Steric effects in acid-catalyzed decomposition and base-catalyzed cyclization of 1-(2-alkoxycarbonyl-phenyl)-3-phenyltriazenes

Article abstract of DOI:10.1135/cccc19960751

Eight derivatives of 1-(2-alkoxycarbonylphenyl)-3-phenyltriazene (R = methyl, ethyl, propyl, isopropyl, butyl, isobutyl, hexyl, and allyl) have been synthesized and their UV-VIS, IR, ¹H and ¹³C NMR spectra measured. The NMR spectra have been interpreted in detail. The kinetics of acid-catalyzed decomposition and base-catalyzed cyclization of the title compounds have been measured in 52.1% w/w methanol at 25.0 °C. The unit reaction order has been verified and the cyclization product has been identified. The pH-profiles obtained have been used to calculate the catalytic rate constants k_A (acid-catalyzed decomposition) and k_B (base-catalyzed cyclization) of all the derivatives; the constants have been interpreted with regard to inductive and steric effects. The catalytic rate constant k_A has been found to be independent of the substituents. The catalytic rate constant k_B depends statistically significantly upon both inductive and steric effects, the sensitivity to the former being more significant. The experimental results and their interpretation confirm the base-catalyzed cyclization mechanism with formation of tetrahedral intermediate as the rate-limiting step.

Full text of DOI:10.1135/cccc19960751

Steric Effects
751
STERIC EFFECTS IN ACID-CATALYZED DECOMPOSITION AND
BASE-CATALYZED CYCLIZATION OF 1-(2-ALKOXYCARBONYL-
PHENYL)-3-PHENYLTRIAZENES
1
2
Oldrich PYTELA and Ales HALAMA
Department of Organic Chemistry, University of Pardubice, 532 10 Pardubice, Czech Republic;
e-mail: 1 pytela@hlb.upce.cz, 2 halama@hlb.upce.cz
Received September 27, 1995
Accepted November 15, 1995
Eight derivatives of 1-(2-alkoxycarbonylphenyl)-3-phenyltriazene (R = methyl, ethyl, propyl, isopropyl,
butyl, isobutyl, hexyl, and allyl) have been synthesized and their UV-VIS, IR, 1H and 13C NMR
spectra measured. The NMR spectra have been interpreted in detail. The kinetics of acid-catalyzed
decomposition and base-catalyzed cyclization of the title compounds have been measured in 52.1% w/w
methanol at 25.0 °C. The unit reaction order has been verified and the cyclization product has been
identified. The pH-profiles obtained have been used to calculate the catalytic rate constants kA (acid-
catalyzed decomposition) and kB (base-catalyzed cyclization) of all the derivatives; the constants have
been interpreted with regard to inductive and steric effects. The catalytic rate constant kA has been
found to be independent of the substituents. The catalytic rate constant kB depends statistically signi-
ficantly upon both inductive and steric effects, the sensitivity to the former being more significant.
The experimental results and their interpretation confirm the base-catalyzed cyclization mechanism
with formation of tetrahedral intermediate as the rate-limiting step.
Key words: Triazenes, acid-catalyzed decomposition of; Triazenes, base-catalyzed cyclization of;
Inductive effect; Steric effects.
The title 1-(2-alkoxycarbonylphenyl)-3-phenyltriazenes of general formula 1 (Scheme 1)
have not been systematically studied yet, only the methyl and ethyl derivatives being
known so far1–3 (1, R = CH3, C2H5). The cognate compounds so far synthesized and
described involve 1-alkyl-3-(2-alkoxycarbonylphenyl)triazenes4, 1-(2-ethoxycarbonyl-
phenyl)-3-aryltriazenes3, 1-(2-aryloxycarbonylphenyl)-3-phenyltriazenes5, and 1-(2-
cyanophenyl)-3-phenyltriazenes6–8. The compounds of type 1 can undergo two
different reactions depending on the catalyst type. In acid media they undergo acid-
COOR
COOR
NHN N
1b
1
2 3
N NNH
1a
SCHEME 1
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

752
Pytela, Halama:
catalyzed splitting of the triazene chain into the respective diazonium salt and amine.
Due to the tautomerism between the two forms 1a and 1b, the splitting can produce
either of the two possible diazonium salts along with the corresponding anilines. This
type of reactions is relatively well known, for a survey see, e.g., our previous communi-
cations9–12. Depending on the substrate structure and on the reaction medium, the cata-
lytic particles involve the proton, nondissociated general acid, and (in aprotic media)
also dimer of general acid9–11. The decomposition connected with splitting of triazene
chain can be described by two mechanisms expressing rather two limit states of a real
process. The first of these mechanisms, which is called A1, involves a formation of
protonated substrate in a rapid pre-equilibrium and its subsequent decomposition in the
rate-limiting step. It is observed in polar protic media predominantly. Evidently, the
reaction is co-assisted by the acid anion as a general base catalyst12. The second mech-
anism, referred to as A-SE2, is characterized by a proton transfer from a general acid to
the substrate in the rate-limiting step, and it is likely to take place in little polar media.
In basic media, the proton is dissociated from N3 atom13 to give an anion behaving as a
nucleophile which attacks the carbonyl of ester group at ortho position to the triazene
chain. This cyclization produces 3-phenyl-3H,4H-benzo[e]-1,2,3-triazin-4-one14
(Scheme 2). The cyclization can also be spontaneous during crystallization from alco-
hols in the presence of small amount of water1,14. The mechanism of base-catalyzed
cyclization of 1-(2-ethoxycarbonylphenyl)-3-aryltriazenes3 and 1-(2-aryloxycarbonyl-
phenyl)-3-phenyltriazenes5 was dealt with in our previous communications. The reac-
tion is likely to go by a BAc2 type mechanism with formation of the tetrahedral
intermediate as the rate-limiting step (Scheme 2). One of the symptoms supporting this
O
C
O
C
RO
RO
N H
N
N
N
N
base, - H
fast
N
O
C
O
RO
C
N
N
N
N
-
RO
N
N
SCHEME 2
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Steric Effects
753
conclusion is the high sensitivity of this reaction to steric effects of substituents at
3-position of the cyclized aryl esters.
The term “steric effects” summarily describes nonbonding repulsion interactions be-
tween the reaction centre and substituent due to overlap of occupied molecular orbi-
tals15–20. The term “kinetic steric effect” is most often understood as a hindrance to the
reagent approach toward the reaction centre19. Steric effects usually result in an in-
crease in the Gibbs activation energy and, hence, retardation of the reaction. On the
other hand, in some reactions the steric requirements in educts are greater than those in
the activated complex, which leads to the so-called steric acceleration of reaction. For
the first time, the steric effect was quantitatively described by Taft15,21 using the ES
constant. Correctness on the ES constant has often been discussed, first of all from the
point of view of involvement of other effects22–25. These drawbacks are avoided by the
steric parameters υ derived by Charton20,26 using the van der Waals radii of atoms. The
steric parameters ΩS were also derived from geometrical arrangement of atoms in the
substituents27. However, differences between the ES and ΩS parameters in their applica-
tions to selected reactions were not much significant27. Another, in principle different
description of steric effect was derived from thermodynamic measurements and called S : it
f
is based on the difference between standard enthalpies of formation of the substituents
bound to tert-butyl and methyl residues28. From the point of view of mechanistic
studies, the quality differences between the parameters given are negligible; it must
only be remembered that the ES constants decrease with increasing steric requirements
of substituent, hence the respective reaction constants have opposite signs to those in
other scales. The correlation relations used for interpretation of experimental kinetic
data evaluate the inductive and/or mesomeric effects along with steric effects.
The aim of the present communication is to evaluate the steric effects in base-cata-
lyzed cyclization of compound 1 and therefrom to decide whether the reaction involves
the formation or the decomposition of the tetrahedral intermediate as the rate-limiting
step (Scheme 2).
EXPERIMENTAL
Syntheses of Alkyl Anthranilates by Reactions of Isatoic Anhydride with Alcohols (Method A)
Sodium (0.5 g, 22 mmol) was dissolved in the chosen alcohol (100 ml) and isatoic anhydride (16.3 g,
100 mmol) was added thereto. The reaction mixture was refluxed until the evolution of carbon dioxide
(foaming) ceased. Then the excess alcohol was distilled off, and the raw alkyl anthranilate was extracted
with benzene (25 ml). The benzene extract was shaken with the same volume of 5% aqueous potas-
sium carbonate and then with the same volume of water, whereafter it was dried with anhydrous
sodium sulfate. Benzene was evaporated in a vacuum evaporator, and the raw ester was refined by
vacuum distillation. The yields and refractive indexes are presented in Table I.
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

754
Pytela, Halama:
Syntheses of Alkyl Anthranilates from Alkyl 2-Nitrobenzoates (Method B)
The respective ester of 2-nitrobenzoic acid (100 mmol) was dissolved in the corresponding alcohol
(in the ratio of ca 1 : 10 v/v) with addition of platinum(IV) oxide (0.2 g). Hydrogen (ca 1 mol) was
introduced under mild pressure into the reaction mixture, and after the reduction was finished the
catalyst and solvent were removed by filtration and distillation, respectively. The raw anthranilate
was purified by vacuum distillation. The yields and refractive indexes are presented in Table I.
Syntheses of 1-(2-Alkoxycarbonylphenyl)-3-phenyltriazenes
In a beaker, alkyl anthranilate (25 mmol) was mixed with hydrochloric acid (20 ml) diluted with
water in the ratio of 1 : 1, the mixture was cooled to 5 °C, and sodium nitrite solution (approximately
5 g NaNO2 in 5 ml water) was added thereinto drop by drop with continuous stirring and keeping the
temperature below 5 °C. The end of diazotization was indicated with the help of potassium iodide-
starch test paper. With intense stirring, the diazonium salt solution prepared was added in portions to
a mixture of aniline (2 g, 22 mmol), ammonium acetate (30 g), and water (40 ml). The separated
solid was collected by suction, washed with water, and dried in air. The raw product was purified by
column chromatography (alumina, benzene) and recrystallized from hexane. The yields of 1-(2-alkoxy-
carbonylphenyl)-3-phenyltriazenes (1) and their melting points are presented in Table II.
Identification of 1-(2-Alkoxycarbonylphenyl)-3-phenyltriazenes
For identification and characterization of 1-(2-alkoxycarbonylphenyl)-3-phenyltriazenes (1) we
measured their UV-VIS, IR, 1H, and 13C NMR spectra. The UV-VIS spectra were measured in methanol
using an HP 8452A (Hewlett–Packard, Diode Array Spectrophotometer). The IR spectra were
1
measured in Nujol suspension using a Specord 75 IR (Zeiss, Jena) apparatus. The H NMR measure-
ments were carried out on an AMX-360 (Bruker) apparatus using a frequency of 360.14 MHz at the
temperature of 298 K; the samples were dissolved in CDCl3 (5% solutions). The chemical shifts are
referenced to the signal of HMDSO standard (δ = 0.05 ppm). The 13C NMR spectra were measured
at the frequency of 90.57 MHz at 298 K in 5% solutions in CDCl3. The chemical shifts are referenced
TABLE I
Methods of preparation, reaction yields, and refractive indexes of the alkyl anthranilates
2-NH2C6H4COOR synthesized
n2D0
(ref.)
n2D0
(ref.)
R
Method Yield, %
R
Method Yield, %
Methyl
Ethyl
A
A
A
B
66.2
50.2
43.8
75.9
1.5830
Butyl
A
B
B
A
40.3
86.3
50.8
45.7
1.5468
(1.582a)
(1.545b)
1.5640
Isobutyl
Hexyl
Allyl
1.5423
(1.564b)
(1.540b)
Propyl
Isopropyl
1.5562
1.5381
–
(1.551b)
1.5478
–
1.5709
–
a Ref.29; b ref.30.
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

Steric Effects
755
to the solvent signal (δ = 77.0 ppm). The CDCl3 used was rid of traces of hydrochloric acid to pre-
vent destruction of the compound measured: deuteriochloroform was shaken with the same volume of
5% aqueous sodium carbonate and then with the same volume of water, dried with anhydrous sodium
sulfate, filtered, and distilled.
Identification of Product of Base-Catalyzed Cyclization of 1-(2-Alkoxycarbonylphenyl)-3-phenyltriazenes
In order to identify the product of base-catalyzed cyclization of the model triazenes we isolated a
small amount of product from the reaction of 1-(2-alkoxycarbonylphenyl)-3-phenyltriazene in
aqueous–methanolic buffer of pH 11.94. The melting point of the product isolated was 149–151 °C
in accordance with that of the expected 3-phenyl-3H,4H-benzo[e]-1,2,3-triazin-4-one (ref.31 gives
m.p. 151–152 °C). Similarly, the positions of UV absorption maxima λ = 291 nm and λ = 315 nm
corresponded to the values published32 λ = 290 nm and λ = 314 nm.
Kinetic Measurements
The reaction media used for the kinetic measurements were aqueous methanolic buffers containing
52.1% (w/w) methanol. The buffers in the range from pH 2.65 to pH 5.27 were obtained by mixing
(at various volume ratios) the solutions of citric acid (0.01–0.1 M) and 0.01 M potassium carbonate.
The buffers in the range from pH 7.53 to pH 9.79 were prepared by mixing various volumes of satu-
rated solutions of sodium hydrogenphosphate and potassium dihydrogenphosphate. The buffers with
pH above 9.79 were prepared by mixing various volumes of saturated sodium hydrogenphosphate
and 0.2 M sodium hydroxide. The ionic strength of the buffers was not adjusted. Their pH values
were measured by means of a glass electrode using an MV 870 Digital (VEB-Präcitronic) pH meter.
Before the measurement itself, about 2 ml of buffer in a cell was placed into the thermostated cell
holder of the apparatus (25 ± 0.1 °C), and after reaching the said temperature, 1–10 µl dioxane solution
of triazene 1 of suitable concentration was added and the mixture was thoroughly mixed. The trans-
mittance–time dependence was recorded for at least four half-lives. The observed rate constants were
evaluated by a known procedure33 and are presented in Table III.
TABLE II
Yields of syntheses, melting points, and spectral characteristics in UV-VIS and IR spectra of 1-(2-
alkoxycarbonylphenyl)-3-phenyltriazenes 1
Yield,
%
ε1 . 10–4
ε2 . 10–4
ν(NH) ν(CO)
R
M.p., °C λ1, nm
λ2, nm
l mol–1 cm–1
l mol–1 cm–1
cm–1
cm–1
Methyl
Ethyl
32
35
28
33
26
32
24
52
64–66
70–72
76–77
77–79
64–66
57–60
53–56
65–68
224
220
220
220
220
220
220
222
2.683
2.220
1.647
1.492
2.023
2.013
2.365
2.504
358
358
358
358
358
358
358
360
2.416
2.512
2.485
2.429
2.553
2.579
2.397
2.557
3 250
3 230
3 217
3 222
3 227
3 240
3 222
3 237
1 678
1 677
1 693
1 690
1 695
1 692
1 677
1 677
Propyl
Isopropyl
Butyl
Isobutyl
Hexyl
Allyl
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Pytela, Halama:
TABLE III
pH dependence of observed rate constant kobs (s–1) of reactions of 1-(2-alkoxycarbonylphenyl)-3-
phenyltriazenes 1
pH
kobs
pH
kobs
pH
kobs
pH
kobs
R = Methyl
2.65
2.84
3.15
3.31
3.52
3.123 . 10–2
1.994 . 10–2
9.256 . 10–3
5.760 . 10–3
3.756 . 10–3
3.90
4.32
4.73
5.27
7.53
1.968 . 10–3
8.222 . 10–4
3.293 . 10–4
9.455 . 10–5
1.532 . 10–4
7.96
8.08
8.27
8.50
8.83
3.185 . 10–4
4.609 . 10–4
7.812 . 10–4
1.278 . 10–3
3.232 . 10–3
9.06
9.20
9.47
9.79
4.624 . 10–3
6.445 . 10–2
1.021 . 10–2
1.826 . 10–2
R = Ethyl
2.65
2.84
3.15
3.31
3.52
2.576 . 10–2
1.843 . 10–2
9.978 . 10–3
6.668 . 10–3
4.132 . 10–3
3.90
4.32
4.73
5.27
8.27
1.821 . 10–3
7.698 . 10–4
3.192 . 10–4
1.074 . 10–4
1.158 . 10–4
8.50
8.68
8.83
9.06
9.20
2.017 . 10–4
3.047 . 10–4
4.065 . 10–4
7.001 . 10–4
9.319 . 10–4
9.47
9.79
1.440 . 10–3
3.865 . 10–3
4.020 . 10–2
7.051 . 10–2
10.93
11.29
R = Propyl
2.65
2.84
3.15
3.31
3.52
2.620 . 10–2
2.074 . 10–2
9.593 . 10–3
6.429 . 10–3
3.790 . 10–3
3.90
4.32
4.73
5.27
8.68
1.863 . 10–3
9.219 . 10–4
3.656 . 10–4
1.014 . 10–4
1.841 . 10–4
8.83
9.06
9.20
9.47
9.79
2.728 . 10–4
4.819 . 10–4
5.976 . 10–4
8.948 . 10–4
2.395 . 10–3
10.26
10.48
10.93
11.41
8.570 . 10–3
6.717 . 10–3
2.618 . 10–2
5.556 . 10–2
R = Isopropyl
2.65
2.84
3.15
3.31
3.52
2.692 . 10–2
1.814 . 10–2
9.821 . 10–3
6.602 . 10–3
3.934 . 10–3
3.90
4.32
4.73
5.27
9.47
1.839 . 10–3
9.79
10.26
10.48
10.72
10.93
2.555 . 10–4
7.761 . 10–4
1.197 . 10–3
1.994 . 10–3
4.353 . 10–3
11.29
11.42
11.84
11.94
9.077 . 10–3
1.467 . 10–2
3.155 . 10–2
4.471 . 10–2
7.219 . 10–4
3.086 . 10–4
1.320 . 10–4
1.101 . 10–4
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Steric Effects
757
TABLE III
(Continued)
pH
kobs
pH
kobs
pH
kobs
pH
kobs
R = Butyl
2.65
2.84
3.15
3.31
3.52
2.577 . 10–2
1.863 . 10–2
8.264 . 10–3
5.087 . 10–3
3.467 . 10–3
3.90
4.32
4.73
5.27
8.83
1.552 . 10–3
7.301 . 10–4
3.051 . 10–4
7.110 . 10–5
2.291 . 10–4
9.06
9.20
3.707 . 10–4
4.557 . 10–4
8.923 . 10–4
1.957 . 10–3
4.973 . 10–3
10.48
10.72
10.93
11.29
8.474 . 10–3
1.521 . 10–2
3.091 . 10–2
7.092 . 10–2
9.47
9.79
10.26
R = Isobutyl
2.65
2.84
3.15
3.31
3.52
2.806 . 10–2
2.021 . 10–2
8.878 . 10–3
5.852 . 10–3
3.455 . 10–3
3.90
4.32
4.73
5.27
9.20
1.604 . 10–3
9.43
9.79
3.814 . 10–4
8.457 . 10–4
2.203 . 10–3
3.613 . 10–3
6.143 . 10–3
10.93
11.26
11.41
1.329 . 10–2
2.885 . 10–2
4.770 . 10–2
9.044 . 10–4
3.208 . 10–4
1.039 . 10–4
1.725 . 10–4
10.26
10.48
10.72
R = Hexyl
2.65
2.84
3.15
3.31
3.52
2.752 . 10–2
1.972 . 10–2
8.644 . 10–3
6.221 . 10–3
3.412 . 10–3
3.90
4.32
4.73
5.27
8.83
1.431 . 10–3
7.821 . 10–4
3.143 . 10–4
1.060 . 10–4
1.971 . 10–4
9.06
9.20
2.924 . 10–4
3.092 . 10–4
8.094 . 10–4
1.771 . 10–3
4.277 . 10–3
10.48
10.93
11.29
5.227 . 10–3
2.325 . 10–2
4.460 . 10–2
9.47
9.79
10.26
R = Allyl
2.65
2.84
3.15
3.31
3.52
2.562 . 10–2
2.093 . 10–2
9.185 . 10–3
6.362 . 10–3
3.315 . 10–3
3.90
4.32
4.73
5.27
7.96
1.570 . 10–3
6.825 . 10–4
3.011 . 10–4
1.064 . 10–4
3.251 . 10–4
8.08
8.27
8.50
8.68
8.83
4.980 . 10–4
7.725 . 10–4
1.132 . 10–3
2.043 . 10–3
3.083 . 10–3
9.06
9.20
4.590 . 10–3
4.978 . 10–3
1.102 . 10–2
2.680 . 10–2
5.902 . 10–2
9.47
9.79
10.26
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Pytela, Halama:
RESULTS AND DISCUSSION
Spectra of 1-(2-Alkoxycarbonylphenyl)-3-phenyltriazenes
The UV-VIS spectra of the triazenes studied were measured in methanol; they show
two absorption maxima: one in the region of 220 nm and the other (which is charac-
teristic of triazenes) in the region of 360 nm. The values of individual derivatives are
presented in Table II. The IR spectra are characterized by valence vibrations ν(N–H) in
the region from 3 217 cm–1 to 3 250 cm–1 and ν(C=O) in the region from 1 677 cm–1 to
1 695 cm–1 (see Table II).
1
The H and 13C NMR spectra (Tables IV and V) are of primary importance for prov-
ing and characterizing the 1-(2-alkoxycarbonylphenyl)-3-phenyltriazenes 1. For an un-
ambiguous assignment of 1H and 13C signals in the NMR spectra of the
1-(2-ethoxycarbonylphenyl)-3-phenyltriazene we used the H,H-COSY, H,C-COSY
technique and two INEPT 13C experiments with selective transfer of polarization from
H-2′ and H-5′ protons. For the remaining derivatives the assignment was carried out on
the basis of analogy between the individual spectra. From the NMR spectra it is ob-
vious that the variation of alkyl groups does not affect the values of chemical shifts of
signals of protons and carbon atoms in the aromatic region and those of carbonyl carb-
on atom signals.
RO
7’
C O
6’
5’
7
H
4’
N
1’
N N
2’
3’
1
2
6
3
5
4
1b
However, it is possible to make an important conclusion with respect to the possible
tautomeric forms of the model triazenes. For this purpose, it is possible to adopt the
known spectral data (13C NMR) on the tautomers of 3-methyl-1-(4-nitrophenyl)triazene
(2a, 2b, ref. 34) and on the triazenes whose tautomerism is prevented by substituents (3, 4,
refs34,35). The tautomerism of triazenes mostly affects the chemical shifts of carbons
separated by the least number of bonds from the –N=N–NHR or –NH–N=NR group
(R = alkyl or aryl). These particularly include the carbons C-1, C-2, and C-1′ and C-2′.
The following chemical shifts are given34 for the tautomer 2a: δ(C-1) = 148.81, δ(C-2) =
120.48, and for the tautomer 2b: δ(C-1′) = 140.74, δ(C-2′) = 112.86. For the triazenes
whose tautomerism is prevented by substituents at N-3 atom, e.g., the following 13C NMR
spectra are known: 3,3-dimethyl-1-(4-nitrophenyl)triazene 3 (δ(C-1) = 155.84, δ(C-2) =
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

Steric Effects
759
120.34, ref.35) and 1-phenyl-3-methylphenyltriazene 4 (δ(C-1) = 150.2, δ(C-2) = 121.2,
δ(C-1′) = 145.0, δ(C-2′) = 116.8, ref.34). From the δ(C-1) and δ(C-2) values of com-
pounds 2a, 3, 4 it follows that the chemical shifts of carbon atoms close to –N=N–
NR1R group (R = H or alkyl, R1 = alkyl or aryl) are found approximately in the
intervals of 148–156 ppm and 120–121 ppm for C-1 and C-2, respectively. On the other
hand, in the cases of 2b and 4 the values of δ(C-1′) and δ(C-2′) of the carbon atoms
close to –NR–N=NR1 group (R = H or alkyl, R1 = alkyl or aryl) are approximately in
the regions of 141–145 ppm and 113–117 ppm for C-1′ and C-2′, respectively. From
the spectral data obtained by us it follows that 1-(2-alkoxycarbonylphenyl)-3-phenyl-
triazenes 1 (δ(C-1) = 149.5–149.7, δ(C-2) = 121.6–121.7, δ(C-1′) = 143.9–144.1, δ(C-2′) =
114.4–114.5) exist in the tautomeric form 1b under the conditions of measurements.
This statement is directly supported also by the found high values of δ(NH) (12.36–12.46)
1
in the H NMR spectra which indicate the participation of H-7 hydrogen in intramole-
cular hydrogen bond with the free electron pair at the carbonyl oxygen atom resulting
in formation of a six-membered cycle (1b).
3
2
2’
3’
4
1
4’
1’
O2N
O2N
N NNHCH3 O2N
NHN NCH3
2a
2b
3
2
3
2
2’
3’
4
1
1
1’
4
4’
N NN(CH3)2
N N N
CH3
3
4
Verification of Reaction Molecularity and Evaluation of Catalytic Rate Constants
The reaction order of acid-catalyzed decomposition and base-catalyzed cyclization of
the triazenes 1 studied were evaluated, with maintaining the reaction conditions, by the
method of two initial concentrations in the case of the methyl derivative (pH 3.90),
ethyl derivative (pH 4.73), hexyl derivative (pH 9.20), and allyl derivative (pH 9.06).
The corresponding n values are 1.033, 1.033, 1.045, and 0.961, hence the reactions are
of the first order in the substrate.
The pH dependence of observed rate constants (Table III), i.e. the pH profile, is
characterized by two line segments corresponding to the acid and base catalyses (nega-
tive and positive slopes, respectively). The catalytic rate constants of the two processes
were calculated as the average value of sum of (kA, acid catalysis) or difference between
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

760
Pytela, Halama:
(kB, base catalysis) the values of the observed rate constants kobs and pH: the respective
results are given in Table VI.
Interpretation of Steric Effects
The catalytic rate constants kA of acid-catalyzed decomposition of 1-(2-alkoxycarbonyl-
phenyl)-3-phenyltriazenes 1 given in Table VI gave no statistically significant correla-
tion with any combination of substituent constant σ* and some other of the constants
describing the steric effects in the same Table. This conclusion is not unexpected since
the change of alkyl group practically changes neither the electron density at the ni-
trogen atoms of the triazene chain nor steric situation in its environment.
On the other hand, correlations of the catalytic constants kB with a combination of the
substituent constant σ* and steric constants from Table VI were statistically significant
as it can be seen from Eqs (1) through (4).
TABLE IV
Values of 1H and 13C NMR shifts (δ, ppm) of protons and carbon atoms of the basic skeleton of
1-(2-alkoxycarbonylphenyl)-3-phenyltriazenes 1b
Atom
Methyl
Ethyl
Propyl Isopropyl
Butyl
Isobutyl
Hexyl
Allyl
H-2/H-6
H-3/H-5
H-4
7.60
7.40
7.59
7.59
7.38
7.59
7.39
7.59
7.59
7.38
7.60
7.40
7.59
7.39
7.26
7.37
7.25
7.40
7.25
7.28
7.26
7.26
7.26
7.27
H-7
12.36
7.91
12.41
7.89
12.41
7.89
12.46
7.90
12.40
7.88
12.40
7.89
12.41
7.91
12.36
7.89
H-2′
H-3′
7.50
7.48
7.47
7.47
7.46
7.47
7.50
7.47
H-4′
6.99
6.98
6.97
6.98
6.95
6.97
7.00
6.96
H-5′
7.99
7.99
7.99
7.98
7.96
7.99
8.00
8.00
C-1
149.5
121.6
128.9
127.7
143.9
114.4
134.4
120.9
131.0
111.8
167.7
149.6
121.7
128.9
127.8
144.0
114.4
134.3
121.0
131.1
112.1
167.3
149.6
121.7
129.0
127.8
144.0
114.4
134.3
121.0
131.0
112.1
167.3
149.7
121.7
129.0
127.8
144.0
114.4
134.2
120.9
131.0
112.5
166.8
149.6
121.6
128.9
127.7
143.9
114.4
134.3
120.9
131.0
112.1
167.3
149.6
121.6
129.0
127.8
144.0
114.4
134.3
121.0
131.1
112.1
167.2
149.7
121.7
129.0
127.8
144.0
114.5
134.4
121.0
131.1
112.3
167.4
149.6
121.7
129.0
127.8
144.1
114.4
134.5
121.0
131.0
112.7
166.9
C-2/C-6
C-3/C-5
C-4
C-1′
C-2′
C-3′
C-4′
C-5′
C-6′
C-7′
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

Steric Effects
761
log kB = –(11.33 ± 0.07) + (7.11 ± 0.89)σ* + (0.545 ± 0.131)ESc
n = 7, s = 0.079, R = 0.9941
log kB = –(10.83 ± 0.20) + (8.52 ± 0.88)σ* – (0.995 ± 0.339)υ
n = 7, s = 0.103, R = 0.9900
log kB = –(7.56 ± 0.43) – (18.4 ± 1.6)ΩS
n = 7, s = 0.123, R = 0.9821
log kB = –(11.41 ± 0.05) + (6.33 ± 0.70)σ* – (0.337 ± 0.054)S
(1)
(2)
(3)
(4)
f
n = 7, s = 0.056, R = 0.9971
TABLE V
Values of 1H and 13C NMR chemical shifts (δ, ppm) of protons and carbon atoms in alkyl chain of
1-(2-alkoxycarbonylphenyl)-3-phenyltriazenes 1
Alkyl
Nuclide CH3
CH2
CH2
CH2
CH2
CH2
CH
CH=
CH2=
Methyl
Ethyl
1H
13C
1H
13C
1H
13C
1H
13C
1H
13C
1H
13C
1H
13C
3.92
52.1
1.38
14.2
1.01
10.5
1.37
21.9
0.95
13.7
1.00
19.7
0.90
14.0
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
4.36
–
–
–
–
–
61.2
1.77
22.0
–
–
–
–
–
Propyl
Isopropyl
Butyl
4.26
–
–
–
–
66.7
–
–
–
–
–
–
–
–
–
–
–
5.24
–
–
–
68.8
1.45
1.72
4.29
–
–
–
–
19.2
4.08
71.1
1.33
22.6
30.6
64.9
–
–
Isobutyl
Hexyl
–
–
–
–
–
–
2.07
–
–
27.8
1.33
1.44
1.76
4.31
–
–
25.7
28.6
31.5
65.4
5.24
5.39
Allyl
1H
–
–
4.79
65.6
–
–
–
–
–
–
–
–
–
–
6.00
13C
131.9
118.6
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

762
Pytela, Halama:
From the equations it is obvious that except for Eq. (3) the correlations are very
good, both electronic and steric effects being interpreted. The multicollinearity between
the interpreting variables is small (Eq. (2)) to medium (Eqs (1) and (4)), hence the
reaction constants (regression coefficients) obtained can be considered valid. The nega-
tive sign of reaction constant ρ* means that the cyclization is accelerated by electron
acceptor substituents, the effect of substituents on dissociation of N–H bond (Scheme 1)
being negligible in similar way as that in the acid-catalyzed decomposition. Therefrom
it unequivocally follows that the attack of carbon atom of ester carbonyl group by the
nitrogen atom N3 is rate limiting. The sensitivity of this elementary reaction expressed
by the reaction constant is roughly twice as large as that in the aminolysis of alkyl
acetates38 and as much as three times as large as that of the base-catalyzed hydrolysis
of methyl carboxylates36. Therefrom it can be deduced that the negative charge is little
localized at the nitrogen atom N3 and this nitrogen atom is a weak nucleophile only.
The sign at the rate constants describing the sensitivity to steric effect (in all the rela-
tions (1) through (4)) leads to an unequivocal conclusion that increased steric demands
of substituents at the reaction centre retard the reaction. This fully agrees with the
above-mentioned conclusions concerning the formation of tetrahedral intermediate as
the rate-limiting step. The magnitude of steric reaction constants indicates a small to
medium sensitivity to steric effect17,26,27,28,39. This conclusion is somewhat different
from those obtained earlier for the cyclizations of 1-(2-aryloxycarbonylphenyl)-3-phenyl-
triazenes5 where the sensitivity to steric effect was very high due obviously to the bul-
kiness of aromatic nucleus.
TABLE VI
Catalytic rate constants log kA for acid-catalyzed decomposition and log kB for base-catalyzed cycli-
zation of 1-(2-alkoxycarbonylphenyl)-3-phenyltriazenes 1 in 52.1% (w/w) aqueous methanol at 25 °C
and the corresponding constants σ* (refs36,37), ESc (ref.25), υ (ref.20), ΩS (ref.27), and S (ref.28) used
f
for their interpretation
R
log kA
log kB
σ*
ESc
υ
ΩS
S
f
Methyl
Ethyl
1.163
1.165
1.178
1.169
1.103
1.153
1.140
1.138
–11.41
–12.25
–12.47
–13.35
–12.51
–12.87
–12.62
–11.41
0.000
–0.100
–0.115
–0.190
–0.130
–0.125
–0.140b
–0.180
–0.059
–0.353
–0.626
–1.024
–0.624
–1.236
–0.628
0.52
0.56
0.68
0.76
0.68
0.98
0.68
0.206
0.256
0.273
0.304
0.272
0.295
0.272
0.00
0.86
0.89
2.29
0.86
1.86
0.85
Propyl
Isopropyl
Butyl
Isobutyl
Hexyla
Allyl
a The values for pentyl were used; b assessed value.
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

Steric Effects
763
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