Photochemistry of N-acetyl-, N-trifluoroacetyl-, N- mesyl-, and N-tosyldibenzothiophene sulfilimines

Article abstract of DOI:10.1021/jo702654q

(Chemical Equation Presented) Time-resolved infrared (TRIR) spectroscopy, product studies, and computational methods were applied to the photolysis of sulfilimines derived from dibenzothiophene that were expected to release acetylnitrene, trifluoroacetylnitrene, mesylnitrene, and tosylnitrene. All three methods provided results for acetylnitrene consistent with literature precedent and analogous experiments with the benzoylnitrene precursor, i.e., that the ground-state multiplicity is singlet. In contrast, product studies clearly indicate triplet reactivity for trifluoroacetylnitrene, though TRIR experiments were more ambiguous. Product studies suggest that these sulfilimines are superior sources for sulfonylnitrenes, which have triplet grounds states, to the corresponding azides, and computational studies shed light on the electronic structure of the nitrenes.

Full text of DOI:10.1021/jo702654q

Photochemistry of N-Acetyl-, N-Trifluoroacetyl-, N- Mesyl-, and
N-Tosyldibenzothiophene Sulfilimines
Vasumathi Desikan,† Yonglin Liu,‡ John P. Toscano,*,‡ and William S. Jenks*,†
Department of Chemistry, Iowa State UniVersity, 3760 Gilman Hall, Ames, Iowa 50011, and Department
of Chemistry, Johns Hopkins UniVersity, 3400 North Charles Street, Baltimore, Maryland 21218
wsjenks@iastate.edu; jtoscano@jhu.edu
ReceiVed December 18, 2007
Time-resolved infrared (TRIR) spectroscopy, product studies, and computational methods were applied
to the photolysis of sulfilimines derived from dibenzothiophene that were expected to release acetylnitrene,
trifluoroacetylnitrene, mesylnitrene, and tosylnitrene. All three methods provided results for acetylnitrene
consistent with literature precedent and analogous experiments with the benzoylnitrene precursor, i.e.,
that the ground-state multiplicity is singlet. In contrast, product studies clearly indicate triplet reactivity
for trifluoroacetylnitrene, though TRIR experiments were more ambiguous. Product studies suggest that
these sulfilimines are superior sources for sulfonylnitrenes, which have triplet grounds states, to the
corresponding azides, and computational studies shed light on the electronic structure of the nitrenes.
Introduction
although the same interaction exists for the closed-shell singlet
state, alkoxycarbonylnitrenes have triplet ground states, the
singlet stabilization apparently being subtly less important.3,7
The chemistry of nitrenes is, if anything, more complex than
that of their isoelectronic carbene cousins.1–6 For example, while
only the closed-shell singlet is usually relevant for carbenes,
the open-shell singlet can be an important state for nitrenes.
Moreover, while the ground-state for simple alkylnitrenes is of
triplet multiplicity (by tens of kcal/mol), relatively slow
intersystem crossing rates and rapid reactions of the singlet often
make detection of the triplet ground-state difficult. In cases such
as methylnitrene, rearranged imines, rather than nitrenes, are
detected even at cryogenic temperatures.3
R-Ketonitrenes, such as benzoylnitrene (2a), on the other
hand, have singlet ground states, due to a very strong stabilizing
interaction between the oxygen lone pair and the nominally
empty orbital on nitrogen. This results in a narrowing of the
O-C-N angle and strong bonding interactions. By contrast,
Despite their structural similarity to the carbonyl case, the
available evidence is that toluenesulfonylnitrene (2e) has a triplet
ground state, as established by the observation of a low
temperature EPR signal on photolysis of tosyl azide.3,8,9 A triplet
EPR signal was also observed for mesylnitrene at very low
temperature obtained in the analogous fashion.10,11 Nonetheless,
the chemistry of sulfonylnitrenes is not well established because
of problems with the sulfonylazide precursor. These have often
led to difficult mixtures with hard-to-characterize precipitates.4
Other problems also exist. Despite the apparent triplet multiplic-
† Iowa State University.
‡ Johns Hopkins University.
(7) Liu, J.; Mandel, S.; Hadad, C. M.; Platz, M. S. J. Org. Chem. 2004, 69,
(1) 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.
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2005, 13, 235–356.
8583–93.
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(3) Platz, M. S. React. Intermed. Chem. 2004, 501–559.
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Wiley-Interscience: New York, 1985; Vol. 3, p 305-332.
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84, 3220–1.
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842–843.
4398 J. Org. Chem. 2008, 73, 4398–4414
10.1021/jo702654q CCC: $40.75 2008 American Chemical Society
Published on Web 05/29/2008

Sulfonyl- and Carbonylnitrenes
CHART 1
ity of the sulfonylnitrene ground states, thermolysis or photolysis
of sulfonyl azides generally leads to negligible amounts of
triplet-nitrene-derived products.8,12–18 Complexities deriving
from the possibility of reactions of the excited states of the azide
(as opposed to nitrene reactivity) and from the hazards associated
with smaller azides (e.g., mesyl azide) have been noted.13,17,19–22
Finally, SO2 extrusion is observed under some circumstances.
For these reasons, sulfonyl azides have been called only
“occasionally useful” precursors to sulfonylnitrenes.8,23 Thus,
other precursors are clearly valuable in elucidating the chemistry
of sulfonylnitrenes.
We recently reported the photolysis of N-benzoyl diben-
zothiophene sulfilimine 1a, which generates dibenzothiophene
(DBT) and the nitrene 2a.24 Both singlet and triplet benzoylni-
trene were directly observed by time-resolved infrared (TRIR)
spectroscopy and confirmed byproduct studies. We have also
recently reported product studies demonstrating the formation
of carbenes from similarly derived S-C ylides of dimethyl
malonate with thiophene derivatives.25
Nearly simultaneous with our publication regarding ben-
zoylnitrene, Morita and co-workers reported photochemically
induced S-N cleavage of several N-tosyl and N-acyl thianthrene
sulfilimine derivatives.26 Intra- and intermolecular trapping of
presumed nitrene intermediates by thianthrene and diphenyl
sulfide was noted. Tosyl amide was also formed in limited yield
from the N-tosyl derivatives and other reactions related to
stereochemical inversion of the thianthrene derivatives.26
Much earlier, the first chemical trapping evidence of nitrene
formation from sulfilimines arose from photolysis of the N-tosyl
sulfilimine of dimethyl sulfide and a few related derivatives,27–29
though in these instances, the nitrene provided the chromophore,
rather than the sulfide. As a matter of principle, the use of either
thianthrene or dibenzothiophene should represent an improve-
ment over dimethyl sulfide for the basic reason that the
chromophore does not need to be tied to the nitrene being
produced.
These compounds are analogs of those studied by Morita, using
dibenzothiophene, rather than thianthrene, as the base sulfide.
Though we do not directly compare the two systems here, the
photochemistry of the thianthrene derivatives26 is more complex.
The starting materials and potential products deriving from
the photolyses are shown in Chart 1. The compounds are
numbered such that sulfilimine 1a gives rise to nitrene 2a, which
in turn gives rise to potential products 3a, 4a, 5a, and so on.
We report here a systematic study using a set of N-substituted
analogues of 1a designed to produce different carbonylnitrenes
and sulfonylnitrenes from the dibenzothiophene chromophore.
(12) Hoyle, C. E.; Lenox, R. S.; Christie, P. A.; Shoemaker, R. A J. Org.
Chem. 1983, 48, 2056–2061.
(13) Abramovitch, R. A.; Knaus, G. N.; Uma, V. J. Am. Chem. Soc. 1969,
91, 7532–3.
Results and Discussion
(14) Abramovitch, R. A.; Takaya, T J. Org. Chem. 1972, 37, 2022–9.
(15) Abramovitch, R. A.; Takaya, T. J. Org. Chem. 1973, 38, 3311–6.
(16) Abramovitch, R. A.; Bailey, T. D.; Takaya, T.; Uma, V. J. Org. Chem.
1974, 39, 340–345.
Time-Resolved IR Studies. Nanosecond TRIR methods were
used to try to detect the various nitrenes directly. The TRIR
spectrometer used faces the limitation that its time resolution
is approximately 50 ns; intermediates that are shorter lived than
that cannot be observed. Photolyses of three sulfilimines were
carried out (1b, 1c, and 1d). The mesyl derivative 1d was chosen
over the tosyl derivative 1e to reduce the complexity of the
spectrum.
(17) Abramovitch, R. A.; Knaus, G. N.; Uma, V. J. Org. Chem. 1974, 39,
1101–1106.
40.(18) Abramovitch, R., A.; Challand, S. R.; Yamada, Y. J. Org. Chem. 1975,
18.(19) Abramovitch, R. A.; Roy, J.; Uma, V. Can. J. Chem. 1965, 43, 3407–
(20) Abramovitch, R. A.; Roy, J.; Uma, V. Can. J. Chem. 1965, 43, 3407.
(21) Horner, L.; Christmann, A. Chem. Ber. 1963, 96, 388–398.
(22) Lwowski, W.; Scheiffele, E. J. Am. Chem. Soc. 1965, 87, 4359–4365.
(23) Lwowski, W. In Azides and Nitrenes; Scriven, E. F. V. , Ed.; Academic
Press: Orlando, FL, 1984.
N-Acetyl Dibenzothiophene Sulfilimine, 1b, and Acetyl-
nitrene, 2b. Recent results with 1a,24 which leads to singlet
and triplet benzoylnitrene, provided an anticipated framework
from which to interpret results from 1b. TRIR difference spectra
obtained on 266 nm laser photolysis of 1b in CD3CN and in
CH2Cl2 are shown in Figure 1. Corresponding kinetic traces
obtained from 266 nm laser photolysis are shown in Figures 2
and 3.
(24) Desikan, V.; Liu, Y.; Toscano, J. P.; Jenks, W. S. J. Org. Chem. 2007,
72, 6848–6859.
(25) Stoffregen, S. A.; Heying, M.; Jenks, W. S J. Am. Chem. Soc. 2008, .
in press.
(26) Fujita, T.; Kamiyama, H.; Osawa, Y.; Kawaguchi, H.; Kim, B. J.; Tatami,
A.; Kawashima, W.; Maeda, T.; Nakanishi, A.; Morita, H. Tetrahedron 2007,
63, 7708–7716.
Methyl isocyanate was observed at 2255 cm-1. It was
produced faster than the 50 ns resolution of the instrument
(Figure 2a), consistent with an excited state of 1b (or an
(27) Oae, S.; Furukawa, N. Sulfilimines and Related DeriVatiVes; American
Chemical Society: Washington, D.C., 1983; Vol. 179.
(28) Hayashi, Y.; Swern, D Tetrahedron Lett. 1972, 19, 1921–1924.
(29) Hayashi, Y.; Swern, D. J. Am. Chem. Soc. 1973, 95, 5205–10.
J. Org. Chem. Vol. 73, No. 12, 2008 4399

Desikan et al.
FIGURE 1. TRIR difference spectra averaged over the time scales indicated following 266 nm laser photolysis of 1b (1 mM) in argon-saturated
3
(a) CH2Cl2 and (b) CD3CN. The red bar indicates the B3LYP/6-31G(d)-calculated IR frequency (scaled by 0.96) of triplet acetylnitrene, 2b.
electronically or vibrationally excited nitrene) being its source,
as was found for the analogous product from 1a (Scheme 1).24
In dichloromethane, a peak at 1518 cm-1 grows in with a
∼230 ns time constant and decays over a few microseconds
(Figure 2d). We attribute it to 32b. B3LYP/6-31G(d) calculations
on acetylnitrene predict a strong IR band at 1774 cm-1 for the
singlet nitrene and 1500 cm-1 for the triplet nitrene (frequencies
scaled by 0.9630).
Singlet acetylnitrene was not observed due to an overlapping
IR band near 1776 cm-1. This band grows in at the same rate
as the triplet nitrene, and the rate depends on the initial
concentration of the precursor. Figure 2b shows that this band
does not decay noticeably within several microseconds, which
is not compatible with it being the singlet nitrene. Because of
these kinetic observations, the 1776 cm-1 band is therefore
attributed to the reaction product between the singlet acetylni-
trene and DBT or the starting material. Finally, another band
at 1680 cm-1 was observed (Figure 2c), which is believed to
arise from the singlet nitrene reaction with dichloromethane.
In CD3CN, the band at 1776 cm-1 was weak and a new,
strong band at 1648 cm-1 was observed. The 1648 cm-1 band
was attributed to the formation of acetonitrile-ylide from the
singlet acetylnitrene. A strong vibration for this species was
calculated at 1694 cm-1, in reasonable, but not outstanding,
agreement with the observed value. The decreased intensity of
the 1776 cm-1 band is attributed to the competition between
acetonitrile ylide formation and reaction of the singlet nitrene
with DBT. A decreased intensity of the 1776 cm-1 band enabled
us to observe a kinetic trace at 1780 cm-1 (albeit not of high
(30) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502–16513.
4400 J. Org. Chem. Vol. 73, No. 12, 2008

Sulfonyl- and Carbonylnitrenes
FIGURE 2. Kinetic traces obtained at (a) 2255 cm-1, (b) 1776 cm-1, (c) 1680 cm-1, and (d) 1518 cm-1 after 266 nm laser photolysis of 1b (1 mM)
in argon-saturated CH2Cl2. The dotted-line curves are experimental data and the black solid curves are the calculated best fit to a single-exponential
function.
SCHEME 1. Proposed Photochemical Processes for 1b
Based on IR Data
FIGURE 3. Kinetic traces obtained at (a) 1648 cm-1, (b) 1515 cm-1
,
and (c) 1780 cm-1 after 266 nm laser photolysis of 1b (1 mM) in argon-
saturated CD3CN. The dotted-line curves are experimental data and
the black solid curves are the calculated best fit to a single-exponential
function.
species, as long as this equilibrium is maintained. This model
comports well with the appearance time constant of the 1518
cm-1 band observed in dichloromethane (Figure 2d, first order
growth time constant ca. 230 ns) and 1515 cm-1 band observed
in acetonitrile (Figure 3b, time constant ca. 130 ns). Since at
room temperature, an energy difference of ∼1.4 kcal/mol
corresponds to a 10:1 population ratio, we presume this is an
approximate upper limit for the energy difference between the
two nitrene multiplicities. Detection of the higher energy triplet
would become very difficult with lower populations. Product
studies (vide infra) are also consistent with mainly singlet
chemistry, as can be understood in terms of greater rate constants
for reaction from the singlet nitrene than from the triplet, in
addition to the greater equilibrium population of the lower
energy singlet state.
quality) that decays at the same rate as the growth of the 1515
cm-1 band and the ylide band. The decay trace at 1780 cm-1 is
attributed to singlet acetylnitrene; we believe that the singlet
nitrene is the most likely source for the ylide and at least most
of the triplet nitrene, as shown in Scheme 1.
Based on the lack of a literature report of a triplet EPR
spectrum of acetylnitrene at low temperature and recent calcula-
tions at the CBS-QB3 level, 7 we presume that the energies of
the singlet and triplet states of acetylnitrene are very similar
but that the singlet is slightly lower. Thus, population of the
triplet acetylnitrene by the singlet is presumably an endothermic
relaxation toward that thermal equilibrium. The triplet nitrene
is then depleted by reactions of either the singlet or triplet
J. Org. Chem. Vol. 73, No. 12, 2008 4401

Desikan et al.
FIGURE 4. TRIR difference spectra observed over the time scales indicated following 266 nm laser photolysis of 1c (1 mM) in argon-saturated
(a) CH2Cl2 and (b) CH3CN.
N-Trifluoroacetyl Dibenzothiophene Sulfilimine, 1c, and
Trifluoroacetylnitrene, 2c. Similar TRIR measurements were
performed using 1c as the photochemical precursor. Representa-
tive difference spectra are shown in Figure 4 in (a) Ar-saturated
dichloromethane and (b) Ar-saturated acetonitrile. Kinetic traces
for the positive bands in dichloromethane, observed at 1660,
1715, and 2270 cm-1, are shown in Figure 5, while kinetic traces
for the positive bands observed at 1688, 1218, and 2270 cm-1
in acetonitrile are shown as Figure 6. (The kinetic trace at 1460
cm-1 is essentially identical to that at 2270 cm-1 observed in
argon-saturated dichloromethane in that the band is permanent
on this time scale.)
band is linearly dependent upon the concentration of metha-
nol, indicating a second-order reaction between the carrier
and methanol, with a second-order rate constant of kmethanol
) 5.0 × 106 M-1 s-1. This rate constant is comparable with
that of singlet benzoylnitrene reaction with methanol (6.5 (
0.4 × 106 M-1 s-1).24
On the basis of these results, the most reasonable assignment
to the 1660 cm-1 band is to singlet trifluoroacetynitrene (12c).
However, we face the difficulty that the vibrational frequencies
nearest 1660 cm-1 predicted by the B3LYP/6-31G(d) calcula-
1
tions (scaled by 0.96) for 2c are not close to this value, as
shown in Table 1. The listed frequencies are the two that are
closest to 1660 cm-1 for each multiplicity of the nitrene. Like
for the singlet, there is not a triplet nitrene frequency predicted
In dichloromethane, the negative band observed at 1624 cm-1
is due to depletion of precursor 1c. A positive band observed
at 1660 cm-1 is formed at a rate faster than our instrumental
time resolution (50 ns) and decays at a first-order rate of 6.5
× 104 s-1, as shown in Figure 5a. (Note the 1660 cm-1 band
partly overlaps with the precursor depletion band observed
at 1624 cm-1.) Additionally, the decay rate of this band is
unaffected by the presence of oxygen, suggesting that it is
due to a singlet species. The decay rate of the 1660 cm-1
near 1660 cm-1
.
Another potential assignment for the 1660 cm-1 band is the
dimerization product of two nitrenes, i.e., the azo compound.
(See, for example, ref 31.) However, we eliminate that product
on kinetic grounds. The appearance of the band in less than 50
ns is not reasonable for a bimolecular reaction of low concentra-
4402 J. Org. Chem. Vol. 73, No. 12, 2008

Sulfonyl- and Carbonylnitrenes
FIGURE 5. Kinetic traces observed at (a) 1660, (b) 1715, and (c) 2270
cm-1 following 266 nm laser photolysis of 1c (1 mM) in argon-purged
CH2Cl2. The dotted-line curves are experimental data and the solid lines
are calculated best fits to a single-exponential function.
tion species. Moreover, the decay of the band (Figure 5a) on
the time scale of tens of microseconds would have produced a
product band, presumably acyl radicals, that would have been
32,33
easily detectable above 1800 cm-1
.
The reactivity of the
1660 cm-1 band (vide infra) also does not comport well with
the azo compound as a potential assignment.
FIGURE 6. Kinetic traces observed at (a) 1688 cm-1 from -0.4 to
3.6 µs, (b) 1218 cm-1 from -0.4 to 3.6 µs, (c) 1688 cm-1 from -100
to 900 µs, (d) 1218 cm-1 from -100 to 900 µs, and (e) 2270 cm-1
following 266 nm laser photolysis of 1c (1 mM) in argon-purged
CH3CN. The dotted-line curves are experimental data and the solid
lines are calculated best fits to a single-exponential function.
Another positive band is observed at 1715 cm-1. Its growth
rate (kobs ) 6.5 × 104 s-1, Figure 5b) matches the decay of the
1660 cm-1 band. This band is not observed in acetonitrile, nor
is its appearance rate affected by the concentration of the
precursor. It is thus assigned to a reaction product between
dichloromethane and the 1660 cm-1 intermediate.
1
3
TABLE 1. Frequencies Nearest 1660 cm-1 for 2c and 2c, As
In dichloromethane, trifluoromethyl isocyanate 12c is detected
at 2270 and 1460 cm-1, in good agreement with the 2255 cm-1
value observed for 12b.34 Again, its rate of growth is faster
than instrumental resolution (Figure 5c), indicating that its
precursor is not the relaxed singlet nitrene, but more likely the
excited-state precursor 1c, or potentially an excited nitrene. In
acetonitrile, it is detected similarly at 2270 cm-1 (Figure 6e).
In acetonitrile, no band attributable to singlet nitrene was
observed. However, we assign two new positive bands observed
at 1688 and 1218 cm-1 (Figure 6) to the acetonitrile ylide 18c,
Predicted by B3LYP/6-31G(d) Calculations
frequency,
cm-1
scaled frequency,
cm-1 (0.96)
assignment
12c
32c
1356
1841
1310
1553
1301
1768
1258
1491
CO stretch, combination
CN stretch, combination
CC off-line stretch
CO stretch
frequencies of 1660, 1688, 1715, and 1218 cm-1. The growth
rates of the ylide bands at 1688 and 1218 cm-1 is the same as
the decay of the 1660 cm-1 band, indicating a kinetic
parent-daughter relationship between the bands. (Note that the
1218 cm-1 band of ylide 18 overlaps with another instantaneous
growth band.) Additionally, the rate of decay of the 1660 cm-1
band is linearly dependent on the concentration of acetonitrile,
from which a second order rate constant of 1.5 × 107 M-1
s-1 can be derived. This is 2 orders of magnitude larger than
the rate constant reported for benzoylnitrene and acetonitrile
((3.4 ( 0.6) × 105 M-1 s-1).7 The 1715 cm-1 band previously
attributed to reaction between singlet nitrene and dichlo-
romethane (Figure 5b) is also observed with added acetoni-
trile (Figure 7c). However, its growth rate is correspondingly
faster, reflective of additional singlet nitrene reactivity with
acetonitrile.
1
formed from singlet nitrene 2c reaction with acetonitrile, in
good agreement with B3LYP/6-31G(d) calculated frequencies
of 1706 (scaled by 0.96)35 and 1241 cm-1. Both bands are
formed more rapidly than instrumental response, and decay,
presumably to oxadiazole 10c, with a first-order rate of 6.5 ×
102 s-1
.
In order to confirm this assignment and check for self-
consistency, we examined the behavior of the 1660 cm-1 band
in dichloromethane solutions with smaller added amounts of
acetonitrile. Kinetic traces for solutions in dichloromethane
containing 1 mM acetonitrile are shown in Figure 7 at
(31) Sankaranarayanan, J.; Bort, L., N.; Mandel, S. M.; Chen, P.; Krause,
J. A.; Brooks, E. E.; Tsang, P.; Gudmundsdottir, A. D. Org. Lett. 2008, 10,
937–940.
(32) Neville, A. G.; Brown, C. E.; Rayner, D. M.; Lusztyk, J.; Ingold, K. U.
J. Am. Chem. Soc. 1991, 113, 1869–70.
We did not detect any transients easily assignable to the triplet
nitrene, but product studies presented below are clearly con-
sistent with triplet nitrene reactivity. Absent the mismatched
frequency calculations, it would be straightforward to assign
(33) Brown, C. E.; Neville, A. G.; Rayner, D. M.; Ingold, K. U.; Lusztyk, J.
Aust. J. Chem. 1995, 48, 363–79.
(34) Nyquist, R. A.; Jewett, G. L. Appl. Spectrosc. 1992, 46, 841–2.
(35) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502–16513.
J. Org. Chem. Vol. 73, No. 12, 2008 4403

Desikan et al.
tion based on the chemical results that ignores the vibrational
calculations, i.e., that the 1660 cm-1 band is due to the singlet
nitrene.
N-Mesyl Dibenzothiophene Sulfilimine, 1d, and Mesyl-
nitrene, 2d. The TRIR difference spectra obtained on laser
photolysis of 1d in Ar-flushed CD3CN are shown in Figure 8,
and corresponding kinetic traces at 1330 cm-1 and 1150 cm-1
are shown in Figure 9. Similar data were obtained in dichlo-
romethane. B3LYP/6-31G(d) calculations on triplet mesylnitrene
(32d) in Cs symmetry predict a strong IR band at 1241 and 1058
cm-1. As is evident from the figure, these bands were not
observed, though the latter might be obscured by the observed
peak near 1150 cm-1. On computational examination, the singlet
nitrene has a stationary point with Cs symmetry, but it is a
transition state connecting two identical structures of low
symmetry. Optimizing the structure in C1 symmetry allows the
minimum to be found (vide infra), and characteristic strong IR
bands are predicted at 1134 cm-1 and 1023 cm-1. Neither of
these frequencies was observed.
As is evident from Figures 8 and 9, we observed two strong
and permanent bands at 1330 and 1150 cm-1. We tentatively
assign these bands to sulfonylazepine 19 (Scheme 3) arising
from the attack of singlet nitrene onto the DBT nucleus, based
on the IR measurements of sulfonyl azepine 20 by Paquette.36
The reported IR frequencies for N-mesylazepine in CHCl3 are
1330 and 1155 cm-1, which is in good agreement with the
observed TRIR signals. (In order to be consistent with product
data, we must also assume that the azepine slowly rearranges
to the net C-H insertion products 5c.) Furthermore, it is long
established that 20 and related derivatives easily rearrange to
the corresponding N-phenylmethanesulfonamides, and product
studies (vide infra) show formation of such rearranged products,
e.g., 5.17,19 Both these bands were produced at a time scale faster
than the instrument resolution (50 ns). We infer that the short-
lived singlet nitrene is responsible for this chemistry, assuming
the assignment is correct. We did not observe the triplet nitrene
by TRIR, but these negative results do not rule out its formation.
Product Studies. Photolyses of the various sulfilimines were
carried out in Ar-purged solvents with initial concentrations of
1-4 mM. (A few experiments were carried out with intention-
ally added O2.) As described below, some samples were
analyzed directly by 1H NMR, using deuterated solvents, while
others required concentration before 1H NMR or HPLC analysis.
The yield of DBT was generally quite high (>90%) when the
photolysis was carried out to high conversion. Yields in the
tables below are reported relative to DBT formation (or DBT
+ 5 when relevant), however, because DBT proved much easier
to precisely quantify when using the chromatographic methods.
Also, we believed that DBT formation was the more direct
stand-in for nitrene precursor formation37 than loss of 1 for any
case in which some 1 might be consumed by other means.
N-Acetyl Dibenzothiophene Sulfilimine, 1b. Representative
product studies for photolysis of 1b are shown in Table 2. The
observed products indicate a predominance of singlet chemistry.
For example, stereochemistry of the alkene is preserved in the
formation of aziridines from 4-octenes in acetonitrile (entries 1
and 2).38 Potential products 4b, 10b, and 12b were not detected
in these experiments.
FIGURE 7. Kinetic traces observed at (a) 1660, (b) 1688, (c) 1715,
and 1218 cm-1 following 266 nm laser photolysis of 4 (1 mM) in argon-
purged CH2Cl2 in the presence of added CH3CN (1 mM). The dotted-
line curves are experimental data, and the solid lines are calculated
best fits to a single-exponential function.
SCHEME 2. Potential Reaction Scheme for 1c, Based on
1
Assignment of the 1660 cm-1 Band to Singlet Nitrene 2c
the 1660 cm-1 band to 12c, with the µs time scale decay of the
singlet nitrene in dichloromethane presumably attributable to
reaction with solvent. There would be only a minor if any
contribution from intersystem crossing to the triplet nitrene,
which is not kinetically observed. The more rapid reaction with
the nucleophilic end of acetonitrile might be rationalized on
the basis that the CF3 group makes the nitrene more electro-
philic. This possibility is represented as Scheme 2, an otherwise
by now conventional diagram that does not address the interplay
between the two nitrene multiplicity states. It should be noted
that it does not necessarily eliminate the unobserved triplet
3
nitrene 2c; there simply is not strong evidence for it from the
TRIR experiments.
Given the lack of a singlet nitrene signal calculated near 1660
cm-1, we must also entertain the possibility that the 1660 cm-1
band represents the triplet nitrene. Under this scenario, one must
assume that the apparent singlet reactivity (acetonitrile, metha-
nol) is due to rapid equilibration of the two spin states of the
nitrene, which draws down the concentration of the triplet. We
must also assume unusually rapid triplet nitrene formation and
an unusually slow reaction between 32c and O2. The latter might
be plausible because of the electron-withdrawing nature of the
CF3 group, but is not attractive. Instead, we favor the interpreta-
(36) Paquette, L. A.; Kuhla, D. E.; Barrett, J. H.; Haluska, R. J. J. Org. Chem.
1969, 34, 2866–2878.
(37) As seen below, the excited sulfilimine can produce the nitrene and the
isocyanate competitively.
4404 J. Org. Chem. Vol. 73, No. 12, 2008

Sulfonyl- and Carbonylnitrenes
FIGURE 8. TRIR difference spectra averaged over the time scales indicated following 266 nm laser photolysis of 1d (1 mM) in argon-saturated
CD3CN.
FIGURE 9. Kinetic traces obtained at (a) 1330 cm-1 and (b) 1150 cm-1 after 266 nm laser photolysis of 1d (1 mM) in argon-saturated CD3CN.
SCHEME 3
for formation of the isocyanate, as seen by formation of 13b
and reported in the TRIR section. These results are broadly
consistent with those we recently reported for benzoylnitrene
generated by photolysis of 1a.39
In CD3CN without any additional trapping agent, where
evaporation before NMR analysis is not necessary, the cyclized
product 10b and methyl isocyanate 12b are detected directly,
along with an unidentified adduct. The assignment of 12b is
based on the appearance of a methyl singlet at 2.98 ppm in the
1H NMR. The unidentified compound (in entries 4-6) has a
methyl peak at 2.31 ppm, consistent with an acetamide
derivative, presumably an adduct to DBT.
Photolysis in acetonitrile with methylcyclohexane provides
the same unidentified adduct. (Detection could be done directly
by NMR, as the methylcyclohexane signals did not interfere
with the necessary region, and GC-MS was also used.) It also
affords some C-H insertion product due to reaction with
methylcyclohexane. The major isomer was the illustrated form
of 14b, i.e., the result of the insertion into the single tertiary
CH position (as determined with authentic sample). None of
the product due to insertion into a primary C-H bond was
observed (authentic sample), but other minor GC-MS peaks
were assigned to some of the 10 possible secondary C-H
insertions. Authentic samples were not obtained for these. The
high selectivity for tertiary insertion is consistent with very early
work by Lwowski using different precursors and dimethylcy-
The experiments with octene required evaporation of most
of the solvent (and octene) for either chromatographic or NMR
analysis. For this reason, it cannot be ruled out that volatile
products such as 4b, 10b, and 12b were formed, but not
detected. Entry 4 shows that 10b and 12 are formed in the
absence of alkene, where detection can be done without the
evaporation step.
The reaction with i-PrOH as solvent is dominated by O-H
insertion, rather than hydrogen abstraction (entry 3), also
consistent with singlet nitrene chemistry. Entry 3 shows evidence
(38) GC-MS analysis indicated very small quantities of isomeric compounds,
also of mass 281. These were interpreted as deriving from small isomeric
impurities in the octene.
(39) Desikan, V.; Liu, Y.; Toscano, J. P.; Jenks, W. S. J. Org. Chem. 2007,
72, 6848–6859.
J. Org. Chem. Vol. 73, No. 12, 2008 4405

Desikan et al.
TABLE 2. Product Yields from Photolysis of 1b
product yields (%), relative to DBT
entry
solvent
λex, nm
ΦDBT
cis-3b
trans-3b
4b
6b
10b
12b
13b
14bc
Ue
1
2
10% cis-4-octene in CH3CNa
10% trans-4-octene in CH3CNa
iPrOH-d8
320
320
320
320
0.49
0.69
0.62
54
0
0
61
ndb
nd
<1
nd
nd
nd
nd
3
72
19f
4e
5e
6e
CD3CN
24
27
61
30
31
6
22
25
22
10% MCHg in CD3CN
10% MCH in CD3CN
320
0.91
0.62
5
6
350 Xd
a Product yields determined from 1H NMR integration of the concentrated photolysate. b Not determined. Volatile materials that were not observed
after evaporation of solvent, but may have been in the mixture originally. c Overall yield of 14b and isomers of 14b. The illustrated tertiary product 14b
identified by comparison with authentic sample using GC. Other isomers of 14 were identified on the basis of their GC-MS data. d Xanthone (ca. 4
mM) sensitized photolysis. e Uncharacterized product: percentage reported from 1H NMR integration of the peak at 2.31 ppm. f 13b contains the
requisite 8 deuteria when prepared from iPrOH-d8. g Methylcyclohexane.
TABLE 3. Product Yields on Photolysis of 1ca
product yields (%) relative to DBTa
entry
solvent
ΦDBT
purge
cis-3c
trans-3c
trans/cis
10cb
4c
1
2
3
4
5
6
7
0.89 M cis-4-octene in CD3CNc
0.89 M trans-4-octene in CD3CNc
0.89 M cis-4-octene in CD3CNc
0.89 M trans-4-octene in CD3CNc
0.012 M cis-4-octene in CD3CNc
0.012 M trans-4-octene in CD3CNc
i-PrOH-d8
0.08
0.09
Ar
Ar
O2
O2
Ar
Ar
Ar
14
10
12
8
7
6
34
37
28
29
17
18
2.4
3.5
2.3
3.8
2.3
3.1
5
7
11
12
44
46
4
tr
tr
majord
a Excitation at 320 nm. Yields determined from 19F NMR integration of photolysate spectra, using Freon 113 as an internal standard. Trace yields are
indicated as “tr”. Unquantified, but GC-MS identified yields are identified as “+”. b Tentatively identified as 10e based on 19F NMR. The methyl
group, of course, is deuterated. See text. c Other uncharacterized product peaks also observed. d Difficult to quantify precisely, ca.70%. There were many
small, uncharacterized peaks.
clohexane. In that work, the retention of the methyl stereo-
chemistry in the inserted product was used as evidence for
singlet-based chemistry of the nitrene.40 (Hadad and Platz report
computed barriers of 14-19 kcal/mol, depending on method,
for the insertion of 2b into the 2-position of propane. )
Xanthone-sensitized photolysis (entry 6) also leads to the
formation of the tertiary C-H insertion product 14b and a
greater proportion of the cyclized product 10b over methyl
isocyanate than when direct irradiation is used. The presence
of any isocyanate at all may be evidence for some direct
photolysis, even under these conditions, according to the TRIR
experiments. However, the predominance of the singlet reactions
is also consistent with the equilibration between the singlet and
triplet nitrene states, as proposed in the TRIR section.
by 21 ppm from that of trifluoromethyl isocyanate, eliminating
that as a possibility. Although we are unaware of any literature
preparation or unambiguous characterization of 10c, three other
related compounds were found that had similar 19F chemical
shifts: Thus, by chemical analogy to the previous derivatives,
7
1
and spectroscopic analogy, we assign the observed compound
and its -66.8 ppm 19F NMR peak to the structure 10c. The
fact that the relative yield of 10c increases on addition of O2
implies that it is a singlet product, consistent with the proposed
mechanism.
Again, the alkene to aziridine stereochemical retention test
was used to probe for triplet nitrene reactivity. Based on Platz’s
data for benzoylnitrene, 7 at an alkene concentration of ∼1 M,
a competition would be observed between addition to the alkene
and trapping of the nitrene by acetonitrile. This seemed like an
appropriate regime in which to begin the singlet vs triplet
addition experiments. (We do not know if the ylide 18c would
react with the alkene to form the aziridine but are confident
that stereochemistry would be retained if so.)
N-Trifluoroacetyl Dibenzothiophene Sulfilimine, 1c. The
results of product studies with 1c contrast dramatically with
those of 1b. They unambiguously demonstrate the intervention
3
of a triplet intermediate, presumably 2c.
1
The lack of a signature peak in the H NMR of products
derived from 1c and their volatility led to the use of 19F NMR
as the principle analytical method for the experiments reported
in Table 3. With this technique, neither concentration of the
sample nor removal of octene was necessary, and the samples
could be analyzed directly after photolysis.
A compound with the mass of [2c + acetonitrile] was
observed from all experiments conducted in acetonitrile and
never in the absence of acetonitrile. This was taken to be the
adduct 10c, formed from the ylide 18c, as observed by TRIR.
The assignment of the identity 10c is based on spectroscopy
and chemical analogy. The observed 19F signal (δ -66.8) differs
The results in Table 3, with incomplete retention of alkene
stereochemistry in the aziridine products confirm the presence
3
of 2c in the reaction mixture (entries 1-4). The cis-aziridine
was identified by both 1H and 19F spectra, compared to authentic
samples. The trans-aziridine was identified by spectroscopic
analogy to other cis -and trans-aziridine pairs.41
The simplest interpretation of the different trans/cis ratios
from cis- and trans-4-octenes is that both singlet and triplet
nitrene reactivity is being observed. The alternate interpretations
(40) Lwowski, W. Nitrenes; John Wiley & Sons, Inc.: New York, 1970.
(41) Tanner, D.; Birgersson, C.; Gogoll, A.; Luthman, K. Tetrahedron 1994,
50, 9797–9824.
4406 J. Org. Chem. Vol. 73, No. 12, 2008

Sulfonyl- and Carbonylnitrenes
SCHEME 4
that only the triplet nitrene reacts with alkene, but without
complete loss of stereochemistrysseems unlikely, due to the
TRIR experiments that demonstrate the existence of 12c in the
mixture.
Because the addition of a singlet nitrene to an olefin is
expected by all literature precedent to have a much higher rate
constant than does addition of a triplet nitrene to an olefin, the
very observation of the mixed stereochemistry of the aziridines
implies that the ground-state of CF3CON is probably a triplet.
We cannot eliminate the possibility that the branching between
the two spin states of the nitrene is essentially irreversible, i.e.,
1
3
that, once formed, both 2c and 2c react faster in some other
way than they interconvert. However, the TRIR data are
3
interpreted as providing a thermal population of 2c on the
submicrosecond time scale. We believe that it is reasonable for
bimolecular reaction between the singlet nitrene and the alkene
to be competitive with this at these high alkene concentrations.
The standard probe for simultaneous reactivity from singlet
and triplet nitrenes of determining the ratio of trans/cis-aziridines
from varying cis-alkene concentrations was employed.42 Varia-
tion in the trans/cis ratio would imply that capture of the nitrene
by the alkene is kinetically competitive with intersystem
crossing. An invariant ratio can indicate either very rapid spin-
equilibration (leading to an apparent “single intermediate”) or
very slow spin-equilibration, where the initial singlet/triplet
branching ratio is maintained throughout the trapping reaction.
In the former case of rapid spin-equilibration, the expected result
would be a predominance of stereochemical retention, because
the singlet trapping reaction is expected to be much faster than
the triplet reaction. For loss of stereochemistry to predominate,
as observed in Table 3, a triplet ground-state is required in order
that its higher population outweigh the kinetic advantage
otherwise held by the singlet.
That triplet reactivity is significant is further implied by the
observation of trifluoroacetamide, 4c. Seen in small amounts
in the acetonitrile/alkene experiments, it becomes the major
product in i-PrOH. The expected singlet nitrene adduct 13c was
not identified, but we cannot rule out its formation because of
analytical limitations. Many small 19F signals were observed,
and we cannot eliminate the possibility of some formation of
the O-H insertion or any other specific products. Nonetheless,
the fact that better than half the mass balance is accounted for
by trifluoroacetamide again implies an ultimate predominance
in triplet nitrene chemistry.
Taken together, the product studies point strongly toward a
predominance of triplet nitrene chemistry in some solvents, and
singlet nitrene chemistry in others. Though the computations’
disagreement with the observed IR bands leads to an ambiguity,
we favor an interpretation based on assigning the 1660 cm-1
IR peak to the singlet nitrene. This implies a certain branching
We believe the lower trans/cis ratio (at lower [alkene]) for
entry 6, compared to entry 2, is experimentally significant. The
ratio of trans/cis-3 can be determined directly by NMR
integration without adding in additional errors related to
calculating the yields shown in Table 3, and the trends were
clear and reproducible. We may infer that less singlet nitrene
is being captured by the lower octene concentration. Note also
that the relative proportion of 10c increases with lower alkene
concentrations, as reaction between the singlet nitrene and
acetonitrile solvent becomes a larger fraction of the total
processes available to the singlet nitrene. The drop in the
absolute yield of trans-3c in entry 6 (compared to entry 2) or
cis-3c in entry 5, compared to entry 1, further implies that at
least some of the triplet nitrene derives from the singlet nitrene,
just as implied by the TRIR results.
1
ratio of 1c* that always provides some minimum amount of
triplet nitrene by way of 31c* and that more triplet nitrene may
be formed if the singlet nitrene is not immediately trapped by
solvent. The triplet nitrene is probably lower in energy than
the singlet by 3-4 kcal/mol (see below). We acknowledge the
difficulty of this otherwise ordinary interpretation with not
finding an appropriate frequency near 1660 cm-1 in the
computational data.
N-Mesyl Dibenzothiophene Sulfilimine, 1d. The mesyl
derivative 1d mainly yielded products attributable to the triplet
nitrene on photolysis. The principle results are given in Table
4. It should be noted that one of the byproducts of sulfonyl
azide chemistry can be a pseudo-Curtius rearrangement leading
to compounds of the form R-NdSO2. We found no positive
evidence for such a rearrangement, which would inevitably
involve products with R-N bonds (CH3 here or C7H7 for 1e,
below). Moreover, entries 4 and 5 in Table 4 show that at least
under the correct conditions, all of the material (carbene,
isocyanate, etc.) with formula RNO2S can be accounted for
without invoking the Curtius-type product.
The TRIR experiments did not allow for direct observation
of the nitrene in either multiplicity but did suggest products
that derive from the singlet. Since we expected the triplet to be
the ground state, we believed both singlet and triplet nitrene
reactivity might be observed under the right conditions.
In acetonitrile/octene (entries 1-3), the trans to cis ratio of
aziridines indicates equilibration of stereochemistry in large part,
and thus at least mainly triplet addition. In these acetonitrile/
Addition of O2 (entries 3 and 4) also provides a small change
in product distribution that we believe is experimentally
significant. It lowers the overall yield of aziridine (accompanied
by an increase in 10c), and also marginally raises the retention
of stereochemistry from the alkene to the aziridine, albeit more
so when the starting material is trans. Incomplete quenching of
31c would provide this result, assuming that the reactions set
out in Scheme 2 is elaborated as in Scheme 4. There is not
enough information to determine whether O2 additionally
quenches the triplet nitrene or whether 18c can lead to
cis-aziridines.
(42) McConaghy, J. S.; Lwowski, W. J. Am. Chem. Soc. 1967, 89, 4450–
4456.
J. Org. Chem. Vol. 73, No. 12, 2008 4407

Desikan et al.
TABLE 4. Product Yields from Photolysis of 1d
Product yields (%), relative to DBTa
solvent
λ, nm
ΦDBT
cis-3d
trans-3d
t/c
4d
5d
8d
15
16
17
1
2
3
4
5
6
7
8
10% cis-4-octene in CH3CN
10% trans-octene in CH3CN
10% trans-octene in CH3CN
i-PrOH
320
320
320
320
320
320
0.08
0.09
0.04
Ar
Ar
O2
Ar
Ar
Ar
Ar
Ar
28
34
27
57
62
57
2.0
1.8
2.1
15b
12b
9b
100
99
81
80
76
CD3OD
e
10% cyclohexane in CH3CN
50% cyclohexane in CH2Cl2
10% cyclohexane in CH3CN
0.06
0.94
<8c
6c
+
+
+
320
+
+
+
350, Xd
<8c
a Product yields determined from 1H NMR integration of the concentrated photolysate. b Combined percentages of 4d and 5d reported due to
overlapping CH3 protons in the 1H NMR spectrum of the concentrated photolysate (entries 1-3 only). c Upper limit reported due to other overlapping
peaks in the 1H NMR. d Xanthone (ca. 4 mM) sensitized photolysis. e Formed in the photolysis and characterized by GC-MS, but not rigorously
quantified.
TABLE 5. Product Yields from Photolysis of 1d in Varying
a pathway for ground-state triplet nitrene formation that passes
through the singlet.
cis-4-Octene Concentrations in Dichloromethanea
entry
[cis-4-octene], M
aziridine yieldb, %
trans/cis
The addition of O2 decreases the overall quantum yield of
product formation (Table 6), presumably because of quenching
of the excited-state of the sulfilimine. Additionally, the fraction
of identified products, relative to DBT formation is also lower
in the presence of O2. This implies an additional role for oxygen.
Maloney, et al. have reported fast quenching of triplet sulfo-
nylnitrenes by O2 (ca. 109 M-1 s-1) based on a transient UV
assignment from photolysis of tosyl azide.8 We suggest that
both the sulfilimine excited-state and the nitrene are quenched
under these conditions.
The standard experiment of examining the effect of alkene
concentration on the degree of stereochemical retention in the
aziridine was carried out. After some initial experiments gave
unexpected results, a new set of carefully controlled measure-
ments were done, all using cis-4-octene as the precursor. As
expected, the total yield of aziridine increases with cis-octene
concentration. However, a small but reproducible loss of
retention was seen as the alkene concentration was raised (Table
7). Control photolyses of tosyl azide in the presence of cis-4-
octene gave intractable mixtures.
1
2
0.012
0.89
74 ( 2
97 ( 3
1.87 ( 0.05
1.71 ( 0.05
a Yields determined from 1H NMR integration of the concentrated
photolysate. Excitation at 320 nm with Ar purge to remove O2.
Approximately 18% of 4c was obtained in entry 1. b Relative to DBT
formation.
octene mixtures, both methanesulfonamide 4d (the double
hydrogen abstraction product) and an adduct between the
mesylnitrene and DBT (5d) were observed in modest yield.
Because of overlap in the NMR spectrum of the corresponding
methyl groups and the multiple peaks in the aromatic region,
the two product yields could not be further distinguished. The
adduct was identified as a formal C-H insertion product by
the appearance of HPLC peaks that had UV spectra essentially
identical to DBT. We did not determine the regiochemistry of
the insertion.
Variation of the alkene concentration gave results consistent
with the triplet nitrene deriving from the singlet. As shown in
Table 5, raising the initial cis-alkene concentration from 12 to
890 mM not only increased the total aziridine yield but also
increased the retention of stereochemistry modestly. This implies
that at least some of the triplet nitrene is formed by way of a
higher energy singlet nitrene, and that intersystem crossing from
the singlet nitrene down to the ground state is kinetically
competitive with addition by the singlet nitrene to the alkene.
On direct photolysis of 1d in methanol (Table 4, entry 5), a
nearly quantitative yield of methansulfonamide was obtained.
These results are in contrast to those reported by Shingaki et
al. for photolysis of methanesulfonyl azide.43 In 1:1 cyclohex-
ane/CH2Cl2, they report 18.3% of the C-H insertion product
8d and 38.4% of H-abstraction product 4d. In ethanol, however,
they report a 48% of the O-H inserted product and 43.3% of
H-abstraction product. These differences may be due either to
complexities from the sulfonyl azide precursors or perhaps due
to different proportions of initially formed singlet and triplet
nitrene from the different precursors.
The possibility of a singlet ground-state for tosylnitrene can
be dismissed. However, a firm explanation for the data in Tables
6 and 7 is not trivial: (1) If the main precursor for triplet
3
1
3
tosylnitrene 2e is 2e and intersystem crossing to 2e is slow
or essentially irreversible, the increased cis-alkene concentration
should lead to increased cis-aziridine due to the high reactivity
of singlet nitrenes with alkenes. This is contradicted by
experiment. (2) If the main precursor for 32e is 12e and
intersystem crossing is extremely rapid and reversible relative
to alkene addition, alkene concentration should not have much
of an effect on the aziridine ratio. This is also contrary to the
3
reported result. (3) If 1b (i.e., the excited-state of the sulfil-
imine) reacts with alkene in competition with dissociation to
form 32e, then one has to postulate that this reaction favors the
loss of stereochemistry more than does the reaction between
32e and the alkene. This seems unlikely, but is possible, and
also requires that no more than a minimal amount of aziridine
is formed by singlet channels.
Another potential explanation is shown in Scheme 5. If the
triplet nitrene is born mainly through the triplet excited-state
of the sulfilimine and kS[cis-octene] > kST, then the uphill
intersystem crossing step leads to some aziridine with retention.
In the limit where kTS becomes essentially irreversible because
the singlet nitrene is rapidly quenched, the degree of retention
N-Tosyl Dibenzothiophene Sulfilimine, 1e. Upon photolysis
of N-tosyl dibenzothiophene sulfilimine, 1e, in the presence of
cis- or trans-4-octene, we obtained mixed-stereochemistry
aziridines (entries 1-4, Table 6). A small degree of stereo-
chemical retention is noted. This would again be consistent with
of stereochemistry is related to kTS/kT[octene]+kO [O2]+kTS}.
2
(43) Shingaki, T.; Inagaki, M.; Torimoto, N.; Takebayashi, M. Chem. Lett.
1972, 1181–4.
If this relation holds, then increasing the octene concentration
4408 J. Org. Chem. Vol. 73, No. 12, 2008

Sulfonyl- and Carbonylnitrenes
TABLE 6. Product Yields from Photolysis of 1e
3e
Product yields (%), relative to DBT
entry
solvent
λ, nm
ΦDBT
cis
trans
trans/ cis
4ea
8eb
15
16
17
1
2
3
4c
5
6
7
10% cis-4-octene in CH3CN
10% cis-4-octene in CH3CN
10% trans-4-octene in CH3CN
10% trans-4-octene in CH3CN
10% cyclohexane in CH3CN
50% cyclohexane in CH2Cl2
10% cyclohexane in CH3CN
320
320
320
320
0.16
0.09
0.14
0.09
0.16
Ar
O2
Ar
O2
Ar
28
20
24
19
52
45
55
40
1.9
2.3
2.3
2.2
d
320
90
95
90
trd
tr
tr
+
+
+
320
+
+
+
350, Xe
0.73
Ar
a Product yields determined from 1H NMR integration of the concentrated photolysate. b Identified by GC-MS. c GC-MS indicates two significant
peaks with m/z ) 169. Xanthone (ca. 4 mM) sensitized photolysis. d tr ) trace, by GC; “+” ) significant but not rigorously quantified (GC).
TABLE 7. Product Yields from Photolysis of 1e in Varying
TABLE 8. Product Yields from Photolysis of 1e in Alcohols
Concentrations of cis-4-Octene in Dichloromethanea
product yields (%), relative to DBTa
entry
[cis-4-octene], M
total aziridinesb, %
trans/cis
entry
solvent
4e
6e
7e
9e
11e
1c
2
3
4
5
0.012
0.071
0.16
0.89
4.30
(43 ( 8)
(70 ( 4)
(74 ( 3)
(83 ( 4)
91
1.55 ( 0.06
1.72 ( 0.04
1.81 ( 0.05
1.93 ( 0.05
1.9
1
2
3
4
77% t-BuOH in CH3CNa
57% t-BuOH in benzene
i-PrOH
80
42
100
98
0
0
25
0
MeOHb
0
a Product yields determined from 1H NMR integration of the
concentrated photolysate. All samples Ar purged and λex ) 320 nm.
b GC-MS trace of the photolysate indicates a trace amount of an isomer
of 7e.
a Excitation at 320 nm, with Ar purging. b Relative to DBT formation,
by NMR. c Significant amounts of 4e were also formed when 0.012 M
of cis-4-octene was used.
SCHEME 5
nitrene and the singlet excited-state of the azide have been
proposed as the key reactive intermediate.21,22 We reproduced
this result. However, as shown in Table 8, photolysis of 1e in
methanol results instead in nearly quantitative yield of the
double-hydrogen abstraction product tosamide 4e.
A similar result was found in i-PrOH. Tosyl azide has been
reported to produce tosamide quantitatively on photolysis in
i-PrOH, but the suggestion has been made that this is due to
radical chain chemistry.4
In order to remove the ambiguity regarding radical chain
mechanisms, we photolyzed 1e in solvent mixtures containing
t-BuOH. None of the OH insertion product (9e) was formed,
and again a high yield of tosamide was observed with aceto-
nitrile as the cosolvent (entry 1). With benzene as a cosolvent
(entry 2), tosamide was formed with smaller yield, and the
formal benzene CH insertion adduct 11e was formed competi-
tively. We assume that this adduct is formed by singlet nitrene
addition to benzene via the azepine in much the same way that
a certain amount of singlet nitrene reacts with the octenes, as
described by Scheme 4.
The previously discussed results from tosyl azide photolysis
in methanol have been complemented by much higher yields
of insertion product (8e) from tosyl azide in cyclohexane12 than
we observed on photolysis of 1e in the presence of cyclohexane.
Instead, we observed mainly the reduction product (Table 6).
We suggest that the sum of our evidence is in favor a much
larger proportion of 1e leading to triplet tosylnitrene without
intervention of the singlet tosylnitrene than is the case for
photolysis of tosylazide.
lowers the retention of stereochemistry because intersystem
crossing to the singlet nitrene is competitively suppressed. For
a ∆EST of about 10 kcal/mol, kTS would likely be no faster than
∼106 s-1. Under the same model, addition of O2 could also
lower the degree of retention if triplet nitrene trapping were
fast enough; this is observed when comparing entries 4 and 2
of Table 6 to entries 3 and 1, respectively.
In order to probe for singlet reactivity of tosylnitrene
besides the partial retention of stereochemistry of the
aziridines, we investigated both alkane solvents and alcohols.
(Based on lack of radical coupling products and on retention
of stereochemistry on insertion, previous workers have
attributed alkane insertions observed on thermolysis or pho-
tolysis of tosyl azide to reactions of the singlet nitrene.12,44
)
Entries 5 and 6 in Table 6 indicate that only traces of
N-cyclohexyltosamide 8e were observed on photolysis of 1e in
cyclohexane/acetonitrile or cyclohexane/dichloromethane. In-
stead, the double-hydrogen abstraction product tosamide 4e was
found in high yield. (Reduction of the nitrene is normally
considered a triplet nitrene reaction.) Perhaps unsurprisingly,
sensitization of 1e with xanthone (entry 7) also did not produce
C-H insertion product 8e, again leading mainly to tosamide.
Photolysis of tosyl azide in methanol has been reported to
produce the formal OH insertion adduct 7e. Both the singlet
Computational Studies. Theoretical studies on formylnitrene
(2 h) and acetylnitrene (2b) have been published.7,45–47Two
major features may be extracted from these data: (1) The
singlet state of these nitrenes is highly stabilized by an
(45) Pritchina, E. A.; Gritsan, N. P.; Maltsev, A.; Bally, T.; Autrey, T.; Liu,
Y.; Wang, Y.; Toscano, J. P. Phys. Chem. Chem. Phys. 2003, 5, 1010–1018.
(46) Pritchina, E. A.; Gritsan, N. P.; Bally, T. Russ. Chem. Bull. 2005, 54,
525–532.
(44) Breslow, D. S.; Sloan, M. F.; Newburg, N. R.; Renfrow, W. B. J. Am.
Chem. Soc. 1969, 91, 2273–2279.
(47) Zabalov, M. V.; Tiger, R. P. Russ. Chem. Bull. 2005, 54, 2270–2280.
J. Org. Chem. Vol. 73, No. 12, 2008 4409

Desikan et al.
TABLE 9. Computed ∆ES-T of Nitrenes 2b-e at Different Levels of Theorya
methodb
CH3CON
CF3CON
CH3SO2N
PhSO2N
14.5
B3LYP/6-31G(d,p)
1.9
6.3
6.9
4.7
6.3
4.7
14.1
15.5
12.6
13.2
12.6
12.6
12.4
5.1
B3LYP/6-311G+(d,p)//B3LYP/6-31G(d,p)
B3LYP/6-311G(3df,2p)//B3LYP/6-31G(d,p)
B3LYP/aug-cc-pV(D+d)Z//B3LYP/6-31G(d,p)
B3LYP/aug-cc-pV(T+d)Z//B3LYP/6-31G(d,p)
B3LYP/aug-cc-pV(Q+d)Z//B3LYP/6-31G(d,p)
MP2/6-31G(d,p)
MP2/6-311G(3df,2p)//B3LYP/6-31G(d,p)
MP2/aug-cc-pVTZ//B3LYP/6-31G(d,p)
CASSCF/6-31G(d,p)e
2.1 (4.9)c
0.3
13.1
13.8
1.7
0.0
0.1
4.8
d
-1.3
-3.9
-4.7
6.1
-0.8
3.7
-8.2
-9.3
2.0
-4.8
-0.2
4.1
4.4
6.7
MCQDPT/6-31G(d,p)//CASSCF/6-31G(d,p)e
CR-CC(2,3)/6-311G(3df,2p)//B3LYP/6-31G(d,p)f
CR-CC(2,3)/aug-cc-pV(D+d)Z//B3LYP/6-31G(d,p)
CR-CC(2,3)L/G3Large*//B3LYP/6-31G(d,p)
CCSD(T)/6-311+G(d,p)//B3LYP/6-31G(d)
CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d,p)
12.2
15.5
12.8
(1.9)c
(1.6)c
a A positive value implies a triple ground state. All energies in kcal/mol. b Positive ∆ES-T values indicate triplet ground states. Spherical harmonics
were used on the polarization functions. All data are∆H (298 K), with temperature corrections unscaled and taken from the Hessian calculated at the
optimization method. c Values in parentheses are from ref 7. d Triplet acetylnitrene always gave an imaginary frequency corresponding to the methyl
rotations and hence could not locate a minimum on the triplet nitrene surface. e The (6,5) active space for CH3CON includes the carbonyl π-system, a
lone pair on the oxygen and filled and empty p-orbitals on nitrogen. The (22,15) active space for mesylnitrene includes all the oxygen and nitrogen lone
pairs and the S-O and S-N σ bonds. The active spaces are illustrated in the Supporting Information. f CR-CC(2,3) is also known as CR-CCSD(T)_L.
interaction between the in-plane lone pair on O and the empty
in-plane orbital on N, resulting in the shortening in the N-O
distance and narrowing of the O-C-N angle, relative to the
triplet; and (2) The calculated singlet-triplet energy gap is
rather method-dependent. For example, among several meth-
ods, only CBS-QB3 correctly predicts a singlet ground-state
for acetylnitrene.7
The relative stability of the triplet state of these acylnitrenes
was overestimated by several kcal/mol by B3LYP/6-31(d).46
The consistency of this error, along with experimental results,
was used to conclude that the ground-state of benzoylnitrene is
a singlet with this relatively inexpensive method.
proximately 2 kcal/molson the basis set (at least at the B3LYP
level) for any of the present nitrenes. This is, presumably, due
to fairly efficient cancelation of errors when comparing the
singlet and triplet species.53 Especially notable is that data
obtained with the 6-311G(3df,2p) basis set are within about 1
kcal/mol of those from the aug-cc-pV(T+d)Z basis set, and often
much closer than that. The cc basis set is computationally much
more expensive, but of similar (though not identical) quality in
terms of polarization and having a triple-ꢀ valence shell. Thus,
we infer that the data for the coupled cluster calculations at the
latter, larger basis set would be similar to the ones reported in
Table 9.
We wished to use computational methods to corroborate with
the conclusions that might be drawn from the product studies
reported above for tosylnitrene, mesylnitrene, and trifluoro-
acetylnitrene. The implementation of B3LYP (and other com-
putational methods) is slightly different in GAMESS than in
GAUSSIAN; thus we undertook the use of GAMESS with
several different methods and basis sets in order to calibrate
In contrast, the correlation energy picked up by each method
for the different multiplicity states varies; thus, the ∆EST values
are more dependent on method than basis set. As noted
previously, we expect that the density functional values are too
positive by several kcal/mol.
Some specific discussion of the CASSCF calculations is
required. Careful choice of active space is required to ensure
that the calculations are on equal footing for singlet and triplet
states of the nitrenes. For the two carbonylnitrenes, an active
space of 6 electrons in 5 orbitals was sufficient. These included
the carbonyl π system, a lone pair on the oxygen, and the filled
and empty p-orbitals on N at the beginning of the calculation.
These orbitals are approximately preserved in the triplet states
(see the Supporting Information). In the singlet state, the
geometric distortion leads to formation of a partial single bond
between the N and O, but the general locations of the orbitals
is the same. The geometries obtained by optimizing at the
CASSCF are quite similar to those obtained at the B3LYP level,
and vibrational analysis showed them to be minima. For
mesylnitrene, after extensive experimentation, the only satisfac-
tory active space was 22 electrons in 15 orbitals (Supporting
Information). This corresponds to a full valence active space
on the SO2N fragment of the molecule. Again, the resulting
7
the results obtained against those of Hadad and Platz (HP),
and then expanded to several other methods for the four
compounds under consideration here. In addition to the density
functional method, we calculate energy differences using ROHF
reference functions at the MP2 and completely renormalized
coupled cluster (CR-CC(2,3)48,49 methods. Additionally, we
examined selected molecules using multireference MP2 methods
with MCSCF reference functions.50,51 The results of these
calculations are given in Table 9, where the values in parentheses
are taken from HP.
As can be seen immediately from Table 9, there is a wide
variance in the calculated ∆ES-T values for any given com-
pound.52 However, in contrast to results for formylnitrene,46
there is only a fairly modest dependence of ∆ESTsap-
(48) Piecuch, P.; Kucharski, S. A.; Kowalski, K.; Musial, M. Comput. Phys.
Commun. 2002, 149, 71–96.
10.(49) Piecuch, P.; Wloch, M. J. Chem. Phys. 2005, 123, 224105-1–224105-
(50) Nakano, H. Chem. Phys. Lett. 1993, 207, 372–378.
(53) In our experience, simplification of a molecule by reducing the first
substituent methyl to a hydrogen atom, as in H2SO for (CH3)2SO, for example,
often leads to surprisingly different computational behavior of the simplified
system, compared to the carbon-substituted analogues.
(51) Nakano, H. J. Chem. Phys. 1993, 99, 7983–7992.
(52) It should also be noted that these are gas-phase calculations. Polar
solvents would presumably favor the singlet state by a few kcal/mol.
4410 J. Org. Chem. Vol. 73, No. 12, 2008

Sulfonyl- and Carbonylnitrenes
Table 10. Also shown are the Mulliken bond orders, taken from
the B3LYP/6-31G(d,p).55
The geometries and bond orders reveal an interesting story. The
S-O bond lengths and bond orders for 3CH3SO2N and 3PhSO2N
are in reasonable agreement with “ordinary” sulfonyl compounds;
the bond orders of approximately 1.7 include the traditional σ
bond and the interaction represented either as the second bond
in many drawings or the “ylidic” portion of the bond if written
in that style. (See, for example, ref 56.) However, the bond
orders are dramatically different for the singlet sulfonylnitrenes.
One S-O bond remains approximately the same as before, but
the other is lengthened and weakened. The S-N bond is
shortened and strengthened to be above the level of a “single”
bond, and of course a much more significant O-N bond order
index is achieved: 0.75 vs <0.2 for the other O-N or for the
triplet states. This set of results can be rationalized by the
following picture for the singlet nitrene state, based on the ylide
representation of sulfur oxides:
FIGURE 10. Calculated geometries for mesylnitrene, calculated at the
B3LYP/6-31G(d,p) level. The view down the S-C axis obscures the
carbon atom; the methyl hydrogens are visible.
geometries were very similar to those from B3LYP, and
vibrational analysis proved the structures to be minima.
As noted by HP, the B3LYP density functional overestimates
the relative stability of the triplet state by several kcal/mol,
leading to an incorrect prediction of ground-state multiplicity
for acetylnitrene. The true ∆EST for acetylnitrene, while its exact
magnitude remains unknown, is surely slightly negative. (As
we discuss in the TRIR section, we believe it most likely to be
between 0 and -1.4 kcal/mol.) We obtain a singlet ground-
state using multireference MP2 but the magnitude is larger than
we expect.
Despite this uncertainty in the absolute value of ∆EST, the
data in Table 9 show that the increments from one compound
to the next are approximately the same, regardless of the
computational method. The ∆EST values for CF3CON are
consistenly 4-5 kcal/mol more positive than those for CH3CON.
Clearly the singlet state of CF3CON is somewhat less stabilized
by the adjacent carbonyl than is the case for CH3CON. Taken
with the experimental TRIR results for CH3CON that show that
the triplet is positioned no more than about 1 kcal/mol above
the triplet, we can infer that ∆EST for CF3CON is ca. 4 kcal/
mol.54 This is consistent with the product study data as well,
which also imply a triplet ground-state for CF3CON. The single
calculation that gives these results most closely (CR-CC(2,3)/
6-311G(3df,2p)//B3LYP/6-31G(d,p)) may also very well be the
best quality calculation shown in Table 9.
The O-N bond order indices for the singlet carbonylnitrenes
are fractionally higher than those for the sulfonylnitrenes,
perhaps reflective of the greater singlet-state stabilization in the
carbonylnitrenes. On going from the triplet to singlet state, the
carbonyl bonds lengthen and the bond order indices drop
significantly, from about 1.9 to nearly 1.1. Additionally, the C-N
bond order rises substantially, and of course the O-N bonding
order index goes up from a small value (<0.2) to almost 0.9. The
lengths change in concert with these bond order indices. Taken
together, these structural parameters confirm that the structural
character of the singlet carbonylnitrene resembles the substituted
oxazirene more than the dipolar structure that more obviously
shows the origin of the stabilizing interaction: The OCN bond angle
It also seems safe to suggest that the computations show that
both mesyl and tosylnitrene (or its stand-in for Table 9,
benzenesulfonylnitrene) have triplet ground states, consistent
with earlier low temperature EPR studies.3,8–11 The magnitude
of the energy gap is probably close to the 12 kcal/mol figure
obtained from the coupled-cluster calculations.
for 1CF3CON is marginally less acute than that for 1CH3CON, and
the O-N distance is correspondingly a bit longer. These geometric
parameters correlate well with the observed trends in ∆EST data
for 2b and 2c, though we will not make the error of ascribing cause
and effect with this level of analysis.
The comparison of the carbonylnitrenes to the sulfonylni-
trenes, however, does merit discussion. The question with the
sulfonylnitrene is whether the adjacent sulfonyl oxygen atom
could provide the level of stabilization to the closed-shell state
of the nitrene as is seen from adjacent carbonyls. In retrospect,
perhaps the negative result might have been predicted, based
on standard S-N and S-O lengths, vs C-N and C-O bond
lengths. Simply put, the expansion of the S-N and S-O lengths
(relative to the C-N and C-O lengths in the carbonylnitrenes)
means that the third edge of that triangle must also be longer,
Furthermore, the “insulation” provided by the sulfonyl group
means that the ∆EST is not so different between the mesylnitrene
and tosylnitrene. It is not practical yet to carry out extraordinarily
high level calculations for benzenesulfonylnitrene. However, the
consistency of the data at the B3LYP and MP2 level with those
of mesylnitrene give us confidence that higher level calculations
would give similar results.
Figure 10 illustrates the geometric result common to both
mesylnitrene and benzenesulfonylnitrene: the triplet states have
symmetric geometries common to other sulfonyl derivatives,
but the singlet nitrenes have distorted geometries that indicate
that there is significant bonding interaction between the O and
N. Key geometric parameters for all four nitrenes are given in
(55) The Mulliken bond orders are better behaved with modest basis sets
such as 6-31G(d,p). There are other, more sophisticated models of bond order,
but these values serve as very adequate comparative and qualitative guides.
(54) The singlet-triplet energy gap is undoubtedly also somewhat dependent
on the solvent.
J. Org. Chem. Vol. 73, No. 12, 2008 4411

Desikan et al.
TABLE 10. Key Calculated Geometric Parametersa
BOIa
nitrene
rCdO or rS-O, Å
rC-N or rS-N, Å
rO-N, Å
rS-O′, Å
OCN or OSN (deg)
1CH3CON
3CH3CON
1CF3CON
3CF3CON
1CH3SO2N
3CH3SO2N
1PhSO2N
3PhSO2N
1.31 (1.12)
1.23 (1.88)
1.30 (1.15)
1.22 (1.87)
1.56 (1.06)
1.47 (1.70)
1.56 (1.05)
1.47 (1.68)
1.26 (1.66)
1.40 (1.12)
1.26 (1.64)
1.39 (1.12)
1.58 (1.29)
1.72 (0.89)
1.58 (1.27)
1.71 (0.92)
1.77 (0.87)
2.25 (0.18)
1.81 (0.87)
2.27 (0.17)
1.77 (0.75)
2.56 (0.12)
1.76 (0.76)
2.55 (0.12)
86.9
117.7
89.8
121.4
68.9
106.7
68.3
106.4
1.45 (1.78)
1.47 (1.70)
1.46 (1.76)
1.47 (1.68)
a All data obtained from B3LYP/6-31G(d,p) optimizations, including bond order indicies (BOI). Mulliken bond orders are given in parentheses.
assuming a common angle. However, the O-N distances in
the singlet sulfonylnitrenes and carbonylnitrenes are comparable.
The O-S-N angle in 2c and 2d is more acute than in the
carbonylnitrenes by almost 20 degrees. One presumes, then, that
there is a higher price to pay for the short O-N distance due to
an increased angle strain.
But if we are to even qualitatively rely on strain as an
argument, we must at address how much strain is introduced
by the O-X-N contraction. We cannot know that with certainty
from the current data. However, the geometric changes from
triplet to singlet states can be used as a guide: the carbonylni-
trene OCN bond angle contracts by a 31-32° on going from
triplet to singlet, whereas the sulfonyl nitrenes have a corre-
sponding 38-39° contraction. A more complete energy surface
calculation would be required to establish the causality of this
association. However, the fact that the difference between the
two types of functionality is in the bond angle, rather than the
N-O distance is clearly established.
singlet, and that it is the only of the four nitrenes reported here
for which that is true. From the intensity and growth rate of the
TRIR signal attributed to the triplet nitrene, we estimate that
the triplet state is no more than about 1 kcal/mol above the
singlet ground state.
N-Trifluoroacetyl Dibenzothiophene Sulfilimine. Calcula-
tions on trifluoroacetylnitrene imply a more positive ∆EST than
for acetylnitrene by about 5 kcal/mol; we thus take it to be about
4 kcal/mol. The product studies indicate a predominance of
triplet-derived products (reduction product, loss of stereochem-
istry in formation of aziridines, etc.). Despite the ambiguity in
the direct detection of the intermediates, we can tentatively infer
a triplet ground-state for the nitrene from these results. We have
no reason to believe that the usual pattern of faster rate constants
for singlet reactions over triplet reactions would be broken and
thus assume that the triplet products indicate that the nitrene
spends significant time in the triplet state. While that could be
assigned also to initial formation of the nitrene in the triplet
state, followed by slow intersystem crossing to the ground
singlet, that hypothesis seems to be contradicted by the data in
Table 3. Those data indicate that the singlet nitrene is (at least
a partial) precursor to the triplet nitrene, and not the reverse.
N-Mesyl Dibenzothiophene Sulfilimine and Mesylnitrene. It is
unfortunate that we were unable to detect either nitrene on
photolysis of this sulfilimine in the TRIR data because this
degrades the certainty with which the chemistry can be assigned
to that intermediate. However, based on the precedent of the
acetyl and benzoyl analogs, along with the product studies, we
are willing to conclude that the nitrene is formed. Computational
results indicate a triplet ground-state by perhaps as much as 10
kcal/mol. However, the product studies clearly indicate a certain
amount of “singlet product” in the form of some retention of
stereochemistry in the aziridines. To rationalize these product
distributions, it is necessary to rely on the precedent of slow
singlet-to-triplet intersystem crossing of related nitrenes,3,24 and
the assumption that reaction of the singlet nitrene is kinetically
competitive with downhill intersystem crossing.
Summary
The data presented here indicate that the dibenzothiophene
sulfilimine platform is a useful one for observation of nitrenes
with electron-withdrawing substituents. Direct observation by
TRIR spectroscopy of two nitrenes (benzoylnitrene and acetylni-
trene) now complement the product studies. While not quantita-
tive, the mass balances reported here are at least in line with
and much better than many reports of product studies using azide
precursors. In the following paragraphs, we attempt to correlate
information derived from each of our techniquessspectroscopic,
product studies, and computationalsto provide insight to the
dynamics of nitrene formation from sulfilimines 1b-e and
comment, as appropriate, on the chemistry of the nitrenes
themselves.
N-Acetyl Dibenzothiophene Sulfilimine. Previous work on
this nitrene is consistent with a singlet ground state. Certainly,
nothing in our product studies contradicts this; we see no
evidence of triplet reactivity. The computations we have carried
out are in line with those of HP, in that they are very method-
dependent and predict ∆EST to be only a few kcal/mol,
regardless of sign. Of the four compounds we examined, only
this one gave only singlet nitrene products, and its calculated
∆EST is also the most negative among them. Importantly, the
triplet-sensitized products are also singlet-derived, which implies
that intersystem crossing from the triplet nitrene to the singlet
nitrene is downhill (and faster than reaction by the triplet). The
evidence from the TRIR data suggests that the triplet state of
the nitrene is populated slowly (on the sub-µs time scale), which
we take to be thermal population from the singlet nitrene. We
thus conclude also that the ground-state of acetylnitrene is a
N-Tosyl Dibenzothiophene Sulfilimine and Tosylnitrene. The
computational results for tosylnitrene and mesyl nitrene are so
similar that they indicate that there is little electronic com-
munication between the phenyl (or methyl) and the nitrene
center. We again conclude that a triplet ground-state is correct
for this nitrene, consistent with the previous reports of EPR
data from photolysis of tosyl azide at low temperature. Again,
however, the product data show some singlet chemistry. In this
instance, we see an unusual dependence of the aziridine
stereochemistry on the concentration of the alkene, in which
raising the concentration of the alkene leads to a decrease in
retention. In Scheme 4, we presented a speculative explanation
4412 J. Org. Chem. Vol. 73, No. 12, 2008

Sulfonyl- and Carbonylnitrenes
dispersion. Azoxybenzene was used as the actinometer.64 Samples
were placed in a 1 cm square quartz cell mounted on a holder such
that all the light exiting the monochromator hits the sample directly.
Except as noted, initial concentrations were 1-4 mM and the
solutions were bubbled with Ar for at least 10 min to remove O2
prior to photolysis. The progress of reactions was monitored by
HPLC, using a diode array UV/vis detector with a C18 reversed-
phase column for separation. All reported yield data represent at
least duplicate experiments, and most were carried out in triplicate
or greater. Experiments done with octene were either done at an
approximate 10% ((approximately 0.5%, by volume) or at precisely
measured concentrations of alkenes as stated in the text.
for this phenomenon, in which we suggest that most of the
nitrene is formed initially in the triplet state. With sufficiently
slow triplet reactivity with the alkene and modest ∆EST (e10
kcal/mol), one could imagine thermal population of the singlet
nitrene on the µs time scale, which would accommodate the
data easily. However, it does require that little if any singlet
nitrene is formed on direct photolysis, a result that is not
common to the other analogs in this series. That said, the alcohol
trapping results (Table 7), which show no typical singlet O-H
insertion product, regardless of conditions, is also unique to this
compound.
Quantum yields for appearance of dibenzothiophene were
determined using HPLC detection with reactions carried out only
to low conversion. Product yields were determined from reactions
run to nearly complete conversion of starting materials. Identifica-
Experimental Section
Time-Resolved IR Methods. The TRIR experiments have been
conducted following the method of Hamaguchi and co-workers,
as previously described.57–59 In short, the broadband output of a
MoSi2 IR source is crossed with excitation pulses from either a
Q-switched Nd:YAG laser (266 nm, 90 ns, 0.4 mJ) operating at
200 Hz or a Nd:YAG laser (266 nm, 5 ns, 2 mJ) operating at 15
Hz. Changes in IR intensity are monitored using an ac-coupled
mercury/cadmium/tellurium (MCT) photovoltaic IR detector, ampli-
fied, digitized with an oscilloscope, and collected for processing.
The experiment is conducted in dispersive mode with a commercial
spectrometer.
Computational Methods. Vibrational frequencies for the TRIR
experiments were calculated at B3LYP/6-31G(d), using the
GAUSSIAN 9860 suite of programs. All other computations were
carried out using the GAMESS61 suite. The output of the GAMESS
calculations was visualized using MacMolPlt.62 Initial geometries
for the GAMESS runs were obtained from semiempirical or HF
runs done with Spartan.63 Triplet calculations were based on ROHF
wave functions. The coefficients and exponents for the G3Large
basis set were obtained from http://chemistry.anl.gov/compmat/
g3theory.htm. All stationary points, except as noted, are confirmed
as energy minima on the potential energy surface by vibrational
frequency analysis. The notation G3Large* refers to the use of the
G3Large basis set on key atoms S, O, and N and the use of
6-31G(d) on carbon and hydrogen.
1
tion and quantification were done by a combination of H NMR,
19F-NMR and GC-MS (EI, DB-5 column) on concentrated reaction
mixtures. All compounds, save for trans-3, were compared to
authentic samples. Previous literature41 with both cis and trans
isomers of analogues provided clear precedent for assignment of
1
the aziridine stereochemistry by H NMR.
Materials. Products 7e,65 8e,66 8d,66 10b,67 and 1166 were
prepared using literature methods. The compounds cis-3 were
prepared from 4-octene by analogy for a preparation based on
3-hexene.41 Compound 6b was prepared using a method given for
the benzoyl analogue.68 Its water solubility and relatively high
volatility made purification beyond about 75% difficult, but the
NMR data were clear, and the photolyzates were clearly identified
as identical by spiking the samples with the synthetic mixture.
Similarly, urethane 13, prepared by treatment of isopropoxycarbonyl
imidazole69 with excess methylamine in water, could not be
completely purified, but was spectroscopically and chromatographi-
cally identified. NMR data for previously unreported aziridines and
6b are given in the Supporting Information.
N-Acetyl Dibenzothiophene Sulfilimine, 1b. To a solution of
trifluoroacetic anhydride, (0.14 mL, 1 mmol) in dichloromethane
(40 mL) at -78 °C, was slowly added dibenzothiophene S-oxide
(0.1 g, 0.5 mmol) in dichloromethane (3 mL). After cooling and
stirring at -78 °C for 1 h, powdered acetamide (80 mg, 1.3 mmol)
was added as the solid. After 2 h, the reaction was warmed to -20
°C and then quenched with ice-water. The reaction mixture was
extracted with saturated sodium bicarbonate, and the organic layer
was dried and concentrated to give the crude product. The title
compound was obtained in 30% yield after silica chromatography
with a gradient of ethyl acetate in dichloromethane: 1H NMR
(CDCl3) δ 8.01 (d, J ) 7.2 Hz, 2H), 7.83 (d, J ) 6.9 Hz, 2H),
7.62 (t, J ) 7.8 Hz, 2H), 7.52 (t, J ) 7.5 Hz, 2H), 2.18 (s, 3H);
13C NMR (CDCl3) δ 24.2, 123.7, 129.0, 130.6, 133.3, 138.5, 138.4,
182.2; HRMS calcd for C14H11NOS 241.0561, found 241.0562.
N-Trifluoroacetyl Dibenzothiophene Sulfilimine, 1c. The
compound was prepared as was 1b, save for use of trifluoroaceta-
Steady-State Photolysis Methods. Solvents were spectral grade
and were used without further purification. The quantum yield
measurements were carried using a 75 W Xe arc lamp fitted to a
monochromator set to the specified wavelength with (12 nm linear
(56) McCulla, R. D.; Cubbage, J. W.; Jenks, W. S. J. Phys. Org. Chem.
2002, 15, 71–77.
(57) Iwata, K.; Hamaguchi, H. Appl. Spectrosc. 1990, 44, 1431–7.
(58) Yuzawa, T.; Kato, C.; George, M. W.; Hamaguchi, H. Appl. Spectrosc.
1994, 48, 684–90.
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mide in place of acetamide: 65% yield; H NMR (CDCl3) δ 8.18
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(d, J ) 7.8 Hz, 2H), 7.95 (d, J ) 7.8 Hz, 2H), 7.74 (t, J ) 7.8 Hz,
2H), 7.60 (t, J ) 7.5 Hz, 2H); 13C NMR (CDCl3) δ, 117.1 (q, J )
286.2 Hz), 122.9, 129.28, 130.7, 133.6, 135.9, 138.7, 169.0 (q, J
) 35.3 Hz); HRMS calcd for C14H8F3NOS 295.0279, found
295.0283.
N-Mesyl Dibenzothiophene Sulfilimine, 1d. The procedure
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J. Org. Chem. Vol. 73, No. 12, 2008 4413

Desikan et al.
chromatography on silica with 15% ethyl acetate in dichlo-
romethane. The yield of the pure compound was 54%: H NMR
δ 21.8,122.7, 126.9, 127.8, 128.5, 129.6, 130.3, 133.2, 137.7, 137.5;
HRMS calcd for C19H15NO2S2 353.0544, found 353.0546.
1
(CDCl3) δ 3.08 (s, 3H), 7.6 (t, J ) 7.5 Hz, 2H), 7.711 (t, J ) 7.5
Hz, 2H), 7.93 (d, J ) 7.2 Hz, 2H), 8.03 (d, J ) 7.5 Hz, 2H); 13C
NMR (CDCl3) δ 43.5, 122.9, 127.7, 130.5, 133.4, 137.6, 138.3;
HRMS calcd for C13H11NO2S 277.0231, found 277.0222.
Acknowledgment. The authors at Iowa State (CHE-0211371)
and at Johns Hopkins (CHE-0518406) thank the National
Science Foundation for support of this research.
N-Tosyl Dibenzothiophene Sulfilimine, 1e. Chloramine T (1.53
g, 6.57 mmol) and DBT (1.00 g, 5.43 mmol) were added to 15 mL
of methanol. To this solution was added, dropwise, 1 mL of glacial
acetic acid. The resulting solution was heated to 50 °C with stirring.
After about 14 h, the reaction mixture was poured into 10% aqueous
NaOH on ice. This solution was filtered and recrystallized from
Supporting Information Available: Spectroscopic charac-
terization of compounds and computational data, including
coordinates and absolute energies. This material is available free
of charge via the Internet at http://pubs.acs.org.
1
methanol and water to give the title compound in 33% yield: H
JO702654Q
NMR (CDCl3) δ 2.44 (s, 3H), 7.83 (d, J ) 8.4 Hz, 2H), 7.87 (d,
J ) 9 Hz, 2H), 7.65 (t, J ) 7.8 Hz, 2H), 7.63 (t, J ) 6.3 Hz, 2H),
7.46 (t, J ) 7.5 Hz, 2H), 7.25-2.28 (m, 2H); 13C NMR (CDCl3)
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2116.
4414 J. Org. Chem. Vol. 73, No. 12, 2008