the fraction of reaction occurring by â-scission
at each temperature, the â-scission of the 2,2-diphenylcyclo-
pentoxyl radical (9‚) was used as an external standard.
Solutions of precursors 4 and 9 were matched for absorbance
at 355 nm and subjected to LFP at temperatures ranging from
50 to 10 °C. Kinetic traces were collected at 334 nm, and
after subtracting a small contribution for the 4-nitroben-
zenethiyl radical at this wavelength, the intensities were
compared to estimate the percent reaction occurring by
â-scission. The remainder of reaction of 4‚ was assumed to
be 1,5-abstraction from the cyclobutyl ring. The amount of
fragmentation was found to vary from 40% at 50 °C to 21%
at 10 °C. These data were extrapolated to lower temperatures
to obtain estimates of the percent of â-scission down to -20
°C. Multiplying the observed rate constant by the fraction
of reaction occurring by each pathway yielded rate constants
for â-scission and 1,5-abstraction over the temperature range
51 to -22 °C. Arrhenius plots (Figure 3b) of the two sets of
data gave the Arrhenius functions in eqs 2 and 3 for
â-scission and 1,5-hydrogen abstraction, respectively. As
with 3‚, the log A value for â-scission is within the range
expected. The log A value for 1,5-hydrogen abstraction is
consistent with the much higher entropic penalty incurred
in organizing the six-membered transition state required for
abstraction.
Figure 2. (a) Time-resolved spectra from LFP of 3 at 25 (b), 45
(O), and 105 ns ([) . (b) Time-resolved spectra from LFP of 4 at
31 (b), 71 (O), and 200 ns ([). In both cases a reference spectrum
produced by the photolysis of 2 was subtracted.
Variable temperature data (51 to -22.7 °C) were collected
for 4‚. The resulting Arrhenius plot is shown in Figure 3a
along with the Arrhenius plot resulting from the cleavage
of 3‚. The Arrhenius plot for 4‚ is clearly nonlinear. In
addition, the signal intensity of the 330 nm absorbance was
found to decrease as the sample temperature was reduced.
These observations are consistent with the presence of two
competing pathways (â-scission and 1,5 hydrogen abstrac-
tion) for the disappearance of 4‚. At higher temperatures,
â-scission dominates due to its higher Ea and log A. As the
log (k/s-1) ) (12.73 ( 0.36) - (8.17 ( 0.28)/θ (2)
log (k/s-1) ) (10.0 ( 0.23) - (3.91 ( 0.30)/θ
(3)
The 1,5-hydrogen abstraction process was further inves-
tigated using precursor 10 (Scheme 3), which cleaved to form
10‚ when photolyzed under LFP conditions. Subsequent 1,5-
abstraction gave 11‚ which rapidly ring opened to 12‚. At
(9) B3LYP/6-31G** calculations on non-phenylated analogues of 3‚ and
4‚ predict that cleavage of the 3-cyclopropyl-2-methyl-2-propoxyl radical
to give cyclopropyl-carbinyl radical and acetone should be more exothermic
by 1.8 kcal/mol than cleavage of the 3-cyclobutyl-2-methyl-2-propoxyl
radical to give the cyclobutylcarbinyl radical and acetone. Recent theoretical
work (ref 3e) has shown that B3LYP/6-31G** accurately calculates
activation barriers (typically within 2 kcal/mol) for alkoxy radical â-scission.
(10) (a) Newcomb, M.; Johnson, C. C.; Manek, M. B.; Varick, T. R. J.
Am. Chem. Soc. 1992, 114, 10915-10921. (b) Newcomb, M.; Choi, S. Y.;
Horner, J. H. J. Org. Chem. 1999, 64, 1225-1231. (c) Choi, S. Y.; Horner,
J. H.; Newcomb, M. J. Org. Chem. 2000, 2, 4447-4449.
Org. Lett., Vol. 2, No. 21, 2000
3371

Scheme 3
Table 1. Comparison of Enthalpy of Reaction (∆Hrxn) with
Activation Energy for â-Scission of Alkoxyl Radicals 3‚, 4‚,
13‚, 14‚, 15‚, 16‚, and 17‚14
radical
∆Hrxn
Ea
ref
13•
13•
13•
13•
14•
15•
16•
17•
3•
4.8
4.8
4.8
4.8
3.2
1.6
-1.2
1.9
0.54
2.7
11.6
11.3
12.4
12.1
9.2
7.5
5.3
7.6
6.8
1e
1e
3a
4b
4b
4b
4b
4b
20 °C, 1,5-abstraction occurred at 2.7 × 107 s-1. A variable
temperature study gave the Arrhenius function in eq 4.
this work
this work
4•
8.2
log (k/s-1) ) (10.36 ( 0.13) - (4.93 ( 0.17)/θ (4)
this trend was reproduced well by DFT calculations (B3LYP/
6-31G**), which show product-like transition states with
C-C bond lengths of greater than 2.0 Å for the bond
undergoing â-scission.
Activation parameters for tert-butoxyl radical cleavage
have been reported by Fischer and Tsentalovitch.1e,3a Their
data can be combined with the recent kinetic data reported
by Nakamura to evaluate the relationship of rate and reaction
enthalpy.4b The enthalpies of formation for the radicals in
Scheme 4 are either available experimentally or can be
Scheme 4
Figure 4. (a) Plot of reaction enthalpy against activation energy
for alkoxyl radical â-scission. (s): correlation including tert-
butoxyl data. (- - -): correlation excluding tert-butoxyl data. The
numbers refer to specific alkoxyl radicals (Scheme 4 and Table 1).
estimated reliably from group additivity methods.11-13 Ac-
tivation energies can be estimated for the Nakamura data
by assuming a log A of 13 for the cleavage process. In a
similar manner, enthalpies of reaction for the â-scission of
radicals 3‚ and 4‚ can be calculated by group additivity. The
results are summarized in Table 1 and shown in Figure 4.14
A strong correlation is seen between reaction enthalpy and
activation energy. The slopes in Figure 4 (1.0 including tert-
butoxyl data and 0.8 excluding the tert-butoxyl points) are
indicative of reactions controlled almost solely by enthalpy.
The reaction enthalpy is influenced primarily by two fac-
tors: stability of the radical leaving group and destabilization
of the alkoxyl radical by steric interactions. A comparison
of the cleavages of 14‚, 17‚, and 4‚, where the product radical
is in each case an unstabilized primary radical, reveals that
the reaction enthalpy is affected strongly by steric interac-
tions. Both 4‚ and 17‚ are destabilized by gauche-butane type
interactions which serve to reduce the reaction enthalpy
relative to 14‚. An examination of Figure 4 reveals that
cleavage of tert-butoxyl (13‚) is distinctly slower than
predicted by the other data points. In passing we note that
This work has demonstrated that 4-nitrobenzenesulfenate
esters are excellent precursors for the LFP (355 nm)
generation of alkoxyl radicals. This has allowed us to study
the â-scission and 1,5-abstraction processes of several
alkoxyl radicals. The rates of â-scission of radicals 3‚ and
4‚ and other alkoxyl radicals correlate well with reaction
enthalpies.
Acknowledgment. This work was supported by a grant
from the National Science Foundation (CHE-9981746).
Supporting Information Available: Synthetic procedures
and spectral data for compounds 1, 2, 3, 4, 9, and 10. Rate
constants from variable temperature LFP studies of 3, 4, and
10. This material is available free of charge via the Internet
at http://pubs.acs.org.
OL006469G
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(14) The data summarized here were obtained in aromatic hydrocarbon
solvents (benzene, fluorobenzene, cumene, and (trifluoromethy)lbenzene).
The relatively small differences in solvent polarity should have a minimal
effect on rates of â-scission.
(11) Heats of formation for the alkoxy radicals were estimated using a
bond dissociation energy of 105 kcal/mol for the corresponding alcohol.
The heats of formation of the alcohols were estimated by group additivity.
In three cases, 12‚ 13‚, and 14‚, experimental heats of formation were
available for the alcohols (ref 12) and found to match group additivity within
0.2 kcal.
(12) Wiberg K. B.; Hao, S. J. Org. Chem. 1991, 56, 5108-5110.
3372
Org. Lett., Vol. 2, No. 21, 2000