974
F. Barba et al. / Electrochemistry Communications 12 (2010) 973–976
to afford azobenzenes as main products together with small amount
of diarylamines and biaryls.
3d: di-m-tolylamine C14H15N
MS (m/e, relative intensity): 197 (M+, 100), 196(33), 182(47),
181(82), 180(47), 167(45), 115(9), 91(20), 77(13), 51(10).
2. Experimental section
3e: bis(2,6-dimethylphenyl)amine C16H19N
MS (m/e, relative intensity): 225(M+, 55), 210(100), 195(61),
180(28), 131(26), 118(37), 91(46), 77(27), 51(14).
3f: 1,1′-(4,4′-azanediylbis(4,1-phenylene))di-ethanone C16H15NO2
MS (m/e, relative intensity): 253(M+, 27), 238(39), 167(100), 139
(23), 83(13), 63(7).
The electroactive diazoniumtetrafluoroborates were prepared
according to conventional methods (Organic Reactions. Ed Krieger, R.E.
publishing company. Huntington N.Y. 1977, vol.V, pp198–228).
The electrochemical reductions were performed under potentio-
static conditions in a concentric cell with two compartments
separated by a low porosity (D4) glass frit diaphragm and equipped
with a magnetic stirrer. The temperature was maintained constant at
0 °C with a cryostat. A mercury pool electrode (15 cm2) was used as
the cathode, a platinum plate (5 cm2) as the anode, and a saturated
SCE electrode as the reference. The SSE (solvent-supporting-electro-
lyte) system was dry acetonitrile containing 0.1 M lithium perchlo-
rate. The catholyte was 60 mL of SSE and the anolyte 20 mL of SSE.
K2CO3 was added to the anodic compartment in order to neutralize
the generated perchloric acid.
3g: dimethyl-4,4′-azanediyldibenzoate C16H15NO4
MS (m/e, relative intensity): 285(M+, 47), 254(51), 194(23),
167(100), 166(90), 140(30), 139(57), 111(18), 83(31).
3h: bis(4-bromophenyl)amine C12H9Br2N
MS (m/e, relative intensity): 329(M++4, 7), 327(M++2, 12),
325(M+, 7), 167(100), 139(19), 84(14), 50(8).
3. Results and discussion
The aryldiazonium tetrafluoroborate (1) (1.0 mmol) was added, in
small solid portions over 3 h, to the cathodic compartment, to be
electrolyzed at a constant potential of −1.0 V(vs SCE).
The cathodic reduction of different diazonium salts (1), at the
reduction potential of the second voltammetric peak of these systems
(−1.0 V, vs SCE), has been performed in a divided cell. The formation
of diarylazo compounds (2) as the main product is explained
according to Scheme 1.
Once the reaction was finished the solvent of the cathodic solution
was removed under reduced pressure. The residue was extracted with
ether/H2O. The organic phase was dried over MgSO4 and concentrated
by evaporation. The resulting solid was purified by chromatography
on silica gel 60 (35–70 mesh) in a (22×2.5 cm) column, using mixture
CHCl3/Hex (4:1) as eluent. The physical and spectroscopical proper-
ties of the obtained azobenzenes (2) were identical to those described
in the literature. The experimental MS spectra of 2 and 3 are given:
The following experimental procedure was required to get the
azoderivatives:
a) the diazonium salt had to be added in small solid portions during
the electrolysis to avoid a high concentration of radicals at the
electrode surface, and the faster dimerization to biaryls instead of a
further reduction of the radical to the desired anion (Scheme 2).
b) The temperature of the cell was maintained at 0 °C during the
electrolysis, with the help of a cryostat, in order to avoid thermal
decomposition of the diazonium salt.
2a: 1,2-diphenyldiazene C12H10N2
MS (m/e, relative intensity): 182(M+, 52), 152(30), 105(74),
77(100), 51(71).
2b: 1,2-bis(4-methoxyphenyl)diazene C14H14N2O2
MS (m/e, relative intensity): 242(M+, 23), 135(66), 107(100),
92(45), 77(93), 63(38).
The obtained yields in diarylazo compounds are summarized in
Table 1.
2c: 1,2-di-o-tolyldiazene C14H14N2
MS (m/e, relative intensity): 210(M+, 5), 167 (7), 119(5), 91
(100), 65(47).
2d: 1,2-di-m-tolyldiazene C14H14N2
MS (m/e, relative intensity): 210(M+, 20), 165(14), 119 (23),
91(100), 65(22).
2e: 1,2-bis(2,6-dimethylphenyl)diazene C16H18N2
MS (m/e, relative intensity): 238(M+, 4), 195(15), 105(100),
103(50), 91(9), 77(49), 51(18).
Scheme 1. Formation pathway of diarylazo compounds (2).
2f: 1,1′-(4,4′-(diazene-1,2-diyl)bis(4,1-phenylene))di-ethanone
C16H14N2O2
MS (m/e, relative intensity): 266(M+, 4), 167(33), 149(100),
119(46), 91(94), 65(15).
2g: dimethyl 4,4′-(diazene-1,2-diyl)dibenzoate C16H14N2O4
MS (m/e, relative intensity): 298(M+, 6), 163(53), 135(100),
119(17), 103(34), 77(13).
Scheme 2. Formation of biaryls.
2h: 1,2-bis(4-bromophenyl)diazene C12H8Br2N2
MS (m/e, relative intensity): 342(M++4, 3) 340(M++2, 6),
338(M+, 3), 185(56), 183(56), 157(100), 155(100), 75(25), 50
(32).
Table 1
Obtained yields of 1,2-diaryldiazenes (2).
3a: diphenylamine C12H11
N
Ar:
Yield of 2 (%)
M.p. (°C)
MS (m/e, relative intensity): 169 (M+, 100), 168(80), 154(3),
141(6), 115(7), 84(6), 77(3), 51(7).
a: C6H5
65
69
52
66
72
63
59
75
67 [Lit25:68]
b: 4-MeO-C6H4
c: 2-Me-C6H4
d: 3-Me-C6H4
e: 2,6-diMe-C6H3
f: 4-MeCO-C6H4
g: 4-MeOOC-C6H4
h: 4-Br-C6H4
163 [Lit26:163–64]
54 [Lit27:54–56]
52 [Lit28:50–52]
48 [Lit29:46–47]
193 [Lit30:194]
3b: bis(4-methoxyphenyl)amine C14H15NO2
MS (m/e, relative intensity): 229 (M+, 27), 214(100), 199(14),
171(13), 154(21), 143(34), 115(20).
3c: di-o-tolylamine C14H15
N
196–198 [Lit31:198–199]
203 [Lit32:205]
MS (m/e, relative intensity): 197(M+, 100), 180(98), 167(63),
106(34), 91(19), 77(22), 51(12).