p-Nitrobenzenesulfenate esters as precursors for laser flash photolysis studies of alkyl radicals

Article abstract of DOI:10.1021/jo026218g

A series of p-nitrobenzenesulfenate esters was used in laser flash photolysis (LFP) studies to generate alkoxyl radicals that fragmented to give the (2,2-diphenylcyclopropyl)methyl radical. Rate constants for the β-scission reactions increased as a function of the carbonyl compound produced in the fragmentation reaction in the order CH₂O < MeCHO < Me₂CO < PhCHO < Ph₂CO and increased with increasing solvent polarity. For alkoxyl radicals that fragment to produce benzaldehyde and benzophenone, the β-scission reactions are faster than 1,5-hydrogen atom abstractions when the incipient carbon radical is as stable as a secondary alkyl radical, and this entry to carbon radicals can be used in LFP kinetic studies.

Full text of DOI:10.1021/jo026218g

SCHEME 1
p-Nitr oben zen esu lfen a te Ester s a s
P r ecu r sor s for La ser F la sh P h otolysis
Stu d ies of Alk yl Ra d ica ls
Martin Newcomb,* Pierre Daublain, and
J ohn H. Horner*
Department of Chemistry, University of Illinois at Chicago,
845 West Taylor Street, Chicago, Illinois 60607
men@uic.edu
Received J uly 20, 2002
Abstr a ct: A series of p-nitrobenzenesulfenate esters was
used in laser flash photolysis (LFP) studies to generate
alkoxyl radicals that fragmented to give the (2,2-diphenyl-
cyclopropyl)methyl radical. Rate constants for the â-scission
reactions increased as a function of the carbonyl compound
produced in the fragmentation reaction in the order CH2O
< MeCHO < Me2CO < PhCHO < Ph2CO and increased with
increasing solvent polarity. For alkoxyl radicals that frag-
ment to produce benzaldehyde and benzophenone, the
â-scission reactions are faster than 1,5-hydrogen atom
abstractions when the incipient carbon radical is as stable
as a secondary alkyl radical, and this entry to carbon
radicals can be used in LFP kinetic studies.
esters are relatively stable and can be purified by silica
gel chromatography with little decomposition.2,4
The series of PNBS esters 1 was prepared from
reactions of the corresponding alcohols with 4-nitroben-
zenesulfenyl chloride in the presence of Et3N. Upon
photolysis with 355-nm light, precursors 1 react as shown
in Scheme 1. The initial photolysis gives alkoxyl radicals
2. â-Scission of radicals 2 produces a carbonyl compound
and the (2,2-diphenylcyclopropyl)methyl radical (3). Radi-
cal 3 is an ultrafast “reporter” radical11 that ring opens
with a rate constant of 5 × 1011 s-1 at ambient temper-
ature12 to give the 1,1-diphenyl-3-butenyl radical (4) that
is readily detected in the UV due to its strong long
wavelength absorbance with λmax at ca. 335 nm.13 The
rates of signal growth for radical 4 give the rates of the
â-scission reactions of the alkoxyl radicals. Because
radical 3 was produced in all reactions, the kinetic effects
of the substituents on the â-scission reaction can be
evaluated.
Laser flash photolysis (LFP) kinetic studies of radicals
are conveniently conducted when one can generate the
desired radical in high yield from a laser photolysis
reaction, but many photochemical bond-cleavage reac-
tions are not useful for LFP. Small quantum yields and/
or rapid electron-transfer processes following the bond
homolysis reaction result in low yields of transients.
When high-energy light is required for radical production,
experimental difficulties are encountered if additives or
the radical transients absorb laser light. We report here
an evaluation of p-nitrobenzenesulfenate (PNBS) esters
as precursors of carbon-centered radicals in LFP studies.
Benzenesulfenates react with stannyl radicals in radi-
cal chain reactions to give alkoxyl radicals,1 and PNBS
esters have been used as alkoxyl radical precursors in
steady-state photolysis reactions2,3 and in LFP studies.4
The PNBS esters are yellow-colored compounds that have
a long wavelength absorbance centered at ca. 350 nm.
The photochemical homolysis reaction gives an alkoxyl
radical and the p-nitrobenzenethiyl radical, and previous
LFP studies demonstrated that the cleavage reactions
of PNBS esters occur with high efficiency using 355-nm
laser light (third harmonic of a Nd:YAG laser).4 Our
interest in exploring PNBS esters as sources of alkyl
radicals for kinetic studies was based on the facility of
alkoxyl radical â-scission reactions, which have been
extensively studied for the tert-butoxyl radical5-8 and the
cumyloxyl radical,9,10 and the observation that PNBS
Reactions with PNBS esters 1 were conducted in
(trifluoromethyl)benzene (trifluorotoluene, TFT), aceto-
nitrile, and acetic acid. Figure 1 shows the time-resolved
growth spectrum following photolysis of precursor 1b
where the formation of the diphenylalkyl radical 4 is
apparent from its characteristic spectrum with λmax at
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Chem. 2002, 67, 2266-2270.
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H.; Musa, O. M.; Martinez, F. N. J . Am. Chem. Soc. 1996, 118, 8505-
8506.
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Scaiano, J . C., Ed.; CRC Press: Boca Raton, FL, 1989; Vol. 2, pp 3-11.
10.1021/jo026218g CCC: $22.00 © 2002 American Chemical Society
Published on Web 11/06/2002
J . Org. Chem. 2002, 67, 8669-8671
8669

and 2c was apparent; this phenomenon is well docu-
mented for â-scission reactions of the tert-butoxyl5-7 and
cumyloxyl9,10,15 radicals.
The change in the rate constants for â-scission for the
series of radicals 2 is noteworthy. For radicals 2a -c, the
trend mirrors the activation energies for â-scission of the
methyl radical from the series of radicals ethoxyl, iso-
propoxyl, and tert-butoxyl.8 A strong acceleration in the
fragmentation rate results from replacement of a methyl
group on the R-carbon with a phenyl group; e.g., radical
2d reacts about 50 times faster than radical 2b at 22 °C
in acetonitrile. The direction, if not the magnitude, of this
effect was expected because the rate constant for â-scis-
sion of a methyl radical from cumyloxyl in acetonitrile is
10 times larger than that for â-scisssion of a methyl
radical from the tert-butoxyl radical in the same sol-
vent.7,9,10 The second phenyl group at the R-carbon in
radical 2e resulted in further acceleration of the â-scis-
sion reaction to an unknown extent because the instru-
ment limit was exceeded. Phenyl group migration in the
1,1-diphenylethoxyl radical to give the 1-phenyl-1-phe-
noxyethyl radical, a neophyl-like rearrangement reaction,
is faster than â-scission of the methyl radical,16 but the
rate constant for phenyl migration in acetonitrile at
F IGURE 1. Time-resolved spectrum from the reaction of
radical 2b in acetonitrile. The time slices are at 70, 100, 200,
and 350 ns with the spectrum at 50 ns subtracted to give a
baseline. Symbols on the 350-ns trace show the wavelengths
that were monitored. The inset shows the kinetic trace at 334
nm.
TABLE 1. Ra te Con sta n ts for F r a gm en ta tion of Alk oxyl
Ra d ica ls 2a
temp
kTFT
(s-1
kACN
(s-1
kAcOH
(s-1
radical byproduct (°C)
)
)
)
ambient temperature is only (2-3) × 106 s-1 17-19
,
about
2a
2b
2c
2d
CH2Oc
MeCHO
Me2CO
PhCHO
22 3.8 × 105
7.5 × 105
5.2 × 106
3 orders of magnitude smaller than that for â-scission of
radical 2e.
22 6.0 × 106
1.1 × 107
>2 × 108
22 2.2 × 107 d 5.7 × 107
-30
7.5 × 107
5 × 108 e
If PNBS esters are to be used as sources of carbon
radicals, the â-scission reactions must be faster than
intra- and intermolecular hydrogen atom transfer reac-
tions. Unlike the â-scission reactions that have rate
constants strongly dependent on the stability of the
incipient radical, hydrogen atom abstractions are rela-
tively insensitive to the stability of the radical product.
For example, rate constants for 1,5-hydrogen atom
transfer in radicals 5 and 6 at 20 °C in trifluorotoluene
were 2.7 × 107 and 1.2 × 107 s-1, respectively,4 even
though the C-H bond energies should differ by 4 kcal/
mol or more.
22
-40
22
2e
Ph2CO
g2 × 108
g1.3 × 109 e
a
Observed rate constants; typical errors are 3% of the stated
b
value. TFT ) R,R,R-trifluorotoluene; ACN ) acetonitrile; AcOH
) acetic acid. c In aqueous acetonitrile solutions, radical 2a
fragmented at 22 °C with the following rate constants: k (ACN:
water, v:v) ) 9.0 × 105 s-1 (95:5), 1.5 × 106 s-1 (80:20), and 3.2 ×
106 s-1 (50:50). Reference 4. e Rate constants for 2d and 2e at
d
22 °C were estimated from the low-temperature kinetic values;
see text.
ca. 335 nm.13 In principle, â-scission of radicals 1b and
1c might occur in part with elimination of methyl
radicals, but that pathway was not important. For
reactions of 1b and 1c, there was no obvious reduction
in the yields of radical 4 relative to the byproduct
p-nitrobenzenethiyl radical as determined from the ratio
of the ultimate signal intensities at 335 nm (for radical
4) and the instantaneous signal intensities at 490 nm
(for the p-nitrobenzenethiyl radical).
The observed rate constants for â-scission of radicals
2 are given in Table 1. Phenyl-substituted radicals 2d
and 2e, which fragment to give benzaldehyde and
benzophenone, respectively, reacted faster than the
instrument response at 22 °C. A rate constant for reaction
of 2d could be obtained at -30 °C in acetonitrile, and we
estimated a rate constant of k ) 5 × 108 s-1 for reaction
of 2d at 22 °C from this kinetic value and a log A term of
12.4.14 Alkoxyl radical 2e fragmented faster than the
instrument limit even at -40 °C, and we estimated a
lower limit for this fragmentation reaction of k g 1.3 ×
109 s-1 at 22 °C. A relatively large solvent polarity effect
on the rate constants for â-scission of radicals 2a , 2b,
When alkoxyl radical 2c was generated in THF at 20
°C, the signal intensity from product radical 4 was
reduced to 40% of that observed when 2c was produced
in the nonreactive solvent trifluorotoluene. This indicates
that 60% of the radical reacted with THF. The observed
rate constant in THF at 20 °C, which is the sum of all
first-order and pseudo-first-order rate constants for reac-
tions of 2c, was kobs ) 7.7 × 107 s-1. Therefore, the
(15) Erben-Russ, M.; Michel, C.; Bors, W.; Saran, M. J . Phys. Chem.
1987, 91, 2362-2365.
(16) Howard, J . A.; Ingold, K. U. Can. J . Chem. 1969, 47, 3797-
3801.
(17) Falvey, D. E.; Khambatta, B. S.; Schuster, G. B. J . Phys. Chem.
1990, 94, 1056-1059.
(18) Banks, J . T.; Scaiano, J . C. J . Phys. Chem. 1995, 99, 3527-
3531.
(14) A log A value of 12.4 was determined for â-scission of radical
2c in triflouormethylbenzene; see ref 4.
(19) Avila, D. V.; Ingold, K. U.; Dinardo, A. A.; Zerbetto, F.; Zgierski,
M. Z.; Lusztyk, J . J . Am. Chem. Soc. 1995, 117, 2711-2718.
8670 J . Org. Chem., Vol. 67, No. 24, 2002

(CH), 34.7 and 35.2 (C), 38.0 and 38.1 (CH2), 85.0 and 85.5 (C),
120.0 and 120.1 (CH), 124.0 (CH), 125.88 and 125.9 (CH), 126.5
and 126.6 (CH), 127.4 (CH), 127.8 (CH), 128.3 (CH), 128.4 (CH),
130.3 and 130.5 (CH), 141.0 and 141.1 (C), 144.98 and 145.04
(C), 146.7 (C), 152.3 and 152.4 (C).
calculated pseudo-first-order rate constant for reaction
with THF is 4.6 × 107 s-1, and that for â-scission is k )
3.1 × 107 s-1. There is good agreement in the value for
the â-scission reaction in THF with that in TFT as
expected given that the polarities of THF and TFT are
similar.20,21 Using a molarity for THF of 12.3 M, the
second-order rate constant for reaction of 2c with THF
is 3.7 × 106 M-1 s-1, which is in good agreement with
the reported second-order rate constant of 4.6 × 106 M-1
s-1 for reaction of the tert-butoxyl radical with THF at
25 °C.22
The â-scission reactions of radicals 2d and 2e, which
eliminate benzaldehyde and benzophenone, respectively,
are more than 1 order of magnitude faster than typical
1,5-hydrogen atom abstractions and reaction with neat
THF. Therefore, PNBS esters that give such alkoxyl
radicals should be useful as carbon-radical precursors
when the incipient carbon radical is as stable as a
secondary alkyl radical. It is possible that this entry to
carbon radicals will not be useful for primary alkyl
radicals, however, due to the strong dependence of the
â-scission reaction on the stability of the product radical.
2-(2,2-Dip h en ylcyclop r op yl)-1,1-d im eth yleth yl 4-Nitr o-
ben zen esu lfen a te (1c). The general procedure was used with
500 mg (1.88 mmol) of 1-(2,2-diphenylcyclopropyl)-2-methyl-2-
1
propanol to yield 1c as a red oil (535 mg, 68% yield). H NMR:
δ 0.80 (dd, J ) 14.4, 10.4 Hz, 1H), 1.30 (s, 3H), 1.33 (m, 1H),
1.36 (s, 3H), 1.70 (m, 1H), 2.08 (dd, J ) 4.3, 3.1 Hz, 1H), 7.09-
7.30 (m, 12H), 8.09 (d, J ) 9.1 Hz, 2H). 13C NMR: δ 21.7 (CH2),
22.0 (CH), 25.3 (CH3), 25.7 (CH3), 33.6 (C), 42.3 (CH2), 87.2 (C),
120.1 (CH), 120.2 (CH), 123.8 (CH), 123.9 (CH), 125.8 (CH), 126.4
(CH), 127.6 (CH), 128.2 (CH), 130.3 (CH), 141.2 (C), 144.8 (C),
146.8 (C), 153.9 (C).
2-(2,2-Dip h en ylcyclop r op yl)-1-p h en yleth yl 4-Nitr oben -
zen esu lfen a te (1d ). The general procedure was used with 300
mg (9.5 mmol) of 2-(2,2-diphenylcyclopropyl)-1-phenylethanol to
yield 1d as an orange solid (312 mg, 70% yield). Mp: 40 °C dec.
The product was a mixture of diastereoisomers that was not
further purified. 1H NMR of the mixture: δ 1.19, 1.33, 1.47, 1.67,
1.89 and 2.20 (m, 5H), 4.57 and 4.65 (t, J ) 7.1 and 6.5 Hz, 1H)
7.07-7.39 (m, 15H), 7.97 and 8.09 (d, J ) 8.9 Hz and J ) 8.9
Hz, 2H). 13C NMR of the mixture: δ 20.0 and 21.1 (CH2), 22.1
and 22.9 (CH), 35.1 and 35.6 (C), 38.4 and 39.2 (CH2), 90.1 and
90.8 (CH), 120.1 (CH), 120.2 (CH), 120.3 (CH), 124.1 and 124.2
(CH), 126.0 (CH), 126.7 (CH), 127.4 (CH), 128.0 (CH), 128.4 (CH),
128.6 (CH), 130.2 (CH), 130.6 (CH), 140.1 and 140.4 (C), 140.8
and 141.1 (C), 145.0 and 145.1 (C), 146.5 and 146.6 (C), 151.5
and 151.6 (C).
2-(2,2-Dip h en ylcyclop r op yl)-1,1-d ip h en yleth yl 4-Nitr o-
ben zen esu lfen a te (1e). The general procedure was used with
750 mg (1.9 mmol) of 2-(2,2-diphenylcyclopropyl)-1,1-diphenyle-
thanol to give 1e as an orange solid (783 mg, 75% yield). Mp:
55 °C dec. 1H NMR: δ 0.77 (m, 1H), 0.91 (m, 1H), 1.43-1.60
(m, 2H), 2.93 (d, J ) 11.9 Hz, 1H), 6.91 (d, J ) 6.6 Hz, 2H),
7.06-7.30 (m, 20H), 7.91 (d, J ) 9.0 Hz, 2H). 13C NMR: δ 21.3
(CH2), 21.9 (CH), 34.1 (C), 39.3 (CH2), 91.9 (C), 121.6 (CH), 121.7
(CH), 123.3 (CH), 123.4 (CH), 125.8 (CH), 126.4 (CH), 127.7 (CH),
127.9 (CH), 128.2 (CH), 130.2 (CH), 141.3 (CH), 142.0 (CH), 142.5
(CH), 144.9 (CH), 146.7 (CH), 151.5 (C).
La ser fla sh p h otolysis stu d ies were performed with Ap-
plied Photophysics LK50 and LK60 kinetic spectrometers by
conventional methods. In brief, solutions of precursors in the
appropriate solvent were prepared such that the absorbance at
355 nm was ca. 0.5. The solutions were allowed to flow through
a 1 cm × 1 cm quartz cell and were irradiated with 355-nm light
(Nd:YAG, third harmonic) in 5- or 7-ns pulses of 50-90 mJ . Data
were collected at individual wavelengths via oscilloscopes with
temporal resolution up to 125 ps. The measured photomultiplier
response was 2 × 108 s-1, which represents the kinetic limit of
the instruments.
Exp er im en ta l Section
Gen er a l Meth od for th e P r ep a r a tion of Alk yl 4-Ni-
tr oben zen esu lfen a tes. To a stirred solution of the appropriate
alcohol (1 equiv) and distilled triethylamine (1.5 equiv) in
anhydrous CH2Cl2 at 0 °C was added dropwise a solution of
4-nitrobenzenesulfenyl chloride (1.2 equiv) in CH2Cl2. After 10
min, the solution was warmed to room temperature and stirred
for an additional 15 min. After addition of water to the reaction
mixture, the organic layer was washed with 10% aqueous NH4-
Cl solution and saturated NaCl solution, dried over MgSO4, and
concentrated in vacuo. Column chromatography on silica gel
(hexanes/EtOAc) yielded the desired 4-nitrobenzenesulfenate
ester as a reddish oil or solid in high yield.
2-(2,2-Dip h en ylcyclop r op yl)et h yl 4-Nit r ob en zen esu l-
fen a te (1a ). The general procedure was used with 300 mg (1.3
mmol) of 2-(2,2-diphenylcyclopropyl)ethanol to yield 1a as a red
oil (370 mg, 75% yield). 1H NMR: δ 1.20 (t, J ) 7.0 Hz, 1H),
1.29 (d, J ) 7.2 Hz, 2H), 1.81 (m, 2H), 3.94 (t, J ) 6.4 Hz, 2H),
7.10-7.32 (m, 12H), 8.12 (d, J ) 9.1 Hz). 13C NMR: δ 20.1 (CH2),
22.3 (CH), 31.7 (CH2), 35.2 (C), 79.0 (CH2), 119.8 (CH), 119.9
(CH), 124.2 (CH), 124.3 (CH), 125.9 (CH), 126.5 (CH), 127.7 (CH),
128.3 (CH), 130.4 (CH), 141.0 (C), 145.1 (C), 146.6 (C), 151.5
(C).
2-(2,2-Dip h en ylcyclop r op yl)-1-m eth yleth yl 4-Nitr oben -
zen esu lfen a te (1b). The general procedure was used with 300
mg (1.2 mmol) of 1-(2,2-diphenylcyclopropyl)-2-propanol to yield
1b a red oil (375 mg, 78% yield). A mixture of diastereoisomers
was obtained, whose NMR peaks were mostly overlapping. 1H
NMR of the mixture: δ 0.98 and 1.17 (m, 1H), 1.29-1.38 (m, 5
H), 1.75 and 1.99 (m, 2H), 3.85 (m, 1H), 7.13-7.31 (m, 12H),
8.04 and 8.11 (d, J ) 9.1 Hz and J ) 9.0 Hz, 2H). 13C NMR of
the mixture: δ 20.4 (CH3), 20.5 and 21.1 (CH2), 22.3 and 22.4
Ack n ow led gm en t. This work was supported by a
grant from the National Science Foundation (CHE-
0296027).
Su p p or tin g In for m a tion Ava ila ble: Experimental de-
tails for preparation of the alcohols used in the preparation of
PNBS esters 1 and 1H and 13C NMR spectra of the alcohols
and PNBS esters 1. This material is available free of charge
via the Internet at http://pubs.acs.org.
(20) Reichardt, C. Chem. Rev. 1994, 94, 2319-2358.
(21) ET(30) solvent polarity values are as follows: THF, 37.4;
trifluoromethylbenzene, 38.5; acetonitrile, 45.6; acetic acid, 51.7. See
ref 20.
(22) Laarhoven, L. J . J .; Mulder, P. J . Phys. Chem. B 1997, 101,
73-77.
J O026218G
J . Org. Chem, Vol. 67, No. 24, 2002 8671