Gas-phase thermolysis of benzotriazole derivatives. Part 2: Synthesis of benzimidazo[1,2-b]cinnolines, a novel heterocyclic ring system, by pyrolysis of benzotriazole derivatives. Kinetic and mechanistic study

Article abstract of DOI:10.1016/j.tet.2003.09.009

Gas-phase pyrolysis of benzotriazolyl ketones and their arylhydrazones gave indole, benzimidazole and benzimidazo[1,2-b]cinnoline derivatives via interesting novel routes. The homogeneous first-order gas-phase pyrolysis of 10 arylhydrazono derivatives of α-benzotriazol-1-yl ketones was investigated over the temperature range 420-530 K. Five of these substrates were 2-(arylhydrazono)-2-(benzotriazol-1-yl)-1-phenylethanone derivatives (Series 1), and the remaining five were the corresponding acetone analogues (Series 2). The values of the Arrhenius frequency factor (logA/s^-1) for the pyrolysis of the compounds of Series 1 and 2 were, respectively, 12.27±1.44 and 9.07±1.20, while the values of the Arrhenius activation energy (E_a/kJmol^-1) were 132.8±8.4 and 123.2±23.0, respectively. Besides, reaction pathways are offered to rationalize the kinetic results and to account for the products of pyrolysis of the substrates under study, namely: (i) extrusion of N₂ and formation of a 1,3-biradical reactive intermediate leading to substituted imidazoles (Series 1); (ii) intramolecular nucleophilic addition, cyclization and subsequent fragmentation with or without loss of H₂O/N ₂ fragments (Series 1 and 2).

Full text of DOI:10.1016/j.tet.2003.09.009

Tetrahedron 59 (2003) 9455–9464
Gas-phase thermolysis of benzotriazole derivatives. Part 2:
Synthesis of benzimidazo[1,2-b]cinnolines, a novel heterocyclic
ring system, by pyrolysis of benzotriazole derivatives.
Kinetic and mechanistic study
*
Hicham H. Dib, Nouria A. Al-Awadi, Yehia A. Ibrahim and Osman M. E. El-Dusouqui
Department of Chemistry, Kuwait University, P.O. Box 5969, Safat 13060, Kuwait
Received 7 April 2003; revised 11 August 2003; accepted 4 September 2003
Abstract—Gas-phase pyrolysis of benzotriazolyl ketones and their arylhydrazones gave indole, benzimidazole and benzimidazo[1,2-b]-
cinnoline derivatives via interesting novel routes. The homogeneous first-order gas-phase pyrolysis of 10 arylhydrazono derivatives of
a-benzotriazol-1-yl ketones was investigated over the temperature range 420–530 K. Five of these substrates were 2-(arylhydrazono)-2-
(benzotriazol-1-yl)-1-phenylethanone derivatives (Series 1), and the remaining five were the corresponding acetone analogues (Series 2).
The values of the Arrhenius frequency factor (log A/s21) for the pyrolysis of the compounds of Series 1 and 2 were, respectively, 12.27^1.44
and 9.07^1.20, while the values of the Arrhenius activation energy (Ea/kJ mol21) were 132.8^8.4 and 123.2^23.0, respectively. Besides,
reaction pathways are offered to rationalize the kinetic results and to account for the products of pyrolysis of the substrates under study,
namely: (i) extrusion of N2 and formation of a 1,3-biradical reactive intermediate leading to substituted imidazoles (Series 1);
(ii) intramolecular nucleophilic addition, cyclization and subsequent fragmentation with or without loss of H2O/N2 fragments (Series 1
and 2).
q 2003 Elsevier Ltd. All rights reserved.
1. Introduction
dazines,6 benzimidazo[2,1-a]isoquinolines,7 benzimidazo-
[2,1-b]benzothiazoles,8 phenanthridines,9 benzoxazoles,7,10
and quinolines11 as well as other heterocyclic systems
became readily available.
Many derivatives of benzotriazole (BT) have been shown to
undergo pyrolytic transformation mainly through initial loss
of N2 and subsequent intramolecular cyclization leading to
facile clean synthesis of a wide range of heterocyclic
compounds.1,2 Thus, substituted benzotriazoles of the
general formula A were reported to pyrolyse into the
corresponding condensed heterocyclic system B.
In the present work, we wish to report further synthetic
application and novel pyrolytic products of benzotriazole
derivatives, and to complement our earlier kinetic and
mechanistic study on the gas-phase thermolysis of a-benzo-
triazol-1-yl ketones,1 by providing data related to the effect
of the arylhydrazono substituent on reactivity, and its
influence on the course of the thermal gas-phase elimination
reaction of these ketones. The 10 arylhydrazono a-benzo-
triazol-1-yl ketones under consideration (Scheme 1) include
five 2-arylhydrazono-1-phenylethanone derivatives (Series
1a–e) and five acetone analogues (Series 2f–j).
2. Results and discussion
2.1. Synthesis
In this way, several derivatives of indole,1,3 carbazole,4
isomeric pyridoindoles,4d,5 pyrido[1,2-a]benzimidazoles,5
benzimidazo[1,2-a]pyrimidines,6 benzimidazo[1,2-b]pyri-
Compounds 1a–e and 2f–j were prepared by coupling
1-(benzotriazol-1-yl)-acetophenone and 1-(benzotriazol-1-yl)-
acetone with the appropriate aryldiazonium chloride in
basic ethanolic media. Good yields ($55%) of 1a–e, 2i and
Keywords: benzotriazole ketones; arylhydrazono group; pyrolysis; kinetics;
mechanism.
*
Corresponding author. Tel.: þ965-4845098; fax: þ965-4836127;
e-mail: nouria@kuc01.kuniv.edu.kw
0040–4020/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2003.09.009

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H. H. Dib et al. / Tetrahedron 59 (2003) 9455–9464
established between the syn and anti isomers under the
influence of the solvent (CDCl3), and the relative strength of
the hydrogen bonds between the NH group and the nitrogen
of benzotriazole for the syn isomer, and the bond between
the hydrogen of the NH group and the carbonyl oxygen for
the anti isomer. 13C NMR spectra show a larger downfield
shift of the carbonyl signal in the case of anti than for syn.
Scheme 1. 2-Arylhydrazone-2-(benzotriazol-1-yl)-1-phenylethanone
(1a–e) and their acetone analogues (2f–j).
2.2. Pyrolysates and mechanism of pyrolytic reactions
Reaction products from complete gas-phase pyrolysis of
substrates 1a–e (Scheme 2) and 2f–j (Scheme 3) were
obtained at optimal reactor conditions of temperature,
pressure (0.06 mbar) and substrate residence time com-
patible with .98% reaction established for kinetic runs. The
pyrolysates were carefully analyzed and their constituents
fully characterized using preparative thin layer chroma-
tography (PLC), GC/MS/MS, 1H and 13C NMR techniques.
2j were obtained using ethanolic sodium hydroxide
solution. Synthesis of 2f–h and 2j was achieved using
sodium acetate in lieu of NaOH. Attempts to prepare 2f–h
using ethanolic sodium hydroxide gave a mixture of
products, from which (in the case of 2h) the formazan 3c
was isolated in pure form. Formazan (3c), prepared for the
first time in the present work, was identified by its MS: Mþz
1
at m/z¼369, H NMR (CDCl3): signals at d¼2.43 (s, 6H,
2CH3), 7.29 (d, J¼7.1 Hz, 4H, ArH), 7.47 (t, J¼7.5 Hz, 1H,
ArH), 7.60 (m, 5H, ArH), 7.91 (d, J¼8.4 Hz, 1H, ArH), 8.19
(d, J¼8.3 Hz, 1H, ArH), 14.80 (br s, 1H, NH); and 13C
NMR (CDCl3): signals at d¼21.77, 112.26, 119.44, 120.33,
124.69, 128.61, 130.64, 133.03, 136.83, 139.37, 145.34,
146.13. The presence of 12 carbon signals in the 13C NMR
indicates the existence of 3c in an equilibrium state of the
strongly H-bonded structures I and II whereby the two aryl
groups appear equivalent in the NMR spectrum. It appears
that in the case of these compounds (2f–h), the presence
of strong base led to further azo-coupling of the Japp–
Klingemann type in which the acetyl group is replaced by
the p-substituted arylhydrazone.12,13
The pyrolysates from the gas-phase elimination reaction of
the arylhydrazonophenylethanone a-benzotriazol-1-yl
ketones 1a–e included products whose MS showed Mzþ
peaks corresponding to a lost fragment of 28 mass units
indicating extrusion of molecular nitrogen from the ketone
substrates. Based on MS, 1H and 13C NMR analyses, these
products were identified as the (1-anilino-1H-benzimidazol-
2-yl)phenylmethanone compounds 4a–e (Scheme 2). The
pyrolysates also contained 2-benzoylbenzimidazole (5),14
aniline and p-substituted anilines (6a–e).15 Typically,
spectral characterization of compound 4d (Scheme 2)
showed 1H NMR signals at d¼6.52 (d, J¼8.7 Hz, 2H,
H10), 7.14 (d, J¼8.7 Hz, 2H, H11), 7.47 (t, J¼7.9 Hz, 1H,
H5), 7.51–7.54 (m, 3H, H6,15), 7.61 (d, J¼7.9 Hz, 1H, H7),
7.66 (t, J¼7.4 Hz, 1H, H16), 8.00 (d, J¼8.1 Hz, 1H, H4),
8.11 (br s, 1H, NH), 8.37 (d, J¼8.1 Hz, 2H, H14); NOE
difference spectra by irradiation at d¼8.11 showed
enhancement at d¼6.52 corresponding to H10. From
H,H-COSY experiment, H11 was readily assigned by
examining the cross-peak with H10. 1H NMR of the sample,
after adding D2O, indicated disappearance of the NH peak
NMR spectra of the substrates 1a–e and 2f–j in CDCl3
show dynamic flux over time until a syn/anti isomeric
equilibrium is reached. At equilibrium the ratio of the two
isomers in the NMR spectra vary from one substrate to
another. For example, the equilibrium ratio (syn/anti) is 5:1
and 5:2 in the case of 1b and 2g, respectively. These results
could be explained on the basis of an equilibrium being

H. H. Dib et al. / Tetrahedron 59 (2003) 9455–9464
9457
Scheme 2. Gas-phase pyrolysis (0.06 mbar) of 2-arylhydrazono-2-(benzotriazol-1-yl)-1-phenylethanone (1a–e).
Scheme 3. Gas-phase pyrolysis (0.06 mbar) of 2-arylhydrazono-1-(benzotriazol-1-yl)-propan-2-one (2f–j).

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H. H. Dib et al. / Tetrahedron 59 (2003) 9455–9464
Scheme 4. Pyrolytic reaction pathway involving 1,3-biradical reactive intermediate.
confirming the presence of the exchangeable NH proton.
From 13C, HMBC and HMQC experiments, the different
carbon signals of 4d were assigned at d¼111.24 (C7),
115.53 (C10), 122.79 (C4), 124.94 (C5), 127.51 (C6), 127.85
(C12), 128.92 (C15), 129.85 (C11), 131.58 (C14), 134.51
(C16), 135.67 (C7a), 135.97 (C13), 139.53 (C3a), 144.84 (C2),
146.18 (C9), 185.95 (CvO). MS spectrometry of 4d gave
[Mzþ] at m/z¼347/349 (35/15%), base peak at 105, and
major fragment ions at 77 and 91.
former set of compounds (4a–e) were isolated from their
pyrolysates and then subjected to further pyrolysis. In each
case, 5 and the corresponding substituted aniline (6)
constituted the products of reaction. It is instructive to
note that a similar 1,3-biradical pathway has also been
postulated for the thermal gas-phase elimination reaction
of the parent a-benzotriazol-1-yl ketones without an aryl-
hydrazono substituent,1 and a range of other BT com-
pounds.16 The noteworthy observation in the present
mechanistic investigation is that no reaction products
corresponding to 4f–j, 5 or 6f–j (Schemes 2 and 4,
RvCH3) were detected in the pyrolysates of the arylhydra-
zone a-benzotriazol-1-yl ketones 2f–j. In this case a
complete departure from the 1,3-biradical pathway has
occurred.
A plausible mechanism is outlined in Scheme 4 to account
for the presence of compounds 4a–e, 5 and 6a–e among the
products of pyrolysis of substrates 1a–e. It includes
extrusion of a stable N2 molecule, formation of a 1,3-bi-
radical reactive intermediate and subsequent cyclization and
fragmentation processes. Besides, and to confirm 4a–e as
the likely precursors of 5 and 6a–e (Schemes 2 and 4), the
Further, the pyrolysates from the thermolysis of the
Scheme 5. Mechanism of cinnoline (8a–j) formation.

H. H. Dib et al. / Tetrahedron 59 (2003) 9455–9464
9459
substrates of both series 1a–e and 2f–j contained reaction
products that gave MS spectra indicating loss of 46 mass
units attributed to loss of H2O and N2. These products were
established to be the phenyl- and methyl-4b,5,11-triaza-
benzo[b]fluorene (i.e. benzimidazo[1,2-b]cinnoline) 8a–j
derivatives (Scheme 5). The structure of this novel ring
system was confirmed by full proton and carbon signal
assignment of compound 8g (Scheme 3) as a representative
It is to be noted that the yields of the cinnoline compounds
from 1a–e were noticeably lower than those from 2f–j,
probably due to competition from the 1,3-biradical pathway
which operates only for the substrates of series 1a–e.
The formation of the cinnolines (8a–j) during the pyrolysis
of the substrates of both series 1a–e and 2f–j follows the
mechanism proposed in Scheme 5. In this mechanism,
intramolecular nucleophilic addition involving the arylhy-
drazono group and the ketone carbonyl moiety is followed
by cyclization and subsequent fragmentation in which
(H2O) and (N2) molecular species are lost, in that order.
Initial threshold loss of (N2) was ruled out, since, as noted
earlier, no reaction products indicative of a 1,3-biradical
mechanism were observed in the case of the compounds of
series 2f–j. It was, however, not possible to isolate the
intermediates 7a–j suggested in the present mechanism.
The tendency of the arylhydrazono (ArNHNv) group to
cause a switch in the mechanism of thermal gas-phase
elimination reactions has been reported for the gas-phase
thermolysis of derivatives and analogues of 3-arylhydra-
zono-pentan-2,4-diones.17,18
1
example, based on H NMR, 13C NMR, NOE-difference
spectra, the 2-D H,H-COSY, HMQC and HMBC of 8g.
Thus, NOE difference spectra (by irradiation at the CH3
signal at d¼3.06) showed enhancement at d¼7.94 corre-
1
sponding to H1. From coupling constants in the H NMR
spectra and the cross-peaks in the H,H-COSY experiment,
all the protons were assigned at d¼3.06 (s, 3H, CH3), 4.07
4
3
(s, 3H, OCH3), 7.19 (dd, J¼2.5 Hz, J¼8.9 Hz, 1H, H3),
7.61–7.68 (m, 2H, H8 and 9), 7.94 (d, J¼2.5 Hz, 1H, H1),
8.05 (d, J¼8.9 Hz, 1H, H4), 8.17 (d, J¼7.2 Hz, 1H, H7),
8.38 (d, J¼7.2 Hz, 1H, H10). From 13C NMR, HMBC and
HMQC experiments the different carbon signals of 8g were
assigned at d¼21.89 (CH3), 56.37 (OCH3), 99.89 (C1),
112.64 (C3), 114.54 (C7), 122.28 (C10), 124.84 (C9), 125.86
(C8), 130.38 (C4a), 130.76 (C6a), 131.65 (C4), 142.08 (C11a),
144.48 (C10a), 151.73 (C12a), 160.30 (C2,12). MS analysis
gave [Mzþ] at m/z¼263.
Further, analysis of the pyrolysates from 1a–e and 2f–j has
established the formation of the anilide and p-substituted
Scheme 6. Mechanism of anilide (9a–j) formation.

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H. H. Dib et al. / Tetrahedron 59 (2003) 9455–9464
Table 2. Rate coefficients (k/s21) and Arrhenius parameters at 500 K of
substrates 2f–j
Substrate T/K
104k/s21 log A /s21 Ea/kJ mol21 104k500 K/s21
2f
486.45 1.483 11.87^0.15 146.2^1.55 3.947
495.35 2.926
504.15 5.093
513.25 9.632
522.15 17.68
530.85 30.83
2g
2h
2i
444.60 1.176
457.25 2.516
470.65 5.291
484.15 11.25
497.55 20.58
7.842^0.14 100.2^1.21 23.84
470.75 1.670
481.85 2.833
494.50 6.229
506.95 10.79
518.25 20.52
8.110^0.33 107.3^3.10
7.958
Figure 1. Arrhenius plot for pyrolysis of 2-(benzotriazol-1-yl)-1-phenyl-2-
(phenyl-hydrazono)ethanone (1a).
anilides 9a–j (Scheme 6). MS spectrometric and spectro-
photometric characterization of these products is in
agreement with literature data.19–23 The mechanism offered
to account for the formation of the anilides is presented in
Scheme 6, and is in accord with feasible mechanisms
reported for the pyrolysis of a-acylhydrazones.24,25 The
proposed reaction pathway proceeds either through a polar
intermediate (route i), or through a thermally-allowed 1,5-H
shift followed by electrocyclization (route ii). Route
(i) involves intramolecular nucleophilic addition between
the arylhydrazono group and the ketone carbonyl function.
Both routes suggest the formation of 1-cyanobenzotriazole
(10), which could not, however, be detected in the product
464.35 1.053
475.25 2.112
499.15 8.686
510.65 16.60
522.75 33.03
9.351^0.09 118.5^0.962 9.307
8.203^0.04 103.3^0.377 26.07
2j
450.35 1.687
462.15 3.422
473.55 6.445
486.05 12.87
497.70 23.21
of reaction. As a result, 1-cyanobenzotriazole (10) was
prepared following literature procedures26 and subjected
to both kinetic and preparative pyrolytic investigation.1
1-Cyanobenzotriazole was found to pyrolyse faster than any
of the substrates under study to give charred and as yet non-
identifiable products. However, kinetic observations serve
to explain the secondary decomposition of 1-cyanobenzo-
triazole under the conditions of pyrolysis of compounds
1a–e, 2f–j.
Table 1. Rate coefficients (k/s21) and Arrhenius parameters at 500 K of
substrates 1a–e
Substrate T/K 104k/s21 log A/s21
Ea/kJ mol21 104k500 K/s21
1a
443.15 1.660
452.75 3.501
461.75 6.769
469.40 12.20
478.20 21.56
485.45 34.70
11.49^0.13 139.5^1.13
90.68
2.3. Kinetic analysis
Pyrolysis of the present substrates (1a–e, 2f–j) is a
homogeneous, first-order gas-phase elimination reaction
free of reactor surface effects. The temperature range over
which the substrates were studied was 420–530 K. The
internal standard used in the chromatographic analysis
of the kinetic runs was either chlorobenzene for (1c, 2i), 1,3-
dichlorobenzene for (1d, 2g, 2h), or 1,2,4-trichlorobenzene
for the remaining five substrates. The solvent used was
acetonitrile which was also used in aqueous solution as
eluent for HPLC analysis, and the wavelength used for
HPLC monitoring was 256 nm. The substrates behaved
well kinetically, gave reproducible rate coefficients, and
Arrhenius plots which were linear for up to $90% reaction
with high correlation coefficients (0.998^0.001). A repre-
sentative Arrhenius plot is given in Figure 1 for 2-(benzo-
triazol-1-yl)-1-phenyl-2-(phenylhydrazono)ethanone (1a).
The kinetic data and the values of the Arrhenius log A/s21
and Ea/kJ mol21, and the rate coefficients at 500 K of the 10
substrates under study are summarized in Table 1 (Series
1a–e) and Table 2 (Series 2f–j). The Arrhenius parameters
are in the range expected for thermal gas-phase first-order
reactions.27
1b
1c
1d
419.75 1.351
428.35 3.352
438.15 7.367
447.85 16.32
457.65 41.52
13.71^0.43 141.2^3.64 915.4
12.23^0.18 134.4^1.72 153.9
12.12^0.09 134.4^0.753 121.1
433.35 1.062
443.25 2.400
453.05 5.636
463.15 11.54
472.85 23.94
424.95 0.414
434.55 0.918
445.95 2.330
455.35 5.066
463.75 9.795
473.45 19.62
483.55 40.92
1e
438.45 1.046
448.05 2.391
459.45 4.266
470.05 8.999
481.25 22.50
492.25 46.28
10.83^0.42 124.4^3.64
68.75

H. H. Dib et al. / Tetrahedron 59 (2003) 9455–9464
9461
Although the reaction pathways (Schemes 4–6) offered
earlier to account for product formation in the pyrolysis of
the present arylhydrazono a-benzotriazol-1-yl ketones
(1a–e and 2f–j) and their ketone analogues1 appear
elaborate and complex, the results of the kinetic investi-
gation of these compounds are, on the other hand, both
informative and simple to rationalize. The arylhydrazono
group, when substituted in place of the a-methylene
hydrogens of the a-benzotriazolyl phenylethanone and
acetone compounds, produces a dramatic change in
mechanism from a 1,3-biradical pathway either partially
or completely to a mechanism involving intramolecular
nucleophilic addition, followed by cyclization with or
without fragmentation. Incorporation of the arylhydrazono
group into the BT ketones also leads to very large rate
enhancement. For example, 2-(benzotriazol-1-yl)-1-phenyl-
2-(phenylhydrazono)ethanone (1a) is ca. 103-fold more
reactive than the unsubstituted 2-(benzotriazol-1-yl)-1-
phenylethanone.1 The rate enhancement factor for the
acetone counterpart is 8.6£102. It has already been noted
that substitution of the arylhydrazono (ArNHNv) group at
the 3-position of the 2,4-pentandione produces not only a
change-over in the mechanism of gas-phase pyrolysis but
also leads to ca. 257-fold increase in reactivity.17,18 The
arylhydrazonophenylethanone BT ketones (1a–e) are more
reactive than their acetone analogues (2f–j) by factors
ranging between ca. 3–38. Similar rate differences have
been observed for the unsubstituted phenylethanone and
acetone BT ketones.1 In both cases, the origin of this order
of reactivity is related to the effect of the electron-
withdrawing phenyl group in the phenylethanone BT
kinetic runs and data analysis were reported in earlier
communications.28
Synthesis. The hydrazones 2f–h, j were prepared according
to a modified literature procedure using sodium acetate
instead of sodium hydroxide as base for the synthesis of
compounds 2f–h.29,30
4.1. General procedure for synthesis of 1a–e, 2i
To a cold solution of each of 1-(benzotriazol-1-yl)acetone
and 1-(benzotriazol-1-yl)acetophenone (0.01 mol) in etha-
nol (100 ml) was added sodium hydroxide (2.0 g) for the
preparation of 1a–e, and sodium acetate (4.0 g) for the
preparation of 2i. The mixture was then treated gradually
with stirring at room temperature with a solution of the
aryldiazonium salt (prepared from 0.01 mol of the aryl-
amine and the appropriate quantities of hydrochloric acid
and sodium nitrite). The product, which separated on
standing, was collected by filtration and crystallized from
ethanol.
4.1.1. 2-(Benzotriazol-1-yl)-1-phenyl-2-(phenylhydrazono)-
ethanone, 1a. The title compound was obtained as yellow
crystals in 60% yield, mp 2008C. MS: m/z¼341 (Mþz). IR
1
(KBr): n 3434 (NH), 1626 cm21 (CO). H NMR (CDCl3):
˜
d¼7.10 (t, J¼7.2 Hz, 1H, Ar-H); 7.24 (d, J¼8 Hz, 2H,
Ar-H), 7.36 (t, J¼7.6, 8 Hz, 2H, Ar-H), 7.43 (d, J¼8.4 Hz,
1H, Ar-H), 7.47 (d, J¼7.6 Hz, 1H, Ar-H), 7.57 (t, J¼7.6 Hz,
1H, Ar-H), 7.62 (d, J¼7.6 Hz, 2H, Ar-H), 7.70 (t, J¼7.6,
7.2 Hz, 1H, Ar-H), 8.09 (d, J¼8.4 Hz, 1H, Ar-H), 8.22
(d, J¼7.2 Hz, 2H, Ar-H), 10.48 (br, 1H, NH). 13C NMR
(CDCl3): d¼112.45, 115.52, 120.41, 124.33, 125.51,
126.46, 128.59, 129.37, 130.02, 131.06, 132.72, 133.14,
137.08, 142.14, 145.31, 184.66. C20H15N5O (341.4): calcd
C 70.37, H 4.43, N 20.52; found C 70.60, H 4.42, N 20.40.
`
ketones vis-a-vis the electron-donating methyl moiety of
the acetone counterparts. It is of interest to note that there
seems to be no opposing substituent effect from the para
position of the aryl moiety of the arylhydrazono group
consistent with the electron-donating character of the
methyl and methoxy substituents and the electron-with-
drawing character of the chloro and nitro substituents. In
fact all these substituents promote reactivity albeit by a
modest rate factor of 1.34–10.1. It is conceivable that the
effects of these substituents on the reactivity of the present
substrates combine ground-state and long-range electronic
effects.1
4.1.2. 2-(Benzotriazol-1-yl)-2-[(4-methoxyphenyl)hydra-
zono]-1-phenylethanone, 1b. The title compound was
obtained as orange crystals in 55% yield, mp 1568C. MS:
m/z¼371 (Mþz). IR (KBr): n 3434 (NH), 1631 cm21 (CO).
˜
1H NMR (CDCl3): d¼3.80 (s, 3H, MeO), 6.90 (d, J¼8.8 Hz,
2H, Ar-H), 7.25 (d, J¼8.8 Hz, 2H, Ar-H), 7.36 (t, J¼7.6 Hz,
1H, Ar-H), 7.43 (d, J¼8.4 Hz, 1H, Ar-H), 7.54 (t, J¼7.6 Hz,
2H, Ar-H), 7.59 (d, J¼7.2, 7.6 Hz, 1H, Ar-H), 7.68 (t, J¼
7.6, 8.4 Hz, 1H, Ar-H), 7.85 (d, J¼8 Hz, 1H, Ar-H), 8.20
(d, J¼7.6 Hz, 2H, Ar-H), 10.62 (br, 1H, NH). 13C NMR
(CDCl3): d¼55.99, 111.93, 115.29, 117.05, 119.74, 125.31,
125.74, 128.50, 129.14, 131.00, 132.81, 133.02, 136.15,
137.42, 145.12, 156.91, 184.75. C21H17N5O2 (371.4): calcd
C 67.92, H 4.61, N 18.86; found C 68.19, H 4.62, N 18.73.
3. Conclusion
Replacement of methylene hydrogen atoms by an aryl-
hydrazono group at the a-position of ketone moieties
substituted at the N(1) position of BT led to a substantial
increase in rates of pyrolysis of BT and a change-over in the
mechanism of the thermal gas-phase elimination reaction.
In addition, this structural modification also made it possible
to obtain novel condensed heterocyclic ring systems by an
essentially clean and environmentally friendly gas-phase
elimination process.
4.1.3. 2-(Benzotriazol-1-yl)-1-phenyl-2-(p-tolylhydrazono)-
ethanone, 1c. The title compound was obtained as reddish
brown crystals in 60% yield, mp 1718C. MS: m/z¼355
1
(Mþz). IR (KBr): n 3432 (NH), 1630 cm21 (CO). H NMR
˜
(CDCl3): d¼2.34 (s, 3H, CH3), 7.14 (d, J¼8.8 Hz, 2H,
Ar-H), 7.15 (t, J¼8.8 Hz, 2H, Ar-H), 7.43 (d, J¼8.4 Hz, 1H,
Ar-H), 7.47 (d, J¼8 Hz, 1H, Ar-H), 7.56 (d, J¼8 Hz, 1H,
Ar-H), 7.59 (t, J¼8.4, 7.6 Hz, 2H, Ar-H), 7.69 (t, J¼7.2,
7.6 Hz, 1H, Ar-H), 8.10 (d, J¼8.4 Hz, 1H, Ar-H), 8.21
(d, J¼7.2 Hz, 2H, Ar-H), 10.38 (br, 1H, NH). 13C NMR
(CDCl3): d¼21.25, 112.25, 115.60, 120.15, 125.41, 126.07,
4. Experimental
General. Experimental techniques and instrumentation were
described in Part 1,1 and the procedures for conducting

9462
H. H. Dib et al. / Tetrahedron 59 (2003) 9455–9464
Table 3. Analytical pyrolysis temperature (T/K) and pyrolysate composition
Cpd
T/K
Pyrolysates and yields (%)
1a
1b
1c
1d
1e
2f
2g
2h
2i
498
478
468
488
498
538
523
528
533
523
4a (50%)
4b (27%)
4c (35%)
4d (48%)
4e (42%)
514 (,1%)
514 (,1%)
514 (,1%)
514 (,1%)
514 (,1%)
6a15 (,1%)
6b15 (,1%)
6c15 (,1%)
6d15 (,1%)
6e15 (,1%)
8a (5%)
9a21 (18%)
9b19,20 (12%)
9c19 (11%)
9d19,20 (4%)
9e19,20 (23%)
9f21 (21%)
8b (,1%)
8c (,1%)
8d (,1%)
8e (,1%)
8f (12%)
8g (48%)
8h (14%)
8i (5%)
9g22 (28%)
9h19,23 (29%)
9i19,23 (30%)
9j22 (5%)
2j
8j (5%)
128.53, 129.25, 130.54, 131.06, 132.85, 132.97, 134.13,
137.24, 139.93, 145.24, 184.68. C21H17N5O (355.4): calcd
C 70.97, H 4.82, N 19.71; found C 71.27, H 4.77, N 19.72.
prepared according to procedures published earlier in the
literature.26
4.2. General procedure for pyrolysis of 1a–e, 2f–j and
4a–e
4.1.4. 2-(Benzotriazol-1-yl)-2-[(4-chlorophenyl)hydra-
zono]-1-phenylethanone, 1d. The title compound was
obtained as reddish brown crystals in 55% yield, mp
Each of the substrates (0.2 g) was introduced in the reaction
tube (1.5£12 cm Pyrex), cooled in liquid nitrogen, sealed
under vacuum (0.06 mbar) and placed in the pyrolyser for
900 s at a temperature comparable to that used for complete
pyrolysis in the kinetic studies. The pyrolysate was then
separated into its constituents by preparative TLC
(MERCK,12 PSC-Platten 20£20 cm, Silica gel 60 F254
2 mm) using chloroform: petroleum ether (40:60) in 80:20
ratio as eluent, and each constituent was collected, analyzed
1908C. MS: m/z¼375/377 (Mþz) (20/11%). IR (KBr): n
˜
3434 (NH), 1629 cm21 (CO). 1H NMR (CDCl3): d¼7.17 (d,
J¼8.8 Hz, 2H, Ar-H), 7.32 (d, J¼8.8 Hz, 2H, Ar-H), 7.42
(d, J¼8.4 Hz, 2H, Ar-H), 7.47 (t, J¼7.6 Hz, 1H, Ar-H), 7.57
(t, J¼7.6 Hz, 1H, Ar-H), 7.61 (t, J¼7.6 Hz, 1H, Ar-H), 7.71
(t, J¼7.6 Hz, 1H, Ar-H), 8.10 (d, J¼8.4 Hz, 1H, Ar-H),
8.19 (d, J¼8.4 Hz, 2H, Ar-H), 10.59 (br, 1H, NH). 13C
NMR (CDCl3): d¼112.51, 116.64, 117.23, 120.42, 125.62,
126.68, 128.65, 128.91, 129.26, 129.48, 130.06, 131.02,
133.33, 136.92, 140.88, 184.59. C20H14ClN5O (375.8):
calcd C 63.92, H 3.75, N 18.63; found C 63.92, H 3.83, N
18.32.
1
and characterized. The techniques used include H NMR,
13C NMR and GC/MS.
Temperatures of analytical pyrolysis, constituents of
pyrolysates and percentage yield of substrates 1a–e and
2f–j, and of further pyrolysis of compounds 4a–e are given
in Table 3. This table also includes useful references for
characterization data.
4.1.5. 2-(Benzotriazol-1-yl)-2-[(4-nitrophenyl)hydra-
zono]-1-phenylethanone, 1e. The title compound was
obtained as brown crystals in 50% yield, mp 2178C. MS:
m/z¼386 (Mþz). IR (KBr): n 3434 (NH), 1655 cm21 (CO).
˜
1H NMR (CDCl3): d¼7.30 (d, J¼7.2 Hz, 2H, Ar-H), 7.40
(d, J¼8.4 Hz, 1H, Ar-H), d¼7.52 (t, J¼7.6 Hz, 1H,
Ar-H),7.59 (d, J¼7.6 Hz, 1H, Ar-H), 7.64 (t, J¼7.6, 8 Hz,
2H, Ar-H), 7.77 (t, J¼7.6, 7.2 Hz, 1H, Ar-H), 8.16
(d, J¼8 Hz, 1H, Ar-H), 8.21 (d, J¼8.4 Hz, 2H, Ar-H),
8.26 (d, J¼7.6 Hz, 2H, Ar-H), 11.15 (br, 1H, NH). 13C
NMR (CDCl3): d¼112.46, 115.06, 120.44, 125.96, 126.34,
128.07, 128.91, 129.85, 131.10, 132.31, 134.01, 136.32,
143.53, 145.11, 147.62, 184.62. C20H14N6O3 (386.4):
calcd C 62.17, H 3.65, N 21.75; found C 62.40, H 3.76, N
21.63.
4.3. MS and NMR characterization
4.3.1. 1-Anilino-2-benzoylimidazole, 4a. MS: m/z¼313
1
(Mþz). H NMR (CDCl3): d¼6.58 (d, J¼7.6 Hz, 2H), 6.93
(t, J¼7.2, 7.6 Hz, 1H), 7.19 (t, J¼7.6 Hz, 2H), 7.44–7.66
(m, 6H), 8.00 (d, J¼8 Hz, 1H), 8.14 (br s, 1H, NH), 8.36 (d,
J¼8.4 Hz, 2H).
4.3.2. 1-p-Toluedino-2-benzoylimidazole, 4c. MS: m/z¼
327 (Mþz). 1H NMR (CDCl3): d¼2.22 (s, 3H, CH3), 6.49 (d,
J¼8.4 Hz, 2H, H10), 6.98 (d, J¼8.4 Hz, 2H, H11), 7.46–
7.65 (m, 2H, 6H5,6,(4 or 7),15,16), 7.99 (d, J¼8 Hz, 1H,
4.1.6. 1-(Benzotriazol-1-yl)-1-[(4-chlorophenyl)hydra-
zono]propan-2-one, 2i. The title compound was obtained
as yellow crystals in 55% yield, mp 1868C. MS: m/z¼313/
H
4 or 7), 8.10 (br s, 1H, NH), 8.35 (d, J¼8.4 Hz, 2H, H14).
4.3.3. 1-p-Chloroaniline-2-benzoylimidazole, 4d. MS:
m/z¼347/349 (Mþz) (35/15%), base peak at 105, and
315 (Mþz) (20/8%). IR (KBr): n 3433 (NH), 1659 cm21
˜
1
(CO). 1H NMR (CDCl3): d¼2.78 (s, 3H, Me), 7.30
(d, J¼7.2 Hz, 2H, Ar-H), 7.36 (d, J¼7.2 Hz, 2H, Ar-H),
7.41 (t, J¼8 Hz, 2H, Ar-H), 7.56 (t, J¼8, 7.2 Hz, 1H, Ar-H),
7.95 (d, J¼8.4 Hz, 1H, Ar-H), 10.22 (br, 1H, NH). 13C
NMR (CDCl3): d¼25.65, 112.08, 116.72, 119.77, 125.42,
127.46, 129.32, 129.51, 130.02, 133.05, 140.81, 145.13,
190.22. C15H12ClN5O (313.8): calcd C 57.42, H 3.86, N
22.32; found C 57.61, H 3.91, N 22.26.
major fragment ions at 77 and 91. H NMR (CDCl3): d¼
6.52 (d, J¼8.7 Hz, 2H, H10), 7.14 (d, J¼8.7 Hz, 2H, H11),
7.47 (t, J¼7.9 Hz, 1H, H5), 7.52–7.54 (m, 3H, H6,15), 7.61
(d, J¼7.9 Hz, 1H, H7), 7.66 (t, J¼7.4 Hz, 1H, H16), 8.00 (d,
J¼8.1 Hz, 1H, H4), 8.11 (br s, 1H, NH), 8.37 (d, J¼8.1 Hz,
2H, H14). 13C NMR (CDCl3): d¼111.24 (C7), 115.53 (C10),
122.79 (C4), 124.94 (C5), 127.51 (C6), 127.85 (C12), 128.92
(C15), 129.85 (C11), 131.58 (C14), 134.51 (C16), 135.67
(C7a), 135.97 (C13), 139.53 (C3a), 144.84 (C2), 146.18 (C9),
185.95 (CvO).
4.1.7. 1-Cyanobenzotriazole, 10. The title compound was

H. H. Dib et al. / Tetrahedron 59 (2003) 9455–9464
9463
4.3.4. 2-Methoxy-12-phenylbenzimidazo[1,2-b]cinnoline,
8b. MS: m/z¼325 (Mþz). 1H NMR (CDCl3): d¼4.10 (s, 3H,
tion was filtered to ensure that a homogeneous solution was
obtained.
3
4
OCH3), 7.23 (dd, J¼8.8 Hz, J¼2.4 Hz, 1H, H3), 7.47–
7.53 (m, 2H, H8,9), 7.58 (d, J¼6.8 Hz, 1H, H16), 7.63
(t, J¼6.8 Hz, 2H, H15), 8.01 (d, 4J¼2.4 Hz, 1H, H1),
8.20 (d, J¼8.8 Hz, 1H, H4), 8.22 (d, J¼6.8 Hz, 1H, H7 or 10),
8.44 (d, J¼6.8 Hz, 1H, H7 or 10), 8.71 (d, J¼7.2 Hz, 2H,
H14).
The weight ratio of the substrate with respect to the internal
standard was calculated from the ratio of the substrate peak
area to the peak area of the internal standard. The kinetic
rate was obtained by tracing the rate of disappearance of the
substrate with respect to the internal standard as follows.
4.3.5. 2-Methyl-12-phenylbenzimidazo[1,2-b]cinnoline,
1
An aliquot (0.2 ml) of each solution containing the substrate
and the internal standard was pipetted into the reaction tube
which was then cooled sufficiently (liquid nitrogen) sealed
and placed in the pyrolyzer for 6 min under non-pyrolytic
conditions. A sample was then analyzed using the HPLC
probe with the UV detector at wavelength of 256 nm, and
the standardization value (Ao) was then calculated. Several
HPLC measurements were obtained with an accuracy of
$2%. The temperature of the pyrolysis block was then
raised until approximately 10% pyrolysis was deemed to
occur over 900 s. This process was repeated after each
10–158C rise in the temperature of the pyrolyzer until
$90% pyrolysis takes place. The relative ratio of the
integration values of the sample and the internal standard
(A) at the pyrolysis temperature was then calculated.
A minimum of three kinetic runs were carried out at each
10–158C rise in the temperature of the pyrolyzer to ensure
reproducible values of (A). Treatment of the kinetic data has
been detailed elsewhere.31–33
8c. MS: m/z¼309 (Mþz). H NMR (CDCl3): d¼2.72 (s,
3H, CH3), 7.25 (d, J¼8 Hz, 2H, H3), 7.45 (t, J¼8.8 Hz,
2H, H15), 7.54 (t, J¼7.6 Hz, 1H, H16), 7.58–7.62 (m, 3H,
H
8 or 9 and 14), 7.85–7.87 (m, 1H, H8 or 9), 8.01 (d, J¼
8.4 Hz, 1H, H7 or 10), 8.17 (s, 1H, H1), 8.63 (d, J¼8.4 Hz,
1H, H7 or 10).
4.3.6. 2-Chloro-12-phenylbenzimidazo[1,2-b]cinnoline,
8d. MS: m/z¼329 (Mþz) (60%), 331 (Mþ2) (43%). 1H
NMR (CDCl3): d¼7.54–7.62 (m, 5H, H3,4,8,9), 6.93 (d,
J¼7.6 Hz, 2H, H15), 8.03 (d, J¼8.4 Hz, 1H, H7 or 10), 8.05
(t, J¼7.2 Hz, 1H, H16), 8.24 (d, J¼8 Hz, 1H, H14), 8.38 (d,
J¼1.2 Hz, 1H, H1), 8.36 (d, J¼8.4 Hz, 1H, H8 or 9).
4.3.7. 2-Methoxyl-12-methylbenzimidazo[1,2-b]cinno-
1
line, 8g. MS: m/z¼263 (Mþz). H NMR (CDCl3): d¼3.06
4
(s, 3H, CH3), 4.07 (s, 3H, OCH3), 7.19 (dd, J¼2.5 Hz,
3J¼8.9 Hz, 1H, H3), 7.61–7.68 (m, 2H, H8 and 9), 7.94 (d,
J¼2.5 Hz, 1H, H1), 8.05 (d, J¼8.9 Hz, 1H, H4), 8.17 (d,
J¼7.2 Hz, 1H, H7), 8.38 (d, J¼7.2 Hz, 1H, H10). 13C NMR
(CDCl3): d¼21.89 (CH3), 56.37 (OCH3), 99.89 (C1), 112.64
(C3), 114.54 (C7), 122.28 (C10), 124.84 (C9), 125.86 (C8),
130.38 (C4a), 130.76 (C6a), 131.65 (C4), 142.08 (C11a),
144.48 (C10a), 151.73 (C12a), 160.30 (C2,12).
Acknowledgements
This work was supported by Kuwait University through
research grant # SC01/99 and ANALAB and SAF grants #
GS01/01 and GS03/01.
4.3.8. 2,12-Dimethylbenzimidazo[1,2-b]cinnoline, 8h.
1
MS: m/z¼247 (Mþz). H NMR (CDCl3): d¼2.71 (s, 3H,
CH3), 3.09 (s, 3H, CH3), 7.25 (d, 3J¼8.4 Hz, 1H, H3), 7.63–
7.66 (m, 2H, H8 and 9), 8.03 (d, J¼8.4 Hz, 1H, H4), 8.19 (d,
J¼6.8 Hz, 1H, H7 or 10), 8.32 (s, 1H, H1), 8.48 (d, J¼6.8 Hz,
1H, H7 or 10).
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