Competition between azido cleavage and triplet nitrene formation in azidomethylacetophenones

Article abstract of DOI:10.1071/CH10331

Photolysis of p- and m-azidomethylacetophenone (1a, 1b) in argon-saturated solutions yields predominantly imine 2a, 2b, whereas irradiation of 1a, 1b in oxygen-saturated solutions results in heterocycles 3a, 3b, aldehydes 4a, 4b and nitriles 5a, 5b. Density functional theory calculations place the energy of the first and second excited state of the triplet ketones (T_1K and T _2K) in 1a, 1b in close proximity to each other. The triplet transition state for cleaving the CN bond in 1a, 1b to form azido and benzyl radicals 1aB, 1bB is located only 3 kcal mol^-1 (1 kcal = 4.184 kJ) above T_1K, indicating that azido cleavage is feasible. The calculations place the energy of the triplet azido group (T_A) in 1a, 1b ～25 kcal mol^-1 below T_1K; thus, this process is also easily accessible via energy transfer. Further, the transition state barrier for T_A to expel N₂ and form triplet nitrenes is less than 1 kcal mol^-1 above T_A in 1a, 1b. Laser flash photolysis of 1a, 1b reveals the formation of the triplet excited ketones of 1a, 1b, which decay to form benzyl radicals 1aB, 1bB and triplet alkylnitrenes. The triplet ketones and the benzyl radicals are quenched with molecular oxygen at rates close to diffusion, whereas the triplet nitrenes react more slowly with oxygen (～5 × 10⁵ M^-1s^-1). We conclude that the triplet alkylnitrenes intercept the benzyl radicals to form 2 in argon-saturated solution, whereas the benzyl radicals are trapped to form 4 in oxygen-saturated solution; thus, the triplet nitrenes react with oxygen to form 3. CSIRO 2010.

Full text of DOI:10.1071/CH10331

RESEARCH FRONT
Full Paper
CSIRO PUBLISHING
www.publish.csiro.au/journals/ajc
Aust. J. Chem. 2010, 63, 1645–1655
Competition Between Azido Cleavage and Triplet Nitrene
Formation in Azidomethylacetophenones
A
A
Ranaweera A. A. Upul Ranaweera, Yu Zhao, Sivaramakrishnan
Muthukrishnan, Christopher Keller, and Anna D. Gudmundsdottir
A
A
A B
,
A
Department of Chemistry, University of Cincinnati, Cincinnati,
OH 45221-0172, USA.
B
Corresponding author. Email: anna.gudmundsdottir@uc.edu
Photolysis of p- and m-azidomethylacetophenone (1a, 1b) in argon-saturated solutions yields predominantly imine 2a, 2b,
whereas irradiation of 1a, 1b in oxygen-saturated solutions results in heterocycles 3a, 3b, aldehydes 4a, 4b and nitriles 5a,
5b. Density functional theory calculations place the energy of the first and second excited state of the triplet ketones (T1K
and T2K) in 1a, 1b in close proximity to each other. The triplet transition state for cleaving the C–N bond in 1a, 1b to form
azido and benzyl radicals 1aB, 1bB is located only 3 kcal molꢀ1 (1 kcal ¼ 4.184 kJ) above T1K, indicating that azido
cleavage is feasible. The calculations place the energy of the triplet azido group (TA) in 1a, 1b ,25 kcal molꢀ1 below T1K
;
thus, this process is also easily accessible via energy transfer. Further, the transition state barrier for TA to expel N2 and
form triplet nitrenes is less than 1 kcal molꢀ1 above TA in 1a, 1b. Laser flash photolysis of 1a, 1b reveals the formation of
the triplet excited ketones of 1a, 1b, which decay to form benzyl radicals 1aB, 1bB and triplet alkylnitrenes. The triplet
ketones and the benzyl radicals are quenched with molecular oxygen at rates close to diffusion, whereas the triplet nitrenes
react more slowly with oxygen (,5 ꢁ 105 Mꢀ1 sꢀ1). We conclude that the triplet alkylnitrenes intercept the benzyl radicals
to form 2 in argon-saturated solution, whereas the benzyl radicals are trapped to form 4 in oxygen-saturated solution; thus,
the triplet nitrenes react with oxygen to form 3.
Manuscript received: 9 September 2010.
Manuscript accepted: 10 November 2010.
Introduction
primarily results in photo-initiated cleavage to form azido and
benzyl radicals.[18] Triplet alkylnitrenes are highly unreactive
because they do not abstract an H atom from the solvent or
react with their precursor; instead, they dimerize to form azo-
dimers.[16,19–21] Triplet alkylnitrenes, however, react efficiently
with benzoyl radicals to form amides,[17] and can be trapped
with triplet oxygen to form nitro compounds.[16]
To investigate further the ability of triplet alkylnitrenes to
act as radical scavengers, we investigated the photochemistry of
p- and m-azidomethylacetophenones (1a and 1b). These mole-
cules can be expected to both undergo energy transfer to form
triplet alkylnitrenes (1aN, 1bN) and cleave to form azido and
benzyl radicals (1aB, 1bB). We used transient spectroscopy and
density functional theory calculations to show that photolysis of
1a, 1b result in the formation of triplet alkylnitrenes and benzyl
radicals. The major photoproducts in argon-saturated solutions
are 2a, 2b, prompting us to conclude that triplet alkylnitrenes
1aN, 1bN trap benzyl radicals 1aB, 1bB to form 2a, 2b.
Nitrenes are reactive intermediates that have a monovalent
nitrogen atom.[1–3] The non-bonding electrons on the mono-
valent nitrogen atom can either be paired, denoting a singlet
nitrene, or unpaired, representing a triplet nitrene. Singlet
nitrenes are highly reactive intermediates that insert into sur-
rounding chemical bonds and have therefore been used in
various applications, such as photoaffinity labelling,[4,5] cross-
linking of polymers,[6–9] and modification of surfaces.[10–12] In
contrast, triplet nitrenes are generally less reactive, and owing to
their high-spin characteristics, they have potential as building
blocks for organic magnets.[13,14] Triplet alkylnitrenes have
only been investigated sporadically because direct irradiation of
alkyl azides does not yield triplet alkylnitrenes, but rather imine
products from a concerted rearrangement of the singlet excited
state of the alkylazides.[15] Formation of triplet alkylnitrenes
from alkylazides requires triplet sensitization.[16,17] We have
reported that the irradiation of simple a-azidopropiophenones,
which have a built-in triplet sensitizer, results in selective for-
mation of triplet alkylnitrene intermediates.[16] Intramolecular
triplet sensitization of alkyl azides, however, does not always
lead to formation of alkylnitrenes. For example, photolysis of
a-azidoacetophenone derivatives results in both triplet energy
transfer to form triplet alkylnitrenes and a-cleavage to form
benzoyl radicals.[17] Furthermore, photolysis of b-azido pro-
piophenone derivatives that have a b-phenyl substituent
Results
Product Studies
Photolysis of 1a, 1b in argon-saturated methanol solutions
resulted in the formation of imines 2a, 2b (Scheme 1). In
comparison, photolysis of 1a, 1b in oxygen-saturated chloro-
form or methanol yields mainly heterocycles 3a, 3b, aldehydes
4a, 4b, and acetyl benzonitriles 5a, 5b (Scheme 2).
Ó CSIRO 2010
10.1071/CH10331
0004-9425/10/121645

RESEARCH FRONT
1646
R. A. A. U. Ranaweera et al.
We studied the effect of concentration on the product ratio
from photolyzing 1b in oxygen-saturated methanol (Fig. 1). We
found that more 3b and less 5b was formed as the concentration
of 1b was increased, whereas formation of 4b was not signifi-
cantly affected. Thus, we theorize that, on photolysis, 1b forms
the triplet state of the ketone, which undergoes energy transfer to
form the triplet excited state of the azido moiety in 1b and falls
apart to yield triplet alkylnitrene 1bN (Scheme 3). In competi-
tion with energy transfer, 1b also undergoes cleavage of the
azido group to form radicals 1bB. We propose that triplet nitrene
1bN reacts with radical 1bB to form imine 2b in argon-saturated
solution. We did not observe products due to dimerization of
1aB, 1bB because triplet nitrenes 1aN, 1bN must trap 1aB, 1bB
more efficiently than they dimerize. However, in oxygen-
saturated methanol or chloroform, 1bB is trapped with oxygen
to form 4b, allowing nitrene 1bN to react with oxygen to form 3b
and 5b. Corsaro et al. have shown that benzonitrile oxide and
benzonitrile undergo cycloaddition to form 3,5-diphenyl-[1,2,4]
oxadiazole.[22] Therefore, we propose that 1bN reacts with
oxygen to form the corresponding nitrile oxide and oxygen
abstracts hydrogen atoms from 1bN to yield 5b, which adds to
the acetylbenzonitrile oxide to form 3b. We also theorize that
the reactivity of 1a is similar to 1b.
radicals,[23] we photolyzed 1,3-diphenylpropan-2-one and ada-
mantyl azide in methanol. By irradiating through a Pyrex filter,
we ensured that only 1,3-diphenylpropan-2-one absorbed the
light and not adamantyl azide. The only product observed was
1,3-diphenylethane, and therefore we conclude that benzyl
radicals do not react with the alkyl azides (Scheme 4). Further-
more, we photolyzed 1b and 1,3-diphenypropan-2-one in
methanol and GC-MS analysis of the reaction mixture showed
formation of a product that corresponds to trapping 1bN with
benzyl radical to form 6 (Scheme 5).
Molecular Modelling
All structures were optimized using Gaussian09[24] at the
B3LYP[25,26] level of theory and with the 6-31þG(d) basis set
at the Ohio Super Computer Center. Time-dependent density
functional theory (TD-DFT)[27–31] calculations of 1a showed
that the first and second excited states of the triplet ketone (T1K
and T2K respectively) were located at 73 and 76 kcal molꢀ1
(1 kcal ¼ 4.184 kJ) above its ground state (S0). Inspection of the
molecular orbitals shows that T1K and T2K have (n,p*) and
(p,p*) configurations respectively (Table 1). The effect of
solvation was calculated using the self-consistent reaction field
(SCRF) method with the polarization continuum model (PCM)
with acetonitrile as the solvent.[32–35] The calculated energies of
We speculated whether 1aB, 1bB can react with 1a, 1b
to form 2a, 2b. Because photolysis of 1,3-diphenylpropan-2-
one has been reported to result in the formation of benzyl
0.8
0.7
0.6
0.5
O
O
O
hν
N3
N
2a
1a
O
hν
N
O
O
0.05
0.10
0.15
0.20
N3
[1b] [M]
1b
2b
Fig. 1. The effect of the concentration of 1b on the product ratio in oxygen-
saturated methanol.
Scheme 1. Photolysis of 1a, 1b in argon-saturated methanol.
O
O
O
O
O
hν
N
N
O
N3
CN
1a
3a
4a
5a
O
O
O
O
O
O
N
hν
N
CN
5b
Scheme 2. Photolysis of 1a, 1b in oxygen-saturated methanol and chloroform.
O
N3
O
1b
3b
4b

RESEARCH FRONT
Azido Cleavage and Triplet Nitrene Formation
1647
5
3
O
O2
O
.
N
.
O
.
N
O
O
O
O2
ϪN2
*
*
N3
O
O
N:
TA of 1
1N
O
O
N3
N3
.
R
2
1
TK of 1
.
N
RH
O
.
O2
1B
O
O
4
O
.
Scheme 3. Proposed reaction mechanisms for 1a and 1b.
h ν
O
N3
1,3-Diphenylpropan-2-one
1,3-Diphenylethane
1-Adamantyl azide
Scheme 4. Photolysis of 1,3-diphenylpropan-2-one and 1-adamantyl azide.
O
O
O
h ν
ϩ
ϩ
+
O
H
N
N
N3
1,3-Diphenylpropan-2-one 1b
O
2b
1,3-Diphenylethane
6
Scheme 5. Photolysis of 1,3-diphenylpropan-2-one and 1b.
Table 1. Energies of T1K and T2K in 1a, 1b using time-dependent density functional theory (TD-DFT) and UB3LYP calculations
1a 1b
TD-DFT
Gas-phase
UB3LYP
Gas-phase
TD-DFT
Gas-phase
UB3LYP
Gas-phase
CH3CN
CH3CN
70
CH3CN
CH3CN
68
(n,p*)
(p,p*)
73
76
78
75
68
74
77
78
77
69

RESEARCH FRONT
1648
R. A. A. U. Ranaweera et al.
T1K and T2K in acetonitrile are 75 and 78 kcal molꢀ1; thus,
solvation does not change these values significantly. Similarly,
TD-DFT calculations place T1K and T2K in 1b 74 and
77 kcal molꢀ1 respectively in the gas phase and 77 and
78 kcal molꢀ1 respectively in acetonitrile above S0. Thus, the
energies of T1K and T2K in 1a, 1b in acetonitrile are comparable.
Furthermore, the calculated energies for T1K in 1a and 1b are
similar to the measured triplet state energy for the analogous
m-methylacetophenone and p-methylacetophenone.[36,37]
We also optimized T1K of 1a, 1b at the UB3LYP level of
theory in the gas phase. The optimized structure of T1K of 1a, 1b
˚
has carbonyl groups with a bond length of 1.32 A, which is
consistent with the triplet ketones having an (n,p*) configura-
tion;[38,39] visualization of the molecular orbitals of T1K in 1a,
1b further support this assignment. The bond lengths and angles
of the azido group of T1K of 1a, 1b are similar to those observed
for the S0 of 1a, 1b, thus indicating that the triplet excited state
is localized on the aryl ketone moiety. The calculated energies
of the optimized T1K in 1a, 1b are 68 and 69 kcal molꢀ1 above
S0, respectively, which is somewhat lower than those estimated
from the TD-DFT calculations. However, we have previously
shown that the B3LYP/6-31Gþ(d) calculation underestimates
the energies of triplet ketones with the (n,p*) configura-
tion.[21,40] In addition, solvation does not significantly change
the energies of T1K in 1a, 1b.
T
2K (π,π*) 76
TS
71
T1K (n,π*) 68–73
TS
54
.
Optimization of the triplet configuration of the azido moi-
eties (TA) in 1a, 1b in the gas phase shows that the N1–N2 and
˚
N2–N3 bonds are lengthened to 1.45 and 1.18 A respectively
44
44
TA
1aB ϩ N3
˚
compared with 1.24 and 1.14 A in S0 of 1a, 1b. Furthermore, the
10
1aN ϩ N2
S0 0
azido groups are bent because the N1–N2–N3 bonds have an
angle of 1208 in the triplet state. The energies of TA of 1a, 1b are
44 and 45 kcal molꢀ1 above S0. Thus, the triplet energies and
optimized structures of TA in 1a, 1b are similar to those reported
previously.[21] The bond length and angles of the carbonyl group
in TA of 1a, 1b are similar to those we observed for the S0 of 1a,
1b, thus indicating that the triplet excited state is localized on the
azido group.
T
2K (π,π*) 77
TS
72
T
1K (n,π*) 69–74
TS
44
56
1bB + N3
.
We calculated the transition state for forming
triplet alkylnitrene (1aN, 1bN) from the TA of 1a, 1b in the
gas phase and found that it was located at less than 1 kcal molꢀ1
above the TA of 1a, 1b. Intrinsic reaction coordinate (IRC)[41]
calculations were performed to correlate these transition states
with the TA of 1 and 1N.[42–44]
44
TA
10
1bN ϩ N2
S0 0
We calculated the triplet transition state for cleavage of the
azido group in 1a, 1b in the gas phase. These triplet transitions
states are located at 3 kcal molꢀ1 above T1K in 1a, 1b. IRC
Fig. 2. Calculated stationary points on the energy surface of (a) 1a, and
(b) 1b to form 1N and 1B at the UB3LYP and TD-B3 LYP levels of theory.
The energies are in kcal molꢀ1
.
(a)
40 ϫ 10Ϫ3
TK of 1a
1aN
1aB
20
0
300
350
400
450
500
550
600
Wavelength [nm]
(b)
TK of 1b
40 ϫ 10Ϫ3
1bN
1bB
20
0
300
350
400
450
500
550
600
Wavelength [nm]
Fig. 3. Calculated major absorption spectral features (above 300 nm) for T1K of 1a, 1b, 1aN, 1bN and 1aB, 1bB in acetonitrile.
f is the calculated oscillator strength for the electronic transitions.

RESEARCH FRONT
Azido Cleavage and Triplet Nitrene Formation
1649
calculations correlate these transitions state with a triplet state
that has elongated C–N–N bonds and the 1B and azido radicals
instead of with the T1K of 1. This process is similar to what has
been reported for a-cleavage in triplet ketones, where the
characteristic state for an a-cleavage has an (n,s*) nature; in
contrast, the excitation is into the triplet (n,p*) state of the
ketone chromophore.[45–47]
1bB, and triplet nitrenes 1aN, 1bN. The calculated spectra in
acetonitrile are displayed in Fig. 3.
Laser Flash Photolysis
We used laser flash photolysis (LFP) to identify the inter-
mediates formed on photolysis of 1a, 1b. LFP (excimer laser,
l ¼ 308 nm, 17 ns) of 1a in argon-saturated acetonitrile pro-
duces a broad transient absorption with lmax between ,350
and 420 nm due to the absorption of T1K of 1a, which decays
with a rate constant of 3.2 ꢁ 106 sꢀ1 (Fig. 4). We assign this
transient absorption to T1K of 1a because the triplet ketone in
p-methylacetophenone has similar absorption. In oxygen-
saturated acetonitrile, T1K of 1a becomes shorter-lived and
decays with a rate constant of 1 ꢁ 107 sꢀ1 due to quenching by
oxygen. Simultaneously with the decay of T1K of 1a, a new
transient absorption with lmax at ,350 nm forms with the same
In Fig. 2, stationary points on the triplet surface of 1a, 1b are
plotted. The transition state barrier for breaking the C–N bond
in 1a, 1b to form 1aB, 1bB is only 3 kcal molꢀ1 above T1K
,
and therefore the azido cleavage should be easily accessible
at ambient temperatures. Because the energies of T1K and T2K
are similar, we expect them to be in equilibrium and thus energy
transfer and azido cleavage to be competitive from these triplet
excited states.
Finally, we used TD-DFT calculations to calculate the
UV-absorption spectra of T1K of 1a, 1b, benzyl radicals 1aB,
(a)
0.02–0.08 μs
0.08–2.10 μs
2.10–4.00 μs
4.00–4.26 μs
0.15
0.10
0.05
0.00
350
400
Wavelength [nm]
450
500
(b) 0.3
0.2
0.1
0.0
0
2
4
6
8
10 ϫ 10Ϫ6
Time [s]
0.15
0.10
0.05
0.00
(c)
0
20
40
60
80
100 ϫ 10Ϫ6
Time [s]
Fig. 4. (a) Transient spectra obtained by laser flash photolysis of 1a in argon-saturated acetonitrile over 50 ms. (b) Kinetic
trace obtained at 390 nm. (c) Kinetic trace obtained at 330 nm.

RESEARCH FRONT
1650
R. A. A. U. Ranaweera et al.
(a)
60 ϫ 10Ϫ3
40
20
0
300
350
400
450
500
550
600
Wavelength [nm]
(b)
30 ϫ 10Ϫ3
0.02–0.80 μs
0.80–2.10 μs
2.10–3.98 μs
3.98–7.26 μs
20
10
330
340
350
360
370
380
390
400
Wavelength [nm]
Fig. 5. Transient spectra obtained from laser flash photolysis of 1b in argon-saturated acetonitrile over (a) 2.5 ms, and
(b) 10 ms.
(a)
5 ϫ 10Ϫ3
0
0
1
2
3
4
5
Time [ms]
30 ϫ 10Ϫ3
(b)
20
10
0
0.5
1.0
1.5
2.0
2.5
Time [ms]
Fig. 6. Kinetic traces from laser flash photolysis of 1b at 380 nm (a) in argon-saturated and (b) in oxygen-saturated acetonitrile.
rate constant; we assign this absorption to 1aB and 1aN (Fig. 4).
We base this assignment on the similarity of this transient
absorption to the calculated absorption spectra of 1aB and 1aN
(see Fig. 3). On shorter timescales, we observe a decay that can
be fitted with a mono-exponential function to yield a lifetime of
,30 ms and assign it to 1aB. On a longer timescale or on a
millisecond timescale, we observe a decay that cannot be fitted
with a mono-exponential function. We assign this decay to 1aN,

RESEARCH FRONT
Azido Cleavage and Triplet Nitrene Formation
1651
O
O
O
O
H
N
h ν
radicals are trapped to form an aldehyde and in the absence
of benzyl radicals, the triplet nitrenes decay by reacting with
oxygen and form 3a, 3b. Thus, triplet alkylnitrenes 1aN, 1bN
are inert or stable intermediates that have lifetimes on the order
of milliseconds, as they do not react with the solvent or their
precursor, but rather intercept radicals such as oxygen and 1aB,
1bB. Therefore, the reactivity of 1a, 1b is similarly to what
we have previously reported for a-azidoacetophenone 7, which
undergoes a-cleavage to form benzoyl radicals and energy
transfer to yield triplet alkylnitrenes 7N (Scheme 6).[17,21b]
Nitrene 7N did not dimerize to form azo-dimers but rather
reacted with benzoyl radicals to form 7P. However, the reac-
tivity of 7N was further complicated because it was photolabile
and underwent a-cleavage to form a benzoyl radical.
N3
.
ϩ
N:
Ar
Ar
Ar
Ar
Ar
O
7
7N
7P
Scheme 6. Photolysis of 7.
O
N3
O
hν
.
.
ϩ
N3
Ph
Ph
Ph
Ph
8
8B
Scheme 7. Photolysis of 8.
We have shown previously that photolysis of azide 8
(Scheme 7) in solution results mainly in azido cleavage to form
azido radical and benzyl radical 8B.[21b] The major difference
between 8 and 1a, 1b is that the triplet ketone in 8 also undergoes
deactivation via b-phenyl quenching in competition with pro-
duct formation. In more detail, propiophenone derivatives with
b-phenyl substituents undergo efficient deactivation of their
triplet ketones. The mechanism for b-phenyl quenching has
not yet been fully characterized;[53–55] however, it has been
proposed that the deactivation is due to charge transfer from the
b-phenyl ring to the electron-deficient triplet ketone.[56] How-
ever, more recently Bucher performed calculations to support
that the b-phenyl quenching is due to addition of the carbonyl
oxygen to the ipso- or ortho-carbon atoms of the b-phenyl ring,
which is followed by intersystem crossing and relaxation to the
ground state.[57] The b-phenyl quenching in 8 presumably limits
the energy transfer to form the corresponding triplet alkylnitrene
but does not yield azido cleavage.
and because 1aN decays by several different routes (Scheme 3),
its decay cannot be fitted with a mono-exponential function. The
assignment of the transient absorption of 1aB and 1aN is further
supported by LFP of 1a in oxygen-saturated acetonitrile. In
oxygen-saturated acetonitrile, we did not observe transient
absorption due to 1aB, presumably because 1aB reacts with a
rate of the order of diffusion with oxygen. In comparison, the
triplet nitrene 1aN has a lifetime of ,200 ms in oxygen-saturated
acetonitrile; by assuming the concentration of oxygen is
0.0091 M,[48] we can estimate that 1aN reacts with oxygen with
a rate of 5 ꢁ 105 Mꢀ1 sꢀ1
.
LFP of 1b resulted in a transient absorption with lmax at
355 nm, which decayed with a rate constant of 5 ꢁ 106 sꢀ1; we
assign this transient absorption to T1K of 1b (Fig. 5). As the
transient absorption at 380 nm decays, a new transient is formed
with the same rate constant at lmax ,340 nm. We assign this
transient absorption to 1bB and 1bN based on their calculated
spectra (Fig. 3). Benzyl radical 1bB decays at a rate constant of
,3 ꢁ 104 sꢀ1. In comparison, the decay of 1bN is observed over
a longer timescale and cannot be fitted as a mono-exponential
function. In oxygen-saturated acetonitrile, the transients due to
T1K of 1b and 1bB are quenched, whereas the absorption due
to 1bN decays with a rate constant of 5.3 ꢁ 103 sꢀ1 (Fig. 6).
Because the concentration of O2 in a saturated acetonitrile can
be assumed to be ,9 ꢁ 10ꢀ3 M,[48] 1bN can be estimated to
Conclusion
LFP of 1a, 1b result in triplet nitrenes 1aN, 1bN and benzyl
radicals 1aB, 1bB. DFT calculations further support the
photoreactivity of 1a, 1b. The triplet nitrenes are stable inter-
mediates that act as radical scavengers and trap the benzyl
radicals to form 2a, 2b. In the absence of benzyl radicals, the
triplet nitrenes react with oxygen and form 3a, 3b.
react with oxygen at a rate of 6 ꢁ 105 Mꢀ1 sꢀ1
.
We have previously estimated that triplet b-nitrenopropio-
phenones react somewhat more slowly with oxygen or with a
rate of 5 ꢁ 104 Mꢀ1 sꢀ1. In comparison, Liang and Schuster
reported that triplet p-nitrophenylnitrene reacts with oxygen
Experimental and Computational Methods
Synthesis of 1a, 1b
Azides 1a and 1b were synthesized by brominating the corre-
sponding methylacetophenone derivatives, followed by reaction
with sodium azide, as described below.
at a rate of less than 2 ꢁ 105 Mꢀ1 s
.
ꢀ1 [49] However, Gritsan and
Pritchina estimated this same rate to be somewhat larger.[50,51]
The rates for triplet alkyl- and arylnitrenes reacting with oxygen
are, however, in agreement with computational work by Liu
et al., who reported that triplet alkylnitrenes and triplet phenyl-
nitrenes can be expected to react similarly with oxygen.[52]
Thus, LFP of 1a and 1b gives similar results; the main
difference is that the T1K of 1b is somewhat longer-lived than
the T1K of 1a, presumably because the energy of T1K is lower for
1a than 1b. LFP supports the theory that 1a, 1b form long-lived
triplet nitrenes (1aN, 1bN) in competition with azido cleavage to
form 1aB, 1bB.
1-(3-Bromomethylphenyl)ethanone
3-Methylacetophenone (1.02 mL, 7.5 mmol), N-bromosuccini-
mide (1.34 g, 7.5 mmol) and benzoyl peroxide (0.036 g,
0.15 mmol) were dissolved in CCl4 (25 mL) and refluxed for
12 h. The reaction mixture was filtered and concentrated under
vacuum to afford a yellow oil. Purification was performed on a
silica column with a solvent system of hexane/ethyl acetate (9:1)
to give 1-(3-bromomethylphenyl)ethanone (0.96 g, 4.5 mmol,
60% yield). nmax (neat)/cmꢀ1 3003, 2965, 2921, 1685, 1601,
1586, 1440, 1357, 1277, 1217, 1191, 797, 691, 585. dH
(400 MHz, CDCl3) 2.62 (s, 3H), 4.53 (s, 2H), 7.46 (t, J 7.6, 1H),
7.61 (d, J 7.6, 1H), 7.90 (d, J 7.6, 1H), 7.98 (s, 1H). dC (100 MHz,
CDCl3) 26.70, 32.52, 128.35, 128.72, 129.19, 133.62, 137.64,
138.45, 197.55. m/z (EI) 214, 212 (Mþ), 199 (10), 197 (10), 133
(100), 118 (19), 90 (26).
Discussion
LFP demonstrated that 1a, 1b form both triplet nitrenes 1aN,
1bN and benzyl radicals 1aB, 1bB on irradiation. We propose
that triplet alkylnitrenes 1aN, 1bN trap the benzyl radicals to
form 2a, 2b, because in oxygen-saturated solution the benzyl

RESEARCH FRONT
1652
R. A. A. U. Ranaweera et al.
1-(3-Azidomethylphenyl)ethanone 1b
with argon for 20 min. The test tube was irradiated with a
medium-pressure Hg arc lamp through a Pyrex filter for 21 h.
GC-MS and NMR analysis of the reaction mixture showed
the formation of imine 2b as the only major photoproduct
(86%) and some remaining 1b (14%). The solvent was
removed under vacuum and the crude product mixture was
purified on an aluminium oxide column, eluting with dichloro-
methane, which was gradually increased to 5% methanol in
dichloromethane. Imine 2b hydrolyzed on the column to yield
3-acetylbenzaldehyde and 3-acetylbenzylamine. Thus, 2b was
characterized by spectroscopy prior to any purification.
2b: nmax (neat)/cmꢀ1 3351, 3032, 3064, 3003, 2922, 2856,
1682, 1645, 1601, 1504, 1404, 1434, 1358, 1273, 1190, 1082,
1021, 958, 915, 799, 730, 692, 650, 588. dH (400 MHz, CDCl3)
2.63 (s, 3H), 2.60 (s, 3H), 4.89 (s, 2H), 7.44–7.54 (m, 3H), 7.85–
8.03 (m, 4H), 8.33 (s, 1H), 8. 49 (s, 1H). dC (100 MHz, CDCl3)
26.73, 64.65, 127.22, 127.77, 128.22, 128.83, 129.01, 130.47,
132.50, 132.81, 136.41, 137.36, 137.47, 139.66, 161.40, 197.68,
198.17. m/z (EI) 279 (Mþ, 38%), 278 (14), 236 (12), 133 (100),
118, 90 (26), 89 (29). m/z (HRMS) calc. for C18H17NO2:
302.1157 [M þ Naþ]. Found: 302.1164.
1-(3-bromomethylphenyl)ethanone (0.96 g, 4.5 mmol) was
dissolved in acetonitrile (25 mL), NaN3 (2.93 g, 45 mmol) was
subsequently added to the solution and the resulting solution
was refluxed for 2 h. The reaction mixture was filtered and
concentrated under vacuum to afford 1-(3-azidomethylphenyl)
ethanone (0.79 g, 4.5 mmol, 99% yield). nmax (neat)/cmꢀ1 3065,
3004, 2929, 2877, 2101, 1686, 1603, 1586, 1438, 1358, 1273,
1185, 796, 693. dH (400 MHz, CDCl3) 2.62 (s, 3H), 4.43 (s, 2H),
7.48–7.55 (m, 2H), 7.91–7.94 (m, 2H). dC (100 MHz, CDCl3)
26.68, 54.44, 127.82, 128.27, 129.19, 132.65, 136.12, 137.63,
197.66. m/z (EI) 175 (Mþ, 10), 133 (76), 132 (90), 118 (10), 105
(13), 104 (67), 90 (13), 78 (21), 77 (100). m/z (HRMS) calc. for
C9H9N3ONa: 198.0640 [M þ Na]þ. Found: 198.0643.
1-(4-Bromomethylphenyl)ethanone
4-Methylacetophenone (5.01 g, 37.4 mmol), N-bromosuccini-
mide (8.05 g, 45.2 mmol), and benzyl peroxide (0.071 g,
0.326 mmol) were dissolved in benzene (300 mL). The solution
was degassed by bubbling argon through it for 20 min. This
solution was photolyzed with a Honoiva Mercury lamp via a
Pyrex sleeve. Once GC-MS spectroscopy indicated that
4-methylacetophenone was depleted, the reaction mixture was
filtered through glass wool, and washed with distilled water
and saturated aqueous sodium bicarbonate solution. The organic
layer was dried with anhydrous magnesium sulfate and was
concentrated under vacuum to yield an oil that was characterized
as 1-(4-bromomethylphenyl)ethanone (6.72g, 33.1mmol, 88.5%
yield). The characterization of the product was confirmed by
obtaining IR and 1H NMR spectra and the spectral data matched
with those reported earlier.[58]
3-Acetylbenzylamine
n
max (neat)/cmꢀ1 3608, 3583, 3354, 3062, 3002, 2923, 2854,
1682, 1602, 1584, 1359, 1275, 1189, 795. dH (400 MHz, CDCl3)
2.62 (s, 1H), 3.95 (s, 2H), 7.44 (t, J 7.6, 1H), 7.54 (d, J 7.6, 1H),
7.84 (d, J 8.0, 1H), 7.93 (s, 1H). dC (100 MHz, CDCl3) 26.73,
46.16, 126.850, 126.96, 128.80, 131.95, 137.42, 143.68,
198.27. m/z (EI) 149 (Mþ, 14), 148 (27), 134 (24), 132 (19),
106 (100), 105 (14), 104 (24), 89 (14), 79 (22), 78 (15), 77. m/z
(HRMS) calc. for C9H11NO: 150.0918 [M þ Hþ]. Found:
150.0919.
Mp 38–408C (lit.[59] 40–428C). nmax (CH2Cl2)/cmꢀ1 1683.
dH 2.60 (s, 3H), 4.50 (s, 2H), 7.48 (d, J 8.0, 2H), 7.92 (d, J 8.0,
2H). dC (CDCl3, 100 MHz) 26.69, 32.14, 128.84, 129.25, 136.90
142.81, 197.39.
Photolysis of 1b in Oxygen-saturated Chloroform
Azide 1b (211.7 mg, 1.21 mmol) was dissolved in chloro-
form (62 mL) in a Pyrex test tube. Then the solution was
purged with oxygen for 30 min. The test tube was photolyzed
under a medium-pressure Hg arc lamp through a Pyrex filter
for 10.5 h. GC-MS of the reaction mixture showed formation
of 3b, 4b and 5b in the ratio of 1.4:1:1.4. The solvent was
removed under vacuum and the resulting yellow solid was
dissolved in methanol, which precipitated 3b (43.2 mg,
0.141 mmol, 23% yield). The mother liquor was separated on
a silica column eluting with dichloromethane to yield 1b
(16.7 mg, 0.095 mol, 8% recovery), 3-acetylbenzaldehyde
(7.3 mg, 0.049 mmol, 4.0% yield) and 3-acetylbenzonitrile
(18.2 mg, 0.13 mmol, 11% yield).
1-(4-Azidomethylphenyl)ethanone 1a
1-(4-Bromomethylphenyl)ethanone (5.0 g, 24.6 mmol) was
dissolved in a mixture of ethanol (150 mL) and acetic acid
(1.5 mL) and cooled in an ice bath. To this solution was added
dropwise sodium azide (10 g, 156 mmol), dissolved in a minimal
amount of water. The resulting mixture was stirred in an ice bath
for 2 h and stored at 48C for 72 h. The reaction mixture was
extracted with diethyl ether (100 mL) and the organic layer
was washed with water, then dried with anhydrous magnesium
sulfate and concentrated under vacuum. The resulting oil was
purified via a silica column using an ethyl acetate and hexane
mixture as the eluent to yield 1-(4-azidomethylphenyl)ethanone
(505 mg, 2.89 mmol, 11.7% yield). The H NMR spectrum of
1-(4-azidomethylphenyl)ethanone matched that found in the
literature.[60]
nmax (CDCl3)/cmꢀ1 3005, 2102, 1684, 1609, 1574, 1412,
1359, 1267, 1182, 1116, 1075, 1017, 956, 847, 814, 694, 660,
595. dH (CDCl3, 400 MHz) 2.63 (s, 3H), 4.44 (s, 2H), 7.43 (d, J
8.0, 2H), 7.99 (d, J 8.0, 2H). dC (CDCl3, 100 MHz) 26.67, 54.24,
128.15, 128.88, 136.93, 140.58, 197.53.
1
1-{3[5-(3-Acetylphenyl)-[1,2,4]oxadiazol-3-yl]-phenyl}
ethanone 3b
n
max (neat)/cmꢀ1 1678, 1603, 1411, 1353, 1246, 919, 799,
678, 586. dH (400 MHz, CDCl3) 2.72 (s, 3H), 2.74 (s, 3H), 7.65
(t, J 7.6, 1H), 7.71 (t, J 7.6, 1H), 8.15 (d, J 7.6, 1H), 8.23 (d, J
8.0, 1H), 8.40 (d, J 7.6, 1H), 8.44 (d, J 8.0, 1H), 8.81 (s, 1H),
8.77 (s, 1H). dC (100 MHz, CDCl3) 26.81, 124.77, 127.34,
127.59, 128.12, 129.44, 129.71, 130.88, 131.85, 132.30,
137.789, 137.97, 168.51, 175.21, 196.79, 197.44. m/z (EI)
306 (Mþ), 291 (100), 221, 147, 138, 130, 118, 102, 91, 76, 65.
m/z (HRMS) calc. for C18H14N2O3: 329.0917 [M þ Naþ].
Found: 329.0902.
Preparative Photolysis
Photolysis of 1b in Argon-saturated Methanol
Azide 1b (83 mg, 4.7 mmol) was dissolved in methanol
(21 mL) in a Pyrex test tube. The solution was purged

RESEARCH FRONT
Azido Cleavage and Triplet Nitrene Formation
1653
Preparative Photolysis of 1a in Oxygen-saturated Toluene
1,3-Diphenylethane: m/z (EI) 182 (Mþ), 165, 152, 139, 128,
115, 104, 91 (100%), 77, 65, 51.
1-(4-Azidomethylphenyl)ethanone (0.459 g, 2.6 mmol) was
dissolved in dry, distilled toluene (100 mL). The solution was
photolyzed for 15 h using a Honoiva Mercury lamp via a Pyrex
sleeve. The reaction was monitored by HPLC and the irradiation
was stopped when the HPLC showed formation of two new
major products, 4a (17%) and 3a (24%) and remaining starting
material (31%). HPLC yielded recovered starting material
(25 mg, 0.14 mmol, 30% yield), 4-acetylbenzaldehyde (5 mg,
0.0368 mmol, 16% yield) and 1-{4[5-(4-acetylphenyl)-[1,2,4]
oxadiazol-3-yl]phenyl}ethanone 3a (10 mg, 0.0328 mmol, 44%
yield).
1-Adamantyl azide: m/z (EI) 177 (Mþ), 148, 135 (100%),
120, 115, 106, 93, 84, 79, 70, 65, 58, 53.
1,3-Diphenylpropan-2-one: m/z (EI) 210 (Mþ), 178, 165,
152, 119, 103, 91 (100%), 77, 65, 51.
Photolysis of 1b and 1,3-Diphenylpropan-2-one
in Argon-saturated Methanol
Azide 1b (40 mg, 0.2 mmol) and 1,3-diphenylpropan-2-one
(60 mg, 0.3 mmol) were dissolved in methanol (3 mL) in a Pyrex
test tube. The solution was purged with argon for 10 min and
irradiated with a medium-pressure Hg arc lamp through a Pyrex
filter for 13 h. GC-MS analysis of the reaction mixture showed
formation of 1,3-diphenylethane as the only major photoproduct
(41%), a smaller amount of 2b (7%) and 6 (6%) and some
remaining starting materials.
1-{4[5-(4-Acetylphenyl)-[1,2,4]oxadiazol-3-yl]phenyl}
ethanone 3a
nmax (CH2Cl2)/cmꢀ1 2924, 1693, 1589, 1526, 1406, 1357,
1267, 960, 845, 760, 714, 600, 474. dH (CDCl3, 400 MHz) 2.17
(s, 6H), 8.29–8.10 (dd, 8.5 and 8.5, 4H), 8.36–8.34 (dd, 8.5 and
8.5, 4H). dC (CDCl3, 100 MHz) 29.45, 127.53, 128.22, 128.58,
128.73, 139.28, 139.86, 196.85, 197.20. m/z (EI) 306 (Mþ 16%),
291 (100), 147 (30), 104 (30). m/z (HRMS) calc. for
C18H14N2O3: 306.1004. Found: 306.1081.
1,3-Diphenylethane: m/z (EI) 182 (Mþ), 165, 152, 139, 128,
115, 104, 91 (100%), 77, 65, 51.
6: m/z (EI) 239 (Mþ), 162, 148, 133, 118, 111, 106, 91
(100%), 77, 65.
2b: m/z (EI) 279 (Mþ), 264, 246, 236, 165, 144, 133, 118,
104, 90, 77, 65.
Photolysis of 1a and 1b
Calculations
Methanol solutions of 1b (0.06 and 0.18 M) were purged with
argon for 10 min and irradiated. GC-MS analysis of the reaction
mixture showed only formation of 2b in significant amounts.
Similarly, GC-MS analysis of 0.06 and 0.18 M methanol solu-
tions of 1a after irradiation showed only formation of 2a. Similar
results were obtained in argon-saturated acetonitrile and chloro-
form solutions.
Oxygen-saturated chloroform solutions of 1a (0.06 M) and
1b (0.06, 0.12, and 0.18 M) were photolyzed and the reaction
mixture was analyzed by GC-MS. The conversion was kept
below 30%. The major products observed were 3a, 3b, 4a, 4b,
and 5a, 5b.
All geometries were optimized as implemented in the Gaus-
sian09 programs,[24] at the B3LYP level of theory and with the
6-31Gþ(d) basis set.[25,26] All transition states were located
at the UB3LYP level of theory, and each transition state was
confirmed to have one imaginary vibrational frequency by
analytical determination of the second derivatives of the energy
with respect to internal coordinates.[61] IRC calculations were
used to verify that the located transition states corresponded to
the attributed reactant and product.[41–44] Vertical UV absorp-
tion spectra were calculated at the TD-B3LYP level with the
6-31þG(d) basis set using the optimized B3LYP/6-31þG(d)
geometry for the S0 state. The effect of solvation was calculated
using the SCRF method with the PCM with acetonitrile as the
solvent.[32–35]
Photolysis of 1,3-Diphenylacetone in Argon-saturated
Methanol
LFP
1,3-Diphenylacetone (60 mg, 0.3 mmol) was dissolved in
methanol (3 mL) in a Pyrex test tube. The solution was purged
with argon for 10 min and irradiated with a medium-pressure Hg
arc lamp through a Pyrex filter for 13 h. GC-MS analysis of the
reaction mixture showed the formation of 1,3-diphenylethane as
the only photoproduct (100%).
LFP was done with an excimer laser (308 nm, 17 ns). The system
has been described elsewhere.[62] Stock solutions of azides 1a
and 1b were prepared with spectroscopic-grade acetonitrile,
such that the solutions had an absorption between 0.6 and 0.8
at 308 nm. Typically, ,1 mL of the stock solution was placed
in a 10 ꢁ 10 mm wide, 48 mm long cm quartz cuvette and was
purged with argon or oxygen for 5 min. The rates were obtained
by fitting an average of three to eight kinetic traces.
1,3-Diphenylethane: m/z (EI) 182 (Mþ), 176, 165, 152, 139,
128, 115, 104, 98, 91 (100%), 86, 77, 65.
Photolysis of 1-Adamantyl Azide and 1,3-
Diphenylpropan-2-one
Acknowledgements
We thank the National Science Foundation and the Ohio Supercomputer
Center for supporting this work. R.A.A.U.R. thanks the University of Cin-
cinnati Research Council for a fellowship and S.M. thanks the Chemistry
Department at the University of Cincinnati for a Harry Mark fellowship.
Adamantyl azide (60 mg, 0.3 mmol) and 1,3-diphenylpro-
pan-2-one (60 mg, 0.3 mmol) were dissolved in methanol
(3 mL) in a Pyrex test tube. The solution was purged with argon
for 10 min and irradiated with a medium-pressure Hg arc lamp
through a Pyrex filter for 4 h. GC-MS analysis of the reaction
mixture showed formation of 1,2-diphenylethane as the only
major photoproduct (27%) and remaining adamantyl azide
(45%) and 1,3-diphenylpropan-2-one (28%).
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