Gas-phase thermolysis of benzotriazole derivatives. Part 3: Kinetic and mechanistic evidence for biradical intermediates in pyrolysis of aroylbenzotriazoles and related compounds

Article abstract of DOI:10.1016/j.tet.2005.06.028

Gas-phase pyrolysis (static and FVP) of 1-aroylbenzotriazoles gave the corresponding substituted benzoxazole, benzimidazole, benzamide, N-phenylbenzamide, phenanthridin-6(5H)-one derivatives and 1- cyanocyclopentadiene. The present kinetic and mechanistic findings also provide further evidence of the involvement of biradical or carbene reactive intermediates in the reaction pathway of gas-phase pyrolysis of benzotriazoles.

Full text of DOI:10.1016/j.tet.2005.06.028

Tetrahedron 61 (2005) 8257–8263
Gas-phase thermolysis of benzotriazole derivatives.
Part 3: Kinetic and mechanistic evidence for biradical
intermediates in pyrolysis of aroylbenzotriazoles and
related compounds
Nouria A. Al-Awadi,
*
Bobby J. George, Hicham H. Dib, Maher R. Ibrahim, Yehia A. Ibrahim
and Osman M. E. El-Dusouqui
Chemistry Department, Kuwait University, PO Box 5969, Safat 13060, Kuwait
Received 19 December 2004; revised 25 May 2005; accepted 9 June 2005
Available online 1 July 2005
Abstract—Gas-phase pyrolysis (static and FVP) of 1-aroylbenzotriazoles gave the corresponding substituted benzoxazole, benzimidazole,
benzamide, N-phenylbenzamide, phenanthridin-6(5H)-one derivatives and 1-cyanocyclopentadiene. The present kinetic and mechanistic
findings also provide further evidence of the involvement of biradical or carbene reactive intermediates in the reaction pathway of gas-phase
pyrolysis of benzotriazoles.
q 2005 Elsevier Ltd. All rights reserved.
1. Introduction
1
,2
The results reported in earlier papers in this series have
combined mechanistic and kinetic investigations to probe
the gas-phase pyrolytic reactions of a range of benzotriazole
(
environmentally friendly and facile alternative for the
BT) heterocycles. Besides, the reactions provided an
1
,2
synthesis of several novel condensed heterocycles.
Much research has recently been directed towards the
study of the products of gas-phase pyrolysis, including flash
3
–7
vacuum pyrolysis (FVP), of numerous BT compounds. In
these reactions, thermolysis of the benzotriazoles appears to
involve formation of reactive biradical intermediates, loss
of molecular nitrogen, and intramolecular cyclization
leading to interesting condensed heterocycles. This investi-
gation is now extended to include the gas-phase thermolysis
of 1-aroylbenzotriazoles 1a–e, and the present results are
compared with our earlier findings on related BT
derivatives; their homologue 1-phenyl-2-(benzotriazol-1-
yl)ethanone 2 with analogue 2-(benzotriazol-1-yl)-1-
phenyl-2-(2-phenylhydrazono)ethanone 3 (Scheme 1).
Previous work, further showed that pyrolysis of a number
of 1-acylbenzotriazoles under different thermolytic con-
Scheme 1. Aroylbenzotriazoles 1a–e, homologue 2, and analogue 3.
ditions including FVP gives the corresponding benzoxazole
derivatives in ca. 10–40% yield alongside other products
depending on the nature of the substituents on the acyl
8–14
moiety.
An important feature of our kinetic analysis of
these reactions concerns the magnitude of rate enhancement
attributed to extended p-conjugative interactions involving
the biradical system and the substituents at the a-N(1)
Keywords: Aroylbenzotriazoles; Pyrolysis; Biradical intermediates; Kin-
etics; Mechanism.
*
Corresponding author. Tel.:C965 4845098; fax: C965 4816482; e-mail:
nouria@kuc01.kuniv.edu.kw
1
,2
radical centre.
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.06.028

8258
N. A. Al-Awadi et al. / Tetrahedron 61 (2005) 8257–8263
Scheme 2. Pyrolysates from thermolysis of 1a–e.
1
1
2
. Results and discussion
benzene and biphenyl derivatives as reported earlier.
Addition of two hydrogen atoms to I explains the formation
of N-phenylamides 7. The second route (ii), which explains
the formation of 2-arylbenzimidazoles 5 involves N–N bond
cleavage of the benzotriazole ring to give biradical II, which
rearranges into the 3-aryl-1,2,4-benzotriazine-2-oxides VII
through the intermediacy of VI. Conversion of some
2
.1. Pyrolysate composition and reaction pathway
Reaction products from complete static pyrolysis of 1a–e
Scheme 2) were obtained at optimal reactor conditions of
(
K2
temperature (300–340 8C), pressure (6!10 mbar) and
substrate residence time compatible with R98% reaction
established from kinetic runs. These products were
separated by preparative HPLC and characterized as the
corresponding arylbenzoxazole 4, arylbenzimidazole 5,
substituted benzamide 6, N-phenylbenzamide 7,
phenanthridin-6(5H)-one derivatives 8 and 1-cyanocyclo-
pentadiene 9. On the other hand, compounds 4, 8 and 9 were
the only characterized products upon FVP of 1a–e at 600 8C
and 0.2 Torr. Towards this end, the pyrolysates were
qualitatively and quantitatively analysed by HPLC (these
1
-substituted benzotriazoles into benzotriazine derivatives
5
were reported. Rearrangement of VII into VIII followed
by loss of (NO) radical and capture of (H) gives 5. It is to be
noted that conversion of benzotriazin-2-oxide into benzi-
15
midazole derivatives has been documented. The isolated
benzamide derivatives has only been proved after HPLC
separation and most probably were produced by hydrolysis
of the N-aroylcyclopentadienylketeneimines XI.
2.2. Kinetic data and comparative molecular reactivity
1
yields were also calculated by H NMR), and the results are
recorded in Table 1. The structures of the pyrolysis products
The rates of gas-phase thermolysis of the 1-aroylbenzo-
triazoles 1a–e were measured over the temperature range
400–505 K with an average range of 54 K per substrate, in
order to ensure reliable activation parameters for the first-
1
13
were established using GC–MS, LCMS, H and C NMR,
and FT-IR spectroscopy, and compared with authentic
samples. Scheme 3 illustrates possible mechanistic routes to
the formation of 4–9 obtained in the present pyrolytic
studies of substrates 1a–e. The first route (i) involves
1
6
order gas-phase elimination process of these compounds.
The kinetic data is given in Table 2. Each rate constant
recorded is an average from at least three independent
evaluations of the rate at each reaction temperature, and
which are in agreement to within G2% rate spread. The
Arrhenius parameters were obtained from strictly linear
correlations over O85% reaction. A typical Arrhenius plot
is given in Figure 1 for compound 1a (with a correlation
extrusion of N to give the biradical intermediate I, which
2
either cyclizes to give benzoxazole derivatives 4, or
phenanthradinone derivatives 8 or rearranges to the N-aroyl-
cyclopentadienylketeneimines XI through the carbene inter-
mediate as reported. Further fragmentation of XI under
FVP leads to the formation of 1-cyanocyclopentadiene 9,
1
1
Table 1. Pyrolysis products of 5–10(a–e) (% yield from Static/FVP)
Substrate 1a–e
Pyrolysis
Pyrolysis products (% Yields)
4
a–e
5a–e
6a–e
7a–e
8a–e
9
Static
FVP
Static
FVP
Static
FVP
Static
FVP
21
15
4
16
3
13
5
14
20
—
—
14
—
11
—
5
—
4
—
24
—
22
—
5
—
22
—
18
—
5
—
—
3
2
7
3
5
—
4
—
—
5
—
1
—
5
—
3
1
1
1
1
1
a, X]OCH
3
—
15
—
23
—
6
—
6
—
b, X]CH
3
c, X]H
d, X]Cl
Static
FVP
—
4
e, X]NO
2
18, 15 (4c)

N. A. Al-Awadi et al. / Tetrahedron 61 (2005) 8257–8263
8259
Scheme 3. Pathways of gas-phase pyrolysis of aroylbenzotriazoles 1a–e.
K1 K1
Table 2. Rate coeffecients (k/s ), Arrhenius parameters and rate constants (k/s ) at 500 K of compounds 1a–e
4
10 k/s
K1
K1
K1
K1
Compound
T/K
log A/s
a
E /kJ mol
k
500 K/s
k
rel
K3
1
a
450.50
2.309
3.440
5.567
10.81
20.14
35.52
0.585
1.072
2.058
3.999
6.764
12.89
21.94
2.344
3.658
6.997
12.63
19.58
0.958
1.975
3.955
7.243
11.73
16.92
32.86
1.310
2.625
6.100
7.886
16.37
29.00
51.26
8.013G0.07
100.49G0.66
3.29!10
0.14
4
4
4
4
5
57.30
66.05
77.75
89.75
01.95
K3
1b
445.40
8.773G0.30
110.52G2.68
1.70!10
0.07
4
4
4
4
4
5
54.30
62.30
74.65
84.25
94.75
04.65
K2
K2
1
1
c
405.25
6.942G0.42
7.633G0.32
82.175G3.38
89.120G2.60
2.29!10
2.11!10
1.00
0.92
4
4
4
4
15.15
25.20
35.25
45.25
d
400.55
4
4
4
4
4
4
10.65
20.35
30.35
40.35
49.75
59.65
K2
1e
400.15
8.083G0.40
91.709G3.31
3.19!10
1.39
4
4
4
4
4
4
10.25
20.15
30.1
40.2
49.95
60.55

8260
N. A. Al-Awadi et al. / Tetrahedron 61 (2005) 8257–8263
mechanism has been used in many investigations to
explain the course of numerous gas-phase pyrolysis of
1
,2,18
BT compounds,
the present investigation and its
predecessors represent the first kinetic study in support
of the proposed mechanism of pyrolysis.
2
. The half-life of biradicals generated photochemically
was found to be longest for radicals conjugated to
unsaturated moieties. This structural effect was ration-
alized in terms of resonance stabilization extended by
1
9
conjugation. The results of the kinetic study reported
here, and in the earlier two papers in the series confirm
this structural relation, and provide a kinetic rationale for
the proposed biradical mechanism and the effect of
structure and substituents on both rate and reaction
product.
2
3
3
. The aroylbenzotriazoles 1 are ca. 2!10 –3!10 -fold
more reactive than their homologue 2. This large
decrease in reactivity is the direct result of loss of
conjugation consequent to the intervention of a
methylene unit between the radical centre and the
unsaturated substituent moiety, and hence, loss of
extended resonance stabilization of the biradical
intermediate. Conjugative resonance stabilization also
accounts for the relatively high rate of pyrolysis of
Figure 1. Arrhenius plot for pyrolysis of benzotriazol-1-yl 4-methoxy-
phenyl ketone 1a.
coefficient of 1.000), and the correlation coefficients for the
five substrates under study averaged 0.998G0.002. The
K1
K1
value of the log A/s
expected for homogeneous, unimolecular gas-phase
and E /kJ mol
are in the range
a
1
elimination processes. The Arrhenius parameters and
7
K1
rate constants (k/s ) calculated at 500 K for substrates
1
1
a–e are summarized in Table 2. To facilitate comparison of
1-cyanobenzotriazole 10, which is of a magnitude (kZ
K2 K1
molecular reactivities, the rate constants are also tabulated
in Figure 2 together with the rate data of related BT
2.00!10
s
the aroylbenzotriazoles 1a–e.
) comparable to the rates of reaction of
1
,2
compounds reported in earlier publications. The reaction
mechanism (Scheme 3) and the kinetic data (Fig. 2) allow
the following conclusions and structure/reactivity corre-
lations to be made.
4. A comparison of the relative rate coefficients (k ) for
rel
pyrolysis at 500 K of compounds 1a–e recorded in
Table 2 indicates little variation in rates, and there is no
Hammet type correlation of substituent effects from the
aryl moiety.
1
. The 1,3-biradical intermediate I proposed in this study
Scheme 3), has earlier been suggested to account for the
results of gas-phase pyrolysis of a-N-benzotriazolyl
5. Compound 3 offers an interesting example of how
structural factors moderate molecular reactivities. This
molecule has an unsaturated moiety, which is in direct
conjugation with the radical centre of the proposed
biradical reaction intermediate. However, this moiety is
also cross-conjugated with the remaining part of the
molecule, a structural factor, which limits the effective-
ness of direct conjugation between the radical centre and
the unsaturated moiety and, hence, reduces resonance
stabilization of the intermediate. This structure/
reactivity relation is borne out by the observed relative
reactivities (Fig. 2). Thus, the reactivity of substrate 3
lies inbetween that of the aroylbenzotriazoles 1, which
(
1
,2
ketones and their arylhydrazono derivatives.
reaction pathways of the present aroylbenzotriazoles
a–e (Scheme 3) also include 1,5-biradical intermediate
The
1
II. Formally, loss of a stable (N ) molecule by II
2
results in the formation of I. However, both reactive
intermediates are subject to stabilizing resonance effects.
It has further been suggested that, the BT 1,3-biradical
intermediate I might exist as a canonical form in
1
8
resonance with a carbene resonance structure. It is of
particular interest to note that although the biradical
K1
Figure 2. Rate constants (k/s ) at 500 K of aroylbenzotriazoles and related compounds.

N. A. Al-Awadi et al. / Tetrahedron 61 (2005) 8257–8263
8261
exhibit direct conjugation and effective resonance
stabilization, and their homologue 2, which altogether
lacks this stabilizing effect. The variation in reactivity
between each two of the three sets of substrates [1, 2 and
3] involves two orders of magnitude difference in rate of
reaction.
Eurotherm tube furnace MTF-12/38A to a temperature of
600 8C, the temperature being monitored by a Pt/Pt-13%Rh
thermocouple situated at the centre of the furnace. The
products were collected in a U-shaped trap cooled in liquid
nitrogen. The whole system was maintained at a pressure of
K2
10 Torr by an Edwards Model E2M5 high capacity rotary
oil pump, the pressure being measured by a Pirani gauge
situated between the cold trap and the pump. Under these
conditions the contact time in the hot zone was estimated to
be y10 ms. The different zones of the products collected in
3
. Experimental
1
the U-shaped trap were analyzed by H NMR, LCMS and
GC–MS. Relative and percent yields were determined from
3.1. Synthesis. General procedure for synthesis of
N-aroylbenzotriazoles 1a–e
1
H NMR. Identity of compounds obtained were confirmed
by comparison of their H NMR with data of products
separated from preparative HPLC.
1
Benzotriazole (5.95 g, 0.05 mol) in dichloromethane
(
100 mL) was cooled to 0 8C and triethylamine (9 mL, 0.
6 mol) was added from a syringe, followed by addition of
0
3.3. Reaction products from complete gas-phase
pyrolysis of substrates 1a–e
the aroyl chloride (0.055 mol). The mixture was stirred at
room temperature for 30 min, and then HCl (2 M, 100 mL)
was added to the mixture. The organic solution was
washed twice with water and dried over anhydrous
Na SO . The solvent was then evaporated and the product
3
crystals, mp 99–100 8C (lit.
2
.3.1. 2-(4-Methoxyphenyl)benzoxazole (4a). White
98–99 8C). LCMS: m/zZ
26,27
1
66 (MC1). H NMR (CDCl ): d 3.89 (s, 3H, OCH ), 7.03
2
4
3
3
was recrystallized from ethanol to give the corresponding
1
(
(
d, 2H, JZ9.0 Hz), 7.30–7.35 (m, 2H), 7.55 (m, 1H), 7.74
m, 1H), 8.20 (d, 2H, JZ9.0 Hz).
a–e.
3.1.1. 1-p-Methoxybenzoylbenzotriazole (1a). White
crystals from ethanol, yield 95%, mp 115–116 8C (lit.
3
.3.2. 2-p-Tolylbenzoxazole (4b). White crystals, mp 112–
2
0
26,28
1
113 8C (lit.
NMR (CDCl ): d 2.42 (s, 3H, CH ), 7.31–7.34 (m, 4H), 7.56
113–114 8C). LCMS: m/zZ210 (MC1). H
1
08–109 8C).
3
3
(
m, 1H), 7.76 (m, 1H), 8.14 (d, 2H, JZ8.2 Hz).
3
.1.2. 1-p-Methylbenzoylbenzotriazole (1b). White
crystals from ethanol, yield 90%, mp 127–128 8C (lit.
2
0,21
3
.3.3. 2-Phenylbenzoxazole (4c). White crystals, mp 101–
1
23–24 8C).
26,28,29
1
02 8C (lit.
102–103 8C). LCMS: m/zZ196 (MC1).
H NMR (CDCl ): d 7.33–7.37 (m, 2H), 7.51–7.54 (m, 3H),
1
3
3.1.3. 1-Benzoylbenzotriazole (1c). White crystals from
petroleum ether (60–80), yield 95%, mp 115–116 8C (lit.
2
0,22
7.56–7.59 (m, 1H), 7.80 (m, 1H), 8.29 (m, 2H).
1
12 8C).
.1.4. 1-p-Chlorobenzoylbenzotriazole (1d). White
crystals from petroleum ether (60–80), yield 98%, mp
3
crystals, mp 146–147 8C (lit.
m/zZ232 (MC3), 230 (MC1). H NMR (CDCl ): d
.3.4. 2-(4-Chlorophenyl)benzoxazole (4d). White
148–149 8C). LCMS:
2
6,28
3
1
3
2
2
7.34–7.40 (m, 2H), 7.53 (d, 2H, JZ8.4 Hz), 7.61 (m, 1H),
1
38–139 8C (lit. 138–139 8C).
7.79 (m, 1H), 8.22 (d, 2H, JZ8.4 Hz).
3.1.5. 1-p-Nitrobenzoylbenzotriazole (1e). White crystals
from ethanol, yield 96%, mp 196–198 8C (lit.
2
2,23
3.3.5. 2-(4-Nitrophenyl)benzoxazole (4e). White crystals,
2
6
mp 264–265 8C (lit. 263–264 8C). LCMS: m/zZ240 (MC
1
93–194 8C).
1
1
). H NMR (CDCl ): d 7.48–7.54 (m, 6H), 8.44 (d, 2H, JZ
3
8.2 Hz).
3
.2. Pyrolysis. General procedure for pyrolysis
3
White crystals, mp 223–224 8C (lit.
LCMS: m/zZ225 (MC1). H NMR (CDCl ): d 3.84 (s,
.3.6. 2-(4-Methoxyphenyl)-1H-benzimidazole (5a).
222–225 8C).
(a) Static pyrolysis. Each of the substrates 1a–e (0.2 g) was
introduced into the reaction tube (1.5!12 cm Pyrex),
3
0
1
cooled in liquid nitrogen, sealed under vacuum
(
3
3
H, OCH ), 7.24 (d, 2H, JZ8.8 Hz), 7.48 (m, 2H), 7.74 (m,
0.06 mbar) and then placed in the pyrolyzer for 15 min at
30, 340, 310, 300, 300 8C for 1a–e, respectively. The
pyrolysate was then separated by preparative HPLC using
3
2H), 8.08 (d, 2H, JZ8.8 Hz), 12.80 (br, 1H, NH).
3
C
ABZ column and an eluent of suitable composition
(acetonitrile and water). The collected solutions of the
pyrolysate fractions were evaporated and each fraction was
3.3.7. 2-p-Tolyl-1H-benzimidazole (5b). Colorless
crystals, mp 269–270 8C (lit. 270–272 8C). LCMS:
3
0
1
m/zZ209 (MC1). H NMR (DMSO-d
CH ), 7.16 (m, 2H), 7.32 (d, 2H, JZ8.0 Hz), 7.56 (m, 2H),
.05 (d, 2H, JZ8.0 Hz), 12.80 (br, 1H, NH).
): d 2.33 (s, 3H,
6
1
subjected to H NMR and GC–MS and LCMS analysis.
3
8
(b) Flash vacuum pyrolysis (FVP). The apparatus used was
similar to the one, which has been described in our recent
3.3.8. 2-Phenyl-1H-benzimidazole (5c). White crystals,
2
4,25
31a,32
publications.
The sample was volatilized from a tube in
mp 289–290 8C (lit.
(MC1). H NMR (CDCl ): d 7.29 (m, 3H), 7.45 (m, 3H),
286–289 8C). LCMS: m/zZ266
1
a B u¨ chi Kugelrohr oven through a 30!2.5 cm horizontal
fused quartz tube. This was heated externally by a Carbolite
3
7.68 (m, 2H), 8.12 (m, 2H).

8262
N. A. Al-Awadi et al. / Tetrahedron 61 (2005) 8257–8263
3
.3.9. 2-(4-Chlorophenyl)-1H-benzimidazole (5d). White
7.41 (m, 2H), 7.63 (m, 2H), 7.79 (br, 1H, NH), 8.04 (m, 2H),
8.35 (m, 2H).
3
0
crystals, mp 289–290 8C (lit. 290–292 8C). LCMS: m/zZ
1
31 (MC3), 229 (MC1). H NMR (DMSO-d ): d 7.19 (m,
2
2
8
6
H), 7.57 (m, 2H), 7.58 (d, 2H, JZ8.4 Hz), 8.18 (d, 2H, JZ
.4 Hz), 12.98 (br, 1H, NH).
3.3.21. 9-Methoxyphenanthridin-6(5H)-one (8a). Color-
3
8
less crystals, mp 235 8C (lit. 235–236 8C). LCMS: m/zZ
26 (MC1).
2
3
.3.10. 2-(4-Nitrophenyl)-1H-benzimidazole (5e). White
3
2
crystals, mp 297–298 8C (lit. 297–299 8C). LCMS: m/zZ
2
8
3.3.22. 9-Methylphenanthridin-6(5H)-one (8b). LCMS:
m/zZ210 (MC1).
1
39 (MC1). H NMR (DMSO-d ): d 6.80–7.74 (m, 5H),
.25–8.57 (m, 4H).
6
3
.3.23. Phenanthridin-6(5H)-one (8c). White crystals, mp
3
9
3
(
.3.11. 4-Methoxybenzamide (6a). LCMS: m/zZ152
290–292 8C (lit. 293–294 8C). LCMS: m/zZ195 (MC1).
1
MC1). H NMR (CDCl ): d 3.45 (s, 3H, CH ), 5.88 (br
1
3
3
H NMR (DMSO-d ): d 7.26 (t, 1H, JZ7.8 Hz), 7.36 (t, 1H,
6
d, 2H, NH ), 6.93 (d, 2H, JZ8.5 Hz), 7.56 (d, 2H, JZ
JZ8.0 Hz), 7.49 (t, 1H, JZ8.0 Hz), 7.65 (t, 1H, JZ7.5 Hz),
7.84 (t, 1H, JZ8.0 Hz), 8.32 (d, 1H, JZ8.0 Hz), 8.37 (d,
2
3
1b
8
.5 Hz).
1
NH).
H, JZ8.0 Hz), 8.51 (d, 1H, JZ8.0 Hz), 11.70 (br, 1H,
3
1
.3.12. 4-Methylbenzamide (6b). LCMS: m/zZ136 (MC
1
). H NMR (CDCl ): d 2.42 (s, 3H, CH ), 5.80 (br d, 2H,
3
3
3
1c
NH ), 7.27 (d, 2H, JZ7.9 Hz), 7.73 (d, 2H, JZ7.9 Hz).
3.3.24. 9-Chlorophenanthridin-6(5H)-one (8d). White
crystals from hexane, LCMS: m/zZ232 (MC3), 230
(MC1). H NMR (CDCl ): d 7.36 (t, 1H, JZ7.8 Hz),
2
1
1
3
.3.13. Benzamide (6c). LCMS: m/zZ122 (MC1).
H
3
NMR (CDCl ): d 5.80 (br d, 2H, NH ), 7.48 (t, 2H, JZ
7.46 (d, 1H, JZ8.4 Hz), 7.60 (t, 1H, JZ8.0 Hz), 7.82 (t, 1H,
JZ7.8 Hz), 8.10 (d, 1H, JZ8.0 Hz), 8.22 (d, 1H, JZ
3
2
3
1d
7
.3 Hz), 7.57 (t, 1H, JZ7.3 Hz), 7.76 (d, 2H, JZ7.3 Hz).
4
0
8
.0 Hz), 8.41 (d, 1H, JZ8.0 Hz), 10.28 (br, 1H, NH).
3
3
7
.3.14. 4-Chlorobenzamide (6d). LCMS: m/zZ158 (MC
1
), 156 (MC1). H NMR (CDCl ): d 5.90 (br d, 2H, NH ),
3
2
3.3.25. 9-Nitrophenanthridin-6(5H)-one (8e). LCMS:
3
3
1
.45 (d, 2H, JZ8.3 Hz), 7.78 (d, 2H, JZ8.5 Hz).
m/zZ241 (MC1). H NMR (CDCl ): d 7.36 (t, 1H, JZ
3
7
.8 Hz), 7.46 (m, 1H), 7.64 (m, 1H), 7.78 (t, 1H, JZ7.8 Hz),
3
.3.15. 4-Nitrobenzamide (6e). LCMS: m/zZ167 (MC1).
7.95 (d, 1H, JZ8.0 Hz), 8.38 (d, 1H, JZ8.0 Hz), 8.51 (d,
1H, JZ8.0 Hz), 10.25 (br, 1H, NH).
1
40
H NMR (CDCl ): d 5.72, 6.11 (br d, 2H, NH ), 8.0 (d, 2H,
JZ8.4 Hz), 8.34 (d, 2H, JZ7.4 Hz).
3
2
3
1e
C
.3.26. Cyanocyclopentadiene (9). MS: m/zZ91 (M ,
3
10%), H NMR (CDCl ): d 3.26 (m, 2H), 6.65 (m, 2H), 7.25
(m, 1H).
1
3.3.16. 4-Methoxy-N-phenylbenzamide (7a). White
crystals, mp 170–171 8C (lit. 169–170 8C). LCMS:
m/zZ228 (MC1). H NMR (CDCl ): d 3.88 (s, 3H), 6.68
3
3
4
41
1
3
(
d, 2H, JZ8.6 Hz), 7.15 (t, 1H, JZ7.8 Hz), 7.43 (t, 2H, JZ
3.4. Kinetic runs and data analysis
8
.0 Hz), 7.58 (d, 2H, JZ7.6 Hz), 7.82 (d, 2H, JZ8.6 Hz),
7
.89 (br, 1H, NH).
Stock solution (7 mL) is prepared by dissolving 6–10 mg of
the substrate in acetonitrile as solvent to give a concen-
tration of 1000–2000 ppm. Internal standard is then added,
the amount of, which is adjusted to give the desired peak
area ratio of substrate to standard (2.5:1). The solvent and
the internal standard are selected because both are stable
under the conditions of pyrolysis, and because they do not
react with either substrate or product. The internal standard
used in this study, is chlorobenzene. Each solution is filtered
to ensure that a homogeneous solution is obtained.
3
.3.17. 4-Methyl-N-phenylbenzamide (7b). White
3
4
crystals, mp 149–150 8C (lit. 150 8C). LCMS: m/zZ212
1
(
MC1). H NMR (CDCl ): d 2.44 (s, 3H), 7.17 (t, 1H, JZ
3
7.3 Hz), 7.29 (d, 2H, JZ8.0 Hz), 7.39 (t, 2H, JZ8.0 Hz),
7.66 (d, 2H, JZ7.8 Hz), 7.79 (d, 2H, JZ8.0 Hz), 7.88 (br,
1H, NH).
3
1
.3.18. N-Phenylbenzamide (7c). White crystals, mp 159–
3
5
1
60 8C (lit. 159–161 8C). LCMS: m/zZ198 (MC1). H
NMR (CDCl ): d 7.11 (t, 1H, JZ7.3 Hz), 7.36 (t, 2H, JZ
The weight ratio of the substrate with respect to the internal
standard is 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:
3
7.6 Hz), 7.53 (t, 2H, JZ7.0 Hz), 7.60 (t, 1H, JZ7.2 Hz),
7.80 (d, 2H, JZ8.2 Hz), 7.97 (d, 2H, JZ7.4 Hz), 10.3 (br,
1H, NH).
3
.3.19. 4-Chloro-N-phenylbenzamide (7d). Colorless
crystals, mp 195–196 8C (lit. 195–196 8C). LCMS:
3
6
An aliquot part (0.2 mL) of each solution containing the
substrate and the internal standard is pipetted into the
reaction tube, which is then placed in the pyrolyzer for
6 min under non-thermal conditions. A sample is then
analyzed using the HPLC probe with the UV detector at
1
m/zZ234 (MC3), 232 (MC1). H NMR (DMSO-d ): d
6
7
.12 (t, 1H, JZ7.3 Hz), 7.34 (t, 2H, JZ7.8 Hz), 7.60 (d, 2H,
JZ8.4 Hz), 7.75 (d, 2H, JZ8.0 Hz), 7.98 (d, 2H, JZ
8
.4 Hz), 10.30 (br, 1H, NH).
wavelength of 256 nm, and the standardization value (A ) is
o
3.3.20. 4-Nitro-N-phenylbenzamide (7e). Colorless
crystals, mp 217–218 8C (lit. 217–218.5 8C). LCMS:
then calculated. Several HPLC measurements are obtained
with an accuracy of R2%. The temperature of the pyrolysis
block is then raised until approximately 10% pyrolysis is
3
7
1
m/zZ244 (MC1). H NMR (CDCl ): d 7.21 (m, 1H),
3

N. A. Al-Awadi et al. / Tetrahedron 61 (2005) 8257–8263
8263
deemed to occur over 900 s. This process is repeated after
each 10–15 8C rise in the temperature of the pyrolyzer until
O85% pyrolysis takes place. The relative ratios of the
integration values of the sample and the internal standard
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reaction temperature following each 10–15 8C rise in the
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Acknowledgements
1
This work was supported by Kuwait University through
research grant # SC02/04 and ANALAB and SAF grants #
GS01/01 and GS03/01.
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