Photolysis and Pyrolysis of Phenyltetrazoles: Formation of Phenylcarbodiimide, N-Phenylnitrile Imine, Phenylnitrene, Indazole, and Fulvenallene

Article abstract of DOI:10.1002/ejoc.201901271

Photolysis of 1-phenyltetrazole 2b at either 193 or 266 nm in a Ne matrix as well as flash vacuum pyrolysis (FVP) of the same compound with isolation of the products in Ne matrix generate phenylcarbodiimide PhN=C=NH 6 very efficiently as the principal product in an almost pure state. Minor amounts of phenyl azide 8 and 1-azacyclohepta-1,2,4,6-tetraene 10, the product of ring expansion of phenylnitrene, are also formed, and the amounts of both substances increase as a function of photolysis time. The photolysis of 2-phenyltetrazole 1b in a Ne matrix at 266 nm for 0.5 min generates the allenic N-phenylnitrile imine PhN=N⁽⁺⁾=CH^(–) 3b, absorbing strongly and cleanly at 2021 cm^–1 in the IR spectrum. At longer photolysis times peaks ascribed to phenyl azide, phenylnitrene, 1-azacycloheptatetraene, and phenylcarbodiimide 6 become prominent. FVP of 2-phenyltetrazole yields indazole 15 as the major product together with a small amount of phenylcarbodiimide 6. Indazole formation is ascribed to cyclization of the nitrile imine in the ground state. It is not formed on FVP of 1-phenyltetrazole in agreement with the computational result that the thermodynamically most favourable path for rearrangement of the putative N-phenylimidoylnitrene 5b yields phenylcarbodiimide 6 rather than phenylnitrile imine 3b. Further pyrolysis of indazole affords a minor yield of fulvenallene 18.

Full text of DOI:10.1002/ejoc.201901271

A Journal of
Accepted Article
Title: Photolysis and Pyrolysis of Phenyltetrazoles: Formation of
Phenylcarbodiimide, N-Phenylnitrile Imine, Phenylnitrene,
Indazole, and Fulvenallene
Authors: Xiaofang Zhao, Qian Liu, Ruijuan Feng, Xiaoqing Zeng, and
Curt Wentrup
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201901271
Link to VoR: http://dx.doi.org/10.1002/ejoc.201901271
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10.1002/ejoc.201901271
European Journal of Organic Chemistry
Photolysis and Pyrolysis of Phenyltetrazoles: Formation of Phenylcarbodiimide,
N-Phenylnitrile Imine, Phenylnitrene, Indazole, and Fulvenallene
Xiaofang Zhao,[a] Qian Liu,[a] Ruijuan Feng,[a] Xiaoqing Zeng,*[a] and Curt Wentrup*[b]
[a]
X. F. Zhao, Q. Liu, R. J. Feng, and Prof. Dr. X. Q. Zeng, College of Chemistry, Chemical
Engineering and Materials Science, Soochow University, Suzhou 215123, P.R. China
E-mail: xqzeng@suda.edu.cn
Prof. Dr. C. Wentrup, School of Chemistry and Molecular Biosciences, The University of
[b]
Queensland, Brisbane, Queensland 4072, Australia
E-mail: wentrup@uq.edu.au; http://researchers.uq.edu.au/researcher/3606
Supporting information for this article and the ORCID identification numbers of the authors
can be found on the WWW under https://doi.org/10.1002/ejoc….
Abstract:
Photolysis of 1-phenyltetrazole 2b at either 193 or 266 nm in a Ne matrix as well as flash
vacuum pyrolysis (FVP) of the same compound with isolation of the products in Ne matrix
generate phenylcarbodiimide PhN=C=NH 6 very efficiently as the principal product in an
almost pure state. Minor amounts of phenyl azide 8 and 1-azacyclohepta-1,2,4,6-tetraene 10,
the product of ring expansion of phenylnitrene, are also formed, and the amounts of both
substances increase as a function of photolysis time.The photolysis of 2-phenyltetrazole 1b in
a Ne matrix at 266 nm for 0.5 min generates the allenic N-phenylnitrile imine PhN=N(+)=CH(-)
3b, absorbing strongly and cleanly at 2021 cm-1 in the IR spectrum. At longer photolysis
times peaks ascribed to phenyl azide, phenylnitrene, 1-azacycloheptatetraene, and
phenylcarbodiimide 6 become prominent. FVP of 2-phenyltetrazole yields indazole 15 as the
major product together with a small amount of phenylcarbodiimide 6. Indazole formation is
ascribed to cyclization of the nitrile imine in the ground state. It is not formed on FVP of 1-
phenyltetrazole in agreement with the computational result that the thermodynamically most
favourable path for rearrangement of the putative N-phenylimidoylnitrene 5b yields
phenylcarbodiimide 6 rather than phenylnitrile imine 3b. Further pyrolysis of indazole
affords a minor yield of fulvenallene 18.
1
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1. Introduction
2- and 5-Substituted tetrazoles 1 are important precursors for the generation of nitrile imines
3, both thermally and photochemically, for use in 1,3-dipolar cycloaddition chemistry.[1] The
synthesis, chemistry and rearrangements of nitrile imines have been reviewed,[2] and these
species can in principle exist in allenic (A) or propargylic (P),[3] or even carbenic[4] forms (C)
(Figure 1), which can be distinguished by IR spectroscopy. The C-phenylnitrile imine 3a,
generated by photolysis of 5-phenyltetrazole 1a, is a unique case, where both allenic and
propargylic forms were observed directly in Ar matrices, and both isomerized
photochemically to the 1H-diazirine 4a.[5]
R1
R2
R2
N
N
R1
N
R1 C N
C
C
N
N
R2
propargylic (P)
allenic (A)
carbenic (C)
Figure 1. Structures of nitrile imines.
The further rearrangement to carbodiimides 6 is known to occur very efficiently in a
highly exothermic reaction on both thermolysis and photolysis of nitrile imines, and this is
believed to take place via short-lived imidoylnitrenes 5 (Scheme 1).[6]
Imidoylnitrenes have been postulated frequently[7] as intermediates in reactions of 1-
substituted tetrazoles 2 but have been observed only recently by ESR spectroscopy.[8]
A
reason for their elusive nature can be found in the very low calculated activation barriers for
ring closure of the open-shell singlet imidoylnitrenes 5 to 1H-diazirines 4 (2-8 kcal/mol) and
the 1,2-shift leading to carbodiimides 6 (2-16 kcal/mol).[6]
To better understand the relationships between these reactive intermediates, we now
report the photochemistry and flash vacuum pyrolysis (FVP) of two isomers of 1a, namely 1b
and 2b.
2
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European Journal of Organic Chemistry
R5
N
R5
N
N
N
N
N
R
R
N
N
1
2
hν or Δ
-N2
-N2
hν or Δ
R5
R5
hν or Δ
~R'
R
R N
N C N R5
C N N
R
N
N
R5
N
R
3
4
6
5
a: R5 = Ph, R = H
b: R5 = H, R = Ph
Scheme 1. Principal reactive intermediates formed from aryltetrazoles.
2. Results and Discussion
2.1. 1-Phenyltetrazole
1-Phenyltetrazole 2b was isolated in neon or argon matrices at 4 K and irradiated with an ArF
laser at 193 nm. The resulting IR spectrum (Figure 2) was shown by comparison with
authentic samples generated or deposited in neon (or argon) matrices to contain four new
products, viz. phenylcarbodiimide PhN=C=NH 6, phenylcyanamide Ph-NH-CN 7, phenyl
azide PhN3 8 and 1-aza-cyclohepta-1,2,4,6-tetraene 10. Phenylcarbodiimide 6 was by far the
major product, observable within three seconds of photolysis with its characteristic main
absorptions at 2167.5 and 2134.3 cm–1 in Ne, and at 2166.6 and 2128.4 cm–1 in Ar matrices.
3
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European Journal of Organic Chemistry
Figure 2. (A) IR difference spectrum showing the photolysis of 1-phenyltyetrazole 2b (bands
pointing downward) on irradiation at 193 nm (pulsed ArF laser, 7.5 mJ, 3 Hz, 15 s) in Ne
matrix at 2.8 K (expanded by a factor 2 along the ordinate). (B) IR difference spectrum
showing the photolysis of 1-phenyltetrazole (bands pointing downward) on irradiation at 193
nm (7.5 mJ, 3 Hz, 32 min) in Ne matrix at 2.8 K (expanded 20 times along the ordinate). (C-
E) Calculated IR spectra for PhNCNH 6, triplet Z-HC(=NPh)N, and singlet HC(=NPh)N 5b.
Assignments: (a) 1-phenyltetrazole 2b, (b) PhNCNH 6 (2167.5 and 2134.3 cm–1), (c)
Ph(H)NCN 7 (2259.5 cm–1), (d) PhN3 8, (e) 1-azacycloheptatetraene 10 (1898.2 cm–1), (*)
1733.6 cm–1 band tentatively assigned to 1H-diazirine 4b (see text). Peaks ascribed to
compounds (b) to (e) were assigned based on separate depositions and literature reports.[3,9]
The formation of these products was also observd on analogous photolysis of 2b at
266 nm (see Figures S1 and S2, Supporting Information). Essentially the same products are
formed at 193 and 266 nm, but the signals due to PhN3 and 1-azacyclpheptatetraene (d and e
in Figure S1) are more pronounced at 266 nm. The temporal evolution of the IR spectra is
illustrated in Figures S3 – S5, Supporting Information.
The photolysis of 2b is expected to lead initially to the open-shell singlet
imidoylnitrene 5b, which rearranges rapidly[6] to phenylcarbodiimide 6, i.e. the same product
as obtained from 1a.[3,5] The carbodiimide 6 is known to isomerize in part to
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10.1002/ejoc.201901271
European Journal of Organic Chemistry
phenylcyanamide 7 on matrix photolysis.[3,5] The formation of 8 and 10 is ascribed to
cycloreversion of 2b to HCN and PhN3, photolysis of the latter to phenylnitrene 9, and ring
expansion to 10 (Scheme 2).[9] It is worth noting that phenylcyanamide 7 is a second source
of triplet PhN 9 by photochemical loss of HCN.[10]
H
Ph
N
N
4b
H
H
N
~H
hν
hν
hν
Ph
N C N
H
Ph-NH-CN
N
N
N
N
Ph
N
-N2
Ph
7
6
5b
2b
-HCN
hν
hν
hν
+
N3
N
HCN
-N2
N
8
9
10
Scheme 2. Matrix photochemistry of 1-phenyltetrazole 2b
The photolyses of 1- and 2-phenyltetrazoles 2b and 1b can give the same products by
interconversion of the nitrile imine 3b, the 1-phenyl-1H-diazirine 4b, and the imidoylnitrene
5b. Calculations[6] indicate that open-shell singlet (OSS) imidoylnitrenes are protected by
barriers of 2-8 kcal/mol toward cyclization to diazirines, and 2-16 kcal/mol for rearrangement
to carbodiimides. The diazirine is protected by barriers of about 14 and 50 kcal/mol toward
rearrangement to the imidoylnitrene and nitrile imines, respectively. Thus, in principle it
should be possible to observe the diazririne 4b and the imidoylnitrene 5b, but the low
activation barriers would make them very prone to rearrangement, and the very efficient
formation of carbodiimide 6 indicates that it will be very difficult to observe the nitrene 5b. It
is seen in Figure 2 that infrared bands potentially assignable to either the Z-triplet nitrene or
the singlet nitrene are too weak for a reliable assignment. The calculated IR spectra of the
triplet Z-and E-nitrenes are very similar, and their energies differ by only 0.06 kcal/mol. The
singlet nitrene lies 9.8 kcal/mol higher (for the calculated IR spectra and energies, see the
Supporting Information). However, an additional, broad band (g) at 1734 cm–1 was observed
at brief photolysis times (Figure 2(A)). It was observed already after 1 s at 193 nm but
disappeared after longer photolysis times (Figure 2(B). For an expanded view, see Figure S5
in the SI). It was also seen within five minutes in the photolysis at 266 nm, but not after
5
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European Journal of Organic Chemistry
extended photolysis.
The 1734 cm–1 band could correspond to the strongest calculated band of the diazirine
4b (1718 cm–1, B3LYP/6-311++G(3df,3dp)). Weak absorptions similar to this, often broad or
structured, have been observed for several 1H-diazirines.[3,5,11] However, the band is too weak
to allow the detection of other infrared bands potentially belonging to the diazirine, and the
attribution must, therefore, remain tentative.
The Z-isomer of imidoylnitrene PhN-CH-N 5b could be expected to cyclize to
benzimidazole 12 (Scheme 3).[6] Putative imidoylnitrenes formed from 1,5-disubstituted
tetrazoles have been reported to undergo such cyclizations, sometimes thermally, sometimes
photochemically, but usually in yields inferior to those of the carbodiimides.[7a] We
performed flash vacuum pyrolysis (FVP) of 2b at 600 oC with isolation of the products in Ne
matrix at 2.8 K. The phenylcarbodiimide 6b was clearly formed as the only major product
(Figure S6). In contrast, the benzimidazole 12 and its 3H-tautomer 11 could at most be
present in trace amounts, and there was also no evidence for the formation of these
compounds in the 193 nm photolysis (Figure S6).
N
H
N
N
N
Ea 22
~1,5-H
-15
N
N
H
H
3b
12 -76
11 -32
Scheme 3. Potential formation of benzimidazole from Z-N-phenylimidoylnitrene. Relative
ground state and activation energies in kcal/mol at the B3LYP/6-311G(d,p) level.[6]
The formation of phenylcarbodiimide and absence of benzimidazole are in agreement
with the calculated activation barriers: 15.7 kcal/mol for the rearrangement to the
carbodiimide and 21.8 kcal/mol for the cyclization to benzimidazole at the B3LYP/6-
311G(d,p) level.[6] Other things being equal, this would correspond to a rate ratio of ~30 at
o
600 C, which would make 12 barely detectable. Furthermore, there is an entropic bias
against the cyclization, which requires the Z-isomer of the nitrene.
2.2. 2-Phenyltetrazole
N-Phenylnitrile imine 3b has been generated for preparative use by photolysis of 2-
phenyltetrazole 1b in benzene solution with a high-pressure Hg lamp and trapped in 1,3-
6
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10.1002/ejoc.201901271
European Journal of Organic Chemistry
dipolar cycloaddition reactions with methyl acrylate and methacrylate.[12] We isolated 1b in a
neon matrix at 4 K and irradiated it with the 266 nm laser for 30 s. The resulting IR spectrum
shows a prominent new species (f) in an almost pure state (Eq. 1 and Figure 3).
N
hν
N
(1)
Ph-N=N=CH
N
-N2
N
3b
Ph
1b
The carrier of this spectrum is ascribed to N-phenylnitrile imine 3b (Eq. 1) on the basis of
excellent agreement with the calculated IR spectrum (Table S2, Supporting Information). The
main absorption at 2021 cm–1 indicates an allenic structure (cf. Figure 1(A)), and this is in
accord with the calculated structures at the B3LYP/6-31G*, CBS-QB3, and QCISD/6-31G*
levels.[3] The 2021 cm–1 absorption is also in agreement with the observation of a single line at
2014 cm–1 in a PVC matrix.[13]
0.03
f
0.02
f
f
0.01
f
f
f
D
A
f
*
f
f
f
0.00
a'
a'
a'
a'
a'
a'
a'
a'
a'
a'
a'
a'
a'
-0.01
a'
a'
a'
-0.02
2400
2100
1800
1500
/ cm-1
1200
900
600
n
Figure 3. IR difference spectrum showing the change of the Ne matrix-isolated 2-
phenyltetrazole 1b upon irradiation at 266 nm (10 mW, 0.5 min). Assignments: (a') 2-
phenyltetrazole 1b, (f) Ph-N=N(+)=C(-)H 3b, * = H2O.
A similar IR spectrum of 3b was obtained on photolysis of 2-phenyltetrazole at 193
nm in Ne matrix at 2.8 K (Figure S7, SI). Subsequent irradiations with blue light and at 365
7
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10.1002/ejoc.201901271
European Journal of Organic Chemistry
nm showed that the intensity of the nitrile imine PhNNCH 3b is strongly reduced at 365 nm.
The 1-azacyclohepta-1,2,4,6-tetraene 10 was partially destroyed at 365 nm.
Thus, irradiation of 2-phenyltetrazole 1b yields the nitrile imine PhN=N(+)=CH(-) 3b as
the principal product. Photolysis of 1b for more than 15 min resulted in further products
(Figures 4-6 and Scheme 4). A competing cleavage of the tetrazole yielded HCN and phenyl
azide 8, and then PhN 9 and 1-azacycloheptatetraene 10. PhN=C=NH 6 and PhNHCN 7 are
formed according to Scheme 1. Further photolysis for 47.5 min resulted in further decrease of
the bands (f) ascribed to 3b, and increased intensities of the bands (g) and (e) due to
phenylnitrene and 1-azacycloheptatetraene (Figure 5). Additional photolysis of this matrix
with blue light yielded an almost pure spectrum of 1-azacycloheptatetaene 10 (Figure 5). The
comparatively large amounts of phenylnitrene 9 and 1-azacycloheptatetraene 10 formed in
these photolyses suggest the direct dissociation of the nitrile imine 3b to PhN 9 and HCN[3,13]
(Scheme 4). The temporal evolution of the infrared bands is illustrated in Figure 6 (see also
Figure S8, SI).
f
e
e/g
0.1
d
f
d
f
ee
e
f
b
e
c
g
f g
e
g
e
0.0
DA
a'
a'
a'
a'
-0.1
a'
a' a'
a'
a'
a'
a'
a'
a'
-0.2
a' a'
a'
1500
n / cm-1
a'
1200
2400
2100
1800
900
600
Figure 4. IR difference spectrum showing the change of the Ne matrix-isolated 2-
phenyltetrazole 1b following irradiation at 266 nm (10 mW) for 5 min. Assignments: 2-
phenyltetrazole (a'), PhN=C=NH 6 (b), PhNH-CN 7 (c), PhN3 8 (d), PhN 9 (g), 1-
azacycloheptatetraene 10 (e), PhNNCH 3b (f). Assignment of peaks ascribed to (b) to (e)
were made as in Figure 2.[3,9]
8
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European Journal of Organic Chemistry
H
H
~H
Ph
Ph
N
N C N
H
N
N
N
Ph
5b
6
4b
hν
H
N
hν
Ph-NH-CN
N
N
N N C
H
-N2
N
7
Ph
3b
Ph
hν
-HCN
1b
-HCN hν
hν
hν
hν
HCN
+
N
N3
-N2
N
9
8
10
Scheme 4. Matrix photochemistry of 2-phenyltetrazole 1b.
e
e
e
1.6
1.2
#
e
e
#
#
e
e e
DA
g
g
g
f
0.8
0.4
0.0
g
g
g
#
#
d
d
g
#
#
b
f
e
c
e e
g
a'
a'
a'
D
-0.4
A
a'
a'
a'
a'
a'
a'
a' a' a'
a'
a' a' a'
-0.8
3200
2800
2400
2000
/ cm-1
1600
1200
800
n
Figure 5. Lower trace: IR difference spectrum showing the change of 2-phenyltetrazole 1b in
Ne matrix upon further irradiation at 266 nm (10 mW) for 47.5 min. Upper trace: IR
difference spectrum showing the change of the same matrix upon blue light irradiation (20
mW, 10 min). Assignments: 2-phenyltetrazole (a'), PhNCNH 6 (b), Ph(H)NCN 7 (c), PhN3 8
(d), 1-azacycloheptatetaene 10 (e), PhNNCH 3b (f), PhN 9 (g). The bands labeled with # at
3142 and 3129 cm–1 may be assigned to (f). The carriers of the bands at 3196, 2087 and 2092
cm–1 are currently unknown.
9
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N - phenylnitrilimine (f)
phenylcyanamide (c)
phenyl azide (d)
phenylcarbodiimide (b)
didehydroazepine (e)
phenyl nitrene (g)
2-phenyltetrazole (a')
5.4
4.8
4.2
3.6
3.0
2.4
1.8
1.2
0.6
0.0
0
5
10
15
20
25
t (min)
30
35
40
45
50
Figure 6. Temporal changes of the IR band intensities for the products of the 266 nm
photolysis of 2-phenyltetrazole 1b.
Tetrazole 1b was also subjected to FVP at 800 oC with subsequent matrix isolation of
the products in neon matrix at 4 K (Figure 7; see also Figure S9).
Figure 7. (A) IR spectrum of 2-phenyltetrazole 1b in Ne matrix at 2.8 K. (B) IR spectrum of
the products of FVP of 2-phenyltetrazole at 800 oC (Ne-matrix at 2.8 K). (C) IR spectrum of
10
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10.1002/ejoc.201901271
European Journal of Organic Chemistry
indazole 15 in Ne matrix at 2.8 K. The main product of pyrolysis is indazole 15. Bands at
2133 and 2167 cm–1 are due to phenylcarbodiimide 6.
In the FVP of 2-phenyltetrazole 1b, weak-to-medium intensity bands assigned to
PhN=C=NH 6 and PhNHCN 7 are formed as expected (Figure S9). But in this case the
cyclization of the N-phenylnitrile imine 3b to indazole 15 has a lower calculated barrier (27
kcal/mol) than the rearrangement to phenyldiazirine 4b (40 kcal/mol)[6] and hence N-
phenylimidoylnitrene 5b and phenylcarbodiimide 6. Therefore, indazole 15 is in fact the
main product (Scheme 5) as clearly seen in the IR spectrum of the pyrolysate (Figure 7).
Thus, carbodiimide formation is favoured in the excited state (photolysis) reaction, but
indazole formation is favoured in the ground state. Indazole itself is largely unaffected by
FVP under these conditions (Figure S10, Supporting Information), but photolysis at 266
causes changes, probably due to formation of tautomers 14 and/or 13, but this phenomenon
was not investigated in detail.
N
hν
Ea 27
N
Ph-N=N=CH
N
-N2
FVP
N
3b
Ph
1b
H
H
H
N
H
~1,5-H
~1,5-H
4
~1,5-H
0.3
N
N
NH
8
N
N
N
N
H
15 -53
13 -33
14 -48
12 -4
~1,5-H
H
H
FVP
-N2
N
N
C CH2
H
N2
17
18
FVP
16
20
19
Scheme 5. The nitrile imine – indazole – fulvenallene path. Calculated relative energies and
activation energies in kcal/mol at the B3LYP/6-311G(d,p) level.[6]
It is noteworthy that indazole is not formed on FVP of 1-phenyltetrazole 2b in
11
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agreement with the computational finding that the activation barrier for rearrangement of the
putative N-phenylimidoylnitrene 5b to phenylcarbodiimide 6 is 15.7 kcal/mol, whereas it is
about 50 kcal/mol for rearrangement to phenylnitrile imine 3b.[6]
According to Scheme 5, the FVP of 1b yields initially 3H-indazole 13. Therefore,
fulvenallene 18 should also form via the diazo compound 16 and the carbene 17, or less
likely via phenyldiazomethane 20.[14,15,16] Bands at 1935 (1930), 1488, and 887 cm–1 in Figure
7 and Figure S9) can be assigned to fulvenallene (lit. 1929, 1488, and 886 cm–1).[17] Under our
pyrolysis conditions, the yields of rearrangement of fulvenallene to ethynylcyclopentadienes
19 are very low,[14,15] and in fact no bands ascribable to 19 are observable in Figures 7 and S9.
3. Conclusion
Matrix photolysis of 1-phenyltetrazole 2b at 193 or 266 nm generates phenylcarbodiimide
PhN=C=NH 6 very efficiently. It is believed to be formed by facile rearrangements of
transient 1-phenyl-1H-diazirine 4b and N-phenylimidoylnitrene 5b. In addition, minor
amounts of phenyl azide 8 and 1-azacyclohepta-1,2,4,6-tetraene 10 are formed, the latter
being the rearrangement product of phenylnitrene 9. Phenyl azide is the result of a
cycloreversion of the tetrazole, but phenylnitrene 9 can also be formed by photolysis of
phenylcyanamide 7, which is formed by isomerization of the carbodiimide 6. The
carbodiimide 6 is also the principal product of flash vacuum pyrolysis (FVP) of tetrazole 2b.
The rearrangement of the putative imidoylnitrene 5b to carbodiimide 6 has a calculated
activation barrier of ~16 kcal/mol, and it takes place to the exclusion of the potential
cyclization to benzimidazole 12 with a calculated barrier of ~22 kcal/mol.
Analogous matrix photolysis of 2-phenyltetrazole 1b generates N-phenylnitrile imine
3b almost exclusively. However, on further photolysis phenylcarbodiimide 6, phenyl azide 8,
phenylnitrene 9 and 1-azacyclohepta-1,2,4,6-tetraene 10 are formed, the latter in substantial
amounts. The FVP of 2-phenyltetrazole produces a minor amount of phenylcarbodiimide 6,
but the main product is now indazole 15, formed by cyclization of the nitrile imine 3b.
Notably, 1-phenyltetrazole 2b does not yield 3b and also not indazole on FVP. Indazole
undergoes minor decomposition to fulvenallene 18 under the FVP conditions.
Experimental
Phenyltetrazoles 1b and 2b and indazole 15 were commercial samples. Matrix IR spectra
were recorded on a Fourier transform infrared (FT-IR) spectrometer (Bruker 70V) in the
12
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10.1002/ejoc.201901271
European Journal of Organic Chemistry
attenuated total reflectance mode using transfer optics. A KBr beam splitter and a liquid
nitrogen-cooled mercury cadmium telluride (MCT) detector were used in the 4000–500 cm–1
region. Scans were recorded with a resolution of 0.5 cm–1. For deposition and pyrolysis,
samples were sublimed into the pyrolysis tube in a current of Ne (or in some cases Ar) with
an estimated sample/matrix gas ratio ≈ 1 : 1000.
The deposition/sublimation/pyrolyses tube was a quartz tube divided in two zones: FVP zone
o.d. 3.0 mm, i.d. 0.6 mm, length 14 mm; sublimation zone o.d. 5.6 mm, i.d. 3.7 mm, length
14.8 mm. The distance between the two zones was 22 mm, and the heating was provided by a
tantalum wire, o.d. 0.4 mm, length 25 mm, with a maximal temperature of 800 oC.
The resulting mixture was immediately deposited (~2 mmol/h) onto a Rh-plated copper block
matrix support at 2-4 K for Ne or 10-25 K for Ar, which was subsequently cooled to 2.8 K
for Ne or 10 K for Ar for recording of IR spectra and photolyses. The system was maintained
in high vacuum (~10–6 Pa). Photolyses were performed using an ArF excimer laser (Gamlaser
EX5/250, 193 nm), a Nd3+:YAG laser (266 nm, 532 nm, MPL-F-266, 10mW), UV lamp (365
nm), and a high-power flashlight (Boyu T648, 20 W) with adjustable sources of purple light
(400±20 nm).
Calculations
The calculations of structures, energies and IR spectra were carried out using the B3LYP
functional[18] with the 6-311++G(3df,3pd) basis set implemented in the Gaussian 09
software.[19] Scaling factors were not used for IR specgtroscopic assignments.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (21673147)
and by the Queensland Cyber Infrastructure Foundation at The University of Queensland
(QCIF g01).
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10.1002/ejoc.201901271
European Journal of Organic Chemistry
Graphical abstract
Matrix photolyses and flash vacuum pyrolyses have been used to generate the title
compounds from 1- and 2-phenyltetrazoles. Specifically, phenylcarbodiimide 6 and N-
phenylnitrile imine 3 are obtained in states of high purity, and there are three routes to
phenylnitrene.
N
N
N
N
N
N
Ph
Ph
N
N
H
H
2
1
Ph
Ph
Ph
N
4
hν/Δ
N
N
hν/Δ
N
5
N C N
H
C N N
H
Ph
6
3
X. Q. Zeng, Q. Liu, X. F. Zhao, C. Wentrup
Page nr – page nr
Photolysis and Pyrolysis of Phenyltetrazoles: Formation of Phenylcarbodiimide,
N-Phenylnitrile Imine, Phenylnitrene, Indazole, and Fulvenallene
Key topic: Tetrazoles photolysis and FVP
16
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