1004
Russ.Chem.Bull., Int.Ed., Vol. 51, No. 6, June, 2002
Chauzov et al.
Accomplishing the discussion of effects caused by the
rearrangement 7 6, we note that its occurrence
ally transforms the pyrazole ring into a withdrawing subꢀ
stituent.
also explains the above mentioned influence of the reꢀ
placement of the solvent (CH Cl instead of MeCN) on
For evaluation of preparative aspects of Nꢀarylation
of pyrazoles, note that 1 and 2 are compounds, whose
syntheses by the classical methods of the organic chemꢀ
istry are difficult, and therefore, the proposed oneꢀstep
method for their preparation from available initial subꢀ
stances seems useful. However, products 2 form, as a
rule, in a mixture with 1 and are hydrolytically unstable
in most cases. That is why, their isolation in the indiꢀ
vidual form is difficult. Therefore, the electrochemical
Nꢀarylation of pyrazoles is of greatest interest as a method
for synthesis of products of the type 1 by the undivided
amperostatic electrolysis of reaction mixtures containing
a considerable DMB excess (see Table 2, entry 1), with
acid additives (entries 9 and 10), or using CH Cl as
solvent (entries 11 and 19). Note, however, that the first
route is accompanied by the intense anodic oxidation of
DMB and affords a great amount of resins. Two other
methods have no these disadvantages and were used as
most appropriate to prepare 1 in the individual form (see
Experimental).
2
2
the ratio of products 1a,b and 2a,b. The known ability of
1
1
СH Cl to stabilize efficiently radical cations is eviꢀ
2
2
dently a result of the poor ionizing and polarizing propꢀ
erties inherent in this solvent (dielectric constants for
MeCN and СH Cl2 are 37 and 9, respectively). These
2
properties substantially decrease the reactivity of ionic or
dipolar nucleophiles present in an electrolyte solution.
For the reactions considered, this is equivalent, in fact,
to a decrease in the nucleophilicity of the pyrazole
coreactant and, according to the above reasons, should
favor the predominant formation of 1a,b due to the rearꢀ
rangement during Nꢀarylation of highly basic pyrazoles
in CH Cl .
2
2
2
2
The decrease in the yield of 1b in the DMP—DMB
system when MeOH is used instead of MeCN as a solꢀ
vent (MeOH is close to MeCN in dielectric constant but
simultaneously is much more basic and, evidently, more
nucleophilic) is probably associated in part with the
"
switching off" of the rearrangement step because the
formation of intermediates 5b and further 7b is hindered
because of a multiply higher concentration of MeOH
over that of DMP.
Experimental
1H NMR spectra of solutions of samples in
DMSOꢀd —CCl (1 : 1 v/v) mixture were recorded on a Bruker
a
Finishing the discussion of the mechanistic aspects
of Nꢀarylation of pyrazoles, we have to concentrate atꢀ
tention on the problem of stability of the products under
the anodic oxidation conditions. In our opinion, the
latter noticeably affects both the composition of the tarꢀ
get products and its dependence on the amount of elecꢀ
tricity passed. Comparison of the structures of arylation
products 1 and 2 shows that a relatively high oxidation
potential can be expected only for 2, which is confirmed,
to some extent, by the oxidation potential of a similar
structure of 1,1,4,4ꢀtetramethoxycyclohexaꢀ2,5ꢀdiene
presented in Table 1. On the contrary, the structure of
products 1 allows us to assume relatively low oxidation
potentials for them. In the absence of electronꢀacceptor
groups in the pyrazole substituents, it seems probable
that the oxidation potentials of these compounds are
comparable or even lower than that for DMB. Indeed,
the CV curves of solutions of 1b contain three oxidation
peaks, whose potentials (see Table 1) are comparable to
those of the first and subsequent peaks in the CV curves
of DMB. This implies that orthoꢀsubstitution products 1
formed by amperostatic electrolysis can undergo elecꢀ
trochemical transformations to a noticeable extent. The
lower yields of 1a are evidently due to its easier oxidaꢀ
tion compared to that of 1b (see Table 2). In fact, the
molecule of the latter is nonꢀcomplanar due to the reꢀ
pulsion between the orthoꢀmethoxy group in the phenyl
ring and methyl groups in the pyrazole ring, which virtuꢀ
6
4
ACꢀ300 instrument.
Redox characteristics of the objects under study were deꢀ
termined by the CV method in a glass cell, whose temperature
was maintained constant (25 °С), using a PIꢀ50ꢀ1.1 potentioꢀ
stat with a PRꢀ8 programmer. A Pt wire 1 mm in diamꢀ
eter coated with a Teflon shell was used as the working
electrode. The reference electrode was Ag/0.1 M AgNO3.
The supporting electrolyte was 0.1 M Bu NClO4 in the solꢀ
vent used.
Amperostatic electrolysis of solutions of DMB (2 mmol) in
reaction mixtures (45 mL) with different compositions was
carried out in a glass undivided cell with a constant temperaꢀ
ture (20—21 °C) using a magnetic stirrer and axial cylindrical
4
2
Pt electrodes with surface areas of 12.3 cm (cathode) and
3
7.2 cm2 (anode) in an argon atmosphere at the controlled
current (I = 50 mA), passing 1 or 2 F electricity per mole of
DMB (see Table 2). After cessation of electrolysis, the solvent
was distilled off on a rotary evaporator at the temperaꢀ
ture ≤100 °C (25 Torr), and the residue was analyzed by
1
H NMR. Reactants and solvents for this experiment, its appaꢀ
ratus design, and typical experimental procedure were similar
to those described previously. The spectral characteristics of
1
compounds 1b, 2a—c used for their identification and the proꢀ
cedure for determination of the current yield of 1 and 2 (calcuꢀ
lation per twoꢀelectron transformation of DMB) based on the
spectral data without their isolation from solution were pubꢀ
1
lished previously.
A porous glass diaphragm was used for electrolysis with
divided cathodic and anodic spaces. The cell volume and ratios
of surface areas of the Pt electrodes were approximately the
Chauzov
