3-pyridazinylnitrenes and 2-pyrimidinylnitrenes

Article abstract of DOI:10.1021/jo4027897

Mild flash vacuum thermolysis of tetrazolo[1,5-b]pyridazines 8T generates small amounts of 3-azidopyridazines 8A (8aA, IR 2145, 2118 cm^-1; 8bA, 2142 cm^-1). Photolysis of the tetrazoles/azides 8T/8A in Ar matrix generates 3-pyridazinylnitrenes 9, detected by ESR spectroscopy (9a: D/hc = 1.006; E/hc = 0.003 cm^-1). Cyanovinylcarbenes 11, derived from 4-diazobut-2-enenitriles 10, are also detected by ESR spectroscopy (11a: D/hc = 0.362; E/hc = 0.021 cm^-1). Carbenes 11 rearrange to cyanoallenes 12 and 3-cyanocyclopropenes 13. Triazacycloheptatetraenes 20 were not observed in the photolyses of 8. Photolysis of tetrazolo[1,5-a]pyrimidines/2- azidopyridmidines 18T/18A in Ar matrices at 254 nm yields 2-pyrimidinylnitrenes 19, observable by ESR, UV, and IR spectroscopy (19a: ESR: D/hc = 1.217; E/hc = 0.0052 cm^-1). Excellent agreement with the calculated IR spectrum identifies the 1,2,4-triazacyclohepta-1,2,4,6-tetraenes 20 (20a, 1969 cm ^-1; 20b, 1979 cm^-1). Compounds 20 undergo photochemical ring-opening to 1-isocyano-3-diazopropenes 23. Further irradiation also causes Type II ring-opening of pyrimidinylnitrenes 19 to 2-(cyanimino)vinylnitrenes 21 (21a: D/hc = 0.875; E/hc = 0.00 cm^-1), isomerization to cyaniminoketenimine 25 (2044 cm^-1), and cyclization to 1-cyanopyrazoles 22. The reaction mechanisms are discussed and supported by DFT calculations on key intermediates and pathways. There is no evidence for the interconversion of 3-pyridazinylnitrenes 9 and 2-pyrimidinylnitrenes 19.

Full text of DOI:10.1021/jo4027897

Article
pubs.acs.org/joc
3‑Pyridazinylnitrenes and 2‑Pyrimidinylnitrenes
Sebastian Torker, David Kvaskoff, and Curt Wentrup*
School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane, Queensland 4072, Australia
S
* Supporting Information
ABSTRACT: Mild flash vacuum thermolysis of tetrazolo[1,5-b]pyridazines
8T generates small amounts of 3-azidopyridazines 8A (8aA, IR 2145,
2118 cm−1; 8bA, 2142 cm−1). Photolysis of the tetrazoles/azides 8T/8A in
Ar matrix generates 3-pyridazinylnitrenes 9, detected by ESR spectroscopy
(9a: D/hc = 1.006; E/hc = 0.003 cm−1). Cyanovinylcarbenes 11, derived
from 4-diazobut-2-enenitriles 10, are also detected by ESR spectroscopy
(11a: D/hc = 0.362; E/hc = 0.021 cm−1). Carbenes 11 rearrange to cyano-
allenes 12 and 3-cyanocyclopropenes 13. Triazacycloheptatetraenes 20 were
not observed in the photolyses of 8. Photolysis of tetrazolo[1,5-a]pyrimidines/
2-azidopyridmidines 18T/18A in Ar matrices at 254 nm yields 2-pyrimidinylnitrenes 19, observable by ESR, UV, and IR
spectroscopy (19a: ESR: D/hc = 1.217; E/hc = 0.0052 cm−1). Excellent agreement with the calculated IR spectrum identifies the
1,2,4-triazacyclohepta-1,2,4,6-tetraenes 20 (20a, 1969 cm−1; 20b, 1979 cm−1). Compounds 20 undergo photochemical ring-
opening to 1-isocyano-3-diazopropenes 23. Further irradiation also causes Type II ring-opening of pyrimidinylnitrenes 19 to 2-
(cyanimino)vinylnitrenes 21 (21a: D/hc = 0.875; E/hc = 0.00 cm−1), isomerization to cyaniminoketenimine 25 (2044 cm−1),
and cyclization to 1-cyanopyrazoles 22. The reaction mechanisms are discussed and supported by DFT calculations on key inter-
mediates and pathways. There is no evidence for the interconversion of 3-pyridazinylnitrenes 9 and 2-pyrimidinylnitrenes 19.
INTRODUCTION
RESULTS AND DISCUSSION
1. 3-Pyridazinylnitrenes. Tetrazolopyridazines exist ex-
■
■
In recent papers, we have demonstrated the occurrence of two
types of ring-opening reactions of (hetero)aromatic nitrenes
and carbenes: Type I causing ring-opening to nitrile ylides,
exemplified by 3-pyridylcarbene 1a,1 3-pyridylnitrene 1b,1 and
2-pyrazinylnitrene 1c2 (Scheme 1); and Type II causing ring-
opening to diradicals, nitrenes, or carbenes, exemplified by the
formation of vinylnitrene 6 and ketenimine 7 on photolysis,
and glutacononitrile on flash vacuum thermolysis (FVT) of
2-pyridylnitrene 4 and its isomer 3-pyridazinylcarbene, as well
as similar reactions of 1-isoquinolylnitrene and 2-quinoxalinyl-
nitrene (Scheme 1).1
clusively in the tetrazole forms 8T (8aT, R = H; 8bT, R = CH3,
Scheme 2) in the solid state at room temperature. Co-
deposition of the unsubstituted 8a with Ar at 25 K also pro-
duced largely the unchanged tetrazole 8aT, but a small amount
of the azide valence tautomer 8aA (2145, 2118, 1585, 1431,
1293 cm−1) is formed on gentle FVT at 250 °C (Figure 1). The
calculated energy difference between the tetrazole and the azide
is 5.5 kcal/mol (see below). Therefore, if equilibrium was at-
tained at 250 °C there would only be ca. 0.5% azide present; at
400 °C there would be ca. 2% azide. FVT at higher tem-
peratures, up to 400 °C, clearly produces more of the azide, but
the pyrolysis products now start to form also, primarily cyano-
allene 12a (1976 and 2229 cm−1) and 3-cyanocyclopropene
(cyclopropene-3-carbonitrile) 13a (2241 and 1664 cm−1)
(Figure S1). The azide 8aA was not observed previously4 be-
cause FVT was not used. As described below, the ring-opening
of tetrazolo[1,5-a]pyrimidines to the azides is much more
favorable.
For the purpose of ESR spectroscopy, a mixture of tetrazole
and azide 8aT/8aA was generated by FVT at 250 °C as above
and deposited in an Ar matrix. Subsequent photolysis at 222 or
254 nm caused formation of the nitrene 9a as revealed by ESR
spectroscopy (D/hc = 1.006; E/hc = 0.003 cm−1; Figure 2).
The D value fits our correlation with calculated spin densities
(D−ρ correlation).6 The nitrene disappeared after further
photolysis of the matrix at 289 nm for 45 min. We were able to
There have been previous reports on 3-pyridazinylnitrene.3−5
Flash vacuum thermolysis (FVT) of tetrazolo[1,5-b]pyridazine
8aT at 305−380 °C produced diazobutenenitrile 10a and the
products of N2 elimination, viz., cyanoallene 12a and 3-cyano-
cyclopropene 13a, which, particularly in the higher tempera-
ture regime, rearrange partially to propargyl cyanide 14a and
tetrolonitrile 15a; these four products were isolated in a
combined yield of 99% (Scheme 2).3 The first two products,
12a and 13a, were also obtained on matrix photolysis.4,5a As we
will show, similar products are obtained from the dimethyl
derivative 8b, but, in addition, isomerization to the conjugated
nitriles 16 and 17 becomes possible. We now report the direct
detection of 3-azidopyridazine and 3-pyridazinylnitrene 9a, its
ring-opening reaction and ensuing chemistry, and its relation-
ship with 2-pyrimidinylnitrene 19a (Scheme 3). These two
nitrenes, 9a and 19a, undergo Type I and Type II ring-opening,
respectively.
Received: December 16, 2013
Published: January 23, 2014
© 2014 American Chemical Society
1758
dx.doi.org/10.1021/jo4027897 | J. Org. Chem. 2014, 79, 1758−1770

The Journal of Organic Chemistry
Article
Scheme 1. Type I and Type II Ring-Opening Reactions
Scheme 2. Formation and Reaction of
3-Pyridazinylnitrenes 9
see nitrene 9a by ESR (and likewise nitrene 9b as described
below), because we used FVT of the tetrazoles to produce
enough of the azides, which then photolyzed to the nitrenes.
Hill and Platz4 did not use FVT and therefore saw neither
azides nor nitrenes.
Scheme 3. Formation and Reactions of
2-Pyrimidinylnitrenes
It is seen in Figure 2 that an additional, strong signal is
present at 5962 G. As reported previously, the same or a very
similar signal is formed from 1,2,3-triazolo[1,5-a]pyridazine,
and we assigned it to propargylene, HCCCH, formed by
elimination of 2N2 + HCN.7 In the present case, it is formed on
matrix photolysis as well as FVT; on FVT at 500 °C with
isolation of the product in Ar matrix, it is the only species
observable by ESR spectroscopy. This signal remains after
further photolysis of the matrix in Figure 2 at 289 nm for 45
min, whereas nitrene 9a disappears under these conditions.
In the IR spectrum, two conformers of diazo compound
10Z,E are observed at 2086 and 2072 cm−1 within the first few
minutes of irradiation (Figure S2) in agreement with previous
observations.4 Difference spectra clearly show that the azide
peaks diminish as the diazo peaks grow (Figure S1). When the
azide has been depleted, the diazo compound starts to
disappear also due to photoreactions forming more cyanoallene
12a (1976, 2229 cm−1) and cyclopropenecarbonitrile 13a
(2241, 1664 cm−1) (Figure S1). When no azide is present in
the original matrix due to deposition at room temperature,
photolysis still causes formation of the diazo compound and
depletion of the tetrazole without any azide being observed. It
is possible that the tetrazole undergoes photochemical ring-
opening to the azide, which photolyzes as fast as it is formed, or
that the tetrazole affords the diazo compound directly (see
Mechanism section below). When both azide and tetrazole are
present in the matrix, the azide is depleted first.
When the diazo compound is photolyzed in the ESR
cryostat, signals ascribed to cyanovinylcarbenes 11a are rapidly
1759
dx.doi.org/10.1021/jo4027897 | J. Org. Chem. 2014, 79, 1758−1770

The Journal of Organic Chemistry
Article
Figure 1. IR spectra following deposition of tetrazolo[1,5-b]pyridazine 8a in Ar matrices. (a) FVT at 100 °C (only tetrazole 2aT present). (b) FVT
of 2aT at 250 °C (small amount of azide 8aA formed (2141, 2118 cm−1)). (c) Calculated IR spectrum of tetrazolo[1,5-b]pyridazine 8aT. (d)
Calculated spectrum of 3-azidopyridazine 8aA (all calculations at the B3LYP/6-31G* level with wavenumbers scaled by a factor 0.9613).
The presence of cyanoallene (1976 cm−1) was not consi-
dered in the previous work,4 where a very weak absorption at
1981 cm−1 was assigned to the seven-membered ring 20a.4,8
This species, 1,3,4-triazacyclohepta-1,2,4,6-tetraene (20a, see
Scheme 3), was not observed as a product of either photolysis
or thermolysis of 8a in our work, nor were the photolysis
products arising from 20a, which are described in Section 2.
20a is formed and observed at 1969 cm−1 in the isomeric 2-
pyrimidinylnitrene (19) system described in Section 2 below.
FVT of 8a at 400−600 °C with Ar matrix isolation of the
products confirmed the previously reported results from pre-
parative FVT:3 12a, 14, and 15 are the principal products, with
only little 3-cyanocyclopropene 13a surviving under these con-
ditions (Figure 3 and Figure S5).
The dimethyl derivative 8bT behaved in a manner very sim-
Figure 2. ESR spectrum (Ar, 20 K) of 3-pyridazinylnitrene 9a
ilar to that of 8a, forming a trace of the azide 8bA (2142 cm−1)
resulting from photolysis of 8aT/8aA in Ar matrix at λ ≥ 254 nm for 1
on mild FVT at 200−250 °C (Figure S6). One minute’s pho-
min. The mixture of 8aT/8aA was prepared by FVT of 8aT at 250 °C
in a stream of Ar at 10−4 h Pa. 9a: 7209, 7360 G; D/hc = 1.060; E/hc =
0.0030 cm−1. Also present is carbene 11a (4639, 5443 G; D/hc =
0.3616; E/hc = 0.0209 cm−1) derived from diazo compound 10a. The
field positions used to calculate D/hc and E/hc are marked with
an x. The signal at 5962 G is due to a different species (see text).
Ho = 3471.1 G.
tolysis at 254 or 222 nm caused the formation of nitrene 9b
(7081 G; D/hc = 1.036; E/hc = 0.0025 cm−1) and carbene 11b
(4576 and 5320 G; D/hc = 0.341; E/hc = 0.020 cm−1) as
observed by ESR spectroscopy (Figure 4). The signals of both
species diminish on further photolysis and have all but disap-
peared after 45 min (Figure S7 and Scheme S1, Supporting
Information). This compound cannot easily form HC3H, and
the major signal near 6000 G in Figure 2 is, therefore, strongly
reduced in Figure 4. Minor, unidentified signals are present at
4788 and 5995 G.
detected in the ESR spectrum (D/hc = 0.362; E/hc = 0.021
cm−1; Figure 2). In the UV spectrum, the diazo compound
gives rise to an absorption at ca. 300−355 nm (Figure S3).
Continued photolysis at 313 nm causes the diazo bands in the
IR to disappear in concert with the 300−355 nm UV band. At
the same time, the usual photolysis products, cyanoallene 12a
(1976, 2227 cm−1) and 3-cyanocyclopropene 13a (1664, 2241
cm−1), are formed (Figure S4; see also Figure 3).
In the IR spectrum, diminution of the tetrazole bands and
formation of diazo compound 10b (2046, 2207, 1464, 1330,
1032 cm−1) were observed (Figure S8). A band at 325 nm in
the UV spectrum is also assigned to the diazo compound
1760
dx.doi.org/10.1021/jo4027897 | J. Org. Chem. 2014, 79, 1758−1770

The Journal of Organic Chemistry
Article
Figure 3. (a) Products of flash vaucum thermolysis (FVT) of 8aT at 600 °C with isolation of the thermolysate in Ar matrix at 20 K; bands marked
with an x belong to cyanoallene 12a, those with an y to but-3-ynenitrile 14, and those with a z to but-2-ynenitrile 15; (b) calculated spectrum of 12a;
(c) calculated spectrum of 14; (d) calculated spectrum of 15 (B3LYP/6-31G*, wavenumbers scaled by a factor 0.9613). Very little 3-
cyanocyclopropene 13a (labeled Cc,1664 cm−1) survives under these FVT conditions (compare Figures S4 and S5).
FVT of 8bT at 450−700 °C afforded dimethylcyanoallene
12b and dimethylcyanocyclopropene 13b, as well as a mixture
of nitriles identified as the s-E- and s-Z-2-methylpentadieneni-
triles 16 and 17 on the basis of comparison with the calculated
IR spectra (Figure S11).
2. 2-Pyrimidinylnitrenes. In contrast to tetrazolopyrida-
zines, tetrazolo[1,5-a]pyrimidines 18T undergo easy ring-opening
to the azides 18A already on sublimation at 60−70 °C. The
azide−tetrazole equilibria of these and related compounds have
been the subject of detailed NMR studies.9 These compounds
exist in the tetrazole forms in the solid state, but entropy-
dominated tautomerization to the azides takes over above room
temperature. However, B3LYP calculations erroneously predict
the azides to be of lower energy than the tetrazoles. Subli-
mation of 18aT through the FVT tube held at 200 °C and
deposition of the vapor in an Ar matrix affords the essentially
pure azide 18aA (Scheme 3; Figure 5).
2-Pyrimidinylnitrenes 19 are obtained under both FVT and
Figure 4. ESR spectrum 4,6-dimethyl-2-pyrazinylnitrene 9b (7081 G;
D/hc = 1.036; E/hc = 0.0025 cm−1) and 4-cyano-3-penten-2-ylidene
11b (4576 and 5320 G; D/hc = 0.341; E/hc = 0.020 cm−1) (Ar, 20 K).
Ho = 3471.2 G. Ordinate: intensities in arbitrary units. For the
temporal evolution of the spectrum, see Figure S7 in Supporting
Information.
photolysis conditions and directly observed by ESR spectros-
copy (19a:10 D/hc = 1.217; E/hc = 0.0052 cm−1; 19b: D/hc =
1.2; E/hc = 0.0) (Figure 6). For this purpose, the starting
materials 18a and 18b are either deposited in an Ar matrix and
then photolyzed to generate 19, or 18 is subjected to FVT in
order to generate more of the azides 18aA and 18bA, which
then decomposes thermally to the nitrenes 19a and 19b. The
latter are then isolated in Ar matrixes, and the ESR spectra are
recorded.
The D values of 19a and 19b fit our correlation with calcu-
lated spin densities.6 The triplet nitrene 19a is also observed in
the matrix UV spectrum, where it features a typical vibrational
progression at 390−422 nm (Figure 7) in agreement with the
previous report.8 The absorptions around 331 nm present in
(Figure S9). In the IR spectrum, weak bands due to 1,3-
dimethyl-3-cyanocyclopropene 13b can be seen to form at
2226 and 1793 cm−1 already during the initial photolysis at
222 nm. These bands grew together with bands due to dimeth-
ylcyanoallene (2-methyl-2,3-pentadienenitrile) 12b on further
photolysis at 254 or 222 nm (Figure S10). In this case, the
cyanocyclopropene 13b is the principal product, with only a
little of the dimethylcyanoallene 12b forming.
1761
dx.doi.org/10.1021/jo4027897 | J. Org. Chem. 2014, 79, 1758−1770

The Journal of Organic Chemistry
Article
Figure 5. Ar matrix depositions of tetrazolo[1,5-a]pyrimidine 18a through the FVT oven. (a) FVT oven at 70 °C (tetrazole 18aT is in equilibrium
with azide 18aA); (b) FVT oven at 200 °C (equilibrium almost completely on the side of azide 18aA); (c) calculated spectrum of tetrazolo-
[1,5-a]pyrimidine 18aT (B3LYP/6-31G*, wavenumbers scaled by a factor 0.9613); (d) calculated spectrum of 2-azidopyrimidine 18aA.
Figure 6. ESR spectrum resulting from FVT of tetrazolo[1,5-a]pyrimidine/2-zidopyrimidine 18aT/18aA at 500 °C with Ar matrix-isolation of the
resulting 2-pyrimidinylnitrene 19a: X2 7326 G; Y2 7597 G. Half-field signal 1683 G (inset: 0−3000 G). Ho = 3374.7 G. Strong signals around g = 2
(3375 G) are ascribed to adventitious doublet radicals and H atoms. The origin of the weak signal at 8137 G is unknown.
Figure 7 are ascribed to diazo compound 23a as discussed
below.
2-quinazolinylnitrene, also affords only a very weak band due to
the seven-membered ring carbodiimide, 1,2,4-triazabenzo[e]-
cyclohepta-2,3,5,7-tetraene 27a, but this compound becomes
very prominent when a phenyl group is added (27b) (eq 1).6
Bucher et al. have reported the ring expansion of an
s-triazinylnitrene to the corresponding tetraazacycloheptate-
traene.11 The band reported by Platz et al.4,8 at 1970 cm−1 is
most likely identical with our band at 1969 cm−1 assigned to
20a, but their band reported at 2045 cm−1 is assigned to the
open-chain ketenimine 25a (Scheme 3) as described below.
By knowing the conditions for observation of 2-pyrimidi-
nylnitrene by ESR and UV spectroscopy, its IR spectrum8 can
also be assigned. It is observed following brief photolysis at
either 254 or 222 nm (Figure 8). Most interestingly, a weak IR
band at 1969 cm−1, which formed at the same time, is in ex-
cellent agreement with the calculated cumulenic stretch of the
seven-membered-ring carbodiimide 20a: 1967 cm−1 at the
B3LYP/6-31G* level (Figure 8). Note that the benzo-analog,
1762
dx.doi.org/10.1021/jo4027897 | J. Org. Chem. 2014, 79, 1758−1770

The Journal of Organic Chemistry
Article
Figure 7. Ar matrix UV absorption spectra during photolysis of 2-azidopyrimidine 18aA/tetrazole 18aT at 254 nm at 8 K − not background
corrected; spectrum 1 (bottom): directly after deposition (70 °C), azide 18aA + tetrazole 18aT show absorption <306 nm; spectra 2, 3, and 4 from
bottom: after 15 min, 45 min, and 2.5 h photolysis at 254 nm; and spectrum 5 (top) after additional photolysis at 222 nm for 18 min. The structured
band at 393−422 nm is assigned to 2-pyrimidinylnitrene 19a. A band at ca. 331 nm may be ascribed to diazo compound 23a. Abscissa in nm.
Figure 8. Result of matrix photolysis of 2-azidopyrimidine 18aA for 5 min at 254 nm. (a) Difference spectrum showing the depletion of
2-azidopyrimidine (negative peaks) and formation of triplet 2-pyrimidinylnitrene 19a (positive bands) and of 1,2,4-triaza-cyclohepta-1,2,4,6-tetraene
20a (positive bands); (b) calculated spectrum of triplet 2-pyrimidinylnitrene 19a; (c) calculated spectrum of 1,2,4-triaza-cyclohepta-2,3,5,7-tetraene
20a (B3LYP/6-31G*, wavenumbers scaled by a factor 0.9613).
(Scheme 3). The intensity of the 1969 cm−1 band of 20a
(actually a double band at 1969 and 1972 cm−1) first increases,
then decreases again after 15 min photolysis at 254 nm. The
2044 cm−1 band (25a) increases more slowly, then also disap-
pears after 2 h of photolysis. All the while, the 2269 cm−1 band
(22a) increases slowly (Figure S13).
The other prominent band at 2135 cm−1 (Figure 9) is as-
cribed to 3-diazo-1-isocyanopropene 23a. The UV−vis absorp-
tion centered at ca. 331 nm (Figure 7) may also be ascribed to
23a. The IR spectrum of 23a changes in the course of further
photolysis at 313 nm (Figures S12 and S13) by forming new
Another two important bands develop at 2044 and 2135 cm−1
after a few minutes of photolysis of 18aA (Figures 9 and S12).
The first, at 2044 cm−1, is assigned to the ketenimine 25a, for-
med in a Type II ring-opening (25a: 2044, 2187, 2203, 3429,
3416 cm−1; calc 2058, 2199, 3386 cm−1 (B3LYP/6-31G*))
1763
dx.doi.org/10.1021/jo4027897 | J. Org. Chem. 2014, 79, 1758−1770

The Journal of Organic Chemistry
Article
Figure 9. IR spectrum (a) after 1 h photolysis of azide 18a with 222 nm (20 K); (b) after 2.5 h photolysis of azide 18a at 254 nm (20 K). (c)
Calculated spectrum of cyanimino-ketenimine 25a (B3LYP/6-31G*). (d) Calculated spectrum of 1-cyanopyrazole 22a (B3LYP/6-31G*,
wavenumbers scaled by a factor 0.9613). Interpretations: 1969 cm−1 band = 20a; 2044 cm−1 = 25a; 2270 cm−1 = 22a. The broad band at 2135/2133
cm−1 is ascribed to diazo-isocyanide 23a.
Figure 10. ESR spectrum resulting from 2 h UV photolysis of the 2-pyrimidinylnitrene 19a isolated in Ar matrix. 2-Pyrimidinylnitrene 19a: X2 7326
G. Y2 7597 G. Half-field signal 1683 G. 4-Cyano-4-azabutadien-1-ylnitrene 21a: X2 5408 G. Y2 5648 G. The signals of nitrene 21a are already clearly
present after 5 min of photolysis. The strong band at g = 2 (ca. 3375 G) is due to adventitious monoradicals. The origin of the broad signal at 6551
G, which grows as a function of photolysis time, is unknown. Inset 600−1950 G range. Ho = 3374.5 G.
absorptions on the high-frequency side of the band. First,
compound 23 can exist in several conformeric forms. Second,
photochemical decomposition of the diazo compound is
expected to form products such as isocyanopropadiene, iso-
cyanocyclopropene, and so forth, all absorbing near 2135 cm−1.
We postulate that isocyanides 23 are formed by photochemical
ring-opening of the triazacycloheptatetraenes, 20 → 23
(Scheme 3), which is analogous to eq 1. This is discussed
further in the context of the dimethyl derivative 23b below.
Continued photolysis of the matrices derived from 18a and
18b at 254 or 313 nm caused increased development of bands
readily assigned to the 1-cyanopyrazoles 22a (2270 cm−1;
Figure 9) and 22b (2259 cm−1), respectively, by comparison
with the cyanopyrazoles formed by FVT.3
1764
dx.doi.org/10.1021/jo4027897 | J. Org. Chem. 2014, 79, 1758−1770

The Journal of Organic Chemistry
Article
Figure 11. (a) Flash vacuum thermolysis of tetrazolo[1,5-a]pyrimidine/2-azidopyrimidine 18a at 700 °C and subsequent isolation in an Ar matrix at
20 K; bands marked with an x belong to 1-cyanopyrazole 22a, those with an y are assigned to (2-cyanovinyl)cyanamide 26, and those with a z to
cyanoallene 12a; (b) calculated spectrum of 1-cyanopyrazole 22a (B3LYP/6-31G*, wavenumbers scaled by a factor 0.9613); (c) calculated spectrum
of (2-cyanovinyl)cyanamide 26; (d) calculated spectrum of cyanoallene 12a (pyrolysis product of 22a). Note that 2-aminopyrimidine is not formed
under these FVT conditions.
Moreover, a second nitrene is formed on UV photolysis of
18aT/18aA and observed in the ESR spectrum (Figure 10). Its
ZFS parameters are in good agreement with expectations for
the open-chain vinylnitrene 21a (D/hc = 0.542, E/hc = 0.0056
cm−1) and very similar to those of the isomeric nitrene NC−
CHN−CHCH−N formed by ring-opening of 2-pyrazi-
nylnitrene.12 The D value fits very well on the D−ρ corre-
lation.12 A D value of 0.5 is much too low to be associated with
an aromatic or heteroaromatic nitrene. This nitrene, 21a, is a
likely precursor of both ketenimine 25a and 1-cyanopyrazole
22a, whereby 22a may also form directly from nitrene 19a in a
concerted ring contraction (Scheme 3).7,13
FVT of 18aT/18aA at 700 °C with isolation of the products
in Ar matrix affords 1-cyanopyrazole 22a3 together with new
absorptions at 2243, 2222, and 1649 cm−1, which are assigned
to N-(cyanovinyl)cyanamide 26, i.e., a tautomer of ketenimine
25a (Scheme 3 and Figure 11). This is analogous to the forma-
tion of glutacononitriles from 2-pyridylnitrenes (Scheme 1).1
Cyanoallene 12a is also formed in this FVT reaction (Figure 11);
this is due to a secondary pyrolysis of 1-cyanopyrazole14 22a
and has nothing to do with the pyridazinylnitrene reactions in
Scheme 2. 2-Aminopyrimidine is formed in the preparative
FVT reaction of 18aT/18aA due to hydrogen abstraction by
the triplet nitrene,3 but it is not formed under the high vacuum
conditions of the present FVT experiments in the presence of
argon as a carrier gas.
Prolonged photolysis caused the formation of 1-cyano-3,5-
dimethylpyrazole 22b (Figure S18), which is also the product
of FVT.3 Formation of a ketenimine 25b from the vinylnitrene
21b is unlikely, as it would require a 1,2-shift of a methyl
group. While not impossible, such a reaction undoubtedly has
higher activation energy than the formation of azirene 24b. The
closure of open-shell singlet vinylnitrene to 1-azirene is said
to be barrierless,15 but we calculate, in fact, a small barrier of
5 kcal/mol, at the CASPT2 level. Azirenes are sometimes ob-
served in the matrix photolysis of vinyl azides.16 Furthermore,
singlet (2-acyl)vinylnitrenes are found to cyclize to azirenes.17
The development of weak bands at 2212, 2202, 1628, and
1244 cm−1 in the photolysis of 18b at 313 nm for 2 h may be
ascribed to azirene 24b (calculated CN and CN
absorptions 2202, 1629, and 1222 cm−1), but this compound
was not investigated in detail (Figures S19−S20).
Another prominent band at 2099 cm−1 develops on
photolysis of 18b at 254 nm and decreases again on photolysis
at 313 nm (Figures S19−S21). In analogy with the similar
observation of 23a, this is assigned to the diazo isocyanide 23b
formed by ring-opening of the triazacycloheptatetraene 20b
(1979 cm−1) (Scheme 3). Compound 23b has a calculated IR
absorption at 2089 cm−1 as the only strong band in the IR
spectrum (Figure S21). A corresponding maximum at 322 nm
was observed in the UV spectrum (Figure S16). When 18bA was
photolyzed at 313 nm from the beginning, neither the diazo band
at 2099 cm−1 nor the UV band at 322 nm formed; instead, the
bands tentatively ascribed to azirene 24b developed immediately.
There is good precedence for ring-opening reactions of the type
20 → 23: 5-phenyl-1,3,4-triazabenzocyclohepta-1,2,4,6-tetraene
The 2,6-dimethylpyrimidinyl azide 18bA is formed by mild
FVT of the tetrazole 18bT (Figure S14). The nitrene 19b is
observed by IR, UV, and ESR spectroscopy (Figures 12 and
S15−S17). A weak signal ascribed to triazacycloheptatetraene
20b is observed at 1979 cm−1 (Figures 12 and S15).
1765
dx.doi.org/10.1021/jo4027897 | J. Org. Chem. 2014, 79, 1758−1770

The Journal of Organic Chemistry
Article
nitrenes, azirines, and seven-membered rings as well as the
transition states connecting them at B3LYP/6-31G* and
CASPT2(8,8)//CASSCF(8,8)6-31G* levels of theory and
demonstrated that the trends of the two types of calculation
are in broad agreement.
The calculations demonstrate that the interconversion of all
the molecules in the top row of Scheme 4 is possible in prin-
ciple, but our experimental results clearly show that the inter-
conversion of the nitrenes 9 and 19 does not happen. Part of
the reason can be found in the relative activation barriers. The
ring-opening of triplet 2-pyrimidinylnitrene 19 to the observed
triplet iminovinylnitrene 21 is a very favorable process with a
modest activation energy (32 kcal/mol). The triazacyclohepta-
tetraene 20 must necessarily be a singlet. Its formation has a
somewhat higher overall barrier because of the higher barrier
toward the diazirene 33 (43 kcal/mol), which may be ascribed
to lone pair−lone pair repulsion in the transition state as well as
potential antiaromaticity of the diazirene. When 20 does form,
it too could undergo ring-opening to 21, but the activation
barrier of 53 kcal/mol is much higher than that for the nitrene
19. A barrier of 42 kcal/mol between 34a and 9a also prevents
the interconversion of 19a and 9a.
The ring-opening of 20 to 1-isocyanodiazopropene 23 also
has a high calculated barrier, ca. 54 kcal/mol, but it must be
kept in mind that the calculations are for the ground-state
reactions, whereas this reaction is in fact photochemical. In the
case of 2-quinazolinylnitrene, the analogous interconversion
of triazacycloheptatetraene 27 and isocyano-diazo compound
28 (eq 1) was clearly demonstrated to be a photochemical
6
process not surprisingly, since a formal double bond is
broken.
For 3-pyridazinylnitrene 9a, ring-opening to diazo compound
10a is very feasible for both the singlet and the triplet states.
The thermal activation barriers are 4 and 10 kcal/mol, respec-
tively, with respect to the singlet and triplet states of nitrene 9
(Scheme 4). The tetrazole 8aT may also photolyze directly to
10a in a process with concerted ring-opening and N2 elimi-
nation. The calculated thermal barrier is 45 kcal/mol, but again,
the reaction is photochemical. The internal reaction coordinate
calculation for the singlet ground-state reaction 8aT → 10a
features an inflection point, which corresponds roughly to the
nitrene (see Supporting Information).
Figure 12. (a) Calculated IR spectrum of 5,7-dimethyl-1,2,4-
triazacyclohepta-2,3,5,7-tetraene 20b. (b) Calculated spectrum of
triplet 4,6-dimethyl-2-pyrimidinylnitrene 19b (all calculations
B3LYP/6-31G*, wavenumbers scaled by a factor 0.9613). (c) IR
spectrum of the products of photolysis of 2-azido-4,6-dimethylpyr-
imidine 18bA with 254 nm for 1 min at 20 K. Positive peaks: triplet
4,6-dimethyl-2-pyrimidylnitrene 19b and 5,7-dimethyl-1,2,4-triaza-
cyclohepta-2,3,5,7-tetraene 20b (1979 cm−1); the negative peaks are
due to the consumed 4,6-dimethyl-2-azidopyrimidine 18bA. For more
details see Figure S15.
Thermal ring expansion of 3-pyridazinylnitrene 9 to the
1,2,4-triazacycloheptatetraene 20 is potentially feasible, but not
observed. It would require an activation energy of ca. 42 kcal/
mol for the open-shell singlet state. The potential ring expan-
sion in the other direction, to the 1,2,3-triazacycloheptatetraene
36, would require a substantially higher barrier, 48 kcal/mol. If
36 did form, it would exist in a very shallow minimum with a
low barrier toward ring-opening to 10.8 There is no evidence
for the formation of 36. As described above, the final products
in the two systems can be understood in terms of rearrange-
ments of cyanovinylcarbene 11 (to cyanoallene 12, cyanocy-
clopropene 13, propargyl cyanide 14, and tetrolonitrile 15),
isocyanodiazopropene 23 (to unidentified isocyanides, e.g.,
isocyanoallene and isocyanocyclopropene), and (cyanimino)-
vinylnitrene 21 (to 1-cyanopyrazole 22, ketenimine 25, cyana-
mide 26, and azirene 24).
27b undergoes reversible ring-opening to o-isocyanophenyl-
(phenyl)diazomethane 28b (eq 1).6
Furthermore, Chapyshev has investigated the matrix pho-
tolysis of azido-s-triazine 29 and suggested the ring-opening of
tetraazacycloheptatetraene 31 (1951 cm−1) to the isocyano
diazo compound 32 (2087 cm−1) (eq 2).18
3. Mechanism. Thermal reaction paths were explored
by calculating the relative energies of some of the species of
interest at the (U)B3LYP/6-31G* level of theory. Energies of
1
open-shell singlet nitrenes (S1, A″) were estimated using the
Cramer-Ziegler method.19 The relevant data for the ‘a’ series
(unsubstituted compounds) are collected in Scheme 4. Very
similar relative energies apply to the dimethyl (‘b’) series. Hill
and Platz4 have already carried out detailed calculations on the
CONCLUSION
■
Mild flash vacuum thermolysis (FVT) of tetrazolopyridazines
8T generates a small amount of the azide valence tautomers
8A. Both matrix photolysis and FVT of 8T/8A give rise to
1766
dx.doi.org/10.1021/jo4027897 | J. Org. Chem. 2014, 79, 1758−1770

The Journal of Organic Chemistry
Article
a
Scheme 4. Calculated Reaction Paths
a
Energies of minima and transition structures relative to 19a T0 at the (U)B3LYP/6-31G* level (ZPVE corrected) in kcal/mol.
3-pyridazinylnitrenes 9, which were detected by ESR spectros-
copy in the photolysis reaction. Neither the azides nor the
nitrenes have been observed previously. Matrix photolysis
with IR observation reveals ring-opening of the nitrenes to
diazovinyl cyanides 10, which again lose N2 to generate
(cyanovinyl)carbenes 11. The carbenes 11 were also detected
by ESR spectroscopy. These vinylcarbenes rearrange primarily
to cyanoallenes 12 and 3-cyanocyclopropenes 13.
Tetrazolo[1,5-a]pyrimidines 18T easily ring open to the
azide valence tautomers 18A; both photolysis and FVT gen-
erate 2-pyrimidinylnitrenes 19, which were observed by Ar
matrix ESR, UV, and IR spectroscopies. A second nitrene, inter-
preted as the ring-opened vinylnitrene 21a, is formed rapidly on
photolysis of matrix-isolated 19a, and this is the logical pre-
cursor of (cyanimino)ketenimine 25a, which is observed in the
IR spectrum. Another photolysis end product is 1-cyanopyr-
azoles 22, which is also formed on FVT. Azirene 24 is sug-
gested as a photolysis product from 18b. Weak IR absorptions
are ascribed to 1,2,4-triazacyclohepta-2,3,5,7-tetraenes 20a and
20b (1969 and 1979 cm−1, respectively). The development of
strong bands in the 2100 cm−1 region on extended photolysis is
in accord with ring-opening of 20 to (isocyanovinyl)diazo-
methanes 23.
An interconversion of 3-pyridazinylnitrenes 9 and 2-pyrimidi-
nylnitrenes 19 is, in principle, energetically possible, but other
reactions described above are more favorable, with the con-
sequence that the interconversion is not observed by either
ESR or IR spectroscopy. In the case of the related isomeric pair,
3-pyridazinylcarbene 1d and 2-pyridylnitrene 4, there was also
very tenuous evidence for interconversion.7 This is in contrast
to the well-established interconversion of phenylnitrene and
2-pyridylcarbene, which takes place both thermally1 and pho-
tochemically.20 The reason can be found in the fact that ring-
opening reactions are much more facile in heterocyclic than in
carbocyclic carbenes and nitrenes.1,12 This is true especially
for Type I ring-openings, which dominate the reactions of
3-pyridazinylcarbenes and -nitrenes, whereas they are not possi-
ble at all in carbocyclic systems.
COMPUTATIONAL METHOD
■
The energies and vibrational frequencies of the various species were
calculated at theB3LYP/6-31G* level of theory using the Gaussian 09
suite of programs.21 Reported energies include zero-point vibrational
energy corrections. The vibrational frequencies were scaled by a factor
0.9613.22 Further details are given in the Supporting Information.
EXPERIMENTAL SECTION
■
Matrix isolation experiments employed an apparatus consisting of an
FVT oven containing a 10 cm long, 0.7 cm I.D. electrically heated
quartz tube suspended in a vacuum chamber (2.0 × 10−6 mbar)
directly flanged to the cold head of a closed cycle liquid He cryostat
with a wall-free flight path of ca. 3 cm between the exit of the quartz
tube and the cold target (10−25 K).12 The quartz tube was unpacked
and not chemically treated or seasoned. The cold target was KBr for
IR spectroscopy, quartz or KBr for UV−vis spectroscopy, and a Cu
rod for ESR spectroscopy.12 Tetrazolo[1,5-b]pyridazines 8a,b and
tetrazolo[1,5-a]pyrimidines 18a,b were sublimed at 60−90 °C in a
stream of Ar, passing the vapor through the FVT oven, either at room
temperature or at the indicated temperature before depositing the ma-
terial on the cold target, usually at 20 K, to form a matrix. Afterward,
the target was cooled to 8−10 K. IR spectra were recorded with
1 cm−1 resolution. The deposited compounds were photolyzed with
different wavelength, usually starting with 254 nm. Photolysis was car-
ried out using a 75 W low pressure Hg lamp (254 nm), a 1000 W high
pressure Xe/Hg lamp equipped with a monochromator and appro-
priate cutoff filters, or excimer laser lamps operating at 222 nm
(25 mW/cm2) and 308 nm (50 mW/cm2). In FVT experiments, the
tetrazoles/azides were pyrolyzed at 400−700 °C, and the products
were isolated in Ar matrixes as above.
1767
dx.doi.org/10.1021/jo4027897 | J. Org. Chem. 2014, 79, 1758−1770

The Journal of Organic Chemistry
Article
Tetrazoles 8aT, 18aT, and 18bT were prepared according to
literature procedures.3,9a
m, 1367 m, 1342 m, 1334 w, 1309 w, 1276 m, 1211 w, 1168 w, 1127
m, 1076 w, 1054 m, 1039 m, 1019 w, 986 m, 947 m, 901 w, 875 m,
826 w, 766 w, 709 w, 634 m, 528 m, 513 w, 485 m, 472 w; calc.
(B3LYP/6-31G*) 3045 (8), 3027 (24), 2997 (10), 2990 (21), 2938
(24), 1598 (33), 1553 (100), 1482 (50), 1451 (38), 1442 (21), 1431
(50), 1386 (17), 1343 (25), 1321 (27), 1317 (28), 1267 (33), 1195
(4), 1151 (8), 1111 (30), 1039 (23), 1033 (16), 1002 (10), 976 (19),
965 (10), 928 (6), 861 (20), 809 (10), 738 (1), 689 (1), 619 (4), 609
(0), 548 (1), 521 (7), 502 (6), 472 (8), 362 (7), 284 (1), 240 (4), 218
(0), 174 (0), 147 (1), 109 (1).
Mild FVT at 250 °C caused formation of a small amount of the
azide 8bA, which is only identified by its strongest band, λ−1/cm−1
2142; calc. (B3LYP/6-31G*) 2201 (100), 1582 (3), 1532 (6), 1460
(7), 1447 (2), 1394 (12), 1385 (5), 1374 (27), 1353 (2), 1306 (23),
1223 (2), 1175 (1), 1157 (1), 1107 (1), 1033 (1), 1002 (1), 970 (1),
945 (1), 883 (1), 802 (1), 611 (2), 578 (1).
After a few minutes of photolysis at either 254 or 222 nm a strong
band assigned to 4-diazo-2-methyl-pent-2-enenitrile 10b was observed.
IR (Ar, 20 K) 10b λ−1/cm−1 2935 w, 2207 w, 2046 s, 1603 w, 1465 w,
1320 w, 1032 w; calc. (B3LYP/6-31G*) 3021 (1), 3011 (3), 2979 (7),
2926 (13), 2227 (8), 2103 (100), 1600 (14), 1472 (7), 1451 (2), 1445
(2), 1389 (2), 1352 (4), 1345 (13), 1207 (1), 1038 (1), 1022 (6), 980
(1), 871 (1), 821 (1), 759 (2), 637 (1), 598 (2), 577 (1).
Azidoyridazines 8aA and 8bA were only observed following mild
FVT of the tetrazoles (up to 400 °C). Consequently, ESR spectra of
pyridazinylnitrenes 9a and 9b were best observed by photolysis of the
matrix-isolated thermolyzates, where azides were detectable by IR
spectroscopy. D/hc and E/hc values were calculated from ESR spectra
using Wasserman’s equations and method of defining field positions.23
5,7-Dimethyltetrazolo[1,5-b]pyridazine 8bT. A solution of
1.59 g (11.15 mmol) 3-chloro-4,6-dimethyl-pyridazine24 and 1.45 g
(22.3 mmol) sodium azide in 20 mL 10% EtOH containing 11.2 mmol
HCl was refluxed for 1 h. After cooling to 0 °C, the crystals formed
were filtered off and purified by sublimation at 115−120 °C/0.005 hPa,
yielding 1.38 g 8bT (83%), mp 136−138 °C. The substance recrys-
tallizes from EtOH/water 9/1, mp 135−138 °C. 1H NMR (CDCl3) δ
7.15 (s, 1 H), 2.76 (s, 3 H), 2.68 (s, 3 H). 13C NMR (CDCl3) δ 156.9,
143.2, 136.7, 125.1, 21.4, 16.5; IR (KBr) 3058 m, 2965 w, 2928 w,
1606 m,1575 s, 1564 m, 1483 s, 1437 s, 1428 s, 1418 m, 1397 w,
1382 s, 1359 m, 1343 s, 1336 w, 1267 s, 1211 m, 1166 m, 1130 s,
1080 w, 1064 s, 1043 m, 1037 m, 1025 w, 995 s, 949 m, 903 s, 826 m,
766 m, 709 w, 638 s, 529 s, 515 w, 487 s cm−1; MS (EI) m/z 149 (M+,
1%), 92 (19), 78 (8), 66 (100), 51 (27). Anal. Calcd for C6H7N5: C
48.32, H 4.73, N 46.96. Found: C 48.03, H 4.65, N 46.94.
Further photolysis at either 254 or 222 nm caused appearance of
new bands, which are assigned to 1,2-dimethyl-cycloprop-2-enecarbo-
nitrile 13b and 2-methyl-penta-2,3-dienenitrile 12b. In the following,
bands labeled with an x belong to 13b, those with an y to 12b, and those
with no label may belong to both: IR (Ar, 20 K) 12b + 13b λ−1/cm−1
2988, 2936, 2226, 1968 y, 1793 x, 1453, 1376, 1206, 1153, 1090 x,
1069 x, 1002, 958 x, 918 x, 790 y, 719 x, 704 x, 607 y, 501 y, 473 y, 439 x.
calc. (B3LYP/6-31G*) for 13b 3167 (3), 3010 (70), 2992 (57), 2986
(27), 2930 (100), 2249 (55), 1807 (43), 1467 (15), 1457 (16), 1449
(36), 1446 (40), 1380 (7), 1195 (9), 1132 (51), 1080 (41), 1058 (9),
1024 (5), 982 (19), 939 (15), 901 (16), 711 (52), 699 (62), 640 (11),
566 (3), 555 (4), 423 (24); calc. (B3LYP/6-31G*) for 12b 3030 (61),
3009 (6), 2995 (41), 2982 (53), 2938 (69), 2930 (100), 2248 (70), 1984
(61), 1472 (12), 1461 (25), 1453 (28), 1450 (28), 1389 (4), 1375 (24),
1207 (55), 1146 (17), 1065 (4), 1032 (2), 1025 (2), 985 (55), 888 (7),
784 (49), 674 (11), 603 (18), 578 (6), 497 (25), 472 (7).
FVT of Tetrazolo[1,5-b]pyridazine 8a. FVT of tetrazolo[1,5-
b]pyridazine 8aT was carried out at 600 °C, and three major products
were identified after isolation in an Ar matrix, namely, cyanoallene 12a,
3-butynenitrile (propargyl cyanide) 14 and 2-butynenitrile (tetroloni-
trile) 15. Very little 3-cyanocyclopropene 13a is formed under these
conditions, but it is identifiable by its absorption at 1664 cm−1. Other
products may form as well, since not all IR bands are assigned. IR (Ar,
20 K) cyanoallene 12a λ−1/cm−1 2227 m, 1976 s, 859 s, 840 m (this
band appears at 843 upon photolysis, probably due to different depo-
sition conditions); 3-cyanocyclopropene 13a λ−1/cm−1 1664 vw;
3-butynenitrile 14 λ−1/cm−1 3318 s, 1434 w, 1313 m, 981 w, 660 s,
624 m, 595 w; calc. (B3LYP/6-31G*) 3359 (100), 2953 (1), 2922 (5),
2288 (5), 2170 (1), 1422 (12), 1307 (24), 959 (11), 870 (12), 624
(70), 596 (83), 352 (3), 342 (9), 302 (8), 134 (5); 2-butynenitrile
15 λ−1/cm−1 2274 s, 1415 m, 1027 w, 505 w; calc. (B3LYP/6-31G*)
2997 (6), 2931 (7), 2315 (100), 1438 (15), 1382 (1), 1020 (7),
536 (5).
FVT of 6,8-Dimethyltetrazolo[1,5-b]pyridazine 8bT. FVT with
Ar matrix isolation of the products was performed at temperatures
ranging from 450 to 700 °C in intervals of 50 °C. Three major pro-
ducts were identified by comparison with the calculated spectra: 1,2-
dimethylcycloprop-2-enecarbonitrile 13b, 2-methylpenta-2,3-dieneni-
trile 12b, and the s-Z and s-E-isomers of 2-methylpenta-2,4-dienenitrile
16 and 17. The ratio of 12b and 13b changes with increasing tem-
perature in favor of the cyanoallene (12b). In Figure S11, bands
marked with an x belong to 1,2-dimethylcycloprop-2-enecarbonitrile
13b, and those with an y to 2-methylpenta-2,3-dienenitrile 12b. Bands
assigned to 16 and 17: IR (Ar, 20 K) λ−1/cm−1 3122 m, 3024 w, 2986
m, 2969 m, 2935 m, 2225 s, 1656 m, 1635 w, 1595 m, 1451 s, 1441 s,
1420 s, 1391 w, 1382 m, 1321 w, 1301 w, 1033 m, 1110 s, 995 s, 926 s,
770 w, 675 w, 549 w; calc. (B3LYP/6-31G*) for 16: 3129 (32), 3068
Matrix Isolation Experiments, IR Peak Listings, and
Calculated Vibrational Wavenumbers. (B3LYP/6-31G*).21 All
calculated wavenumbers in this document are scaled by 0.9613. Cal-
culated relative peak intensities are normalized to 100.
Tetrazolo[1,5-b]pyridazine 8aT and 3-Azidopyridazine 8aA.
8aT was sublimed at 60 °C in an Ar stream and deposited on a KBr
window at 20 K. An IR spectrum of pure tetrazole was recorded. IR
(Ar, 20 K) tetrazole 8aT λ−1/cm−1 3117 m, 3102 w, 3086 w, 3061 m,
1619 w, 1542 m, 1492 m, 1474 w, 1373 m, 1347 s, 1334 w, 1324 w,
1272 w, 1256 w, 1133 w, 1076 w, 1019 w, 1006 w, 985 w, 913 w, 812
m, 802 m, 762 m, 754 w, 648 w, 588 w, 563 w, 444 w; calc. (B3LYP/
6-31G*) 3107 (6), 3095 (7), 1524 (41), 1476 (29), 1363 (7), 1326
(10), 1308 (99), 1262 (33), 1238 (15), 1144 (5), 1113 (7), 1063 (39),
1002 (23), 965 (16), 880 (15), 786 (100), 752 (13), 730 (9), 581 (4),
548 (5), 433 (4).
After mild FVT at 250 °C small amounts of 3-azidopyridazine 8aA
were observed as well. IR (Ar, 20K) azide 8aA λ−1/cm−1 2163, 2145,
2118, 1585, 1431, 1293, 696; calc. (B3LYP/6-31G*) 3093 (2), 3080
(1), 2202 (100), 1570 (16), 1418 (27), 1373 (10), 1316 (37), 1087
(3), 995 (1), 791 (6), 678 (3), 536 (2).
Photolysis of the matrix-isolated tetrazole 8aT with 254 nm light for
2 min at 20 K produced a strong double peak at 2086/2072 cm−1
which is assigned to the stretching vibration of the diazo group of
4-diazobut-2-enenitrile 10a: IR (Ar, 20 K) λ−1/cm−1 2214 w, 2086 s,
2072 s, 1592 m, 1582 m, 1417 m, 1362 m, 1236 m, 1164 w, 789 w, 703
w, 495 w; calc. (B3LYP/6-31G*) 3089 (1), 3072 (1), 2232 (8), 2133
(100), 1589 (24), 1417 (12), 1365 (10), 1232 (5), 1163 (1), 970 (1),
768 (3), 698 (7), 579 (4), 548 (1), 503 (11).
Further photolysis at 222, 254, or 313 nm gave rise to several new
peaks, which are assigned to 3-cyanocyclopropene 13a and buta-2,3-
dienenitrile (cyanoallene) 12a. The ratio of these two products changes
with the wavelength. Irradiation at 254 nm produces distinctively
stronger bands at 2227 and 1976 cm−1 (cyanoallene) compared to
bands 2241 and 1664 cm−1 (cyanocyclopropene). IR (Ar, 20 K)
3-cyanocyclopropene 13a λ−1/cm−1 2995 m, 2241 m, 1664 m, 1352 w,
986 s, 949 w, 888 m, 623 s, 543 m; calc. (B3LYP/6-31G*) 3162 (5),
2990 (37), 2261 (22), 1691 (33), 1331 (8), 1113 (6), 1011 (8), 974
(77), 932 (11), 876 (21), 800 (2), 619 (100), 554 (8), 535 (29);
cyanoallene 12a λ−1/cm−1 2227 w, 1976 s, 858 s, 843m; calc. (B3LYP/
6-31G*) 3043 (1), 2259 (28), 1985 (100), 1428 (3), 1315 (1), 976
(1), 922 (12), 854 (85), 842 (40), 628 (9).
6,8-Dimethyltetrazolo[1,5-b]pyridazine 8bT and 2-Azido-
4,6-dimethylpyridazine 8bA. Compound 8bT was sublimed at
65 °C in an Ar stream, and was deposited on a KBr window at 20 K.
An IR spectrum then showed almost pure tetrazole 8bT: IR (Ar, 20 K)
λ−1/cm−1 3066 w, 3049 w, 3103 w, 2974 w, 2936 w, 2927 w, 1612 m,
1579 s, 1569 m, 1490 s, 1460 w, 1444 s, 1431 s, 1418 w, 1396 m, 1383
1768
dx.doi.org/10.1021/jo4027897 | J. Org. Chem. 2014, 79, 1758−1770

The Journal of Organic Chemistry
Article
(45), 3053 (7), 3046 (14), 3038 (25), 2987 (29), 2938 (37), 2239
(100), 1642 (14), 1597 (15), 1461 (17), 1449 (21), 1418 (23), 1391
(12), 1328 (7), 1288 (5), 1199 (5), 1130 (3), 1037 (1), 993 (80), 920
(90), 904 (21), 776 (3), 657 (17), 573 (3), 552 (26); calc. (B3LYP/
6-31G*) for 17: 3128 (27), 3060 (14), 3046 (31), 3019 (28), 2984
(34), 2931 (48), 2240 (72), 1644 (5), 1598 (17), 1464 (15), 1445
(19), 1418 (20), 1386 (6), 1315 (5), 1286 (7), 1203 (3), 1110 (3),
1040 (17), 1019 (50), 1002 (36), 915 (100), 892 (3), 757 (1), 673
(25), 605 (7), 567 (2).
Tetrazolo[1,5-a]pyrimidine 18aT and 2-Azidopyrimidine
18aA. Tetrazole 19aT was sublimed at 60 °C in a stream of Ar and
deposited on a KBr target at 22 K. The matrix was then cooled to 8 K
and an IR spectrum of the mixture of tetrazole 18aT and azide 18aA
was recorded. After mild FVT up to 250 °C, only azide 18aA was
observed in the matrix. The following bands are assigned: IR (Ar, 8 K)
tetrazole 18aT λ−1/cm−1 3108 w, 3086 w, 1623 s, 1529 m, 1508 s,
1486 w, 1457 w, 1375 w, 1340 w, 1337 w, 1333 m, 1331 m, 1267 w,
1263 w, 1231 w, 1117 w, 1084 w, 1075 w, 977 w, 924 w, 789 w, 784 m,
780 m, 773 w, 576 w, 565 w, 458 w, 444 w, 419 w; calc. (B3LYP/
6-31G*) 3118 (1), 3068 (20), 1610 (100), 1514 (69), 1489 (96),
1399 (35), 1359 (1), 1312 (43), 1245 (24), 1210 (22), 1139 (3), 1108
(25), 1068 (24), 1012 (3), 960 (15), 945 (1), 925 (0), 892 (10), 769
(42), 755 (38), 680 (0), 642 (1), 566 (5), 551 (5), 434 (3); IR (Ar,
20 K). Azide 18aA 3058 w, 3052 w, 2178 w, 2136 s, 1583 w, 1569 s,
1565 m, 1557 w, 1440 w, 1407 s, 1401 m, 1356 w, 1340 w, 1325 s,
1317 m, 1303 w, 1281 w, 1221 w, 1185 w, 1164 w, 1076 w, 1033 w,
858 w, 801 w, 792 w, 701 w, 635 w, 554 w, 495 w, 413 w; calc.
(B3LYP/6-31G*) 3060 (5), 2204 (100), 1574 (26), 1547 (17), 1424
(3), 1396 (71), 1340 (43), 1264 (1), 1230(1), 1180 (1), 1056 (0), 838
(1), 787 (5), 777 (2), 686 (2), 620 (1), 542 (2), 405 (1).
Photolysis of the azide 18aA for a few minutes at either 222 or
254 nm caused the azide to disappear, and two intermediate species
were formed. By comparison with the calculated IR spectra (B3LYP/
6-31G*), they were identified as the 2-pyrimidinylnitrene 19a and
1,2,4-triaza-cyclohepta-2,3,5,7-tetraene 20a. IR (Ar, 20 K) nitrene
19a λ−1/cm−1 3018 w, 1531 s, 1361 m, 1231 m, 795 m, 624 m; calc.
(B3LYP/6-31G*) 3058 (41), 1531 (4), 1498 (100), 1405 (1), 1349
(37), 1227 (8), 1212 (15), 1073 (2), 1038 (2), 961 (1), 938 (5), 848
(0), 781 (49), 759 (1), 610 (15), 558 (6).
Because of its low concentration, only the strongest band of
the triazacycloheptatetraene 20a is assigned: λ−1/cm−1 1969 s; calc.
(B3LYP/6-31G*) 1968 (100), 1587 (3), 1520 (8), 1356 (4), 1284
(3), 1233 (6), 1154 (1), 990 (10), 936 (1), 919 (1), 896 (2), 848 (1),
768 (9), 708 (3), 667 (4), 623 (2), 541 (7).
After photolysis of azide 18aA at 222 nm for 2 h at least two new
products were formed. The following bands are assigned to cyan-
iminoketenimine 25a (conformer(s) unspecified): IR (Ar, 20 K)
25a λ−1/cm−1 N−H: 3299 w, 3289 w; NCN: 2203 w, 2187 w; C
CN: 2044 s; 740 m, 727 w; calc. (B3LYP/6-31G*) 3386 (15), 3045
(4), 2200 (16), 2058 (100), 1587 (53), 1398 (18), 1138 (0), 987 (7),
947 (1), 880 (2), 848 (78), 804 (6), 787 (22), 620 (4), 515 (6).
The peak ascribed to 1,2,4-triazacyclohepta-2,3,5,7-tetraene 20a at
1969 cm−1 is still present after this photolysis. A strong, broad band at
2135 cm−1 is ascribed to 3-diazo-1-isocyanopropene 23a; calc. (B3LYP/
6-31G*) λ−1/cm−1 3116 (1), 3101 (2), 3081 (1), 2127 (100) (CNN),
2098 (17) (NC), 1611 (8), 1417 (12), 1365 (4), 1242 (3), 1177 (2),
982 (3), 782 (1), 700 (5), 553 (4).
After photolysis of azide 18aA at 254 nm for 2.5 h further bands
were formed, which are assigned to 1-cyanopyrazole 22a: IR (Ar,
20 K) λ−1/cm−1 2270 s, 1411 m, 1404 s, 1330 m, 1198 w, 1078 w,
934 m, 769 m. The spectrum was identical with that of the previously
characterized 1-cyanopyrazole.3
Further photolysis of the matrices at 313 nm caused growth of
bands at 2269 (22a), 2130−2140 cm−1 (ascribed to isocyanides derived
from 23a), while bands at 2203, 2187, 2044 (25a) and 1969 cm−1
(20a) decreased.
showed only azide 18bA: IR (Ar, 20 K) λ−1/cm−1 3009 w, 3002 w,
2979 w, 2937 w, 2171 w, 2159 w, 2142 s, 2123 s, 1638 w, 1593 s,
1561 m, 1555 m, 1546 w, 1535 w, 1447 m, 1439 w, 1436 w, 1424 w,
1392 m, 1373 w, 1353 s, 1344 s, 1321 w, 1235 m, 1223 w, 1200 m,
1175 w, 1096 w, 1034 w, 991 w, 964 w, 935 w, 840 w, 782 w, 742 w,
632 w, 567 w, 545 w; calc. (B3LYP/6-31G*) 3024 (5), 3001 (2), 2942
(3), 2201 (100), 1570 (47), 1543 (33), 1454 (17), 1446 (3), 1416 (2),
1387 (6), 1377 (3), 1356 (71), 1328 (28), 1249 (3), 1220 (1), 1160
(1), 1028 (2), 1019 (1), 971 (1), 944 (2), 920 (2), 827 (1), 767 (2),
723 (1), 622 (1), 547 (1), 542 (1), 528 (1).
After photolysis at 254 nm for a few minutes at 20 K the azide 18bA
was disappearing, and new bands appearing were assigned to triplet
4,6-dimethyl-2-pyrimidylnitrene 19b and 5,7-dimethyl-1,2,4-triazacy-
clohepta-2,3,5,7-tetraene 20b. IR (Ar, 20 K) 4,6-dimethyl-2-
pyrimidylnitrene 19b: λ−1/cm−1 1555 s, 949 w, 837 w, 763 w, 586 w,
533 w (values for bands between 1555 and 949 cm−1 cannot be given
due to overlapping (negative) peaks of the azide); calc. (B3LYP/
6-31G*) 3024 (12), 3004 (6), 2944 (7), 1535 (100), 1498 (3), 1455
(12), 1451 (23), 1446 (10), 1382 (2), 1343 (14), 1317 (9), 1157 (2),
1061 (1), 1028 (6), 957 (1), 930 (7), 918 (1), 830 (3), 753 (5), 608
(0), 583 (3), 522 (3), 514 (3). Again, because of its low concentra-
tion, only the strongest band is assigned to the 5,7-dimethyl-1,2,4-
triazacyclohepta-2,3,5,7-tetraene 20b: λ−1/cm−1 1979; calc. (B3LYP/6-
31G*) 3047 (2), 3027 (3), 2988 (6), 2934 (6), 1981 (100), 1597
(11), 1538 (15), 1451 (8), 1439 (8), 1382 (2), 1372 (1), 1308 (6),
1216 (2), 1181 (5), 1132 (4), 1036 (1), 1024 (2), 986 (2), 884 (1),
842 (5), 809 (6), 771 (2), 670 (0), 627 (3), 584 (2), 538 (2).
After 2 h of photolysis at 254 nm no azide was left in the matrix, and
new bands had formed. 1-Cyano-3,5-dimethylpyrazole 22b was iden-
tified by comparison with previously reported data.3 IR (Ar, 20 K)
λ−1/cm−1 3008 w, 2974 w, 2937 w, 2259 s, 1354 m, 1107 m, 964 m,
799 m, 784 w, 479 w; calc. (B3LYP/6-31G*) 3146 (1), 3031 (7), 2988
(14), 2937 (20), 2275 (100), 1583 (33), 1481 (1), 1460 (6), 1453 (6),
1449 (8), 1441 (7), 1402 (23), 1377 (11), 1340 (65), 1215 (1), 1140
(1), 1082 (15), 1036 (3), 1010 (3), 970 (2), 949 (15), 784 (13), 761
(4), 643 (0), 605 (1), 592 (1), 562 (1), 541 (1), 467 (5). A band at
2099 cm−1 is assigned to 4-diazo-2-isocyano-2-pentene 23b; calc.
(B3LYP/6-31G*) λ−1/cm−1 3067 (2), 3019 (4), 2975 (6), 2923 (11),
2098 (100) (CN2), 2091 (21) (NC), 1626 (4), 1473 (6), 1466 (2),
1450 (2), 1442 (2), 1388 (1), 1351 (10), 1337 (3), 1213 (1), 1127
(1), 1038 (1), 1025 (5), 978 (1), 844 (2), 822 (1), 784 (1), 636 (1),
564 (1), 539 (1), 499 (1); 5,7-dimethyl-1,2,4-triaza-cyclohepta-2,3,5,7-
tetraene 20b is still present after this photolysis (1979 cm−1). Some
unidentified peaks were present in the nitrile/isonitrile region at 2200,
2191, 2175, 2155, and 2035 cm−1, and between 1600 and 1350 cm−1.
Further photolysis at 313 nm (2.5 h) caused bands at 2099 and
1979 cm−1 to disappear. New peaks growing in their place were
attributed to 1-(2-methyl-3H-aziren-3-yl)ethylidenecyanamide 24b: IR
(Ar, 20 K) λ−1/cm−1 2212 s, 2202 s, 1806 w, 1628 s, 1244 s; calc.
(B3LYP/6-31G*) 3053 (5), 3031 (4), 3021 (19), 2979 (5), 2945 (6),
2930 (3), 2202 (55), 1807 (15), 1629 (100), 1445 (19), 1439 (22),
1374 (22), 1371 (16), 1353 (22), 1222 (92), 1194 (8), 953 (11), 940
(12), 874 (6), 819 (7), 584 (5), 508 (5). Further bands appearing in
the isocyanide region at 2083 and 2073 cm−1 could belong to com-
pounds derived from 23b such as 3-isocyano-1,3-dimethyl-cyclo-
propene and 2-isocyanopenta-2,3-diene; calc. (B3LYP/6-31G*) 3-
isocyano-1,3-dimethyl-cyclopropene: λ−1/cm−1 3017 (18), 3012 (13),
2995 (14), 2987 (7), 2934 (18), 2931 (17), 2119 (100), 1793 (13),
1467 (2), 1455 (5), 1449 (11), 1447 (11), 1381 (3), 1377 (2), 1232
(12), 1107 (28), 1098 (23), 1063 (3), 1024 (3), 988 (6), 951 (4), 881
(11), 725 (20), 712 (11), 646 (6), 525 (4); calc. (B3LYP/6-31G*)
2-isocyanopenta-2,3-diene: λ−1/cm−1 3036 (8), 3028 (13), 3003 (3),
2991 (9), 2979 (11), 2935 (14), 2928 (21), 2114 (100), 1993 (1),
1474 (3), 1459 (4), 1453 (5), 1446 (5), 1376 (4), 1214 (25), 1141
(22), 1067 (1), 1034 (1), 987 (8), 890 (7), 792 (9), 679 (3), 585 (8).
5,7-Dimethyltetrazolo[1,5-a]pyrimidine 18bT and Azide
18bA. Compound 18bT was sublimed at 60 °C in an Ar stream
and subjected to mild FVT at 200 °C with deposition of the products
on a KBr target at 20 K. The IR spectrum of the resulting matrix
1769
dx.doi.org/10.1021/jo4027897 | J. Org. Chem. 2014, 79, 1758−1770

The Journal of Organic Chemistry
Article
(19) Johnson, W. T. G.; Sullivan, M. B.; Cramer, C. Int. J. Quantum
Chem. 2001, 85, 492−508.
ASSOCIATED CONTENT
* Supporting Information
Additional Ar matrix IR, UV and ESR spectra, NMR spectra of
8bT, and computational details. This material is available free of
charge via the Internet at http://pubs.acs.org.
■
S
(20) Chapman, O. L.; Sheridan, R. S.; LeRoux, J. P. J. Am. Chem. Soc.
1979, 101, 3691−3692.
(21) Frisch, M. J. et al. Gaussian 09; Gaussian, Inc., Pittsburgh, PA,
1998 ( the full citation is available in the Supplementary Information).
(22) (a) Wong, M. W. Chem. Phys. Lett. 1996, 256, 391−394.
(b) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502−16512.
(23) Wasserman, E.; Snyder, L. C.; Yager, W. A. J. Chem. Phys. 1964,
41, 1763−1772.
AUTHOR INFORMATION
Corresponding Author
*E-mail: wentrup@uq.edu.au
Notes
■
(24) Levisalles, J. Bull. Soc. Chim. France 1957, 1004−1008.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the Australian Research
Council, the Centre for Computational Molecular Science at
The University of Queensland, and the National Computing
Infrastructure facility financed by the Australian Government
(MAS grant g01). S. Torker is indebted to the Technical
University of Graz, Austria, for a scholarship. We thank Mr. Ian
A. Davis,5b Dr. Ales Reisinger, and Dr. Ullrich Mitschke for
early, exploratory experiments.
REFERENCES
■
(1) Wentrup, C. Acc. Chem. Res. 2011, 44, 393−404.
(2) Addicott, C.; Wong, M. W.; Wentrup, C. J. Org. Chem. 2002, 67,
8538−8546.
(3) (a) Wentrup, C.; Crow, W. D. Tetrahedron 1970, 26, 4915−4924.
(b) Wentrup, C.; Crow, W. D. Tetrahedron 1971, 27, 1566.
(4) Hill, B. T.; Platz, M. S. Phys. Chem. Chem. Phys. 2003, 5, 1051−
1058.
(5) (a) Torker, S. Rearrangements of 2-Pyrimidylnitrene and 3-
Pyridazylnitrene. Ring Expansion and Ring Cleavage; Diploma Thesis;
The University of Queensland (Australia) and Technical University of
Graz (Austria), 2003. (b) Davis, I. A. The Synthesis, Photolysis and
Pyrolysis of some Tetrazoloazines; B.Sc. Honours Thesis; The University
of Queensland (Australia), 1986.
(6) Kvaskoff, D.; Bednarek, P.; George, L.; Waich, K.; Wentrup, C. J.
Org. Chem. 2006, 71, 4049−4058.
(7) Kvaskoff, D.; Bednarek, P.; Wentrup, C. J. Org. Chem. 2010, 75,
1600−1611.
(8) Cerro-Lopez, M.; Gritsan, N.; Zhu, Z.; Platz, M. S. J. Phys. Chem.
A 2000, 104, 9681−9686.
(9) (a) Temple, C., Jr.; Montgomery, J. A. J. Org. Chem. 1965, 30,
826−829. (b) Temple, C., Jr.; Thorpe, M. C.; Coburn, W. C.;
Montgomery, J. A. J. Org. Chem. 1966, 31, 935−938. (c) Wentrup, C.
Tetrahedron 1970, 26, 4969−4983. (d) Shestakova, T. S.; Shenkarev,
Z. O.; Deev, S. L.; Chupakhin, O. N.; Khalymbadzha, I. A.; Rusinov, V.
L.; Arseniev, A. S. J. Org. Chem. 2013, 78, 6975−6982.
(10) Kuzaj, M.; Luerssen, H.; Wentrup, C. Angew. Chem., Int. Ed.
̈
Engl. 1986, 25, 480−482.
(11) Bucher, G.; Ziegler, F.; Wolf, J. J. Chem. Commun. 1999, 2113−
2114.
(12) Wentrup, C.; Kvaskoff, D. Aust. J. Chem. 2013, 66, 286−296.
(13) Ring contraction to 5-membered ring nitriles may take place by
several different mechanisms, (a) concerted, and (b) stepwise via ring
opening.1,7
(14) Wentrup, C.; Crow, W. D. Tetrahedron 1971, 27, 361−366.
(15) (a) Klessinger, M.; Bornemann, C. J. Phys. Org. Chem. 2002, 15,
514−518. (b) Bornemann, C.; Klessinger, M. Chem. Phys. Lett. 2000,
259, 263−271.
(16) Finnerty, J.; Mitschke, U.; Wentrup, C. J. Org. Chem. 2002, 67,
1084−1092.
(17) Gamage, D. W.; Li, Q.; Upul Ranaweera, R. A. A.; Sarkar, S. K.;
Weragoda, G. K.; Carr, P. L.; Gudmundsdottir, A. D. J. Org. Chem.
2013, 78, 11349−11356.
(18) Chapyshev, S. V. Mendeleev. Comm. 2003, 53−55.
1770
dx.doi.org/10.1021/jo4027897 | J. Org. Chem. 2014, 79, 1758−1770