Photolysis of α-azidoacetophenones: Direct detection of triplet alkyl nitrenes in solution

Article abstract of DOI:10.1021/jo034674e

We report the first detection of triplet alkyl nitrenes in fluid solution by laser flash photolysis of α-azido acetophenone derivatives, 1. Azides 1 contain an intramolecular triplet sensitizer, which ensures formation of the triplet alkyl nitrene by bypassing the singlet nitrene intermediate. At room temperature, azides 1 cleave to form benzoyl and methyl azide radicals in competition with triplet energy transfer to form triplet alkyl nitrene. The major photoproduct 3 arises from interception of the triplet alkyl nitrene with benzoyl radicals. The triplet alkyl nitrene intermediates are also trapped with molecular oxygen to yield the corresponding 2-nitrophenylethanone. Laser flash photolysis of 1 reveals that the triplet alkyl nitrenes have absorption around 300 nm. The triplet alkyl nitrenes were further characterized by obtaining their UV and IR spectra in argon matrices. ¹³C and ¹⁵N isotope labeling studies allowed us to characterize the C-N stretch of the nitrene intermediate at 1201 cm^-1.

Full text of DOI:10.1021/jo034674e

P h otolysis of r-Azid oa cetop h en on es: Dir ect Detection of Tr ip let
Alk yl Nitr en es in Solu tion
Pradeep N. D. Singh, Sarah M. Mandel, Rachel M. Robinson, Zhendong Zhu, Roberto Franz,
Bruce S. Ault, and Anna D. Gudmundsd o´ ttir*
Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221-0172
anna.gudmundsdottir@uc.edu
Received May 20, 2003
We report the first detection of triplet alkyl nitrenes in fluid solution by laser flash photolysis of
R-azido acetophenone derivatives, 1. Αzides 1 contain an intramolecular triplet sensitizer, which
ensures formation of the triplet alkyl nitrene by bypassing the singlet nitrene intermediate. At
room temperature, azides 1 cleave to form benzoyl and methyl azide radicals in competition with
triplet energy transfer to form triplet alkyl nitrene. The major photoproduct 3 arises from
interception of the triplet alkyl nitrene with benzoyl radicals. The triplet alkyl nitrene intermediates
are also trapped with molecular oxygen to yield the corresponding 2-nitrophenylethanone. Laser
flash photolysis of 1 reveals that the triplet alkyl nitrenes have absorption around 300 nm. The
triplet alkyl nitrenes were further characterized by obtaining their UV and IR spectra in argon
matrices. 13C and N isotope labeling studies allowed us to characterize the C-N stretch of the
15
-
1
nitrene intermediate at 1201 cm
.
The pursuit of organic magnetic materials has sparked
renewed interest in triplet nitrene intermediates, which
are ideal candidates because of their high-spin proper-
2
allowed methyl azide to react with metastable N in a
flowing afterglow and were then able to obtain and
analyze the UV emission spectrum of triplet methyl
1
6
8
ties. It is presumed that the direct photolysis of alkyl
nitrene in the gas phase. Ferrante and Platz et al. have
obtained a UV absorption spectrum of triplet alkyl
nitrene in matrices at low temperature. 9
azides does not lead to the formation of triplet alkyl
nitrene intermediates; instead, alkyl azides are consid-
ered to undergo a rearrangement from the excited state
of the azide to form imine derivatives with loss of
2
nitrogen. This is supported by the fact that there are
only a few examples where products have been isolated
that could be attributed to the decomposition of alkyl
3
azides into alkyl nitrene intermediates. However, physi-
cal measurements have been reported that support the
existence of triplet alkyl nitrenes in matrices and in the
gas phase.4 Wasserman, for example, obtained EPR
spectra of triplet alkyl nitrenes in a matrix by sensitized
photolysis of alkyl azides.4 Engelking and co-workers
-8
(1) (a) Rajca, A. Chem. Rev. 1994, 94, 871. (b) Iwamura, H. Adv.
Phys. Org. Chem. 1990, 26, 179. (c) Magnetic Properties of Organic
Materials; Lahti, P. M., Ed.; Marcel Dekker: New York, 1999. (d)
Nicolaides, A.; Tomioka, H.; Murata, S. J . Am. Chem. Soc. 1998, 120,
1
1
1530. (e) Subhan, W.; Rempala, P.; Sheridan, R. S. J . Am. Chem. Soc.
998, 120, 11528.
By photolyzing azidoaryl ketone derivatives, which
contain an intramolecular triplet sensitizer, we have
succeeded in obtaining products that can be attributed
to the trapping of alkyl nitrenes in bimolecular reac-
tions.10 The intramolecular triplet sensitization ensures
efficient formation of the triplet alkyl nitrene by bypass-
(2) (a) Abramovitch, R.; Kyba, E. P. J . Am. Chem. Soc. 1971, 93,
1
1
537. (b) Moriarty, R. M.; Serridge, P. J . Am. Chem. Soc. 1971, 93,
534. (c) Moriarty, R. M.; Reardon, R. C. Tetrahedron 1970, 26, 1379.
(3) (a) Banks, R. E.; Berry, D.; McGlinchey, M. J .; Moore, G. J . J .
Chem. Soc. C 1970, 1017. (b) Pancrazi, A.; Khuong-Huu, Q. Tetrahe-
dron 1975, 31, 2041
(4) (a) Barash, L.; Wasserman, E.; Yager, W. A. J . Am. Chem. Soc.
1
967, 89, 3931. (b) Wasserman, E.; Smolinsky, G.; Yager, W. A. J . Am.
Chem. Soc. 1964, 86, 3166. (c) Smolinsky, G.; Wasserman, E.; Yager,
W. A. J . Am. Chem. Soc. 1962, 84, 3220.
(7) (a) Shang, H. R.; Gao, R. M.; Ying, L. M.; Zhao, X. S.; Tang, Y.
Q. Chem. Phys. Lett. 1997, 267, 345. (b) Ying, L. M.; Xia, Y.; Shang,
H. R.; Zhao, X. S.; Tang, Y. Q. J . Chem. Phys. 1996, 105, 5798.
(8) Ferrante, R. F. J . Chem. Phys. 1987, 86, 25.
(9) Gritsan, N. P.; Likhotvorik, I.; Zhu, Z.; Platz, M. S. J . Phys.
Chem. A 2001, 105, 3039.
(
5) Radziszewski, J . G.; Downing, J . W.; J awdosiuk, M.; Kovacic,
P.; Michl, J . J . Am. Chem. Soc. 1985, 107, 594.
6) (a) Chapell, E. L.; Engelking, P. C. J . Chem. Phys. 1988, 89, 6007.
b) Carrick, P. G.; Brazier, C. R.; Bernath, P. F.; Engelking, P. C. J .
(
(
Am. Chem. Soc. 1987, 109, 5100. (c) Carrick, P. G.; Engelking, P. C.
J . Chem. Phys. 1984, 81, 1661.
(10) Mandel, S. M.; Krause Bauer, J . A.; Gudmundsdottir, A. D. Org.
Lett. 2001, 3, 523.
1
0.1021/jo034674e CCC: $25.00 © 2003 American Chemical Society
Published on Web 09/25/2003
J . Org. Chem. 2003, 68, 7951-7960
7951

Singh et al.
SCHEME 1
SCHEME 2
ing the singlet manifold of the azide precursor. Triplet
alkyl nitrenes are presumably the ground state, as triplet
methyl nitrene is 31 kcal/mol lower in energy than singlet
Resu lts a n d Discu ssion
P r od u ct Stu d ies. We photolyzed azides 1a -f in
toluene solutions at ambient temperature and kept the
conversion below 20%. In all instances amide 3 was
formed as the major product along with some acetophe-
none 4 and benzaldehyde, 5 (see Scheme 1). We have
previously reported the product studies from photolysis
of 1a and 1c but these compounds are included here for
comparison.10 Formation of amide 3 suggests that azides
1
1
methyl nitrene. Since a spin barrier prohibits the triplet
nitrenes from rearranging to singlet imine products,
triplet alkyl nitrenes can be expected to be long-lived and
thus directly detectable. In this present study we have
extended our previous work10 and set out to detect triplet
alkyl nitrenes directly in fluid solution with time-resolved
laser flash photolysis and to further characterize these
intermediates by obtaining their UV and IR spectra in
matrices. We specifically wanted to characterize the C-N
1
undergo R-cleavage to form benzoyl radicals in compe-
tition with nitrene formation (see Scheme 2). Benzoyl
radicals do not readily abstract H atoms from appropriate
1
5
stretch in the alkyl nitrene intermediates by using
and 13C labeling.
N
12
solvents but are quenched rapidly by molecular oxygen.
(12) (a) Neville, A. G.; Brown, C. E.; Rayner, D. M.; Lusztyk, J .;
Ingold, K. U. J . Am. Chem. Soc. 1991, 113, 1869. (b) Brown, C. E.;
Neville, A. G.; Rayner, D. M.; Ingold, K. U.; Lusztyk, J . Aust. J . Chem.
1995, 48, 363.
(
11) Travers, M. J .; Cowles, D. C.; Clifford, E. P.; Ellison, G. B. J .
Chem. Phys. 1999, 111, 5349.
7
952 J . Org. Chem., Vol. 68, No. 21, 2003

Photolysis of R-Azidoacetophenones
SCHEME 3
SCHEME 4
SCHEME 5
Thus 3 can be formed similarly from interception of
triplet nitrene, 2, by a benzoyl radical, whereas benzoyl
radicals slowly abstract an H atom from toluene to form
a minor amount of benzaldehyde, 5. It is also possible
that triplet alkyl nitrenes 2 fragments into benzoyl and
imine radical. Acetophenone 4 can be formed as a
secondary photoproduct from photolysis of azo dimer 6,
which is presumably formed by dimerization of two
triplet nitrene intermediates. We were, however, not
successful in isolating azo dimer 6, presumably because
the concentration of 6 is minimal since it reacts ef-
ficiently. Nevertheless, triplet alkyl nitrenes can be
expected to dimerize to form azo dimers in a manner
azide, 7. Since azide 7 does not absorb light above 350
nm, benzoyl radicals can be formed in its presence by
irradiating through the 349 nm filter benzoin methyl
ether, 8, that undergoes R-cleavage readily. We specifi-
cally selected 8 as the precursor for the benzoyl radical,
since its excited state has been estimated to decay with
a rate of more than 1010
s
-1
and thus intermolecular
triplet sensitization of azide 7 with 8 can be mini-
mized.1
4,15
Photolyzing 8 in the presence of 7 did not lead
to any azide reactivity, but only the expected photoprod-
ucts from 8 (see Scheme 4). The photoproducts from 8,
5, 9, 10, 11, and 12 were formed in the same ratio as
when azide was not present. In contrast, we have shown
that triplet-sensitized photolysis of azide 7 mainly leads
to formation of di-adamantan-1-yl-1,2-diazene, presum-
1
3
similar to triplet phenyl nitrenes. It is, however, also
possible that acetophenone is formed from â-cleavage of
azide 1a .
1
5
ably via triplet adamantyl nitrene formation.
It is also likely that 3 can be formed through a
nonnitrene mechanism if the benzoyl radical formed from
the irradiation of azide 1 reacts with a second azide
molecule (see Scheme 3). To test this hypothesis we
formed benzoyl radicals in the presence of admantanyl
Effect of Tem p er a tu r e on P r od u ct F or m a tion . We
investigated how the temperature of the photolysis
affected the product ratio (see Scheme 5). Photolysis at
100 °C yielded benzaldehyde and 3a as at room tem-
(
14) Lewis, F. D.; Lauterbach, R. T.; Heine, H. G.; Hartmann, W.;
(
13) (a) Schrock, A. K.; Schuster, G. B. J . Am. Chem. Soc. 1984, 106,
228. (b) Marcinek, A.; Levya, E.; Whitt, D.; Platz, M. S. J . Am. Chem.
Soc. 1993, 115, 8609. (c) Schuster, G. B.; Platz, M. S. Adv. Photochem.
992, 17, 70.
Rudolph, H. J . Am. Chem. Soc. 1975, 97, 1519.
5
(15) R. F. Klima and A. D. Gudmundsdottir. Unpublished results.
Triplet sensitization of 1-azidoadamantane with acetophenone yields
mainly di-adamantan-1-yl-1,2-diazene.
1
J . Org. Chem, Vol. 68, No. 21, 2003 7953

Singh et al.
SCHEME 6
pereature, but in addition products 9, 10, and 12 were
formed. These compounds must originate from benzoyl
radicals. Dibenzyl 9 is presumably formed from dimer-
ization of two benzoyl radicals, and 10 must come from
trapping of a benzoyl radical with toluene radicals.
Similarly, 13 must come from trapping of the nitrene 2a
with two benzoyl radicals. Thus at higher temperature
R-cleavage must be favored over triplet energy transfer
since more of the products can be attributed to formation
of benzoyl radicals. Photolysis at -63 °C gave 4a , 3a , and
manifolds. We estimated that the triplet energy and the
electronic configuration of the lowest excited ketone in
azides 1 are the same as for the analogous ketones
without azido substituents (see Scheme 1).2 Thus the
first excited triplet state of azides 1a -c can be expected
to have an n,π*configuration whereas in azides 1d -f the
π,π* excited state is lower in energy. We speculate that
in azides 1, the R-cleavage comes from the n,π* triplet
excited states whereas the triplet energy transfer takes
place from the π,π* excited state. Photolysis of azides 1
yields a similar amount of R-cleavage products, presum-
ably since the first and second triplet excited states are
in equilibrium with each other,17 and thus we observe
reactivity from both. Even though azides 1d -f have the
first excited triplet state with a π,π* configuration, they
do not yield less of R-cleavage products, because the
triplet energy of their π,π* state is lower than that in
azides 1a -c and thus the triplet energy transfer is more
endothermic and slower, allowing for the R-cleavage to
compete. Azides 1 can be compared to substituted vale-
rophenone derivatives, which undergo intramolecular
H-atom abstraction from their n,π* excited triplet state,
whether the electron configuration of the first excited
triplet state is n,π* or π,π*.17 However, it possible that
triplet alkyl nitrenes 2 fragment into benzoyl and imine
radicals and thus care should be taken in correlating the
product ratio from azides 1 with the electron configura-
tion of the lowest triplet excitated state of the ketone.
2
1
2 as the major products. Diketone 12 must come from
dimerization of two acetophenone radicals. Hence, pho-
tolysis at lower temperature yielded less of products
coming from R-cleavage and more of products that must
come from formation of acetophenone radicals. The
product ratio indicates that the activation energy of
R-cleavage must be higher than that for energy transfer
to form triplet alkyl nitrene and the presumable â-cleav-
age.
Oxygen Tr a p p in g of th e Rea ctive In ter m ed ia tes
F or m ed u p on P h otolysis of Azid e 1a . We studied the
photolysis of azide 1a in oxygen-saturated solutions to
trap the intermediates formed with oxygen. Irradiating
azide 1a in an oxygen-saturated toluene solution at ∼25
°
C yielded benzoic acid as the major product and lesser
amounts of acetophenone 5a and 2-nitrophenylethanone,
5 (see Scheme 6). Formation of benzoic acid is expected
1
since benzoyl radicals are efficiently trapped by molecular
oxygen.12 Formation of 15 demonstrates that we have
trapped triplet alkyl nitrene 2a with oxygen just as
triplet aryl nitrenes are trapped with molecular oxygen
to form nitroaryls.16 Photolysis at ca. -63 °C of azide 1a
in toluene saturated with oxygen yielded nitro compound
La ser F la sh P h otolysis: Tr a n sien t Sp ectr a . We
studied azide 1a by laser flash photolysis in methylene
chloride (Lambda Physik, excimer laser, 17 ns, 150 mJ ,
3
08 nm), recording the spectrum immediately after the
laser pulse over a window of 600 ns. A moderately broad
transient with a maximum around 300 nm was observed
upon irradiating azide 1a (Figure 1). We have assigned
this absorbance to the triplet alkyl nitrene 2a (λmax ∼ 300
nm) and benzoyl radical (λmax ) 370 nm) based on the
following:
The most intense absorption band near 300 nm in both
triplet phenyl nitrene and NH radicals has been at-
z y
tributed mainly to the n f n transition on the nitrogen
1
5 as the major product. Thus, the major intermediate
formed at ca. -63 °C must be nitrene 2a , which is
trapped by molecular oxygen to yield 15. Additionally,
since the combined yield of 4a and 12 decreases in the
presence of molecular oxygen we can speculate that these
acetophenone-based products are not formed directly by
â-cleavage of 1 but rather result from nitrene 2a .
Presumably, nitrene 2a undergoes dimerization to form
6
in the absence of molecular oxygen, whereas in the
presence of oxygen nitrene 2a is trapped and thus any
secondary photoproducts that might come for dimer 6 are
decreased.
Electr on Con figu r a tion of th e Excited Sta tes of
Azid e 1 a n d P r od u ct F or m a tion . We analyzed the
product formation taking into account the electron con-
figuration of the triplet excited state of the ketone in
azides 1, since R-cleavage is generally thought to arise
from the n,π* excited state, whereas triplet energy
transfer can take place from both the n,π* and the π,π*
(
17) (a) Wagner, P. J .; Park, B.-S. Photoinduced Hydrogen Atom
Abstraction by Carbonyl Compound. In Org. Photochem. 1991, 11, 227.
Ed. A. Padwa, New York, M. Dekker. (b) Wagner, P. J .; Sibert, E. J . J .
Am. Chem. Soc. 1981, 103, 7329. (c) Wagner, P. J .; Kemppainen, A.
E.; Schott, H. E. J . Am. Chem. Soc. 1973, 95, 5604.
(18) Huggenberger, C.; Lipscher, J .; Fischer, H. J . Phys. Chem. 1980,
84, 3467.
(
19) Gudmundsdottir, A. D. Unpublished results. Laser flash pho-
tolysis of â-azides propiophenone gives a transient spectrum that does
not have any benzoyl radical absorption present.
(20) Fairchild, P. W.; Smith, G. P.; Crosley, D. R.; J effries, J . B.
Chem. Phys. Lett. 1984, 107, 181.
(21) Gritsan, N. P.; Zhu, Z.; Hadad, C. M.; Platz, M. S. J . Am. Chem.
Soc. 1999, 121, 1202.
(22) Murov, S. T.; Carmichael, I.; Hug, G. L. Handbook of Photo-
chemistry, 2nd ed.;, Marcel Dekker: New York, 1993.
(
16) Abramovitch, R. A.; Challand, S. R. J . Chem. Soc., Chem.
Commun. 1972, 964.
7
954 J . Org. Chem., Vol. 68, No. 21, 2003

Photolysis of R-Azidoacetophenones
F IGURE 1. Transient spectra of azide 1a .
F IGURE 2. Transient spectrum of 1a 200 ns after the laser
pulse under O and Ar.
2
SCHEME 7
atom (see Scheme 7) and does not involve the benzylic
π-system in phenyl nitrene.20,21 Therefore, we can expect
triplet alkyl nitrene to absorb somewhat similarly as
triplet phenyl nitrenes despite the fact that alkyl nitrene
lacks a π-system. Furthermore, the absorption spectra
of triplet trifluoromethyl nitrene and triplet methyl
nitrene in matrices also have absorptions around 300
nm.8 Triplet methyl nitrene has maximum absorption
at 280 nm in matrices. The laser flash apparatus used
in our studies is limited to detection above 300 nm and
thus we cannot determine whether nitrene 2a also
absorbs strongly at 280 nm in solution. Benzoyl radicals
have a weak UV absorbance with λmax ) 370 nm that
extents to 600 nm thus giving the transient spectra of
F IGURE 3. Transient spectra of 1f: (A) immediately after
the laser pulse and (B) 6900 ns after the laser pulse.
,9
1
a a broad appearance compared to that of the triplet
1
8,19
aryl nitrenes.
On a microsecond time scale, the
transient with a maximum absorbance around 300 nm
is unaffected in solutions saturated with oxygen. It can
be concluded that the rate of interception of the triplet
4
-1
alkyl nitrene with oxygen must be slower than 10 s
.
In contrast, when we obtained transient spectra 200 ns
after the laser pulse in oxygen-saturated solutions, the
benzoyl radical absorption is depleted (see Figure 2).
The transient spectrum of azide 1a in methylene
chloride does not exhibit absorption due to the triplet
ketone, presumably because the triplet energy is ef-
ficiently transferred to the azido group at a rate faster
than the time resolution of the laser flash apparatus. In
contrast (Figure 3), the transient spectrum of azide 1f,
taken immediately after the laser pulse in a 200-ns
window, reveals transient absorption of both the triplet
ketone (λmax ) 420-450 nm) and the triplet alkyl nitrene
F IGURE 4. Kinetic traces (A) of 1a at 320 nm and (B) of 1f
at 320 and 430 nm.
(λ
max ≈ 300 nm). A transient spectrum acquired 6900 ns
exothermic and slower than that in 1a , enabling the
triplet ketone absorption and the triplet nitrene to be
detected.
Kin etics. The absorption of nitrene 2a was monitored
at 320 nm and it formed faster than the time resolution
of the laser flash photolysis apparatus (see Figure 4a).
Therefore energy transfer from the triplet ketone to the
azido group must be faster than 17 ns. Stern-Volmer
after the laser pulse, in a 600-ns window, shows only the
triplet alkyl nitrene and benzoyl radical absorption.
Assuming that the triplet energy of the ketone moieties
in azides 1 is the same as in the analogous ketones, the
triplet energy of the ketone moiety in 1a and 1f is 311
and 254 kJ /mol, respectively.22 Thus the lower triplet
ketone energy in 1f makes the triplet energy transfer less
J . Org. Chem, Vol. 68, No. 21, 2003 7955

Singh et al.
F IGURE 6. Syn and gauche conformers of 1a .
F IGURE 5. Rate of triplet energy transfer in azides 1.
treatment of the quenching of the absorbance at 320 nm
with isoprene in ethanol gave a linear plot with a slope
of 0.9. Assuming that the energy transfer is diffusion
9
-1 -1
controlled or k
q
is between 1 and 10 × 10 M
s , the
lifetime of the triplet ketone can be estimated to be
between 0.9 and 0.09 ns. For azide 1f, the triplet ketone
6
-1
absorbance at 430 nm decays with a rate of 2 × 10 s
in methylene chloride (see Figure 4b) and the nitrene
absorption at 320 nm forms at the same rate. The triplet
ketone moieties in azides 1a and 1f are substantially
shorter lived than the triplet ketone in the analogous
acetophenone and p-phenylacetophenone, which have a
lifetime of 0.3 and 5 µs, respectively, in methylene
chloride. The short triplet lifetimes of the ketone azides
are presumably due to efficient energy transfer to the
azido moiety.
The triplet energy transfer in azide 1f must be endot-
hermic, which allows the energy transfer in azide 1f to
be measured. We set out to estimate the triplet energy
of the azido group in azides 1 by measuring the rate of
triplet energy transfer in azides 1 and comparing it to
their triplet ketone energies. For azides 1b and 1c we
used Stern-Volmer quenching with isoprene to estimate
the lifetime of the triplet ketone, whereas for azides 1d
and 1e we measured directly the rate of the energy
transfer. As these compounds do not show any phospho-
rescence emission due to the efficient intramolecular
quenching of the triplet ketone, we estimate that the
triplet energies of the ketones are the same as those of
the analogous ketones without R-azido substituents.22 In
Figure 5 we have plotted the rates of triplet energy
transfer versus the triplet energy of the ketone. From
this graph we can approximate that the triplet energy of
the alkyl azide group must be slightly higher than 310
kJ /mol. This is in an agreement with Lewis et al., who
estimated the triplet energy of simple alkyl azides to be
between 314 and 335 kJ /mol.23
F IGURE 7. Syn and gauche conformers of 2a .
SCHEME 8
energy minima on the potential energy surfaces. In contrast,
the anti conformer had one negative frequency for a vibration
associated with the rotation of the -CH - group. This
2
indicates that the anti conformer corresponds to either a saddle
point or maximum on the potential energy surface. As such,
this conformer was not investigated any further. The calcu-
lated energy difference between the gauche and the syn
conformers was 1.4 kcal/mol.
Triplet nitrene 2a was also investigated at the CASSCF-
(6,6)/6-31G* level of theory. The active space consisted of six
electrons and six molecular orbitals, which were the two half-
filled p orbitals on the nitrogen, the lowest occupied π and σ
orbitals, and the corresponding antibonding orbitals. At this
level of theory, the syn and gauche conformers again were
found to be local minima on the potential energy surfaces, with
an energy difference of 0.8 kcal/mol. Infrared spectra, including
13
15
those of compounds with C (R position) and N labels (see
Scheme 8), were calculated for nitrene 2a at both the B3LYP/
6-31G* and CASSCF(6,6)/6-31G* levels of theory, and will be
discussed below.
Ma tr ix Isola tion of P h otolysis of 1a
We deposited azide 1a into argon matrices in several
different experiments, entraining the vapor pressure over
a sample of the pure azide in flowing argon and deposit-
ing the resulting sample on a 14 K cryogenic surface. The
infrared spectrum of such a sample showed a strong,
Th eor etica l Ca lcu la tion s
To identify possible intermediates in this study, we did
theoretical calculations at several different levels of theory for
azide 1a and nitrene 2a . Geometry optimization of azide 1a
at the B3LYP/6-311G level of theory resulted in two low-energy
conformations with the azide group in either the syn or the
gauche position (see Figure 6). The calculated energy difference
between these two conformers is 0.4 kcal/mol. The geometries
of the syn, gauche, and anti conformers of triplet nitrene 2a
were also optimized at the B3LYP/6-31G* level of theory (see
Figure 7). The harmonic frequency calculations indicated that
the syn and gauche conformers had all positive frequencies,
demonstrating that these conformers correspond to local
-
1
broad azide band at 2111 cm with a shoulder at 2104
-1
cm (see Figure 8). These frequencies fit reasonably well
with the calculated frequencies for the syn and gauche
conformers of 1a , respectively, and thus we assign them
accordingly. On the basis of the relative intensities of the
-1
2
104- and 2111-cm
features, we estimated the syn
conformer to be approximately three times as abundant
in the argon matrix as the gauche conformer.
Irradiation of azide 1a in argon matrices led to
complete destruction of bands due to the azide, and the
7
956 J . Org. Chem., Vol. 68, No. 21, 2003

Photolysis of R-Azidoacetophenones
F IGURE 8. IR spectra of azide 1a in argon matrices: before (red) and after irradiation (blue).
most intense and distinct. All three bands shifted to lower
energy upon 13C substitution, whereas the bands at 536
-
1
15
and 1201 cm also shifted to lower energy with
N
substitution, as shown in Table 1. In contrast, the
remainding product bands did not shift upon isotopic
substitution, confirming that they are associated with
vibrations of the acetophenone moiety. This observation
limits the number of possibilities of the absorbing species.
Candidates include the triplet nitrene, the singlet ni-
trene, imine 17, and the R-cleavage products. We can rule
out imine 17 since we do not observe any N-H stretching
-
1
mode between 3100 and 3500 cm , despite the fact that
the vibrational mode should absorb strongly in this
region. This allows us to exclude the formation of singlet
alkyl nitrene in matrices as well. Ellison et al. have
shown that there is no barrier separating the rearrange-
ment of singlet methyl nitrene into an imine and thus
we would expect singlet nitrene to undergo a 1,2-H shift
to form imine 17 or intersystem cross to the ground-state
triplet nitrene 2a .11 Furthermore, we can also rule out
R-cleavage in argon matrices since we do not observe any
benzoyl radical formation, which has a strong absorption
-
1
12
at 1828 cm in fluid solutions. We speculated, though,
that if R-cleavage took place at low temperature then the
methyl azide radical could expel a nitrogen molecule and
rearrange into an imine radical. The argon matrices
would hold the benzoyl and imine radicals in close
approximation and allow them to recombine to form
imine 16 (see Scheme 9). However, imine 16 is not formed
in argon matrices, because our calculations show that the
frequency of the carbonyl stretch would be shifted 27 and
F IGURE 9. (I) IR spectra of 2a in argon matrices. (II)
Expansion of regions A, B, and C. Blue spectra are of 2a with
-
1
13
15
1
8 cm upon C and N substitution, respectively.
1
3
a
1
C isotope in the R-position. Green spectra are of 2a with a
We calculated the infrared spectrum of the syn and
15 14
:1 mixture of N and N isotopes. Red spectra are of 2a with
the gauche conformers of triplet nitrene 2a at several
levels of theory, for comparison to these three bands. The
C-N stretch and the C-C-N bending modes are ex-
normal isotopes. (III) Simulated spectra (B3LYP, 631G*) of
syn (blue) and gauche (red) conformers of 2a .
growth of many new features attributable to photoprod-
uct(s). Many of these occurred very near bands of the
parent azide, which is expected since the acetophenone
moiety should not be significantly affected by changes
due to irradiation. This point is also validated by our
calculations (see Figure 9). We observed several distinct
pected to be very sensitive to 13C and N substitution,
15
whereas the CH
2
wag should be sensitive to 1 C but not
3
1
5
N substitution. The C-N stretches of nitrenes have
been observed between 1000 and 1075 cm , for example,
-1
the C-N stretch of triplet methyl nitrene has been
-
1
-1
new bands upon irradiation, at 536, 939, and 1201 cm
(
reported at 1040 cm
whereas the C-N stretch of
see Figure 9). Of these, the band at 1201 cm- was the
1
tetrafluoropyridine-4-nitrene has been observed at 1272
J . Org. Chem, Vol. 68, No. 21, 2003 7957

Singh et al.
TABLE 1. Com p a r ison of Exp er im en ta l a n d Th eor etica l Ba n d P osition s a n d Isotop ic Sh ift for Nitr en e 2a
band position
CASSCF
13C shift
15N shift
B3LYPa
exptl
B3LYP
CASSCF
exptl
B3LYP
CASSCF
exptl
6
9
067
63
09
360
1002
1145
539
939
1201
-5
-7
-17
-2
-8
-16
-2
-6
-9
-1
-1
-15
-5
-1
-15
-3
0
-6
1
a
The calculated IR frequencies are not scaled. In the Supporting Information we have listed the calculated IR frequencies for B3LYP/
6
311++G*, which are generally not scaled. There is not a significant shift in the IR frequencies of these three bands with use of either
one of these basis sets.
SCHEME 9
cm-
(
1 6,24
.
Our calculations place this mode between 1067
-
1
B3LYP/6-31G*) and 1145 cm (CASSCF/6-31G*). On
-
1
the basis of the location of the 1201-cm band and its
sensitivity to both 13C and 15N substitution, we assign it
to the C-N stretch of the syn conformer of nitrene 2a .
Since azide 1a is primarily trapped in the syn conforma-
tion we expect it to produce primarily the syn conformer
of nitrene 2a . We assigned the 939-cm band to the CH
wagging motion of the -CH N group; the calculated
-1
2
2
position and isotopic shifts are in good agreement with
the experimental values.
Less is known experimentally about the C-C-N
bending mode, although it should occur at substantially
lower frequency. For comparison, the C-CdO bending
-
1 25
mode of acetone occurs near 535 cm
.
The bending
-1
mode is calculated at 663 cm at the B3LYP/6-31G* level
-1
of theory. While this is higher than the 536-cm experi-
mental band, the calculated isotopic shifts are reasonable.
On the other hand, calculations at the CASSCF/6-31G*
level substantially underestimated this band position, but
again the calculated isotopic shifts are in good agreement.
Overall, while theoretical calculations have some dif-
ficulty calculating absolute band positions for the nitrene
moiety, the calculated isotope shifts for these three modes
are quite good, and confirm identification of nitrene 2a .
The UV/vis spectrum obtained after photolysis of azide
F IGURE 10. The UV absorbance spectra of azide 1a in argon
matrices: (a) before photolysis, (b) after photolysis, and (c) the
difference of spectrum b - spectrum a.
benzoyl radicals is favored whereas at lower temperature
more of the triplet alkyl nitrenes are formed. The triplet
alkyl nitrenes 2 are intercepted with both molecular
oxygen and benzoyl radicals. The triplet alkyl nitrenes
can be detected directly with laser flash photolysis and
they have a lifetime longer than microseconds in solution.
The triplet alkyl nitrene intermediates were further
characterized by obtaining their UV and IR spectrum in
argon matrices. The UV absorption spectrum of triplet
alkyl nitrene 2a in argon matrices has maximum absorp-
tion at 280 nm. The IR spectrum of nitrene 2a in argon
1
a in argon matrices shows no absorption due to benzoyl
radical formation (see Figure 10) and it does resemble
the UV absorption spectrum of triplet methyl nitrene.6
When comparing the transient spectra of nitrene 2a
obtained with the laser flash apparatus in fluid solutions
and in matrices it must be kept in mind that the laser
flash apparatus is limited to detection above 300 nm.
Thus the laser flash photolysis catches the trail of the
UV absorption of nitrene 2a , whereas the transient
spectrum of 2a in argon matrices has λmax at 280 nm.
-
1
matrices has a weak C-N stretch at 1201 cm , which
shifted on 15N and C isotope labeling.
13
Con clu sion
Exp er im en ta l Section
Ca lcu la tion s.: The geometries of azide 1a were optimized
by the B3LYP method as implemented in the Gaussian 98
programs, using the 6-311G basis set. The geometries and IR
spectra of nitrene 2a were optimized by the B3LYP and
CASSCF theory, using the 6-31G* basis set. The calculated
IR frequencies are not scaled. All the calculations were
We have shown that irradiation of azides 1 leads to
the formation of benzoyl radicals and triplet alkyl ni-
trenes 2. At higher temperatures R-cleavage to form
(
23) Lewis, F. D.; Saunders, W. H., J r. J . Am. Chem. Soc. 1968, 90,
033.
24) Chapyshev, S. V.; Kuhn, A.; Wong, M. W.; Wentrup, C. J . Am.
Chem. Soc. 2000, 122, 1572.
25) Marcou, A.; Roche, D. C. R. Acad. Sci. Ser. B 1973, 276, 599.
7
32
preformed with the Gaussian 98 software package.
(
(
(26) Ault, B. S. J . Am. Chem. Soc. 1978, 100, 2426.
7
958 J . Org. Chem., Vol. 68, No. 21, 2003

Photolysis of R-Azidoacetophenones
La ser F la sh P h otolysis. Laser flash photolysis was carried
7.99 (d, 8 Hz, 2H) ppm. 13C NMR (60 MHz, CDCl
117.5, 128.5, 132.5, 137.3, 192.2 ppm. MS (ESI) 159.1 (M
).
2-Azido-(4′-biph en yl-4-yl)eth an on e, 1f. Mp (ethyl acetate/
3
) δ 55.2,
+
out with an Excimer laser (308 nm, 17 ns). The system has
-
2
7
been described in detail elsewhere. The rate of the energy
transfer for azides 1d -f was measured directly. We did not
detect the absorption of the triplet excited state of the ketone
chromophore in azides 1a -c. However, we succefully manged
to reduce the nitrene absorption at 320 nm with isoprene,
because the precursor or the triplet excited ketone is being
quenched. Standard Stern-Volmer treatment of the quenching
of the absorbance at 320 nm with isoprene gave a linear plot.
By assuming that the energy transfer from the triplet excited
N
2
2
8
hexanes) 78-82 °C (lit. mp 88-88.5 °C). IR (KBr) 2099, 1683
-
1 1
cm . H NMR (250 MHz, CDCl ) δ 4.59 (s, 2H), 7.45 (m, 3H),
3
7.64 (d, 5 Hz, 2H), 7.71 (d, 8 Hz, 2H), 7.98 (d, 8 Hz, 2H) ppm.
C NMR (60 MHz, CDCl ) δ 54.59, 126.7, 126.9, 127.6, 128.1,
128.6, 132.6, 138.9, 146.2, 192.8 ppm.
1
3
3
P r ep a r a tive P h otolysis of 1. Formation of acetophenone
4
and benzaldehyde 5 was confirmed by authentic injection
state of the ketone to isoprene was diffusion controlled, or k
q
on GC and GC-MS and HPLC. Products 3a -f were isolated
and characterized.
P h otolysis of 1a . A solution of azide 1a (80 mg, 0.5 mmol)
in toluene (2 mL) was degassed with argon and irradiated for
9
-1 -1
is 5 × 10 M
s
, the lifetime of the triplet ketone could be
2
8
estimated from the slope of the Stern-Volmer graph.
Ma tr ix Isola tion s. Matrix isolation studies were done on
2
6
conventional equipment.
2
6 h. GC analysis of the reaction mixture showed no remaining
Gen er a l P r oced u r es. Photolysis were carried out with a
50-W mercury lamp through Pyrex filters. Toluene was dried
and distilled over sodium prior to use.
1
a . The solvent was removed under vacuum and the resulting
4
oil purified on a silica column eluted with 10% ethyl acetate
in hexane. The column yielded three fractions: benzaldehyde
P r ep a r a tion of Azid es 1a -f. Azides 1 were synthesized
(15 mg, 0.14 mmol, 28%), acetophenone (11 mg, 0.09 mmol,
2
8
as described. In a typical reaction 2-bromoacetophenone (2.0
g, 0.010 mol) was dissolved in ethanol (50 mL) and glacial
acetic acid (2 mL). To this mixture was added a solution of
sodium azide (1.30 g, 0.204 mol) in water (5 mL) and the
resulting mixture was refrigerated overnight. The reaction was
neutralized with aqueous saturated sodium bicarbonate solu-
tion and extracted with ethyl acetate. The organic layer was
1
8%), and amide 3a (26 mg, 0.11 mmol, 44%). The total
1
isolated yield of photoproducts is 90%. The H NMR, IR, and
melting point of N-(2-oxo-2-phenyl-ethyl)benzamide, 3a , were
10
identical with those in the literature.
P h otolysis of 1b. A solution of 1b (916 mg, 1.5 mmol) in
toluene (80 mL) was photolyzed overnight. GC analysis of the
reaction mixture showed no remaining starting material. The
solvent was removed under vacuum and the resulting oil was
purified on a silica column eluted with hexane in ethyl acetate.
The major product isolated was 4-chloro-N-[2-(4-chlorophenyl)-
4
washed with brine and dried over MgSO and the solvent was
removed under vacuum. The resulting solid was recrystallized
from ethanol in an ice bath (1.32 g, 8.2 mmol, 82% yield). The
1
H NMR, IR spectra and the melting point of 2-azido-1-
2
-oxoethyl]benzamide, 3b (65 mg, 0.21 mmol, 30% yield). Mp
phenylethanone, 1a , are identical with those in the litera-
(
1
7
ethyl acetate/hexane) 172-174 °C. IR (KBr) 3367, 1691, 1635,
1
0,28
ture.
-1
1
094 cm . H NMR (250 MHz, CDCl
3
) δ 4.92 (d, 6 Hz, 2H),
The labeled 1a C was synthesized in the same manner from
.23 (s, 1H), 7.47 (m, 6 Hz, 5H), 7.80 (d, 8 Hz, 2H), 8.0 (d, 8
1
3
2
-bromo-1- C-acetophenone. The 1:1 mixture of 1a N1:1a N3
13
Hz, 2H) ppm. C NMR (60 MHz, CDCl
1
1
3
) δ 47.0, 128.8, 129.1,
1
5
was prepared by the same method with N-labeled sodium
azide.
29.1, 129.2, 129.6, 129.7, 132.5, 133.0, 138.4, 141.1, 166.5,
93.3 ppm. GCMS 307/309 (M , 2%), 168/167 (5%), 139/141
+
Azides 1b-f were synthesized in approximately 80% yield
(100%).
1
by the same procedure as azide 1a . The H NMR, IR spectra,
P h otolysis of 1c. A solution of 1c (200 mg, 0.84 mmol) was
3
3
and melting points of 2-azido-(4′-bromophenyl)ethanone, 1b,
photolyzed until all the GC analyzis showed only 5% remaining
starting material. The solvent was removed under vacuum and
the resulting oil purified with silica column eluted with 5%
ethanol in hexane. Three fractions were collected: 3c (77 mg,
1
0,28
2
[
-azido-(4′-chlorophenyl)ethanone, 1c,
and 2-azido-1-benzo-
1,3]dioxol-5-ylethanone, 1e, are identical with those in the
literature.
3
4
2
-Azido-(4′-cyan oph en yl)eth an on e, 1d. Mp (ethyl acetate/
0
5
.20 mmol, 47% yield), 4c (12 mg, 0.06 mmol, 7% yields), and
c (3 mg, 0.017 mmol, 2% yields). The H NMR, C NMR, IR,
-
1
1
hexanes) 127-130 °C. IR (KBr) 2232, 2107, 1693 cm
NMR (250 MHz, CDCl ) δ 4.57 (s, 2 H), 7.82 (d, 8 Hz, 2H),
.
H
1
13
3
and melting point of 4-bromo-N-[2-(4-bromophenyl)-2-oxoethyl]-
1
0
benzamide, 3c, were identical with those in the literature.
(
27) Gritsan, N. P.; Zhai, H. B.; Yuzawa, T.; Karweik, D.; Brooke,
J .; Platz, M. S. J . Phys. Chem. A 1997, 101, 2833.
28) Turro, N. J . Modern Molecular Photochemistry; University
Science Books: Sausalito, CA, 1991; Chapter 8.
29) (a) Boyer, J . H.; Straw, D. J . Am. Chem. Soc. 1952, 74, 4506.
b) Boyer, J . H.; Straw, D. J . Am. Chem. Soc. 1953, 75, 1642.
30) (a) Hegarty, D.; Robb, M. A. Mol. Phys. 1979, 38, 1795. (b) Robb,
P h otolysis of 1d . A solution of 1d (900 mg, 4.8 mmol) in
toluene (20 mL) was bubbled with argon and irradiated
overnight. Insoluble polymers formed upon photolysis which
were filtered and discarded. HPLC showed formation of 3d
and 11d . The solvent was removed under vacuum to yield oil
that was purified on silica gel eluted with ethyl acetate in
hexane.
(
(
(
(
M. A.; Niazi, U. Rep. Mol. Theory 1990, 1, 23. (c) Anderson, K.; Roos,
B. O. In Modern Electron Structure Theory; World Scientific Publish-
ing: Singapore, 1995; Part 1, Vol. 2, p 55.
4
-Cya n o-N -[2-(4-cya n op h e n yl)-2-oxye t h yl]b e n za m -
id e,, 3d , was isolated in 6% yield (39.2 mg, 0.14 mmol). IR
(31) (a) Gross, E. K. U.; Kohn, W. Adv. Quantum Chem. 1990, 21,
-
1
1
2
55. (b) Casida, M. E. In Recent Advances in Density Functional
Methods; World Scientific Publishing: Singapore, 1995; Vol. 1, 55. (c)
Wiberg, K. B.; Stratmann, R. R.; Frisch, M. J . Chem. Phys. Lett. 1998,
(CHCl
3
) 3407, 2230, 1704, 1641, 1607, 1404 cm . H NMR
(250 MHz, CDCl
3
) δ 8.14 (d, 8 Hz, 2H), 8.05 (d, 8 Hz, 2H),
13
7
.9-7.6 (m, 4 H), 7.25 (s, 1H), 4.98 (d, 3 Hz, 2H) ppm.
C
2
97, 60.
32) Frisch, M. J .; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
NMR (60 MHz, DMSO-d
6
) δ 47.0, 118.2, 118.4, 128.2, 128.3,
(
1
32.6, 132.7, 138.2, 139.7, 165.5, 195.0 ppm. HRMS (ESI) calcd
Robb, M. A.; Cheeseman, J . R.; Zakrzewski, V. G.; Montgomery, J . A.,
J r.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J .; Barone, V.;
Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Cliford,
S.; Ochterski, J .; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma,
K.; Malick, D. K.; Rbuck, A. D.; Raghavachari, K.; Foresman, J . B.;
Cioslowski, J .; Ortiz, J . V.; Stafanov, B. B.; Liu, G.; Liashenko, A.;
Piskorz, P.; Komaromi, I.; Goperts, R.; Martin, R. L.; Fox, D. J .; Keith,
T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.;
Challacombe, M.; Gill, P. M. W.; J ohnson, B.; Chen, W.; Wong, M. W.;
Andres, J . L.; Head-Gordon, M.; Replogle, E. S.; Pople, J . A. Gaussian
+
for C17
-(2-Ben zyla m in o-a cetyl)ben zon itr ile, 11d , was isolated
in 1% yield (13.5 mg, 0.054 mmol). IR (CHCl ) 3406, 2233,
10 3 2
H N O (M - 1) 288.0773, found 288.0773.
4
3
-
1
1
1
4
667, 1607, 1521, 1422 cm
.65 (s, 2H), 4.84 (s, 2H) ppm. C NMR (60 MHz, CDCl
.
H NMR (250 MHz, CDCl
3
) δ
) δ
1
3
3
38.7, 56.8, 115.6, 117.4, 127.7, 127.8, 128.7, 132.7, 132.8, 134.5,
137.7, 197.1 ppm.
P h otolysis of 1e. A solution of 3e (250 mg, 1.22 mmol) in
toluene (50 mL) was photolyzed overnight. The solvent was
removed under vacuum and the resulting oil purified on a
silica column to yield four fractions: 1e (40 mg, 0.20 mmol,
9
4, Revision D.2; Gaussian, Inc.: Pittsburgh, PA, 1998.
33) Nair, V.; Nair, L. G.; George, T. G.; Augustine, A. Tetrahedron
000, 56, 7607.
34) Ackrell, J .; Muchowski, J . M. J . Org. Chem. 1986, 51, 3374.
(
2
(
J . Org. Chem, Vol. 68, No. 21, 2003 7959

Singh et al.
1
1
6%), 4e (15 mg, 0.091 mmol, 7% yield), 5e (35 mg, 0.23 mmol,
9% yield), and 3e (60 mg, 0.18 mmol, 30% yield). The physical
was photolyzed at -63 and 100 °C. The reaction mixture was
analyzed by GC and GC/MS. Formation of 1,2-diphenyletha-
none (9) and 1,4-diphenylbutane-1,4-dione (12) was verified
with commercially available authentic samples. Formation of
N-benzyl-N-(2-oxo-2-phenylethyl)benzamide (13) and 2-ben-
zylamino-1-phenylethanone (11) was established by synthesiz-
properties of (2-benzo[1,3]dioxol-5-yl-2-oxo-ethyl)amide benzo-
1,3]dioxole-5-carboxylic acid, 3e, are as follows. IR (CHCl
[
1
4
3
)
677, 1657, 1604, 1254 cm-1
1
3
. H NMR (250 MHz, CDCl ) δ
.84 (d, 4 Hz, 2H), 6.04 (s, 2H), 6.08 (s, 2H), 6.88 (m, 2H), 7.15
1
3
36
(s, 1H), 7.40 (m, 4H), 7.63 (d, 8 Hz, 2H) ppm. C NMR (60
ing these compounds following literature procedures.
MHz, CDCl
3
) δ 192.3, 166.5, 152.7, 150.5, 148.5, 148.0, 129.2,
P h otolysis of 1a in Tolu en e Sa tu r a ted w ith Oxygen .
An oxygen-saturated solution of 1a (0.01 M) in toluene was
photolyzed at 30 and -63 °C. GC analysis of the reaction
mixture showed formation of benzoic acid, acetophenone,
2-nitrophenylethanone, and 3a along with unreacted starting
material. The formation of benzoic acid, acetophenone (4a ),
2-nitrophenylethanone (15), and 3a was verified by GC/MS
and authentic samples.36
1
4
28.2, 124.4, 121.8, 108.3, 108.0, 107.9, 107.6, 102.1, 101.7,
6.6 ppm. HRMS calcd for C17H13NO Na, [M + 23] , 350.0641,
6
+
found 350.0665.
P h otolysis of 1f. A solution of 1f (298 mg, 1.25 mmol) in
toluene (60 mL) was photolyzed overnight. A TLC showed no
remaining starting material. The solvent was removed under
vacuum and the resulting oil purified on a silica column eluted
with ethyl acetate in hexane. Three fractions were collected:
4
f (2.4 mg, 0.013 mmol, 1%), p-phenyl acetophenone 5f (35
Ack n ow led gm en t. A.D.G. is grateful to the Uni-
versity of Cincinnati Research Council Faculty Research
Support. We also acknowledge the finical support from
the National Science Foundation (CHE No. 0093622)
and the Petroleum Research Foundation (ACS-PRF No.
#35809). S.M.M. thanks the University of Cincinnati
Research Council for financial support. We thank
Professor M. S. Platz at The Ohio State University for
the use of his laser flash apparatus.
mg, 0.18 mmol, 14% yields), and 3f (45 mg, 0.12 mmol, 20%
yields). The spectroscopic characterization of biphenyl-4-
carboxylic acid (2-biphenyl-4-yl-2-oxoethyl)amide, 3f, is listed
below. Mp 257-8 °C dec (lit. mp 259 °C). IR (KBr) 3388, 1646,
1
7
3
4
-
1
1
3
603 cm . H NMR (250 MHz, CDCl ) δ 4.81 (d, 4 Hz, 2H),
.39 (m, 10H), 7.72 (d, 8 Hz, 3H), 7.87 (d, 8 Hz, 3H) ppm.
An a lytica l P h otolysis of Azid es 1. Azides 1 were dis-
solved in toluene, degassed by bubbling argon through the
solution, and photolyzed through Pyrex filter. The conversion
of the starting material was kept below 20%. HPLC, GC
analysis, and H NMR of the reaction mixtures were used to
estimate the product ratio for 3, 4, and 5.
1
Su p p or tin g In for m a tion Ava ila ble: Cartesian coordi-
nates, number of imaginary frequencies, IR frequencies, and
P h otolysis of 1-Azid oa d m a n ta n e a n d Meth yl Ben zoin ,
. A solution of 8 (17 mg, 0.1 mmol) and 1-azidoadmantane
total energy of 1a , 2a , and 16; the Stern Volmer graph for
8
isoprene quenching of azides 1a -c and 1H NMR spectra of
(22.6 mg, 0.1 mmol) in 2 mL of toluene was degassed with
products 3; the rate of intramolecular energy transfer in 1.
This material is available free of charge via the Internet at
http://pubs.acs.org.
argon and photolyzed with a 450-W Mercury arc lamp through
a 349-nm filter. A solution of 8 (0.1 M) in toluene was also
prepared and photolyzed simultaneously. The major products
coming from photolysis of 8 were characterized by GC/MS as
J O034674E
5
a , benzil (9), 1,2-diphenylethanone (10), 1-methoxy-1,2-
diphenylethane (12), and 1,2-dimethoxy-1,2-diphenylethane
11).
P h otolysis of 1a in Tolu en e a t Low Tem p er a tu r e. A
solution of 1a (0.02 M) and hexadecane (0.005 M) in toluene
(
35) Balaban, A. T.; Bally, I.; Frangopol, P. T.; Bacescu, M.;
Cioranescu, E.; Birladeanu, L. Tetrahedron 1963, 19, 169.
36) Fuhrman, H.; Haber, H.; Haupt, M.; Henning, H.-G. J . Prakt.
Chem. 1982, 6, 1055.
(
(
7
960 J . Org. Chem., Vol. 68, No. 21, 2003