2-Pyridylnitrene-1,3-diazacyclohepta-1,2,4,6-tetraene rearrangements in the trifluoromethyl-2-pyridyl azide series

Article abstract of DOI:10.1021/ja9541645

Photolysis of Ar matrix isolated trifluoromethyl-substituted 2-pyridyl azides/tetrazolo[1,5-a]pyridines at 12-18 K causes rapid and mostly clean conversion to the corresponding 1,3-diazacyclohepta-1,2,4,6-tetraenes (4D, 5D, 5,6D, and 4,6D) absorbing near 2000 cm^-1 in the IR. In the latter case, the intermediate 3,5-bis(trifluoromethyl)-2-pyridylnitrene (4,6N) was observed by both ESR and IR spectroscopy and converted to the diazacycloheptatetraene 4,6D in the course of 90 min of UV irradiation. The 2-pyridylnitrenes were generally observable by ESR spectroscopy (D/hc ~ 1.05-1.10; E/hc ~ 0.0 cm^-1) following both photochemical and thermal (FVP) generation from the 2-azidopyridines. Irradiation of the Ar matrix isolated mixtures of nitrenes and diazacycloheptatetraenes also caused development of weak carbene transitions (D/hc ~ 0.40-0.45; E/hc ~ 0.006 cm^-1) in the ESR spectra.

Full text of DOI:10.1021/ja9541645

J. Am. Chem. Soc. 1996, 118, 4009-4017
4009
2-Pyridylnitrene-1,3-Diazacyclohepta-1,2,4,6-tetraene
Rearrangements in the Trifluoromethyl-2-pyridyl Azide Series1a
Richard A. Evans,1c Ming Wah Wong, and Curt Wentrup*
Contribution from the Chemistry Department, The UniVersity of Queensland,
Brisbane, Queensland 4072, Australia
ReceiVed December 11, 1995X
Abstract: Photolysis of Ar matrix isolated trifluoromethyl-substituted 2-pyridyl azides/tetrazolo[1,5-a]pyridines at
12-18 K causes rapid and mostly clean conversion to the corresponding 1,3-diazacyclohepta-1,2,4,6-tetraenes (4D,
5D, 5,6D, and 4,6D) absorbing near 2000 cm-1 in the IR. In the latter case, the intermediate 3,5-bis(trifluoromethyl)-
2-pyridylnitrene (4,6N) was observed by both ESR and IR spectroscopy and converted to the diazacycloheptatetraene
4,6D in the course of 90 min of UV irradiation. The 2-pyridylnitrenes were generally observable by ESR spectroscopy
(|D/hc |∼1.05-1.10; |E/hc|∼0.0 cm-1) following both photochemical and thermal (FVP) generation from the
2-azidopyridines. Irradiation of the Ar matrix isolated mixtures of nitrenes and diazacycloheptatetraenes also caused
development of weak carbene transitions (|D/hc|∼0.40-0.45; |E/hc| ∼0.006 cm-1) in the ESR spectra.
The ring expansion of 2-pyridylcarbenes2,3 (1) to azacyclo-
heptatetraene 2, the interconversion of 1, 2, and phenylnitrene
3, and the related 2-pyridylnitrene automerization4 (4 a 5 a
6) were the first examples of the fascinating rearrangements of
carbenes and nitrenes.5
However, IR spectroscopic observations of 28 and 59 in
cryogenic matrices and of 2 in solution10 make it clear that the
long-lived species are the cumulenic forms shown.
Laser flash photolytic (LFP) and product studies in solution
at room temperature were also interpreted in terms of 1-aza-
cyclohepta-1,2,4,6-tetraene (1,2-didehydroazepine) (2) in equi-
1
librium with the singlet nitrene 3.10,11 The bicyclic azirine 9,
predicted12 to lie on the reaction coordinate from 13 to 2 but of
higher energy than 2, has neither been observed directly nor
trapped in reactions of phenyl azide. There is, however, some
trapping evidence for the bicyclic azirine in the photolysis of a
substituted phenyl azide.13 In contrast, in the photolysis of
naphthyl and pyrenyl azides, it is the tricyclic azirines that are
being trapped (e.g., 10a and 11 from 2- and 1-naphthylnitrenes,
respectively) to the exclusion of the dehydroazepines.11,14 There
In the early publications, the seven-membered ring intermedi-
ates were thought to be carbenes (7 and 8) rather than the
cumulenes 2 and 5.2,3 Photolysis of an Ar matrix containing
the triplet nitrene 34 as well as 5 produces a carbene, observed
by ESR spectroscopy and tentatively identified as carbene 8.6
The existence of a phenyl derivative of 7 in equilibrium with
13 (and 2) is suggested from solution trapping experiments.7
X Abstract published in AdVance ACS Abstracts, April 15, 1996.
(1) (a) Hetarylnitrenes, Part X. For Part IX, see ref 1b. For Part VIII,
see ref 9. (b) Evans, R. A.; Wentrup, C. J. Chem. Soc., Chem. Commun.
1992, 1062. (c) Present address: CSIRO, Division of Chemicals and
Polymers, Clayton, Victoria 3169, Australia.
(2) Crow, W. D.; Wentrup, C. Tetrahedron Lett. 1968, 6149.
(3) Wentrup, C. J. Chem. Soc., Chem. Commun. 1969, 1386.
(4) Wentrup, C.; Crow, W. D. J. Chem. Soc., Chem. Commun. 1969,
1387.
(5) (a) Wentrup, C. Top. Curr. Chem. 1976, 62, 175-251. (b) Chapman,
O. L. Pure Appl. Chem. 1979, 51, 331-339. (c) Jones, W. M. In
Rearrangements in Ground and Excited States; deMayo, P., Ed.; Academic
Press: New York, 1980; Chapter 3. (d) Wentrup, C. In ReactiVe Intermedi-
ates; Abramovitch, R. A., Ed.; Plenum Press: New York, 1980; Vol. 1,
Chapter 4. (e) Wentrup, C. AdV. Heterocycl. Chem. 1981, 28, 233-361.
(f) Wentrup, C. In Azides and Nitrenes; Scriven, E. F. V., Ed.; Academic
Press: New York 1984; Chapter 8, pp 395-432. (g) Wentrup, C. ReactiVe
Molecules; Wiley: New York, 1984; Chapter 4.
(8) Chapman, O. L.; LeRoux, J.-P. J. Am. Chem. Soc. 1978, 100, 282.
Chapman, O. L.; Sheridan, R. S.; LeRoux, J.-P. Ibid. 1978, 100, 6245. Recl.
TraV. Chim. Pays-Bas. 1979, 98, 334.
(9) Wentrup, C.; Winter, H.-W. J. Am. Chem. Soc. 1980, 102, 6159.
(10) (a) Li, Y.-Z.; Kirby, J. P.; George, M. W.; Poliakoff, M.; Schuster,
G. B. J. Am. Chem. Soc. 1988, 110, 8092. (b) Marcinek, A.; Leyva, E.;
White, D.; Platz, M. S. J. Am. Chem. Soc. 1993, 115, 8609.
(11) Schuster, G. B.; Platz, M. S. AdV. Photochem. 1992, 117, 69-143.
(12) For MNDO calculations of the C6H5N potential energy surface, see
ref 10a. For CNDO/2 and Extended Hu¨ckel calculations, see: Gleiter, R.;
Rettig, W.; Wentrup, C. HelV. Chim. Acta 1974, 57, 2111. For thermo-
chemical calculations, see: Wentrup, C. Tetrahedron 1974, 30, 1301. For
MINDO/2 and INDO calculations, see: Shillady, D. D.; Trindle, C. Theor.
Chim. Acta 1976, 43, 137. For force field-SCF calculations, see: Wentrup,
C.; Mayor, C.; Becker, J.; Lindner, H. J. Tetrahedron 1985, 41, 1601.
(13) Younger, C. G.; Bell, R. A. J. Chem. Soc., Chem. Commun. 1992,
1359.
(6) Wentrup, C.; Kuzaj, M.; Lu¨erssen, H. Angew. Chem., Int. Ed. Engl.
1986, 25, 480.
(7) Murata, S.; Sugawara, T.; Iwamura, H. J. Chem. Soc., Chem.
Commun. 1984, 1198. Cf. Murata, S.; Sugawara, T.; Iwamura, H. J. Am.
Chem. Soc. 1985, 107, 6317.
(14) Schrock, A. K.; Schuster, G. B. J. Am. Chem. Soc. 1984, 106, 5234.
S0002-7863(95)04164-3 CCC: $12.00 © 1996 American Chemical Society

4010 J. Am. Chem. Soc., Vol. 118, No. 17, 1996
EVans et al.
Chart 1
Figure 1. Difference FTIR spectrum of 5D (positive peaks) generated
by Ar matrix photolysis of 5A/5T (negative peaks).
2-hydrazinopyridines with hydrazine hydrate, followed by
diazotization. 2-Azido-6-trifluoromethyl-pyridine (6A) could
only be obtained by method (ii). The hydrazinopyridines are
unstable solids that can be purified by sublimation but discolor
even on storage at -20 °C. Compounds 4T, 6A, and 4,6A have
been discussed in a patent,21 but few data were reported. 6A
and 4,6A exist as azides in the neat liquid state at room
temperature. However, all the azide-to-tetrazole isomerizations
are exothermic, with the enthalpies of the azides being higher
is IR spectroscopic evidence for both the tricyclic azirines and
the benzodehydrodiazepines from Ar matrix photolysis of the
naphthyl azides,15 but neither could be detected by time resolved
IR spectroscopy in solution on the µs time scale.10 The UV
active transients observed in LFP experiments were assigned
to 10a and 11.14 Azirino[2,3-c]isoquinoline (10b) has been
observed by Ar matrix IR spectroscopy following photolysis
of 3-isoquinolyl azide.5f
than those of the tetrazoles (∆H°isom ) -13 to -30 kJ mol-1
;
∆S°isom ) -50 to -59 ( 8 J mol-1 K-1), as determined by
1
variable temperature H NMR spectroscopy in DMSO-d6 or
CDCl3 solution.1b,22 Thus, the existence of the azides is an
entropic effect. For those compounds that exist as azides in
the liquid state, the tetrazole isomers were detectable by 1H and
13C NMR spectroscopy in DMSO solution. Conversely, the ring
opened azide forms of 5T, 3,5T, and 4,5T were detectable by
IR spectroscopy in CHCl3 solution. The azide corresponding
to 5T was detectable by 1H NMR in TFA-d solution (5T:5A )
4.9:1 at 22 °C), and for 3,5T and 4,5T the azide forms were
also observed in CDCl3 solution by 13C NMR spectroscopy (see
Experimental Section). Sublimation of the tetrazoles, usually
at 40-50 °C, caused significant ring opening to the azides as
revealed by the IR spectra of the Ar matrix isolated materials,
except in the case of 4T which only showed weak azide bands
in the 2100 cm-1 region under these conditions (see spectra
below). This conversion to azides facilitated the subsequent
matrix photolysis, but the tetrazole forms also photolyzed with
elimination of N2.
The first-formed intermediates in all these studies, the singlet
nitrenes (e.g., 13) have not been observed. Triplet nitrenes are
observable under quite different conditions by ESR spectroscopy
in crystals, organic glasses at 77 K, or Ar matrices at e15
K6,8,16,17 but are not usually observed by IR spectroscopy. In
the original matrix photolysis work on phenyl azide, only
dehydroazepine 2 was observed, not the nitrene 3.8 However,
3
Hayes and Sheridan18 were able to observe 3 directly by IR
spectroscopy, using 334 nm irradiation of phenyl azide. The
nitrene was converted to 2 on irradiation at 485 nm.
Here we wish to report full details of our studies of
trifluoromethyl-substituted 2-pyridyl azides.1b These com-
pounds are of particular interest due to their considerable
synthetic potential, being excellent precursors for high-yielding
preparations of stable 1H- and 5H-1,3-diazepines.19 We also
describe an example of the direct IR spectroscopic observation
of a 2-pyridylnitrene (3,5N)20 and its photoisomerization to a
didehydrodiazepine (4,6D).20
2. 1,3-Diaza-5-trifluoromethylcyclohepta-1,2,4,6-tetraene
(5D). Tetrazole 5T was sublimed at 40-50 °C and deposited
with Ar at 15 K on a BaF2 disk. The infrared spectrum (Figure
1, negative peaks) demonstrated partial ring opening to the azide
5A (2155, 2128 cm-1). Photolysis for 6 min with the unfiltered
light of a 1000 W high pressure Hg-Xe lamp caused complete
consumption of the 5A/5T mixture and formation of a new
compound absorbing strongly at 1997 cm-1 and assigned as
the cyclic carbodiimide 5D (Figure 1, positive peaks, and
Scheme 1). This is some 135-160 cm-1 below the normal
value for linear carbodiimides.23 The assignment is based on
comparison with previously reported 1,3-diazacyclohepta-1,2,4,6-
tetraenes9,24 as well as B3LYP/6-31G* calculations, which
reproduces the experimental spectra extremely well (Table 1).25
Results and Discussion
1. Synthesis. The 2-azidopyridines/tetrazolo[1,5-a]pyridines
used in this study (Chart 1)20 were prepared from the corre-
sponding 2-chloropyridines by two methods: (i) direct nucleo-
philic substitution with NaN3 in DMF or (ii) conversion to the
(15) Dunkin, I. R., Thomson, P. C. P. J. Chem. Soc., Chem. Commun.
1980, 499.
(16) Wasserman, E. Prog. Phys. Org. Chem. 1971, 8, 319. Platz, M. S.
In Azides and Nitrenes; Scriven, E. F. V., Ed., Academic: New York, 1984;
Chapter 7.
(17) Leyva, E.; Munoz, D.; Platz, M. S. J. Org. Chem. 1989, 54,
5938. Mahe´, L.; Izuoha, A.; Sugawara, T. J. Am. Chem. Soc. 1992, 114,
7904.
(18) Hayes, J. C.; Sheridan, R. S. J. Am. Chem. Soc. 1990, 112, 5881.
(19) (a) Reisinger, A.; Wentrup, C. J. Chem. Soc., Chem. Commun. 1996,
in press. (b) Reisinger, A.; Wentrup, C. Unpublished results.
(20) Formulas are numbered according to the following system: 4A is
4-trifluoromethyl-2-azidopyridine. 4T is the corresponding tetrazolo[1,5-
a]pyridine, and 4N is the corresponding 2-pyridylnitrene; 34N is the triplet
nitrene; 5D is 5-trifluoromethyl-1,3-diazacyclohepta-1,2,4,6-tetraene, etc.
(21) Rogers, R. B.; Gerwick, B. C.; Egli, E. A. US Patent 4,474,599
(1984); Chem. Abstr. 1985, 102, P45956g.
(22) Evans, R. A. PhD Thesis, The University of Queensland, 1992.
(23) IR of open chain carbodiimides: Williams, D. H.; Fleming, I.
Spectroscopic Methods in Organic Chemistry; McGraw-Hill: London, 1995.
Palomo, C.; Mestres, R. Synthesis 1981, 5, 373. Cyclic carbodiimides:
Richter, R.; Tucker, B.; Ulrich, H. J. Org. Chem. 1983, 48, 1694. Molina,
P.; Alajarin, M.; Sanchez-Andrada, P.; Elguero, J.; Jimeno, M. L. J. Org.
Chem. 1994, 59, 7306, and references therein.

Trifluoromethyl-2-pyridyl Azide Series
Table 1. Calculated (B3LYP/6-31G*) and Experimental IR Spectra of 5D, 4D, 6D and Triplet 3,5Na,b
5D 4D 4,6D
expt freq calc freq calc inten expt freq calc freq calc inten expt freq calc freq calc inten
J. Am. Chem. Soc., Vol. 118, No. 17, 1996 4011
3,5N
expt freq
calc freq calc inten
3119
3103
3078
1987
1588
1550
1364
1286
1278
2
9
0
353
7
22
10
21
289
3101
3094
3068
1984
1611
1552
1354
1283
1276
10
2
2
357
7
9
16
18
242
3123
3104
1991
1619
1558
1334
1289
1288
1247
1
2
372
9
28
27
105
35
490
3121
3073
1564
1472
1429
1362
1321
1283
1241
1
5
12
61
7
2000 s
1997 s
1992 s
1337 w
3
1338 w
1276 s
146
11
508
1319 m
1294 m
1311 m
1280 s
1277cs
1237
1177
1170
35
4
181
1229
1177
1166
20
202
11
1225
1182
1174
15
234
184
1208 m
1182 m
1196
1185
1167
106
79
143
1192 s
1200 s
1184 m
1143 s
985 m
1190 m
1186c m
1170 s
1137
1021
966
956
919
256
15
113
45
4
12
1
1164 s
1040 m
1006 w
1158
1013
989
922
916
244
128
62
5
12
10
3
1162
1142
1041
1014
968
227
237
12
143
87
2
27
(1171)d m
1159 m
1164
1152
1141
1056
1008
939
278
209
66
64
108
2
1150 m
1041 m
985 w
1075 w-m
1029 w-m
886
880
895
849e
831f
876g
916h
30
a Frequency in cm-1 and intensity in km mol-1
d 3,5A and 4,6D both have bands at 1171 cm-1, thus making the assignment of a band at 1171 cm-1 for 3,5N uncertain. e No calculated band with
intensity >28 km mol-1 below 849 cm-1 f No calculated band with intensity >33 km mol-1 below 831 cm-1 g No calculated band with intensity
>27 km mol-1 below 876 cm-1 h No calculated band with intensity >30 km mol-1 below 916 cm-1
.
b Calculated frequencies scaled by 0.9613 (ref 30). c These bands may be due to matrix sites.
.
.
.
.
precursors, the azide 6A and the tetrazole 3T, the latter
tautomerizing in part to 3A on sublimation/deposition (Scheme
2 and Figures 3 and 4). 6A was converted completely to 4D
after 12 min of irradiation, giving major bands at 1992 s, 1319
m, 1294 m, 1192 s, 1164 vs (νCF), 1040 m, and 1006 w cm-1
(the band at 1294 cm-1 is not seen in the difference spectrum,
Figure 3, as it is coincident with a stronger band due to the
azide 6A). Again, there is good agreement with B3LYP/6-31G*
calculations (Table 1).
4D was likewise obtained from 3T/3A in 2 min (Figure 4).
The same seven bands as above were clearly identified, although
the reaction in this case was less clean. Unidentified bands
appeared at 1310 m, 1208 m, 1175 m, and 1150 m cm-1 on
complete conversion of 3T/3A. They must arise from a reaction
of 3T/3A or the related nitrene or azirine and not from the
carbodiimide 4D since they are not formed from the clean
sample of 4D described above (Figure 3), and they stop
developing once 3T/3A have been fully consumed. A weak
signal at 2041 cm-1 also appeared, but this grew more slowly
and was unrelated to the other species. Except for this case,
all the azides/tetrazoles were cleanly and virtually quantitatively
converted to cyclic carbodiimides.
Figure 2. Difference FTIR spectrum (15 K) of 5D (positive peaks)
generated by Ar matrix photolysis of 4T (negative peaks).
Other prominent bands in the IR spectrum of 5D are at 1311
m, 1184 m, 1143 s (νCF), and 985 m cm-1
.
The same carbodiimide (5D) was generated by 20 min broad-
band photolysis of tetrazole 4T (Figure 2). This tetrazole only
gave a small amount of azide 4A on sublimation (Figure 2,
negative peaks). The IR spectra of 5D from the two sources
(5A and 4T) were identical, as required by the chemistry
depicted in Scheme 1.
Comparison of the four spectra of carbodiimides makes it
clear that one carbodiimide is generated from the two precursors
in Scheme 1; a different carbodiimide is generated from the
two precursors in Scheme 2.
4. 5,6-Bis(trifluoromethyl)-1,3-diazacyclohepta-1,2,4,6-
tetraene (5,6D). The tetrazole/azide mixture 4,5T/4,5A (Scheme
3) was converted after 2 min of broad-band photolysis to the
carbodiimide 5,6D (Figure 5). The latter absorbed at 2010 m,
1336 s, 1307 s, 1183 s, 1167 s (νCF), 991 m, 970 w, and 875 w
cm-1. For symmetry reasons, there is only one azide/tetrazole
precursor to this molecule.
Prolonged irradiation of carbodiimide 5D (30-60 min)
resulted in the growth of a small peak at 2066 cm-1 concurrent
with decomposition of 5D. The 2066 cm-1 signal always
remained weak to medium, and no other peak could be
correlated with it. On further irradiation with the full light of
a 75 W low pressure Hg lamp, a nitrile absorption developed
at 2235 cm-1 19b The carrier of the 2066 cm-1 signal may be
.
a ketenimine. We have seen such signals in several other cases
(see also 4D and 4,6D below). The carrier of the 2235 cm-1
signal may be a cyanopyrrole in analogy with the results of
FVP experiments.1b These secondary photolyses are under
further investigation.
5. 4,6-Bis(trifluoromethyl)-1,3-diazacyclohepta-1,2,4,6-
tetraene (4,6D) and 3,5-bis(trifluoromethyl)-2-pyridylnitrene
(3,5N). The carbodiimide 4,6D was obtainable from two
precursors, 4,6A and 3,5T (Scheme 4). 4,6A underwent clean
3. 1,3-Diaza-4-trifluoromethylcyclohepta-1,2,4,6-tetraene
(4D). This compound was again produced from two isomeric
(24) Wentrup, C.; The´taz, C.; Tagliaferri, E.; Lindner, H. J.; Kitschke,
B.; Winter, H.-W; Reisenauer, H. P. Angew. Chem., Int. Ed. Engl. 1980,
19, 566.
(25) Further calculations on the C5H4N2 energy surface and substituted
species will be published by Wong, M. W.; Wentrup, C.

4012 J. Am. Chem. Soc., Vol. 118, No. 17, 1996
EVans et al.
Scheme 2
Figure 3. Difference FTIR spectrum (15 K) of 4D (positive peaks)
generated by Ar matrix photoysis of 6A (negative peaks).
Figure 4. Difference FTIR spectrum (15 K) of 4D (positive peaks)
generated by Ar matrix photolysis of 3A/3T (negative peaks).
Scheme 1
Scheme 3
Figure 5. Difference FTIR spectrum (15 K) of 5,6D (positive peaks)
generated by Ar matrix photolysis of 4,5T/4,5A (negative peaks).
Tetrazole 3,5T underwent ring opening to azide 3,5A on
sublimation/deposition (Figure 7). Photolysis converted it to
the same carbodiimide as above (4,6D), but a distinct intermedi-
ate was obtained in this process (bands marked B in Figure 7).
Continuous broad-band irradiation for 85 min converted the
intermediate B fully to the carbodiimide 4,6D (bands marked
C in Figure 7). All the azide/tetrazole 4,6A/4,6T had been
consumed within the first 10-12 min of photolysis, and the
and complete conversion to the carbodiimide 4,6D during 6
min of photolysis. This product absorbs at 2000 s, 1279 s,
1277 s, 1200 s, 1190 w, 1186 m, 1171 s, 1151 m (this and
four preceding bands νCF), 1041 m, and 985 w cm-1 (Figure
6). The experimental and calculated values are in good
agreement (Table 1).

Trifluoromethyl-2-pyridyl Azide Series
J. Am. Chem. Soc., Vol. 118, No. 17, 1996 4013
Figure 6. Difference FTIR spectrum (15 K) of 4,6D (positive peaks)
generated by Ar matrix photolysis of 4,6A (negative peaks).
Scheme 4
Figure 7. Lower trace: Difference spectrum showing 3,5T/3,5A
(negative peaks, marked A) and a mixture of nitrene 3,5N (peaks
marked B) and 4,6D (peaks marked C), produced by 5 min photolysis
(Ar, 15 K). Upper trace: After 85 min of photolysis; X ) unidentified
product; C ) 4,6D.
Figure 8. First order decay toward zero intensity of the negative XY2
ESR signal of 3,5N during continuous irradiation at 20 K. Inset: the
XY2 signal at 7395.3 G (microwave frequency 9.72779 MHz).
inset in Figure 8). Continuous broad-band irradiation within
the ESR cavity caused this signal to decay over a 90 min period,
i.e., a time scale very close to that observed for intermediate B
by IR spectroscopy under the same conditions. The decay
obeyed a first-order rate law with k = 2 × 10-4 s-1 (Figure 8).
The close correspondence between IR and ESR spectroscopic
behavior virtually identifies the intermediate B as the triplet
nitrene, 3,5N. Furthermore, the observed IR spectrum is in very
good agreement with B3LYP/6-31G* calculated data (Table 1).
We assume that the ring expansion 3,5N f 4,6D takes place
on the singlet energy surface and hence it requires intersystem
crossing (ISC) to the (not observed) singlet nitrene. It is possible
that the ring expansion of singlet 3,5N is slightly retarded
because of steric hindrance by the neighboring CF3 group, thus
making ISC to the triplet ground state more competitive with
ring expansion. However, a similar effect was not observed
for 3N.
bands of intermediate B reached maximum intensity within the
first 8-10 min of photolysis. The additional amount of
carbodiimide formed during the 85 min of photolysis must
therefore come from intermediate B. Difference spectra clearly
revealed the disappearance of B in concert with formation of
4,6D. In addition, the long term irradiation also caused
formation of new bands marked X in Figure 7, due to secondary
reaction of either B or 4,6D.
Intermediate B cannot be an azirine of type 9 as such
compounds are expected to have IR absorptions in the vicinity
of 1750 cm-1 5f,15
The nitrene 3,5N is a likely candidate in
.
analogy with the experiment of Hayes and Sheridan on
phenylnitrene.18 This was confirmed by observation of the ESR
spectrum of triplet 3,5N. For this purpose, 3,5T/3,5A was
deposited at 20 K with Ar on the Cu rod target of an ESR
cryostat. Photolysis as above generated a typical arylnitrene
XY2 signal centered at 7335.5 G (midpoint) (|D/hc| ) 1.108
cm-1; E/hc = 0.00) within the first 5 min of photolysis (see
6. Thermally Produced Nitrenes and Diazacycloheptatet-
raenes. All the nitrenes described in this study (Schemes 1-4)

4014 J. Am. Chem. Soc., Vol. 118, No. 17, 1996
Table 2. Zero Field Splitting Parameters of Nitrenesa
EVans et al.
nitrene
|D/hc|
E/hc
3N
4N
5N
6N
1.0768
1.0826
1.0865
1.0575
1.1080
0.00
0.00
0.00
0.00
0.00
3,5N
a Ar matrix, 20 K. D/hc and E/hc in cm-1
.
Figure 9. Computed structure of 1,3-diazacyclohepta-1,2,4,6-tetraene
(5) (B3LYP/6-31G*).
have been observed by ESR spectroscopy by irradiation of Ar
matrices of the deposited azides/tetrazoles, and they are therefore
expected to have triplet ground states. Moreover, the same
nitrenes were also generated by flash vacuum thermolysis (FVT)
(200-400 °C) of the azides/tetrazoles, deposited in Ar matrices,
and observed by ESR spectroscopy. Some D and E values are
collected in Table 2. The cyclic carbodiimides 4D, 5D, 4,6D,
and 5,6D were also observed by Ar matrix IR spectroscopy
following FVP at 360-410 °C, giving spectra identical with
the ones reported above. At FVP temperatures above 450 °C,
the carbodiimides were replaced by trifluoromethyl-substituted
cyanopyrroles. Details of these rearrangements1b will be the
subject of a separate publication.
cyclic carbodiimide 5 (1,3-diazacyclohepta-1,2,4,6-tetraene) is
predicted to have a chiral equilibrium geometry, with C2
symmetry (Figure 9). The NdCdN bond angle deviates from
linearity by just 15° and the CNC angle is rather acute (104°).
The corresponding bond angles in the allene moiety of 1,2,4,6-
cycloheptatetraene are 146° and 114°, respectively (B3LYP/6-
31G*). This suggests that the nitrogen atoms in 5 aid in
alleviating the considerably strained cumulenic bonds. Intro-
duction of one or two CF3 substituents in 5 (i.e., compounds
4D, 5D, 4,6D, and 5,6D) leads to small changes in the molecular
geometry (see supporting information).
7. Photochemically Produced Carbenes. From the IR and
ESR experiments described above, we know that photolysis
produces 1,3-diazacycloheptatetraenes within a few minutes, and
triplet nitrenes are formed at the same time. Both of these
species are also formed on FVP. Photolysis of the thermally
deposited Ar matrices containing a mixture of the triplet nitrenes
and the diazacycloheptatetraenes in the cavity of the ESR
spectrometer caused development of very weak signals typical
of carbenes in some cases. These signals were usually too weak
for accurate measurement and assignment but resemble those
previously reported for 8 and other 1,3-diazacycloheptatrie-
nylidenes.6 For example, photolysis of a matrix containing
36N (observed by ESR) and 4D (observed by IR in sepa-
rate experiments) gave weak signals for which zero field
splitting parameters |D/hc| ) 0.4501 cm-1 and |E/hc| )
0.0059 cm-1 were derived. These values are similar to those
of 8 and of cycloheptatrienylidene itself and unlikely to be due
to pyridyl- or pyrimidylcarbenes.6,26 We speculate that the
carbenes are the 1,3-diazacycloheptatrienylidenes, generated
either by excitation of the singlet 1,3-diazacycloheptatetraenes
to the triplet states or by photochemical rearrangement of the
triplet nitrene, although further experimentation will be neces-
sary.
The parent 2-pyridylnitrene (4) is predicted to have a triplet
ground state and a substantial singlet triplet gap. The bicyclic
azirine is calculated to be a stable equilibrium structure, ca. 53
kJ mol-1 above the cyclic carbodiimide 5, but the barrier toward
ring opening to 5 is only ca. 14 kJ mol-1, thus making this
azirine virtually unobservable. 5 lies slightly below the triplet
nitrene. For the CF3-substituted systems, the carbodiimides are
also calculated to be the most stable species. For instance, the
bis-CF3-substituted carbodiimide 4,6D is predicted to lie 19 kJ
mol-1 below the triplet nitrene 3,5N.25 Finally, we note that
the calculated B3LYP/6-31G* IR spectra for the carbodiimides
and nitrene (Table 1) are in very good agreement with
experiment. In particular, the calculated B3LYP frequencies
for the triplet nitrene 3,5N are significantly better than those
predicted at the UHF/6-31G* level.
Conclusion
1,3-Diazacyclohepta-1,2,4,6-tetraenes 4D, 5D, 5,6D, and 4,-
6D are formed very efficiently and rapidly on UV photolysis
of the corresponding azide and/or tetrazole precursors. The
corresponding triplet nitrenes are observable by ESR spectros-
copy both under matrix photolysis and under flash vacuum
thermolysis conditions (4N, 5N, 6N, 3N, and 3,5N). Nitrene
3,5N was also observed by IR spectroscopy; it decayed in the
course of ca. 90 min on broad-band UV irradiation with
concomitant formation of the diazacycloheptatetraene 4,6D as
observed by IR spectroscopy. The nitrene decay took place in
the same time interval when monitored under the same
photochemical conditions by ESR spectroscopy (k = 2 × 10-4
s-1). Photolysis of the mixtures of triplet nitrenes and diaza-
cycloheptatretraenes also produced very weak carbene signals
in the ESR spectra, tentatively assigned to 1,3-diazacyclohep-
tatrienylidenes.
8. Computational Results. In order to gain some insight
into the structures of the seven-membered ring carbodiimides
and the relative energies of the intermediates, ab initio calcula-
tions were carried out at the B3LYP/6-31G* level of theory.
As with the carbon analogue (1,2,4,6-cycloheptatetraene),27 the
(26) (a) It has been shown that the ESR spectrum reported by Chapman
and McMahon26b was incorrectly assigned to cycloheptatrienylidene.26c,d
Although treatment of tropone tosylhydrazone with n-BuLi, followed by
thermolysis, yields an isolable diazo compound,26b it is now clear that the
diazo compound is not diazocycloheptatriene. The material obtained by
reaction of tropone tosylhydrazone with n-BuLi does not regenerate tropone
tosylhydrazone on hydrolysis, nor does it produce an appreciable quantity
of heptafulvalene on thermolysis or photolysis.26c In contrast, the tropone
tosylhydrazone salts prepared with NaH or LiH indeed regenerate tropone
tosylhydrazone on hydrolysis and produce heptafulvalene on thermolysis
and photolysis.26c Thermolysis/matrix deposition of these Na or Li salts
directly onto the ESR cold tip, followed by photolysis, provided Kuzaj,
Lu¨erssen, and Wentrup with triplet cycloheptatrienylidene.6 (b) McMahon,
R. J.; Chapman, O. L. J. Am. Chem. Soc. 1986, 108, 1713. (c) Blanch, R.
J. Ph.D. Thesis, The University of Queensland, Brisbane, 1991. Blanch, R.
J.; Wentrup, C. To be published. (d) Chapman, O. L. Personal communica-
tion, 1989. McMahon, R. J. Personal communication, 1995.
(27) For example, see: (a) Waali, E. E. J. Am. Chem. Soc. 1981, 103,
3604. (b) Radom, L.; Schaefer, H. F. Vincent, M. A. NouV. J. Chim.
1980, 4, 411. (c) Matzinger, S.; Bally, T.; Patterson, E. V.; McMahon, R.
J. Am. Chem. Soc. 1996, 118, 1535. (d) Wong, M. W.; Wentrup, C. J. Am.
Chem. Soc. Submitted. Schreiner, P. R.; Karney, W. L.; Schleyer, P. v. R.;
Borden, W. T.; Hamilton, T. P.; Schaefer, H. F. J. Am. Chem. Soc.
Submitted.

Trifluoromethyl-2-pyridyl Azide Series
J. Am. Chem. Soc., Vol. 118, No. 17, 1996 4015
br, 1625 s, 1577 w, 1526 m, 1332 vs, 1193, 1110 s, 1077 m, 1003 w,
936 m, 822 s sh, 507 w cm-1; HRMS m/z 177.0516 (C6H6F3N3 requires
177.05138).
Experimental and Computational Section
Calculations. Density functional calculations were carried out using
the Gaussian 92/DFT series of programs.28 Geometry optimizations
were performed at the B3LYP/6-31G* level.29,30 The B3LYP formula-
tion30 of density functional theory corresponds to Becke's three-
parameter exchange functional30a in combination with the Lee-Yang-
Parr correctional functional.30b Harmonic frequencies and infrared
intensities were determined at these optimized geometries. The directly
calculated frequencies were scaled by a factor of 0.9613 to account
for the overestimation of vibrational frequencies at this level of theory.31
Improved relative energies were obtained through B3LYP/6-311+G**
calculations,29,30 based on the B3LYP/6-31G* optimized geometries.
2-Hydrazino-4-trifluoromethylpyridine: This compound was par-
ticularly unstable and decomposed even when stored at -30 °C: wax-
like white solid; yield 70%; mp 38 °C. 1H NMR (CDCl3) δ 8.24 (d,
3
3J ) 5.4 Hz, 1H, H6), 7.01 (br s, 1H, H3), 6.84 (dm, J ) 5.39 Hz,
1H, H5), 6.27 (br s, 1H, NH), 3.84 br s, 2H, NH2); 13C NMR (CDCl3)
2
δ 161.7 (C2), 148.9 (C6), 139.8 (q, J(CF) ) 33.6 Hz, C4), 123.0 (q,
3
1J(CF) ) 273.0 Hz, C7), 109.3 (q, J(CF) ) 3.7 Hz, C5), 102.6 (q,
3J(CF) ) 4.3 Hz, C3); IR (KBr) 3316 m br, 1620 m, 1575 m, 1449 m,
1337 s, 1129 vs, 1083 m, 984 w, 882 m, 816 m, 670 sh m cm-1; HRMS
m/z 177.0535 (C6H6F3N3 requires 177.05138).
General Experimental Apparatus. 1H and 13C NMR spectra were
recorded on a JEOL GX-400, mass spectra on a Kratos MS25RFA,
ESR spectra on a Bruker ER2000, UV spectra on a Varian Cary 1, and
FTIR spectra on a Perkin Elmer 1720X spectrometer, the latter with 1
cm-1 resolution. Photolyses were carried out using a Hanovia 1000
W high-pressure Xe-Hg lamp equipped with a 150 mm water filter
and a monochromator or cutoff filters if required. Matrix isolation
used Ar (99.999%) and an Air Products Displex DE202 cryostat for
ESR or IR spectroscopy. A Leybold-Heraeus ROK 10-300 cryostat
was also used for IR spectroscopy. FVP equipment was as previously
described.32 The 1,3-diazacycloheptatetraenes were stable enough for
direct isolaltion at 77 K, without any matrix host, and a liquid N2 cooled
cryostat equipped with BaF2 windows was used for this purpose. The
inner targets for matrix deposition were BaF2 for all IR spectra and
electrolytically refined Cu for ESR spectroscopy. Ar matrices were
deposited at temperatures between 12 and 23 K, and spectra were
recorded at the lowest accessible temperature (10-14 K).
General Procedure for the Synthesis of 2-Hydrazinopyridines.
The appropriate mono- or bis(trifluoromethyl)-2-chloropyridine (Ishi-
hara Sangyo Kaisha, Tokyo, or Maybridge, UK) (ca. 0.08 mol) was
added to a large excess of hydrazine hydrate (Ajax Chemicals) (ca. 70
mL), and the mixture was gently heated. Enough ethanol (40 mL)
was added to make the solution homogeneous and the solution was
gently refluxed (2-3 h) until TLC analysis (silica gel/chloroform)
showed none of the starting 2-chloropyridine remained. When the
reaction was complete, the heat was removed, and as much water and
excess hydrazine was removed in vacuo as possible. The resultant
slurry was cooled and extracted with ether (3 × 150 mL). The
combined ether extracts were dried (MgSO4) and evaporated to give a
solid, the crude 2-hydrazinopyridine. The 2-hydrazinopyridines were
purified by gentle sublimation (ca. 40 °C, 10-4 mbar). They were all
unstable compounds and readily discolored in a few hours after
purification. Refrigeration at -20 °C slowed decomposition/decol-
oration but did not prevent it.
2-Hydrazino-6-trifluoromethylpyridine: wax-like white solid;
yield 65%; mp 53 °C. 1H NMR (CDCl3) δ 7.56 (apparent t, J ) ca.
7-8 Hz, 1H, H4), 6.98 (d, 3J ) 7.0 Hz, 1H, H5), 6.90 (d, 3J ) 8.6 Hz,
1H, H3), 6.40 br s, 1H, NH), 3.86 br s, NH2); 13C NMR (CDCl3) δ
2
161.2 (C2), 146.0 (q, J(CF) ) 31.6 Hz, C6), 138.2 (C4), 121.6 (q,
3
1J(CF) ) 273.6 Hz, C7), 110.4 (q, J(CF) ) 3.4 Hz, C5), 110.4 (q,
3J(CF) ) 1.3 Hz, C3); IR (KBr) 3377 m sh, 3275 m, 1648 m, 1608 m,
1470 m, 1363 s, 1277 s, 1190 s, 1130 vs, 1099 s, 1073 m, 986 m, 932
w, 886 w, 804 s cm-1
177.05138).
. HRMS m/z 177.0514 (C6H6F3N3 requires
2-Hydrazino-3-trifluoromethylpyridine: wax-like white solid;
3
yield 64%; mp 71-73 °C. 1H NMR (CDCl3) δ 8.32 (br d, J ) 4.8
3
3
Hz, 1H, H6), 7.58 (dm, J ) 7.7 Hz, 1H, H4), 6.61 (ddm, J (H5,H4)
3
) 7.5 Hz, J (H5,H6) ) 4.8 Hz, 1H, H5), 6.51 (br s, 1H, NH), 4.03
(br s, 2H, NH2); 13C NMR (CDCl3) δ 156.3 (q, 3J (CF) ) 1.4 Hz, C2),
151.1 (C6), 134.9 (q, 3J (CF) ) 4.7 Hz, C4), 123.9 (q, 1J (CF) ) 271.6
Hz, C7), 112.5 (C5), 108.4 (q, 2J (CF) ) 32.3 Hz, C3); IR (KBr) 3303
br m, 1613 m, 1583 m, 1508 m, 1471 s, 1435 m, 1309 s, 1151 m,
1110 vs, 1030 s, 767 m, 488 w cm-1; HRMS m/z 177.0516 (C6H6F3N3
requires 177.05138).
2-Hydrazino-4,6-bis(trifluoromethyl)pyridine: wax-like white
solid; yield 80%; mp 79-80 °C. 1H NMR (CDCl3) δ 7.34 (s, 1H,
H5), 7.31 (br s, 1H, NH), 7.15 (s, 1H, H3), 3.95 (br s, 2H, NH2); 13C
2
NMR (CDCl3, H coupled) δ 162.0 (d, J (CH) ) 6.0 Hz, C2); 147.5
2
2
(q, J (CF) ) 35.0 Hz, C6), 141.0 (q, J (CF) ) 34.3 Hz, C4); 122.4
1
3
1
(qt, J(CF) ) 272.9 Hz, J (CH) ) 4.0 Hz, C7), 121.0 (qd J (CF) )
3
1
3
273.6 Hz, J (CH) ) 3.0 Hz, C8), 106.4 (dq, J (CH) ) 168.7 Hz, J
(CF) ) 3.4 Hz, C3), 105.6 (d of sept, 1J (CH) ) 172.1 Hz, 3J (CF) )
3.4 Hz, C5); IR (KBr) 3265 br m, 1660 m, 1587 w, 1465 m, 1396 m,
1297 m, 1278 vs, 1185 vs, 1149 s, 1112 s, 952 m, 931 m, 896 m, 856
m, 846 m, 685 s cm-1; HRMS m/z 245.0389 (C7H5F6N3 requires
245.03877).
General Procedure for the Synthesis of Tetrazolo[1,5-a]pyridines/
2-Azidopyridines from 2-Hydrazinopyridines. The appropriate
mono- or bis(trifluoromethyl)-2-hydrazinopyridine (ca. 0.034 mol) was
added to 20 mL of water, and enough concentrated hydrochloric acid
(ca. 5 mL) was added to give the solution of pH of ca. 1. The solution
was then cooled to 0-5 °C and a precooled solution of 2.5 g (0.036
mol) of sodium nitrite in 10 mL of water was added dropwise. Mild
frothing generally occurred on the addition of each drop.
Caution: The 2-hydrazinopyridines have significant vapor pressure
at atmospheric pressure and may be toxic. Inhalation of their vapor
may cause nausea for some hours. 2-Hydrazino-4-trifluoromethylpy-
ridine was particularly potent.
2-Hydrazino-5-trifluoromethylpyridine: wax-like white solid; yield
88%; mp 62-63 °C. 1H NMR (CDCl3) δ 8.32 (br s, 1H, H(C-6), 7.59
The tetrazoles 5T, 4T, and 3T precipitated from the solutions as
formed. The precipitate was collected, dried, and readily purified by
sublimation.
3
4
(dd, J ) 8.9 Hz, J ) 2.4 Hz, 1H, H(C-3/4)), 6.95 (br s, 1H, NH),
6.74 (d, J ) 8.9 Hz, 1H, H(C-4/3)), 3.79 (br s, 2H, NH2); 13C NMR
3
1
(CDCl3, H coupled), δ 162.9 (br s, C2), 145.5 (dq, J(CH) ) 181.0
The 2-azidopyridines 6A and 4,6A separated as oils. After the
complete addition of the sodium nitrite solution, the reaction mixture
was extracted with ether (3 × 20 mL). The extracts were dried and
evaporated to give the crude 2-azidopyridines. These were purified
by vacuum distillation using a Bu¨chi Kugelrohr apparatus.
6-Trifluoromethyltetrazolo[1,5-a]pyridine (5T)/(5A): white solid;
purified by sublimation (90 °C (melt), 0.05 mbar); yield 61%, mp 67
3
1
3
Hz, J (CF) ) 4.0 Hz, C6), 134.5 (dq, J(CH) ) 166.0 Hz, J(CF) )
1
2
3.4 Hz, C4), 124.4 (q, J(CF) ) 270.2 Hz, C7), 116.6 (q, J(CF) )
32.9 Hz, C5), 105.9 (d, J(CH) ) 168.1 Hz, C3); IR (KBr) 3303 m,
1
(28) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio
Molecular Orbital Theory; Wiley: New York, 1986.
(29) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.;
Johnson, B. G.; Wong, M. W.; Foresman, J. B.; Robb, M. A.; Head-Gordon,
M.; Replogle, E. S.; Gomperts, R.; Andres, J. L.; Raghavachari, K.; Binkley,
J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.; DeFrees, D. J.; Baker, J.;
Stewart, J. J. P.; Pople, J. A. GAUSSIAN 92/DFT, Gaussian Inc.:
Pittsburgh, PA, 1993.
(30) (a) A. D. Becke, J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang,
W.; Parr, R. G. Phys. ReV. B 1988, 37, 785.
(31) Wong. M. W. Chem. Phys. Lett. Submitted.
(32) Kappe, C. O.; Wong, M. W.; Wentrup, C. J. Org. Chem. 1995, 60,
1686.
3
°C. 1H NMR (CDCl3) δ 9.29 (br s, 1H, H5), 8.25 (apparent d, J
(H8,H7) ) 10.2 Hz, 1H, H8), 7.90 (dd, 3J (H7,H8) ) 10.0 Hz, 4J
1
(H7,H5) ) 1.9 Hz, 1H, H7); H NMR (TFA-d), tetrazole isomer 5T:
3
3
δ 9.26 (1H, H5), 8.32 (d, J (H8,H7) ) 8.72 Hz, 1H, H8), 8.07 (d, J
(H7,H8) ) 8.72 Hz, 1H, H7); azide isomer 5A: δ 8.63 (s, 1H, H6),
3
3
8.56 (d J (HH) ) 8 Hz, 1H), 7.84 (d, J (HH) ) 8 Hz, 1H); ratio
tetrazole:azide 5T:5A (22 °C) ) 4.9:1. 13C NMR (CDCl3) δ 148.8
3
3
(C8a), 128.3 (q, J (CF) ) 24 Hz, C7), 124.7 (q, J (CF) ) 5.5 Hz,
C5), 122.1 (q, 1J (CF) ) 272.2 Hz, C9), 117.2 (C8). Selected irradiation

4016 J. Am. Chem. Soc., Vol. 118, No. 17, 1996
EVans et al.
of H7 (δ 7.90) in the proton coupled 13C NMR spectrum revealed (i)
2-Azido-4,6-bis(trifluoromethyl)pyridine (4,6A)/(4,6T): clear oil;
1
3
1
1
no J(CH) coupling for J(CF) quartet at δ 128.8 (C7); (ii) J(CH)
coupling for 3J(CF) quartet at δ 124.7 (C5), and (iii) 1J(CH) coupling
for singlet at 117.2 (C8); 13C NMR (TFA-d), tetrazole isomer 5T: δ
purified by Kugelrohr distillation (50 °C, 0.15 mbar); yield 49%; H
NMR (CDCl3) δ 7.60 (s, 1H, H5), 7.20 (s, 1H, H3); 1H NMR (DMSO-
d6), tetrazole isomer 4,6T: δ 9.29 (s, 1H, H8), 8.40 (s, 1H, H6); azide
isomer 4,6A: δ 8.00 (s, 1H, H5), 7.70 (s, 1H, H3), ratio tetrazole:
azide 4,6T: 4,6A (22 °C) 1:9.8; 13C NMR (CDCl3), azide isomer
3
150.5 (C8a), 135.5 br s, C7), 128.7 (q, J (CF) ) 5.4 Hz, C5), 127.4
(q, 2J (CF) ) 37.0 Hz, C6), 124.9 (q, 1J (CF) ) 272.2 Hz, C9), 117.2
3
2
2
(C8); azide isomer 5A: δ 158.6 (s, C2), 148.3 (q J (CF) ) 2.7 Hz,
4,6A: δ 156.9 (C2); 149.0 (q, J (CF) ) 36.3 Hz, C6), 142.7 (q, J
C6), 141.2 (br s, C4?), 121.4 (s, C3), other C not observed above
baseline noise; IR (KBr) 3091 w, 3034 m, 1650 m, 1552 w, 1333 vs,
1293 m, 1186 s, 1169 s, 1150 s, 839 m, 683 m, 583 m cm-1; mass
spectrum (E.I.) m/z 188 (43%, M•+), 160 (100, M•+ -N2), 141 (32),
140 (41), 113 (10), 91 (14), 69 (41), 64 (32); HRMS m/z 188.0313.
(C6H3F3N4 requires 188.03098). Anal. Calc. for C6H3F3N4: C, 38.30;
H, 1.60; N, 29.80. Found: C, 38.08; H, 1.51; N, 30.19.
(CF) ) 35.0 Hz, C4), 121.8 (q, 1J (CF) ) 273.6 Hz, C8), 120.4 (q, 1J
3
(CF) ) 274.9 Hz, C7), 113.4 (q, J (CF) ) 3.0 Hz, C3), 112.3 (sept.,
3J (CF) ) 3.4 Hz, C5); selected irradiation of H3 (δ 7.25) in proton
coupled 13C NMR spectrum revealed (i) no 1J(CH) coupling for 3J(CF)
1
3
quartet at δ 113.4 (C3), and (ii) J(CH) coupling for J(CF) septet at
112.3 (C5). 13C NMR (DMSO-d6), tetrazole isomer 4,6T: δ 149.0
(C8a), 132.2 (q, 2J (CF) ) 35.6 Hz, C7), 126.3 (q, 2J (CF) ) 39.7 Hz,
C5), 122.0 (q, 1J (CF) ) 272.9 Hz, C9), 121.9 (q, 1J (CF) ) 258.1 Hz,
C10), 119.8 (q, 3J (CF) ) 4.7 Hz, C8), 113.9 (br s, C6); azide isomer
7-Trifluoromethyltetrazolo[1,5-a]pyridine (4T): white solid; puri-
fied by sublimation (50 °C, 0.05 mbar); yield 83%; mp 56 °C. 1H
2
2
3
4,6A: δ 156.1 (C2), 147.2 (q, J (CF) ) 35.6 Hz, C6), 141.5 (q, J
NMR (CDCl3) δ 9.07 (d, J (H5,H6) ) 7.3 Hz, 1H, H5), 8.41 (br s,
1
1
3
4
(CF) ) 34.3 Hz, C4), 121.7 (q, J (CF) ) 273.6 Hz, C8) (q, J (CF)
1H, H8), 7.49 (dd, J (H6,H5) ) 7.3 Hz, J (H6,H8) ) 1.6 Hz, 1H,
H6); 13C NMR (CDCl3) δ 147.7 (s, C8a), 134.0 (q, 2J (CF) ) 35.4 Hz,
C7), 127.0 (C5), 121.8 (q, 1J (CF) ) 273.4 Hz, C9), 114.1 (q, 3J (CF)
) 4.9 Hz, C8), 112.7 (q, 3J (CF) ) 2.7 Hz, C5); selected irradiation of
3
) 274.3 Hz, C7), 114.7 (q, J (CF) ) 3.0 Hz, C3), 113.1 (br s, C5);
selected irradiation of H8 (δ 9.29) of the tetrzole isomer 4,6T in proton
1
3
coupled 13C NMR spectrum revealed no J (CH) coupling for the J
(CF) coupled quartet at δ 119.8, thus confirming 1H NMR assignments.
IR (KBr) 2187 w and 2142 s, 1624 m, 1585 w, 1440 m, 1380 vs, 1286
s, 1274 s, 1274 s, 1149 s, 1096 m, 989 m, 930 m, 885 m, 845 m, 713
m, 686 m, sh cm-1; IR (CHCl3) 2187 m, 2141 vs, 1623 m, 1585 m,
1450 m, 1439 m, 1417 m, 1383 vs, 1275 vs, 1096 m, 1047 m, 989 m,
930 s, 885 s, 848 m cm-1. Sample was too volatile to allow standard
E.I. mass spectrum to be obtained. GC-mass spectrum (E.I.) m/z 256
(8, M•+), 228 (27, M•+ - N2), 209 (4), 189 (17), 159 (8), 69 (100).
General Procedure for Synthesis of the Bis(trifluoromethyl)-
tetrazolo[1,5-a]pyridines/2-Azidopyridines. The appropriate 2-chlo-
robis(trifluoromethyl)pyridine (ca 9.2 g; 0.037 mol) was dissolved in
36 mL of DMF (AR grade, Ajax Chemicals) and 2 × 3 g (total 0.09
mol) of sodium azide was added slowly over 1 h. The mixture was
stirred overnight at 70 °C. The solution was then cooled and slowly
added, with vigorous stirring, to 150 mL of ice-cold water. The
resultant precipitate was collected and washed thoroughly with water,
air dried, and further purified by sublimation.
1
H6 (δ 7.49) in the proton coupled 13C NMR spectrum revealed (i) J
(CH) coupled for singlet at δ 127.0 (C5), (ii) 1J(CH) coupling for 3J(CF)
1
quartet at δ 114.1 (C8), and (iii) no J(CH) coupling for quartet at δ
112.7 (C6); IR (KBr) 3091 m, 3034 m, 1650 m, 1551 w, 1333 vs,
1186 vs, 1094 m, 1055 m, 839 m, 683 m, 583 m, 456 m cm-1; mass
spectrum (E.I.) m/z 188 (48%, M•+), 169 (6, M•+-F), 160 (100, M•+
-
N2), 141 (18, M•+ -N2-F), 140 (26), 133 (8), 91 (13), 69 (32), 64 (28);
HRMS m/z 188.0318 (C6H3F3N4 requires 188.03098). Anal. Calc for
C6H3F3N4: C, 38.30; H, 1.60; N, 29.80. Found: C, 37.98; H, 1.44; N,
30.13.
2-Azido-6-trifluoromethylpyridine (6A)/(6T): clear oil; purified
by Kugelrohr distillation (50 °C, 0.01 mbar); yield 31%; 1H NMR
(CDCl3) δ 7.8 (apparent t, 3J(HH) ) 7.9 Hz, 1H, H4), 7.42 (d, 3J
(H5,H4) ) 7.3 Hz, 1H, H5), 6.98 (d, 3J (H3,H4) ) 7.9 Hz, 1H,
1
3
H3); H NMR (DMSO-d6), tetrazole isomer 6T; δ 8.57 (dd, J (H8,-
H7) ) 8.9 Hz, 4J (H8,H6) ) 0.6 Hz, H8), 8.05 (m, obscured by azide
isomer, H7?), 7.99 (d?, obscured by azide isomer, H6); azide isomer
6A: δ 8.05 (apparent t, obscured by tetrazole isomer, H4), 7.65 (d, 3J
6,8-Bis(trifluoromethyl)tetrazolo[1,5-a]pyridine (3,5T)/(3,5A): white
solid, purified by sublimation (100 °C (melt), 0.1 mbar); yield 48%;
3
(H5,H4) ) 7.6 Hz, 1H, H5), 7.24 (d, J (H3,H4) ) 8.3 Hz, 1H, H3);
1
ratio tetrazole: azide 6T:6A (22 °C) ) 1:2.3; 13C NMR (CDCl3), azide
mp 97-98 °C; H NMR (CDCl3), tetrazole isomer 3,5T: δ 9.40 (s,
2
1H, H5), 8.13 (s, 1H, H7); azide isomer 3,5A: δ 8.71 (s, 1H, H6),
8.08 (s, 1H, H4) ratio tetrazole: azide (22 °C) 3.2:1.
isomer 6A: δ 155.3 (C2), 147.4 (q, J (CF) ) 35.0 Hz, C6), 139.8
(C4), 121.0 (1J (CF) ) 273.6 Hz, C7), 117.1 (C3), 116.4 (q, 3J (CF) )
3.4 Hz, C5); selected irradiation of H3 (δ 6.98) in proton coupled
13NMR spectrum revealed (i) 1J (CH) coupling for singlet (C4), (ii) no
13C NMR (CDCl3) tetrazole isomer 3,5T: δ 145.5 (C8a), 127.9 (q,
1
3J (CF) ) 5.4 Hz, C5); 127.0 (m, C7), 121.6 (q, J (CF) ) 272.9 Hz,
1J (CH) for singlet at δ 117:1 (C3), and (iii) J (CH) coupling for J
1
3
1
C10/C9), 120.7 (q, J (CF) ) 273.6 Hz, C9/C10), 120.6 (2J (CF) )
(CF) quartet at δ 116.4 (C5). 13C NMR (DMSO-d6), tetrazole isomer
37.7 Hz, C8/C6), 120.5 (2J (CF) ) 37.7 Hz, C6/C8); azide isomer
2
2
6T: δ 149.0 (C8a), 132.5 (C7), 124.6 (q, J (CF) ) 39.0 Hz, C5),
3,5A: δ 155.8 (C2), 149.0 (q, J (CF) ) 4.0 Hz, C6), 133.8 (C4),
1
3
1
2
120.2 (C8), 119.4 (q, J (CF) ) 272.9 Hz, C9), 117.9 (q, J (CF) )
122.8 (q, J (CF) ) 271.6 Hz, C7/C8), 122.6 (q, J (CF) ) 35.0 Hz,
4.7 Hz, C6); azide isomer 6A: δ 154.3 (C2), 145.6 (q, 2J (CF) ) 35.0
1
2
C5), 121.4 (q, J (CF) ) 271.0 Hz, C8/C7), 115.9 (q, J (CF) ) 4.3
Hz, C3); IR (KBr) 3109 m, 3073 w, 1657 m, 1591 m, 1386 m, 1360
m, 1339 m , 1279 vs, 1139 s, 1074 s, 974 m, 898 m, 779 m, 733 m,
704 m, 657 m cm-1; IR (CHCl3) 2145 m, 1655 m, 1618 w, 1583 m,
1513 m, 1470 m, 1425 w, 1382 w, 1361 m, 1350 m, 1333 s, 1304 s,
1282 vs, 1161 vs, 1123 m, 1095 m, 1098 m, 1053 m, 970 m, 924 m,
1
Hz, C6), 141.5 (s, C4), 121.0 (q, J (CF) ) 274.1 Hz, C7), 117.9 C3;
117.2 (q, J (CF) ) 2.7 Hz, C5); selected irradiation of H8 (δ 8.57,
3
tetrazole isomer) in coupled 13C NMR spectrum revealed (omitting azide
isomer) (i) 1J (CH) coupling for singlet at δ 132.5 (C7), (ii) no 1J (CH)
for singlet at δ 120.2 (C8), and (iii) 1J (CH) coupling for 3J (CF) quartet
at 117.9 (C6); IR (CHCl3) 2135 vs, 1603 m, 1459 m, 1347 m, 1288 s,
1216 vs, 1193 m, 1150 m, 813 w cm-1; IR (KBr) 2198 w, 2133 vs,
1604 m cm-1; mass spectrum (E.I.) m/z 188 (38%, M•+), 160 (100,
M•+ - N2), 141 (21, M•+ - N2 - F), 140 (20), 131 (13), 113 (11), 69
(100); HRMS m/z 188.0309 (C6H3F3N4 requires 188.03098).
886 m cm-1; mass spectrum (E.I.) m/z 256 (16, M•+), 228 (43, M•+
-
N2), 209 (16), 189 (11), 69 (100); HRMS m/z 256.0180 (C7H2F6N4
requires 256.01837).
6,7-Bis(trifluoromethyl)tetrazolo[1,5-a]pyridine (4,5T)/(4,5A): white
solid; purified by Kugelrohr distillation (75-80 °C, 0.1 mbar); yield
56%; mp 46-47 °C; 1H NMR (CDCl3), tetrazole isomer (4,5T); δ 9.40
(s, 1H, H5), 8.62 (s, 1H, H8); azide isomer (4,5A): δ 8.80 (s, 1H,
H6), 7.18 (s, 1H, H3); ratio tetrazole: azide 4,5T:4,5A (22 °C) ) 2.2:
1; 13C NMR (CDCl3), tetrazole isomer (4,5T) δ 147.8 (C8a), 131.0 (q,
2J (CF) ) 36.3 Hz, C7), 128.0 (q, 3J (CF) ) 8.1 Hz, C5), 121.3 (q, 1J
(CF) ) 274.6 Hz, C9), 121.0 (q, 1J (CF) ) 274.3 Hz, C10), 117.7 (q,
2J (CF) ) 36.0 Hz, C6), 117.5 (3J (CF) ) 7.1 Hz, C8); selective
decoupling of H5 (δ 9.40) of the tetrazole isomer in the proton coupled
8-Trifluoromethyltetrazolo[1,5-a]pyridine (3T): white solid; puri-
fied by sublimation (90 °C, 0.05 mbar); yield 85%; mp 99 °C; 1H
NMR (CDCl3) δ 9.10 (d, 3J (H5,H6) ) 6.6 Hz, 1H, H5), 8.10 (dm, 3J
3
(H7,H6) ) 7.3 Hz, 1H, H7), 7.47 (apparent t, J (HH) ) 7.0 Hz, 1H,
3
H6); 13C NMR (CDCl3) δ 145.2 (C8a), 130.6 (q, J (CF) ) 4.7 Hz,
C7), 128.8 (C5), 121.4 (q, 1J (CF) ) 272.2 Hz, C9), 119.0 (q, 2J (CF)
) 37.0 Hz, C8), 115.6 (C6); IR (KBr) 3129 m, 3057 m, 1634 m, 1583
m, 1496 m, 1428 m, 1364 s, 1321 s, 1249 m, 1239 m, 1187 m, 1164
m, 1139 vs, 1096 s, 1047 m, 1002 m, 990 m, 984 m, 959 m, 809 w
cm-1; mass spectrum (E.I.) m/z 188 (55%, M•+), 160 (100, M•+ - N2),
141 (17, M•+ - N2 - F), 140 (18), 113 (7), 91 (7), 69 (41); HRMS
m/z 188.0310 (C6H3F3N4 requires 188.03098).
1
3
13C NMR spectrum revealed (i) no J (CH) coupling for the J (CF)
coupled quartet at δ 128.0 (C5), and (ii) 1J (CH) coupling for 3J (CF)
coupled quartet at δ 117.5 (C8); azide isomer (4,5A): δ 159.7 (C2),
3
2
148.9 (q, J (CF) ) 6.7 Hz, C6), 138.6 (qm, J (CF) ) 35.6 Hz, C4),

Trifluoromethyl-2-pyridyl Azide Series
J. Am. Chem. Soc., Vol. 118, No. 17, 1996 4017
1
122.3 (1J (CF) ) 273.0 Hz, C8), 121.1 (q, J (CF) ) 274.6 Hz, C7),
Complete conversion to 4D was achieved in 5 min of broad-band
irradiation. The formation of byproducts is described in section 3.
4,5T had IR (Ar, 20 K): 2138 m (νazide), 1393 m, 1335 s, 1321 m,
1391 s, 1281 m, 1273 m, 1229 m, 1205 m, 1192 vs, 1185 m, 1170 s,
1154 w, 1141 w, 1101 w, 1091 w, 975 w, 898 w cm-1. It was converted
completely to 5,6D during 3 min of broad-band irradiation, as described
in section 4.
4,6A had IR (Ar, 15 K): 2146 s, 1441 w, 1420 w, 1389 vs, 1362
m, 1290 m, 1277 vs, 1216 m, 1203 m, 1186 m, 1173 s, 1165 m, 1159
m, 1132 m, 933 m, 882 m, 758 cm-1. It was converted completely to
4,6D in 6 min of irradiation. An additional 90 min of irradiation
appeared to cause ca. 30% destruction of 4,6D with no specific new
absorptions appearing.
3,5T underwent significant ring opening to the azide (3,5A) during
deposition. IR (Ar, 18 K): 2148 s, 1623 m, 1592 m, 1475 m, 1427 m,
1353 m, 1306 vs, 1279 vs, 1268 m, 1248 w, 1171 m, 1166 m, 1154 m,
1097 w, 1054 m, 938 w, 781 w cm-1. Five minutes of broad-band
irradiation caused complete conversion of the precursor to carbodiimide
4,6D together with an intermediate B as described in section 5. B had
IR (Ar, 11 K): 1337 m, 1276 s, 1208 m, 1182 s, 1159 m, 1075 m,
1029 m cm-1. The bands due to species B disappeared during a further
80 min of irradiation, with concomitant increase in the peaks due to
carbodiimide 4,6D. Unknown decomposition products (Ì) were formed
during this time as well: 2065 w, 1271 m, 1199 m, 1176 w, 1161 m
cm-1. The carbodiimide 4,6D had IR (Ar, 18 K): 2000 s, 1280 s,
1200 s, 1190 w, 1186 m, 1171 s, 1151 m, 1041 m, 985 w cm-1. After
a total of 16 h of irradiation, complete destruction of all the species
had occurred, with only weak bands in the νCF region of the IR spectrum
remaining (1285, 1213, 1181 cm-1).
2
3
118.6 (qm, J (CF) ) 34.0 Hz, C5), 112.0 (q, J (CF) ) 6.1 Hz, C3);
selective decoupling of H3 (δ 7.18) of the azide isomer in the proton
coupled 13C NMR spectrum revealed (i) J (CH) coupling for the J
1
3
(CF) coupled quartet at δ 148.9 (C6), and (ii) no 1J (CH) coupling for
3
the J (CF) coupled quartet at δ 112.0; IR (KBr) 3083 w, 1661 w,
1540 w, 1459 m, 1335 s, 1317 s, 1276 s, 1264 s, 1170 vs, 1130 m,
1095 m, 1069 w, 996 m, 981 m, 889 m, 759 m, 715 m, 632 m, 625 w
cm-1; IR (CHCl3), 2135 s (νazide), 1657 m, 1611 m, 1559 w, 1535 w,
1457 m, 1387 s, 1316 s, 1274 s, 1093 m, 1028 m, 978 m, 895 m cm-1
;
mass spectrum (EI.) m/z 256 (46, M•+), 228 (100, M•+ - N2), 209
(48), 189 (17), 159 (25), 139 (26), 69 (40); HRMS m/z 256.0184
(C7H2F6N4 requires 256.01837). Anal. Calcd for C7H2F6N4 C, 32.80;
H, 0.78; N, 21.80. Found C, 32.21; H, 0.82; N, 21.81.
Matrix Isolation. Solid samples (ca 10 mg) were sublimed at ca.
40-50 °C from a Knudsen cell and deposited with Ar at 20 K and ca.
10-4 mbar in the course of ca. 50 min. Liquids were admitted through
a needle valve.
5T containing a smaller amount of 5A had the following IR (Ar, 15
K): 2155 m, 2118 m (νazide), 1659 m, 1605 m, 1689 m, 1346 s, 1334
s, 1289 s, 1187 s, 1170 m, 1158 vs (νCF) s, 1147 w, 1087 m, 1079 m,
821 w cm-1
.
Six minutes of broadband irradiation caused complete conversion
of the precursor to give exclusively 5D as described in section 2.
Thirty-six minutes of irradiation caused ca. 40% destruction of 5D to
unknown products. No significant new peak appeared except a band
at 2066 cm-1
.
4T underwent ring opening to a very small amount of 4A (2135
cm-1) under analogous conditions. 4T, IR (Ar, 18 K): 1498 m, 1379
m, 1346 s, 1340 m, 1288 s, 1262 m, 1234 w, 1194 vs, 1656 vvs (νCF),
Repetition of this experiment with monochromatic light (267 nm
which is λmax of 3,5T in EtOH) had the same outcome, but the reaction
was slower.
1122 w, 1077 w, 1051 w, 989 w, 941 w, 891 w cm-1
.
Twenty minutes broad-band irradiation caused complete and exclu-
sive conversion to 5D as described in section 2. An additional 90 min
of irradiation caused almost complete destruction of 5D, with only a
weak band at 2066 and a broad feature at 1180 cm-1 appearing.
The use of monochromatic irradiation (λ ) 268 nm, which is the
Acknowledgment. This research was supported by the
Australian Research Council. We thank Mr. I. A. Davis for
exploratory studies, Dr. P. Kambouris and Dr. T. Mosandl for
help with some experiments, and Dr. R. J. Blanch, Dr. J.
Andraos, and Mr. G. G. Qiao for technical assistance.
λ
max of 4T in EtOH) gave the same result, with formation of 5D from
4T in 20 min. No intermediate was observed.
6A was matrix isolated in 25 min and had IR (Ar, 15 K): 2137 vvs,
1606 m, 1578 w, 1461 m, 1443 w, 1434 m, 1351 s, 1342 w, 1291 s,
1262 w, 1195 m, 1169 vs, 1160 m, 1149 w, 1121 m, 1079 w, 1033 w,
992 w, 916 w, 863 m cm-1. It was converted fully to 4D during 12
min of broadband irradiation, as described section 3. 4D underwent
ca. 30% decomposition in the course of an additional 16 min of
Supporting Information Available: Calculated structures
(BLYP/6-31G*) of 4D, 5D, 4,6D, and 5,6D (1 page). This
material is contained in many libraries on microfiche, im-
mediately follows this article in the microfilm version of the
journal, can be ordered from the ACS, and can be downloaded
from the Internet; see any current masthead page for ordering
information and Internet access instructions.
irradiation, causing development of a weak band at 2065 cm-1
.
3T had IR (Ar, 15 K): 2163 m, 2157 m, 2136 m (νazide), 1624 m,
1443 w, 1435 s, 1368 m, 1334 m, 1310 s, 1299 m, 1231 m, 1180 m,
1173 vs, 1160 s, 1150 m, 1123 m, 1087 m, 1037 m, 953 m cm-1
.
JA9541645