Anodic thiocyanation of mono-and disubstituted aromatic compounds

Article abstract of DOI:10.1016/j.electacta.2010.05.035

The in situ and environmentally friendly thiocyanation (no use of toxic oxidizing agents) electrochemical thiocyanation of aromatic compounds involving various derivatives of anisole and aniline to afford aromatic thiocyanates have been studied in organic

Full text of DOI:10.1016/j.electacta.2010.05.035

Electrochimica Acta 55 (2010) 5854–5859
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Anodic thiocyanation of mono- and disubstituted aromatic compounds
Anna Gitkis, James Y. Becker∗,1
Department of Chemistry, Ben-Gurion University of the Negev, Beer Sheva 84105, Israel
a r t i c l e i n f o
a b s t r a c t
Article history:
The in situ and environmentally friendly thiocyanation (no use of toxic oxidizing agents) electrochemical
thiocyanation of aromatic compounds involving various derivatives of anisole and aniline to afford aro-
matic thiocyanates have been studied in organic acidic media. The initial electrochemical step involves
anodic oxidation of thiocyanate anion to its radical (SCN), followed by dimerization to thiocyanogen
(SCN)2. The latter is polarized by the acidic solvent and attacks the aromatic nucleus of the substrate
to afford the corresponding thiocyanate derivative. The sole thiocyanate products obtained in each case
shows high regio-selectivity (no ortho isomer was observed) for the monosubstituted aromatics and high
isomer-selectivity (no isothiocyanate isomer was detected) for both mono- and disubstituted aromatics.
Received 19 March 2010
Received in revised form 4 May 2010
Accepted 8 May 2010
Available online 15 May 2010
Keywords:
Anodic thiocyanation
Aromatic thiocyanates
Thiocyanogen
© 2010 Elsevier Ltd. All rights reserved.
Green chemistry
1. Introduction
In comparison, the electrochemical method affords a “green”
oxidizing agent – the anode. Previously we have shown [21] that
Aryl thiocyanates are useful precursors for agrochemicals, dyes,
insecticides and drugs [1–3]. In organic synthesis, they are also used
as a convenient source of ArS− for introducing sulfur functional
groups in various organic molecules [4–7]. Chemical synthesis
of aryl thiocyanates can be performed via both electrophilic and
radical reactions [8]. In the radical process (SCN) is produced by
N-thiocyanatosuccinimide (the analog of NBS) and followed by an
attack on the aromatic nucleus [9]. However, this reaction is accom-
panied by by-products that are difficult to remove from the reaction
mixture. The electrophilic reaction involves an initial oxidation of
thiocyanate anion and its recombination to the thiocyanogen dimer
(SCN)2. Then a polarization of the S–S bond is required in order
to generate a positive charge on one of the sulfur atoms, in order
to allow an electrophilic attack on the aromatic ring. Oxidation of
the thiocyanate anion has been carried out by various oxidizing
reagents, such as halogens [3,10], metal oxidants [11–13] and many
other organic and inorganic oxidants [14–20]. The drawback is that
these reactions involve the use of large amounts of the oxidizing
agent or toxic metal thiocyanate.
under acidic conditions the anodic oxidation of thiocyanate anion
to its radical favours dimerization over polymerization. In addition,
the electrochemical thiocyanation of anisole affords both regio- and
isomer-selectivities in good yields (Scheme 1).
In the present work we wish to expand the scope of the elec-
trochemical thiocyanation reaction to other aromatic derivatives
under similar conditions. Scheme 2 describes the various mono-
and disubstituted aromatic substrates that have been investigated
for this purpose.
2. Experimental
2.1. General
Aromatic compounds were supplied by Aldrich and Alfa and
used without further purification. Analytical grade glacial acetic
(supplied by Frutarom Co.) and formic 97% (supplied by Gadot
Co.) acids were used without further purification. For electro-
chemical measurements and electrolyses, a Princeton Applied
Research (PAR) Potentiostat/Galvanostat Model 173 and PAR
Universal Programmer Model 175, or a computerized PAR Potentio-
stat/Galvanostat Model 273A, were employed. Cyclic voltammetry
(CV) measurements were performed in a conventional three-
electrode cell. The working electrode was a platinum disk (ca. 1 mm
diameter), the reference electrode was Ag/AgCl (in 3 M NaCl), and
the auxiliary electrode was a Pt cylindrical gauze or wire.
Furthermore, the chemical thiocyanation reaction is far from
being selective because usually a mixture of isomers is formed, e.g.,
para and ortho isomers by thiocyanation of aromatic derivatives
[20], and thiocyanate and isothiocyanate isomers by addition to
alkenes [3].
For controlled potential electrolysis (CPE), an H-type two-
∗
Corresponding author. Tel.: +972 8 6461197; fax: +972 8 6472943.
E-mail address: becker@bgu.ac.il (J.Y. Becker).
ISE member.
compartment cell equipped with
a medium glass frit as a
1
membrane was used. The anode compartment contained a polished
0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2010.05.035

A. Gitkis, J.Y. Becker / Electrochimica Acta 55 (2010) 5854–5859
5855
cyanate group at 2155 cm−1
;
1H NMR (in CDCl3, 200 MHz) [21] ı:
3.83 (s, 3H, OCH3), 6.95 (d, J = 8.9 Hz, 2H, 2-H, 6-H), 7.51(d, J = 8.9 Hz,
2H, 3-H, 5-H); 13C NMR (in CDCl3, 200 MHz) ı: 55.5 (OCH3), 111.6
(SCN), 113.9, 117.4, 132.1, 135.4 (4 different aromatic carbons);
MS: m/z (%): M+ 165 (100), 150 (75), 139 (15), 122 (50), 95 (13), 63
(18).
2.1.1.2. 1-Methyl-4-thiocyanatobenzene
(4-thiocyanotoluene).
Scheme 1. Electrochemical thiocyanation of anisole as sample of substrate.
Solid (m.p. 40–41 ◦C [22]); IR: a sharp peak of thiocyanate group
at 2155 cm−1 1H NMR (in CDCl3, 200 MHz) ı: 2.50 (s, 3H, CH3),
;
7.05–7.23 (m, 4H, ring protons); MS: m/z (%): M+ 149 (100), 116
(75), 91 (90), 65 (18).
2.1.1.3. 1,3-Dimethoxy-4-thiocyanatobenzene. Oil [18]; IR: a sharp
peak of thiocyanate group at 2153 cm−1, 3013, 2938, 2838, 1595,
1481, 1306, 1205, 1166, 1071; 1H NMR (in CDCl3, 200 MHz) ı: 3.77
(s, 3H, OCH3), 3.85 (s, 3H, OCH3), 6.46 (s, 1H, 2-H), 6.48 (d × d, J = 8.2,
2.5 Hz, 1H, 6-H), 7.38 (d, J = 8.2 Hz, 1H, 5-H); 13C NMR (in CDCl3,
200 MHz) ı: 55.6 (OCH3), 56.1 (OCH3), 111.3 (SCN), 97.8, 101.1,
104.5, 107.9, 132.1, 135.3 (6 aromatic carbons); MS: m/z (%): M+
195 (100), 180 (45), 152 (25), 95 (10), 69 (12).
Scheme 2. Aromatic substrates.
silver wire quasi-reference electrode, immersed in electrolyte solu-
tion in a glass cylinder equipped with a fine glass frit at its end. Both
compartments contained either glacial acetic acid and 0.5 M LiClO4
(for entries 1, 10, 11, Table 1) or a mixture of formic and acetic
acids (1:1) and 0.1 M LiClO4 (for entries 2–9 and 12, Table 1) The
choice of solvent-electrolyte medium depends on the optimized
final outcome. Glacial acetic acid does not conduct electricity well
unless a higher concentration of electrolyte was used. The aromatic
substrates, and thiocyanate salt, NH4SCN (dried at 80 ◦C under vac-
uum, for 24 h), were added to the anode compartment. A magnetic
stirrer stirred the mixture during electrolysis (1–3 days) and for an
additional 24 h after it stopped. Electrolysis was terminated after
passing 1.5–2.2 F/mol.
CPE was conducted by controlling the potential at 1.25 V (vs.
Ag wire, which corresponds to ∼1.05 V vs. Ag/AgCl). This applied
potential is based on the observation that the thiocyanate anion
is oxidized at ∼1 V (vs. Ag/AgCl) in acetic acid [21]. A platinum
foil (5 cm2) working electrode and a stainless-steel counter elec-
trode were used. The anode compartment contained 5 mmol of
aromatic substrate and 2 mmol of NH4SCN, both dissolved in 30 ml
of solvent-electrolyte solution. Initial current was typically ∼5 mA
in acetic acid or >20 mA in mixtures of formic/acetic acids and at the
end of electrolysis it reached a value of ∼1 mA. Pulsing (to 0 V for
0.5 s, every 50 s) was required during electrolysis to avoid passiva-
tion of the working electrode surface, probably due to the formation
of the insulating polymer, parathiocyanogen. The reaction mixture
was filtered and treated twice with 30 ml of saturated aqueous NaCl
and 30 ml of CH2Cl2. After phase separation, the organic layer was
washed with three portions of saturated NaHCO3 solution, then
dried over MgSO4, and filtered. The solvent CH2Cl2 was evaporated
by a rotavapor until reaching a final volume of ∼1 ml. A sample of
the residue was checked by glc and the yield of the product was
determined by a calibration curve of an external standard consist-
ing of 1-methoxy-4-thiocyanatobenzene [21].
2.1.1.4. 1,2-Dimethoxy-4-thiocyanatobenzene. Solid m.p. 48–50 ◦C
(m.p. 49–50 ◦C [23]); IR: a sharp peak of thiocyanate group at
2155 cm−1, 3007, 2933, 2830, 1600, 1502, 1453, 1238, 1170; 1H
NMR (in CDCl3, 200 MHz) ı: 3.90–3.92 (s + s, 6H, OCH3), 6.88 (d,
J = 8.4 Hz 1H, 6-H), 7.05 (d, J = 2.2 Hz, 1H, 3-H), 7.15 (d × d, J = 8.4,
2.2 Hz, 1H, 5-H); 13C NMR (in CDCl3, 200 MHz) ı: 55.9 (OCH3), 56.0
(OCH3), 111.3 (SCN), 111.9, 113.7, 114.3, 125.1, 149.9, 150.7 (6 aro-
matic carbons); MS: m/z (%): M+ 195 (100), 180 (45), 152 (15), 125
(12), 94 (30).
2.1.1.5. 1,4-Dimethoxy-2-thiocyanatobenzene. Solid m.p. 67–69 ◦C
(m.p. 68–69 ◦C [23]); IR: a sharp peak of thiocyanate group at
2155 cm−1, 3002, 2935, 2830, 1600, 1502, 1450, 1240, 1166, 1037;
1H NMR (in CDCl3, 200 MHz) ı: 3.80 (s, 3H, OCH3), 3.87 (s, 3H,
OCH3), 6.87–6.88 (m, 2H, 2-H, 6-H), 7.12 (d, J = 1 Hz, 1H, 5-H); 13C
NMR (in CDCl3, 200 MHz) ı: 55.0 (OCH3), 55.8 (OCH3), 110.3 (SCN),
111.6, 113.0, 113.9, 114.7, 149.4, 153.3 (6 aromatic carbons); MS:
m/z (%): M+ 195 (80), 180 (100), 152 (25), 107 (10), 79 (12).
2.1.1.6. 1-Methoxy-3-methyl-4-thiocyanatobenzene. Solid
m.p.
1H
41–42 ◦C; IR: a sharp peak of thiocyanate group at 2153 cm−1
;
NMR (in CDCl3, 200 MHz) ı: 2.51 (s, 3H, CH3), 3.81 (s, 3H, OCH3),
6.78 (d × d, J = 8.6, 2.8 Hz, 1H, 6-H), 6.86 (d, J = 2.8 Hz, 1H, 2-H), 7.54
(d, J = 8.6 Hz, 1H, 5-H); 13C NMR (in CDCl3, 200 MHz) ı: 21.1 (CH3),
55.4 (OCH3), 111.4 (SCN), 114.6, 115.4, 118.6, 134.0, 137.3, 142.8
(6 aromatic carbons); MS: m/z (%): M+ 179 (100), 164 (50), 136
(20), 109 (18), 77 (17).
2.1.1.7. 1-Methoxy-2-methyl-4-thiocyanatobenzene. Oil (yellowish
[23]); IR: a sharp peak of thiocyanate group at 2155 cm−1 1H NMR
;
(in CDCl3, 200 MHz) ı: 2.22 (s, 3H, CH3), 3.85 (s, 3H, OCH3), 6.84 (d,
J = 8.5 Hz, 1H, 6-H), 7.36 (d, J = 2.5 Hz, 1H, 3-H), 7.38 (d × d, J = 8.6,
2.5 Hz, 1H, 5-H); 13C NMR (in CDCl3, 200 MHz) ı: 16.1 (CH3), 55.5
(OCH3), 111.2 (SCN), 111.4, 112.9, 129.6, 131.2, 134.1, 159.4 (6s,
ring carbons); MS: m/z (%): M+ 179 (100), 164 (80), 148 (15), 109
(15), 78 (20).
Constant current electrolyses were performed in a divided H-
type two-compartment cell equipped with a medium glass frit as
a membrane. Both electrodes were made of Pt foils (5 cm2) and
distant from each other by 3–5 mm. The volume of the electrolyte
solution (1:1 AcOH–HCOOH and 0.1 M LiClO4) in each compartment
was 20 ml and the ratio between substrate (5 mmol) and NH4SCN
(2 mmol) was 5:2. After passing the desired Coulombs, the solution
mixture was treated as before.
2.1.1.8. 1-Methoxy-4-methyl-2-thiocyanatobenzene. Oil (yellowish
[23]); IR: a sharp peak of thiocyanate group at 2155 cm−1 1H NMR
;
(in CDCl3, 200 MHz) ı: 2.32 (s, 3H, CH3), 3.88 (s, 3H, OCH3), 6.82
(d, J = 8.2 Hz, 1H, 6-H), 7.14 (d × d, J = 8.2, 2.0 Hz, 1H, 3-H), 7.36 (d,
J = 2.0 Hz, 1H, 5-H); 13C NMR (in CDCl3, 200 MHz) ı: 18.8 (CH3),
55.5 (OCH3), 113.2 (SCN), 111.3, 113.6, 129.0, 133.5, 136.1, 148.5 (6
2.1.1. Characterization of products
2.1.1.1. 1-Methoxy-4-thiocyanatobenzene (4-thiocyanatoanisole).
Solid m.p. 40–41 ◦C (m.p. 43–44 ◦C [20]); IR: a sharp peak of thio-

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A. Gitkis, J.Y. Becker / Electrochimica Acta 55 (2010) 5854–5859
Table 1
Electrochemical thiocyanation of substituted anisole, toluene and aniline derivatives under CPE conditionsa.
Entry
Substrateb
Ep(ox)/Vc
Product
Yield (%)
80%d
1
1.95
2
2.24
15%e
3
1.55
74%
4
5
6
7
1.49
66%
12%e
38%
60%
1.35 (E1/2, quasi-reversible)
1.74
1.79
8
1.60
10%e
9
2.15
13%e

A. Gitkis, J.Y. Becker / Electrochimica Acta 55 (2010) 5854–5859
5857
Table 1 (Continued )
Entry
Substrateb
Ep(ox)/Vc
Product
Yield (%)
42%
10
0.80 (E1/2)
11
0.83 (E1/2
)
15%f
12
1.36g
15%f
a
The oxidation peak potential of thiocyanate anion is ∼1 V (vs. Ag/AgCl). All substrates in entries 1–9 have peak potentials in the range of 1.35–2.2 V (vs. Ag/AgCl), well
above the oxidation potential of the thiocyanate anion. As to entries 10–12, see Section 3. The proper experimental conditions for each entry are described in Section 2. The
initial current was typically ∼5 mA in acetic acid and >20 mA in mixtures of acetic/formic acids.
b
No thiocyanation products were detected when the substrate contains a deactivating substituent such as, Cl, NHCOMe or NO2.
Oxidation peak potentials (vs. Ag/AgCl) were measured by CV in solutions of acetic–formic acids (1:1)–0.1 M LiClO4 on a glassy carbon working electrode; scan rate:
c
50 mV/s.
d
This result is taken from Ref. [21].
The rest is mostly unreacted starting material.
A formylation reaction occurred at the amino group converting the substrate to its formamide derivative, prior to thiocyanation.
This relatively high value is attributed to the oxidation of the formamide derivative, PhNHCHO.
e
f
g
aromatic carbons); MS: m/z (%): M+ 179 (100), 164 (30), 151 (55),
136 (15), 109 (15), 78 (35).
13C NMR (in CDCl3, 200 MHz) ı: 111.9 (SCN), 113.7, 114.4, 114.5,
114.7, 120.0, 120.9, 120.1, 168.3, 169.1 (aromatic carbons, for the
two forms); MS: m/z (%): M+ 178 (100), 150 (30), 118 (40), 106 (10),
80 (12).
2.1.1.9. 1,3-Dimethyl-4-thiocyanatobenzene. Oil (b.p. 133–134 ◦C at
12 Torr [24]); IR: a sharp peak of thiocyanate group at 2155 cm−1
;
1H NMR (in CDCl3, 400 MHz) ı: 2.33 (s, 3H, CH3), 2.45 (s, 3H, CH3),
7.06 (d, J = 8 Hz, 1H, 6-H), 7.11 (s, 1H, 2-H), 7.49 (d, J = 8, Hz, 1H, 5-H);
13C NMR (in CDCl3, 400 MHz) ı: 20.3 (CH3), 23.5 (CH3), 110.7 (SCN),
128.3, 129.0, 132.1, 132.6, 139.6, 140.7 (6 aromatic carbons); MS:
m/z (%): M+ 163 (100), 135 (90), 121 (15), 103 (20), 91 (20), 77 (25).
2.1.1.13. 1-Thiocyanatomethyl-2-methylbenzene. Oil; IR: a sharp
peak of thiocyanate group at 2147 cm−1
;
1H NMR (in CDCl3,
200 MHz) ı: 2.42 (s, 3H, CH3), 4.22 (s, 2H, CH2SCN), 7.27–7.28 (m,
4H, ring protons), 13C NMR (in CDCl3, 200 MHz) ı: 18.9 (CH3), 36.4
(CH2SCN), 111.8 (SCN), 126.5, 129.2, 130.1, 131.2, 131.8, 136.7 (6
aromatic carbons); MS: m/z (%): M+ 163 (5), 134 (3), 121 (3), 105
(100), 77 (20).
2.1.1.10. N,N-dimethyl-4-thiocyanatoaniline. Solid m.p. 70–72 ◦C
(m.p. 72–74 ◦C [25]); IR: a sharp peak of thiocyanate group at
2147 cm−1, 3452.5, 3010, 2942.5, 1615, 1510, 1367.5, 1112.5; 1H
NMR (in CDCl3, 200 MHz) ı: 3.02 (s, 6H, N(CH3)2), 6.68 (d, J = 9 Hz,
2H, 2-H, 6-H), 7.43 (d, J = 9 Hz, 2H, 3-H, 5-H); 13C NMR (in CDCl3,
200 MHz) ı: 40.1 (N(CH3)2), 112.6 (SCN), 111.5, 114.6, 132.8, 136.1
(4 aromatic carbons); MS: m/z (%): M+ 178 (100), 161 (15), 152
(20), 145 (35), 136 (13), 119 (13).
3. Results and discussion
Previously we have shown [21] that polar acidic solvents
promote polarization of the S–S bond in the electrochemically gen-
erated thiocyanogen dimer (SCN)2. The partial positively charged
species could then attack aromatic substrates to yield the desired
thiocyanation products, according to the mechanism outlined in
Scheme 3.
It has been postulated that the reason for the selectivity of the
thiocyanation of anisole to be both regio-selectivity (no ortho iso-
mer) and isomer-selectivity (no isothiocyanate) could be attributed
to the bulkiness and nature (it ‘blocks’ the nitrogen site), respec-
tively, of the thiocyanation species ‘Z’ mentioned in Scheme 3. In
addition, it is noteworthy that although the formal exhaustive elec-
trolysis requires 1 F/mol NH4SCN whereas in practice, 1.5–2 F have
passed. As can be seen from Scheme 3, more electricity is consumed
due to further oxidation of regenerated SCN−.
2.1.1.11. N-ethyl-4-thiocyanatoaniline. Solid m.p. 53–54 ◦C (m.p.
52–44 ◦C [26] IR: a sharp peak of thiocyanate group at 2155 cm−1
;
1H NMR (in CDCl3, 200 MHz) ı: 1.27 (t, J = 7.1 Hz, 3H, NCH2CH3),
3.16 (q, J = 7.2 Hz, 2H, NCH2CH3), 3.91 (s (broad), 1H, HN), 6.57 (d,
J = 8.8 Hz, 2H, 2-H, 6-H), 7.43 (d, J = 9 Hz, 2H, 3-H, 5-H; 13C NMR (in
CDCl3, 200 MHz) ı: 14.4 (NCH2CH3), 37.9 (NCH2CH3), 112.4 (SCN),
106.5, 113.4, 134.6, 150.0 (4 aromatic carbons); MS: m/z (%): M+
178 (45), 163 (100), 152 (10), 138 (10), 105 (15), 63 (10).
2.1.1.12. N-(4-thiocyanatophenyl)formamide. Oil (yellowish); IR: a
sharp peak of thiocyanate group at 2155 cm−1
;
1H NMR (in CDCl3,
In the present study, we have examined the in situ electro-
chemical thiocyanation of various aromatic compounds, including
substituted methoxybenzenes and anilines in the presence of
200 MHz) ı: 6.74 (m), 7.16 (m), 7.36 (m), 7.54 (m), 7.64 (m) (ring
H for the two forms), 8.4 (m), 8.7 (m) (for the two forms NHCHO);

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A. Gitkis, J.Y. Becker / Electrochimica Acta 55 (2010) 5854–5859
Scheme 3. A plausible mechanism for electrochemical thiocyanation.
electrochemically generated thiocyanogen. The outcome of thio-
cyanation products emerged from the mono- and disubstituted
aromatic substrates studied is summarized in Table 1. All substrates
in entries 1–9 have peak potentials in the range of 1.35–2.2 V (vs.
Ag/AgCl), well above the oxidation potential of the thiocyanate
anion (∼1 V vs. Ag/AgCl). As to the aniline derivatives in entries
10–12, see a relevant discussion later on. In general, the results
indicate that a high isomer-selectivity was achieved because all
products are exclusively thiocyanates with no formation of isothio-
cyanate isomers. This trend is in line with our previous observation
in the case of anisole [21].
The electron density on the aromatic substituted ring plays an
important role on the yield of the thiocyanation product. The pres-
ence of a strong electron-donating substituent such as a methoxy
group, highly activates the aromatic ring to give a good yield of
para substituted thiocyanate (Table 1, entry 1). However, the effi-
ciency of the thiocyanation reaction decreases considerably when
a weaker electron-donating substituent is employed (a methyl
group, entry 2).
attributed to the smaller activating effect of the methyl group with
respect to the methoxy group (ꢀp values for methyl and methoxy
groups are −0.31 and −0.78, respectively [27].
+
The observation that m-xylene gave a low yield of a thiocyanate
product (13%, entry 9) with a complex mixture of side-chain prod-
ucts and parathiocyanogen (SCN)n suggests that intermediate ‘Z’
is a weak electrophile. In addition, the electrophilic thiocyanation
process could be sensitive not only to the electron-donating abil-
ity of the substituent, but also to steric hindrance (Me vs. OMe).
Therefore, stronger activating groups, or at least as strong as the
methoxy substituent, are to be examined. Accordingly, amino sub-
+
stituents (ꢀp values for NH2 and NMe2 groups are −1.3 and −1.7,
respectively) appear to be good candidates for this purpose. How-
ever, a drawback of their use stem from the fact that aniline, and
its N-alkyl and N,N-dialkyl substituted derivatives are oxidized in
the range of 0.7–1 V [28] and therefore, all compete with the oxi-
dation of the thiocyanate anion. Consequently, for this very reason
and others (e.g., transformation of the substrates to other types of
products [29]), the yield of the thiocyanate products is not expected
to be high. Indeed, N,N-dimethylaniline gave only 42% of p-SCN-
C6H4NMe2 (entry 10). In the case of aniline and N-ethylaniline
(entries 11 and 12), in addition to the competition problem, both
substrates undergo hydroformylation (whenever formic acid is
present) presumably prior to thiocyanation, which cause the aro-
matic ring to be less susceptible towards an electrophilic attack.
This further diminishes the yield of the desired products. Appar-
ently, in both cases the major products were C6H4NHCOH and
C6H4N(Et)COH, respectively. N-ethylaniline afforded only 15% of
p-NCS-C6H4NHEt whereas aniline itself yielded 15% of the respec-
tive hydroformylated products, p-NCS-C6H4NHCOH (it is likely that
the hydroformylation took place prior the initial thiocyanation
reaction, causing a deactivation of the aromatic nucleus towards
thiocyanation). Notably that thiocyanation of N-ethylaniline and
toluidines was previously carried out under different experimen-
tal conditions (constant current, aqueous ethanol–HCl solution and
low temperature) in a two-step reaction (first electrochemical gen-
eration ofthiocyanogen andthen addition ofthe substrate)afforded
good yields (∼60%) of the desired thiocyanation products [30].
Finally, it is noteworthy that whenever the yield of the thiocya-
nation product goes down, the yield of parathiocyanogen goes up,
as evidenced by the color change in the solution (from colorless to
pale yellow to yellow to orange).
This finding could be accounted to the weak electrophilic nature
of ‘Z’.
In the case of disubstituted anisoles, the para position to one
of the methoxy groups in 1,3-dimethoxybenzene is activated by
both methoxy substituents towards an electrophilic attack. There-
fore, as expected, of the three dimethoxybenzenes, the highest yield
(74%) of the thiocyanate product has been obtained for this isomer
(entry 3). However, in the case of 1,2-dimethoxybenzene, the para
position is now activated by only one methoxy group (and deac-
tivated by the other) and therefore, the yield of the thiocyanate
product decreases (66%) (entry 4). For 1,4-dimethoxybenzene, the
para position is occupied and all available ortho positions for elec-
trophilic attack are activated and deactivated at the same time, in
addition to the steric hindrance applied by the bulkiness of species
‘Z’. As a result, a sharp decrease in the yield of the desired product
(12%) (entry 5) takes place.
A similar trend should be expected for the three isomers of
methylanisoles (entries 6–8), namely, when both substituents exert
an inductive effect at two different sites of the aromatic nucleus,
the more powerful activating group has a dominant influence, and
that is why the thiocyanate group occupies the para position to
the methoxy rather than to the methyl substituent in the case of
m-methylanisole (entry 6). The same argument applies when the
inductive effect of the two substituents opposes each other like in
the case of o-methylanisole (entry 7).
Surprisingly, the ortho isomer of methylanisole afforded a
higher yield (60%) of the thiocyanate product compared to that
obtained from the meta isomer (38%), in contrast to the trend
observed for the corresponding dimethoxybenzene isomers. This
result could be ascribed to the steric hindrance exerted by the
methyl group at its ortho position (in addition to the bulkiness of ‘Z’
described in Scheme 3), which is not applicable in the case of 1,3-
dimethoxybenzene (entry 3). It is noteworthy that the yield from
each isomer (entries 6–8) is lower than that from the correspond-
ing ones of the dimethoxybenzenes (entries 3–5). This outcome is
Attempts to investigate the electrochemical thiocyanation pro-
cess by carrying out electrolyses under constant current conditions
are described in Table 2 for some selective examples. Whereas sim-
ilar regio- and isomer-selectivities have been observed as in the
use of the CPE technique, the reaction has certainly become less
efficient now. Entries 1–3 are confined to anisole at different cur-
rent densities but with the same electricity consumption. Clearly,
the yield of the product p-thiocyanatoanisole increases (from 31
to 64%) with decreasing charge density (from 1 to 10 mA/cm2).
However, the low current density and duration (about 17 h) of elec-
trolysis of 2 mmol of thiocyanate anion by consuming just 1.5 F/mol
(entry 3) is far from being practical. With higher electricity con-

A. Gitkis, J.Y. Becker / Electrochimica Acta 55 (2010) 5854–5859
5859
Table 2
Constant current electrochemical thiocyanation of anisole and some representative disubstituted aromatic derivativesa.
Entry
Substrate
Product
Current density (mA/cm2)
Charge (Coulombs) [F/mol]
Yield (%)
1
2
3
4
10
4
1
300 [1.5]
300 [1.5]
300 [1.5]
450 [2.25]
31
58
64
75
1
5
4
4
300 [1.5]
300 [1.5]
29
13
6
a
In a mixture of acetic and formic acids (1:1), 0.1 M LiClO4 employing Pt electrodes and a divided cell; 5 mmol substrate and 2 mmol NH4SCN were used.
sumption (entry 4) the product’s yield increases to 75% but again,
about 26 h were required to get it. Anyway, just for comparison,
o-dimethoxybenzene (entry 5) and o-xylene (entry 6) were elec-
trolyzed under the same conditions used for anisole in entry 3,
and the results show that the yield of products were 29% (ring-
thiocyanation) and 13% (side-chain thiocyanation, respectively,
considerably lower than what was achieved by controlled potential
electrolysis for these substrates (Table 1). The lower yields could
be attributed to the simultaneous oxidation of the substrate and
the thiocyanate anion, which may lead to other types of products
besides the thiocyanation of the aromatic nucleus.
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An environmentally friendly electrochemical thiocyanation
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Acknowledgment
The authors are grateful to Mrs. E. Solomon for technical assis-
tance.