The versatile reactivity of 2-aryl-4,6-dinitrobenzotriazole 1-oxides in diels-alder type condensations and in σ-complexation - A relationship between superelectrophilicity and pericyclic reactivity

Article abstract of DOI:10.1139/v01-020

A two-pronged study of a series of 2-aryl-4,6-dinitrobenzotriazole 1-oxide substrates (2a-c) is reported: Diels-Alder type pericyclic reactivity and covalent hydration to yield Meisenheimer type hydroxy anionic σ-adducts. The most activated benzotriazole

Full text of DOI:10.1139/v01-020

1617
The versatile reactivity of 2-aryl-4,6-
dinitrobenzotriazole 1-oxides in Diels–Alder type
condensations and in ꢀ-complexation — A
relationship between superelectrophilicity and
pericyclic reactivity
Dominique Vichard, Taoufik Boubaker, François Terrier, Marie-José Pouet, Julian
M. Dust, and Erwin Buncel
Abstract: A two-pronged study of a series of 2-aryl-4,6-dinitrobenzotriazole 1-oxide substrates (2a–c) is reported:
Diels–Alder type pericyclic reactivity and covalent hydration to yield Meisenheimer type hydroxy anionic ꢀ-adducts.
The most activated benzotriazole 1-oxide 2a is found to exhibit both dienophilic and heterodienic behaviour on
treatment with cyclopentadiene, providing a highly functionalized stereoselective diadduct (7). This diadduct is shown
to be the result of two consecutive inverse demand Diels–Alder condensations proceeding through the endo mode with
a trans addition of two cyclopentadiene molecules. Kinetic and thermodynamic measurements of the ease of covalent
hydration of 2a–c to give ꢀ-adducts 4a–c in aqueous solution indicate that the electrophilic reactivity of the 4,6-
dinitrobenzotriazole 1-oxide structure is much closer to that of the superelectrophilic 4,6-dinitrobenzofuroxan (DNBF,
1a) than that of the standard Meisenheimer electrophile 1,3,5-trinitrobenzene (TNB). The pKa values range from 6.70
for the 2-(2>,4>,6>-trinitrophenyl)-4,6-dinitrobenzotriazole 1-oxide (Pi-DNBT, 2a) to 10.73 for the 2-phenyl-4,6-
dinitrobenzotriazole 1-oxide (Ph-DNBT, 2c). The observation that the most activated benzotriazole 1-oxide structure 2a
which resembles 4,6-dinitrobenzofuroxan (DNBF) in superelectrophilic behaviour, is the only one that undergoes Diels–
Alder reactions, points to a direct relationship between superelectrophilic and pericyclic reactivity.
Key words: ꢀ-complexes, arylnitrobenzotriazole 1-oxides, super-electrophiles, Diels–Alder condensations, pericyclic
reactions.
Résumé : O1n6a23réalisé une étude d’une série de substrats 1-oxyde de 2-aryl-4,6-dinitrobenzotriazole (2a–c) et elle a été
menée sur deux fronts: la réactivité péricyclique de type Diels–Alder et l’hydratation covalente conduisant à des
adduits ꢀ, des hydroxy anioniques de type Meisenheimer. On a observé que le traitement de l’oxyde de benzotriazole
le plus activé (2a) avec du cyclopentadiène donne lieu à des comportements à la fois diénophile et hétérodiénique et
qu’il permet d’obtenir un diadduit stéréosélectif hautement fonctionnalisé (7). On a démontré que ce diadduit résulte de
deux condensations consécutives de Diels–Alder ayant des demandes inverses et qu’elles se produisent par le mode
endo avec une addition trans de deux molécules de cyclopentadiène. Des mesures cinétiques et thermodynamiques de
la facilité d’hydratation covalente des composés 2a–c qui conduit à la formation des adduits ꢀ correspondants (4a–c)
indiquent que, en solution aqueuse, la réactivité électrophile de la structure 1-oxyde de 4,6-dinitrobenzotriazole est
beaucoup plus proche de celle du 4,6-dinitrobenzofuroxane (DNBF, 1a) superélectrophile que de celle du 1,3,5-
trinitrobenzène (TNB), l’électrophile de Meisenheimer de référence. Les valeurs de pKa vont de 6,70 pour le 1-oxyde
de 2-(2>4>6>-trinitrophényl)-4,6-dinitribenzotriazole (Pi-DNBT, 2a) à 10,73 pour le 1-oxyde de 2-phényl-4,6-
dinitrobenzotriazole (P-DNBT, 2c). On a observé que la seule structure à subir des réactions de Diels–Alder est la
structure la structure la plus activée du 1-oxyde de benzotriazole (2a), qui ressemble au 4,6-dinitrobenzofuroxnae
Received August 29, 2000. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on October 31, 2001.
Dedicated to Professor Victor Snieckus in recognition of his contributions to Canadian chemistry.
D. Vichard, F. Terrier, and M.-J. Pouet. Institute Lavoiser-Franklin, UPRESA 8086, Université de Versailles, 45 Avenue des
Etats-Unis, 78035 Versailles CEDEX, France.
T. Boubaker. Unité de Recherche de Physico-Chimie Moléculaire, Faculté des Sciences de Monastir, Avenue de l’Environnement
5000, Monatir, Tunisie.
J.M. Dust. Departments of Chemistry and Environmental Science, Sir Wilfred Grenfell College, Corner Brook, NF, A2H 6P9, Canada.
E. Buncel.1 Department of Chemistry, Queen’s University, Kingston, ON K7L 3N6, Canada.
1Corresponding authors (EB) (telephone: (613) 533-2653; fax: (613) 533-6669; e-mail: buncele@chem.queensu.ca); (JD) (telephone:
(709) 637-6330; fax: (709) 639-8125; e-mail: jdust@swgc.mum.ca; (FT) (telephone: 33-1-39-25-44-50; fax: 33-1-39-25-44-52;
e-mail: terrier@chimie.uvsq.fr.).
Can. J. Chem. 79: 1617–1623 (2001)
© 2001 NRC Canada
DOI: 10.1139/cjc-79-11-1617

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Can. J. Chem. Vol. 79, 2001
(DNBF) en comportement supe-électrophile; cette observation suggère qu’il existe une relation directe entre un
comportement superélectrophile et la réactivité péricyclique.
Mots clés : complexes-ꢀ, 1-oxydes d’arylnitrobenzotriazole, superélectrophiles, condensations de Diels–Alder, réactions
péricycliques.
[Traduit par la Rédaction]
Introduction
Vichard et al.
Diels–Alder reactivity towards cyclopentadiene (Cp). This
discovery opens the way to the synthesis of highly
functionalized benzotriazole derivatives with defined stereo-
chemistry. Importantly, the results point to a direct relation-
ship between superelectrophilic and pericyclic behaviour.
So far no firm assessment of the electrophilic reactivity of
2a or related benziotriazole systems, in terms of meaningful
thermodynamic and kinetic data, has been reported. There-
fore, we have carried out a kinetic and thermodynamic study
of the reactions of hydroxide ion in water with the various
substrates 2a–c yielding the hydroxide adducts 4a–c accord-
ing to eq. [1]. Our results reveal that the reactivity of
compounds 2a–c is greater than that of the conventional
Meisenheimer electrophile 1,3,5-trinitrobenzene (TNB) (9),
but only that of 2a is found to approach that of 1a, provid-
ing further evidence of the superelectrophilic behaviour of
structure 2.
Following the discovery of the extremely strong electrophilic
character of the carbocyclic ring of 4,6-dinitrobenzofuroxan
(DNBF, 1a) (1–9), we have been engaged in an effort to
design analogues of 1a in which the oxygen atom in the
furoxan ring is replaced by other electron-withdrawing moi-
eties (10, 11). We thus found that 2-(2>,4>,6>-trinitrophenyl)-
4,6-dinitrobenzotriazole 1-oxide (Pi-DNBT, 2a) is also a
powerful electrophilic heterocycle that undergoes facile ꢀ-
complexation at C-7 of the carbocyclic ring with a number
of nucleophiles, including weak neutral nucleophiles like
water, methanol, or ethanethiol (10a). A contrasting feature
of 2a is that the C-1> carbon of the picryl ring is also suscep-
tible to reaction with nucleophiles, giving rise to an SNAr
displacement reaction via the metastable ꢀ-adduct 3a (10,
11). The ambident (C-7 vs. C-1>) electrophilicity of 2a (and
other members of benzotriazole 1-oxides series, e.g., 2b) has
given new impetus to the study of reactivity–selectivity rela-
tionships in reactions of this and related ambident
electrophiles with ambident nucleophiles (9b, 11).
[1]
Results
Reaction of Pi-DNBT (2a) with cyclopentadiene
Treatment of 2a with excess cyclopentadiene in a
CH2Cl2–THF mixture at room temperature results in nearly
quantitative formation of a product (5 h), which was readily
1
isolated as a pale yellow solid after addition of pentane. H
and 13C NMR spectra of this solid (CD3CN solvent) con-
sisted of only one set of signals, indicating that the reaction
has proceeded with a high stereoselectivity, affording only
the diastereomer 7 (only one enantiomer is shown).
In probing the manifold reactivity of electron-deficient
heteroaromatics like 1a, we have discovered that nitrobenzo-
furoxans are also remarkably versatile reagents in pericyclic
reactions such as the Diels–Alder, being capable of acting as
dienophiles, carbodienes, or heterodienes depending upon
the experimental conditions and reaction partners employed
(12). To test whether this diverse pericyclic reactivity is a
reflection of the low aromaticity of the carbocyclic ring of
such compounds, studies have been undertaken of the 2-aryl-
4,6-dinitrobenzotriazole 1-oxides (2a–c) in which the
electron-withdrawing power of the 2-aryl moiety is system-
atically decreased through the sequential removal of nitro
groups. In this paper, we report that 2a exhibits ambident
The proposed stereochemistry for 7 has been derived from a
detailed NMR analysis which has included COSY, HETCOR, as
well as J-modulation and NOE experiments. In particular, 2D-
NOE experiments have revealed that the H-7, H-10, H-11, and
© 2001 NRC Canada

Vichard et al.
1619
H-14 protons (but not the H-14> proton) are located on the
same side of the dihydrooxazine N-oxide and cyclopentene
[3]
[4]
kobsd = k1OH[OH–]
k–1 = k1OHKe[Ka]–1
3
rings of 7. The values of the related J coupling constants
3
(3JH7-H10 = 5.6, JH10-H11 = 8.9 Hz) agree with the cis
arrangement of the H-7, H-10, and H-11 protons (12a). On the
Values of the various rate and equilibrium parameters for
the formation and decomposition of 4a–c together with the
related parameters for the DNBF adduct 5a, the 4-nitro-
benzofuroxan (NBF, 1b) adduct 5b, and the TNB adduct 6
are given in Table 3 for purposes of comparison.
4
other hand, the observation of a W-type J coupling constant
between the H-5 and H-7 protons (4JH5-H7 = 2.8 Hz) implies a
trans addition of the two Cp molecules (13, 14). In accord with
the strong electron-withdrawing effect of the O-N+-O– frag-
ment of the dihydrooxazine ring, both the sp3 carbon C-11 and
the related H-11 proton of 7 are strongly deshielded (d C11
=
93.02, d H11 = 5.77 ppm in CD3CN) with the C-11 resonance
comparing well with that of the nitro-substituted quaternary
carbon C-4 (d C4 = 93.17 ppm) (15). The close analogy be-
tween the resonances of the protons and carbons of the three
carbocyclic rings of 7 with those of the related DNBF adduct 8
further supports our stereochemical assignment (Tables 1 and
2) (12c). Note that the stereochemistry illustrated in structure 8
was determined independently by X-ray diffraction studies. That
the benzotriazole 2a undergoes addition of 2 mol of cyclo-
pentadiene to give the diadduct 7 was also confirmed by mass
spectrometry (HRMS). Attempts to get precise information on
the reaction sequence leading to 7 by carrying out similar exper-
iments at 0°C have been unsuccessful because of solubility
problems.
Discussion
The Diels–Alder reactivity of Pi-DNBT (2a)
Two main factors have long been suggested as contribut-
ing to the exceptional ease of formation and stability of
adducts such as 5a formed by reaction with the super-
electrophile 1a. The first is the combination of the strong
electron-withdrawing effects of the nitro groups and the
annelated furoxan ring (1–5). The result is not only a high
electron deficiency at C-7 of the carbocyclic ring, and hence
a high susceptibility of this ring position to nucleophilic at-
tack, but also a high stability for the resultant ꢀ-adducts, as a
function of the high electron-delocalizing capability of the
ring structure. The second factor is the relatively low aroma-
ticity of the benzofuroxan system which is expected to
favour covalent addition compared to most common aro-
matic and heteroaromatic systems, e.g., TNB or 3,5-dinitro-
pyridine (9a, 17).
Reactions of the 2-aryl-4,6-dinitrobenzotriazole 1-oxide
series (2a–c) with OH–
All rate and equilibrium measurements pertaining to eq. [1]
were made at 25°C and constant ionic strength of 0.2 mol dm–3
maintained with KCl. Measurements of absorbance at ꢁ max of
the ꢀ-adducts 4a–c (ꢁ max ca. 470 nm) were first made at
equilibrium as a function of pH. These describe clear-cut acid–
base behaviour, as evidenced by the observation of excellent
straight lines with unit slopes fitting eq. [2]. Here, pKa refers to
the reaction of 2a–c with water to produce 4a–c and a proton.
[2]
log [4a–c]/[2a–c] = pH – pKa
Evidence that the aromaticity factor is actually very impor-
tant in determining the electrophilic reactivity of the nitro-
benzofuroxan system has recently come from the discovery
that DNBF is a remarkably versatile reagent in Diels–Alder
reactions. We have shown that it may act as dienophile or
heterodiene, depending upon the reaction partners employed
and the experimental conditions (12, 18). In this regard, an
early report by Kresze and Bathelt (19) that DNBF reacts to
some extent with 2,3-dimethylbutadiene and 1,3-butadiene
to afford the adducts 9 has remained essentially unnoticed in
the literature.
Plots are shown in Fig. 1 and yield the following pKa values:
2a 6.70 ± 0.05, 2b 9.0 ± 0.05, 2c 10.73 ± 0.05. The pH
ranges required for these pKa determinations were conve-
niently covered by using amine buffers made up from N-sub-
stituted aminoethane or propane sulfonic acids, namely TES,
HEPES, TAPS, and CAPS which are known to be rather
unreactive in nucleophilic processes (16).
Using a stopped-flow apparatus, the interconversions of
2a–c and 4a–c were then kinetically investigated under
pseudo-first-order conditions. Solutions of the substrates (ca.
3 × 10–5 mol dm–3) in water containing HCl (1 × 10–3
mol dm–3) were mixed with the appropriate KOH solutions
(2 × 10–3 – 6.1 × 10–2 mol dm–3). In agreement with the
direct equilibrium approach described by eq. [1], only one
relaxation time corresponding to the complete formation of
the adducts 4a–c was observed in all cases. For each of the
reactions studied, plots of kobsd vs. the KOH concentration
afforded excellent straight lines with zero intercepts. This
shows that the k–1 pathway makes a negligible contribution
so that the kobsd values are given by the simple eq. [3]
(Fig. 2). From the slopes of these lines, values of k1OH were
readily determined. Then the k–1 values could be calculated
by means of eq. [4].
© 2001 NRC Canada

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Can. J. Chem. Vol. 79, 2001
Table 1. 13C NMR data for the benzotriazole 1-oxide 2a and the diadducts 7 and 8.a
Compound
C4
C5
124.2
49.04
47.54
C6
C7
C8
C9
C10
C11
C12
C13
2a
137.5
93.17
91.27
C14
144.5
123.11
121.82
C16
119.9
33.53
32.17
C17
125.6b
124.45
111.35
C18
136.3b
145.71
152.69
C19
7c
38.45
36.61
C1>
122.2b
128.3
93.02
91.67
C2>,6>
145.5
147.2 (broad)
128.31
127.52
C3>,5>
140.43
139.36
C4>
150.0
150.4
8d
Compound
C15
2a
7c
8d
126.4
35.34
34.60
55.17
54.56
135.40
134.27
142.33
141.90
47.45
46.11
47.08
46.24
124.44
ad in ppm relative to TMS in Me2SO-d6, unless otherwise specified; numbering is given in the structures 2a, 7, and 8 in the text.
bThese values are based on the analysis of lone distance C—H coupling constants, allowing a complete and more reliable assignment of previously
reported data (11c).
cIn acetonitrile-d3.
dData taken from ref. 12c.
Scheme 1.
Most importantly, strong pericyclic reactivity could be
observed with other highly activated nitrobenzofuroxans,
e.g., the 4-nitro-6-trifluoromethylsulfonyl and 4-aza-6-
nitrobenzo-furoxans for which pKa values of 2.95 and 3.8
have been measured, respectively, for water addition in
aqueous solution, but not with less activated compounds
such as 4-nitrobenzofuroxan itself (pKa = 10.32) (18, 20,
21)2. It thus appears that there is a clear correlation between
the superelectrophilic and pericyclic behaviours in this series
of 10 ꢂ electron heteroaromatic substrates.
Since pericyclic reactions are governed by HOMO–LUMO
interactions (22) and similarly electrophile–nucleophile reac-
tions are governed by the LUMO energy of the electrophilic
partner and the HOMO energy of the nucleophilic partner
(23), this observed correspondence between pericyclic and
superelectrophilic reactivity follows the parallelism in the
fundamental nature of both types of processes. We empha-
size the salient point that it is only the most super-
electrophilic member of the series of benzotriazole 1-oxides,
2a, that also displays Diels–Alder reactivity.
In this context, our finding that 2a reacts with cyclo-
pentadiene (Cp) at room temperature to afford the highly
functionalized stereoselective diadduct 7 supports the idea
that the 4,6-dinitro-substituted carbocyclic ring of the most
activated benzotriazole has a relatively low aromaticity.
Interestingly, neither the N-para-nitrophenyl nor the N-
phenyl analogues 2b and 2c which have pKa values compa-
rable to that of NBF (1b) show similar pericyclic reactivity
upon treatment with cyclopentadiene, thus adding to the evi-
dence of a close relationship between the degree of electron
deficiency of the carbocyclic ring and the Diels–Alder reac-
tivity of this ring.
In the case of the reaction of 1a with Cp, it has been
clearly demonstrated that the formation of the diadduct 8 pro-
ceeds according to Scheme 1 (12c). Following a diastereo-
selective and regioselective normal electron-demand
condensation leading to the formation of the monoadduct 10
under kinetic control (path A), the diadduct 8 is formed as
the result of two consecutive inverse and normal electron de-
mand processes which take place with a high stereo-
selectivity at the C-6—C-7 and C-4—C-5 double bonds of
the carbocyclic ring. This reaction sequence is consistent
with theoretical calculations which point to the activated
6,7-double bond of DNBF as being the preferred site for
normal as well as inverse Diels–Alder reactivity (24). Then,
a second cycloaddition can take place at the remaining acti-
vated C-4—C-5 double bond of the resulting monoadduct
11, akin to the reactivity of nitroolefins (25, 26). Although
we were not able to carry out a similarly detailed study of
the behaviour of 2a at low temperatures (vide supra), it is
reasonable to assume by analogy that the formation of the
diadduct 7 is the result of the two-step sequence of path B in
Scheme 1. That the two condensations leading to 12 and
then to 7 proceed through endo processes with a trans addi-
tion of the two cyclopentadiene molecules is reminiscent of
the situation which prevails in the formation of all DNBF
diadducts (6b, 6c, 18, 20).
The electrophilic character of the 2a–c series
So far, the pKa values of DNBF (3.73) (1a) and TNB
(13.43) (26) represent the extremes in the scale of measuring
2 F. Terrier, G. Moutiers, M. Mokhtari, and J.C. Hallé. Unpublished results.
© 2001 NRC Canada

Vichard et al.
1621
the electron deficiency of aromatic and heteroaromatic sub-
strates as derived from the susceptibility of these compounds
to undergo covalent hydration in aqueous solution (9). In
this regard, Table 3 shows that the pKa values for the three
benzotriazole 1-oxides 2a–c fall in the midrange between
those of DNBF and TNB with the N-phenyl-substituted de-
rivative 2c being the weakest electrophile of the family, as
expected, with a pKa value (10.73) close to that of 4-nitro-
benzofuroxan (10.32). Introduction of nitro groups in the
phenyl ring of 2c has a major effect on the ease of ꢀ-
complexation of the 4,6-dinitro-substituted carbocyclic ring,
as evidenced by the measurement of pKa values of 9.0 and
6.70 for formation of the hydroxy adducts 4b and 4a derived
from the N-(4>-nitrophenyl) and N-(2>,4>,6>-trinitrophenyl)
compounds 2b and 2a, respectively. This corresponds to a
major shift in electrophilic character from that of the refer-
ence electrophile TNB, approaching in the case of 2a the
recognized superelectrophilic character of DNBF (1, 5–9).
Interestingly, an X-ray analysis of 2a has revealed that the
picryl ring and the benzotriazole ring system are in an al-
most orthogonal orientation, due to a strong steric compres-
sion of the adjacent ortho–nitro and N-oxide functions (10a).
This type of steric compression may be reasonably expected
to also prevail in the corresponding adduct 4a. Based on
this, the strong activating effect exerted by the picryl ring on
the susceptibility of the 4,6-dinitro-substituted carbocyclic
ring of 2a to covalent hydration must be the result of induc-
tive and (or) field effects rather than conjugation effects.
Using again the data for the DNBF and TNB as the refer-
ences, Table 3 also reveals that the k1OH rate constants for the
hydroxide ion attack on our three benzotriazole 1-oxides
(2a–c) to generate the adducts 4a–c (eq. [1]), are intermedi-
ate between the two extremes, with relatively minor varia-
tions on going from 2c to 2a. In fact, a most noteworthy
finding is that the k–1 values that measure the tendency of
the three adducts 4a–c to undergo spontaneous decomposi-
tion are all much lower than the value measured for the TNB
adduct 6. On the other hand, it is also of much interest that
the changes in k–1 are essentially responsible for changes in
the pKa values for 2a–c. This is clearly consistent with the
idea that the Marcus intrinsic barrier for the ꢀ-complexation
process increases on going from 2c to 2a. This effect of ni-
tro substitution on the intrinsic barrier may be understood as
the result of charge delocalization being extensive in the re-
sulting adducts 4a–c, but only slightly developed at the tran-
sition state (27). In the case of 4c, the k–1 value is about 30-
fold greater than for the mononitrobenzofuroxan adduct 5b,
suggesting that some electron donation from the mono-
substituted phenyl ring to the heterocyclic framework of the
benzotriazole may occur in this instance.
Conclusions
The present work has shown definitively that the 4,6-
dinitrobenzotriazole 1-oxide structure 2, when appropriately
substituted as in 2a, can exhibit overall ambident pericyclic
reactivity, i.e., normal and inverse electron demand cyclo-
additions take place with high stereoselectivity. This behav-
iour, which was found previously for 1a, may extend to a
wide range of low aromaticity heteroaromatics and investi-
gation of the generality of such reaction is a thrust of inves-
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Can. J. Chem. Vol. 79, 2001
Fig. 1. Variation of the ratio of ionized to non-ionized
benzotriazole 1-oxides 2a–c as a function of pH in aqueous solu-
tion. T = 25°C, I = 0.2 mol dm–3 KCl.
of defined stereochemistry with potential pharmacological
activity.
The kinetic and equilibrium data reported here show that
the electrophilic character of the 4,6-dinitrobenzotriazole 1-
oxide strudcture is closer to that of the nitrobenzofuroxans
(i.e., 1a, 1b) than that of the standard aromatic electrophile
TNB. This result is believed to be largely the reflection of a
rleatively low aromaticity of the carbocyclic ring. Overall,
the finding of a dual electrophilic and pericyclic behaviour
for 2a adds a new facet to the extremely rich chemistry of
the benzotriazole 1-oxide structure (32, 33).
Experimental
Materials
The 4,6-dinitrobenzotriazole 1-oxides 2a and 2b were
available from previous studies (11); the preparation of 2c is
described below. Reagent grade TES (2-N-[tris(hydromethyl)
methyl]aminoethane sulfonic acid), HEPES (N-2-hydroxy-
lethylpiperazine-N>-2-ethanesulfonic acid, (3-N-[tris(hydroxyl-
methyl)methyl]aminopropanesulfonic acid), and CAPS (3-
[cyclohexylamino]propanesulfonic acid) were of the highest
purity obtainable from commerical sources and used without
further purification to prepare buffer solutions if the pH range
required for the pKa determinations of 2a–2c. KOH solutions
were prepared from Titrisol.
Fig. 2. Effect of the hydroxide ion concentration on the ꢀ-
complexation of 2a–c in aqueous solution. T = 25°C, I = 0.2
mol dm–3 KCl.
Synthesis of 2c
1-Chloro-2,4,6-trinitrobenzene (picryl chloride, 3.0 g,
0.012 mol) was dissolved in 95% ethanol (ca. 25 mL) in a
50 mL round-bottomed flask. Phenylhydrazine (2.8 g,
2.4 mL, 0.025 mol) was dripped into the stirring solution.
The brick red picrylhydrazobenzene solidified in the flask,
more ethanol was added (15 mL), and the suspension was
stirred and warmed for 3 h. The final suspension was cooled
to room temperature and the crude product collected by suc-
tion filtration, washed with dilute HCl (0.3 M), and then dis-
tilled water. The product was dried in a vacuum dry-box and
recrystallized from 95% ethanol to give picrylhydra-
zobenzene (2.65 g, 69%), mp 179 to 180°C (lit. (29) 181°C).
Rf (TLC, CH2Cl2) 0.26. Picrylhydrazobenzene (0.50 g,
1.6 mmol) was cyclized in glacial acetic acid to 2-phenyl-4,6-
dinitrobenzotriazole 1-oxide (2c) according to the method of
Joshi and Deorha (30). Yield (after recrystallization from
CH3CN): 0.25 g, 51% (metallic, golden plates), mp 248–
254°C (lit. (31) 249 to 250°C). Rf (CH2Cl2) 0.36.
Table 3. Kinetic and thermodynamic parameters for the formation
and decomposition of hydroxy ꢀ-adducts ion aqueous solution.a
Adduct
pKa
3.75
6.70
9.00
10.73
10.32
13.43
k1OH (dm3 mol–1 s–1)
3.35 × 104
392
680
317
k–1 (s–1)
5ab
4a
4b
4c
5bc
6d
2.6 × 10–6
1.96 × 10–5
3.54 × 10–3
0.19
6 × 10–3
9.8
Synthesis of 7
25
37.5
2-(2>,4>,6>-Trinitrophenyl)-4,6-dinitrobenzotriazole 1-oxide
(2a) (86 mg, 0.197 mmol) was dissolved in a CH2Cl2–THF
mixture and an excess of freshly distilled cyclopentadiene
(1 mL) was added. The solution was maintained under stir-
ring at room temperature for 5 h, then concentrated under
reduced pressure. After addition of an excess of pentane, a
pale yellow solid was isolated and dried under pump vac-
uum. Compound 7 was obtained in 92% yield (103 mg) and
gave satisfactory analytical data. 1H NMR: 300 MHz,
acetonitrile-d3, see Table 2. 13C NMR: (see Table 1).
FAB+-MS mass spectrum (obsd). m/z: 569.1017 (reference
m-nitrobenzyl alcohol), C22H14N8O11 requires 569.0984.
aT = 298 K; I = 0.2 mol dm–3 KCl.
bReference 1a.
cReference 1b.
dReference 25, I = 0.1 mol dm–3 NaCl.
tigations in our laboratories. It is also important to note that
the relative ease of displacement of the picryl moiety from
adducts such as 7 by nucleophiles would afford a facile
route to the synthesis of functionalized triazole deriviatives
© 2001 NRC Canada

Vichard et al.
1623
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Rate and pKa measurements
Stopped-flow determinations were performed on an Applied
Photophysics spectrophotometer, the cell compartment of
which was maintained at 25° ± 0.2°C. Other kinetic and pKa
measurements were made using a Varian Cary 1E spectro-
photometer. All kinetic runs were carried out in triplicate
under pseudo-first-order conditions with a substrate concen-
tration of 2 to 3 × 10–5 mol dm–3. In the case of 2c, the mea-
surements have been made in 90% H2O – 10% Me2SO (v/v)
to overcome solubility problems but this low Me2SO content
does not have any major influence on the discussion of the
results (1b). All rate constants are accurate to ±3%.
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Acknowledgements
We are grateful for the financial support of this research by
CNRS (F.T.), the Natural Sciences and Engineering Research
Council of Canada (NSERC) (E.B.), and the Principal’s
Research Fund of Sir Wilfred Grenfell College (J.M.D.).
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© 2001 NRC Canada