516
Vol. 56, No. 4
anodic oxidation of some catechols and hydroquinones in the
presence of 3-hydroxy-1H-phenalene-1-one (3).17) We ob-
served an interesting diversity in the mechanism (ECE,
ECEC) of electrochemical oxidation of 1a and 1b in the
presence of 3. In the case of 2,5-dihydroxybenzoic acid (1a),
the final product is a quinone derivative, whereas in the case
of 3,4-dihydroxybenzaldehyde (1b), the final product (8b) is
a dihydroxybenzofuran derivative that was obtained after in-
tramolecular Michael addition reaction. Nature stability of p-
quinone in comparison with o-quinone in one hand and sta-
bility of final product arising from two intramolecular hydro-
gen bonding, on the other hand, are responsible for remain-
ing the final product in a quinone form. And finally, although
the experiments were conducted on a relatively small scale,
there is little difficulty in producing larger quantities either
by using larger cells or by running several cells in series.
Experimental
Apparatus Cyclic voltammetry, controlled-potential coulometry and
preparative electrolysis were performed using an Autolab model PGSTAT 20
potentiostat/galvanostat. The working electrode used in the voltammetry ex-
periments was a glassy carbon disc (1.8 mm2 area) and a platinum wire was
used as the counter electrode. The working electrode used in controlled-po-
tential coulometry and macroscale electrolysis was an assembly of four car-
bon rods (31 cm2) and a large platinum gauze constituted the counter elec-
trode. The working electrode potentials were measured versus SCE (all elec-
trodes from AZAR Electrodes).
Reagents All chemicals (2,5-dihydroxybenzoic acid, 3,4-dihhydroxy-
benzaldehyde and 3-hydroxy-1H-phenalene-1-one) were reagent-grade ma-
terials. Sodium acetate, solvents and reagents were of pro-analysis. These
chemicals were used without further purification.
Fig. 6. Cyclic Voltammograms of 0.1 mmol 3,4-Dihydroxybenzaldehyde
(1b) in the Presence of 0.1 mmol 3-Hydroxy-1H-phenalene-1-one (3) at a
Glassy Carbon Electrode during Controlled Potential Coulometry at 0.45 V
versus SCE. After consumption of (a) 0, (b) 6, (c) 13, (d), 20 (e) 27, and (f)
35 C. Scan rate 100 mV sꢁ1. Inset: variation of peak current (IpA1) versus
charge consumed. tꢀ25ꢂ1 °C
Electro-Organic Synthesis of 6a and 8b In a typical procedure, 60 ml
of acetate buffer solution (0.2 M, pHꢀ5.5) in water/acetonitrile (80/20) was
pre-electrolyzed at 0.45 V versus SCE, then 0.20 mmol of 1a or 1b and
0.20 mmol of 3 were added to the cell. The electrolysis was terminated when
the current decayed to 5% of its original value. The process was interrupted
during the electrolysis and the carbon anode was washed in tetrahydrofuran
(THF) in order to reactivate it. At the end of the electrolysis, a few drops of
acetic acid were added to the solution and the cell was placed in refrigerator
overnight. The precipitated solid was collected by filtration and washed sev-
eral times with water. After washing, products were characterized by IR, 1H-
NMR, 13C-NMR, and MS. The Faraday yields of 6a and 8a are more than
95% and the isolated yields of 6a and 8a after washing (in the case of 6a)
and recrystallization (in the case of 8a, the crude product was purified by re-
crystallization from a mixture of THF/acetonitrile in room temperature) are
62 and 67%, respectively.
Characterization of Products 6a and 8b. (6a) (C20H10O6): mp 125—
1
127 °C. H-NMR (300 MHz, DMSO-d6) d: 7.05 (d, Jꢀ14 Hz, 1H, quinone)
and 7.70 (d, Jꢀ13 Hz, 1H, quinone), 7.80—8.00 (m, 2H, aromatic), 8.30—
8.60 (m, 4H, aromatic), 9.8 (broad, 1H, –COOH), 12.85 (broad, 1H, enol-
OH). 13C-NMR, (125 MHz, DMSO-d6) d: 112.6, 115.1, 115.7, 116.3, 119.6,
120.7, 125.4, 125.5, 127.0, 127.4, 129.1, 130.1, 132.0, 132.9, 135.3, 148.3,
151.3, 161.3, 167.5, 178.5. IR (KBr) cmꢁ1: 3408, 3062, 1690, 1630, 1608,
1550, 1460, 1418, 1379, 1226, 1105, 978, 903, 824, 779. MS (EI): m/z (rela-
tive intensity): 346 (Mꢃ·, 2.1), 330 (100), 313 (26.8), 286 (41.5).
Chart 3
the oxidation of the parent-starting molecule (1b). The in-
tramolecular reaction (Chart 3, Eq. 4), performed via a 1,6-
addition reaction, leads to the formation of the final product
8b. The reaction product 8b can also be oxidized at a lower
potential than the starting 3,4-dihydroxybenzaldehyde (1b).
However, overoxidation of 8b was circumvented during the
preparative reaction because of the insolubility of product in
(8b) (C20H10O5): mp ꢄ230 °C (dec). IR (KBr) cmꢁ1: 3484, 3421, 1710,
1627, 1587, 1525, 1451, 1426, 1368, 1343, 1255, 1195, 1076, 893, 785. 1H-
NMR, d ppm (300 MHz, DMSO-d6) d: 6.90 (s, 1H, aromatic), 7.87—8.78
(m, 6H aromatic), 9.56 (broad 1H, OH), 10.26 (broad 1H, OH), 11.33
(broad, 1H, aldehyde). MS (EI): m/z (relative intensity): 330 (Mꢃ·, 2.08),
302 (1.48), 196 (33.3), 163 (11.9), 126 (33.3), 87 (14.3), 63 (26.19), 44
the electrolysis media. The synthesis of 8b has been per- (100).
formed using electrochemical oxidation of 3,4-dihydroxy-
benzaldehyde (1b) in the presence of 3 in water/acetonitrile
Acknowledgments The authors acknowledge to Bu-Ali Sina University
Research Council and Center of Excellence in Development of Chemical
Methods (CEDCM) for supporting this work.
(80/20) solution (pH 5.5, 0.2 M acetate buffer) in a undivided
cell at a potential less than the 0.45 V.
References
1) Blum R. H., Carter S. K., Ann. Int. Med., 80, 249—259 (1974).
2) Kaleem K., Chertok F., Erhan S., Prog. Org. Coating, 15, 63—71
(1987).
Conclusions
The present results complete the previous reports on the