Reactivity of substituted benzoic acids and their complexes with 2,4,6-trinitrotoluene toward diphenyldiazomethane

Article abstract of DOI:10.1134/S1070428016040059

The reactions of 2,4-dihydroxybenzoic and 4-aminobenzoic acids with diphenyldiazomethane are accelerated in the presence of 2,4,6-trinitrotoluene as π-acceptor. The catalytic effect of the latter is determined by the formation of charge-transfer complexes

Full text of DOI:10.1134/S1070428016040059

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2016, Vol. 52, No. 4, pp. 513–515. © Pleiades Publishing, Ltd., 2016.
Original Russian Text © A.V. Ryzhakov, V.P. Andreev, 2016, published in Zhurnal Organicheskoi Khimii, 2016, Vol. 52, No. 4, pp. 526–528.
Reactivity of Substituted Benzoic Acids and Their Complexes
with 2,4,6-Trinitrotoluene toward Diphenyldiazomethane
A. V. Ryzhakova and V. P. Andreevb
a Institute of Northern Water Problems, Karelian Research Center, Russian Academy of Sciences,
pr. Aleksandra Nevskogo 50, Petrozavodsk, 185030 Russia
b Petrozavodsk State University, pr. Lenina 33, Petrozavodsk, 185910 Russia
e-mail: andreev@psu.karelia.ru
Received January 27, 2016
Abstract—The reactions of 2,4-dihydroxybenzoic and 4-aminobenzoic acids with diphenyldiazomethane are
accelerated in the presence of 2,4,6-trinitrotoluene as π-acceptor. The catalytic effect of the latter is determined
by the formation of charge-transfer complexes with the acids, which facilitates proton abstraction and stabilizes
the corresponding anions.
DOI: 10.1134/S1070428016040059
It is known that formation of strong donor–acceptor
complexes by organic and inorganic compounds dra-
matically changes chemical properties of the compo-
nents due to redistribution of electron density in their
molecules [1]. It is much more difficult to obtain
experimental proofs for variation of the reactivity of
compounds involved in weak π,π-complexes or
charge-transfer complexes (CTC) as compared to the
free species. Charge-transfer complexes are formed via
electron transfer from the HOMO of the donor mole-
cule to the LUMO of acceptor; however, insofar as the
degree of charge transfer is usually not high, such
adducts are considerably weaker than those formed via
dative covalent bonding. Therefore, most π,π-com-
plexes decompose partially or completely by the action
of nucleophiles or electrophiles and even some
solvents before chemical reaction. Penn and Lin [2]
reported that the complexation of benzyl phenyl ethers
with 2,3-dichloro-5,6-dicyanobenzoquinone facilitates
cleavage of the C–O bond in the former. As noted in
[3], acetolysis of 2,4,7-trinitrofluoren-9-yl p-toluene-
sulfonate and hydrolysis of 4-nitrobenzyl bromide are
accelerated in the presence of various π-donors (such
as phenanthrene) capable of forming CTC with the
substrate. Another example is cis–trans isomerization
of 4-nitrostilbene in the complex with N,N-dimethyl-
aniline [4].
We previously showed that π–π complexation of
heteroaromatic N-oxides with such electron-deficient
compounds as haloanils and tetracyanoethylene
facilitates nucleophilic substitution in the aromatic ring
and in some cases enables those reactions which are
impossible with the free substrates [5, 6].
In this work we studied the reactions of substituted
benzoic acids 1a–1c, as well as of their charge-transfer
complexes with 2,4,6-trinitrotoluene (2), with di-
phenyldiazomethane (3) in toluene–dioxane (4 :1),
which afforded esters 4a–4c (Scheme 1).
2,4,6-Trinitrotoluene (2) is a fairly strong π-accep-
tor, whereas acids 1a and 1b exhibit electron-donating
properties due to the presence of hydroxy or amino
Scheme 1.
R
R
O
Ph
NO2
COOH
Me
2
O
Ph
+
Ph2CN2
–N2
R'
R'
O2N
NO2
1a–1c
3
4a–4c
2
R = R′ = OH (a), R = H, R′ = NH2 (b), O2N (c).
513

RYZHAKOV, ANDREEV
514
groups in the aromatic ring. Acids 1a and 1b with 2
form π-complexes which exist in solution as equilib-
rium mixture with the free components. The com-
plexes can be detected by spectrophotometry by the
appearance of a charge transfer band at λ 420–480 nm,
which overlaps the absorption band of 2. The light
yellow color of a solution of 2 strengthened on
addition of acid 1a or changed to greenish yellow on
addition of 1b. The absorbance of 2 at λ 460 nm
decreased to almost zero, so that it was possible to
determine quantitative characteristics of the complexes
by the Beneshi–Hildebrand method [7]. For this
purpose, the dependence ca L/D = f(l/cd.) was plotted,
where ca and cd are the concentrations of the acceptor
and donor, respectively, D is the optical density, and L
is the cell path length (cm). The resulting plots were
straight lines with a correlation coefficient of 0.99. The
linear character of these plots indicated 1:1 stoichi-
ometry of the CTCs, and their stability constants and
molar absorption coefficients at the CTB maximum
(λ 460 nm) were calculated from the coefficients of the
linear equations: 1a·2: Kst = 0.71, ε460 = 24; 1b·2:
Kst = 4.50, ε460 = 56. Thus, complex 1b ·2 is much
more stable than 1a·2 due to the presence of a very
strong electron-donating substituent, amino group, in
the para position of the benzene ring of 1b. We failed
to detect by spectrophotometry CTC of 2,4,6-trinitro-
toluene with acid 1c containing an electron-with-
drawing nitro group.
donor properties of various compounds. Diazoalkanes
react with acids to give the corresponding esters. The
rate-determining step in this reaction is protonation of
diazo compound with formation of unstable diazonium
ion [8]. The use of diphenyldiazomethane is advan-
tageous due to its intense color, so that the reaction rate
can be monitored by spectrophotometry at λ 525 nm.
Trinitrotoluene 2 did not react with 3: the optical
density of a solution containing compounds 2 and 3
did not change over several hours.
The reactions of acids 1a–1c with diphenyldiazo-
methane 3 in the absence of π-acceptor 2 were carried
out at a 1-to-3 molar ratio of 10:1. In this case, the
process may be considered to be pseudo-unimolecular,
and it can be described by first-order kinetic equation.
The apparent rate constants (see table) were calculated
by Eq. (1):
k = 1/τ ln(D0/D),
(1)
where D0 and D are the optical densities at the initial
moment and at a time τ. The reaction with acid 1a was
characterized by the highest rate; obviously, intramo-
lecular hydrogen bond involving the 2-hydroxy group
increases the proton-donor power of 1a. Acid 1c
reacted with 3 at a higher rate than 1b, which is
consistent with electron-withdrawing effect of the nitro
group in the former.
The reactions of acids 1a–1c with diphenyldiazo-
methane (3) in the presence of π-acceptor 2 were
studied under analogous conditions. The concentration
of 2, as well as of acids 1a and 1b, considerably
exceeded the concentration of 3. The apparent catalytic
rate constants were also calculated by Eq. (1). As
follows from the data in table, addition of π-acceptor 2
accelerated the reaction, especially with acids 1a and
1b. The reaction rate also significantly increased as the
concentration of 2,4,6-trinitrotoluene changed from
0.04 to 0.06 M, as shown with acid 1a as an example.
The catalytic effect of 2 in the reaction with acid 1c
was weaker than in the case of 1a or 1b since the CTC
of 1c with 2 is very unstable (if formed).
The formation of CTCs with π-acceptor 2 was
expected to enhance proton-donating ability of acids
1a and 1b due to stabilization of negative charge in the
respective anions. This assumption was verified by
comparing the reactivities of 1a and 1b and their CTCs
with 2 in the reaction with diphenyldiazomethane (3).
It is known that aliphatic diazo compounds (alkyl- and
aryldiazomethanes) are sensitive to acids, and they are
sometimes used for quantitative assessment of proton-
Apparent rate constants for the reactions of acids 1a–1c with
diphenyldiazomethane (3) in toluene–dioxane (4:1) at 20°Ca
Acid
Concentration of 2, M
k, s–1
1a
1a
1a
1b
1b
1c
1c
0.000
0.040
0.060
0.000
0.040
0.000
0.040
(1.74±0.04)×10–3
(2.11±0.08)×10–3
(3.05±0.07)×10–3
(7.79±0.07)×10–5
(9.96±0.11)×10–5
(2.28±0.04)×10–4
(2.47±0.05)×10–4
Obviously, two concurrent processes are possible in
the system benzoic acid–π-acceptor–diazo compound:
catalytic and noncatalytic. However, it is difficult to
rigorously determine the kinetics of the catalytic reac-
tion without elucidating its mechanism. Most probably,
π-acceptor forms a complex with acid 1, which
facilitates deprotonation of the latter, i.e., it acts as
an electron-withdrawing substituent but is not cova-
lently bound to the aromatic ring.
a
Concentration of 1a–1c 0.045 M; concentration of 3 0.0045 M.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 52 No. 4 2016

REACTIVITY OF SUBSTITUTED BENZOIC ACIDS AND THEIR COMPLEXES
Such π-acceptors as chloranil or tetracyanoethylene
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515
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