Cathodic reduction of hydroxycarbonyl compound trichloroacetyl esters

Article abstract of DOI:10.1016/j.tet.2003.09.051

New coumarins and new 2-(2′,2′-dichlorovinyl) phenols have been prepared by cathodic reduction under potentiostatic conditions of trichloroacetyl esters of o-hydroxyketones and o-hydroxyaldehydes in aprotic media. Electroreductions of trichloroacetyl esters of α-hydroxy-1,4- naphthoquinone, 3-hydroxy-2-methyl-4-pyrone, methyl salicylate and benzoin have also been investigated.

Full text of DOI:10.1016/j.tet.2003.09.051

Tetrahedron 59 (2003) 9161–9165
Cathodic reduction of hydroxycarbonyl compound
trichloroacetyl esters
*
Dolly, B. Batanero and F. Barba
´ ´
Department of Organic Chemistry, University of Alcala, 28871 Alcala de Henares, Madrid, Spain
Received 25 July 2003; revised 5 September 2003; accepted 16 September 2003
Abstract—New coumarins and new 2-(20,20-dichlorovinyl) phenols have been prepared by cathodic reduction under potentiostatic
conditions of trichloroacetyl esters of o-hydroxyketones and o-hydroxyaldehydes in aprotic media. Electroreductions of trichloroacetyl esters
of a-hydroxy-1,4-naphthoquinone, 3-hydroxy-2-methyl-4-pyrone, methyl salicylate and benzoin have also been investigated.
q 2003 Elsevier Ltd. All rights reserved.
1. Introduction
2. Results and discussion
In previous work, we have described an easy methodology
to prepare heterocycles such as 3-chloro-4-alkyl(or aryl)
coumarins,1,2 or 1-alkyl-4-alkyl(or aryl)-3-chloro quino-
linones3 in good yield by cathodic reduction of trichloro-
acetyl esters of o-hydroxyphenones or o-(N-alkyl)amino-
phenones, respectively. In these cases, the consumption of
the charge was corresponding to a 3e2/substrate molecule,
and after the first electron transfer the electrogenerated
radical-anion undergoes C–Cl cleavage to give a chlorine
radical and an organic anion. The latter is highly stabilized
by the electron withdrawing effect of two halogen atoms and
by the resonance effect of the adjacent carbonyl group.2
Electrolysis at a controlled potential of 21 V (vs SCE) of
trichloroacetyl esters of 1-acetyl-2-naphthol (1a), 2-acetyl-
1-naphthol (1b) and 1-acetyl-4,6-dibromophenol
(1c) yielded 3-chloro-4-methyl-benzo[f]-coumarin (2a),
3-chloro-4-methyl-benzo[h]-coumarin (2b) and 3-chloro-
5,7-dibromo-4-methyl-coumarin (2c), respectively, as
shown in Table 1.
It is a well known fact that aldehydes are more reactive
(vs nucleophiles) as compared to ketones, so we opted to
reduce the esters of salicylaldehydes 1(d–f) to further
improve the yields of the corresponding 3-chloro-
coumarins. Surprisingly, the desired coumarins were not
In the present work, we report one pot synthesis of
3-chlorocoumarins and styrenes by cathodic reduction of
trichloroacetyl esters of phenones and salicylaldehydes,
respectively. An interesting feature of the cathodic
reductions of salicylaldehydes is the formation of some
styrenes, which have never been described in literature. The
electrochemistry has been extensively applied not only
to reduce phenones, but also to trichloroacetyl esters of
a-hydroxy-1,4-naphthoquinone, 3-hydroxy-2-methyl-4-
pyrone, methyl salicylate and o-hydroxyphenyl aldehydes.
Table 1. Transformation of trichloroacetyl esters 1 into the corresponding
coumarins 2
Substrate
Product
Yield (%)
63
(1a)
(2a)
3-Halocoumarins possess hypnotic, insecticidal, and fungi-
cidal properties4,5 and can be used as potent inhibitors to
trigger vasoconstriction.6 However, in the literature only
three methods describe the direct synthesis of 3-chloro-4-
alkylcoumarins4,7 but these syntheses proceed with poor
yields and multiple steps.
45
62
(2b)
(2c)
(1b)
(1c)
Keywords: electrosynthesis; cathodic reduction; constant potential;
coumarins.
*
Corresponding author. Tel.: þ34-91-8854617; fax: þ34-91-8854686;
e-mail: fructuoso.barba@uah.es
0040–4020/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2003.09.051

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Dolly et al. / Tetrahedron 59 (2003) 9161–9165
Scheme 1.
obtained in expected yields. When esters of salicylalde-
hydes were reduced at 21.0 V (vs SCE), 3-chlorocoumarins
2(d–f) and a new product 3 were formed in 40–43% and
20–23% yield, respectively. These new products 3(d–f)
were isolated and identified as 2-(20,20-dichloro-
vinyl)phenols. The formation of 3 can be explained by the
mechanism proposed in Scheme 1.
In the cathodic reduction of salicylaldehyde esters, the
presence of substituents on the aromatic ring does not affect,
as expected, the yield ratio coumarin/styrene.
However, the electroreduction of trichloroacetyl esters of
hydroxy 1,4-naphthoquinone, 3-hydroxy-2-methyl-4-pyrone
and methylsalicylate only afforded the corresponding
hydroxy derivatives. In these cases the expected coumarins
were not formed due to the lower reactivity of these
carbonyl groups. The first electrogenerated anion evolves as
indicated in Scheme 2.
After the first electron transfer, two different conformations
X and Y are obtained for the cyclated anion. The
stereochemical environment of conformation Y facilitates
the formation of phenols 3. In the Y conformation the
negatively charged oxygen atom is proximal to the carbonyl
group. This approximation is possible only if the aldehydic
hydrogen atom is between the two chlorine atoms of the
adjacent carbon. In other words, the molecule adopts a
gauche conformation. However ketones, where the alde-
hydic hydrogen atom is replaced by an alkyl or aryl group,
the molecule in gauche conformation is sterically hindered,
for this reason, molecules prefer the staggered confor-
mation, and consequently only the expected coumarins
2(a–c) are formed. It explains why the corresponding
dichlorostyrenes were not obtained when esters of hydroxy-
ketones were reduced.
It has been reported in the literature that the attempts to
obtain coumarin from methylsalicylate by the chemical
methods have never been successful.7a
In the reduction of 1(a–c) and 1(d–f), where coumarins
were obtained, the experimental charge consumption was
always less than theoretically expected for a 3e2 process,
due to the formation, together with the coumarins, of the
corresponding phenols (Scheme 2) or styrenes, respectively,
in a 1e2 process. In the other cases, where coumarins were
not formed, the experimental charge consumption was the
corresponding to 1e2/substrate molecules.
Scheme 2.

Dolly et al. / Tetrahedron 59 (2003) 9161–9165
9163
Scheme 3.
Finally, the cathodic reduction of trichloroacetyl ester of
benzoin was carried out, leading to the formation of the
corresponding dichloroacetyl ester. It can be explained by
protonation of the first electrogenerated carbanion by the
OH on the highly stabilized enolic form of trichloroacetyl
ester of benzoin (Scheme 3).
1H, J¼9 Hz), 7.32–7.43 (m, 2H), 7.62–7.75 (m, 3H). 13C
NMR (75.4 MHz, CDCl3) d: 25.7, 90.0, 119.3, 124.5, 126.9,
128.1, 128.5, 129.4, 130.4, 131.4, 131.9, 143.6, 160.1,
201.3. MS m/e (relative intensity) EI: 334 (Mþþ4, 7), 332
(Mþþ2, 22), 330 (Mþ, 22), 319 (9), 317 (27), 315 (27), 186
(27), 171 (47), 168 (100), 157 (33), 141 (40), 129 (56), 114
(65).
3.1.2. 2-Acetyl-napthyltrichloroacetate (1b). IR (KBr) n
1
3. Experimental
(cm21): 3070, 1782, 1680, 1627, 1211, 765, 660. H NMR
(300 MHz, CDCl3) d: 2.71 (s, 3H), 7.61–7.66 (m, 2H),
7.87–7.94 (m, 3H), 8.12–8.17 (m, 1H). 13C NMR
(75.4 MHz, CDCl3) d: 29.7, 90.0, 122.2, 125.1, 125.5,
126.7, 127.3, 128.0, 128.2, 129.1, 136.3, 145.0, 160.0,
197.0. MS m/e (relative intensity) EI: 334 (Mþþ4, 2), 332
(Mþþ2, 7), 330 (Mþ, 7), 213 (5), 168 (100), 157 (22), 129
(25), 114 (22).
The electrolyses were carried out using an Amel potentio-
stat, Model 552, connected with an electronic coulombi-
meter-integrator Amel, Model 721.
Mass spectra (EI, ionizing voltage 70 eV) were determined
with a Hewlett–Packard Model 5988A mass-selective
detector equipped with a Hewlett–Packard MS Chem
Station. IR spectra were obtained, as dispersions in KBr,
on a Perkin–Elmer Model 583 spectrophotometer. 1H NMR
and 13C NMR (300 and 75.4 MHz, respectively) spectra
were recorded on a Varian Unity 300 apparatus with
deuteriochloroform as an internal standard. The chemical
shifts are given in ppm.
3.1.3. 2-Acetyl-4,6-dibromo-phenyltrichloroacetate (1c).
1
IR (KBr) n (cm21): 2981, 1760, 1694, 1207, 833, 677. H
NMR (300 MHz, CDCl3) d: 2.57 (s, 3H), 7.86 (d, 1H,
J¼2.3 Hz), 7.94 (d, 1H, J¼2.3 Hz). 13C NMR (75.4 MHz,
CDCl3) d: 29.1, 89.9, 119.2, 121.0, 132.4, 133.0, 139.5,
145.0, 158.8, 194.4. MS m/e (relative intensity) EI:
442 (Mþþ6, 5), 440 (Mþþ4, 10), 438 (Mþþ2, 9), 436
(Mþ, 3), 323 (8), 321 (16), 319 (8), 278 (30), 276 (54), 274
(25), 199 (18), 158 (14), 156 (15), 143 (23), 119 (93), 117
(100).
Melting points were determined on a Reichert Thermovar
microhot stage apparatus, and are uncorrected. Elemental
analyses were performed on a Perkin–Elmer Model 240-B
analyzer. The products were purified by silica gel 60 (35–70
mesh) in a (24£2.5 cm) column, using mixtures CHCl3/
hexane as eluent.
3.1.4. 4-Bromo-2-formyl-phenyltrichloroacetate (1d). IR
(KBr) n (cm21): 3094, 2878, 1784, 1694, 1593, 1209, 1105,
861, 840, 672. 1H NMR (300 MHz, CDCl3) d: 7.25 (d, 1H,
J¼8.4 Hz), 7.80 (dd, 1H, J1¼2.5 Hz, J2¼8.7 Hz), 8.05 (d,
1H, J¼2.5 Hz), 10.1 (s, 1H). 13C NMR (75.4 MHz, CDCl3)
d: 90.0, 121.4, 124.2, 129.2, 133.6, 138.5, 150.2, 159.9,
186.2. MS m/e (relative intensity) EI: 348 (Mþþ4, 2), 346
(Mþþ2, 3), 344 (Mþ, 2), 229 (32), 227 (33), 173 (10), 171
(10), 145 (22), 143 (22), 121 (29), 119 (100), 117 (100), 86
(59), 63 (85).
3.1. General procedure
Treatment of a-hydroxy carbonyl compounds with tri-
chloroacetyl chloride gave rise to electroactive esters (1)
quantitatively. Trichloroacetyl chloride was added dropwise
to a stirred solution of a-hydroxy ketones (or aldehydes)
in dry THF in the presence of equimolar amount of
triethylamine. The crude product was filtered and the
esters were obtained from the solution after removal of the
solvent.
3.1.5. 6-Methoxy-2-formyl-phenyltrichloroacetate (1e).
IR (KBr) n (cm21): 2950, 2846, 2768, 1785, 1700, 1581,
1
1445, 1282, 1253, 1207, 1071, 951, 817, 673. H NMR
(300 MHz, CDCl3) d: 3.9 (s, 3H), 7.24–7.29 (m, 1H), 7.37–
3.1.1. 1-Acetyl-napthyltrichloroacetate (1a). IR (KBr) n
(cm21): 3079, 1783, 1695, 1623, 1211, 856, 835, 801, 746,
7.49 (m, 2H), 10.21 (s, 1H). 13C NMR (75.4 MHz, CDCl3)
1
663. H NMR (300 MHz, CDCl3) d: 2.57 (s, 3H), 7.17 (d,

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Dolly et al. / Tetrahedron 59 (2003) 9161–9165
d: 56.7, 90.0, 118.4, 121.0, 128.2, 128.9, 140.0, 151.2,
160.0, 187.5. MS m/e (relative intensity) EI: 300 (Mþþ4, 1)
298 (Mþþ2, 2) 296 (Mþ, 3), 205 (6), 179 (21), 152 (100),
151 (65), 109 (35), 106 (46), 81 (28).
(22), 216 (65), 181 (52), 153 (80), 152 (100), 76 (26), 63
(20). Anal. calcd for C14H9ClO2: C, 68.71; H, 3.68. Found:
C, 68.67; H, 3.81.
3.2.3. 3-Chloro-5,7-dibromo-4-methyl-coumarin (2c).
62% Yield. Mp 161–1628C. IR (KBr) n (cm21): 3093,
2925, 1743, 1543, 1087, 1001, 866, 750. 1H NMR
(300 MHz, CDCl3) d: 2.56 (s, 1H), 7.67 (s, 1H), 7.88 (s,
1H). 13C NMR (75.4 MHz, CDCl3) d: 16.4, 111.7, 117.5,
122.1, 122.9, 126.7, 137.3, 141.5, 146.0, 147.4, 155.3. MS
m/e (relative intensity) EI: 356 (Mþþ6, 13), 354 (Mþþ4,
65), 352 (Mþþ2, 91), 350 (Mþ, 41), 291 (49), 289 (100),
287 (52), 182 (16), 180 (18), 136 (29), 101 (35), 75 (26).
Anal. calcd for C10H5Br2ClO2: C, 34.04; H, 1.42. Found: C,
34.26; H, 1.52.
3.1.6. 2-Formyl-phenyltrichloroacetate (1f). IR (NaCl) n
(cm21): 2862, 2759, 1791, 1699, 1608, 1211, 822, 754, 674.
1H NMR (300 MHz, CDCl3) d: 7.32 (d, 1H, J¼8.2 Hz), 7.49
(t, 1H, J¼7.4 Hz), 7.7 (dt, 1H, J1¼1.8 Hz, J2¼8 Hz), 7.95
(dd, 1H, J1¼1.8 Hz, J2¼7.7 Hz), 10.12 (s, 1H). 13C NMR
(75.4 MHz, CDCl3) d: 89.4, 122.4, 127.9, 128.0, 131.4,
135.8, 151.0, 160.1, 187.8. MS m/e (relative intensity) EI:
268 (Mþþ2, 1), 266 (Mþ, 1), 230 (2), 177 (4), 175 (6), 169
(4), 167 (13), 149 (100), 119 (66), 117 (67).
3.2. General electrochemical procedure
3.2.4. 3-Chloro-6-bromo-coumarin (2d). 40% Yield. Mp
1618C. IR (KBr) n (cm21): 3039, 1736, 1237, 993, 923, 817,
754. H NMR (300 MHz, CDCl3) d: 7.27 (m, 1H), 7.60–
The electrochemical reductions were carried out under the
following experimental conditions:
1
7.68 (m, 2H), 7.8 (s, 1H). 13C NMR (75.4 MHz, CDCl3) d:
117.8, 118.6, 120.3, 124.0, 129.5, 134.7, 138.7, 151.5,
157.0. MS m/e (relative intensity) EI: 262 (Mþþ4, 20), 260
(Mþþ2, 79), 258 (Mþ, 60), 234 (11), 232 (45), 230 (36),
169 (14), 167 (14), 125 (32), 123 (100), 97 (19), 87 (35), 62
(68). Anal. calcd for C9H4BrClO2: C, 41.62; H, 1.54. Found:
C, 41.91; H, 1.67.
Cathode—mercury pool. Catholyte—LiClO4 (1.7 g, 1.6£
1023 mol) in dry CH3CN (40 mL) and 1 (3£1023 mol).
Anode—platinum plate. Anolyte—LiClO4 (0.42 g,
0.4£1023 mol) in dry CH3CN (10 mL). Electrolysis cell—
divided cell equipped with a magnetic stirrer containing
a piece of glass tubing with a glass frit of medium porosity
at one end (anodic compartment). Solid potassium carbo-
nate (2.1 g, 1.5£1023 mol) was added to the anodic
compartment for in situ neutralization of the generated
perchloric acid.
3.2.5. 3-Chloro-8-methoxy-coumarin (2e). 42% Yield. Mp
1538C (lit.7a,b 1538C). Spectroscopic data (IR, and 1H NMR)
are coincident with those described in literature.7a,b
A constant cathodic potential of 21.0 V (vs SCE) was
applied during electrolysis of salicylaldehydes or ketones.
When the initial current value of 200 mA fell to 20 mA the
reduction was finished. The reaction time was in all the
cases about 1.5 h. At the end, the cathodic solution was
filtered and the solvent was evaporated under reduced
pressure. Extraction by ether/water followed by concen-
tration in vacuo led to the compounds 2 and 3, which were
purified by chromatography in silica gel, using CHCl3/
hexane (1:4, 3:7 or 2:3) as eluent.
13C NMR (75.4 MHz, CDCl3) d: 56.4, 113.8, 118.6, 119.5,
123.0, 125.1, 140.2, 142.2, 147.0, 156.9. MS m/e (relative
intensity) EI: 212 (Mþþ2, 35), 210 (Mþ, 100), 184 (5), 182
(15), 169 (8), 167 (22), 141 (12), 139 (36), 113 (6), 111 (18),
89 (10), 75 (19).
3.2.6. 3-Chloro-coumarin (2f). 43% Yield. Mp 1218C
(lit.4,7a,b 1218C). Spectroscopic data (IR, and 1H NMR) are
coincident with those described in literature.4,7a,b.
13C NMR (75.4 MHz, CDCl3) d: 116.8, 118.8, 122.3, 125.2,
127.4, 132.0, 140.2, 152.5, 157.3. MS m/e (relative
intensity) EI: 182 (Mþþ2, 34), 180 (Mþ, 100), 154 (19),
152 (57), 126 (7),124 (23), 89 (87), 63 (29).
3.2.1. 3-Chloro-4-methyl-benzo[h]-coumarin (2a). 63%
Yield. Mp 2298C. IR (KBr) n (cm21): 3077, 1731, 1635,
1090, 980, 808, 768. 1H NMR (300 MHz, CDCl3) d: 2.66 (s,
3H), 7.6–7.65 (m, 3H), 7.73 (d, 1H, J¼8.6 Hz), 7.85–7.87
(m, 1H), 8.53–8.55 (m, 1H). 13C NMR (75.4 MHz, CDCl3)
d: 16.9, 115.1, 120.5, 122.5, 122.9, 124.9, 127.5, 127.7,
128.9, 134.6, 148.3, 148.7, 157.1. MS m/e (relative
intensity) EI: 246 (Mþþ2, 34), 244 (Mþ, 100), 218 (20),
216 (58), 181 (75), 153 (56), 152 (94), 151 (36), 76 (27), 63
(22). Anal. calcd for C14H9ClO2: C, 68.71; H, 3.68. Found:
C, 68.57; H, 3.71.
3.2.7. 4-Bromo-2-(20,20-dichlorovinyl)-phenol (3d). 23%
Yield. Mp 57–598C. IR (KBr) n (cm21): 3391, 1615, 1404,
1
1175, 1110, 926, 807, 621. H NMR (300 MHz, CDCl3) d:
5.01 (bs, OH), 6.7 (d, 1H, J¼8.7 Hz), 6.92 (s, 1H), 7.28 (dd,
1H, J1¼2.6 Hz, J2¼8.7 Hz), 7.7 (d, 1H, J¼2.6 Hz). 13C
NMR (75.4 MHz, CDCl3) d: 113.0, 117.5, 122.4, 122.9,
132.0, 132.7, 152.1, 164.0. MS m/e (relative intensity)
EI: 270 (Mþþ4, 17), 268 (Mþþ2, 37), 266 (Mþ, 23), 235
(13), 233 (55), 231 (42), 207 (14), 205 (59), 203 (46),
154 (34), 152 (100), 123 (48), 89 (60), 63 (58). Anal. calcd
for C8H5BrCl2O: C, 35.82; H, 1.87. Found: C, 36.03; H,
1.79.
3.2.2. 3-Chloro-4-methyl-benzo[f]-coumarin (2b). 45%
Yield. Mp 136–1388C. IR (KBr) n (cm21): 3081, 2924,
1
1732, 1022, 825, 783, 747. H NMR (300 MHz, CDCl3) d:
3.05 (s, 3H), 7.47 (d, 1H, J¼8.9 Hz), 7.58 (t, 1H, J¼8.0 Hz),
7.66 (t, 1H, J¼8.6 Hz), 7.94 (d, 1H, J¼8 Hz), 7.98 (d, 1H,
J¼8.9 Hz), 8.49 (d, 1H, J¼8.5 Hz). 13C NMR (75.4 MHz,
CDCl3) d: 22.9, 114.5, 117.0, 121.8, 125.0, 125.6, 127.6,
129.3, 129.5, 131.4, 133.4, 149.2, 151.5, 156.4. MS m/e
(relative intensity) EI: 246 (Mþþ2, 33) 244 (Mþ, 99), 218
3.2.8. 6-Methoxy-2-(20,20-dichlorovinyl)-phenol (3e). 23%
Yield. Mp 37–398C. IR (KBr) n (cm21): 3450, 3065, 2940,
1
1614, 1579, 1360, 1266, 1071, 898, 719, 673. H NMR
(300 MHz, CDCl3) d: 3.88 (s, 3H), 5.9 (bs, OH), 6.75–6.9

Dolly et al. / Tetrahedron 59 (2003) 9161–9165
9165
(m, 2H), 7.1 (s, 1H), 7.3–7.42 (m, 1H). 13C NMR
(75.4 MHz, CDCl3) d: 56.3, 110.6, 119.4, 119.9, 120.8,
121.0, 123.2, 143.5, 146.3. MS m/e (relative intensity) EI:
222 (Mþþ4, 7), 220 (Mþþ2, 43), 218 (Mþ, 62), 185 (14),
183 (42), 177 (16), 175 (23), 170 (19), 168 (55), 157 (34),
155 (100), 111 (17). Anal. calcd for C9H8Cl2O2: C, 49.31;
H, 3.65. Found: C, 48.99; H, 3.67.
and Technology. Project BQU2001-1083. B. Batanero
´
thanks to this Ministry for Ramon and Cajal contract.
Dolly thanks Spanish Ministry of Education and Culture for
financial assistance.
References
3.2.8. 2-(20,20-Dichlorovinyl)-phenol (3f). 22% Yield. Mp
40–428C. IR (KBr) n (cm21): 3307, 3037, 1452, 1241,
1098, 912, 745. 1H NMR (300 MHz, CDCl3) d: 5.0 (bs, 1H,
OH), 6.84 (d, 1H, J¼7.9 Hz), 7.0 (t, 1H, J¼7.9 Hz), 7.06 (s,
1H), 7.2–7.3 (m, 1H), 7.64 (d, 1H, J¼7.9 Hz). 13C NMR
(75.4 MHz, CDCl3) d: 115.6, 120.9, 121.4, 123.0, 123.9,
129.9, 130.1, 152.7. MS m/e (relative intensity) EI: 192
(Mþþ4, 4), 190 (Mþþ2, 26), 188 (Mþ, 39), 155 (7), 153
(21), 127 (33), 125 (100), 89 (39), 63 (14). Anal. calcd for
C8H6Cl2O: C, 50.79; H, 3.17. Found: C, 50.83; H, 3.24.
´
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Acknowledgements
This work was supported by the Spanish Ministry of Science