Spectroscopy and kinetics of triplet 4-methylbenzenesulfonylnitrene

Article abstract of DOI:10.1021/jp9528525

Triplet 4-methylbenzenesulfonylnitrene 3 was studied by solution phase laser flash photolysis, low-temperature absorption spectroscopy, and low-temperature EPR spectroscopy. Upon laser flash photolysis of 4-methyl-benzenesulfonyl azide 1 in ethanol, absorption maxima were observed at 325 and 460 nm while in cyclohexane, a single maximum was observed at 310 nm. The short-wavelength absorptions were assigned to the triplet nitrene. There was no evidence that hydroxylic solvents catalyzed intersystem crossing from the singlet to the triplet state of 3. After irradiation of 1 in an EPA glass at 77 K, a band at 313 nm was observed. The zero-field splitting parameters from the EPR spectrum of the nitrene in an EPA glass at 77 K were |D/hc| = 1.471 cm^-1 and |E/hc| = 0 cm^-1. Remarkably, 3 was not stable at 77 K in EPA. Irradiation of 1 also produced a second nitrene species, possibly (4-methylphenyl)nitrene, with D/hc| = 0.9840 cm^-1 and |E/hc| = 0 cm^-1. The decay of the 325 nm transient was first order in ethanol (τ = 8.96 ± 0.48 μs), methanol (τ = 8.18 ± 0.08 μs), and methanol-d₄ (τ = 8.02 ± 1.76 μs).

Full text of DOI:10.1021/jp9528525

5788
J. Phys. Chem. 1996, 100, 5788-5793
Spectroscopy and Kinetics of Triplet 4-Methylbenzenesulfonylnitrene
Jean-Claude Garay,† Vincent Maloney,* Matthew Marlow,† and Phillip Small†
Department of Chemistry, Indiana UniVersity Purdue UniVersity Ft. Wayne, Ft. Wayne, Indiana 46805-1499
ReceiVed: September 26, 1995; In Final Form: December 13, 1995X
Triplet 4-methylbenzenesulfonylnitrene 3 was studied by solution phase laser flash photolysis, low-temperature
absorption spectroscopy, and low-temperature EPR spectroscopy. Upon laser flash photolysis of 4-methyl-
benzenesulfonyl azide 1 in ethanol, absorption maxima were observed at 325 and 460 nm while in cyclohexane,
a single maximum was observed at 310 nm. The short-wavelength absorptions were assigned to the triplet
nitrene. There was no evidence that hydroxylic solvents catalyzed intersystem crossing from the singlet to
the triplet state of 3. After irradiation of 1 in an EPA glass at 77 K, a band at 313 nm was observed. The
zero-field splitting parameters from the EPR spectrum of the nitrene in an EPA glass at 77 K were |D/hc| )
1.471 cm-1 and |E/hc| ) 0 cm-1. Remarkably, 3 was not stable at 77 K in EPA. Irradiation of 1 also
produced a second nitrene species, possibly (4-methylphenyl)nitrene, with |D/hc| ) 0.9840 cm-1 and |E/hc|
) 0 cm-1. The decay of the 325 nm transient was first order in ethanol (τ ) 8.96 ( 0.48 µs), methanol
(τ ) 8.18 ( 0.08 µs), and methanol-d4 (τ ) 8.02 ( 1.76 µs).
Introduction
CHART 1
Unlike the case with carbenes, attempts to observe triplet
nitrenes through laser flash photolytic techniques have been
hindered by two severe problems, inefficient intersystem cross-
ing to the triplet and sluggish triplet reactivity.1-5 Most singlet
nitrenes react more rapidly than they undergo intersystem
crossing to the triplet nitrene. Formation of the triplet is a minor
pathway. In such cases, the triplet concentration is below
detection limits. Often singlet nitrenes undergo rearrangements,
particularly in nonnucleophilic solvents.6,7 In solvents with
sufficient nucleophilicity, the singlet nitrenes are trapped, and
ylides are formed. These products of intra- and intermolecular
reactions can be observed spectroscopically. Laser flash pho-
tolytic techniques have been applied almost exclusively to
arylnitrenes.1-3 The tendency of singlet arylnitrenes, particularly
phenylnitrene, to rearrange to ketenimines led to confusion over
the assignment of transients observed upon flash photolytic
decomposition of aryl azides. Only in the last decade has a
coherent scheme been developed for phenylnitrene reactivity
on the basis of product studies, ambient temperature laser flash
photolysis (LFP), and low-temperature EPR, IR, absorbance,
and luminescence spectra.1 It is possible by carefully choosing
conditions to detect and assign observed transients to triplet
nitrenes, ketenimines, or ylides. The detection of triplet aryl-
nitrenes illustrated the second impediment usually encountered.
Although the products of intermolecular triplet nitrene reactions
such as hydrogen abstraction have been observed in product
studies,6,7 their reactivity has been too sluggish to obtain absolute
rate constants through LFP. These apparent hydrogen abstrac-
tion products may have been formed by photoreduction of the
azide precursor also. Transient lifetimes were dominated by
triplet nitrene dimerization or dependent on precursor concentra-
tion.1,8,9
criteria.10 Previous product studies had shown that the reactions
of (diethoxy)- and (diphenoxyphosphoryl)nitrenes were straight-
forward.11,12 No products attributable to singlet rearrangement
were found. Singlet phosphorylnitrenes react with hydrocarbons
at diffusion control12 and exhibit the lowest selectivity observed
for any nitrene.11b,13 Upon LFP of diphenyl phosphorazidate 7
in ethanol, a transient with an absorption maximum at 340 nm
was observed.10 Although its lifetime was unaffected by the
presence of oxygen, triethylsilane, isoprene, or 1,4-cyclohexa-
diene, the transient decay was first order in ethanol, ethanol-
d6, methanol, and methanol-d4. 4-Methylbenzenesulfonylnitrene
is another possible candidate for study through laser flash
photolytic techniques.14 Relative to other classes of nitrenes,
the singlet states of sulfonylnitrenes are resistant to rearrange-
ment.13 The selectivity of methanesulfonylnitrene is only
slightly greater than that of phosphorylnitrenes,13,15 indicating
that singlet sulfonylnitrenes are highly reactive. As with phos-
phorylnitrenes, those structural features that enhance the reactiv-
It would be desirable to find nitrenes whose singlet states
are resistant to rearrangement and whose triplet states are
sufficiently reactive for the acquisition of absolute rate constants.
Then, substituent effects on the absolute rate constants of triplet
nitrenes could be investigated. Recently, we have reported that
triplet (diphenoxyphosphoryl)nitrene 6 (Chart 1) fulfills these
† All coauthors are undergraduate research assistants.
X Abstract published in AdVance ACS Abstracts, March 1, 1996.
0022-3654/96/20100-5788$12.00/0 © 1996 American Chemical Society

Spectroscopy and Kinetics of Triplet Nitrenes
J. Phys. Chem., Vol. 100, No. 14, 1996 5789
ity of the singlet may also increase the triplet nitrene reactivity
sufficiently for the acquisition of absolute rate constants.
to triplet 4-methylbenzenesulfonylnitrene with success. Upon
formation of the singlet nitrene from the precursor azide,
intersystem crossing to the triplet must be efficient enough to
generate spectroscopically detectable concentrations. Ther-
molysis of 4-methylbenzenesulfonyl azide in cyclohexane
produces 4-methylbenzenesulfonamide in 5% yield.23b It is
uncertain whether the appearance of this product is due to the
triplet nitrene. However, if the sulfonamide does arise from a
triplet nitrene intermediate, a 5% yield could indicate that
sufficient concentrations of the triplet nitrene will be present
for observation by LFP. Also, it must be possible to distinguish
the sulfonylnitrene from other possible species that may appear
in a transient spectrum. Photolysis of 1 may be expected to
proceed along the pathways outlined in Scheme 1. This
representation has been simplified for clarity and does not
include potential decomposition with loss of SO2 by either the
azide or nitrene or the potential rearrangement of the singlet
nitrene. For 1, at ambient temperatures, these appear to be
minor pathways. To apply LFP, intermediates from these
pathways must be undetectable, not interfere with triplet nitrene
absorption, or be readily distinguishable from the triplet.
Although potentially useful, sulfonylnitrenes are not ideal
candidates for study. Unlike phosphorylnitrenes whose reactions
are relatively simple, both sulfonylnitrenes and sulfonyl azides
exhibit a rich chemistry.13,16 It is often unclear whether the
products of thermolytic or photolytic decomposition of sulfonyl
azides arise from a sulfonylnitrene or the azide itself. EPR
spectroscopy provides strong evidence that sulfonylnitrenes are
generated from the photolysis of sulfonyl azides at low
temperatures.17 At ambient temperatures in solution, the
situation is more complex. Lwowski classified photolysis of
azides as an occasionally useful method of generating sulfo-
nylnitrenes.18 There are three possible sources of complications;
alternate mechanistic pathways may yield apparent nitrene
products, rearrangement of singlet sulfonylnitrene, and elimina-
tion of SO2 from either the azide precursor or the nitrene itself.
Both ground state and excited state azides may react to yield
the same products as nitrenes. Sulfonyl azides may undergo a
1,3-dipole addition with some olefins at ambient temperature
followed by elimination of nitrogen to give aziridines.13,16
Addition of a sulfonylnitrene to a double bond would also
produce an aziridine. Normally, elevated temperatures are
required for the 1,3-dipole addition. Often the photolysis of
sulfonyl azides with olefins results in tar formation and
polymerization.13 Triplet nitrenes produce the corresponding
amines and amides through consecutive hydrogen abstractions.
It has long been known that reduction of excited state azides
gives the same products.13 Radicals react with azides in the
presence of suitable hydrogen donors such as 2-propanol to yield
the apparent products of triplet nitrenes.19 Maslak demonstrated
that the phosphoramidate observed upon sensitized photolysis
of diethyl phosphorazidate was solely attributable to the triplet
nitrene.12b Unfortunately, such evidence is unavailable for
sulfonyl azides. Although sulfonylnitrenes are resistant to
rearrangement relative to other nitrenes, evidence for a photo-
Curtius rearrangement has been found. Gas phase pyrolysis of
benzenesulfonyl azide produced azobenzene.20 Lwowski et al.
found products that were consistent with phenyl migration to
nitrogen upon irradiation of benzenesulfonyl azide in methanol.21
Benzenesulfonamide, a potential triplet nitrene product, and
N-methoxybenzenesulfonamide attributed to singlet nitrene
insertion into the methanol O-H bond were also observed.
Alternative mechanisms precluding benzenesulfonylnitrene as
an intermediate were also proposed to explain the isolated
compounds. As an added complication, each product was
shown to be photolabile. The photolysis of 4-methylbenze-
Experimental Section
4-Methylbenzenesulfonyl azide 1 was prepared according to
the method described by Leffler and Tsuno.24b 4-Methylben-
zenesulfonamide 5 (Aldrich) was recrystallized from ethanol.
Ethanol (MCB Reagent) was distilled from Mg under nitrogen.
Cyclohexane (Aldrich spectrophotometric grade) was distilled
from Na under nitrogen. 2-Methyltetrahydrofuran (2-MTHF)
and 1,4-cyclohexadiene (Aldrich) were distilled from LAH under
nitrogen. Isoprene (Aldrich) was dried over LAH and distilled
under nitrogen. Distilled water was passed through a Barnstead
NANOpure II filter. All other materials were used without
further purification.
IR were obtained with a Perkin Elmer 683 infrared spectro-
photometer. 1H NMR were recorded on a Hitachi Perkin Elmer
R-24B spectrometer (60 MHz). Absorption spectra were
obtained on a Cary 1 spectrophotometer. Melting points were
recorded on a Thomas Hoover melting point apparatus.
Laser Flash Photolysis. The laser flash photolysis apparatus
used to acquire transient absorption spectra and lifetimes has
been described previously.10 Samples were irradiated with
pulses from a frequency-quadrupled Nd-YAG laser (266 nm,
<35 mJ, 5 ns) in either suprasil quartz 1 cm path length static
cells or flow cells. Solutions were deaerated with a stream of
deoxygenated nitrogen passing through the static cells or flow
cell reservoir. Solution concentrations were adjusted so that
the absorbance was 0.5-1.0 at 266 nm.
nesulfonyl azide in methanol gave unexpected products.22
A
third characteristic of sulfonyl azides not encountered with
phosphoryl azides was their tendency to eliminate SO2 through
radical chain processes.23,24 Radical initiators were found to
catalyze the decomposition of some sulfonyl azides. However,
4-methylbenzenesulfonyl azide was shown to generate little
SO2 upon thermolysis. In aprotic solvents, C-H insertion
products have been considered to arise from a nitrene
intermediate.21a,23b
Low-Temperature Absorption Spectra. Solutions were
deaerated by four freeze/pump/thaw cycles in a suprasil quartz
0.3 cm path length cell. Absorption spectra at 77 K were
obtained by placing the cell in a quartz Dewar fitted with optical
windows filled with liquid nitrogen. Spectra of the sample
before and after irradiation (254 nm) in the Dewar were then
acquired using the Cary 1 spectrophotometer.
Low-Temperature EPR Spectra. Solutions of 1 (0.1 M)
were deaerated by bubbling nitrogen through them in 4 mm
o.d. suprasil quartz tubes. Samples were placed in a quartz
optical Dewar filled with liquid nitrogen and irradiated in a mini-
Rayonet photochemical reactor (Southern New England Ultra-
violet Co.) (254 nm). EPR spectra were obtained using either
a Varian E-112 X-band spectrophotometer or a Bruker ESP 300
X-band spectrophotometer.
Given the complexity of sulfonyl azide and sulfonylnitrene
chemistry, one may question their suitability for LFP. The
example of the carbene fluorenylidene may be useful.2,25,26 The
reactions of the triplet state carbene are readily observable with
LFP. Yet, product studies show that with hydrocarbons, the
reaction of the singlet state predominates. Triplet state reactions
constitute minor pathways. This illustrates that LFP can be
applied effectively to species even when they represent a minor
side reaction. If several criteria are met, LFP may be applied
Irradiation of 4-Methylbenzenesulfonyl Azide in Metha-
nol. Aliquots (1 mL) of a 2.0 × 10-2 M solution of 1 in

5790 J. Phys. Chem., Vol. 100, No. 14, 1996
Garay et al.
SCHEME 1
methanol were deaerated by four freeze/pump/thaw cycles in a
15 mm i.d. suprasil quartz tube. Samples were irradiated for
10 min in the aforementioned mini-Rayonet photochemical
reactor. Photosylates were analyzed by GC or GC-MS. Gas
chromatographic analyses were carried out on a FID-equipped
Shimadzu GC-14A gas chromatograph using a 15 m × 0.53
mm i.d. Rtx-5 capillary column (Restek) in isothermal mode at
110 °C. GC-MS analyses were carried out on a TCD-equipped
HP-5890 Series II gas chromatograph in tandem with a HP-
5971A mass selective detector (Tri-State University). A 30 m
× 0.25 mm i.d. DB-5 capillary column (J & W Scientific) was
used with a temperature program from 150 to 200 °C with a 20
°C/min ramp. He was used as the carrier gas for both
instruments.
Figure 1. Transient absorption spectrum observed 500-540 ns after
266 nm laser flash photolysis of 6.54 × 10-4 M 4-methylbenzene-
sulfonyl azide 1 in ethanol (deoxygenated) at ambient temperature.
Results and Discussion
Laser flash photolysis (266 nm) of 4-methylbenzenesulfonyl
azide 1 in ethanol produced a transient spectrum with a strong
absorption at 325 nm and a substantially weaker band at 460
nm (see Figure 1). Two species were present: one exhibited a
first-order decay (τ325 nm ) 8.96 ( 0.48 µs), while the other
did not decay within 70 µs. Both species appeared within the
laser pulse. The 460 nm absorption was too weak to obtain
reliable kinetic data, but its lifetime was comparable to the 325
nm band. LFP of 1 in cyclohexane produced a transient
spectrum with a maximum at 310 nm (see Figure 2). Although
the transient absorption extended beyond 450 nm, a distinct band
could not be discerned in this region.
Transient absorption signal intensities were consistently
weaker in hydroxylic solvents relative to those in cyclohexane.
Enhanced triplet yields were observed in alcohols for
(diphenoxyphosphoryl)nitrene,10,11b pentafluorophenylnitrene,27
and 2,6-difluorophenylnitrene.28 Platz has suggested that hy-
droxylic solvents catalyze intersystem crossing from the singlet
to triplet state.1 If the transients observed in ethanol and
cyclohexane can be assigned to 3, then the catalytic effect is
negligible for this nitrene. A linear relationship between
transient signal intensity at 310 nm and laser energy/pulse over

Spectroscopy and Kinetics of Triplet Nitrenes
J. Phys. Chem., Vol. 100, No. 14, 1996 5791
Figure 4. 2.73 × 10-3 M 4-methylbenzenesulfonyl azide 1 in EPA at
Figure 2. Transient absorption spectrum observed 200-1200 ns after
266 nm laser flash photolysis of 6.8 × 10-4 M 4-methylbenzenesulfonyl
azide 1 in cyclohexane (deoxygenated) at ambient temperature.
77 K: (a) before irradiation and (b) after irradiation for 10 min at 254
nm.
) 1.471 cm-1, |E/hc| ) 0 cm-1) were in reasonable agreement
with those obtained by Smolinsky et al.17a in fluorolube (|D/
hc| ) 1.442 cm-1, |E/hc| ) 0 cm-1). Both in EPA and in
fluorolube, the peak corresponding to the overlap of the X2 and
Y2 transitions was broad (300-350 G). In fluorolube, the
nitrene was reported to be stable for 18 h, while in EPA it slowly
decayed as had been noted in the matrix absorption spectra.
Due to the higher concentrations of azide necessary for the EPR
experiment, the red color of the matrix was more pronounced
after irradiation. The color faded to orange over a time period
that corresponded with the disappearance of the EPR signal. A
second nitrene species was observed with 3 upon irradiation of
the glass. Its ZFSs were |D/hc| ) 0.9840 cm-1 and |E/hc| )
0 cm-1. After prolonged irradiation (5 min) of 1, the ratio of
peak areas of the second nitrene species to 3 increased. These
ZFSs approximate those found for (4-methylphenyl)nitrene (|D/
hc| ) 0.9761 cm-1).17c Prolonged irradiation of 1 (up to 45
min) in the optical Dewar employed for matrix absorption
spectroscopy did not result in the appearance of any new bands.
Considering that the 313 nm band was weak and that irradiation
into the absorption Dewar was inefficient relative to the EPR
Dewar, it possible that a secondary photoproduct may not be
observable in the matrix absorption spectra. Triplet phenylni-
trene was shown to be light sensitive and photorearranged
readily.1 It would be tempting to assign the second species to
(4-methylphenyl)nitrene arising from a photochemical reaction
of 3. The much longer lifetime of the nitrene at 77 K would
allow for multiphotonic processes. As previously described,
the arylnitrene product, azobenzene, was isolated in the pyrolysis
of benzenesulfonyl azide.20 Elimination of SO2 was observed
in the thermolysis of various alkyl- and arylsulfonyl azides.23,24
No alkyl- or arylnitrene products were isolated, however, and
radicals were believed to catalyze decomposition of the azide.
Further EPR and products studies are being conducted to identify
the second species and determine whether it is a photoproduct
of 3 or formed concomitantly with it. Nonetheless, the EPR
signal attributable to 3 and the 313 nm band in the matrix
absorption spectrum are formed under similar conditions, and
both are unstable in EPA at 77 K.
Figure 3. Plot of transient absorption at 310 nm vs laser power (mJ)
upon flash photolysis of 6.33 × 10-4 M 4-methylbenzenesulfonyl azide
1 in cyclohexane (deoxygenated) at ambient temperature.
a 14 mJ range was observed (see Figure 3), indicating that the
transient arises from a monophotonic process. The nonzero
intercept is surprising. The transient absorbance at zero power
roughly corresponds to that of the unidentified long-lived species
and suggests that it is not a photoproduct.
The presence of oxygen or acid had no observable effect on
the peak positions or their relative signal intensities in the
spectrum in Figure 1. Transient spectra from LFP have been
recently reported for arylnitrenium ions in acidic media.29 No
new transients were observed, and no enhancement of the 460
nm absorption was detected upon LFP of 1 in 70% ethanol/
30% water using a 1:1 acetate buffer (2.0 mM). Similarly, there
was no observable difference in the transient spectrum in a
solution of 1 in 70% ethanol/30% water adjusted to a pH of
approximately 2.6 with H2SO4. Failure to observe a nitrenium
ion from a highly electron deficient nitrene is not surprising.
Irradiation (10 min, 254 nm) of 1 in an EPA glass (2:5:5
ethanol, 2-methylbutane, diethyl ether) at 77 K produced the
spectrum in Figure 4 with an absorption maximum at 313 nm.
This band overlapped with a tailing absorption whose maxima
were obscured by the precursor. The species responsible for
the 313 nm transient was not stable in EPA at 77 K. Also, it
disappeared rapidly when the glass was annealed. Upon
irradiation, the glass turned red. After warming to ambient
temperature, the solution was yellow.
Abstraction of hydrogen by 3 from the solvent will produce
the amidyl radical 4. Conceivably, it could be the species
responsible for the transient spectra in ethanol and cyclohexane.
The most convenient method for generating such radicals is to
irradiate the corresponding amine or amide in di-tert-butyl
peroxide. The tert-butoxy radicals initially formed can abstract
hydrogen from the nitrogen to give the radical which can
detected through LFP.30 Strong absorption by the peroxide
Irradiation (75 s, 254 nm) of 1 in an EPA glass at 77 K
produced a spectrum consistent with 4-methylbenzenesulfo-
nylnitrene. The zero-field splitting parameters (ZFSs) (|D/hc|

5792 J. Phys. Chem., Vol. 100, No. 14, 1996
Garay et al.
despite the greater electron-withdrawing ability of the sulfonyl
substituent relative to the phosphoryl substituent.31 Due to the
low number of runs, the lifetime in methanol-d4 is less reliable
than in methanol. The difference between lifetimes is probably
negligible and shows that 4-methylbenzenesulfonylnitrene does
not exhibit a significant kinetic isotope effect (KIE). Liang and
Schuster reported that the reaction of triplet (4-nitrophenyl)-
nitrene with N,N-dimethyl-tert-butylamine had a surprisingly
low KIE.8 It was postulated that a rate-limiting electron transfer
from the amine to the highly electron deficient nitrene to produce
a radical ion intermediate had occurred. Small or negligible
isotope effects would be expected for such a mechanism. No
KIE was observed for the lifetimes of triplet 6 with alcohols.10
This mechanism is currently under investigation for 3 and 6.
The 310 nm transient decay in cyclohexane was too long-lived
to acquire kinetic data on our apparatus. The presence of 1,4-
cyclohexadiene (0.106 M) in cyclohexane had no observable
effect on the lifetime of the transient.
Figure 5. Transient absorption spectrum observed 400-1400 ns after
266 nm laser flash photolysis of 1.9 × 10-3 M 4-methylbenzene-
sulfonamide 5 in ethanol (deoxygenated) at ambient temperature.
The presence of oxygen caused an anomalous decrease in
the lifetime of 3 and its signal intensity in ethanol and
cyclohexane. In air-saturated ethanol and cyclohexane, the
transient signals were weaker. The transient decay was first
order (τ ) 328 ( 44 ns in cyclohexane and τ ) 297 ( 38 ns
in ethanol). Given that the concentration of oxygen in air-
saturated cyclohexane is 2.3 mM,32 the bimolecular rate constant
for oxygen with 3 would be 1.4 × 109 M-1 s-1. In air-saturated
ethanol, the oxygen concentration is 2.07 mM,32 which corre-
sponds to koxygen ) 1.6 × 109 M-1 s-1. Rate constants of this
magnitude between oxygen and a nitrene are unexpected.
Although it is known that nitrenes react with O2,33 the reactions
are slow.8 Liang and Schuster reported that the reaction of (4-
nitrophenyl)nitrene with O2 had a bimolecular rate constant less
than 2 × 105 M-1 s-1. Oxygen was found to have no effect on
the lifetime of triplet (diphenoxyphosphoryl)nitrene.10 The
diffusion control rates at which 3 appears to react with O2 are
perplexing. We can only suggest that sulfonylnitrenes react with
oxygen in a manner unlike other nitrenes. Breslow et al. found
that cleavage of the S-N bond occurred in the thermolysis of
1 in the presence of tert-butyl hydroperoxide to produce the
corresponding sulfinic acid.23a Perhaps, oxygen can react
similarly.
Figure 6. 1.17 × 10-2 M 4-methylbenzenesulfonamide 5 in EPA at
77 K: (a) before irradiation and (b) after irradiation for 15 min at 254
nm.
TABLE 1: Lifetimes of the Transients Assigned to 3 and 6
in Various Solvents
solvent
τ (µs)a of 3
τ (µs) of 6b
ethanol
ethanol-d6
methanol
8.96 ( 0.48
3.8 ( 0.6
4.1 ( 0.8
3.6 ( 0.4
4.0 ( 1.0
Although it is possible to assign the reactive species
responsible for the transient absorption spectra and the matrix
absorption and EPR spectra to 3, the anomalous rate of reaction
with oxygen and the second species observed in the EPR
indicate that care must be taken when making simplistic
comparisons with triplet (diphenoxyphosphoryl)nitrene. With
the latter, the chemistry is relatively straightforward. To
understand why the sulfonylnitrene appears to be so reactive
with O2 and its instability in EPA at 77 K will require further
product studies.
8.18 ( 0.08
8.02 ( 1.76
methanol-d4
a Errors are 2σ. b Reference 10.
below 320 nm limits the use of this method. A less satisfactory
way to generate the amidyl radical would be to directly
photolyze the sulfonamide. Upon LFP of 5 in ethanol, the
transient spectrum consisted of a species with a strong tailing
absorption with a maximum below 280 nm and a second weak
maximum at 460 nm (see Figure 5). At 290 nm, the transient
had a second-order decay (2/kꢀ ) (2.02 ( 0.79) × 107 cm s-1
)
Preliminary results have been obtained for the photolysis of
1 in methanol which are germane to the assignment of the
spectra to 3. Previous product studies involving the photolysis
of arylsulfonyl azides are contradictory. Horner and Christmann
reported that irradiation of 1 in methanol produced 8b, 10b,
12b, and an unidentified product.22 8b is the expected product
of singlet nitrene insertion into the O-H bond. No product of
nitrene insertion into the C-H bond 9b or triplet nitrene product
5 was reported. Minor product 10b was the result of migration
of the methoxy group in 8b. Photolysis of benzenesulfonyl
azide in methanol proceeded somewhat differently.21 The
product of singlet nitrene O-H insertion, 8a, was isolated along
with 11a and 12a. No C-H nitrene insertion product was
observed, but the potential triplet product benzenesulfonamide
in ethanol. A similar spectrum was obtained in EPA at 77 K
(see Figure 6), but the 460 nm peak was either not present or
too weak to be observed. If this species is the amidyl radical
4, then the transients produced upon LFP of 1 may be assignable
to the triplet nitrene.
The decay of the 325 nm transient produced from the nitrene
was first order in hydroxylic solvents (See Table 1). If this
transient can be assigned to triplet 4-methylbenzenesulfonylni-
trene, then the lifetimes demonstrate that it is less reactive than
triplet (diphenoxyphosphoryl)nitrene 6. The |D/hc| value of 3
is smaller than that of triplet 6 (|D/hc| ) 1.5408 cm-1),10
indicating greater spin delocalization in the former. Thus 3
would be expected to be more stable and less reactive than 6

Spectroscopy and Kinetics of Triplet Nitrenes
J. Phys. Chem., Vol. 100, No. 14, 1996 5793
(2) Platz, M. S.; Maloney V. M. In Kinetics and Spectroscopy of
Carbenes and Biradicals; Platz, M. S., Ed.; Plenum: New York, 1990;
Chapter 8 and references therein.
(3) Platz, M. S. In Azides and Nitrenes; Academic Press: Orlando,
FL, 1984; Chapter 7 and references therein.
(4) Thrush, B. A. Proc. R. Soc. London, Ser. A 1956, 253, 143.
(5) Reviews of laser flash photolysis studies of carbenes can be found
in ref 2 and in: Moss, R. A.; Turro, T. J. In Kinetics and Spectroscopy of
Carbenes and Biradicals; Platz, M. S., Ed.; Plenum Press: New York, 1990;
Chapter 7.
(6) Scriven, E. F. V., Ed. Azides and Nitrenes; Academic Press:
Orlando, FL, 1984.
(7) Lwowski, W., Ed. Nitrenes; Wiley Interscience: New York, 1970.
(8) Liang, T.-Y.; Schuster, G. B. J. Am. Chem. Soc. 1987, 109, 7803.
(9) Kobayashi, T.; Ohtani, H.; Suzuki, K.; Yamaoka, T. J. Phys. Chem.
1985, 89, 776.
(10) Houser, M.; Kelley, S.; Maloney, V.; Marlow, M.; Steininger, K.;
Zhou, H. J. Phys. Chem. 1995, 99, 7946.
(11) (a) Breslow, R.; Herman, F.; Schwabacher, A. W. J. Am. Chem.
Soc. 1984, 106, 5359. (b) Breslow, R.; Feiring, A.; Herman, F. J. Am. Chem.
Soc. 1974, 96, 5937.
was isolated in low yield. No 10a due to rearrangement of 8a
was obtained. Instead, a product consistent with singlet nitrene
rearrangement, 11a, was isolated in this case. In our hands,
irradiation of 1 in methanol for 15 min produced only two
products on the basis of GC and GC-MS analysis. From its
mass spectrum and comparison to an authentic sample, 5 was
one of the photoproducts. Since 4-methylbenzenesulfonamide
can arise from alternative pathways that do not involve the triplet
nitrene as an intermediate, its presence is not confirmation of
the assignment of the transient species to 3. However, its
complete absence would have been strong evidence against such
an assignment. The second product had a molecular ion peak
with m/z ) 201. This is consistent with structures 8-11b. The
mass spectrum has a strong molecular ion which precludes 9b.
Assignment of the second product between the three remaining
possibilities cannot be made with complete confidence on the
basis of the mass spectrum alone until comparison with authentic
samples are made. However, the simple O-H insertion product
is the most reasonable candidate. Further investigations are
underway. The significance of these preliminary results is that
the potential triplet product is present.
(12) (a) Maslak, P.; Sczepanski, J. J.; Minard, R. D.; Collins, L. A.
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(14) Jackson, James E., Michigan State University. Personal com-
munication.
Conclusions
The transient absorption produced upon LFP of 1 and the
species observed in the matrix absorption and EPR spectra are
tentatively assigned to the triplet nitrene 3. It is comparable in
nature to the triplet state of 6 whose character was shown to be
intermediate between alkyl- and arylnitrenes. The ZFSs and
lower reactivity indicate that electron delocalization is greater
in 3, although this is not reflected in the absorption spectra where
the nitrene’s strongest band in ethanol (325 nm) is blue shifted
relative to that of 6 (345 nm). Several other features distinguish
3 from the phosphorylnitrene. It is unstable in an EPA matrix
at 77 K and exhibits unique reactivity with oxygen. The
detection of a second nitrene species suggests that 3 is
photolabile or that there is an alternate photochemical reaction
for the precursor azide. With the observation of 3 and 6, there
is now the opportunity to compare the effect of structure on
the reactivity of nitrenes through LFP if sufficiently reactive
substrates can be found. Recently, acyl azides have been
employed in polymer surface modification.34 The relatively
simple chemistry of phosphoryl azides suggest that they may
be potentially useful for such applications. The more complex
behavior of sulfonyl azides illustrated here and in the cited
papers make them less attractive candidates for polymer surface
modification.
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Acknowledgment. This work was supported by a Cottrell
College Science Award of Research Corporation (C-3258), for
which we are grateful. The laser flash photolysis system was
funded in part by the National Science Foundation, which we
gratefully acknowledge. We also thank Professor Robert Berger
of Indiana University Purdue University Ft. Wayne for use of
the Cary 1 spectrophotometer, Professor Matthew Platz of The
Ohio State University for use of the Varian E112 X band EPR
spectrophotometer, Professor Eric Finsden at the University of
Toledo for spectra from the Bruker ESP 300 EPR spectropho-
tometer, and Professors Brooks Bigelow and Ira Jones at Tri-
State University for use of the HP-5890 Series II gas chro-
matograph with the HP-5971A mass selective detector. Finally,
we thank Mr. Jen Lung Wang for his assistance at The Ohio
State University.
References and Notes
(1) Schuster, G. B.; Platz, M. S. In AdVances in Photochemistry;
Volman, D., Hammond, G., Neckers, D. Eds.; John Wiley & Sons: New
York, 1992; Vol. 17.
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