Generation of oxynitrenes and confirmation of their triplet ground states

Article abstract of DOI:10.1021/ja062712g

New sulfoximine- and phenanthrene-based photochemical precursors to oxynitrenes have been developed. These precursors have been used to examine the chemistry and spectroscopy of oxynitrenes. The first EPR spectra of oxynitrenes are reported and are consistent with their triplet ground states. Additional support for the triplet ground state of oxynitrenes is provided by trapping and reactivity studies, nanosecond time-resolved IR investigations, and computational studies.

Full text of DOI:10.1021/ja062712g

Published on Web 09/19/2006
Generation of Oxynitrenes and Confirmation of Their Triplet
Ground States
Walter A. Wasylenko,† Naod Kebede,‡ Brett M. Showalter,† Nikita Matsunaga,§
Alexander P. Miceli,† Yonglin Liu,† Lev R. Ryzhkov,*,| Christopher M. Hadad,*, and
John P. Toscano*,†
Contribution from the Department of Chemistry, Johns Hopkins UniVersity, 3400 North Charles
Street, Baltimore, Maryland 21218, Department of Chemistry, Edinboro UniVersity of
PennsylVania, Edinboro, PennsylVania 16444, Department of Chemistry and Biochemistry, Long
Island UniVersity, 1 UniVersity Plaza, Brooklyn, New York 11201, Department of Chemistry,
Towson UniVersity, 8000 York Road, Towson, Maryland 21252, and Department of Chemistry,
The Ohio State UniVersity, 100 West 18th AVenue, Columbus, Ohio 43210
Received April 18, 2006; E-mail: lryzhkov@towson.edu; hadad.1@osu.edu; jtoscano@jhu.edu
Abstract: New sulfoximine- and phenanthrene-based photochemical precursors to oxynitrenes have been
developed. These precursors have been used to examine the chemistry and spectroscopy of oxynitrenes.
The first EPR spectra of oxynitrenes are reported and are consistent with their triplet ground states. Additional
support for the triplet ground state of oxynitrenes is provided by trapping and reactivity studies, nanosecond
time-resolved IR investigations, and computational studies.
reports of sulfur-substituted nitrenes have also appeared.21-31
)
Introduction
In fact, direct observations of oxynitrene intermediates (RON,
R ) H, Cl, Br, or CN) via low-temperature matrix IR
spectroscopy have only recently been reported.12-15 We en-
countered oxynitrene intermediates in our study of the photo-
chemistry of O2-substituted diazeniumdiolates32 during the
course of our development of novel photochemical precursors
to nitric oxide (NO).33-35
Oxynitrenes have been generated by the oxidation of
O-alkylhydroxylamines3-11,36 and by base-catalyzed decomposi-
tion of N-sulfonyl-O-alkylhydroxylamines.4,7 (Analogous oxida-
Although the parent oxynitrene (HON) is an important
intermediate in combustion and atmospheric chemistry,1,2 only
relatively limited experimental investigations of oxynitrenes
have been reported,3-15 in contrast to the more thoroughly
studied carbon- and nitrogen-substituted nitrenes.16-20 (Limited
† Johns Hopkins University.
‡ Edinboro University of Pennsylvania.
§ Long Island University.
| Towson University.
The Ohio State University.
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13142
J. AM. CHEM. SOC. 2006, 128, 13142-13150
10.1021/ja062712g CCC: $33.50 © 2006 American Chemical Society

Generation of the Triplet Ground State of Oxynitrenes
A R T I C L E S
Scheme 1
tion reactions of the corresponding amino compounds have been
scopic methods, a general, efficient photochemical precursor is
required. Thus, on the basis of the work described above, we
have begun our investigations of oxynitrenes with previously
unreported sulfoximine derivatives 1 as well as phenanthrene
derivatives 2, which have been reported,59 but whose photo-
chemistry has not been previously investigated.
used to generate aminonitrenes19,37-40 and sulfenylnitrenes.23,24
)
In the absence of trapping reagents, oxynitrenes undergo
rearrangement reactions to form nitroso compounds that tau-
tomerize (when possible) to oximes. Although evidence for
oxynitrene intermediates in the above studies has been inferred
by alkene trapping to form N-alkoxyaziridines, the intermediacy
of oxynitrenes in these reactions has been questioned.6,7
N-Alkoxyaziridine trapping products generated by oxidation
of O-alkylhydroxylamines in the presence of cis-2-butene or
trans-2-butene have been found to be formed nonstereospecifi-
cally.7 This observation, together with the assumption (based
on analogy with aminonitrenes and semiempirical calculations36)
that oxynitrenes have singlet ground states, led to the conclusion
that oxynitrene intermediates are not likely formed in the oxi-
dation reaction. However, recent high-level calculations indicate
that HON is a ground-state triplet by 15-20 kcal/mol,41-47
consistent with the above trapping studies. Recently, Maier and
co-workers generated HON (and several isotopomers) in low-
temperature argon matrixes by photolysis of matrix-isolated
nitroxyl (HNdO) and concluded that the observed IR spectrum
fits much better to that calculated (B3LYP/6-311++G** and
QCISD/6-311++G**) for triplet HON compared with that of
singlet HON.14
Results and Discussion
Product Analysis. Products of photolysis were preliminarily
identified by GC/MS and then confirmed and quantified by
HPLC and NMR spectroscopy. As has been analogously found
for N-(arylsulfonyl)sulfoximines,60,61 the major photochemical
pathway for sulfoximines 1 is cleavage of the O-N bond (ca.
75%), rather than formation of oxynitrene and dimethyl sul-
foxide (DMSO) (ca. 25%). For example, photolysis (Rayonet,
254 nm) of 1a yields only 25% DMSO, but significant amounts
of benzaldehyde and S,S-dimethylsulfoximine (3) (Scheme 1).
Control experiments indicate that sulfoximine 3 is stable to the
photolysis conditions, indicating that the observed DMSO is
not formed via secondary photolysis.
Alternative precursors to amino- and sulfenylnitrenes include
sulfoximine derivatives26,38,48,49 and benzonorbornadiene deriva-
tives.25,40,50,51 The latter precursors are analogous to recently
reported carbene precursors that extrude an aromatic compound
upon photolysis.52-58 To investigate the fundamental chemistry
of oxynitrenes by time-resolved or low-temperature spectro-
Photolysis (Rayonet, 300 nm) of phenanthrene-releasing
precursors 2, on the other hand, provides phenanthrene in
essentially quantitative yield (97-100%) along with very high
yields of oxynitrene-derived products, which, as shown in
Scheme 2 for benzyl derivative 2a, are strongly dependent on
the presence of oxygen.
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According to the NIST (webbook.nist.gov) database, the experimental
H-NO bond dissociation energy is 49.9 kcal/mol, and this value is
reasonably reproduced at the CBS-QB3 level of theory (∆H(298 K) ) 48.2
kcal/mol). The bond dissociation energy for triplet H-ON is calculated to
be 21.9 kcal/mol at the CBS-QB3 level.
Trapping studies with benzyloxynitrene and phenoxynitrene
precursors 2a and 2c, respectively, were performed using an
alkene (cis-2-butene) or H-atom donor (1,4-cyclohexadiene). No
trapping is observed with 2a; however, photolysis of 2c in neat
cis-2-butene, to yield both cis- and trans-N-phenoxy-2,3-
dimethylaziridine, and in neat 1,4-cyclohexadiene, to yield
O-phenylhydroxylamine, strongly suggests triplet reactivity.
(The difference in reactivity between benzyloxynitrene and
phenoxynitrene is discussed further below.)
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9
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A R T I C L E S
Wasylenko et al.
Figure 1. TRIR difference spectra averaged over the time scales indicated following laser photolysis (266 nm, 5 ns, 2 mJ) of a solution of phenanthrene-
releasing precursor 2a in argon-saturated acetonitrile-d3 overlaid with bars representing B3LYP/6-31G*-calculated IR frequencies (scaled by 0.96) and
relative intensities of triplet benzyloxynitrene 4. The inset shows the kinetics observed at 1420 cm-1. Negative signals are due to depletion of reactant, and
positive signals are due to the formation of new transients or products.
Figure 2. TRIR difference spectra averaged over the time scales indicated following laser photolysis (266 nm, 5 ns, 2 mJ) of a solution of phenanthrene-
releasing precursor 2a in oxygen-saturated acetonitrile-d3. Negative signals are due to depletion of reactant, and positive signals are due to the formation of
new transients or products.
Scheme 2
Time-Resolved IR Studies. Reaction product analyses were
supported by nanosecond time-resolved infrared (TRIR) inves-
tigations. After 266 nm laser photolysis of the phenanthrene-
releasing precursor 2a in argon-saturated acetonitrile-d3 (which
lacks strong C-H bending modes in the spectral region of
interest), a broad, weak signal (kdecay ) 2.5 × 106 s-1) attributed
to triplet benzyloxynitrene 4 is observed from 1330 to 1450
cm-1 (overlapping with precursor depletion bands) in reasonable
agreement with B3LYP/6-31G* calculations (Figure 1). In
oxygen-saturated solutions, this signal is completely quenched
while new and intense IR bands assigned to benzyl nitrate (1286
and 1638 cm-1) and benzaldehyde (1706 cm-1) are observed
(Figure 2). Although we are unable to detect TRIR signals
attributable to methoxynitrene under argon, analogous TRIR data
in oxygen-saturated acetonitrile-d3 (i.e., formation of formal-
dehyde and methyl nitrate) are observed upon 266 nm laser pho-
tolysis of the phenanthrene-releasing precursor 2b (Figure 3).
On the basis of the observation that benzyloxynitrene 4 is
completely quenched by millimolar concentrations of oxygen,
we estimate that the rate constant for oxynitrene reaction with
oxygen is on the order of 109 M-1 s-1. (The concentration of
oxygen in saturated acetonitrile solutions is 9.1 mM.62) This
rate constant is consistent with that for a triplet carbene reaction
with oxygen,63-67 but is in stark contrast to rate constants for
reaction of arylnitrenes with oxygen, which are known to be
very sluggish, typically near 105 M-1s-1 68-73
.
(62) Clark, W. D. K.; Steel, C. J. Am. Chem. Soc. 1971, 93, 6347-6355.
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9
13144 J. AM. CHEM. SOC. VOL. 128, NO. 40, 2006

Generation of the Triplet Ground State of Oxynitrenes
A R T I C L E S
Figure 3. TRIR difference spectra averaged over the time scales indicated following laser photolysis (266 nm, 5 ns, 2 mJ) of a solution of phenanthrene-
releasing precursor 2b in oxygen-saturated acetonitrile-d3. Negative signals are due to depletion of reactant, and positive signals are due to the formation of
new transients or products.
Table 1. Calculated Singlet-Triplet Energy Gapsa (kcal/mol) for
electron donors and help to stabilize resonance forms, such as
B, for the singlet state. Oxygen, on the other hand, is less good
as an electron donor and thereby favors the triplet state. Charge
density difference plots between the singlet and triplet states
for these X-N species are provided in the Supporting Informa-
tion and confirm that the lone pair orbitals on the X donor and
the nitrene N’s p orbitals are the most important.
Heteroatomic Nitrenes (X-N)
computational
method
H2N−N
CH3HN
−N
HO
−N
CH3O
−N
HS
−N
CH3S−N
MP2/6-311G**
CCSD/6-311G**
MRCISD/6-311G** -12.1
BPW91/cc-pVTZ
B3LYP/6-311G**
CBS-QB3
-10.9 -18.9
-11.8 -19.1
23.7
20.0
20.2
18.4
20.5
16.0
14.8
16.8
15.4
9.4
7.7
15.2
-0.8 -11.4
-1.9 -8.7
b
0.4
b
-15.1 -20.5
-12.7 -17.7
-16.8 -22.2
-17.4 -22.9
-5.0 -10.7
-1.0 -7.0
11.5 -11.1 -16.1
10.1 -12.4 -18.1
G2
a Negative values signify that the singlet state is lower in energy.
Geometries have been optimized at the specified level for the (closed-shell)
singlet and the (open-shell) triplet states. b Not determined.
Further insight into the origin of the preference for the triplet
state of the oxynitrene can be obtained with the following
isodesmic reaction:
The reaction of oxynitrenes with oxygen has recently been
examined by computational methods.74 It was concluded that
the rapid reaction of benzyloxynitrene with oxygen is the result
of partial electron transfer in the transition state, followed by
rapid collapse of the “ion pair” to form a nitrene oxide that
subsequently undergoes a facile rearrangement to a nitrate via
a dioxaziridine intermediate. Such a pathway has been pro-
posed previously for oxynitrenes formed from photolysis of
O2-substituted diazenium diolates.32
Spin States of Oxynitrenes. When we first encountered
oxynitrenes, we anticipated by analogy with aminonitrenes that
oxynitrenes would have singlet ground states. As discussed
above, however, high-level calculations have recently indicated
that HON is a ground-state triplet by 15-20 kcal/mol.41-47 We
addressed the energetics of oxynitrenes, as well as their amino
and sulfenyl analogues, by computational methods, and the
results are presented in Table 1, with a positive singlet-triplet
energy gap (∆EST) indicating a triplet ground state. Substitution
of the hydrogen in HON by an electron-donating methyl group
lowers the singlet nitrene energy relative to the triplet energy
in each of the heteroatomic nitrenes investigated. The calcula-
tions at the G2 and CBS-QB3 levels are probably the most
reliable estimates for the singlet-triplet energy gaps as the
singlet states for these nitrenes are well described by a single-
reference, closed-shell wave function (see below). Such stabi-
lization can be rationalized in terms of singlet nitrene resonance
form B and the ability of the X group to donate electron density
to the nitrene’s N center. Amino and sulfenyl groups are better
H2NCH2O-N1,3 f HOCH2NH-N1,3
For this hypothetical reaction, we compute ∆H(0 K) of the
reaction to be -37.8 kcal/mol when comparing the singlet
nitrenes and only -5.4 kcal/mol for the triplet nitrenes at the
CBS-QB3 level. Thus, the more electron-donating amino
substituent does stabilize the singlet and triplet nitrenes, but the
singlet nitrene is more strongly stabilized by the adjacent amino
substituent. This differential substituent effect on the amino-
and oxynitrenes provides for the large change in singlet-triplet
energy gap (∆H(0 K)) between H2NCH2O-N (+11.1 kcal/mol,
favoring the triplet) and HOCH2NH-N (-21.4 kcal/mol,
favoring the singlet) at the CBS-QB3 level. These singlet-triplet
energy gaps are similar to the values for CH3NH-N and
CH3O-N in Table 1.
Calculations at the B3LYP/6-311G** level for the nitrene’s
X-N bond lengths are provided in Table 2 and for other levels
in the Supporting Information. Calculated bond lengths for the
singlet state are consistently shorter than those in the triplet state,
with larger changes observed for amino- and sulfenylnitrenes,
which are predicted to have singlet ground states.
We have also examined the relative energies of closed-shell
and open-shell singlet states for H2N-N, HO-N, and HS-N
by the MCSCF and MRCISD methods (Table 3). In each case,
the closed-shell singlet (1A′) is calculated to be lower in energy
relative to the open-shell singlet (1A′′) by approximately 50,
15, and 27 kcal/mol, respectively. These results are in contrast
to those of phenylnitrene, which is a ground-state triplet by 18
(74) Liu, J.; Hadad, C. M.; Platz, M. S. Org. Lett. 2005, 7, 549-552.
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A R T I C L E S
Wasylenko et al.
Table 2. B3LYP/6-311G**-Calculated Nitrene
Heteroatom-Nitrogen Bond Lengthsa
nitrene
(X N)
singlet X
−
N bond
triplet X
−
N bond
∆
(bond length)
−
length (Å)
length (Å)
(singlet triplet) (Å)
−
H2N-N
CH3HN-N
HO-N
1.212
1.206
1.255
1.228
1.510
1.504
1.343
1.355
1.324
1.310
1.668
1.650
-0.131
-0.149
-0.069
-0.082
-0.158
-0.146
CH3O-N
HS-N
CH3S-N
a Geometries have been optimized at the B3LYP/6-311G** level for the
(closed-shell) singlet and the (open-shell) triplet states.
Table 3. Calculated Relative Energies (kcal/mol) between the 1A′
and 1A′′ States of Heteroatomic Nitrenes (X-N)a
H2N−N
HO
−N
HS−N
1A
′
1A′′
1A
′
1A′′
1A
′
1A′′
MCSCF/6-311G**
MRCISD/6-311G**
0.0
0.0
53.6
45.7
0.0
0.0
15.8
13.7
0.0
0.0
24.4
29.1
a Geometries have been optimized at the specified level for the (closed-
shell) 1A′ and the (open-shell) 1A′′ states (see the Experimental Section for
details on the calculations).
kcal/mol, but whose lowest singlet state is the open-shell
configuration.75 However, these results are completely consistent
with those reported by Liu et al. for other oxynitrenes.74
Low-Temperature ESR Spectroscopy. To confirm experi-
mentally the production of triplet oxynitrenes upon photolysis
of phenanthrene-releasing precursors 2a-c and O2-methyl
diazeniumdiolate 8, these precursors were irradiated in frozen
matrixes and examined by low-temperature EPR spectroscopy.
The results, which represent the first EPR detection of oxyni-
trene intermediates, are shown in Figure 4. In each case, two
high-field oxynitrene absorptions are observed in the region
between 8000 and 10000 G. The zero-field parameters D and
E (Table 4) were derived assuming the g value of each nitrene
was equal to the free electron value (ge). Also included in Table
4 are D and E values previously determined for other repre-
sentative alkyl-, aryl-, and carbonylnitrenes. Interestingly, our
experimentally determined oxynitrene D values (1.96, 1.93, and
1.97 cm-1) are all comparable to that derived for imidogen
Figure 4. Low-temperature EPR spectra obtained after irradiation at 10 K
of the (a) diazenium diolate 8 in methylcyclopentane, (b) methoxynitrene
precursor 2b in methylcyclopentane, (c) phenoxynitrene precursor 2c in
methylcyclohexane, and (d) benzyloxynitrene precursor 2a in methylcy-
clohexane.
Table 4. Nitrene EPR Zero-Field Parameters
R
R
′
D
(cm-
E
(cm-
1
1
(in RO
−N)
(in R′−N)
)
)
ref
CH3 (from 8)
CH3 (from 2b)
Ph
1.96
1.96
1.93
1.97
1.86
1.595
1.720
0.9978
0.9882
1.603
1.57
0.0142
0.0147
0.0141
0.0118
0
this work
this work
this work
this work
76
76
77
76
76
76
79
80
CH2Ph
H
CH3
<0.003
(H-N, D ) 1.86 cm-1 76
and much larger than that of
)
methylnitrene (CH3-N, D ) 1.595 or 1.720 cm-1).76,77
Since the D value is proportional to the inverse cube of the
average distance between the two unpaired electrons,78 smaller
D values in nitrenes are usually interpreted to signify greater
delocalization of spin density away from the nitrogen atom.76
Thus, the oxynitrene D values presented here suggest very little
or no delocalization of electron spin. This observation is
consistent with the UMP2/6-311G**-calculated spin density on
N (1.764 and 1.719) and on O (0.204 and 0.217) for HO-N
and CH3O-N, respectively. However, the oxynitrene E values
determined in this work are significantly different from zero,
much like those of (m-bromophenyl)nitrene or (ethoxycarbonyl)-
nitrene (Table 4). Perhaps the repulsive interaction between the
oxygen lone pairs and the unpaired electrons on nitrogen serve
Ph
<0.002
0.0075
0.0215
0
m-BrPh
EtOC(dO)
Me3Si
(PhO)2P(dO )
1.541
0.0074
to reduce the effective distance between the two unpaired
electrons. Such an interaction may also result in a nonsym-
metrical distribution of the unpaired electrons, thus leading to
an enhanced E value. Another potential explanation involves a
significant contribution from spin-orbit coupling that makes
the g values of oxynitrenes significantly different from ge; thus,
inaccurate D and E are derived since g is assumed to be equal
to ge.76,78
Oxynitrene to Nitroso Rearrangement. The mechanism of
HON (triplet ground state) rearrangement to HNdO (singlet
ground state) has been examined in detail by computational
methods.1,2 Mechanistic possibilities include concerted or step-
wise (via a H-atom and NO) reactions on either a singlet or a
triplet potential energy surface. Results indicate that concerted
(75) Borden, W. T.; Gritsan, N. P.; Hadad, C. M.; Karney, W. L.; Kemnitz, C.
R.; Platz, M. S. Acc. Chem. Res. 2000, 33, 765-771 and references therein.
(76) Wasserman, E. Prog. Phys. Org. Chem. 1971, 8, 319-336.
(77) Carrick, P. G.; Brazier, C. R.; Bernath, P. F.; Engelking, P. C. J. Am. Chem.
Soc. 1987, 109, 5100-5102.
(78) Wertz, J. E.; Bolton, J. R. Electron Spin Resonance: Elementary Theory
and Practical Applications; Chapman and Hall: New York, 1986.
9
13146 J. AM. CHEM. SOC. VOL. 128, NO. 40, 2006

Generation of the Triplet Ground State of Oxynitrenes
A R T I C L E S
Scheme 3
Experimental Section
General Methods. Unless otherwise noted, materials were obtained
from Aldrich Chemical Co., Fisher Scientific, or Cambridge Isotope
Laboratories and were used without further purification. O2-Methyl
1-(N,N-diethylamino)diazen-1-ium-1,2-diolate (8) was kindly provided
by Drs. Joseph E. Saavedra and Larry K. Keefer at the NCI in Frederick,
MD. Dichloromethane was distilled from phosphorus pentoxide and
tetrahydrofuran was distilled from sodium/benzophenone before use.
Dimethyl sulfoxide was distilled from calcium hydride under vacuum.
1H NMR spectra were recorded on a Bruker AMX 300 (300 MHz) or
a Varian Unity Plus 400 (400 MHz) Fourier transform NMR spec-
trometer. 13C NMR spectra were recorded on a Varian Unity Plus 400
(100 MHz) Fourier transform NMR spectrometer. Resonances are
reported in δ units downfield from the peak for tetramethylsilane. Mass
spectra were collected with a VG 70-S mass spectrometer in the fast
atom bombardment (FAB) mode or EI/CI mode with sample induction
via a direct probe, a Kompact Kratos MALDI instrument, or a Shimadzu
QP5050A GC/MS instrument in EI mode with a Shimadzu AOC-20i
autosampler. HPLC analysis was performed on one of two systems:
(1) a Waters Delta 600 system equipped with a model 6000A pump
and a model 2487 dual-wavelength UV detector or (2) a Perkin-Elmer
Series 4 HPLC system with a PE85B variable-wavelength detector.
GC analysis was performed on an HP5790 with an FID detector
equipped with an integrator or a Shimadzu QP5050A GC/MS instru-
ment. Ultraviolet-visible (UV-vis) absorption spectra were obtained
using a Hewlett-Packard 8453 diode array spectrometer. Infrared (IR)
absorption spectra were obtained using a Bruker IFS 55 Fourier
transform infrared spectrometer.
S,S-Dimethyl-N-benzoxysulfoximine (1a). A solution of freshly
prepared tert-butyl hypochlorite83 (3.1 g, 28.5 mmol) in 90 mL of
methylene chloride under nitrogen was cooled using a dry ice/
chloroform bath. A solution of dimethyl sulfoxide (8.8 g, 113 mmol)
in 40 mL of dichloromethane was then added slowly, and the resulting
solution was allowed to stir for 1 h. O-Benzylhydroxylamine (3.6 g,
29.5 mmol) in 40 mL of dichloromethane was added to this mixture;
the solution was allowed to stir for an additional 3 h. This was followed
then by the addition of triethylamine (7.25 g, 72 mmol) in 30 mL of
dichloromethane. After the addition was complete, the mixture was
allowed to warm to room temperature. The mixture was washed with
water (5×) and dried with sodium sulfate, and the solvent was removed.
The residue was chromatographed on silica gel using 70% ethyl acetate/
hexane as an eluent to give 0.90 g of 1a (15%) as a white solid: 1H
NMR (CDCl3) δ 3.06 (6 H, s), 4.87 (2 H, s), 7.38 (5 H, m); 13C NMR
(CDCl3) δ 37.44, 79.29, 127.99, 128.28, 128.90, 137.35; MS (FAB)
200 (M + 1), 222 (M + Na); UV-vis (CH3CN) λmax 210, 260 nm
(ꢀ254 ) 380 M-1 cm-1).84
Scheme 4
rearrangement of HON to HNdO involves a barrier that is
higher than the dissociation barrier (to H• + •NO) on the singlet
surface.1,42 On the triplet surface, the concerted rearrangement
barrier (to triplet HNdO) is calculated to be higher1 or
comparable42 to the dissociation barrier. Maier has obtained
some evidence for the dissociation mechanism of rearrangement
in his low-temperature matrix IR studies of HON.14
We reasoned that if the rearrangement does indeed occur
stepwise, then the rate of rearrangement should depend on the
radical stability. When phenoxynitrene precursor 2c is photo-
lyzed, we find low yields of nitrosobenzene, which if formed
via the stepwise process would be required to proceed through
the very unstable phenyl radical (Scheme 3). The major
decomposition pathway observed for phenoxynitrene is forma-
tion of phenol. We propose that the phenol arises from
dimerization of phenoxynitrene to the unstable hyponitrite ester
9, which subsequently loses nitrogen and forms phenol, as has
been proposed previously.5,81 For comparison, benzyloxynitrene
produces rearrangement products (oximes 5), presumably
involving formation of a stabilized benzyl radical, almost
exclusively (Scheme 2).
To gain further evidence for a stepwise oxynitrene to nitroso
rearrangement, we have also examined by HPLC the products
formed from oxynitrene 10 (formed via photolysis of its corres-
ponding sulfoximine precursor 1c). We find that both oximes
11 and 12 are produced, providing additional evidence for a
stepwise rearrangement process, in this case through an inter-
mediate (2-phenylcyclopropyl)carbinyl radical82 (Scheme 4).
S,S-Dimethyl-N-(4-methylphenoxy)sulfoximine (1b). Ethyl O-
(mesitylsulfonyl)acetohydroxamate (3.0 g, 11 mmol) was dissolved in
6 mL of dioxane, and then 6 mL of perchloric acid (70%) was added
in portions, resulting in the formation of a precipitate. Once the
perchloric acid addition was complete, the resulting mixture was allowed
to stir for 5-10 min. The mixture was then added to ice-water, and
the precipitate was filtered and dried to give 1.89 g of O-(mesitylsul-
fonyl)hydroxylamine.85
p-Cresol (1.5 g, 13.8 mmol) was dissolved in 20 mL of methanol,
and then potassium tert-butoxide (1.5 g, 13.3 mmol) was added. The
mixture was allowed to stir for 5 min, the methanol was removed, and
the residue was taken up in 10 mL of dichloromethane. Then to this
was added the freshly prepared O-mesitylsulfonylhydroxylamine (1.89
g, 8.8 mmol) in 10 mL of dichloromethane under ice cooling. The
mixture was allowed to stir for 1 h, washed with water, and dried with
Conclusions
Phenanthrene-releasing precursors 2a-c have been shown to
be efficient photochemical precursors to oxynitrenes. These
precursors have been used to obtain the first EPR spectra of
oxynitrenes, consistent with their triplet ground states. The triplet
ground state of oxynitrenes is also reflected in trapping (with
oxygen, cis-2-butene, and 1,4-cyclohexadiene) and reactivity
studies, nanosecond TRIR investigations, and computational
studies. In addition, TRIR studies have been used to estimate
that the rate constant for the reaction of benzyloxynitrene 4 with
oxygen is on the order of 109 M-1 s-1, consistent with previous
computational predictions.74
(79) Feuer, H., Ed. The Chemistry of the Nitro and Nitroso Groups, Part 1;
Wiley: New York, 1969.
(83) Walling, C.; McGuinness, J. A. J. Am. Chem. Soc. 1969, 91, 2053-2058.
(84) Heintzelman, R. W.; Bailey, R. B.; Swern, D. J. Org. Chem. 1976, 41,
2207-2209.
(85) Tamura, Y.; Minamikawa, J.; Sumoto, K.; Fujii, S.; Ikeda, M. J. Org. Chem.
1973, 38, 1239-1241.
(80) Houser, M.; Kelley, S.; Maloney, V.; Marlow, M.; Steininger, K.; Zhou,
H. J. Phys. Chem. 1995, 99, 7946-7950.
(81) Boyer, J. H.; Woodyard, J. D. J. Org. Chem. 1968, 33, 3329-3331.
(82) Newcomb, M. Tetrahedron 1993, 49, 1151-1176.
9
J. AM. CHEM. SOC. VOL. 128, NO. 40, 2006 13147

A R T I C L E S
Wasylenko et al.
sodium sulfate, and the dichloromethane was removed. The residue
was chromatographed on silica gel using dichloromethane as an eluent
to obtain 0.97 g of O-(4-methylphenyl)hydroxylamine (90%): 1H NMR
(CDCl3) δ 2.31 (3 H, s), 5.82 (2 H, br s), 7.06 (4 H, m).86
addition of triethylamine (0.73 g, 7.2 mmol) in 15 mL of dichlo-
romethane. Once the addition was complete, the mixture was allowed
to warm to room temperature. The mixture was diluted with 40 mL of
dichloromethane, washed with water (5×), and dried with sodium
sulfate, and the solvent was removed. The residue was chromatographed
on silica gel using 70% ethyl acetate/hexane as an eluent to give 0.124
g (13%) of 1c as an off-white solid: 1H NMR (CDCl3) δ 0.99 (2 H, t,
J ) 5.4 Hz), 1.53 (1 H, m), 1.86 (1 H, m), 3.08 (6 H, s), 3.84 (2 H,
m), 7.07-7.27 (5 H, m); 13C NMR (CDCl3) δ 14.2, 21.6, 22.0, 37.4,
80.8, 125.5, 125.8, 128.2, 142.6; MS (EI) m/z 131 (95), 91 (100); UV-
vis (CH3CN) λmax 195, 220 nm (ꢀ254 ) 430 M-1 cm-1).
1a,9b-Dihydro-1-benzoxy-1H-phenanthro[9,10-b]azirine (2a). 2,2′-
Biphenyldicarboxaldehyde88 (2.32 g, 10.9 mmol) was dissolved in 25
mL of pyridine with 3 Å molecular sieves, and then to this was added
O-benzylhydroxylamine hydrochloride (1.74 g, 10.9 mmol). The
mixture was allowed to stir for 16 h and then was taken up in ethyl
ether. This mixture was washed with water, 10% hydrochloric acid
solution (4×), and a saturated sodium chloride solution. The organic
layer was dried over magnesium sulfate, and the solvent was removed.
The residue was purified by column chromatography on silica gel using
5% ethyl acetate/hexane as an eluent to give biphenyl-2,2′-dicarbox-
aldehyde mono(O-benzyloxime) as a colorless oil (1.1 g, 32%): 1H
NMR (CDCl3) δ 5.13 (2 H, s), 7.25-7.60 (11 H, m), 7.87 (1 H, s),
8.00 (2 H, m), 9.76 (1 H, s).59
A solution of freshly prepared tert-butyl hypochlorite83 (1.3 g, 12
mmol) in 45 mL of methylene chloride under a nitrogen atmosphere
was cooled using a dry ice/chloroform bath. A solution of dimethyl
sulfoxide (4.4 g, 56 mmol) in 20 mL of dichloromethane was added
slowly, and the resulting solution was allowed to stir for 1 h. O-(4-
Methylphenyl)hydroxylamine (1.2 g, 9.7 mmol) in 20 mL of dichlo-
romethane was subsequently added, and the resulting solution was
allowed to stir for an additional 3 h. This was followed by the addition
of triethylamine (2.9 g, 29 mmol) in 15 mL of dichloromethane. Once
the addition was complete, the mixture was allowed to warm to room
temperature. The mixture was diluted with 40 mL of dichloromethane,
washed with water (5×), and dried with sodium sulfate, and the solvent
was removed. The residue was chromatographed on silica gel using
70% ethyl acetate/hexane as an eluent to give 0.41 g of 1b (21%) as
a light brown solid: 1H NMR (CDCl3) δ 2.28 (3 H, s), 3.21 (6 H, s),
1
7.06 (4 H, s); H NMR (CD3CN) δ 2.28 (3 H, s), 3.12 (6 H, s), 7.03
(4 H, m); 13C NMR (CDCl3) δ 20.58, 37.57, 113.74, 129.61, 130.54,
159.59; MS (FAB) m/z 199 (M), 222 (M + Na); UV-vis (CH3CN)
λmax 225, 280 nm (ꢀ254 ) 7446 M-1 cm-1).84
S,S-Dimethyl-N-[(2-phenylcyclopropyl)methoxy]sulfoximine (1c).
Lithium aluminum hydride (0.772 g, 20.3 mmol) was added to a flame-
dried round-bottom flask followed by 45 mL of tetrahydrofuran. To
this slurry was added 2-phenylcyclopropanecarboxylic acid (3.00 g,
18.5 mmol) in 45 mL of tetrahydrofuran. The mixture was allowed to
stir at room temperature for 2.5 h and then at a gentle reflux overnight.
The reaction was quenched with addition of methanol (2 mL), water
(2 mL), and 10% aqueous sodium hydroxide (2 mL). The solids were
then filtered, and the solvent was removed. The residue was taken up
in ethyl ether, washed with water, and dried over magnesium sulfate,
and the solvent was removed to obtain (2-phenylcyclopropyl)methanol
(1.90 g, 70%) as a colorless oil: 1H NMR (CDCl3) δ 0.96 (2 H, m),
1.46 (1 H, m), 1.82 (1 H, m), 3.62 (2 H, m), 7.08 (2 H, m), 7.18 (1 H,
m), 7.27 (2 H, m).
This alcohol (1.60 g, 11 mmol) was combined with N-hydroxy-
phthalimide (1.80 g, 11 mmol), and triphenylphosphine (2.88 g, 11
mmol) in 50 mL of tetrahydrofuran, followed by the dropwise addition
of diisopropyl azodicarboxylate (2.43 g, 12 mmol). The mixture was
then allowed to stir for 2 days. The solvent was removed, and the
residue was chromatographed on silica gel using 15% ethyl acetate/
petroleum ether mixture as an eluent. The product was recrystallized
using dichloromethane/petroleum ether to provide 2-[(2-phenylcyclo-
propyl)methoxy]isoindole-1,3-dione as colorless needles (0.83 g,
26%): 1H NMR (CDCl3) δ 1.04 (2 H, m), 1.63 (1 H, m), 1.90 (1 H,
m), 4.23 (2 H, m), 6.98 (2 H, m), 7.18 (3 H, m), 7.75 (4 H, m).87
This dione (1.3 g, 4 mmol) was dissolved in 15 mL of ethanol and
combined with 0.50 mL of hydrazine hydrate. This solution was heated
at reflux for 1 h and then cooled to room temperature. The solids that
formed were dissolved with 3% aqueous sodium carbonate solution
(100 mL), and then the product was extracted with ethyl ether. The
ethyl ether was removed to obtain O-[(2-phenylcyclopropyl)methyl]-
hydroxylamine as a colorless oil.87
This oxime (2.40 g, 7.6 mmol) was dissolved in 20 mL of methanol
followed by the addition of p-toluenesulfonic acid hydrazide (1.55, 8.3
mmol). The mixture was allowed to stir for 16 h, and the precipitate
that was formed was filtered and dried to give 3.4 g (92%) of
4-methylbenzenesulfonic acid [[2′-[(benzoxyimino)methyl][1,1′-biphe-
nyl]-2-yl]methylene]hydrazide as a white powder: 1H NMR (CDCl3)
δ 2.40 (3 H, s), 5.09 (2 H, s), 7.11 (2 H, m), 7.28-7.38 (11 H, m),
7.62 (1 H, s), 7.79 (1 H, s), 7.81 (2 H, d, J ) 8.3 Hz), 7.97 (2 H, m).59
This hydrazide (3.4 g, 7.0 mmol), dissolved in 35 mL of tetrahy-
drofuran, was added under nitrogen to a slurry of sodium hydride (0.18
g, 7.5 mmol) and 5 mL of tetrahydrofuran. Once the addition was
complete, the mixture was allowed to stir for 2 days. The tetrahydro-
furan was removed, and the reaction mixture was taken up in
dichloromethane and washed with water. The organic layer was dried
over sodium sulfate, and then the solvent was removed. The residue
was purified by column chromatography on silica gel using 5% ethyl
acetate/hexane to obtain 0.87 g (42%) of 2a: 1H NMR (CDCl3) δ 3.70
(2 H, s), 4.85 (2 H, s), 7.27-7.37 (11 H, m), 7.94 (2 H, d, J ) 6.4
Hz); MS (FAB) m/z 300 (M + 1), 322 (M + Na); UV-vis (CH3CN)
λmax 210, 250, 280, 310 nm (ꢀ300 ) 4664 M-1 cm-1); IR (neat) 3061,
3033, 2912, 2841, 1488, 1454, 1354, 1259, 1240, 1202, 1152, 1012
cm-1 59
.
1a,9b-Dihydro-1-methoxy-1H-phenanthro[9,10-b]azirine (2b). 2,2′-
Biphenyldicarboxaldehyde88 (2.75 g, 12.9 mmol) was dissolved in 20
mL of pyridine with 3 Å molecular sieves, and then O-methylhydroxy-
lamine hydrochloride (0.98 g, 11.8 mmol) was added. The mixture was
allowed to stir for 3 days and then taken up in ethyl ether. The ether
layer was washed with water, 10% hydrochloric acid solution (4×),
and a saturated sodium chloride solution. The organic layer was dried
over sodium sulfate, and the solvent was removed. The residue was
purified by column chromatography on silica gel using 5% ethyl acetate/
hexane to give biphenyl-2,2′-dicarboxaldehyde mono-(O-methyloxime)
as a colorless oil (1.0 g, 35%): 1H NMR (CDCl3) δ 3.90 (3 H, s),
7.29 (2 H, m), 7.44-7.64 (4 H, m), 7.78 (1 H, s), 8.02 (2 H, m), 9.76
(1 H, s).59
A solution of freshly prepared tert-butyl hypochlorite83 (0.80 g, 7.4
mmol) in 20 mL of methylene chloride was cooled using a dry ice/
chloroform bath under nitrogen, and a solution of dimethyl sulfoxide
(2.2 g, 28 mmol) in 15 mL of dichloromethane was added slowly; the
resulting solution was allowed to stir for 1 h. Then freshly prepared
O-[(2-phenylcyclopropyl)methyl]hydroxylamine (0.65 g, 4.0 mmol) in
20 mL of dichloromethane was added, and the resulting solution was
allowed to stir for an additional 3 h. This was followed then by the
Biphenyl-2,2′-dicarboxaldehyde mono(O-methyloxime) (1.0 g, 4.1
mmol) was dissolved in 20 mL of methanol, and then p-toluenesulfonic
acid hydrazide (0.85, 4.5 mmol) was added. The mixture was allowed
to stir for 48 h, and the precipitate that was formed was filtered and
(86) Haga, N.; Endo, Y.; Kataoka, K.; Yamaguchi, K.; Shudo, K. J. Am. Chem.
Soc. 1992, 114, 9795-9806.
(87) Grochowski, E.; Jurczak, J. Synthesis 1976, 682-684.
(88) Ligtenbarg, A. G. J.; van den Beuken, E. K.; Meetsma, A.; Veldman, N.;
Smeets, W. J. J.; Spek, A. L.; Feringa, B. L. J. Chem. Soc., Dalton Trans.
1998, 263-270.
9
13148 J. AM. CHEM. SOC. VOL. 128, NO. 40, 2006

Generation of the Triplet Ground State of Oxynitrenes
A R T I C L E S
dried to give 1.15 g (69%) of 4-methylbenzenesulfonic acid [[2′-
[(methoxyimino)methyl][1,1′-biphenyl]-2-yl]methylene]hydrazide as a
white powder: 1H NMR (CDCl3) δ 2.42 (3 H, s), 3.86 (3 H, s), 7.15
(2 H, m), 7.31 (2 H, d, J ) 8.0 Hz), 7.39 (4 H, m), 7.54 (1 H, s), 7.65
(1 H, s), 7.81 (2 H, d, J ) 8.3 Hz), 7.97 (2 H, m).59
NMR (CD3CN) δ 2.95 (6 H, s); MS (EI) m/z 93 (90), 78 (100), 60
(79), 46 (70); IR (neat) 3270, 1211, 1045 cm-1 89
.
syn- and anti-Benzaldehyde Oximes 5. The syn-benzaldehyde
oxime is commercially available (Aldrich), and the anti-benzaldehyde
oxime can be synthesized from the syn-oxime.90 In a three-neck flask,
under nitrogen, equipped with a reflux condenser and a tube to introduce
hydrochloric acid gas, syn-benzaldehyde oxime (2.42 g, 20 mmol) was
dissolved in 12 mL of benzene. The mixture was brought to reflux,
then heating was stopped, and the flow of hydrochloric acid gas was
started. After approximately 5 min a precipitate started to form, and at
this point the flow of hydrochloric acid gas was discontinued. The
mixture was cooled with an ice bath. The solids were collected by
filtration and then washed with benzene and petroleum ether. To a slurry
of the solids in 20 mL of ethyl ether under ice cooling was added a
precooled solution of sodium hydroxide (15 mL, 2.66 M), and the solids
soon dissolved. Once the solids dissolved, a solution of ammonium
chloride (4.0 g, 75 mmol) in 15 mL of water was added. Solid
precipitated and within 1 min went back into solution. The aqueous
and organic layers were then separated, and the aqueous layer was
extracted again with ethyl ether. The organic layers were dried over
MgSO4, and the solvent was removed. The residue was treated with
petroleum ether, and the solids were then filtered to give 1.05 g (43%)
of the anti-oxime: 1H NMR (CD3CN) δ 7.34 (1 H, s), 7.44 (3 H, m),
7.93 (2 H, m), 9.29 (1 H, s); mp 121-122.5 °C (lit.90 mp 129.5-130
°C).
This hydrazide (1.2 g, 2.8 mmol) was dissolved in 15 mL of
tetrahydrofuran and added under nitrogen to a slurry of sodium hydride
(0.074 g, 3.1 mmol) and 10 mL of tetrahydrofuran. Once the addition
was complete, the mixture was allowed to stir for 2 days. The
tetrahydrofuran was removed, and the reaction mixture was taken up
in dichloromethane and washed with water. The organic layer was dried
over sodium sulfate, and then the solvent was removed. The residue
was purified by column chromatography on silica gel using 5% ethyl
acetate/hexane to obtain 0.20 g (32%) of 2b: 1H NMR (CDCl3) δ 3.65
(3 H, s), 3.73 (2 H, s), 7.32-7.38 (4 H, m), 7.59 (2 H, m), 7.97 (2 H,
d, J ) 7.6 Hz); MS (MALDI) m/z 223 (M), 224 (M + 1); UV-vis
(CH3CN) λmax 217, 250, 280, 310 nm (ꢀ300 ) 4545 M-1 cm-1); IR
(neat) 3059, 3028, 2982, 2935, 2893, 2853, 1490, 1467, 1452, 1433,
1378, 1260, 1240, 1202, 1164, 1043, 1015 cm-1 59
.
1a,9b-Dihydro-1-phenoxy-1H-phenanthro[9,10-b]azirine (2c). 2,2′-
Biphenyldicarboxaldehyde88 (3.43 g, 16.2 mmol) was dissolved in 20
mL of pyridine with 3 Å molecular sieves, and then O-phenylhydroxy-
lamine hydrochloride (1.77 g, 16.2 mmol) was added. The mixture was
allowed to stir for 15 h and then was taken up in ethyl ether. This
mixture was washed with water, 10% hydrochloric acid solution (4×),
and a saturated sodium chloride solution. The organic layer was dried
over sodium sulfate, and the solvent was removed. The residue was
purified by column chromatography on silica gel using 10% ethyl
acetate/hexane as an eluent to give biphenyl-2,2′-dicarboxaldehyde
mono(O-phenyloxime) as a colorless oil (2.8 g, 57%): 1H NMR
(CDCl3) δ 7.12 (3 H, m), 7.31 (4 H, m), 7.50-7.68 (4 H, m), 8.12 (2
H, m), 8.17 (1 H, s), 9.83 (1 H, s).59
Biphenyl-2,2′-dicarboxaldehyde mono(O-phenyloxime) (2.8 g, 9.29
mmol) was dissolved in 20 mL of methanol, and then p-toluenesulfonic
acid hydrazide (1.9 g, 10.2 mmol) was added. The mixture was allowed
to stir for 48 h, and the precipitate that was formed was filtered and
dried to give 3.4 g (78%) of 4-methylbenzenesulfonic acid [[2′-
[(phenoxyimino)methyl][1,1′-biphenyl]-2-yl]methylene]hydrazide as a
white powder: 1H NMR (CDCl3) δ 2.36 (3 H, s), 7.02-7.30 (9 H, m),
7.45 (4 H, m), 7.52 (1 H, s), 7.77 (2 H, d, J ) 8.4 Hz), 8.01 (1 H, s),
8.03 (2 H, m).59
Benzyl Nitrate (6). 6 was synthesized according to a literature
procedure.91 The product was distilled under reduced pressure to give
5.2 g of the nitrate as a colorless oil (82%): 1H NMR (CD3CN) δ 5.49
(2 H, s), 7.44 (5 H, m); IR (neat) 1634, 1280 cm-1
.
2-Phenylcyclopropanecarboxaldehyde Oxime (11). To a solution
of (2-phenylcyclopropyl)methanol (0.21 g, 1.4 mmol) in dry dichlo-
romethane under nitrogen was added pyridinium chlorochromate (PCC)
(0.34 g, 1.6 mmol). The mixture was allowed to stir overnight. The
mixture was diluted with dichloromethane and then passed through a
plug of Celite and silica gel. The solvent was removed to give
2-phenylcyclopropanecarboxaldehyde: 1H NMR (CDCl3) δ 1.5-1.8
(2 H, m), 2.17 (1 H, m), 2.65 (1 H, m), 7.07-7.38 (5 H, m), 9.32 (1
H, d, J ) 5.2 Hz).
This aldehyde (0.21 g, 1.4 mmol) was dissolved in 4 mL of pyridine
and hydroxylamine hydrochloride (0.11 g, 1.6 mmol) added. The
mixture was allowed to stir for 1 h. Then the mixture was taken up in
ethyl ether and washed with water, 10% aqueous hydrochloric acid
(2×), and a saturated sodium bicarbonate solution. The organic layer
was dried with magnesium sulfate, and the solvent was removed to
give the oxime 11: 1H NMR (CDCl3) δ 1.25-1.50 (2 H, m), 1.88 (1
H, m), 2.18 (1 H, m), 7.10-7.36 (6 H, m); IR (neat) 3307, 2923, 1651,
1456, 1261 cm-1; MS (EI) m/z 161 (24), 144 (85), 115 (100); UV-vis
(CH3CN) λmax 195, 225 nm (ꢀ220 ) 8615 M-1 cm-1).92
4-Phenylbuten-4-one Oxime (12). To a solution of 4-phenylbuten-
4-ol (0.50 g, 3.4 mmol) in dry dichloromethane under nitrogen was
added PCC (0.80 g, 3.7 mmol). The mixture was allowed to stir
overnight. The mixture was diluted with dichloromethane and then
passed through a plug of Celite and silica gel. The solvent was removed
to give 4-phenylbuten-4-one (0.36 g, 72%): 1H NMR (CDCl3) δ 3.76
(2 H, dt, J ) 6.7 Hz, J ) 1.4 Hz), 5.23 (2 H, m), 6.10 (1 H, m), 7.54
(3 H, m), 7.98 (2 H, m).
This hydrazide (3.4 g, 7.2 mmol) was dissolved in 15 mL of
tetrahydrofuran and added under nitrogen to a slurry of sodium hydride
(0.174 g, 7.2 mmol) in 10 mL of tetrahydrofuran. Once the addition
was complete, the mixture was allowed to stir for 2 days. The
tetrahydrofuran was removed, and the reaction mixture was taken up
in dichloromethane and washed with water. The organic layer was dried
over sodium sulfate, and then the solvent was removed. The residue
was purified by column chromatography on silica gel using 5% ethyl
acetate/hexane to obtain 0.67 g (33%) of 2c: 1H NMR (CDCl3) δ 4.03
(2 H, s), 7.03 (3H, m), 7.26 (2 H, m), 7.36-7.43 (4 H, m), 7.62 (2 H,
d, J ) 7.4 Hz), 8.02 (2 H, d, J ) 6.8 Hz); 13C NMR (CDCl3) δ 49.15,
113.68, 121.42, 123.55, 128.44, 128.64, 129.01, 129.39, 130.81, 131.64,
159.18; MS (MALDI) m/z 285 (M); UV-vis (CH3CN) λmax 200, 215,
250, 280, 310 nm (ꢀ300 ) 4717 M-1 cm-1); IR (neat) 3072, 3037, 3021,
1598, 1588, 1490, 1480, 1454, 1224, 1164, 1074, 1022 cm-1 59
.
This ketone (0.36 g, 2.5 mmol) was dissolved in 2 mL of pyridine
and hydroxylamine hydrochloride (0.19 g, 2.7 mmol) added. The
mixture was allowed to stir for 1 h. The mixture was then taken up in
ethyl ether and washed with water, 10% aqueous hydrochloric acid
(2×), and a saturated sodium bicarbonate solution. The organic layer
S,S-Dimethylsulfoximine (3). To a mixture of dimethyl sulfoxide
(2.2 g, 28 mmol), 5 mL of concentrated sulfuric acid, and 20 mL of
dichloromethane was added sodium azide (2.0 g, 31 mmol) using a
powder addition funnel over 1 h. The mixture was allowed to stir for
16 h. The mixture was poured into ice-water and neutralized with
sodium bicarbonate, and the solvent was removed. Ethanol was added
to the residue, and the solids that formed were filtered (2×). The
ethanolic solutions were combined, and the ethanol was removed. The
residue was then heated with an air bath at 60 °C under reduced pressure
(89) Oae, S.; Iida, K.; Takata, T. Phosphorus Sulfur Relat. Elem. 1981, 12, 103-
113.
(90) Schoenewaldt, E. F.; Kinnel, R. B.; Davis, P. J. Org. Chem. 1968, 33,
4270-4272.
(91) Pattison, F. L. M.; Brown, G. M. Can. J. Chem. 1956, 34, 879-884.
(92) Wu, P. L.; Wang, W. S. J. Org. Chem. 1994, 59, 622-627.
1
to remove dimethyl sulfoxide: 1H NMR (CDCl3) δ 3.08 (6 H, s); H
9
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A R T I C L E S
Wasylenko et al.
was dried with magnesium sulfate, and the solvent was removed to
give the oxime 12: 1H NMR (CDCl3) δ 3.59 (2 H, dt, J ) 6.2 Hz, J
) 1.7 Hz), 5.15 (2 H, m), 5.94 (1 H, m), 7.37 (3 H, m), 7.64 (2 H, m),
8.05 (1 H, br s); IR (neat) 3270 cm-1; UV-vis (CH3CN) λmax 205,
248 nm (ꢀ220 ) 6140 M-1 cm-1).92
Photolysis of Sulfoximines 1a-1c and 3. Irradiations were per-
formed on solutions of reactant (10 or 1 mM) in acetonitrile or
acetonitrile-d3 in 1.0 cm quartz cuvettes, sealed with rubber septa, and
purged with argon for 15 min prior to irradiation. Solutions were
irradiated using a Rayonet reactor equipped with 254 nm low-pressure
mercury arc lamps.
Photolysis of Phenanthrene-Releasing Precursors 2a-2c. Irradia-
tions were performed on solutions of the reactant (1 mM) in acetonitrile
or acetonitrile-d3 in 16 mm Pyrex tubes, sealed with rubber septa, and
purged with argon or oxygen for 15 min prior to irradiation. Solutions
were irradiated using a Rayonet reactor equipped with 300 nm low-
pressure mercury arc lamps.
Analysis of Reaction Mixtures. The products of the photolysis were
preliminarily identified using GC/MS and then confirmed by NMR and
HPLC with a co-injection of authentic material. Products were
quantified by HPLC, GC, and NMR spectroscopy. For HPLC analyses
a Waters C-18 Symmetry analytical column (3.9 × 150 mm), a Waters
C-18 Symmetry analytical column (7.8 × 100 mm), or a Phenomenex
C-18 Luna column (4.6 × 250 mm) was used with acetonitrile/water
mixtures as eluents. The column used for GC analysis was an Alltech
30 m Econo-Cap EC-5 capillary column.
Trapping of Benzoxynitrene and Phenoxynitrene upon Photolysis
of Phenanthrene-Releasing Precursors 2a and 2c in cis-2-Butene.
In a typical experiment oxynitrene precursor 2a or 2c was placed in a
Pyrex tube fitted with a dry ice condenser under nitrogen, and cis-2-
butene was condensed into the tube to obtain a 1 mM solution. The
solution was photolyzed with a Hanovia 450 W water-cooled medium-
pressure mercury arc immersion lamp for 2 h. The reaction mixture
was analyzed by GC/MS, NMR, and HPLC. The photolysis mixture
was also compared with authentic aziridines using co-injection in HPLC.
Comparison with authentic aziridines revealed that no trapping had
occurred for benzoxynitrene and that phenoxynitrene was trapped with
cis-2-butene to provide both the cis- and trans-N-phenoxy-2,3-
dimethylaziridine, albeit in very low yield.
Trapping of the Oxynitrene Intermediate upon Photolysis of
Phenanthrene-Releasing Precursors 2a and 2c by 1,4-Cyclohexa-
diene. A solution of oxynitrene precursor 2a or 2c (10 mM) in 1,4-
cyclohexadiene was degassed and irradiated using a Rayonet reactor
equipped with 350 nm low-pressure mercury arc lamps for 1 h. The
reaction mixtures were analyzed by GC/MS, NMR, and HPLC. The
photolysis mixtures were also compared with authentic O-hydroxy-
lamines (free based from HCl salt with triethylamine) employing co-
injections in HPLC. These analyses demonstrated that benzoxynitrene
was not trapped, but phenoxynitrene was trapped to yield O-phenyl-
hydroxylamine, the double H-atom abstraction product, in approxi-
mately 12% yield.
monitored using an ac-coupled mercury/cadmium/tellurium (MCT)
photovoltaic IR detector (Kolmar Technologies, KMPV11-J1/AC),
amplified, digitized with a Tektronix TDS520A oscilloscope, and
collected on a Macintosh with IGOR for data processing. The
experiment is conducted in dispersive mode with a JASCO TRIR 1000
spectrometer.
Electron Paramagnetic Resonance (EPR) Spectroscopy. A 20,
10, or 5 mM solution of the appropriate precursor in either methylcy-
clohexane or methylcyclopentane was subject to at least three freeze-
pump-thaw cycles and sealed under vacuum in a 4 mm quartz EPR
tube. The samples were placed in the cryostat at 10 K and photolyzed
with an Oriel 250 W medium-pressure mercury arc lamp. The EPR
spectra were obtained on an X-band Bruker EMX spectrometer with a
gun diode as the microwave source running at 9.48 GHz. Samples were
cooled by a continuous flow of helium using an Oxford ESR-900
cryostat with a model ITC 503 temperature controller. EPR spectra
were recorded with a microwave power of 10 mW and modulation
amplitude of 20 G with H0 ) 3382 G. Similar EPR spectra were
obtained on an X-band Varian E12 spectrometer with a TE102 cavity
and a klystron as a microwave source.
Computational Methods. Geometries were fully optimized at the
respective level of theory, and all stationary points were confirmed to
be energy minima by vibrational frequency analyses. The B3LYP,96-98
MP2,99 G2,100 and CBS-QB3101 calculations were performed with
Gaussian 98102 and provided the energy difference between the closed-
shell singlet and the open-shell triplet states for the respective nitrenes.
To compute the relative energy of the closed- and open-shell singlet
states, MCSCF and MRCISD methods were employed with the
MOLPRO program.103 The active spaces for the MCSCF and subse-
quent MRCISD calculations were chosen such that, in the triplet state,
one unoccupied orbital in each symmetry (a′ and a′′ of Cs symmetry)
at the Hartree-Fock configuration was included. For HO-N, CH3O-
N, and HS-N, an active space consisting of eight electrons distributed
over seven molecular orbitals was used. For H2N-N, an active space
consisting of ten electrons distributed over nine molecular orbitals was
used. Atomic spin densities, based on Lo¨wdin charges, of triplet HO-N
and CH3O-N were calculated at the UMP2/6-311G**-optimized
geometry using the UMP2/6-311G** wave functions.
Acknowledgment. We gratefully acknowledge the National
Science Foundation (Grant CHE-0518406) and the Ohio Su-
percomputer Center for generous support of this research.
Supporting Information Available: Experimental details for
the synthesis of authentic samples of N-phenoxy- and N-ben-
zoxy-2,3-dimethylaziridines, computational results for hetero-
atomic nitrenes, and full references for Gaussian 98 and
MOLPRO. This material is available free of charge via the
Internet at http://pubs.acs.org.
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conducted following the method of Hamaguchi and co-workers93,94 as
has been described previously.95 Briefly, the broad-band output of a
MoSi2 IR source (JASCO) is crossed with excitation pulses from either
a Quantronix Q-switched Nd:YAG laser (266 nm, 90 ns, 0.4 mJ)
operating at 200 Hz or a Continuum Minilite II Nd:YAG laser (266
nm, 5 ns, 2 mJ) operating at 15 Hz. Changes in IR intensity are
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