Comparative studies for selective deprotection of the N-arylideneamino moiety from heterocyclic amides: kinetic and theoretical studies. Part 2

Article abstract of DOI:10.1016/j.tet.2006.04.054

4-Benzylideneamino-1,2,4-triazine-3,5(2H,4H)-diones (2-5), 6-styryl-1,2,4-triazine-3,5(2H,4H)-dione (6), and 6-styryl-2,3-dihydro-3-thioxo-1,2,4-triazin-5(4H)-one (7) were synthesized and pyrolyzed in the gas phase. The kinetic effect of changing the substituent on the triazine ring from hydrogen to methyl, phenyl, and styryl was measured. Analyses of the pyrolyzates of 2-5 showed the elimination products to be benzonitrile and the triazine fragment, while the pyrolyzates of 6 and 7 reveal the formation of cis- and trans-cinnamonitriles. Theoretical study of the pyrolysis reactions of 2-5 using an ab initio SCF method was investigated.

Full text of DOI:10.1016/j.tet.2006.04.054

Tetrahedron 62 (2006) 6214–6221
Comparative studies for selective deprotection of the
N-arylideneamino moiety from heterocyclic amides: kinetic
and theoretical studies. Part 2*
*
Nouria A. Al-Awadi, Yehia A. Ibrahim, Hicham H. Dib, Maher R. Ibrahim,
Boby J. George and Mariam R. Abdallah
Chemistry Department, Kuwait University, PO Box 5969, Safat 13060, Kuwait
Received 22 February 2006; revised 30 March 2006; accepted 20 April 2006
Available online 15 May 2006
Abstract—4-Benzylideneamino-1,2,4-triazine-3,5(2H,4H)-diones (2–5), 6-styryl-1,2,4-triazine-3,5(2H,4H)-dione (6), and 6-styryl-2,3-di-
hydro-3-thioxo-1,2,4-triazin-5(4H)-one (7) were synthesized and pyrolyzed in the gas phase. The kinetic effect of changing the substituent
on the triazine ring from hydrogen to methyl, phenyl, and styryl was measured. Analyses of the pyrolyzates of 2–5 showed the elimination
products to be benzonitrile and the triazine fragment, while the pyrolyzates of 6 and 7 reveal the formation of cis- and trans-cinnamonitriles.
Theoretical study of the pyrolysis reactions of 2–5 using an ab initio SCF method was investigated.
Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction
were suggested (Scheme 1). Theoretical studies showed that
it is the carbonyl and not the thione bond that attacks the
hydrogen atom of the arylideneamino groups.
Pyrolytic deprotection of 4-arylideneamino-3(2H)-thioxo-
1,2,4-triazin-5(4H)-one (1) was carried out.2 Based on the
product analysis together with kinetic results and in the
absence of theoretical studies two mechanistic pathways
In this study we have extended the investigation to include 4-
benzylideneamino-1,2,4-triazine-3,5-dione derivatives 2–5,
6-styryl-1,2,4-triazine-3,5(2H,4H)-dione (6), and 6-styryl-
2,3-dihydro-3-thioxo-1,2,4-triazin-5(4H)-one (7).
N
NH
N
NH
N
N
S
H
O
ArC
N
+
N
SH
2. Results and discussion
2.1. Product analysis
O
Ar
N
NH
Ar
N
The substrates 2–7 were prepared and fully characterized by
NMR and MS, as described in the Section 3. The products of
static as well as flash vacuum pyrolysis of each substrate
were analyzed; the constituents of pyrolyzate from 2–5
were ascertained to be benzonitrile3 together with the het-
erocyclic fragments 10a, 10b, 10c, and 6 while FVP of 6
and 7 results in cis- and trans-cinnamonitrile (Table 1).
NH
NH
N
N
S
O
N
H
S
O
1
N
NH
N
N
ArC
N
O
N
N
S
+
HO
S
2.2. Kinetic studies
H
For each substrate, first-order rate coefficients were obtained
at regular temperature intervals. Each rate constant is an av-
erage of at least three independent measurements in agree-
ment to within ꢀ2%. The reactions for which the kinetic
data were obtained have been ascertained to be homo-
geneous, unimolecular, non-catalytic, and non-radical
processes.4,5 Arrhenius plots of the data using first-order
Ar
Scheme 1.
* See Ref. 1.
Keywords: Arylideneamino; Pyrolysis; Kinetics; Reaction mechanism.
* Corresponding author. Tel.: +965 4985537; fax: +965 4816482; e-mail:
nouria@kuc01.kuniv.edu.kw
0040–4020/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2006.04.054

N. A. Al-Awadi et al. / Tetrahedron 62 (2006) 6214–6221
Table 1. Pyrolysis products and % yield of compounds 1–6 under static/FVP pyrolysis
6215
Compound
Substrate
% Yield (static/FVP)
10a–c, 5, 13
9
11
12
N
N
NH
NH
Ph
C
N
N
N
N
(42/43)3
O
N
N
O
O
N
H
O
1
—
—
10a (35/36)12
Ph
H3C
O
N
H3C
N
NH
NH
Ph
C
(32/48)3
N
N
O
O
N
H
O
2
3
4
—
—
10b (53/39)12
Ph
Ph
O
N
N
Ph
NH
NH
Ph
C
N
H
(47/28)3
N
N
O
O
O
—
—
10c (45/46)12
Ph
N
H
CN
H
H
H
PhCH=CH
O
N
PhCH=CH
O
NH
NH
Ph
C
CN
Ph
Ph
(34/38)3
N
N
O
N
H
O
12 (17/10)17
11 (19/13)17
5 (54/24)10
Ph
Ph
N
H
CN
H
H
H
NH
5
6
—
—
—
CN
Ph
Ph
O
N
H
O
11 (18/9)17
12 (14/12)17
Ph
N
HS
CN
1318
H
CN
H
H
NH
H
CN
Ph
Ph
O
N
H
S
11 (38/23)17
12 (25/15)17
rate equation: log k (sꢁ1)¼log AꢁEa kJ molꢁ1 (2.303RT)ꢁ1
were strictly linear over ꢂ95% reaction with correlation
coefficient in the range 0.99ꢀ0.005. The log A, Ea values
and the first-order rate constants of the six compounds under
investigation are given in Table 2.
2. With the exception of compound 5, there is no
considerable difference between the TS-I in the two
suggested pathways.
3. For each compound, formation of P-I from TS-I
requires relatively lower energy via pathway B than
that in pathway A. This means that the reaction pref-
erentially involves C3]O rather than C5]O in the
transition state.
The main features of the kinetic results are as follows:
ꢃ Two alternative pathways could be postulated for the ex-
trusion of PhC^N from 2–5. Pathway (A) involves nu-
cleophilic attack of the C5]O group on the hydrogen of
the arylidene moiety (Scheme 2) while pathway (B) in-
volves the nucleophilic attack of the C3]O on the same
hydrogen atom (Scheme 3). To make the choice between
(A) and (B), we have examined both pathways theoreti-
cally for compounds 2–5.
ꢃ Examination of the free energy of the two suggested
pyrolytic pathways of compounds 2–5 shows that:
1. For hydrogen, methyl, phenyl, and styryl substituted
compounds (2–5), the final product P-II can be
formed from either pathway A or B (Fig. 1).
ꢃ The overall reactivities of compounds 2–5 are the result
of electronic synergism involving bonds a, b, and c, and
the stabilizing influences associated with the substitu-
ents (R) (Scheme 4).
ꢃ The reactivity of compound 3 exceeds that of compound
2 by a factor of 1.39ꢄ104 (Scheme 4), this rate enhance-
ment is attributed to the influence that the methyl substit-
uent has on the delocalization of the lone pair of
electrons on N2 onto the carbonyl oxygen atom of
C3]O group involved in the reaction pathway. This
causes an increase in the protophilicity of the bond a,
this effect of which the delocalization of this lone pair
of electrons on N2 is exerting is further revealed when

6216
N. A. Al-Awadi et al. / Tetrahedron 62 (2006) 6214–6221
Table 2. Rate coefficients (k/sꢁ1), Arrhenius parameters of compounds 1–6
Compound
T/K
104k/sꢁ1
log A/sꢁ1
Ea/k J molꢁ1
140.58ꢀ3.24
k500K/sꢁ1
1
548.35
588.45
598.35
608.65
618.45
1.455
10.66
19.78
28.71
45.88
9.39ꢀ0.29
7.55ꢄ10ꢁ6
2
3
399.45
409.55
419.05
428.85
444.25
0.790
1.942
4.582
11.23
26.41
11.35ꢀ0.68
12.54ꢀ0.07
119.36ꢀ5.44
147.00ꢀ0.70
1.05ꢄ10ꢁ1
2.31ꢄ10ꢁ3
445.45
465.35
475.25
485.55
495.65
0.322
1.728
3.671
8.146
17.23
4
5
456.75
471.25
514.55
519.35
1.332
2.643
18.07
20.77
6.13ꢀ0.10
6.36ꢀ0.07
88.572ꢀ1.02
123.08ꢀ0.89
9.69ꢄ10ꢁ4
4.66ꢄ10ꢁ7
620.95
649.75
663.65
677.85
705.95
719.95
1.390
3.967
6.426
9.750
23.63
35.90
6
639.85
661.55
683.05
704.75
726.45
748.10
770.15
1.125
1.947
3.573
8.391
17.46
29.84
46.17
6.02ꢀ0.33
124.25ꢀ4.41
1.56ꢄ10ꢁ7
N
R
H
N
R
N1
H
N
C
R
N
H
N2
C3
C6
C5
N
C5
C3
O4
+
N4
N9
C5
N4
N9
C3
O8
O7
O7
H11
N4
O7
O8
H11
C10
Ph
C10
Ph
H11
(P-I)
(Reactant)
(TS-I)
R
N
H
R
N
H
(2) R = H
N
N
(3) R = CH3
(4) R = C6H5
(5) R = PhCH=CH
C3
C5
C5
C3
N4
N4
H11
O8
O8
O7
O7
H11
(TS-II)
(P-II)
Scheme 2. Pathway A of the suggested mechanism.
the rate of compound 4 is compared with that of
compound 3.
ꢃ This effect explains also:
(i) The relative reactivity factor of 304.5 of compound
4 over compound 2.
ꢃ Changing (R) from methyl in compound 3 to phenyl in
compound 4 resulted in reducing the molecular reactiv-
ity of compound 4 by a factor of 45.5, this is attributed to
the competitive (opposing) delocalization of the lone
pair of electrons on N2 atom of the triazine ring by the
phenyl group (Scheme 5).
(ii) The comparable reactivities of compounds 4 and 5.
ꢃ A reaction mechanism postulated in Scheme 6 is sug-
gested for the formation of cis- and trans-cinnamonitrile
from compounds 6 and 7 together with traces of HSCN
from compound 7 (Table 1).

N. A. Al-Awadi et al. / Tetrahedron 62 (2006) 6214–6221
6217
N
C
R
N
H
R
H
R
N
H
N
N
N
N
C3
C3
C3
C5
C5
C5
+
N4
O7
N4
O7
O8
O8
O8
N4
N9
O7
N9
H11
H11
H11
(P-I)
C10
Ph
C10
Ph
(Reactant)
(TS-I)
R
H
N
R
H
N
(2) R = H
(3) R = CH3
N
N
C5
C3
C3
C5
(4) R = C6H5
(5) R = PhCH=CH
N4
O7
N4
H11
(TS-II)
O7
O8
O8
H11
(P-II)
Scheme 3. Pathway B of the suggested mechanism.
50
10
50
TS-I
TS-II
TS-II
0
TS-I
Reactant
-50
-100
-150
-200
0
Reactant
-50
-100
-150
-200
-250
P-I
P-I
(2)
(3)
-250
P-II
P-II
-300
50
0
50
TS-I
TS-I
TS-II
TS-I
0
Reactant
Reactant
-50
-50
-100
-150
-100
-150
-200
-250
-300
P-I
P-I
(4)
(5)
-200
-250
P-II
P-II
Reaction Coordinate
Pathway A
Pathway B
Figure 1. Free energy profile evaluated at the HF/6-31G* level for compounds 2–5.
R
O
N
NH
a
This reaction mechanism accounts well for the absence of
any electronic effect of changing X from oxygen in 6 to
sulfur in 7.
N
N
O
H
c
b
Ph
krel
2.3. Computational studies
(3) R = CH3
(2) R = H
7.55 x 10-6
k500K/s-1= 1.05 x 10-1
1.39 x 104
Ab initio molecular orbital studies on the thermolysis of
four-substituted 1,2,4-triazine compounds 2–5, were carried
out in the gas phase using SCF method. Calculations were
carried out to explore the nature of the reaction mechanism
of the unimolecular decomposition of the studied reaction.
Two competitive reaction pathways have been studied.
Calculations have been performed using TITAN and
SPARTAN packages.6,7
(3) R = CH3
(4) R = Ph
2.30 x 10-3
k500K/s-1
=
=
1.05 x 10-1
45.6
(2) R = H
(4) R = Ph
7.55 x 10-6
k500K/s-1
2.30 x 10-3
304.6
(5) R = PhCH=CH
(4) R = Ph
k500K/s-1
9.70 x 10-4
The geometric parameters of the reactants, the transition
states (TS), and the products of the four studied reactions
were fully optimized at the HF/6-31G* level to obtain the
=
2.30 x 10-3
2.4
Scheme 4. Relative reactivities of compounds 2–5.

6218
N. A. Al-Awadi et al. / Tetrahedron 62 (2006) 6214–6221
R
O
X
NH
+
N
N
H
6,7
C
H
CN
H
H
H
NH
St/450 °C
NH
C
O
+
+
H3C
O
N
N
FVP/700 °C
X
NH
Ph
NH
CN
Ph
Ph
HS
C
N
N
N
O
H
O
N
N
O
H
R
X
6:O
PhCH=CH
PhCH=CH
7:S
Ph
(3)
(4)
Scheme 6. Product analyses of compounds 6 and 7.
Scheme 5. Opposing delocalization of the lone pair of electrons on N2 atom
by methyl (3) and phenyl (4) groups.
is represented in Table 3. Electronic energies, zero-point
vibrational energies, enthalpies, and entropies, evaluated at
the HF/6-31G* level of theory, for all the reactants, transi-
tion states, and product involved in the two pathways of
the studied reaction are collected in Table 4. The free energy
energy profiles corresponding to the studied reactions. A
scaling factor8 of 0.9135 for the zero-point vibrational ener-
gies has been used. The main distances of the optimized
structure of the two pathways of the two studied reactions
˚
Table 3. Main distances (A) of the reactants, TS-I, TS-II, TS-III, TS-IV, P-I, and P-II of the two pathways of the four-substituted compounds, calculated at the
HF/6-31G* level
Species
C3–O8
C5–O7
O7–H11
C10–H11
C10–N9
N4–H11
N4–N9
O8–H11
R¼H
Path A
Reactant
TS-I
P-I
TS-II
1.188
1.185
1.190
1.188
1.191
1.193
1.322
1.249
2.614
2.080
0.948
1.317
1.079
1.071
—
1.258
1.260
—
2.448
2.617
3.028
1.323
1.401
1.382
—
4.080
4.586
5.271
3.041
—
—
—
Path B
Reactant
TS-I
P-I
TS-II
P-II
1.187
1.195
1.316
1.253
1.189
1.190
1.184
1.191
1.189
1.191
1.945
2.078
0.953
1.308
2.481
1.059
1.071
—
—
—
1.258
1.260
—
—
—
2.501
2.629
2.281
1.331
1.000
1.401
1.382
—
—
—
1.945
2.078
0.953
1.308
2.481
R¼CH3
Path A
Reactant
TS-I
P-I
TS-II
1.186
1.186
1.191
1.189
1.193
1.195
1.323
1.251
1.815
2.079
0.947
1.316
1.058
1.071
—
1.257
1.260
—
2.554
2.627
3.026
1.324
1.396
1.382
—
4.594
4.589
5.262
3.403
—
—
—
Path B
Reactant
TS-I
P-I
TS-II
P-II
1.196
1.196
1.317
1.254
1.192
1.190
1.185
1.193
1.190
1.191
4.605
4.581
4.492
3.411
2.486
1.064
1.071
—
—
—
1.257
1.259
—
—
—
2.525
2.628
2.279
1.329
0.999
1.396
1.382
—
—
—
1.951
2.072
0.953
1.308
2.481
R¼Phenyl
Path A
Reactant
TS-I
P-I
TS-II
1.181
1.186
1.191
1.189
1.180
1.193
1.316
1.253
1.744
2.086
0.954
1.307
1.038
1.070
—
1.258
1.259
—
2.542
2.647
2.240
1.321
1.403
1.381
—
4.601
4.594
5.256
3.390
—
—
—
Path B
Reactant
TS-I
P-I
TS-II
P-II
1.198
1.196
1.317
1.254
1.192
1.187
1.184
1.192
1.189
1.190
4.504
4.565
4.479
3.386
2.455
1.064
1.071
—
—
—
1.258
1.260
—
—
—
2.490
2.631
2.284
1.331
1.000
1.403
1.383
—
—
—
1.909
2.068
0.953
1.308
2.481
R¼PhCH¼CH
Path A
Reactant
TS-I
P-I
TS-II
1.189
1.186
1.191
1.189
1.192
1.194
1.320
1.254
2.717
2.097
0.949
1.309
1.079
1.071
—
1.257
1.259
—
2.433
2.654
3.013
1.323
1.401
1.381
—
3.944
4.595
4.492
4.348
—
—
—
Path B
Reactant
TS-I
P-I
TS-II
P-II
1.194
1.196
1.313
1.255
1.193
1.187
1.185
1.198
1.191
1.191
4.094
4.569
4.457
3.378
2.448
1.078
1.070
—
—
—
1.258
1.260
—
—
—
2.460
2.627
2.276
1.329
1.000
1.401
1.384
—
—
—
2.575
2.058
0.954
1.309
2.481

N. A. Al-Awadi et al. / Tetrahedron 62 (2006) 6214–6221
6219
Table 4. Total energy, zero-point vibrational energy (ZPE) and thermal correction to enthalpy and entropy at HF/6-31G* for the two pathways of the four studied
reactions
Species
Total energy (Hartree)
ZPE (kcal/mol)
Enthalpy (kcal/mol)
Entropy
R¼H
Pathway A
R
TS-I
P-I
ꢁ750.818948
ꢁ750.816035
ꢁ428.390523
ꢁ428.326590
120.113
120.036
50.859
47.769
127.63
127.075
54.482
51.182
111.907
107.121
77.457
76.326
TS-II
Pathway B
R
TS-I
P-I
TS-II
P-II
ꢁ750.819075
ꢁ750.816123
ꢁ428.398918
ꢁ428.327099
ꢁ428.428879
120.124
120.011
51.036
47.767
51.339
127.677
127.06
54.652
51.215
54.934
112.895
107.211
77.529
76.584
77.531
R¼CH3
Pathway A
R
TS-I
P-I
ꢁ789.847659
ꢁ789.859391
ꢁ467.432060
ꢁ467.370078
138.917
138.634
69.519
66.306
146.924
146.654
74.072
70.756
114.956
114.164
84.270
84.052
TS-II
Pathway B
R
TS-I
P-I
TS-II
P-II
ꢁ789.847626
ꢁ789.859867
ꢁ467.443538
ꢁ467.371790
ꢁ467.472906
138.807
138.607
69.632
66.363
69.945
146.823
146.644
74.203
70.777
74.518
115.701
114.384
84.584
83.687
84.785
R¼Phenyl
Pathway A
R
TS-I
P-I
ꢁ980.323127
ꢁ980.366412
ꢁ657.944562
ꢁ657.880994
174.213
174.408
105.496
102.288
182.91
119.782
129.535
100.159
108.443
184.171
111.736
108.418
TS-II
Pathway B
R
TS-I
P-I
TS-II
P-II
ꢁ980.360354
ꢁ980.366976
ꢁ657.950982
ꢁ657.879006
ꢁ657.980931
174.604
174.413
105.494
102.236
105.843
184.363
184.187
111.774
108.357
112.109
130.6
129.724
100.546
99.479
100.544
R¼PhCH¼CH
Pathway A
R
TS-I
P-I
TS-II
ꢁ1057.258752
ꢁ1057.25448
ꢁ734.828937
ꢁ734.769220
197.233
197.072
128.141
124.933
208.812
208.199
135.741
132.416
145.891
141.538
112.727
112.019
Pathway B
R
TS-I
P-I
TS-II
P-II
ꢁ1057.258991
ꢁ1057.25538
ꢁ734.839730
ꢁ734.767977
ꢁ734.869487
197.102
197.093
128.205
124.949
128.550
207.682
208.241
135.847
132.443
136.184
136.564
142.02
113.125
112.211
113.414
profile for the decomposition process of the two studied
reactions is presented in Figure 1.
pathway A. The difference between the activation energies
of P-I in the two pathways is 44.18 kJ/mol.
Examination of the free energy of the two suggested path-
ways of the four-substituted reactions studied shows that,
for hydrogen, methyl, and phenyl substituted compounds
the final product can be formed from either of the two path-
ways, since there is no large differences between the transi-
tion state one (TS-I) in the two pathways. Just in styryl, it
appears clearly the formation for the final product (P-II)
is favored via pathway B than pathway A, since the six-
membered cyclic transition state (TS-I) is of lower energy
in pathway A than in pathway B. The calculated activation
energies of TS-I are 18.40 and ꢁ0.25 kJ/mol for the reaction
via pathway A and pathway B, respectively. Although
the formation of P-I is more favored for pathway B than
3. Experimental
3.1. Synthesis
All melting points are uncorrected. IR spectra were recorded
in KBr disk using Perkin–Elmer system 2000 FTIR spectro-
photometer. 1H NMR spectra were recorded on a BRUKER
DPX 400 MHz Superconducting NMR spectrometer. Mass
spectra were measured on Vg Auto-spes-q (high resolution,
high performance, tri-sector GC/MS/MS) and with LCMS
using Agilent 1100 series LC/MSD with an API-ES/APCI
ionization mode. Microanalyses were performed on LECO
CHNS-932 Elemental Analyzer.

6220
N. A. Al-Awadi et al. / Tetrahedron 62 (2006) 6214–6221
1
3.1.1. 4-Benzylidineamino-1,2,4-triazine-3,5(2H,4H)-
diones (2–5).
(lit.11 264 ꢅC). H NMR (DMSO-d6) d 12.63 (br, 1H, NH),
12.23 (br, 1H, NH), 7.79 (d, 1H, J 16.4), 7.64 (d, 2H,
J 7.4), 7.42–7.33 (m, 3H), 7.07 (d, 1H, J 16.4).
3.1.1.1. General procedure. A mixture of the appropri-
ate 4-amino-1,2,4-triazine derivatives (5 mmol)9 in acetic
acid (10 mL) together with benzaldehyde (0.5 g, 5 mmol)
and anhydrous sodium acetate (500 mg) was heated under
reflux for 2 h. The solid that precipitated after cooling was
collected and recrystallized from ethanol.
3.1.1.8. 1,2,4-Triazine-3,5(2H,4H)-dione (10a). Mp
282–283 ꢅC (lit.12 283 ꢅC). H NMR (DMSO-d6) d 12.32
(br, 1H, NH), 11.96 (br, 1H, NH), 7.38 (s, 1H).
1
3.1.1.9. 6-Methyl-1,2,4-triazine-3,5(2H,4H)-dione
(10b). Mp 215–216 ꢅC (lit.12 216 ꢅC). 1H NMR (DMSO-d6)
d 11.99 (br, 1H, NH), 11.88 (br, 1H, NH), 2.50 (s, 3H, CH3).
3.1.1.2. 4-Benzylidineamino-1,2,4-triazine-3,5(2H,4H)-
dione (2). Colorless crystals from ethanol, yield 800 mg
(74%), mp 205–207 ꢅC. LCMS m/z¼217 (M+1). IR: 3221,
3111, 3052, 2941, 1714, 1665, 1612, 1570, 1401, 1341,
3.1.1.10. 6-Phenyl-1,2,4-triazine-3,5(2H,4H)-dione
1
1
1230, 1097, 976, 833, 743. H NMR (CDCl3) d 9.75 (br,
(10c). Mp 262–263 ꢅC (lit.12 262 ꢅC). H NMR (DMSO-
1H, NH), 8.68 (s, 1H), 7.92 (d, 2H, J 8), 7.60 (m, 1H), 7.54
(s, 1H), 7.48 (t, 2H, J 7.6). 13C NMR (DMSO) d 172.6,
154.2, 148.1, 136.3, 134.1, 132.9, 130.2, 129.9. Anal. Calcd
for C10H8N4O2 (216.2): C, 55.56; H, 3.73; N, 25.91.
Found: C, 55.56; H, 3.67; N, 25.87.
d6) d 12.52 (br, 1H, NH), 12.11 (br, 1H, NH), 7.82 (d, 2H,
J 7.4, Ar–H), 7.43 (m, 3H, Ar–H).
3.2. Pyrolysis
3.2.1. Static pyrolysis. Each of the substrates 2–7 (200 mg)
was introduced into the reaction tube (1.5ꢄ12 cm Pyrex),
cooled in liquid nitrogen, sealed under vacuum (0.06 mbar)
and then placed in the pyrolyzer for 15 min at a temperature
deemed necessary for complete pyrolysis of the substrate.
The pyrolyzate was then separated by preparative HPLC us-
ing ABZ+ column and an eluent of suitable composition (ace-
tonitrile and water). The collected solutions of the pyrolyzate
fractions were evaporated and each fraction was subjected to
1H NMR, GC–MS, and LC–MS analyses (Table 1).
3.1.1.3. 4-Benzylidineamino-6-methyl-1,2,4-triazine-
3,5(2H,4H)-dione (3). Colorless crystals from ethanol, yield
900 mg (78%), mp 190–192 ꢅC. LCMS m/z¼231 (M+1). IR:
3252, 3189, 2927, 1733, 1684, 1660, 1615, 1423, 1373,
1
1280, 1239, 795, 760, 690. H NMR (CDCl3) d 9.73 (br,
1H, NH), 8.66 (s, 1H), 7.91 (dd, 2H, J 8, 1.4), 7.56 (m,
1H), 7.44 (m, 2H), 2.31 (s, 3H). 13C NMR (CDCl3)
d 169.9, 153.8, 148.1, 144.5, 133.4, 132.1, 129.7, 129.2,
17.3. Anal. Calcd for C11H10N4O2 (230.2): C, 57.39; H,
4.38; N, 24.34. Found: C, 57.35; H, 4.40; N, 24.38.
3.2.2. Flash vacuum pyrolysis (FVP). The apparatus used
was similar to the one, which has been described in our recent
publications.5,13 The sample was volatilized from a tube in
3.1.1.4. 4-Benzylidineamino-6-phenyl-1,2,4-triazine-
3,5(2H,4H)-dione (4). Colorless crystals from ethanol, yield
1.20 g (82%), mp 230–232 ꢅC. LCMS m/z¼293 (M+1). IR:
3224, 3140, 3118, 3033, 2935, 1731, 1683, 1659, 1446,
1415, 1298, 1247, 1167, 768, 685. 1H NMR (CDCl3)
d 9.77 (br, 1H, NH), 8.70 (s, 1H), 7.99 (dd, 2H, J 8, 1.4),
7.94 (d, 2H, J 8.0), 7.57 (m, 1H), 7.53–7.45 (m, 5H). 13C
NMR (DMSO) d 173.0, 153.8, 148.0, 142.4, 134.1, 133.7,
133.0, 130.6, 130.2, 129.9, 129.2, 129.0. Anal. Calcd for
C16H12N4O2 (292.3): C, 65.75; H, 4.14; N, 19.17. Found:
C, 65.55; H, 3.96; N, 19.10.
a Buchi Kugelrohr oven through a 30ꢄ2.5 cm horizontal
¨
fused quartz tube. This was heated externally by
a Carbolite Eurotherm tube furnace MTF-12/38A to a
temperature of 600 ꢅC, the temperature being monitored by
a Pt/Pt–13%Rh thermocouple situated at the center of the fur-
nace. The products were collected in a U-shaped trap cooled
in liquid nitrogen. The whole system was maintained at
a pressure of 10ꢁ2 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
3.1.1.5. 4-Benzylidineamino-6-styryl-1,2,4-triazine-
3,5(2H,4H)-dione (5). Colorless crystals from ethanol, yield
1.30 g (82%), mp 260–262 ꢅC. LCMS m/z¼319 (M+1). IR:
3322, 3236, 3081, 2945, 1723, 1653, 1562, 1418, 1317,
1
collected in the U-shaped trap were analyzed by H NMR,
LC–MS, and GC–MS. Relative and percent yields were de-
1
1
1199, 972, 941, 741. H NMR (DMSO-d6) d 12.90 (br,
termined from H NMR. Identities of compounds obtained
were confirmed by comparison of their H NMR with data
of products separated from preparative HPLC (Table 1).
1
1H, NH), 8.79 (s, 1H), 7.92 (d, 2H, J 7.6), 7.74 (d, 1H, J
16.4), 7.67–7.57 (m, 5H), 7.43–7.33 (m, 3H), 7.14 (d, 1H,
J 16.4). 13C NMR (DMSO) d 173.0, 153.9, 147.8, 140.9,
137.0, 135.0, 134.1, 133.0, 130.2, 130.0, 129.9, 129.1,
128.2, 121.0. Anal. Calcd for C18H14N4O2 (318.3): C,
67.92; H, 4.43; N, 17.60. Found: C, 67.88; H, 4.41; N, 17.58.
3.3. Kinetic runs and data analyses
A stock solution (7 mL) was prepared by dissolving 6–10 mg
of the substrate in acetonitrile as solvent to give a concentra-
tion of 1000–2000 ppm. An internal standard was then
added, the amount of which was 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, 1,3-dichlorobenzene or
1,2,4-trichlorobenzene. Each solution was filtered to ensure
3.1.1.6. 6-Styryl-1,2,4-triazine-3,5(2H,4H)-dione (6).
1
Prepared as reported,10 mp 268–270 ꢅC (lit.9 266 ꢅC). H
NMR (DMSO-d6) d 12.41 (br, 1H, NH), 12.09 (br, 1H,
NH), 7.69 (d, 1H, J 16.4), 7.60 (d, 2H, J 7.4), 7.39 (t, 2H,
J 7.4), 7.33 (m, 1H), 7.05 (d, 1H, J 16.4).
3.1.1.7. 6-Styryl-2,3-dihydro-3-thioxo-1,2,4-triazine-
5(4H)-one (7). Prepared as reported,10 mp 278–280 ꢅC

N. A. Al-Awadi et al. / Tetrahedron 62 (2006) 6214–6221
6221
that a homogeneous solution is obtained. The weight ratio of
References and notes
the substrate with respect to the internal standard was calcu-
lated from the ratio of the substrate peak area to the peak area
of the internal standard. The kinetic ratewas obtained by trac-
ing the rate of disappearance of the substrate with respect to
the internal standard as follows: an aliquot part (0.2 mL) of
each solution containing the substrate and the internal stan-
dard 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 a Waters HPLC probe (pump
model 515, UV detector model 2487), or a Metrohm HPLC
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tor) and UV detector at wavelength of 256 nm, and the stan-
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measurements were obtained with an accuracy of ꢂ2%.
HPLC columns used for the analysis were Supelco (25 cm
length, 4.6 mm ID) ABZ+, LC-8, and LC-18. The tempera-
ture of the pyrolysis (aluminum) block is then raised and
held for ca. 900 s to allow approximately 10% pyrolysis to
take place at this temperature. This procedure is repeated
after each 10–15 ꢅC rise in the temperature of the pyrolyzer
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temperature, following every 10–15 ꢅC rise in the tempera-
ture of the pyrolyzer, in order to ensure reproducible values
of (A). Treatment of the kinetic data has been detailed else-
where.14–16
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The support of the University of Kuwait received through
research grant (SC02/03) and the facilities of Analab/SAF
(GS02/01, GS03/01) is gratefully acknowledged.