Selective formation of triplet alkyl nitrenes from photolysis of β-azido-propiophenone and their reactivity

Article abstract of DOI:10.1021/ja077523s

Photolysis of β-azido propiophenone derivatives, 1, with built-in sensitizer units, leads to selective formation of triplet alkyl nitrenes 2 that were detected directly with laser flash photolysis (λ_max = 325 nm, τ = 27 ms) and ESR spectroscopy (|D/hc| = 1.64 cm^-1, |E/hc| = 0.004 cm^-1). Nitrenes 2 were further characterized with argon matrix isolation, isotope labeling, and molecular modeling. The triplet alkyl nitrenes are persistent intermediates that do not abstract H-atoms from the solvent but do decay by dimerizing with another triplet nitrene to form azo products, rather than reacting with an azide precursor. The azo dimer tautomerizes and rearranges to form heterocyclic compound 3. Nitrene 2a, with an n,π* configuration as the lowest triplet excited state of the its ketone sensitizer moiety, undergoes intramolecular 1,4-H-atom abstraction to form biradical 6, which was identified by argon matrix isolation, isotope labeling, and molecular modeling. β-Azido-p-methoxy-propiophenone, with a π,π* lowest excited state of its triplet sensitizer moiety, does not undergo any secondary photoreactions but selectively yields only triplet alkyl nitrene intermediates that dimerize to form 3b.

Full text of DOI:10.1021/ja077523s

Published on Web 11/23/2007
Selective Formation of Triplet Alkyl Nitrenes from Photolysis
of â-Azido-Propiophenone and Their Reactivity
Pradeep N. D. Singh,† Sarah M. Mandel,† Jagadis Sankaranarayanan,†
Sivaramakrishnan Muthukrishnan,† Mingxin Chang,† Rachel M. Robinson,†
Paul M. Lahti,‡ Bruce S. Ault,† and Anna D. Gudmundsdo´ttir*,†
Contribution from the Department of Chemistry, UniVersity of Cincinnati, Cincinnati, Ohio
45221-0172, and Department of Chemistry, UniVersity of Massachusetts,
Amherst, Massachusetts 01003
Received September 29, 2007; E-mail: Anna.Gudmundsdottir@uc.edu
Abstract: Photolysis of â-azido propiophenone derivatives, 1, with built-in sensitizer units, leads to selective
formation of triplet alkyl nitrenes 2 that were detected directly with laser flash photolysis (λmax ) 325 nm,
τ ) 27 ms) and ESR spectroscopy (|D/hc| ) 1.64 cm-1, |E/hc| ) 0.004 cm-1). Nitrenes 2 were further
characterized with argon matrix isolation, isotope labeling, and molecular modeling. The triplet alkyl nitrenes
are persistent intermediates that do not abstract H-atoms from the solvent but do decay by dimerizing with
another triplet nitrene to form azo products, rather than reacting with an azide precursor. The azo dimer
tautomerizes and rearranges to form heterocyclic compound 3. Nitrene 2a, with an n,π* configuration as
the lowest triplet excited state of the its ketone sensitizer moiety, undergoes intramolecular 1,4-H-atom
abstraction to form biradical 6, which was identified by argon matrix isolation, isotope labeling, and molecular
modeling. â-Azido-p-methoxy-propiophenone, with a π,π* lowest excited state of its triplet sensitizer moiety,
does not undergo any secondary photoreactions but selectively yields only triplet alkyl nitrene intermediates
that dimerize to form 3b.
Introduction
crosses to the triplet, but the light used to decompose the phenyl
azide precursor at low temperature is also absorbed by the
The pursuit of organic magnetic materials has sparked
renewed interest in triplet nitrene intermediates, which are
readily studied models for exchange behavior because of their
high spin properties.1 A limitation, however, is that they are
difficult to form selectively because they and their precursors
are highly reactive.2,3 Thermal activation or direct photolysis
of aryl, carboalkoxy, and phosphoryl azide precursors yields
singlet nitrenes that are highly reactive and can insert into
themselves and other molecules in competition with intersystem
crossing to their triplet ground states. Direct photolysis of the
azide precursors at low temperature favors intersystem crossing
over other singlet reactivity. However, the triplet nitrenes
themselves are sensitive to light. For example, Levya et al.
have demonstrated that photolyzing phenyl azide at low
temperature produces singlet phenyl nitrene that intersystem
phenyl nitrene to form ketenimine.4 Interestingly, Mahe et al.
and Ichimura et al. have successfully formed stable triplet aryl
nitrenes at low temperatures within crystal lattices.5,6
In contrast to aryl nitrenes, triplet alkyl nitrenes have not been
studied extensively, in part because direct irradiation of alkyl
azides in solution leads to the formation of imine products via
a concerted rearrangement of the singlet excited state of the
alkyl azides rather than alkyl nitrenes.7 Some of us have shown
that the photolysis of R-azidoacetophenone, having a built-in
intramolecular triplet sensitizer, leads to intramolecular energy
transfer and the formation of triplet alkyl nitrene intermediates.8,9
This process is complicated because, in competition with energy
transfer to form triplet alkyl nitrenes, the R-azidoacetophenones
undergo R-cleavage to form benzoyl and azido methyl radicals.
The triplet alkyl nitrenes can then be intercepted by the benzoyl
† University of Cincinnati.
‡ University of Massachusetts Amherst.
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120, 11528. (c) Magnetic Properties of Organic Materials; Lahti, P. M.,
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C. M.; Krause, J. A.; Gudmundsdottir, A. D. J. Org. Chem. 2007, 72, 2757.
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A.; Gudmundsdottir, A. D. Org. Lett. 2006, 8, 4207.
9
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J. AM. CHEM. SOC. 2007, 129, 16263-16272
16263

A R T I C L E S
Scheme 1
Singh et al.
Scheme 2. Possible Mechanism for Formation of Photoproducts from Irradiation of Azide 1
radicals to form benzamide products. Thus, we investigated the
photochemistry of azides 1, with a built-in intramolecular
sensitizer that is not able to undergo R-cleavage,10 to verify that
triplet alkyl nitrenes can indeed be formed selectively, and to
subsequently explore the reactivity of triplet alkyl nitrenes in
the absence of radical traps. We prepared azide 1a and 1b, which
have n,π* and π,π* configurations as the lowest triplet excited
state of the ketone moiety, respectively. In this paper we
demonstrate for the first time that triplet alkyl nitrenes can be
formed selectively in solution and that triplet alkyl nitrenes are
unreactive and long-lived intermediates, because they do not
abstract H-atom from the solvent or react with their precursors.
However, prolonged irradiation of the triplet alkyl nitrene
formed from photolysis of azide 1a leads to intramolecular â-H-
atom abstraction to form an imine radical as a secondary
photoproduct. In comparison, the nitrene formed from azide 1b
is not photoreactive, since triplet ketones with π, π* configu-
ration do not undergo intramolecular H-atom abstraction ef-
ficiently.11
Results and Discussion
Product Studies. Photolysis of azide 1a yields 3a as the
major product and smaller amounts of 4a and propiophenone
(see Scheme 1). Pyrazole 3a was characterized based on the
similarity of its 13C and 1H NMR spectra to those of the
1-methyl-5-phenylpyrazole and 1-(4-chloro-phenyl)-3-[3-(4-
(10) Klima, R. F.; Jadhav, A. V.; Singh, P. N. D.; Chang, M.; Vanos, C.;
Sankaranarayanan, J.; Vu, M.; Ibrahim, N.; Ross, E.; McCloskey, S.;
Murthy, R. S.; Krause, J. A.; Ault, B. S.; Gudmundsdottir, A. D. J. Org.
Chem. 2007, 72, 6372.
(11) (a) Wagner, P. J. In Rearrangements in Ground and Excited States; De
Mayeo, P., Ed.; Academic Press: New York, 1980; Vol. 42-3, p 381. (b)
Wagner, P. J. Top. Curr. Chem. 1976, 66, 1.
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Selective Formation of Triplet Alkyl Nitrenes
A R T I C L E S
Figure 2. UV-vis absorbance spectra of azides 1.
Figure 1. Product ratio from irradiation of 1a vs laser intensity.
Scheme 3. Products from Photolysis of 1a in Oxygen-Saturated
Toluene
chlorophenyl)-pyrazol-1-yl]-propan-1-one analogues.12,13 Prod-
uct 4a was identified similarly based on a comparison of its
1
13C and H NMR spectra with the NMR spectra of 3-alkyl-
amino-1-phenyl-propenone derivatives.14
We propose that 3a is formed by the dimerization of two
triplet alkyl nitrene intermediates to form azo compound 5a,
which rearranges with loss of a water molecule as shown in
Scheme 2. Similarly, triplet aryl nitrenes mainly dimerize to
give diaryl diazenes, but triplet aryl nitrenes also abstract an
H-atom from the solvent to form aniline derivatives and are
therefore more reactive than triplet alkyl nitrenes.15 The forma-
tion of propiophenone can be explained by the secondary
photolysis of the azo dimer 5a to give a propiophenone radical
that abstracts a H-atom from the solvent. Similarly, formation
of 4a can be attributed to the propiophenone radical that is
intercepted by nitrene 2a, and abstraction of a H-atom from
the resulting radical 7a-i would lead to the formation of
oxoimino 4a. Zhou et al. have shown that even though
enaminones can exist in the tautomeric iminoenol, oxoimino,
and ketoamino forms, in solution they are mainly present in
the ketoamino form.14 Since, the present studies of nitrene 2a
support that it undergoes intramolecular H-atom abstraction to
form biradical 6, we speculated what products will arise from
biradical 6. Interestingly, trapping of 6 with nitrene 2a might
also yield product 3a, whereas propiophenone radical and
biradical 6 can combine to form 7a-ii which results in formation
of 4a.
In order to verify that propiophenone and 4a are formed upon
secondary photolysis, we photolyzed azide 1a with a high
intensity argon laser jet at three different laser pulse powers,
but keeping the conversion below 10%. Under these experi-
mental conditions we did not observe any propiophenone
formation. As the intensity of the laser pulse was increased,
higher yields of 4a were observed at the expense of 3a (see
Figure 1), demonstrating that 4a must come from secondary
photolysis, presumably of 5a or 2a. It can be speculated that a
higher excited state of the azide 1a is attainable upon irradiation
with the high-intensity laser. However, the second excited triplet
state of the ketone chromophore in azide 1a has a π,π*
configuration which would not undergo efficient intramolecular
â-H-atom abstraction.11
In contrast, photolysis of azide 1b is much more selective
than azide 1a. Azide 1b yields only one major compound, 3b,
which comes from the formation of the azo dimer 5b. No
formation of p-methoxy propiophenone from photolysis of azide
1b was observed. Presumably, the major difference in the UV-
vis spectra of the azo dimers 5a and 5b is that 5b must have a
stronger absorption in the visible region due to the methoxy
substituent. The absorption spectra of azides 1a and 1b
demonstrate this difference as well (see Figure 2). It is known
that the azo chromophore in azomethane has a maximum
absorption at λmax ) 347 nm (ꢀ ) 4.5);16 thus direct absorption
by the azo chromophore in 5a is possible whereas in 5b the
methoxy-acetophenone chromophore absorbs much more strongly
above 300 nm than the azo chromophore. Therefore it stands
to reason that the azo chromophore in 5a can absorb light
directly to form a propiophenone radical and a nitrogen
molecule. Since we do not observe the formation of methoxy
propiophenone from 5b, it can be surmised that the ketone
chromophore in both 5a and 5b can absorb light and transfer
its energy to the azo chromophore which most likely only results
in cis-trans isomerization, whereas direct absorption by the
azo chromophore cleaves the carbon nitrogen bond. Generally,
the dominant reaction from triplet sensitization of simple acylic
azo compounds is cis-trans isomerization rather than decom-
position to nitrogen and radicals.17 Furthermore, we do not
expect nitrene 2b to undergo intramolecular H-atom abstraction
since triplet ketones with π,π* electron configurations do not
abstract H-atoms efficiently.11
Oxygen Trapping of the Reactive Intermediates. We
photolyzed azide 1a in oxygen-saturated toluene solutions and
observed the formation of compounds 8 and 9 (see Scheme 3).
We characterized compound 9 based on its spectroscopic data.
The 13C NMR and the IR spectra of 9 match precisely with the
(12) Pavlik, J. W.; Kebede, N. J. Org. Chem. 1997, 62, 8325.
(13) Sankaranarayanan, J.; Mandel, S. M.; Krause, J. A.; Gudmundsdottir, A.
D. Acta Crystallogr., Sect. E 2007, E63, o721.
(14) Zhuo, J.-C. Magn. Reson. Chem. 1997, 35, 21.
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(b) Reiser, A.; Leyshon, L. J. J. Am. Chem. Soc. 1971, 93, 4051.
(16) Gillam, A. E.; Stern, E. S. An introduction of Electronic Absorption
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A R T I C L E S
Singh et al.
Figure 3. Minimal energy conformer of azide 1a and their calculated relative energies.
Scheme 4. Photolysis of 1a in the Presence of Alkyl Azides
characteristic spectra of other furazan 2-oxide derivatives.18 The
nitro compound 8 comes from trapping nitrene 2a with oxygen
just as triplet aryl nitrenes are trapped with molecular oxygen
to form nitroaryls.19 Compound 9, however, must come from
nitrene 2a reacting with the precursors to 8.
Formation of Azo Dimers. A point of curiosity to us was
how the triplet alkyl nitrene decays. The product studies
demonstrate that nitrene 2a decays either by dimerization or
by reaction with azide 1a. In order to investigate this, we
followed the photolysis of azide 1a in the presence of an alkyl
azide without a built-in sensitizer, which does not absorb light
above 300 nm. If nitrene 2a decays by reacting with an alkyl
azide molecule we should observe cross-products between the
nitrene and the alkyl azide. Photolysis of 1a in the presence of
admantanyl azide or cyclopentyl azide (Scheme 4) did not yield
any new products. Thus triplet alkyl nitrene 2a must be a long-
lived intermediate specifically reactive only toward other radicals
such as 6 and propiophenone radicals.
Molecular Modeling. Molecular modeling studies of the
target azides were done using Gaussian0320 at a B3LYP21 level
of theory and 6-31+G(d) as the basis set. We optimized the
structure of azide 1a and found several conformers and
calculated their IR spectra (see Supporting Information). The
minimal energy conformers of azide 1a that are within 1 kcal/
mol (Figure 3) were used for the calculations described below.
Time dependent calculation (TD-DFT)22 of azide 1a showed
that the lowest triplet state of the ketone moiety (T1) is located
∼75 kcal/mol above its ground state. T1 and the second excited
triplet ketone (T2) are located within a kcal/mol of each other.
The calculated energies of T1 and T2 in azide 1a are similar to
those observed for butyrophenone.23 We optimized the triplet
azide configuration (T1A) of azide 1a and found it is ∼44 kcal/
mol above its ground state. The N1-N2-N3 bond in T1A has a
bond angle of ∼120.6° (Figure 4), and the N1-N2 bond is 1.43
Å, whereas in the ground state of azides 1 the N1-N2 bond
length is 1.23 Å. As a reflection of the increased bond length
of the N1-N2 bond, the calculated IR spectra of T1A does not
have an azide stretching vibration at ∼2200 cm-1 as is seen
for azide 1a, but rather, the N-N stretch is calculated to be
∼1748 cm-1
.
The calculated transition state for forming nitrene 2a and a
nitrogen molecule is approximately 0.1 kcal/mol above the
energy of the triplet azide moiety in azide 1a. Intrinsic reaction
coordinate (IRC) calculation connected the triplet azide and
nitrene 2 with this transition state.24 Thus, the calculations
support that nitrenes 2 can easily be formed via intramolecular
energy transfer from the triplet ketone in azides 1 to the azide
moiety which falls apart to form a nitrogen molecule and a triplet
nitrene.
We optimized the minimal energy conformers of nitrene 2
and obtained their predicted IR spectra (see Figure 5). TD-DFT
calculations were done to calculate the absorption spectra of
nitrene 2a, and in the gas phase the major bands above 290 nm
are listed in Table 1. We used the self-consistent reaction field
(SCRF) method with the integral equation formalism polarizable
continuum model (IEFPCM)25 to calculate the absorption spectra
of nitrene 2a in methanol above 290 nm (Table 1). The
calculated absorption spectrum of nitrene 2b is similar (see
Supporting Information).
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Selective Formation of Triplet Alkyl Nitrenes
A R T I C L E S
Table 1. TD-DFT Computed Major Absorption Spectral Features
(nm, Oscillator Strength) for 2aa,b
spectra of nitrenes 2. We have previously shown that laser flash
photolysis of R-azidoacetophenone results in transient spectra
with broader absorption bands because benzoyl radicals, which
have an absorption band further to the red (λmax ∼370 nm), are
formed concurrently with the triplet alkyl nitrene.8a Thus the
transient spectra of nitrenes 2 are similar to the absorption
spectra obtained for triplet phenyl nitrene and NH radicals that
have an intense absorption band near 300 nm, which is
attributable mainly to the nz f ny transition on the nitrogen
atom.26,27 However, triplet methyl nitrene has a maximum
absorption at ∼280 nm in matrices, whereas the maximum
absorption of triplet trifluoromethylnitrene is shifted to 354
nm.28,29
electronic transition above 290 nm
2a gas phase 305 (0.0071) 298 (0.0051) 292 (0.0493)
MeOH
313 (0.0458) 310 (0.0005) 303 (0.0010) 296 (0.0196)
a
Number in parentheses is the calculated oscillator strength (f) for the
b
electronic transition in nm. Calculated absorption spectrum for vertical
excitation using TD-B3LYP/6-31+G(d).
We measured the decay for nitrene 2a at 330 nm. The nitrene
transient decays on the milliseconds time scale and can be fitted
with first-order expression to have a lifetime of 27 ms in
methanol (see Figure 9). The transient absorbance is quenched
with isoprene, which quenches the triplet excited state of the
ketone precursor. A Stern-Volmer plot of isoprene quenching
is linear with a slope of 0.15. Thus the lifetime of the triplet
excited state of the ketone is between 1.5 and 0.15 ns, assuming
quenching is a diffusion-controlled process with kq between 109
and 1010 M-1 s-1. The triplet ketone in azide 1a is efficiently
quenched by energy transfer to the azido group. The rate of the
energy transfer in azide 1a is similar to what we observed for
intramolecular energy transfer in R-azido acetophenones8a and
much faster than intermolecular energy transfer in alkyl azides.30
In oxygen saturated methanol solution the lifetime of the triplet
nitrene 2 becomes approximately 2 ms. By assuming that the
concentration of the molecular oxygen in methanol at room
temperature is around 0.01 M,31 we can estimate the rate of
quenching triplet nitrenes 2 is around 5 × 104 M-1 s-1. In
comparison, Liang and Schuster reported that triplet p-nitro-
phenyl nitrene reacts with oxygen at rate of less than 2 × 105
M-1 s-1, whereas Gritsan and Pritchina estimate this same rate
constant to be somewhat larger.32,33 These results are, however,
in agreement with molecular modeling by Liu et al., who
demonstrated that triplet alkyl nitrene can be expected to react
similarly with oxygen as triplet phenyl nitrenes.34 Furthermore,
the lifetimes of nitrenes 2 in nitrogen-saturated solutions must
be limited by residual oxygen as well as dimerization.
Figure 4. T1A of 1a.
Figure 5. Minimal energy conformers of nitrene 2a, showing relative
energies.
We also optimized the structure of biradical 6: several
conformers were found and their IR spectra calculated (see
Supporting Information). One of the lowest energy conformers,
6A, has a hydrogen bond between the hydroxyl moiety and the
imine radical, whereas the others are twisted such that hydrogen
bonding cannot occur (see Figure 6). We calculated the transition
state for the intramolecular H-atom abstraction in nitrene 2a
and found it was located 32 kcal/mol above its ground state.
Thus, H-atom abstraction is feasible since TD-DFT calculation
indicated that the triplet ketone moiety is located ∼75 kcal/
mol above the ground state in nitrene 2a. IRC calculation
connected nitrene 2a and the biradical 6 to this transition state.
We also calculated the transition state for acetophenone forming
1-phenylethenol, 10, which is located 69 kcal/mol above the
ground state of acetophenone. Finally, we calculated the
transition state for biradical 6A falling apart to yield hydrocyanic
acid and triplet 1-phenylethenol, 11. This transition state was
28 kcal/mol above the ground state of 6A. IRC calculation
verified that this transition state correlates 6A with hydrocyanic
acid and 11.
ESR Spectroscopy. Electron spin resonance (ESR) spec-
troscopy was used to characterize nitrene 2 further. Azide 1a
was dissolved in ethanol, and the resulting solution was frozen
to form a glassy matrix at 4 K in a LHe cryostat in an X-band
ESR spectrometer and then irradiated in situ through a Pyrex
filter. The ESR spectrum was then recorded between 500 and
11 500 G. The spectrum showed a line at 8400 G (Figure 10),
consistent with triplet state nitrene 2a and similar to spectra of
Laser Flash Photolysis. We studied azides 1 by laser flash
photolysis in methanol (Lambda Physik, excimer laser, 17 ns,
150 mJ, 308 nm), recording the spectra immediately after the
laser pulse during a 200 ns window. The transient spectrum
has a maximum absorbance at ∼325 nm (see Figure 7) that we
assign this absorbance to nitrenes 2 based on the predictions in
Table 2. The calculated electronic transition for nitrene 2a in
methanol (see Figure 8) fits well with the observed transient
(26) Gritsan, N. P.; Zhu, Z.; Hadad, C. M.; Platz, M. S. J. Am. Chem. Soc.
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Singh et al.
Figure 6. Minimal energy conformers of 6, showing relative energies.
The intensity of the lines around 3300 G did not increase after
irradiation and therefore was not be due to dark reactions of
2a. Thus, ESR spectroscopy further supports the conclusion that
photolysis of azide 1a results in formation of nitrene 2a as a
relatively persistent intermediate, since it does not react with
the solvent.
Matrix Isolation: We studied the photochemistry of azide
1a in argon matrices at 14 K to confirm the formation of the
triplet alkyl nitrene intermediate and to determine whether
nitrene 2a undergoes secondary photolysis. We deposited 1a
into argon matrices in several different experiments, entraining
the vapor pressure over a sample of pure azide in flowing argon,
and depositing the resulting sample on a 14 K cryogenic surface.
The infrared spectrum of such a sample showed strong azide
bands at 2136, 2118, 2120, and 2108 cm-1 (Figure 11), which
are all destroyed upon irradiation except the 2136 cm-1 band.
The various azides bands are presumably due to different
conformers of azide 1a, which are also entrapped in different
matrix sites.
Figure 7. Transient spectrum of nitrene 1b in methanol at ambient
temperature.
Table 2. Calculated C-N Stretches (cm-1) of Nitrene 2a
relative
energy
12C
−
14N IR band,
15N
13C
× 3
cm-1 (intensity,
substituted
band, cm-
substituted
band, cm-
1
1
conformer
(kcal/mol)
km/mol)
2a-A
2a-B
0
1.4
1071 (8)
1068 (3)
1033 (25)
894 (3)
1060
1061
1030
891
1058
1047
1011
877
Irradiation of 1a in argon matrices led to the growth of new
absorption bands, mostly occurring very near those of the parent
azide, which is expected since the propiophenone moiety is not
significantly affected by irradiation and the majority of the
observed vibrational bands belong to it. Theoretical calculations
suggest that nitrene 2a is a likely candidate for the new
absorptions. The calculated IR spectra of nitrene conformers
2a-A and B have a weak C-N stretching mode at 1071 and
1068 cm-1, respectively, with calculated intensities of less than
10 km/mol (Table 2). One notes that, in conformer 2a-B, the
C-N stretch is strongly coupled to the C-C stretching modes
calculated at 1033 and 894 cm-1. To aid in identifying these
modes in the experimental spectra, 15N and 13C labeling
experiments were done by synthesizing azide 1a with the
appropriate isotopes as shown in Scheme 5. In experiments with
the natural isotopes, weak bands were observed at 1030 and
913 cm-1; these shifted to 1017 and 912 cm-1 upon 15N labeling
while 13C labeling led to absorptions at 1017 and 906 cm-1
(see Table 3, Figure 12).
We assign the band at 1030 cm-1 to the C-N stretch of
conformers 2a-A and 2a-B, although the C-N stretch is not as
intense as was observed for the nitrene formed upon photolysis
of R-azidoacetophenone.8a Possibly, the C-N stretch in the
R-nitrene is coupled with the adjacent C-O group which gives
the C-N band a higher intensity and a higher vibrational
frequency. The bands at 913 cm-1 is assigned to the calculated
C-C stretch at 898 cm-1 that is coupled to the C-N stretch in
conformer 2a-B. While none of these bands is very intense,
their positions and isotopic shifts fit reasonably with those
other triplet nitrene intermediates.28,29,35 The signal at 8400 G
grew more intense with continued irradiation and did not decay
for hours once irradiation ceased. Cycling the temperature to
40 K and recooling to low temperature did not significantly
reduce the original signal intensity. The signal was readily
observed upon further increase in temperature, until the solvent
matrix became plastic above 77 K. Similar ESR spectra of
nitrene 2a were obtained in toluene and methanol glasses with
only small changes in line position or shape. The spectral zero-
field splitting parameters were estimated by line shape analysis
to be |D/hc| ) 1.64 cm-1 and |E/hc| ) 0.004 cm-1 which is
similar to those measured for n-propyl and n-octyl nitrenes,
reported to be 1.607 and 1.616 cm-1, respectively.35a,b
In addition to the 8400 G peak, the ESR spectra obtained
from photolysis of azide 1a showed formation of lines around
3300 G (Supporting Information), possibly due to intramolecular
H-atom abstraction in nitrene 2a. However, we cannot rule out
that they are due to direct photolytic cleavage of the solvent.
(35) (a) Wasserman, W.; Smolineksy, G.; Yager, W. A. J. Am. Chem. Soc. 1964,
86, 3166. (b) Carrick, P. G.; Brazier, C. R.; Bernath, P. F.; Engelking, P.
C. J. Am. Chem. Soc. 1987, 109, 5101. (c) Radziszewski, J. G.; Downing,
J. W.; Wentrup, C.; Kaszynski, P.; Jawdosiuk, M.; Kovacic, P.; Michl, J.
J. Am. Chem. Soc. 1984, 106, 7996. (d) Radziszewski, J. G.; Downing, J.
W.; Wentrup, C.; Kaszynski, P.; Jawdosiuk, M.; Kovacic, P.; Michl, J. J.
Am. Chem. Soc. 1985, 107, 594. (e) Radziszewski, J. G.; Downing, J. W.;
Wentrup, C.; Kaszynski, P.; Jawdosiuk, M.; Kovacic, P.; Michl, J. J. Am.
Chem. Soc. 1985, 107, 2799.
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Selective Formation of Triplet Alkyl Nitrenes
A R T I C L E S
Figure 8. Orbitals involved in electron transition in nitrene 2a in methanol.
Figure 9. Decay of nitrene 2a at 330 nm in methanol at ambient
temperature.
Figure 11. IR Spectrum of azide 1a in an argon matrix.
Scheme 5. Isotope Substitution of Nitrene 2a
Table 3. Observed IR Bands (cm-1) in 2a That Are Affected by
Isotope Substitution
Figure 10. ESR spectrum of nitrene 2a in ethanol at 10.4 K.
natural
assignment
intensity
isotopes
15N
13C
× 3
calculated for nitrene 2a. In contrast, the remaining product
bands did not shift upon 15N isotopic substitution, confirming
that they are associated with vibrations of the propiophenone
moiety. These experiments further support the formation of
triplet alkyl nitrenes in cryogenic matrices.
C-N
C-N coupled with C-C
weak
weak
1030
913
1017
912
1017
906
due to an O-H stretching mode. Our conclusion is supported
by deuteration experiments, namely that when the R position
was deuterated (R-d2 see Scheme 5), the band at 3526 and 3508
cm-1 did not shift. However, when both the R- and â-positions
were deuterated (R,â-d4), these bands shifted to 2631 and 2594
cm-1, respectively. Additionally, the band at 3302 cm-1 was
not affected by 15N substitution, whereas 13C substitution shifted
it to 3285 cm-1. The 3302 cm-1 band was also not affected
Secondary Photolysis in Matrices: Prolonged irradiation of
the above matrices led to the growth of several new bands. The
most significant of these were at 3526, 3508, 3302, 1721, 1713,
and 725 cm-1 (see Figures 13 and 14, and Table 4). Integration
of these IR bands shows that, within experimental errors, they
are all formed with the same rate. The bands at 3526 and 3508
cm-1 are not shifted by 15N or 13C substitution. This isotopic
behavior, coupled with the location, requires that the bands are
9
J. AM. CHEM. SOC. VOL. 129, NO. 51, 2007 16269

A R T I C L E S
Singh et al.
Figure 12. Argon matrix spectra of 1a (a) before and (b) after irradiation. (c) IR bands that were affected by isotope labeling. Blue shows the photolyzed
spectrum for natural abundances 1a, and green and pink show the spectra 15N and 13C labeled 1a, respectively (see Scheme 5). Arrows indicate peaks that
are assigned to the same vibration.
Figure 14. Part of the IR spectra of azide 1a after 3.5 h of irradiation.
Blue and pink traces show spectra from natural abundance and 13C-enriched
Figure 13. Part of the IR spectrum of 1a in Ar matrix at 14 K before
1a, respectively.
irradiation (a), after 5 h of irradiation (b), and after 11 h of irradiation (c).
Table 4. IR Bands (cm-1) in 6 and HCN That Are Affected upon
Isotope Substitution
when the R position was deuterated, whereas deuteration of both
the R- and the â-position caused a shift to 2626 cm-1
.
natural
Consequently, we assign the secondary photoproducts to be
biradical 6 and hydrocyanic acid based on these observations
as well as comparison to calculations. Hydrocyanic acid has
previously been isolated in argon matrices and exhibits major
assignment
intensity
isotopes
15N
13C
×
3
R-d2
R-d2, â-d2
6 (OH)
6 (OH)
6 (CdN)
6 (CdN)
HCN
moderate
strong
moderate
moderate
strong
3526
3508
1721
1713
3302
725
3526
3508
n/a
n/a
3302
724
3526
3508
1680
1670
3285
719
3526
3508
1721
1713
3302
724
2605
2594
1718
1709
2626
579
bands at 3306 and 721 cm-1 36
The vibrational bands at 3302
.
and 725 cm-1 (see Table 4) are affected by isotope substitution
in exactly the same way as has been reported for hydrocyanic
acid.
HCN
moderate
Conformer 6A has a hydrogen bond between the hydroxyl
moiety and the imine radical, whereas the others, 6B and 6C,
have a structure rotated such that hydrogen bonding cannot occur
(Figure 5). The calculated deuterium shifts of an O-H stretch
in 6B and 6C are 1024 and 1002 cm-1 (Table 5), which is in
(36) Pacansky, J.; Calder, G. V. J. Mol. Struct. 1972, 14, 363.
9
16270 J. AM. CHEM. SOC. VOL. 129, NO. 51, 2007

Selective Formation of Triplet Alkyl Nitrenes
A R T I C L E S
Table 5. Calculated IR Bands (cm-1) in 6
Scheme 7. Ideal Parameters for Intramolecular γ H-Atom
Transfer
conformer
12C-14
N
15N
13C
×
3
R-d2
R-d2, â-d2
6-B
OsH
CdN
OsH
CdN
3688
1708
3744
1718
3688
1686
3744
1695
3688
1671
3744
1680
3688
1704
3744
1714
2686
1691
2724
1703
6-C
Scheme 6
the acetophenone that would result from 11 is obscured by the
carbonyl groups of azide 1a and nitrene 2a and remains
undetected in these experiments. However, isotope labeling rules
out that biradical 6 forms secondary photoproducts 1-phenyle-
thenol, 10, and hydrocyanic acid, since the calculations show
that the CdC in 10 would be expected to be shifted ∼56 cm-1
upon 13C labeling, and no bands exhibit this behavior. The large
expected shift is further supported by Hawkins and Andrews
who demonstrated that the CdC bond in ethenol shifts ∼55
cm-1 with 13C substitution.38 We therefore concluded that
conformer 6A cleaves to yield hydrocyanic acid and acetophe-
none in matrices.
As mentioned before, molecular modeling shows that the
transition state for nitrene 2a to form biradical 6 is located 32
kcal/mol above its ground state. Thus, the ketone chromophore
in nitrene 2a could reasonably absorb another photon and
undergo intramolecular H-atom abstraction to form biradical
6. The calculated transition state for the cleavage of biradical
6A is 28 kcal/mol above the ground state of 6A. The calculated
transition state for 10 to form acetophenone is ∼4 kcal/mol
below the energy of triplet biradical 11. Thus, in matrix a
vibrationally hot biradical 6A cleaves to give hydrocyanic acid
and triplet ketyl radical 11, which intersystem crosses to the
singlet surface and forms acetophenone rather than the less stable
10. In solution the vibrationally hot imine biradical 6 would
give up its excess energy to the surrounding solvent, so
formation of acetophenone is not observed under these condi-
tions.
reasonable agreement with the experimental shifts of 921 and
914 cm-1, respectively, once anharmonicity is taken into
consideration.37 We therefore assign the 3526 and 3508 cm-1
band to the O-H stretch of biradical 6 in the conformers lacking
hydrogen bonding.
The bands at 1721 and 1713 cm-1, having a shift of 41 cm-1
to lower energy upon 13C substitution, are assigned to the Cd
N stretching modes of biradical 6. The observed shifts are in
very good agreement with calculated shifts of approximately
37 cm-1. The 15N shift was calculated as 22 cm-1, placing them
directly under the broad, intense CdO stretching band at 1700
cm-1 where they are obscured. Deuteration of the R-position
does not affect these bands significantly. On the other hand,
deuteration of both the R and â position causes a shift in the
bands to 1718 and 1709 cm-1, respectively. The calculated shifts
of these bands are 19 and 16 cm-1, considerably larger than
observed experimentally. However, the coupling of the CsH
band with the CdN band will be strongly affected by the angle
between the two bonds, and thus small geometric changes will
have large effects on the actual CdN vibrational band.
The matrix spectra support that nitrene 2a undergoes second-
ary photolysis forming biradical 6. It is reasonable to assume
that conformers of biradical 6 that do not have intramolecular
H-atom bonding are not the precursors to hydrocyanic acid in
matrices since the vibrational bands due to biradical 6 and
hydrocyanic acid grow in simultaneously and that hydrocyanic
acid and biradical 6 come from the same precursor. We do not
observe conformer 6A in matrices, since no OH bands were
detected at low frequency as is typical for OH bonds that are
involved in hydrogen bonding. Conformer 6A is best aligned
to cleave, because the R-C-â-C bond-breaking will result in
formation of biradical 11, in which the p orbital on the R-carbon
atom is orthogonal to the ketyl radical, thus allowing for the
best separation of the triplet radical centers (Scheme 6).
Unfortunately, the vibrational band for the carbonyl group of
There are very few examples of intramolecular â-H-atom
abstraction11,39-41 for which the geometric requirement must be
similar to that for intramolecular γ-hydrogen abstraction, since
it relies on a favorable orbital alignment between the carbonyl
oxygen and the C-H bond that is being cleaved.11 In Scheme
7 are shown the ideal ground state geometric parameters for
intramolecular γ-H-atom abstraction.11a,40 For example, Scheffer
et al. have reported intramolecular singlet state â-H-atom
abstraction of enediones in crystals and that the â-H-atom
abstraction occurs from a nearly perfect geometry.40a Also, when
(38) Hawkins, M.; Andrews, L. J. Am. Chem. Soc. 1983, 105, 2523.
(39) Wagner, P. J. Yang photocyclization: coupling of biradicals formed by
intramolecular hydrogen abstraction of ketones. In CRC Handbook of
Organic Photochemistry and Photobiology, 2nd ed.; Horspool, W., Lenci,
F., Eds.; CRC Press: Boca Raton, FL, 2004.
(40) (a) Scheffer, J. R. Org. Photochem. 1987, 8, 247. (b) Gudmundsdo´ttir, A.
D.; Lewis, T. J.; Randall, L. H.; Scheffer, J. R.; Rettig, S. J.; Trotter, J.;
Wu, C.-H. J. Am. Chem. Soc. 1996, 118, 6167.
(41) Yoshioka, M.; Miazoe, S.; Hasegawe, T. J. Chem. Soc., Perkin Trans 1
1993, 2781.
(37) Del Bene, J. E.; Person, W. B.; Szczepaniak, K. J. Phys. Chem. 1995, 99,
10705.
9
J. AM. CHEM. SOC. VOL. 129, NO. 51, 2007 16271

A R T I C L E S
Singh et al.
Scheme 8. Photoreactivity of 12
Summary
Photolysis of azides 1 selectively yields triplet alkyl nitrene
intermediates 2, which preferentially react with another nitrene
molecule to form azo dimers 5. The azo dimers tautomerize
and cyclize to form products 3. Dimer 5a also undergoes
secondary photolysis to form propiophenone radicals, which are
intercepted by nitrene 2a to form 4a. Triplet alkyl nitrenes 2
can be detected directly by laser flash photolysis of azides 1,
which confirms that nitrenes 2 are long-lived intermediates (τ
) 27 ms), because they do not abstract a H-atom from the
solvent and selectively react with other radicals such as other
nitrene molecules or molecular oxygen. ESR spectroscopy
further supports the characterization of nitrene 2a with zero-
field splitting parameters |D/hc| ) 1.64 cm-1, |E/hc| ) 0.004
cm-1. Argon matrix isolation made it possible to further
characterize nitrene 2a and demonstrates that it undergoes a
unique intramolecular â-H-atom abstraction to form biradical
6, which was further characterized with isotope labeling.
R is a bulky substituent, ketone 12 has been shown to undergo
intramolecular â-hydrogen abstraction from the triplet excited
state (Scheme 8).41
Our molecular modeling shows that conformers A and B
of nitrene 2a are geometrically aligned for intramolecular
â-H-atom abstraction since the distances between the â-H-atom
and the oxygen molecule, d, are 2.49 and 2.67 Å in 2a-A
and 2a-B, respectively. The O-H-C angles (θ) are 82° and
92°, and the CdOsH angles (∆) are 82° and 86° for nitrene
conformers 2a-A and 2a-B, respectively. The distance for
intramolecular H-atom abstraction is ideal for the 2a-A and
2a-B conformers, whereas the angles are not ideal, so this is
reflected in a higher calculated activation energy for this â-H-
atom transfer compared to intramolecular γ-H-atom abstrac-
tion.43,44 The calculated transition state for nitrene 2a forming
6 is located ∼37 kcal/mol above the ground state of 2a, which
is, however, below the energy of the triplet ketone in nitrene
2a and thus the intramolecular H-atom abstraction should be
feasible.
Acknowledgment. We thank the National Science Founda-
tion, Petroleum Research Foundation, and the Ohio Supercom-
puter Center for supporting this work. We also thank Professor
M. S. Platz at The Ohio State University, Professor J. C. Scaiano
at the University of Ottawa, and Professor R. M. Wilson at the
University of Cincinnati for access to laser instrumentation.
Supporting Information Available: Preparation of 1, isolation
and characterization of 3 and 4a. Cartesian coordinates, no. of
imaginary frequencies, IR frequencies and total energy of azide
1a, nitrene 2a, 6, 10, 11 and the calculated transition states for
nitrene 2a, 16, and 10. The full IR matrix spectra of nitrene 2a.
Complete ref 20. This material is available free of charge via
the Internet at http://pubs.acs.org.
(42) Wagner, P. J.; Wiegel W. J. Am. Chem. Soc. 1996, 118, 12858.
(43) (a) Severance, D.; Pandey, B.; Morrison, H. J. Am. Chem. Soc. 1987, 109,
3231. (b) Dorigo, A. E.; Houk, K. N. J. Am. Chem. Soc. 1987, 109, 2195.
(c) Dorigo, A. E.; McCarrick, M. A.; Loncharich, R. J.; Houk, K. N. J.
Am. Chem. Soc. 1990, 112, 7508.
(44) Barton, D. H. R.; Beaton, J. M.; Geller, L. E.; Pechet, M. M. J. Am. Chem.
Soc. 1960, 82, 2640.
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