Reaction of PbO2 with Solutions of Methylbenzonitriles

Article abstract of DOI:10.1023/A:1024901819045

The yield of products formed by low-temperature oxidation of methylbenzonitriles with PbO₂ in fluorosulfonic acid solutions was studied in relation to the stabilization factor of the reaction system. The revealed correlation is common for methylbenzonitriles differing in the degree of alkylation and in the oxidation mechanism.

Full text of DOI:10.1023/A:1024901819045

Russian Journal of General Chemistry, Vol. 73, No. 3, 2003, pp. 389 393. Translated from Zhurnal Obshchei Khimii, Vol. 73, No. 3, 2003,
pp. 417 421.
Original Russian Text Copyright
2003 by Kol’tsov, Salfetnikova.
Reaction of PbO with Solutions of Methylbenzonitriles
2
S. I. Kol’tsov and Yu. N. Salfetnikova
St. Petersburg State Institute of Technology, St. Petersburg, Russia
Received March 26, 2001
Abstract The yield of products formed by low-temperature oxidation of methylbenzonitriles with PbO in
2
fluorosulfonic acid solutions was studied in relation to the stabilization factor of the reaction system. The
revealed correlation is common for methylbenzonitriles differing in the degree of alkylation and in the
oxidation mechanism.
It was shown previously [1, 2] that element oxide
layers prepared by molecular layer deposition [3] on
the surface of solid matrices can alter the composition
and reactivity of organic functional groups grafted to
the surface [4]. Due to this fact, e.g., titanium oxide
layers can be used as matrices for preparing peptide
chains [5], and the systems based on vanadium oxide
layers [6], for oxidizing o-xylene and other organic
compounds [7, 8]. Interesting opportunities for study-
ing low-temperature reactions are offered by PbO2,
which, similar to other oxides [3], can be prepared by
molecular layer deposition from volatile tetrachloride
or organic derivatives. Being a powerful oxidant,
and characterized by procedures described in [13].
The purity of the initial compounds and reaction prod-
ucts was checked by TLC on Silufol UV-254 plates.
Voltammograms of oxidation of methylbenzonitriles
were taken in the system HSO F 0.1 M AcOH at
3
76 C on a platinum electrode. The reaction mixtures
were separated by column chromatography on silica
gel (Chemapol, 40 100 m). The analytical data were
consistent with the composition. The product struc-
tures were judged using mass, NMR, and IR spectro-
metric data from [13]. The conditions of oxidation of
methylbenzonitriles and the yields of reaction prod-
ucts are listed in Table 1.
PbO is used for low-temperature synthesis of organic
2
Oxidation of methylbenzonitriles of series 1 (Ta-
ble 1) at 75 C for 5 h, followed by treatment of the
reaction mixture with concentrated HCl, yields chlori-
nated benzonitriles as shown in scheme (1) (by the ex-
ample of 2-methylbenzonitrile). This reaction, as in
the case of other alkyl-substituted benzonitriles, in-
volves formation of intermediate nonprotonated radi-
cal cations [11, 14].
compounds, in particular, by oxidation of aromatic
compounds [9, 10]. These include benzonitriles and
their diverse transformation products, widely used for
preparing important compounds and formulations.
These compounds are also of theoretical interest, since
electrophilic radical cations [11] arising after one-
electron oxidation of nitriles in superacids like HSO3F
[12] should undergo further transformations by differ-
ent pathways depending on their composition and
structure. The most probable transformation pathways
CN
CN
CH3
CH3
CN
+
of methylbenzonitriles in the system HSO F PbO2
3
e
were studied in [13] in relation to the steric and elec-
tron donor acceptor properties of the substituents and
to the reaction conditions. However, some problems
related to control of the synthesis by dosing definite
CN
CH OSO F
CH Cl
2
FSO
2
2
3
HCl
+
(1)
H ,
e
amounts of PbO by molecular layer deposition re-
2
quire particular consideration. This study is aimed at
elucidating the general parameter that governs the
completeness of transformations of methylbenzoni-
triles in the system PbO HSO F.
The substituent in the m- or o-position (Table 1,
run nos. 1, 2) does not affect the degree of conversion,
whereas with the p-substituent the reaction should be
performed at a higher temperature ( 40 C) to attain
the same yield.
2
3
Lead dioxide was of chemically pure grade. Fluo-
1
8
3
rosulfonic acid HSO F (D 1.74 g cm ) was used as
Oxidation of dimethylbenzonitriles (series II) does
not stop at the stage of formation of chlorinated com-
pounds of type A [scheme (1)]. Another transforma-
3
solvent. Three series of methylbenzonitriles differing
in the number of substituents (Table 1) were prepared
1
070-3632/03/7303-0389$25.00 2003 MAIK Nauka/Interperiodica

3
90
Table 1. Conditions of oxidation of methylbenzonitriles with lead dioxide in fluorosulfonic acid
Amount of reagents Reaction conditions
nitrile, PbO2, HSO F, tempera-
KOL’TSOV, SALFETNIKOVA
Product
yield,
%
Run
no.
Series
Compound
Products
time,
h
3
mmol
mmol
ml
ture, C
1
2
3
4
5
6
7
8
9
I
2-Methylbenzonitrile
3-Methylbenzonitrile
4-Methylbenzonitrile
4-Methylbenzonitrile
2,5-Dimethylbenzonitrile
3,5-Dimethylbenzonitrile
3,5-Dimethylbenzonitrile
10
10
8
10
5
4
7
10
5
1
15
15
6
15
3
2
4
10
5
1
15
15
10
15
10
12
10
20
20
20
15
20
75
75
75
40
70
75
70
75
75
75
72
75
5
5
6
5
2
2
2
5
5
5
1.5
2
29
29
20
28
40
17
30
94
57
36
75
87
A
A
A
A
II
A, B, C, D
A, D
A, D
E
E
E
III 2,4,6-Trimethylbenzonitrile
2,4,6-Trimethylbenzonitrile
1
1
1
0
1
2
2,4,6-Trimethylbenzonitrile
2,3,5,6-Tetramethylbenzonitrile
2,3,4,5,6-Pentamethylbenzonitrile
4
5
2
2.5
B, I
I
tion pathway appears for the radical cations [13]: elec-
trophilic attack of the starting molecule, followed by
one-electron oxidation of the dimeric radical cation
and its deprotonation. This process leads to products
of oxidative dimerization (B) and then trimerization
(C), and also to biaryls (D), as shown in scheme (2)
(by the example of 2,5-dimethylbenzonitrile, Table 1,
run no. 5).
CN
H C
3
CN
CN
CN
CN
CN
H C
3
H C
3
H C
3
H C
3
H C
3
+
CH3
+
e
CH Cl
CH2
CH3
2
CH3
CH3
A
B
CN
CN
CN
H3C
CN
H3C
CN
H C
3
H C
3
H C
3
+
(2)
+
CH2
CH2
CH3
CH3
CH3
C
D
Table 1 shows that the structure and yield of the
transformation products depend on the mutual loca-
tion of the substituents. For example, in the case of
In series III of tri-, tetra-, and pentamethylbenzoni-
triles, of most interest is oxidation of symmetrical
2,4,6-trimethylbenzonitrile. Firstly, this reaction gives
a single final product E [scheme (3)] whose yield, at
increased reactant concentrations, is as high as 94%
(Table 1). Secondly, here the cyano group is involved
in the transformations, which is a specific feature of
oxidative transformations of alkylated benzonitriles
3
,5-dimethylbenzonitrile, formation of products of
types B and C is impossible, and only product of
type D is formed as a result of o o dimerization.
CN
CN
[
13]. The radical cation formed in the initial stage of
H C
3
CH3
oxidation (as in the other cases) attacks the CN group
of the starting compound, rather than eliminates pro-
CH3 CH3
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 73 No. 3 2003

REACTION OF PBO2 WITH SOLUTIONS OF METHYLBENZONITRILES
391
ton from the methyl group, and yields dimeric radical
cation F and then [scheme (3)] cation G stable in
,
V
HSO F; its hydrolysis yields the final product, N-aryl-
benzamide (compound E).
3
1
.6
CN
CN
CH3
H C
3
CH3
H C
3
1.4
+
e
CH3
CH3
CN
1
.2
CN
H C
CH3
3
CH3
CH3
+
H C
3
N===C
CH3
1.0
CH3
H
CH3
CH3
F
CN
1
2
3
4
5 q
CH3
CH3
H C
3
Fig. 1. Correlation between the oxidation potential of
methylbenzonitriles and the number
substituents.
+
N
e, H+
q of methyl
C
CH3
CH3
CN
CH3
G
pathways depending on the composition and structure
of the starting methylbenzonitrile. This fact is proba-
bly reponsible for different concentration dependences
of the product yields in series I and III and for the
lack of such a dependence in series II. This compli-
cates the use of oxidation of alkylated benzonitriles
for controllable synthesis with dosed amounts of
CH3
CH3
H C
3
H O
2
H+
N
C
CH3 (3)
CH3
O
CH3
E
PbO on the matrix surface and limits the possibility
2
In the case of tetra- and pentamethylbenzonitriles,
oxidation also gives substituted N-benzylbenzamides
in high yields (Table 1). However, their formation in-
volves deprotonation of m-methyl groups [scheme (4)
for 2,3,5,6-tetramethylbenzonitrile as example] in the
radical cations, followed by oxidation to cation H,
which reacts with the CN group, yielding compound I
as final product:
of comprehensive consideration of the substituent
effects.
At the same time, the chemical transformations in
this series have a common feature, namely, initial
formation of intermediate radical cations [11] which
subsequently undergo various further transformations
[
schemes (1) (3)]. The extent of these transformation
and the attainable product yield depend on the capa-
bility of various methylbenzonitriles to be oxidized
to the radical cation (which can be evaluated by their
oxidation potentials [15]) and on the reaction condi-
tions (component concentrations, temperature, etc.).
Since the temperature conditions are similar in all the
series (Table 1) and the time is sufficient for the reac-
CN
CH3
H C
3
+
CH2
H C
3
H
CN
CH CH
3
tion completion, we consider the product yield
in
CH3
3
H C
3
O
relation to the oxidation potential and concentra-
CH NH C
tions of methylbenzonitrile (m) and PbO (n) only,
H C
3
2
(4)
2
assuming that the amount of the product formed (p) is
proportional to n.
CH CH
3
3
I
Figure 1 shows the correlation between the oxida-
tion potential and the number of substituents q in
the methylbenzonitrile molecules. It is seen that
smoothly decreases from 1.70 V for benzonitrile ( 0,
q = 0) to 0.90 V for fully substituted (q = 5) penta-
methylbenzonitrile.
Oxidation of pentamethylbenzonitrile yields similar
products.
Thus, low-temperature oxidation of methylbenzo-
nitriles with the PbO HSO F system occurs by three
2
3
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 73 No. 3 2003

3
92
KOL’TSOV, SALFETNIKOVA
0). Since p = m, we obtain
=
= c n/m = cR
P, %
00
(where c is the proportionality coefficient).
1
8
1
The stabilization factors R calculated from the
experimental data are listed in Table 2, and the corre-
lation between the stabilization factor and the yield of
reaction products for all the three series of methyl-
benzonitriles is shown in Fig. 2. It is seen that all the
data are satisfactorily fitted by a straight line in the
12
8
0
0
1
9
6
coordinates
R, deviating by no more than 3%.
Mono- and dimethylbenzonitriles (q = 1, 2; series I,
II) have R < 0.25, and the compounds with q = 3 5
(series III), R > 0.25. The observed significant devia-
tions from the correlation for 4-methylbenzonitrile
samples in run no. 3 are probably due to the lower (by
half) n/m ratio (Table 2), and in run no. 4, to a consid-
erably higher reaction temperature ( 40 C, Table 1),
which might affect ; in the calculation of R for this
run, we used the potential obtained at 76 C. For run
nos. 9 and 10, which reveal in combination with run
no. 8 the influence of the 2,4,6-trimethylbenzonitrile
concentration on the yield of oxidation products at
equal reactant ratio, the parameters were calculated
from the logarithmic correlation between and sub-
strate concentration. As seen from Fig. 2, the points
for run nos. 8 10 are also fitted by the same straight
line, deviating by no more than 3%.
5
1
4
2
0
0
1
0
7
2
4
3
6
0
0
.2
0.3
0.4
0.5 R
Fig. 2. Correlation between the product yield
oxidation of methylbenzonitriles and the stabilization
factor R. Point nos. are run nos. in Tables 1 and 2.
in
Presumably, the product amount p will increase as
the methylbenzonitriles will become more susceptible
to oxidation with increasing number of methyl substit-
uents, in direct proportion with the difference
but in inverse proportion with . Let us define the
Thus, the (R) correlation based on the decisive
role of radical cation formation in the initial stage of
0
ratio ( 0
)/
=
as a parameter characterizing
and
oxidation with PbO is common for all the methyl-
2
the substrate reactivity (for benzonitrile,
=
benzonitriles studied, differing in the degree of alky-
lation, despite different reaction mechanisms [schemes
(1) (3)] and diversity of products formed.
0
Table 2. Main parameters of low-temperature oxidation
of methylbenzonitriles with the system PbO HSO F. The
nitriles used and reaction conditions are the same as in
Table 1
2
3
This result also demonstrates the possibility of
using the whole set of methylbenzonitriles with regu-
larly changing number of substituents for studying the
reactivity at negative temperatures of oxide systems
prepared by molecular layer deposition.
Concentration, M
Run
no.
n/m
R
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