“Metal-free” electrooxidative C—H thiocyanation of arenes

Full text of DOI:10.1007/s11172-019-2681-2

Russian Chemical Bulletin, International Edition, Vol. 68, No. 11, pp. 2140—2141, November, 2019
2140
"Metal-free" electrooxidative C—H thiocyanation of arenes
V. A. Kokorekin,a,b E. I. Mel´nikova,a R. R. Yaubasarova,a N. V. Gorpinchenko,b and V. A. Petrosyana
aN. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences,
47 Leninsky prosp., 119991 Moscow, Russian Federation.
Fax: +7 (499) 135 9070. E-mail: kokorekin@yandex.ru
bI. M. Sechenov First Moscow State Medical University
of the Ministry of Health of the Russian Federation (Sechenov University),
Build. 2, 8 ul. Trubetskaya, 119991 Moscow, Russian Federation
Functionally substituted arenes are structural com-
development of such processes. This created pre-requisites
to search for more environmentally friendly and less ex-
pensive electrode materials. Earlier, for thiocyanation of
1,3,5-trimethylpyrazolo[1,5-a]pyrimidine, we successfully
used one of such materials, namely, glassy carbon (GC).6
For development of these studies, there was an un-
doubted practical interest in using GC electrodes for
synthesis of a wider range of aryl thiocyanates that we
carried out via electrooxidative С—Н thiocyanation of
benzene, pyrrole, and pyrazolo[1,5-a]pyrimidine deriva-
tives to yield products 1—15 (Scheme 1).
ponents of numerous practically valuable compounds.
Currently, the most demanded strategy for the synthesis
of substituted arenes is their direct С—Н functionaliza-
tion.1 The most promising trend within this strategy may
involve electrooxidative (anodic) С—Н functionaliza-
tion,2,3 using electric current as an available and environ-
mentally attractive activator.
In recent years, based on anodic С—Н-functionalization
of arenes we developed4—7 an effective system for elec-
trosynthesis of aryl thiocyanates, a promising class of
compounds having a wide range of pharmacological ac-
tivities and being valuable precursors of sulfur-containing
substances.8,9 Meanwhile, the use of platinum electrodes
essentially limited the practical attraction and innovative
Electrolysis was carried out at a temperature of 20—25 °С in
an undivided cell with the supporting electrolyte solution of
0.1 М NaClO4 in MeCN (70 mL) charged with arene (1 mmol)
and NH4SCN (5 mmol) at the anode potential (Еan) of 0.85 V
Scheme 1
E
an is anode potential.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2140—2141, November, 2019.
1066-5285/19/6811-2140 © 2019 Springer Science+Business Media, Inc.

C—H Thiocyanation of arenes
Russ. Chem. Bull., Int. Ed., Vol. 68, No. 11, November, 2019
2141
vs. SCE (approach I, synthesis of products 1—11) or 1.85 V
(approach II, synthesis of products 12—15). An appropriate
electric charge (2—8 F, 193—772 C, as 1 F per 1 mol of thio-
cyanate ion or 2 F per 1 mol of arene) was passed for the full
conversion of the starting arene: 2 F to yield products 1, 5—9;
4 F to yield products 2—4, 10 and 11; 8 F to yield products
12—15. The treatment of the reaction mixture was similar to that
represented in our prior work.6 Earlier described target products
1—15 were identified by their melting points and NMR spectra
according to reference data.4,6,9—11
In general, our suggested tool for direct formation of
С—S bonds has a number of obvious advantages (mild
conditions, available and environmentally friendly mate-
rials, absence of chemical oxidants and metal-containing
catalysts) that make it very promising for use in future.
This work was financially supported by the Russian
Science Foundation (Project No. 19-73-20259).
References
In this work, we confirmed the anodic thiocyanation
regularities which we found earlier.4—7 Thus, approach I
(see Scheme 1) was most effective for easily oxidizable
(Epox = 0.95—1.65 V) arenes being reactive with the elec-
trogenerated thiocyanogen (SCN)2. The yield of products
1—11 was 45—82%. In the case of hydroquinone, instead
of aryl thiocyanate, we obtained its rearrangement product,
namely, benzoxathiolone 4 (yield 52%). For hardly oxidiz-
able (Epox = 1.75—1.90 V) arenes being almost unreactive
with (SCN)2, approach II is most effective (see Scheme 1).
It is accomplished at Epox of arene via ECE mechanism6
and led to target products 12—15 with the yields of
47—60%. Note that in some experiments we observed side
processes (polymerization of thiocyanogen,6 oxidation
of starting arenes to resinous products7, etc.), which
caused higher electricity consumption and lowering the
product yields.
Thus, this work was the first to show feasibility of
"metal-free"1 approaches to electrooxidative С—Н thio-
cyanation of a wide range of high- and low-reactive
(hetero)arenes in an undivided cell with GC electrodes.
As a result, the target aryl thiocyanates 1—15 were syn-
thesized with the yields of 45—82%, whereas products
2—4 were electrochemically obtained for the first time.
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Received August 8, 2019;
in revised form August 26, 2019;
accepted September 3, 2019