Photochemical study of tris(benzotriazol-1-yl)methane

Article abstract of DOI:10.1021/jo061851d

(Chemical Equation Presented) Photodecomposition of tris(benzotrizol-1-yl) methane (1) in benzene gives [1-benzotryazol-1-yl-methylidene]-biphenyl-2- ylamine (2) resulting from the loss of the benzotriazolyl radical and nitrogen followed by addition of benzene. Elimination of the second benzotriazolyl radical from 2 provides the biphenyl-2-ylmethyleneamine radical, which affords phenantridine (3) after ring closure. In contrast, the photolysis of 1 in methanol gives a high yield of benzotriazole (4).

Full text of DOI:10.1021/jo061851d

Photochemical Study of Tris(benzotriazol-1-yl)methane
Dmitry A. Androsov and Douglas C. Neckers*
Center for Photochemical Sciences,† Bowling Green State UniVersity, Bowling Green, Ohio 43403
neckers@photo.bgsu.edu
ReceiVed September 7, 2006
Photodecomposition of tris(benzotrizol-1-yl)methane (1) in benzene gives [1-benzotryazol-1-yl-meth-
ylidene]-biphenyl-2-ylamine (2) resulting from the loss of the benzotriazolyl radical and nitrogen followed
by addition of benzene. Elimination of the second benzotriazolyl radical from 2 provides the biphenyl-
2-ylmethyleneamine radical, which affords phenantridine (3) after ring closure. In contrast, the photolysis
of 1 in methanol gives a high yield of benzotriazole (4).
Introduction
1-phenylbenzotriazole on irradiation gave carbazole in nearly
quantative yield.4 Prolonged irradiation of benzotriazole and its
derivatives gives poor yields of isolable products formed by
nitrogen elimination.5-7 Nevertheless, some derivatives of
benzotriazole have been propagated as a new family of light-
activated DNA cleaving agents (triazole photonucleases).8 Two
nitrogen molecules are lost during the photolysis of 1-ami-
nobenzotriazole. One nitrogen molecule is lost on photolysis,
and another nitrogen is lost thermally.9 Photochemical reaction
of 1-aminobenzotriazole in polymer film was used for modula-
tion of various properties such as its solubility, absorption, and
refractive index.10
We studied tris(benzotriazo-1-yl)methane (1) as a potential
component that can extrude nitrogen molecules. Such a system
could find application as a molecular part of novel polymer
compositions, which can be used for read-only holographic
memories, waveguide lithography, microlenses, and other optical
elements.11-15 In this report we present the mechanisms of
The benzotriazole ring is highly stable to acids and bases,
oxidation and reduction, and heat. Ring opening by pyrolysis
requires forcing conditions (200 °C) and 1-alkylbenzotriazoles
afford complex rearranged products in low yields after elimina-
tion of nitrogen.1 Flash-vacuum pyrolysis (500-700 °C) of
N-vinylbenzotriazoles yields N-phenylketene imines with loss
of nitrogen.2 Photolysis of 1-aryl-substituted benzotriazoles
extrudes nitrogen to form diradicals,3 which then cyclize: thus
† Contribution No. 614.
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150. (c) Hubert, A. J. J. Chem. Soc. C 1969, 1334-1336. (d) Tsujimoto,
K.; Ohashi, M.; Yonezava, T. Bull. Soc. Chem. Jpn. 1972, 45, 515.
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90, 1923.
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Pharmacol. Res. 1997, 20, 197.
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(10) Murase, S.; Horie, K. Macromolecules 1999, 32, 1103.
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10.1021/jo061851d CCC: $37.00 © 2007 American Chemical Society
Published on Web 01/26/2007
1148
J. Org. Chem. 2007, 72, 1148-1152

Tris(benzotriazol-1-yl)methane
SCHEME 1
SCHEME 2
SCHEME 4
SCHEME 3
SCHEME 5
photochemical dissociation of 1. The effect of the nature of the
solvents used and wavelengths employed is discussed.
Results and Discussion
Our attention was initially focused on the presumption that
1 with three benzotriazole rings can liberate nitrogen in a
stepwise fashion. A possible product of the reaction might be
9-phenyl-9H-4,b,9,10-triazaindeno[1,2-a]indene (5). This prod-
uct is a planar cyclic system containing four conjugated rings
(Scheme 1).
benzotriazole (4) could be isolated by using more concentrated
solutions of 1 and a moderate time of irradiation (1-2 h)
(Scheme 3).
Compounds 2 and 3 could be isolated from column chroma-
tography. The structure of 2 was established by reducing it with
LiAlH4 in THF. The problem in the spectroscopic identification
of 2 results from negligible intensity of its molecular ion (m/z
298) in GS/MS and MALDI experiments. Moreover, m/z 269
attributed to [M - N2 - H] is detected by GC/MS and m/z 271
corresponding to [M - N2 + H] is detected by MALDI. In
contrast, the products of reduction of 2 (2-methylaminobiphenyl,
7 and 4) are easily identifiable (Scheme 4).
Compound 1 was prepared from benzotriazole and chloroform
according to a literature procedure (Scheme 2).16
When diluted benzene solutions of 1 were subjected to
prolonged irradiation with 300 nm light, only phenantridine (3)
and 2-aminobiphenyl (6) were observed, in nearly quantative
yield. Other intermediates of the photodecomposition of 1 to
[1-benzotryazol-1-ylmethylidene]biphenyl-2-ylamine (2) and
We also irradiated 1 in deuterated benzene. This produced
the products 2′ and 3′ (Scheme 5). The evidence for this was
(14) Chandross, E. A.; Pryde, C. A.; Tomlinson, W. J.; Wever, H. P.
Appl. Phys. Lett. 1974, 24, 72.
(15) Pham, V. P.; Galstyan, T.; Granger, A.; Lessard, R. A. Jpn. J. Appl.
Phys. 1997, 70, 2940.
(16) Katritzky, A. R.; Yang, Z.; Lam, J. M. Synthesis 1990, 666.
J. Org. Chem, Vol. 72, No. 4, 2007 1149

Androsov and Neckers
FIGURE 1. Photodecomposition of 1 in C6D6 at λ ) 254 nm (C0 ) 4.09 × 10-3 mol/L) (A) and at λ ) 300 nm (C0 ) 1.201 × 0-2 mol/L)
(B) and relative formation of products 2 and 3.
SCHEME 6
SCHEME 7
FIGURE 2. UV spectra of 1 (c) and its photoproducts, recorded in
cyclohexane.
An independent experiment showed that 4 converts into 6 in
benzene (C0 ) 4.2 × 10-3 mol/L) after 12 h of irradiation at
300 nm (Scheme 6).
the resonance of CHdN moiety protons at 8.70 ppm in
Compelling evidence that suggests elimination of the ben-
zotriazolyl radical being the initial step is obtained during
characteristic thermal decomposition of 1. Only the benzotria-
zolyl radical elimination was observed under the conditions of
the GC/MS experiments. The thermal decomposition of 1 in
the GC column at 270 °C affords bis(benzotriazol-1-yl)methane
detected as a broad peak giving the intensive molecular ion m/z
249 (Scheme 7).
semideuterted 2′ and at 9.13 ppm in 3′. This indicates that the
H-atom of the CH moiety of 1 keeps its position through the
reaction. The GC/MS confirmed the incorporation of a single
deuterobenzene molecule resulting in the formation of 2′ (m/z
303) and 3′ (m/z 183).
The analysis of the reaction mixture at differing degrees of
conversion of 1 indicates whether elimination of the benzo-
triazolyl radical or loss of the nitrogen molecule is the initial
step (Scheme 6). Monitoring of the reaction at short times shows
that 4 prevails in the reaction mixture, but in the process the
amount of semideuterated analogue of 2-aminobiphenyl (6)
gradually increases.
Additional evidence for primary benzotriazolyl radical elimi-
nation is provided by the DIP-MS of 1. Only an intense peak
corresponding to benzotriazolyl radical elimination was observed
(m/z 249) and no peak corresponding to nitrogen extrusion was
detected.
The benzotriazolyl radical should be rather stable. Though it
should abstract H-atom from the substrate or solvent, the absence
of biphenyl in the reaction mixture ruled out this abstraction
from benzene.
Profiles of 1 photodecomposition in C6D6 at λ ) 254 nm
(C0 ) 4.091 × 0-3 mol/L) and at λ ) 300 nm (C0 ) 1.20 ×
1
10-2 mol/L) were obtained by using H NMR (Figure 1).
1150 J. Org. Chem., Vol. 72, No. 4, 2007

Tris(benzotriazol-1-yl)methane
SCHEME 8
SCHEME 9
Compound 2 loses 4 resulting in the formation of 3 via the
radical 8. In turn, 4 loses nitrogen upon irradiation and forms
6 (Scheme 10).
Sensitized Experiments. No photodecomposition was ob-
served after irradiation of 1 in benzene (C0 ) 1.36 × 10-3 mol/
L) at 350 nm for 25 h. Using 1 equiv of triplet sensitizer such
as benzophenone or triphenylamine in the presence of air gave
only traces of photodecomposition under the same conditions.
Irradiation of an argon purged 1:1 mixture of 1 and triphenyl-
amine (ET ) 291 kJ/mol17) afforded a mixture of starting
material and 3 in the ratio of 1:1 after 50 h of irradiation. The
same mixture of 1 and 3 in the ratio 1.86:1 was observed after
21 h of irradiation (350 nm) when benzophenone (ET ) 287
kJ/mol17) was used as the sensitizer. Only traces of 2 were
detected in both cases. The considerable absorbance of 2 in the
350 nm region may be the reason for the higher rate of
photodecomposition than the rate of its formation at this
wavelength.
The similarity (absence of new products and pathways of the
reaction) between photodecomposition of 1 in the presence and
absence of triplet sensitizers suggests the reaction passes through
the exited triplet state.
In a proton donating solvent such as methanol, photodecom-
position of 1 results in 4 (yield 47%). Some amount of resinous
products was also observed. However, no anticipated intermedi-
ate products were detected after 48 h of irradiation at 254 nm
as shown Scheme 11. Thus, the only way 1 photodecomposes
in methanol is by extrusion of the benzotriazolyl radical. No
loss of nitrogen takes place.
When irradiated at 254 nm, a larger amount of 2 was formed
in comparison with 300 nm light being used as the irradiation
source (Figure 1). This can be explained by the strong
absorbance of benzene (solvent) in the 254 nm region. In
contrast, benzene does not absorb at 300 nm while the
intermediate imine 2 absorbs intensively at this wavelength. So,
2 does not accumulate in reasonable amounts during 300 nm
irradiation. The UV-vis absorption spectra of 1 and its
photoproducts are presented in Figure 2.
Formation of a yellow intermediate was observed following
irradiation of 1 in dilute MeOH (C0 ) 4.09 × 10-5 mol/L). At
the end of photodecomposition the yellow color disappears
resulting in a clear solution that does not contain 4 (see the
Supporting Information). These results correlate with the
observation of Wirz18 et al., who showed that irradiation of 4
in acidic water gave only a low yield of 2-aminophenol. In
neutral and basic water solutions only resinous products of
2-aminophenol photooxidation were formed.
The main route of photodecomposition of 2 also involves
the initial step of elimination of the benzotriazole ring. The
transfer of H-atom from 7 to the benzotriazolyl radical followed
by cyclization of 8 results in the formation of 3. GC monitoring
of the reaction at short times of irradiation shows that 4 prevails
in the reaction mixture but in the process the amount of 6
gradually increases (Scheme 8).
Replacement of benzene by deuterated benzene does not
influence the nature of 3, but leads to the formation of
semideuterated 2-aminobiphenyl (6′). GC/MS analyses of solu-
tions of 2 in C6H6 and C6D6 (C0 ) 5.45 × 10-3 mol/L) after
24 h of irradiation at 300 nm show only 3 and 6 or 3 and 6′,
respectively (Scheme 9). Formation of 3, 6, and 6′ was also
confirmed by NMR spectroscopy.
Conclusions
In conclusion, it was established that photodecomposition of
1 involves benzotriazol ring elimination affording the bis-
(benzotriazol-1-yl) radical (9). The fate of the radical 9 depends
on the solvent. In benzene, 9 liberates nitrogen molecule and
combines with a molecule of solvent. This leads to the formation
In summary, photodecomposition of 1 proceeds by elimina-
tion of the benzotriazolyl radical giving bis(benzotriazol-1-yl)-
methyl radical (9), which converts to an intermediate radical
(10) after extrusion of nitrogen. Benzene adds to 10 forming
11, which converts to intermediate 2 via abstraction of H-atom.
(17) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochem-
istry; Marcel Dekker, Inc.: New York, 1993.
(18) Wang, H.; Burda, C.; Persy, G.; Wirz, J. J. Am. Chem. Soc. 2000,
122, 5849.
J. Org. Chem, Vol. 72, No. 4, 2007 1151

Androsov and Neckers
SCHEME 10
SCHEME 11
1/8) to give the first fraction (Rf 0.55) [1-benzotryazol-1-ylmeth-
ylidene]biphenyl-2-ylamine 2 (0.092 g; 0.31 mmol), after crystal-
lization from dichloromethane/hexanes [colorless needless; mp
1
120 °C; H NMR (DMSO-d6) δ 7.33-7.59 (m, 11H), 7.92-7.98
(m, 1H), 8.16-8.22 (m, 1H), 9.59 (s, 1H); 13C NMR (DMSO-d6)
δ 114.2, 119.8, 119.9, 126.4, 127.0, 127.2, 128.0, 128.9, 129.8,
130.0, 130.1, 130.4, 136.5, 139.4, 144.5, 145.9, 146.0; m/z (EI+
mode) 298 (M+, 4%), 269 (100), 180 (55), 152 (80); HRMS (EI+
mode) calcd for M+ 298.1218, found 298.1223], and the second
fraction (Rf 0.20) phenantridine 3 (0.04 g; 0.22 mmol) [white solid;
1
mp 100 °C (CAS Registry No. 229-87-8); H NMR (CDCl3) δ
6.63-6.77 (m, 3H), 7.81-7.87 (m, 1H), 8.02 (dd, 1H), 8.53-8.60
(m, 2H), 9.27(s, 1H); 13C NMR (CDCl3) δ 121.8, 122.2, 124.1,
126.3, 127.0, 127.4, 128.6, 128.7, 130.1, 130.9, 132.5, 144.5, 153.5;
m/z (EI+ mode) 179 (M+, 100)].
Reduction of [1-Benzotryazol-1-yl-methylidene]biphenyl-2-
ylamine (2). To the well-stirred mixture of 2 (52 mg; 0.17 mmol)
in 30 mL of dry THF was added LiAlH4 (39 mg; 1 mmol). The
mixture was stirred for 10 min. Several drops of methanol and
several drops of acetic acid were added to neutralize the solution.
Solvent was evaporated under reduced pressure. The residue was
chromatographed (ethyl acetate/hexanes 1/8) to give 2-methylami-
nobiphenyl 7 (CAS Registry No. 14925-09-8) (20 mg; yield 63%)
as the first fraction [Rf 0.85; 1H NMR (CDCl3) δ 2.79 (s, 3H), 3.95
(br s, 1H), 6.69 (d, 1H), 7.09 (dd, 1H), 7.23-7.47 (m, 6H); 13C
NMR (CDCl3) δ 30.7, 109.8, 116.8, 127.1, 127.6, 128.7, 128.8,
129.4, 130.0, 139.5, 146.2; m/z (EI+ mode) 183 (M+, 100), 167
(38)] and benzotriazole 4 (CAS Registry No. 95-14-7) (15 mg; yield
of 3. Methanol as a proton donating solvent promotes trans-
formation of 9 into 4.
Experimental Section
Tris(benzotriazol-1-yl)methane (1). A mixture of benzotriazole
(2.0 g; 0.5 mol), aq NaOH (40%, 50 mL), and Bu4NBr (1.6 g) in
CHCl3 (50 mL) is heated under reflux for 48 h. The reaction mixture
is cooled and the organic material extracted with CHCl3 (2 × 100
mL). The organic layer is washed with H2O (5 × 50 mL) and
concentrated at reduced pressure to give a brown solid. The pure
product (29 g; yield 46.8%) was obtained after crystallization from
CHCl3-MeOH, with the addition of charcoal. White needles, mp
1
72%) as the second fraction [Rf 0.20; H NMR (C6D6) δ 6.96-
7.02 (m, 2H), 7.68 (br s, 2H), 14.35 (br s, 1H); m/z (EI+ mode)
119 (M+, 64), 91 (64), 83 (43), 64 (100)].
Acknowledgment. We thank Dr. Eugene Danilov and Prof.
Michael A. J. Rodgers for valuable discussions. The support of
the Ohio Department of Development Grant TECH 03-054 is
gratefully acknowledged.
1
197-198 °C (lit.16 mp 194-196 °C). H NMR (C6D6) δ 6.75-
6.78 (m, 6H), 6.98-7.01 (m, 3H), 7.75-7.79 (m, 3H), 9.59 (s,
1H). 13C NMR (CDCl3) δ 78.1, 110.0, 120.7, 125.4, 129.5, 132.0,
146.4. m/z (EI+ mode) 367 (M+, 25%), 249 (100), 221 (58), 192
(85), 166 (35), 140 (17).
Supporting Information Available: General information,
[1-Benzotryazol-1-yl-methylidene]biphenyl-2-ylamine (2). A
solution of tris(benzotriazol-1-yl)methane 1 (0.3 g; 0.82 mmol) in
200 mL of benzene was irradiated for 24 h with 254 nm UV light
in a Rayonet photoreactor. Solvent was evaporated under reduced
pressure. The residue was chromatographed (ethyl acetate/hexanes
materials and methods, characterization data, and copies of MS,
1
UV-vis, and H and 13C NMR spectra of all compounds. This
materialisavailablefreeofchargeviatheInternetathttp://pubs.acs.org.
JO061851D
1152 J. Org. Chem., Vol. 72, No. 4, 2007