Dehydrophenylnitrenes: Matrix isolation and photochemical rearrangements

Article abstract of DOI:10.1021/jo061624b

(Chemical Equation Presented) The photochemistry of 3-iodo-2,4,5,6- tetrafluorophenyl azide 8 and 3,5-diiodo-2,4,6-trifluorophenyl azide 9 was studied by IR and EPR spectroscopy in cryogenic argon and neon matrices. Both compounds form the corresponding n

Full text of DOI:10.1021/jo061624b

Dehydrophenylnitrenes: Matrix Isolation and Photochemical
Rearrangements
Wolfram Sander,* Michael Winkler, Bayram Cakir, Dirk Grote, and Holger F. Bettinger
Lehrstuhl fu¨r Organische Chemie II, Ruhr-UniVersita¨t Bochum, UniVersita¨tsstrasse 150,
44780 Bochum, Germany
wolfram.sander@rub.de
ReceiVed August 4, 2006
The photochemistry of 3-iodo-2,4,5,6-tetrafluorophenyl azide 8 and 3,5-diiodo-2,4,6-trifluorophenyl azide
9 was studied by IR and EPR spectroscopy in cryogenic argon and neon matrices. Both compounds form
the corresponding nitrenes as primary photoproducts in photostationary equilibria with their azirine and
ketenimine isomers. In contrast to fluorinated phenylnitrenes, ring-opened products are obtained upon
short-wavelength irradiation of the iodine-containing systems, indicative of C-I bond cleavage in the
nitrenes or didehydroazepines under these conditions. Neither 3-dehydrophenylnitrene 6 nor 3,5-
didehydrophenylnitrene 7 could be detected directly. The structures of the acyclic photoproducts were
identified by extensive comparison with DFT calculated spectra. Mechanistic aspects of the rearrangements
leading to the observed products and the electronic properties of the title intermediates are discussed on
the basis of DFT as well as high-level ab initio calculations. The computations indicate strong through-
bond coupling of the exocyclic orbital in the meta position with the singly occupied in-plane nitrene
orbital in the monoradical nitrenes. In contrast to the ortho or para isomers, this interaction results in
low-spin ground states for meta nitrene radicals and a weakening of the C1-C2 bond causing the kinetic
instability of these species even under low-temperature conditions. 3,5-Didehydrophenylnitrenes, on the
other hand, in which a strong C3-C5 interaction reduces coupling of the radical sites with the nitrene
unit, might be accessible synthetic targets if the intermediate formation of labile monoradicals could be
circumvented.
Introduction
nitrenes results in low-spin, quinoid arrangements. Lahti et al.
described the formation of the quartet ground state molecules
1 and 2 via coupling of a phenylnitrene unit with a stable
π-radical attached to the para position of the phenyl ring.8,9
The three unpaired electrons of these spin systems consist of
one σ-electron localized at the nitrene center, one delocalized
nitrene π-electron, and one delocalized radical π-electron (σππ
triradical). Spin fragment analysis reveals the non-disjoint nature
of these spin systems and consequently the high spin (quartet)
High-spin organic molecules are for both the experimentalist
and theoretician challenging species with potential applications
for the development of new magnetic materials. An important
and successful approach for achieving high-spin states is via
π-coupling in non-Kekule´ molecules.1 In particular m-phenylene
units have proved to be reliable linkers for generating high-
spin states of polyradicals,2 -carbenes,3-5 and -nitrenes.6,7 In
contrast, para or ortho coupling of π-radicals, carbenes, or
(6) Chapyshev, S. V.; Walton, R.; Sanborn, J. A.; Lahti, P. M. J. Am.
Chem. Soc. 2000, 122, 1580.
(7) Sato, T.; Narazaki, A.; Kawaguchi, Y.; Niino, H.; Bucher, G.; Grote,
D.; Wolff, J. J.; Wenk, H. H.; Sander, W. J. Am. Chem. Soc. 2004, 126,
7846.
(1) Berson, J. A. In ReactiVe Intermediate Chemistry; Moss, R. A., Platz,
M. S., Jones, M., Jr., Eds.; Wiley: New York, 2004.
(2) Lahti, P. M.; Liao, Y.; Julier, M.; Palacio, F. Synth. Met. 2001, 122
(3), 485.
(3) Sato, K.; Shiomi, D.; Takui, T.; Hattori, M.; Hirai, K.; Tomioka, H.
Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A 2002, 376, 549.
(4) Teki, Y.; Itoh, K. Magn. Prop. Org. Mater. 1999, 237.
(5) Itoh, K. In NATO ASI Ser., Ser. E 1991, 198, 67.
(8) Lahti, P. M.; Esat, B.; Liao, Y.; Serwinski, P.; Lan, J.; Walton, R.
Polyhedron 2001, 20, 1647.
(9) Serwinski, P. R.; Esat, B.; Lahti, P. M.; Liao, Y.; Walton, R.; Lan,
J. J. Org. Chem. 2004, 69, 5247.
10.1021/jo061624b CCC: $37.00 © 2007 American Chemical Society
Published on Web 01/05/2007
J. Org. Chem. 2007, 72, 715-724
715

Sander et al.
CHART 1
CHART 3
SCHEME 1. Photochemistry of Pentafluorophenyl Azide
10, Matrix Isolated in Argon at 10 K
CHART 2
ground states that have been observed via EPR spectroscopy.9
Recently, we described a novel high-spin molecule 3 with a
quartet ground state formed by para coupling of a phenylnitrene
unit and a phenyl radical center.10 In this system, one π-electron
is coupled to two σ-electrons located formally at the nitrogen
atom and the p-phenyl carbon atom, respectively (σπσ tri-
radical).
the 2,4,6-trifluorophenylnitrene-3,5-diyl 7 with the two radical
and the nitrene centers in the meta position, which combines a
phenylnitrene with a m-benzyne unit. Here we describe the
photochemistry of the phenyl azides 8 and 9 as potential
precursors of the phenylnitrene radicals 6 and 7, respectively,
under the conditions of matrix isolation.
4-Iodo-2,3,5,6-tetrafluorophenyl azide 4 was used as photo-
chemical precursor of 3. Azides are versatile nitrene precursors,
and aryl iodides with a rather labile C-I bond can be used to
generate phenyl radicals. A major problem of aryl iodides as
precursors of radicals in matrices is the rapid thermal recom-
bination of the photochemically generated phenyl radicals and
the iodine atoms residing in the same matrix cages. This in-
cage recombination is suppressed if the radical pairs are
separated by inert gas atoms in the rigid matrix. The yield of
matrix-isolated radicals depends on the constitution of the radical
precursor, on the matrix material, and on the matrix temperature.
Fluorine substitution at the phenyl rings has been shown to
increase the yield of radicals after irradiation in matrices.11
As expected, 320 nm photolysis of 4, matrix isolated in argon
at 3 K, produced triplet ground state nitrene 5 as the primary
photoproduct.10 Subsequent 254 nm photolysis resulted in the
formation of 2,3,5,6-tetrafluorophenylnitrene-4-yl 3 with a
quartet ground state.10,12 The nitrene radical was identified by
comparison of the matrix IR spectrum with spectra calculated
for the lowest lying doublet and quartet states, where the latter
gives a better agreement with the experimental data than the
doublet spectrum.10 The assignment of a quartet ground state
to phenylnitrene radical 3 was later confirmed by EPR measure-
ments.13 The nitrene radical 3 represents the first example of a
novel coupling between a σ-radical and a nitrene unit resulting
in a high-spin state.12
Results and Discussion
Photochemistry of Pentafluorophenyl Azide 10. Phenylni-
trenes are photolabile and on UV irradiation rearrange to azirines
which on subsequent irradiation produce cyclic ketenimines
(didehydroazepines).14 These rearrangements are reversible, and
thus under the conditions of matrix isolation photostationary
equilibria between these three reactive intermediates are ob-
served. Pentafluorophenylnitrene 11 was first matrix isolated
and spectroscopically characterized by Dunkin et al. describing
it as “an arylnitrene that does not undergo ring expansion”.14c
Later, we found azirine 13 as a reversibly formed photoproduct
of nitrene 11.15 Again, ring expansion of nitrene 11 was not
observed. Only recently, Bally et al.16 reported for the related
2,6-difluorophenylnitrene that the missing didehydroazepine is
indeed formed if monochromatic UV light is used in the
irradiation of 2,6-difluorophenylnitrene. Since fluorinated di-
dehydroazepines, such as 12, are labile toward visible light
irradiation (λ > 420 nm) they are not observed if broadband
UV-vis irradiation is used to irradiate the nitrene. The relative
yields of the three reactive intermediates 11-13 strongly depend
on the irradiation conditions. However, even prolonged short-
According to our calculations, a meta coupling between the
phenyl radical and the nitrogen atom should lead to a low-spin
doublet configuration of the resulting phenylnitrene radical.12
It is thus tempting to investigate the corresponding 2,4,5,6-
tetrafluorophenylnitrene-3-yl 6. Another interesting system is
(14) (a) Dunkin, I. R.; Thomson, P. C. P. J. Chem. Soc., Chem. Commun.
1982, 1192. (b) Hayes, J. C.; Sheridan, R. S. J. Am. Chem. Soc. 1990, 112,
5879. (c) Borden, W. T.; Gritsan, N. P.; Hadad, C. M.; Karney, W. L.;
Kemnitz, C. R.; Platz, M. S. Acc. Chem. Res. 2000, 33, 765. (d) Platz,
M. S. In ReactiVe Intermediate Chemistry; Moss, R. A., Platz, M. S., Jones,
M., Jr., Eds.; Wiley: New York, 2004.
(15) Morawietz, J.; Sander, W. J. Org. Chem. 1996, 61, 4351.
(16) Carra, C.; Nussbaum, R.; Bally, T. Chem. Phys. Chem. 2006, 7,
1268.
(10) Wenk, H. H.; Sander, W. Angew. Chem., Int. Ed. 2002, 41, 2742.
(11) (a) Wenk, H. H.; Sander, W. Eur. J. Org. Chem. 1999, 57. (b) Wenk,
H. H.; Sander, W. Chem. Eur. J. 2001, 7, 1837.
(12) Bettinger, H. F.; Sander, W. J. Am. Chem. Soc. 2003, 125, 9726.
(13) Grote, D.; Wenk, H. H.; Sander, W. Unpublished results.
716 J. Org. Chem., Vol. 72, No. 3, 2007

Dehydrophenylnitrenes
increases the concentration of azirines 16 at the expense of
ketenimines 15 (Figure 2). To this point the photochemistry of
14 is in accordance with the photochemical isomerization of
pentafluorophenylnitrene 11.
Short-wavelength UV irradiation (248 nm KrF Excimer Laser
or 254 nm low-pressure mercury arc lamp) results in the
formation of two new products (Figure 3). The first product
exhibits a weak absorption at 2286 cm-1, which is characteristic
of alkynes or nitriles. A medium intensity absorption at 1982
cm-1 is found in the region typical of allenes. The second
product, formed upon prolonged irradiation, exhibits a very
strong absorption at 2352 cm-1. A comparison with calculated
IR spectra of a large number of C6F4N and C6F4NI isomers
allows us to tentatively assign the two compounds to allene 19
and its acetylene isomer 20 (Table 2). Calculated IR spectra of
various other potential side products are given in the Supporting
Information section for comparison. The nitrile absorption in
20 is predicted to be of very low intensity around 2250 cm-1
and is thus not observed experimentaly. Obviously, short-
wavelength irradiation results in the rupture of either the six-
or seven-membered ring and formation of acyclic products.
From the experimental data it is not possible to decide whether
the ring opening involves 6 or 17, or whether both pathways
contribute to formation of acyclic products. Since the electronic
properties of the fluorine and iodine substituents are roughly
comparable, however, the different behavior of 11 and 14 upon
short-wavelength UV irradiation suggests that cleavage of the
C-I bond takes place at the stage of either 14 or 15, and that
recombination with iodine occurs after ring opening
(Scheme 2).
To gain additional insight into the formation of acyclic
photoproducts a number of isomers and reaction paths of nitrene
radical 6 were calculated (Scheme 3). Although the vibrational
spectra of these compounds are usually nicely reproduced at
the DFT level, the doublet-quartet splittings and reaction
barriers should be taken with some caution (vide infra).
According to these calculations, 6 has a doublet ground state
with a 4A′′ state lying 10 kcal mol-1 above. This is in qualitative
agreement with our previous calculations on the parent system.12
The calculated barriers for rearrangement to azirines 21a and
21b are significantly higher, and the latter are less stable than
expected by comparison with fluorinated phenylnitrenes that
have been the subject of high-level experimental and compu-
tational studies.14 In further contrast to the latter, 21a and 21b
occupy very shallow minima on the C6F4N potential energy
surface that disappear as stationary points when zero point
vibrational energy (ZPVE) corrections are taken into account.
The ketenimines 17a and 17b are more stable than 6 by 11 and
9 kcal mol-1, respectively. As discussed in detail for the parent
3-dehydrophenylnitrene prviously,12 the C1-C2 bond of 6 is
weakened by through-bond interaction18 of the exocyclic σ
orbital in the meta position with the singly occupied in-plane
orbital at nitrogen. Accordingly, rupture of this bond requires
only a small barrier of 6 kcal mol-1 leading to the delocalized
π-radical 18 that is 18 kcal mol-1 more stable than 6.
Alternatively, 17b can undergo a ring-opening reaction to the
same intermediate with a barrier of 10 kcal mol-1. Whether
this reaction is a real photochemical process, taking place on
FIGURE 1. Photochemistry of 3-iodo-2,4,5,6-tetrafluorophenyl azide
8, matrix-isolated in neon at 3 K. (a) Difference IR spectrum of a matrix
obtained after short-time photolysis of 8 at λ ) 305-320 nm. (b)
Calculated IR spectrum of triplet nitrene 14. (c) IR spectrum of the
same matrix as in part a after prolonged irradiation. A complex mixture
of 8, 14, and 15 is obtained. Note that no 16 is formed under these
conditions.
wavelength UV irradiation (248 nm Excimer Laser) does not
lead to ring-opened acetylenic compounds.17 This is in contrast
to the chemistry of the phenylnitrenes containing one or two
iodine atoms described in the next section.
Photochemistry of 3-Iodo-2,4,5,6-tetrafluorophenyl Azide
8. UV photolysis of azide 8, matrix isolated in argon at 10 K,
with λ ) 305-320 (high-pressure mercury arc lamp and cutoff
filters) or 308 nm (XeCl Excimer laser) produced 2,4,5,6-
tetrafluoro-3-iodophenylnitrene 14 with an EPR spectrum
characteristic of triplet nitrenes. By simulation of the EPR
spectra the zero field splitting parameters (ZFS) D/hν and E/hν
were determined to be 1.033 and 0.039 cm-1, respectively.
Using matrix IR spectroscopy to analyze the photolysis of 8
reveals a more complex reaction pattern. The experiments were
performed both in argon at 10 K and in neon at 3 K with the
latter showing a higher resolution of the IR spectra. Since
otherwise the results from both matrices are identical, only the
spectra in solid neon are discussed here. UV photolysis (λ )
305-320 nm) of 8 produced nitrene 14 with the most intense
vibration at 1446 cm-1. The measured IR spectrum of 14 agrees
nicely with calculations at the BLYP/6-311G(d,p) level of theory
(Figure 1, Table 1).
Prolonged irradiation at λ > 305 nm leads to the formation
of several additional products that are identified as the dehy-
droazepines 15a and 15b and the azirines 16a and 16b (Scheme
2) by comparison with DFT calculated spectra (Table 1). Nitrene
14, dehydroazepines 15, and azirines 16 are formed in photo-
stationary equilibria and the relative yields of these compounds
strongly depend on the irradiation conditions. Similar to the
pentafluoro derivative (vide supra), irradiation at λ > 420 nm
(17) In contrast to 11, ring-opening reactions of annellated phenylnitrenes
have been observed previously: (a) Maltsev, A.; Bally, T.; Tsao, M. L.;
Platz, M. S.; Kuhn, A.; Vosswinkel, M.; Wentrup, C. J. Am. Chem. Soc.
2004, 126, 237. (b) Kvaskoff, D.; Bednarek, P.; George, L.; Waich, K.;
Wentrup, C. J. Org. Chem. 2006, 71, 4049.
(18) For example: (a) Hoffmann, R.; Imamura, A.; Hehre, W. J. J. Am.
Chem. Soc. 1968, 90, 1499. (b) Hoffmann, R. Acc. Chem. Res. 1971, 4, 1.
(c) Gleiter, R. Angew. Chem., Int. Ed. 1974, 13, 696. (d) Paddon-Row,
M. N. Acc. Chem. Res. 1982, 15, 245. (e) Jordan, K. Theor. Chem. Acc.
2000, 103, 286.
J. Org. Chem, Vol. 72, No. 3, 2007 717

Sander et al.
TABLE 1. IR Spectroscopic Data of 14, 15a, 15b, 16a, and 16be
14 15a
15b
16a
16b
a
a,b
c
c
a
a,b
c
c
a
a,b
c
c
a
a,b
c
c
a
a,b
c
c
V˜exp
Iexp
V˜calc Icalc
V˜exp
Iexp
V˜calc Icalc
V˜exp
Iexp
V˜calc Icalc
V˜exp
Iexp
V˜calc Icalc
V˜exp
Iexp
V˜calc Icalc
402
424
480
518
566
599
627
662
749
798
883
953
3
2
5
6
7
9
1
0
414
455
491
539
569
581
623
664
739
779
909
971
3
5
4
13
2
3
4
1
52
44
69
99
V˜exp
406
463
529
562
597
629
648
668
1
1
8
4
3
2
69
12
447
481
525
559
576
635
650
677
775
935
960
5
0
11
10
1
4
5
25
420
481
550
573
604
634
705
767
0
7
0
3
0
0
2
86
672
693
781
50
5
20
60
697
5
50
10
792
20
40
20
60
d
d
d
d
d
40
83
36
d
832
973
d
5
20
d
d
d
d
d
812
994
50
50
736
889
120 817
124 966
127 843
43 935
210 1005
953
107 926
1071 40
1179 10
1024 54
1130 10
1227 14
1257 20
1280 104
1367 24
1388 237 1531
1473 127 1586
1540 78
1046 60
1181 20
d
1304 100
d
1003 73
1126 42
1224 26
1246 406 1282
1259 70 1324
1312 241 1393 100
1453 203 1524
1510 76 1586
1007 80
1140 10
1255
140 1019d
1076 17
1209 99
1147d
1210d
1084 107 1147d
1130 46
1210d
1223 101 1278
1258 46 1313
1314 261 1386
1454 1547
1089 126
1130 35
1206 32
1246 247
1279d
d
d
d
d
d
d
d
1227 108 1287d
1269 124 1320d
1324 388 1375
1332 50
1419 10
1446 100
1531 50
1587 40
d
1357 40
100 1316 210
d
d
d
d
d
1445 93
1512 135 1615
1830 98 1696
d
d
4
20
40
d
1470 91
1559 88
1669 87
100 1545 183 1642
d 1663 52 1696
1845
1801 207 1872 20
a Ne, 3 K. b Relative intensity based on the strongest absorption. Due to the complexity of the spectra, all experimental intensities are rough estimates.
c BLYP/6-311G(d,p). d Not assigned or tentative assignment because of overlap with other nearby bands. e Frequencies are given in cm-1, calculated Intensities
in km mol-1
.
SCHEME 2. Photochemistry of Nitrene 14, Matrix Isolated in Neon at 3 Ka
a Since 11 is completely stable toward ring-opening upon short-wavelength irradiation, cleavage of the C-I bond in 14 or 15 prior to ring rupture is
likely.
an excited-state potential energy surface, or a hot ground state
reaction cannot be decided conclusively from our experiments.
Anyway, the excess energy available from the photolytic
generation of 6 or 17 cannot be dissipated easily in noble-gas
matrices, and is more than sufficient to trigger the 6 f 18
rearrangement even in the ground state. Subsequent recombina-
tion of 18 with a nearby iodine atom accounts for the formation
of ring-opened compounds 19 and 20 as shown in Scheme 2.
Overall, the DFT calculations support the mechanistic inter-
pretation of the experimental findings and indicate that 6 is
kinetically labile toward ring-opening. In contrast to the ortho
and para isomers, these systems do not qualify as synthetic
targets, even under low-temperature conditions.
Photochemistry of 3,5-Diiodo-2,4,6-trifluorophenyl Azide
9. Matrix-isolated azide 9 was irradiated under conditions
analogous to those of the photolysis of 8 described above. The
photochemistry of 9 proved to be much cleaner than that of 8.
As with the other phenyl azides the primary photoproduct is
the corresponding nitrene 22. The nitrene was characterized by
its EPR and IR spectra. The EPR spectrum is characteristic for
a triplet nitrene with the ZFS parameters D/hν and E/hν of 1.012
and 0.094 cm-1, respectively. The IR spectrum of 22 shows
718 J. Org. Chem., Vol. 72, No. 3, 2007

Dehydrophenylnitrenes
FIGURE 2. Interconversion of didehydroazepines 15 to azirines 16
upon irradiation with visible light. (a) Calculated IR spectrum of 15a.
(b) Calculated IR spectrum of 15b. (c) Difference IR spectrum (neon,
3 K) of a matrix obtained after irradiation of 8 with λ > 305 nm
followed by irradiation at λ > 420 nm. Bands pointing upwards increase
in intensity upon long-wavelength irradiation. Absorptions of 14 are
labeled by dots. (d) Calculated IR spectrum of 16a. (e) Calculated IR
spectrum of 16b.
FIGURE 3. Formation of ring-opened compounds 19 and 20 upon
irradiation at 248 nm. (a) Calculated IR spectrum of 19. (b) IR spectrum
of a matrix obtained after irradiation of 8 with 305-320 nm, followed
by short-time irradiation at 248 nm. (c) Difference IR spectrum of the
same matrix after continued irradiation at 248 nm. Bands pointing
upwards increase in intensity upon prolonged irradiation. Absorptions
of 14 are labeled by crosses for comparison. Most of the remaining
bands belong to 15. (d) Calculated IR spectrum of 20.
strong absorptions at 1536, 1416, and 1303 cm-1 and is nicely
reproduced by DFT calculations (Table 3, Figure 4).
TABLE 2. IR Spectroscopic Data of 19 and 20e
Further irradiation of 22 with 254 or 308 nm UV light resulted
in the formation of only two major new products which were
characterized by IR spectroscopy. One of the products exhibits
a strong absorption at 1852 cm-1 characteristic of ketenimines
and was identified as 1,2-didehydro-3,5,7-trifluoro-4,6-di-
iodoazepine 23 (Table 3, Figure 4). Presumably azirine 24 is
the precursor of 23; however, it could not be directly observed
in our experiments.
19
20
a
a,b
c
c
a
a,b
c
c
v˜exp
Iexp
v˜calc
Icalc
v˜exp
Iexp
v˜calc
Icalc
455
477
519
565
589
603
662
702
916
1007
1084
1201
1245
1384
1604
1970
2225
2
3
5
10
7
491
537
547
583
621
669
868
969
1057
1189
1220
1350
1612
2229
2346
21
1
7
46
20
2
3
The second product is formed almost exclusively on pro-
longed irradiation and shows intense absorptions at 2333, 1995,
1174, 1081, and 854 cm-1. The band at 1995 cm-1 of the second
photoproduct is characteristic of an allene, and by comparison
with the results from DFT calculations this compound was
identified as 2,4,6-trifluorohexa-2,3-dien-5-yne nitrile 25 (Figure
5). Obviously, ring cleavage is a highly efficient process in this
system as well (Scheme 4), and no evidence for the formation
of 7 could be obtained by IR or EPR spectroscopy.
744
968
10
70
d
229
149
252
15
232
196
66
759
905
60
80
70
10
40
d
160
185
189
19
142
86
1075
1139
1268
1316
1022
1102
1279
d
1401
1689
70
80
10
30
13
54
8
1606
1982
2286
70
100
10
71
442
15
2352
100
423
a Ne, 3 K. b Relative intensity based on the strongest absorption. Due to
the complexity of the spectra, all experimental intensities are rough
estimates. c BLYP/6-311G(d,p). d Not assigned or tentative assignment
In analogy to the 3-iodo-tetrafluoro system, we also inves-
tigated the ring fragmentation of 7 at the DFT level. Since
density functional theory erroneously predicts a singlet ground
state for nitrene 7, the DFT calculated energetics for the ring-
opening process 7 f 27 should be considered as tentative only.
According to these calculations, the rearrangement on the triplet
because of overlap with other nearby bands. e Frequencies are given in cm-1
,
calculated intensities in km mol-1
.
potential energy surface requires an energy of 14.3 kcal mol-1
,
To verify a triplet ground state of this system, and to
rationalize the higher barrier for ring-opening, we investigated
the electronic structure of 28 as a model for nitrene 7 in more
detail. A closer examination of substituent effects in these and
and the reaction is less exothermic (-11.6 kcal mol-1) than
the 6 f 18 rearrangement. Thus, the kinetic stability of 7 is
significantly higher than that of monoradical 6.
J. Org. Chem, Vol. 72, No. 3, 2007 719

Sander et al.
SCHEME 3. Energetics of Different Rearrangements of Nitrene Radical 6 Calculated at the UBLYP/6-311G(d,p) Level of
Theory
TABLE 3. IR Spectroscopic Data of 22, 23, and 25d
22
23
25
a
a,b
c
c
a
a,b
c
c
a
a,b
c
c
v˜exp
Iexp
v˜calc
Icalc
v˜exp
Iexp
v˜calc
Icalc
v˜exp
Iexp
v˜calc
Icalc
422
501
547
565
603
625
651
677
865
929
982
1194
1210
1263
1271
1431
1473
1805
4
447
549
578
607
621
662
678
862
1009
1017
1224
1237
1262
1342
1370
1419
1500
2
1
0
4
18
23
6
54
2
59
21
152
20
430
464
494
568
604
623
639
822
1005
1098
1258
1412
1977
2233
2339
3
1
11
1
8
4
2
254
237
197
56
562
634
675
694
915
963
1027
1241
1260
1320
1332
1507
1539
1852
10
10
40
10
60
5
658
60
70
1
6
45
37
19
16
48
92
6
700
901
1051
5
40
30
20
40
49
29
854
40
50
50
20
5
1268
1277
1302
1390
1414
10
40
70
5
1081
1174
1316
1436
1995
100
60
70
60
50
80
204
253
50
129
119
217
19
394
5
80
106
9
138
90
1536
100
2333
100
465
a Ne, 3 K. b Relative intensity based on the strongest absorption. Due to the complexity of the spectra, all experimental intensities are rough estimates.
c BLYP/6-311G(d,p). d Frequencies are given in cm-1, calculated intensities in km mol-1
.
related systems will be given elsewhere. Following our previous
study on dehydrophenylnitrenes,12 the ground and low-lying
excited states of 28 were studied by second-order perturbation
theory (MR-MP2),19 employing a complete active space self-
consistent-field reference wave function (CASSCF).
The ground state structures of 28-31 calculated at the
CASSCF level are given in Figure 6. The distance of the radical
centers in 28 is very similar to that of m-benzyne 31,20 and the
length of the C-N bond is almost identical with that of phenyl
(19) (a) Hirao, K. Chem. Phys. Lett. 1992, 190, 374. (b) Hirao, K. Chem.
Phys. Lett. 1992, 196, 397. (c) Hirao, K. Int. J. Quantum Chem. 1992, S26,
517. (d) Hirao, K. Chem. Phys. Lett. 1993, 201, 59.
720 J. Org. Chem., Vol. 72, No. 3, 2007

Dehydrophenylnitrenes
SCHEME 4. Photochemistry of 9 Leading via Nitrene 22
Finally to Allene 25
CHART 4
FIGURE 4. Photochemistry of azide 9 isolated in a neon matrix at 3
K. (a) Calculated IR spectrum of nitrene 22. (b) Difference spectrum
obtained after irradiation of the matrix at 305-320 nm. Bands pointing
upwards increase in intensity after short-time irradiation. (c) Further
photolysis leads to the formation of didehydroazepine 23. Note that in
this difference spectrum the concentration of 22 remains almost
constant, i.e., the absorptions are very small, because it is formed and
bleached at roughly the same rate until all 9 is consumed. Absorptions
of allene 25 are labeled by crosses. (d) Calculated IR spectrum of
didehydroazepine 23.
CHART 5
excitation energies of 28 given in Table 4. The first excited
singlet state is of A2 symmetry and 16.6 kcal mol-1 above the
3A2 ground state. It corresponds to the first excited singlet state
of 30 for which a very similar singlet-triplet energy splitting
(18.0 kcal mol-1) is found at the same level of theory.12 The
next state of 28 is the 3B1 triplet that is 17.5 kcal mol-1 above
the ground state, and which can be looked upon as the 3B2 triplet
state of m-benzyne 31 interacting with 3A2-30. For the former,
a singlet-triplet gap of 21.3 kcal mol-1 is found at the MR-
3
MP2 level. Similar to the coupling in 29, in B1-28 there is
strong interaction of the two, now uncoupled, electrons of the
m-benzyne moiety with the in-plane nitrene p orbital, reducing
the 3A2-3B1 gap of 28 as compared to the 1A1-3B2 splitting in
3
31. Accordingly, the C(N)-C(H) bonds in B1-28 are again
considerably longer (143.8 pm), and the C-N bond is shorter
FIGURE 5. (a) IR spectrum of 25 generated by long-time irradiation
(λ 305-320 nm) of 9, matrix-isolated in neon at 3 K. (b) Calculated
IR spectrum of allene 25.
(131.5 pm) than in the ground state. The lowest quintet state of
(20) (a) Sander, W. Acc. Chem. Res. 1999, 32, 669. (b) Winkler, M.;
Sander, W. J. Phys. Chem. A 2001, 105, 10422. (c) Sander, W.; Exner, M.;
Winkler, M.; Balster, A.; Hjerpe, A.; Kraka, E.; Cremer, D. J. Am. Chem.
Soc. 2002, 124, 13072. (d) Wenk, H. H.; Winkler, M.; Sander, W. Angew.
Chem., Int. Ed. 2003, 42, 502. (e) Winkler, M.; Cakir, B.; Sander, W.
J. Am. Chem. Soc. 2004, 126, 6135.
nitrene 30. Most importantly, the C(N)-C(H) bonds in 28 are
only modestly lengthened compared with 30, indicating that
through-bond interaction of the exocyclic orbitals at carbon with
the nitrogen in-plane p orbital is much weaker than that in 29.
This conclusion is further supported by the computed
(21) As with the iodotetrafluoro system, a pathway involving tri- or
tetradehydroazepines cannot be excluded.
J. Org. Chem, Vol. 72, No. 3, 2007 721

Sander et al.
FIGURE 6. Structures of 28-31 calculated at the CASSCF/cc-pVTZ level of theory. Heavy-atom bond lengths are given in pm.
TABLE 4. Absolute (au), Relative (kcal mol-1), Zero-Point
drophenylnitrenes. Attempts in this direction are currently in
progress in our laboratory.
Vibrational Energies (kcal mol-1), and Number of Imaginary
Vibrational Frequencies (Nimag) for Various Electronic States of
3,5-Didehydrophenylnitrene 28 as Computed at the CASSCF(10,10)/
cc-pVTZ and MRMP2/cc-pVTZ//CASSCF(10,10)/cc-pVTZ Levels of
Theory
Experimental Section
Synthesis. All phenyl azides were synthesized according to or
in analogy to literature procedures starting from the corresponding
anilines.22
state
CASSCF(10,10)
ZPVE (Nimag)
MR-MP2
3A2
3B1
1A2
5B1
1A1
-283.40191
12.4
42.5 (0)
naa
41.7 (0)
42.8 (0)
naa
-284.38119
17.5
3-Iodo-2,4,5,6-tetrafluoronitrobenzene. H5IO6 (2.27 g, 10.0
mmol) was solved in 70 mL of concentrated H2SO4 and cooled to
0 °C. KI (4.98 g, 30.0 mmol) was added slowly with stirring. The
solution was stirred for another 15 min and then 1.95 g (10 mmol)
of 2,3,4,6-tetrafluoronitrobenzene was added slowly. The solution
was stirred for 30 min at 0 °C and then warmed up to 50 °C for
another 3.5 h. After cooling to room temperature, the solution was
poured onto 500 mL of ice and extracted twice with 300 mL of
MTBE. The organic phase was extracted three times with 500 mL
of Na2S2O3 solution (10%) and dried over MgSO4. The solvent
was removed under vacuum. After chromatographic purification
(pentane) 2.95 g (9.2 mmol, 92%) of 3-iodo-2,4,5,6-tetrafluoroni-
trobenzene was obtained as a yellow solid.
MS (m/z, %) 321 (M+, 75), 291 (10), 263 (20), 148 (100), 117
(25), 98 (30), 69 (10). 13C NMR (400 MHz, [D6]-DMSO) δ 154.3,
151.5, 149.0, 145.9, 143.2, 137.7, 135.2, 125.9, 71.7. IR (Ar, 3 K)
V˜/cm-1 (%) 2229.9 (5), 2196.8 (5), 2130.9 (100), 2112.4 (18),
1634.5 (10), 1492.2 (95), 1478.4 (49), 1307.6 (15), 1223.2 (31),
1012.1 (31), 1001.9 (13), 974.3 (39), 955.3 (6), 807.8 (11), 768.3
(22), 664.5 (5).
17.2
18.1
38.6
16.6
24.8
25.4
a Not available.
28 is already 24.8 kcal mol-1 above the ground state, and will
not be discussed in detail here. Overall, A2-28 can be looked
3
upon as a phenylnitrene, interacting only weakly with a separate
m-benzyne moiety on the opposite side of the ring. The
weakness of through-bond coupling compared to the strong C3-
C5 interaction accounts for the higher kinetic stability of 28
found at the DFT level. On the basis of these arguments, and
given the photolability of 6, it is plausible to assume that the
ring-opening observed experimentally occurs already at the stage
of nitrene radical 26.20 In contrast to 3-dehydrophenylnitrenes,
3,5-didehydrophenylnitrenes might be accessible under low-
temperature conditions, if precursors can be found that avoid
the intermediate formation of the labile m-dehydrophenylni-
trenes.
Calcd C6F4INO2 (320.97): C 22.45, N 4.36. Found: C 20.26,
N 5.96.
3-Iodo-2,4,5,6-tetrafluoroaniline. 3-Iodo-2,4,5,6-tetrafluoroni-
trobenzene (1.56 g, 8.0 mmol) was dissolved in 50 mL of methanol
and 100 mg of Pt/C catalyst was added. The solution was stirred
in a hydrogenator with a hydrogen pressure of 4 bar for 1 h. After
the hydrogenation procedure, the catalyst was filtered off and the
solvent removed. The product was purified by column chromatog-
raphy (pentane) to give 800 mg (2.9 mmol, 81%) of 3-iodo-2,4,5,6-
tetrafluoroaniline as a black oil.
MS (m/z, %) 291 (M+, 100), 164 (40), 148 (65), 69 (30). 13C
NMR (400 MHz, [D6]-DMSO) δ 146.5, 144.2, 140.6, 138.3, 137.7,
123.6, 100.1, 67.6. IR (Ar, 3 K) V˜/cm-1 (%) 2229.9 (5), 2196.8
(5), 2130.9 (100), 2112.4 (18), 1634.5 (10), 1492.2 (95), 1478.4
(49), 1307.6 (15), 1223.2 (31), 1012.1 (31), 1001.9 (13), 974.3 (39),
955.3 (6), 807.8 (11), 768.3 (22), 664.5 (5).
Conclusions
The photochemistry of 3-iodo-2,4,5,6-tetrafluorophenyl azide
8 and 3,5-diiodo-2,4,6-trifluorophenyl azide 9 was investigated
in cryogenic argon and neon matrices, and compared to that of
pentafluorophenyl azide 10. All three compounds produce the
corresponding nitrenes as primary photoproducts in photosta-
tionary equilibria with their azirine and didehydroazepine
isomers. Only the iodine-containing systems undergo a ring-
opening reaction upon prolonged irradiation with UV light,
indicating that cleavage of the C-I bond occurs under these
conditions. Neither 3-dehydrophenylnitrene 6 nor 3,5-didehy-
drophenylnitrene 7 could be detected by IR or EPR spectros-
copy. According to DFT and ab initio computations, the
3-dehydrophenylnitrenes are labile systems that rapidly undergo
a ring-opening reaction that accounts for the formation of the
observed acyclic products. The formal tetraradical 7 possesses
a triplet ground state and should be kinetically more stable than
6, because strong coupling of the formally unpaired electrons
at C3 and C5 reduces the through-bond interaction with the
nitrene moiety that is responsible for the lability of 6. A synthetic
approach to these species requires a precursor chemistry that
circumvents the intermediate formation of the labile 3-dehy-
Calcd C6F4INH2 (290.99): C 24.77, N 4.81. Found: C 22.91,
N 4.38.
3-Iodo-2,4,5,6-tetrafluoroazidobenzene (8). 3-Iodo-2,4,5,6-tet-
rafluoroaniline (0.87 g, 3.0 mmol) was dissolved in 50 mL of
trifluoroacetic acid and cooled to -5 °C. Sodium nitrite (0.24 g,
(22) (a) Poe, R.; Schnapp, K.; Young, M. J. T.; Grayzar, J.; Platz, M. S.
J. Am. Chem. Soc. 1992, 114, 5054. (b) Neenan, T. X.; Whitesides, G. M.
J. Org. Chem. 1988, 53, 2489. (c) Filler, R.; Novar, H. J. Org. Chem. 1961,
26, 2707. (d) Kanakarajan, K.; Haider, K.; Czarnik, A. W. Synthesis 1988,
566.
(23) Dunkin, I. R. Matrix-Isolation Techniques; Oxford University
Press: Oxford, UK, 1998.
722 J. Org. Chem., Vol. 72, No. 3, 2007

Dehydrophenylnitrenes
3.5 mmol), dissolved in 5 mL of water, was added slowly, keeping
the temperature of the solution below 5 °C. After an additional 30
min of stirring at -5 °C, the solution was warmed to room
temperature and poured onto 250 g of ice. The suspension was
extracted twice with 250 mL of MTBE, and the combined organic
layers were washed twice with 250 mL of a NaHCO3 solution.
The organic phase was dried over MgSO4 and the solvent was
removed under vacuum. The product was purified by column
chromatography (pentane) to give 800 mg (2.5 mmol, 84%) of
3-iodo-2,4,5,6-tetrafluoroazidobenzene as a yellow oil.
MS (m/z, %) 317 (M+, 25), 289 (50), 162 (100), 148 (10), 127
(10), 93 (30), 69 (50). 13C NMR (400 MHz, [D6]-DMSO) δ 150.9,
148.5, 147.5, 145.1, 144.5, 142.0, 137.0, 134.3, 114.3, 101.0, 69.0.
IR (Ar, 3 K) V˜/cm-1 (%) 2229.9 (5), 2196.8 (5), 2130.9 (100),
2112.4 (18), 1634.5 (10), 1492.2 (95), 1478.4 (49), 1307.6 (15),
1223.2 (31), 1012.1 (31), 1001.9 (13), 974.3 (39), 955.3 (6), 807.8
(11), 768.3 (22), 664.5 (5).
repeated with saturated NaHCO3 solution. The organic phase was
dried over MgSO4 and the solvent was removed in vacuum. The
product was purified by column chromatography (pentane) to yield
650 mg (1.5 mmol) of 3,5-diiodo-2,4,6-trifluoro-1-azidobenzene
as a bright yellow solid.
MS (m/z, %) 425 (M+, 25), 397 (60), 270 (100), 143 (55), 124
(50), 93 (40), 69 (10). 13C NMR (400 MHz, [D6]-DMSO) δ 158.5,
156.1, 153.6, 113.5, 68.2. IR (Ar, 3 K) V˜/cm-1 (%) 2229.9 (5),
2196.8 (5), 2130.9 (100), 2112.4 (18), 1634.5 (10), 1492.2 (95),
1478.4 (49), 1307.6 (15), 1223.2 (31), 1012.1 (31), 1001.9 (13),
974.3 (39), 955.3 (6), 807.8 (11), 768.3 (22), 664.5 (5).
Calcd C6F3I2N3 (424.89): C 16.96, N 9.89. Found: C 16.84, N
10.67.
Matrix Isolation. Matrix isolation experiments were performed
by standard techniques22 with closed cycle helium cryostats allowing
cooling of a CsI spectroscopic window to 3 or 10 K, respectively.
FTIR spectra were recorded with a standard resolution of 0.5 cm-1
,
using a N2(l)-cooled MCT detector in the range 400-4000 cm-1
.
Calcd C6F4IN3 (316.99): C 22.74, N 13.26. Found: C 20.45, N
14.59.
X-band EPR spectra were recorded from a sample deposited on a
oxygen-free high-conductivity copper rod (75 mm length, 2 mm
diameter) cooled by a closed-cycle cryostat.
3,5-Diiodo-2,4,6-trifluoronitrobenzene. H5IO6 (2.27 g, 10.0
mmol) was solved in 70 mL of concentrated H2SO4 and cooled to
0 °C. KI (4.98 g, 30.0 mmol) was added slowly. After 15 min of
stirring, 1.79 g (10 mmol) of 2,4,6-trifluoronitrobenzene was
dropped slowly into the solution. After 30 min of stirring at 0 °C,
the solution was warmed to 50 °C and stirred for an additional 3.5
h. After cooling to room temperature, the solution was poured onto
500 mL of ice. The aqueous solution was extracted twice with 300
mL of MTBE. The organic phase was washed three times with
500 mL of aqueous Na2S2O3 (10%) and dried with MgSO4. After
removal of the solvent 3.85 g (9.0 mmol, 89%) of 3,5-diiodo-2,4,6-
trifluoro-1-nitrobenzene was obtained as a yellow solid. The crude
product was purified chromatographically.
MS (m/z, %) 429 (M+, 100), 399 (10), 383 (18), 272 (8), 256
(70), 129 (90), 79 (58). 13C NMR (400 MHz, [D6]-DMSO) δ 70.2
(t, 28.0 Hz), 125.4 (dm, 2.9 Hz), 153.7 (d, 256.4 Hz), 156.2 (d,
248.1 Hz), 161.7 (d, 248.1 Hz). IR (Ar, 3 K) V˜/cm-1 (%) 2229.9
(5), 2196.8 (5), 2130.9 (100), 2112.4 (18), 1634.5 (10), 1492.2 (95),
1478.4 (49), 1307.6 (15), 1223.2 (31), 1012.1 (31), 1001.9 (13),
974.3 (39), 955.3 (6), 807.8 (11), 768.3 (22), 664.5 (5).
Calcd C6F3I2NO2 (428.88): C 16.80, N 3.27. Found: C 16.63,
N 3.90.
Broadband irradiation was carried out with mercury high-pressure
arc lamps in housings equipped with quartz optics and dichroic
mirrors in combination with cutoff filters (50% transmission at the
wavelength specified). IR irradiation from the lamps was absorbed
by a 10 cm path of water. For 254 nm irradiation a low-pressure
mercury arc lamp was used. A XeCl Excimer Laser was used for
308 nm, and a KrF Excimer Laser for 248 nm irradiation.
Computational Methods. Optimized geometries and vibrational
frequencies of all species were calculated at the BLYP level24
employing the 6-311G(d,p) polarized valence-triple-ê basis set.25
Tight convergence criteria for gradients and a full (99 590)
integration grid, having 99 radial shells per atom and 590 angular
points per shell, were used throughout. Despite the neglect of
relativistic effects, this approach has been shown to reproduce the
measured IR spectra of several iodoaromatic compounds accur-
ately.11,20c,e A spin-unrestricted formalism was used for all high-
spin systems and for singlet biradicals, whenever an external
instability26 was observed. All DFT calculations were carried out
with Gaussian 98.27 The GAMESS-US program package was used
3,5-Diiodo-2,4,6-trifluoroaniline. 3,5-Diiodo-2,4,6-trifluoroni-
trobenzene (1.29 g, 3.0 mmol) was dissolved in 50 mL of methanol
and 100 mg of Pt/C was added. This solution was stirred in a
hydrogenator with a H2 pressure of 4 bar for 1 h. After hydrogena-
tion, the catalyst was separated and the solvent removed. After
purification by column chromatography (pentane), 1.03 g (2.6
mmol, 86%) of 3,5-diiodo-2,4,6-trifluoroaniline was obtained as a
bright yellow solid.
MS (m/z, %) 399 (M+, 100), 272 (30), 145 (60), 117 (20), 99
(20), 70 (10). 13C NMR (400 MHz, [D6]-DMSO) δ 151.4, 149.1,
66.2. IR (Ar, 3 K) V˜/cm-1 (%) 2229.9 (5), 2196.8 (5), 2130.9 (100),
2112.4 (18), 1634.5 (10), 1492.2 (95), 1478.4 (49), 1307.6 (15),
1223.2 (31), 1012.1 (31), 1001.9 (13), 974.3 (39), 955.3 (6), 807.8
(11), 768.3 (22), 664.5 (5).
(24) (a) Becke, A. D. Phys. ReV. A 1988, 38, 3098. (b) Lee, C.; Yang,
W.; Parr, R. G. Phys. ReV. B 1988, 37, 785.
(25) (a) Glukhovstev, M. N.; Pross, A.; McGrath, M. P.; Radom, L.
J. Chem. Phys. 1995, 103, 1878. (b) Basis sets were obtained from the
Extensible Computational Chemistry Environment Basis Set Database,
Version 6/19/03, as developed and distributed by the Molecular Science
Computing Facility, Environmental and Molecular Sciences Laboratory,
which is part of the Pacific Northwest Laboratory, P.O. Box 999, Richland,
Washington 99352, and funded by the U.S. Department of Energy. The
Pacific Northwest Laboratory is a multiprogram laboratory operated by
Battelle Memorial Institute for the U.S. Department of Energy under contract
DE-AC06-76.RLO 1830. Contact David Feller or Karen Schuchardt for
further information.
(26) Bauernschmitt, R.; Ahlrichs, R. J. Chem. Phys. 1996, 104, 9047.
(27) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels,
A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.;
Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.;
Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick,
D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.;
Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi,
I.; Gomperts, 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.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez,
C.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98; Gaussian,
Inc.: Pittsburgh, PA, 1998.
Calcd C6F3I2NH2 (398.89): C 18.07, N 3.51. Found: C 18.28,
N 3.58.
3,5-Diiodo-2,4,6-trifluorophenyl Azide (9). 3,5-Diiodo-2,4,6-
trifluoroaniline (0.7 g, 1.8 mmol) was dissolved in 50 mL of
trifluoroacetic acid and cooled to -5 °C. Sodium nitrite (0.2 g, 3
mmol), dissolved in 5 mL of H2O, was added within 15 min, while
the temperature of the solution was kept below 5 °C. After 30 min
of stirring at -5 °C, a solution of 0.2 g (3 mmol) of sodium azide
in 5 mL of water was added slowly and the solution was stirred
subsequently for an additional 30 min. After warming to room
temperature, it was poured onto 250 g of ice and extracted twice
with 250 mL of MTBE. The organic phase was extracted twice
with 250 mL of a diluted NaHCO3 solution and the procedure
(28) Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Elbert, S. T.;
Gordon, M. S.; Jensen, J. H.; Koseki, S.; Matsunaga, N.; Nguyen, K. A.;
Su, S. J.; Windus, T. L.; Dupuis, M.; Montgomery, J. A. J. Comput. Chem.
1993, 14, 1347.
(29) Dunning, T. H. J. Chem. Phys. 1989, 90, 1007.
J. Org. Chem, Vol. 72, No. 3, 2007 723

Sander et al.
for ab initio calculations.28 The active spaces for CASSCF
calculations cover the six valence π orbitals, and the formally singly
occupied radical orbitals at nitrogen or carbon, resulting in (8,8)
active spaces for 30 and 31, (9,9) for 29, and (10,10) for 28.
Harmonic vibrational frequencies were computed by finite differ-
ences of analytic first derivatives for the lowest energy state within
a given spin multiplicity. Subsequent multireference second-order
perturbation theory single-point energy computations (MRMP2)19
were performed within the frozen core approximation, using the
same active spaces. Dunning’s cc-pVTZ basis set with spherical
harmonic polarization functions was employed in all computations.29
Acknowledgment. This work was financially supported by
the Deutsche Forschungsgemeinschaft and the Fonds der
Chemischen Industrie.
Supporting Information Available: Calculated IR spectra and
energies of various potential photoproducts, IR spectroscopic data
of 10-13, as well as CASSCF geometries of low-lying states of
28. This material is available free of charge via the Internet at
http://pubs.acs.org.
JO061624B
724 J. Org. Chem., Vol. 72, No. 3, 2007