Electrocarboxylation of chlorinated aromatic compounds

Article abstract of DOI:10.1135/cccc20000862

Chorinated benzenes (1, 4), biphenyls (6, 9), dibenzofurans (10, 15, 17, 18), 2-chlorodibenzo[1,4]dioxine (24) and 1-chloronaphthalene (26) as well as dibenzofuran (12) and naphthalene (27) themselves were transformed into carboxylic acids by galvanostatic electroreduction in the presence of carbon dioxide ("electrocarboxylation"). Dry DMF was used as solvent, zinc or stainless steel as cathode and magnesium as a sacrificial anode in an undivided cell. Hydrogenation of aromatic rings was not observed. However, reductive addition of two molecules of carbon dioxide to form dihydrodicarboxylic acids, e.g. 22 and 29, occurs in the dibenzofuran and naphthalene series.

Full text of DOI:10.1135/cccc20000862

862
Golinske, Voss, Adiwidjaja:
ELECTROCARBOXYLATION OF CHLORINATED AROMATIC
COMPOUNDS+
1
2,
Dirk GOLINSKE , Jürgen VOSS * and Gunadi ADIWIDJAJA
Institute of Organic Chemistry, University of Hamburg, Martin-Luther-King-Platz6,
1
20146 Hamburg, Germany; e-mail: golinske@cip.chemie.uni-hamburg.de,
2
voss@chemie.uni-hamburg.de
Received Jan uary 24, 2000
Accepted April 28, 2000
Presented at the 32nd Heyrovský Discussion on Organic Electrochemistry, Třeš3, June 6–10, 1999.
Ch orin ated ben zen es (1, 4), biph en yls (6, 9), diben zofuran s (10, 15, 17, 18), 2-ch loro-
diben zo[1,4]dioxin e (24) an d 1-ch loron aph th alen e (26) as well as diben zofuran (12) an d
n aph th alen e (27) th em selves were tran sform ed in to carboxylic acids by galvan ostatic
electroreduction in th e presen ce of carbon dioxide (“electrocarboxylation ”). Dry DMF was
used as solven t, zin c or stain less steel as cath ode an d m agn esium as a sacrificial an ode in an
un divided cell. Hydrogen ation of arom atic rin gs was n ot observed. However, reductive addi-
tion of two m olecules of carbon dioxide to form dih ydrodicarboxylic acids, e.g. 22 an d 29,
occurs in th e diben zofuran an d n aph th alen e series.
Key w ord s: Electroreduction ; Electrocarboxylation ; Carboxylic acids; Ch lorin ated aren es;
Diben zofuran s; Electroch em istry.
In th e last years we h ave successfully used th e electroreductive
deh alogen ation of several arom atic1–3 an d aliph atic ch lorides4,5 in m eth a-
n ol an d m eth an ol–water m ixtures as solven ts. Lead proved to be a suitable
cath ode m aterial. In th ese electrolyses, th e proton was th e on ly electroph ile
presen t wh ich could substitute th e h alogen . In th e followin g we presen t
our results on th e electroreduction of several ch lorin ated as well as
un substituted aren es an d h etaren es in DMF as aprotic solven t with carbon
dioxide as electroph ile in an un divided cell with a sacrificial an ode. Th e
form ation of carboxylic acids by th is m eth od h as already been described by
+ Part 33 in th e series Electroreduction of Organ ic Com poun ds; Part 32 see ref.1
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

Chlorinated Aromatic Compounds
863
several auth ors6. Th e electroch em ical process can be described by th e fol-
lowin g overall equation s:
Cath ode:
An ode:
ArCl + 2 e + CO2
Mg – 2 e
→
→
→
ArCO –2 + Cl–
Mg2+
Overall:
ArCl + CO2 + Mg
ArCO –2 + Cl– + Mg2+
Th e outcom e of th e electrolysis is th us iden tical with th e result of a Grig-
n ard reaction . Th e target of th ese in vestigation s was m ain ly to explore its
preparative poten tial. Our studies are, h owever, m ore directed to th e
deh alogen ation of com poun ds such as oligoch loroben zen es, -biph en yls,
-n aph th alen es an d -diben zofuran s. Th ese xen obiotics are h igh ly toxic an d
persisten t an d, th erefore, cause serious en viron m en tal problem s. Microbial
degradation of the m ost dangerous highly chlorinated substances in particular
is proh ibited an d th ere is a dem an d for m eth ods of lowerin g th eir ch lorin e
con ten t7. Carboxylation could, m oreover, h elp in crease th e bioavailability
of th e products as com pared with th e sim ple h ydrodech lorin ation .
EXPERIMENTAL
NMR spectra: Bruker WH 400 (400 MHz for 1H, 100.6 MHz for 13C; δ, ppm ; J, Hz). IR spec-
tra: ATI Mattson “Gen esis”-FT-IR (ν, cm –1). GC-MS: Fison s In strum en ts GC 8000 (equipped
with a 30 m SE 54 colum n ) an MD 800 quadrupole-MS (EI, 70 eV, m/z (rel.%)).
Dim eth ylform am ide (DMF) (Merck), tetrabutylam m on ium brom ide (Fluka), m agn esium
rods (Riedel de Haën ), 1,2,4,5-tetrach loroben zen e (1) (Merck), h exach loroben zen e (4) (Bayer
Leverkusen ), 4-ch lorobiph en yl (6) (Lan caster), 4,4′-dich lorobiph en yl (9) (Lan caster),
diben zofuran (12) (Merck), 2-ch lorodiben zo[1,4]dioxin e (24) (Prom och em ) an d
1-ch loron aph th alen e (26) (Merck) were com m ercially available.
Preparation of Startin g Com poun ds
1-(4-Chlorophenoxy)-2-nitrobenzene was prepared from 4-ch loroph en ol (16 g, 120 m m ol)
an d 2-ch loron itroben zen e ( 15.76 g, 100 m m ol) by a stan dard procedure8,9 in 80% yield
(20.0 g); yellow crystals; m .p. 40–41 °C (95% aqueous eth an ol), ref.8 m .p. 43–43.5 °C. 1H NMR
(CDCl3): 7.96 dd, 1 H, J(3,4) = 8.1, J(3,5) = 1.7 (H-3); 7.53 vtd, 1 H, J(4,5) = 7.9, J(3,5) = 1.7
(H-5); 7.33 AA′XX′, 2 H, J(AX) = J(A′X′) = 8.7, J(AA′) = J(XX′) = 2.8, J(A′X) = J(AX′) = 0.3
(H-3′, H-5′); 7.24 vtd, 1 H, J(3,4,5) = 7.9 J(4,6) = 1.7 (H-4); 7.03 dd, 1 H, J(5,6) = 8.4, J(4,6) =
1.3 (H-6); 6.98 AA′XX′, 2 H, J(AX) = J(A′X′) = 8.7, J(AA′) = J(XX′) = 2.8, J(A′X) = J(AX′) = 0.3
(H-2′, H-6′).
2-(4-Chlorophenoxy)aniline hydrochloride. Th e reduction of 1-(4-ch loroph en oxy)-2-n itro-
ben zen e was ach ieved with h ydrazin ium h ydroxide an d Ran ey n ickel in boilin g eth an ol10
.
Precipitation of th e h ydroch loride was perform ed with con cen trated HCl. Recrystallisation
of th e crude product from eth an ol yielded 76% of grey crystals, m .p. 176–177 °C, ref.8 m .p.
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864
Golinske, Voss, Adiwidjaja:
181–183 °C. IR (KBr): 2 965, 2 940, 2 925, 2 907, 2 740, 2 721, 2 649, 2 345, 1 943, 1 482,
1 093. 1H NMR (CD3OD): 7.56 dd, 1 H, J(3,4) = 8,0, J(3,5) = 1.6 (H-3); 7.49 AA′XX′, 2 H,
J(AX) = J(A′X′) = 8.5, J(AA′) = J(XX′) = 2.8, J(A′X) = J(AX′) = 0.3 (H-3′, H-5′); 7.45 vtd, 1 H,
J(4,5,6) = 7.8, J(3,5) = 1.3 (H-5); 7.27 vtd, 1 H, J(3,4,5) = 7.7, J(4,6) = 1.3 (H-4); 7.19 AA′XX′,
2 H, J(AX) = J(A′X′) = 8.5, J(AA′) = J(XX′) = 2.8, J(A′X) = J(AX′) = 0.3 (H-2′, H-6′); 6.99 dd,
1 H, J(3,4) = 8.0, J(4,6) = 1.3 (H-6).
2-(4-Chlorophenoxy)benzene-1-diazonium tetrafluoroborate.
A solution of 2-(4-ch loro-
ph en oxy)an ilin e h ydroch loride in water an d con cen trated HCl was cooled in an ice bath
with vigorous stirrin g8. Th e h ydroch loride was diazotized at 5 °C by a slow addition of
NaNO2 in H2O. Th e product was precipitated with NaBF4 from th e aqueous solution of th e
diazon ium ch loride in 65% yield, m .p. 148–149 °C, ref.8 m .p. 148–149 °C. IR (KBr): 2 274
(N≡N).
2-Chlorodibenzofuran (10). Th e 2-(4-ch loroph en oxy)ben zen e-1-diazon ium tetrafluoroborate
was added to iron (II) sulfate in boilin g water an d th e product was steam distilled to afford
62% of 10, m .p. 98–99 °C, ref.8 m .p. 102–103 °C, ref.11 m .p. 102 °C. 1H NMR spectrum was
in agreem en t with ref.11
.
4-Chlorodibenzofuran (17). A solution of 12 ( 5 g, 30 m m ol) in dry THF un der N2-atm o-
sph ere was cooled to –60 to –70 °C. Butyllith ium (25 m l, 40 m m ol) was added dropwise
with a syrin ge. Th e m ixture was allowed to warm up to –5 to 0 °C an d stirred for about 1 h .
Th e m ixture was cooled down again to –60 to –70 °C an d a solution of Cl2 in CCl4 (25 m l,
≈36 m m ol) was added dropwise. For an oth er 2 h th e m ixture was stirred at –5 to 0 °C an d
th en wash ed with an aqueous solution of Na2S2O5, wash ed with water an d dried over
MgSO4. Th e crude product was purified by colum n ch rom atograph y (silica gel, petroleum
eth er). Yield: 2.23 g (37%), m .p. 96.5 °C. 1H NMR (CDCl3): 7.93 dd, 1 H, J(8,9) = 8.1, J(7,9) =
1.0 (H-9); 7.83 dd, 1 H, J(2,3) = 8.5, J(1,3) = 1.0 (H-3); 7.64 dd, 1 H, J(6,7) = 8.1, J(6,8) = 1.0
(H-9); 7.49 vtd, 1 H, J(6,7,8) = 8.14, J(7,9) = 1.0 (H-7); 7.45 dd, 1 H, J(1,2) = 8.5, J(1,3) = 1.0
(H-1); 7.36 vtd, 1 H, J(7,8,9) = 8.1, J(6,8) = 1.0 (H-8); 7.26 vt, 1 H, J(1,2,3) = 8.5 (H-2).
2-(3,5-Dichlorophenoxy)-1-nitrobenzene was prepared from 3,5-dich loroph en ol an d
1-ch loro-2-n itroben zen e in th e sam e m an n er as 2-(4-ch loroph en oxy)n itroben zen e in 75%
yield. Yellow crystals, m .p. 78–79 °C (95% eth an ol). 1H NMR (CDCl3): 7.56 dd, 1 H, J(3,4) =
8,0, J(3,5) = 1.6 (H-3); 8.02 dd, 1 H, J(3,4) = 7.9, J(3,5) = 1.8 (H-3); 7.61 ddd, 1 H, J(5,6) =
7.8, J(4,5) = 7.1, J(3,5) = 1.8 (H-5); 7.35 ddd, 1 H, J(3,4) = 7.9, J(4,5) = 7.4, J(4,6) = 1.5 (H-4);
7.15 t, 1 H, J(2′,4′,6′) = 1.5 J (H-4′); 7.14 dd, 1 H, J(4,5) = 7.1, J(4,6) = 1.5 (H-6); 6.90 d, 2 H,
J(2′,6′) = J(4′,6′) = 1.5 (H-2′, H-6′).
2-(3,5-Dichlorophenoxy)aniline hydrochloride. Th e reduction of 1-(3,5-dich loroph en oxy)-
2-n itroben zen e was carried out as described above. Recrystallisation of th e crude product
from eth an ol yielded 91% of 2-(3,5-dich loroph en oxy)an ilin e h ydroch loride as grey crystals,
m .p. 167–168 °C. 1H NMR (m eth an ol-d4): 7.59 dd, 1 H, J(5,6) = 7,9, J(4,6) = 1.6 (H-6); 7.51 ddd,
1 H, J(3,4) = 8.3, J(4,5) = 7.3, J(4,6) = 1.6 (H-4); 7.35 t, 1 H, J(2′,4′,6′) = 1.8 (H-4′); 7.3 ddd,
1 H, J(4,5) = 7.3, J(5,6) = 7.9, J(3,5) = 1.3 (H-5); 7.15 d, 2 H, J(2′,4′) = J(4′,6′) = 1.7 (H-2′,
H-6′); 7.11 dd, 1 H, J(3,4) = 8.3, J(3,5) = 1.3 (H-3).
2-(3,5-Dichlorophenoxy)benzene-1-diazonium tetrafluoroborate. The diazotation of 2-(3,5-dichloro-
ph en oxy)an ilin e h ydroch loride was carried out in th e sam e m an n er as described above. Th e
precipitation of th e product with an aqueous solution of NaBF4 yielded 76% of diazon ium
tetrafluoroborate, m .p. 127–128 °C. IR (KBr): 2 263 (N≡N).
1,3-Dichlorodibenzofuran
(18).
Th e
2-(3,5-dich loroph en oxy)ben zen e-1-diazon ium
tetrafluoroborate was added to iron (II) sulfate in boilin g water an d th e product was
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

Chlorinated Aromatic Compounds
865
steam -distilled to afford 65% of 18, m .p. 118–119 °C. 1H NMR (CDCl3): 8.19 d, 1 H, J(8,9) =
7,6 (H-9); 7.56 d, 1 H, J(5,6) = 8.1 (H-6); 7.51 ddd, 1 H, J(6,7) = 8.1, J(7,8) = 7.1, J(7,9) = 1.5
(H-7); 7.48 d, 1 H, J(2,4) = 1.5 (H-2); 7.40 ddd, 1 H, J(7,8) = 7.1, J(8,9) = 7.6, J(6,8) = 1.5
(H-8); 7.33 d, 1 H, J(2,4) = 1.5 (H-4).
Electrolyses. Gen eral Procedure
Th e electrolyses were carried out at about –5 to 0 °C in an un divided cell sh own in Fig. 1,
with a cylin drical stain less steel gauze or a zin c-plate cath ode con cen trically surroun din g a
rod of m agn esium as an ode. Th e solven t was DMF, wh ich was distilled just before use. A
low con cen tration of Bu4NBr as supportin g electrolyte was required to adjust th e in itial con -
ductivity sin ce ion ic species were produced durin g th e reaction . Th e batch cell h ad to be
cooled to dissipate th e h eat evolved durin g th e electrolysis an d to keep th e tem perature be-
low 0 °C. After bubblin g CO2 for about 30 m in to expel th e residual air, a low overpressure
(0.1 bar) of CO2 was applied. Th e electrodes were con n ected to a power supply an d a con -
stan t curren t of I = 0.25 A was passed th rough th e stirred m ixture. Every h our, a sm all sam -
ple was taken to m on itor th e decrease of th e substrate con cen tration an d th e in crease of th e
product con cen tration with respect to th e ch arge con sum ption . After th e electrolysis, th e re-
action m ixture was acidified with 6 M HCl un der ice-coolin g. All products an d residual sub-
strate were th en extracted th ree tim es with dieth yl eth er. Th e organ ic layer was extracted
with 6 M HCl to rem ove residual DMF an d th en dried over MgSO4. An aliquot part (50%) of
th e organ ic extract was treated with 2 M aqueous NaOH to separate carboxylic acids from
n eutral products. Th e aqueous layer was again acidified (pH con trol) an d extracted with di-
eth yl eth er wh ich , after evaporation , led to th e carboxylic acid or a m ixture of acids. Sam -
ples taken durin g th e electrolysis were acidified with 6 M HCl an d extracted with dieth yl
eth er. A solution of CH2N2 in dieth yl eth er was added dropwise to th e organ ic layer to form
th e m eth yl esters of th e carboxylic acids. Iden tification of th e products was perform ed by
th e GC-MS tech n ique. An aliquot part of 25% of th e organ ic extract was again derivatized
with an eth eral solution of CH2N2 an d supplied with an in tern al stan dard for quan titative
GC an alysis.
5
1
2
3
FIG. 1
Batch cell for electrocarboxylation s. 1 CO2 in -
let, 2 th erm om eter, 3 activated carbon ab-
sorber, 4 cath ode con tact, 5 an ode (Mg rod),
6 cath ode (stain less steel n et or Zn -sh eet), 7
m agn etic stirrin g bar
6
7
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866
Golinske, Voss, Adiwidjaja:
Electrocarboxylation of 1,2,4,5-Tetrachlorobenzene (1)
Startin g m aterial 1.92 g (8.9 m m ol); cath ode m aterial zin c plate; ch arge 2.8 F; recovery 79%;
current efficiency 36% (20% carboxylic acids); products (GC): 1,2,4,5-tetrachlorobenzene (1) 1%,
1,2,4-trich loroben zen e 8%, 1,2-dich loroben zen e 1%, 1,3-dich loroben zen e 2%, 1,4-dich loro-
ben zen e 9%, ch loroben zen e 4%, ben zen e 2%, 2,5-dich lorotereph th alic acid (3) 17%,
2,4,5-trich loroben zoic acid (2) 14%, 2,5-dich loroben zoic acid 6%, 4-ch loroben zoic acid 2%,
3-ch loroben zoic acid 1%, ben zoic acid 3%.
Dimethyl 2,5-dichloroterephthalate. M.p. 134–136 °C, ref.12 m .p. 138–140 °C. 1H NMR
(CDCl3): 7.92 s, 2 H (H-3, H-6); 3.96 s, 6 H (OCH3). 13C NMR (CDCl3): 163.8 (C=O), 133.2
(C-3, C-6), 132.9 (C-2, C-5), 131.5 (C-1, C-4), 52.5 (OCH3). MS: 266 (1) [M+], 264 (16) [M+],
262 (25) [M+], 233 (65) [M+ – OCH3], 232 (11) [M+ – OCH3], 231 (100) [M+ – OCH3], 205 (8)
[M+ – CO2CH3], 203 (12) [M+ – CO2CH3], 190 (11) [M+ – CO2CH3, – CH3], 188 (19) [M+
–
CO2CH3, – CH3], 146 (10) [M+ – 2 CO2CH3], 144 (13) [M+ – 2 CO2CH3,], 111 (15) [C6H2Cl+],
109 (35) [C6H2Cl+], 74 (42), 59 (39) [CO2CH+3].
Electrocarboxylation of Hexachlorobenzene (4)
Startin g m aterial 1.28 g (4.5 m m ol); cath ode m aterial zin c plate; ch arge 6.1 F; recovery 90%;
curren t efficien cy 16% (16% carboxylic acids); products (GC): h exach loroben zen e (4) 7%,
pen tach loroben zen e 1%, pen tach loroben zoic acid (5) 64%, tetrach lorotereph th alic acid 10%,
tetrach loroisoph th alic acid 8%.
Methyl pentachlorobenzoate. M.p. 97–97.5 °C, ref.13 m .p. 98–100 °C. MS: 312 (5) [M+], 310
(16), 308 (23), 306 (14), 281(20) [M+ – OCH3], 279 (65), 277 (100), 275 (63), 251 (12), 249
(18) [M+ – CO2CH3], 247 (12), 215 (9) [M+ – CO2CH3 – Cl], 214 (17), 212 (13), 181 (4) [M+
–
CO2CH3, – 2 Cl], 179 (10), 177 (12), 144 (13) ) [M+ – CO2CH3, – 3 Cl], 142 (21), 107 (11)
[M+ – CO2CH3 – 4 Cl], 106 (4), 59 (8) [CO2CH+3].
Dimethyl tetrachloroterephthalate. M.p. 157 °C, ref.12 m .p. 156–157 °C. 13C NMR (CDCl3):
163.4 (C=O), 136.4 (C-1, C-4), 129.5 (C-2, C-3, C-5, C-6), 53.6 (OCH3). MS: 334 (12) [M+],
332 (26), 330 (18), 305 (11) [M+ – OCH3], 303 (50), 301 (100), 299 (82), 275 (2) ) [M+
–
CO2CH3], 273 (4), 271 (3), 225 (5), 223 (19), 221 (18), 216 (3), 214 (6), 212 (5), 181 (4), 179
(8), 177 (8), 59 (13) [CO2CH+3].
Dimethyl tetrachloroisophthalate. 13C NMR (CDCl3): 163.5 (C=O), 134.0 (C-4, C-6), 132.1
(C-5), 132.6 (C-2), 126.9 (C-1, C-3), 53.6 (OCH3).
Electrocarboxylation of 4-Chlorobiphenyl (6)
A. Startin g m aterial 1.68 g (8.9 m m ol); cath ode m aterial steel gauze; ch arge 2.5 F; recov-
ery 99%; curren t efficien cy 39%; products (GC): 4-ch lorobiph en yl (6) 7%, biph en yl (7) 26%,
biph en yl-4-carboxylic acid (8) 26%, dih ydrobiph en ylcarboxylic acids 40%.
B. Startin g m aterial 1.68 g (8.9 m m ol); cath ode m aterial Pb plate; ch arge 1.8 F, recovery
91%; curren t efficien cy 55%; products (GC): 4-ch lorobiph en yl (6) 8%, biph en yl (7) 23%,
biph en yl-4-carboxylic acid (8) 20%, dih ydrobiph en ylcarboxylic acids 41%.
Biphenyl-4-carboxylic acid (8). M. p. 224 °C, ref.14 m .p. 225–226 °C. 1H NMR ((CD3)2CO):
8.08 AA′XX′, 2 H, J(AX) = J(A′X′) = 8.1, J(AX′) = J(A′X) = 1.9, J(AA′) = J(XX′) = 0.5 (H-3, H-5);
7.68 AA′XX′, 2 H, J(AX) = J(A′X′) = 8.1, J(AX′) = J(A′X) = 1.9, J(AA′) = J(XX′) = 0.5 (H-2, H-6);
7.60 m , 2 H (H-2′, H-6′); 7.38 m , 2 H (H-3′, H-5′); 7.29 m , 1 H (H-4′).
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Chlorinated Aromatic Compounds
867
Electrocarboxylation of 4,4′-Dichlorobiphenyl (9)
Startin g m aterial 2.0 g (8.9 m m ol), cath ode m aterial steel gauze; ch arge, recovery an d cur-
ren t efficien cy could n ot be determ in ed because th e product m ixture was n ot fully an alyzed;
products (GC, relative yields): 4,4′-dich lorobiph en yl (9) 4%, 4-ch lorobiph en yl (6) 3%,
biph en yl (7) 6%, biph en yl-4-carboxylic acid (8) 38%, oth er carboxylic acids 49%.
Electrocarboxylation of 2-chlorodibenzofuran (10)
A. Startin g m aterial 1.803 g (8.84 m m ol); cath ode m aterial steel gauze; ch arge 36 F; recovery
75%; curren t efficien cy 3%; products (GC): 2-ch lorodiben zofuran (10) 49%, diben zofuran
(12) 3%, diben zofuran -4-carboxylic acid (11) 1%, 2-ch lorodiben zofuran carboxylic acid 1%,
diben zofuran -1,4-dicarboxylic acid (13) 7%, 2-ch lorodiben zofuran -1,4-dicarboxylic acid (14)
13%.
B. Startin g m aterial 1.803 g (8.84 m m ol); cath ode m aterial zin c plate; ch arge 8.1 F; recovery
76%; curren t efficien cy 12%; products (GC): 2-ch lorodiben zofuran (10) 6%, diben zofuran
(12) 1%, diben zofuran -4-carboxylic acid (11) 7%, 2-ch lorodiben zofuran carboxylic acid 11%,
diben zofuran -1,4-dicarboxylic acid (13) 30%, 2-ch lorodiben zofuran -1,4-dicarboxylic acid
(14) 20%.
Methyl dibenzofuran-4-carboxylate. M.p. 94.5 °C. 1H NMR (CDCl3): 8.14 dd, 1 H, J(2,3) =
7.6, J(1,3) = 1.5 (H-3); 8.11 dd, 1 H, J(1,2) = 7.6, J(1,3) = 1.5 (H-1); 7.96 dd, 1 H, J(8,9) = 7.6,
J(7,9) = 1.0 (H-9); 7.71 dd, 1 H, J(6,7) = 7.6, J(6,8) = 1.0 (H-6); 7.51 vtd, 1 H, J(6,7,8) = 7.6,
J(6,7) = J(7,8) = 1.0 (H-7); 7.41 vt, 1 H, J(1,2,3) = 7.6 (H-2); 7.38 vtd, 1 H, J(7,8) = J(8,9) = 7.6,
J(6,8) = 1.0 (H-8); 4.05 s, 3 H (OCH3). 13C NMR (CDCl3): 165.4 (C=O), 156.5 (C-5a), 154.9
(C-4a), 129.3 (CH), 127.8 (CH), 126.0 (Cq), 125.3 (CH), 123.2 (CH), 123.1 (Cq), 122.4 (CH),
120.6 (CH), 115.3 (Cq), 112.2 (C-6), 52.3 (OCH3).
Electrocarboxylation of 3-Chlorodibenzofuran11 (15)
A. Startin g m aterial 1.80 g (8.84 m m ol); cath ode m aterial zin c plate; ch arge 3.2 F; recovery
85%; curren t efficien cy 31%; products (GC): 3-ch lorodiben zofuran (15) 3%, diben zofuran
(12) 3%, diben zofuran -3-carboxylic acid (16) 71%, diben zofuran -1,4-dicarboxylic acid (13)
8%.
B. Startin g m aterial 1.80 g (8.84 m m ol); cath ode m aterial steel gauze; ch arge 9.6 F; recovery
99%; curren t efficien cy 13%; products (GC): 3-ch lorodiben zofuran (15) 35%, diben zofuran
(12) 15%, diben zofuran -3-carboxylic acid (16) 27%, diben zofuran -1,4-dicarboxylic acid (13)
22%.
Methyl dibenzofuran-3-carboxylate. 1H NMR (CDCl3): 8.25 d, 1 H, J(2,4) = 1.5 (H-4); 8.06 dd,
1 H, J(1,2) = 8.1, J(2,4) = 1.5 (H-2); 8.00 d, 1 H, J(1,2) = 8.1 (H-1); 7.99 dd, 1 H, J(8,9) = 8.1,
J(7,9) = 1.5 (H-9); 7.61 dd, 1 H, J(6,7) = 8.1, J(6,8) = 1.5 (H-6); 7.52 vtd, 1 H, J(6,7,8) = 8.1,
J(7,9) = 1.5 (H-7); 7.38 vtd, 1 H, J(7,8,9) = 7.6, J(6,8) = 1.5 (H-8); 3.98 s, 3 H (OCH3). MS: 226
(64) [M+], 195 (76) [M+ – OCH3], 167 (30) [M+ – CO2CH3], 139 (100), 113 (18), 98 (18), 87
(26) [C4H7O+2], 74 (28) [C3H6O+2], 63 (51), 59 (18) [CO2CH+3], 50 (34), 39 (42).
Dimethyl dibenzofuran-1,4-dicarboxylate. M.p. 142 °C (diethyl ether). 1H NMR (CDCl3): 8.77 dd,
1 H, J(8,9) = 8.1, J(7,9) = 1.5 (H-9); 8.11 d, 1 H, J(2,3) = 8.7 (H-3); 8.01 d, 1 H, J(2,3) = 8.7
(H-2); 7.72 dd, 1 H, J(6,7) = 8.1, J(6,8) = 1.5 (H-6); 7.58 vtd, 1 H, J(6,7,8) = 8.1, J(7,9) = 1.5
(H-7); 7.42 vtd, 1 H, J(7,8,9) = 8.1, J(6,8) = 1.5 (H-8), 4.08 s, 6 H (OCH3). MS: 284 (74) [M+],
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

868
Golinske, Voss, Adiwidjaja:
253 (81) [M+ – OCH3], 225 (9) [M+ – CO2CH3], 194 (13) [M+ – CO2CH3, – OCH3], 166 (20)
[M+ – 2 CO2CH3], 138 (33), 126 (22), 111 (22), 87 (23), 74 (12), 59 (100) [CO2CH+3], 39 (25).
Electrocarboxylation of 4-Chlorodibenzofuran (17)
Startin g m aterial 1.616 g (8.00 m m ol); cath ode m aterial zin c plate; ch arge 3.5 F; recovery
90%; curren t efficien cy 29%; products (GC): 4-ch lorodiben zofuran (17) 7%, diben zofuran
(12) 3%, diben zofuran -4-carboxylic acid (11) 57%, diben zofuran -1,4-dicarboxylic acid (13)
16%, un iden tified dih ydrodiben zofuran dicarboxylic acids 17%.
Electrocarboxylation of 1,3-Dichlorodibenzofuran (18)
Startin g m aterial 1.897 g (8.00 m m ol); cath ode m aterial zin c plate; ch arge 3.8 F; recovery
95%; curren t efficien cy 38%; products (GC): 1,3-dich lorodiben zofuran (18) 5%,
diben zofuran -1-carboxylic acid (20) 14%, diben zofuran -3-carboxylic acid (16) 11%,
1-ch lorodiben zofuran -3-carboxylic acid (19) 46%, 3-ch lorodiben zofuran -1-carboxylic acid
5%, diben zofuran -1,3-dicarboxylic acid (21) 11%, diben zofuran -1,4-dicarboxylic acid (13) 2%.
Methyl dibenzofuran-1-carboxylate. 1H NMR (CDCl3): 8.81 dd, 1 H, J(8,9) = 7.6, J(7,9) = 1.2
(H-9); 8.02 dd, 1 H, J(2,3) = 8.1, J(2,4) = 1.0 (H-2); 7.77 dd, 1 H, J(3,4) = 8.1, J(2,4) = 1.0
(H-4); 7.59 dd, 1 H, J(6,7) = 8.0, J(6,8) = 1.2 (H-6); 7.52 vtd, 1 H, J(6,7,8,) = 8.0, J(7,9) = 1.2
(H-7); 7.50 vt, 1 H, J(2,3,4) = 8.1 (H-3); 7.39 vtd, 1 H, J(7,8,9) = 7.6, J(6,8) = 1.2 (H-8); 4.10 s,
3 H (OCH3). 13C NMR (CDCl3): 167.1 (C=O), 156.9 (C-4a), 156.7 (C-5a), 128.3 (C-7), 126.2
(C-3, C-9), 125.6 (C-2), 125.3 (Cq), 124.2 (Cq), 122.92 (C-8), 122.88 (Cq), 116.0 (C-4), 111.3
(C-6), 52.2 (OMe). MS: 226 (62) [M+], 195 (65) [M+ – OCH3], 167 (27) [M+ – CO2CH3], 139
(100).
Methyl 1-chlorodibenzofuran-3-carboxylate. 1H NMR (CDCl3): 8.76 dd, 1 H, J(8,9) = 8.2,
J(7,9) = 1.5 (H-9); 7.98 d, 1 H, J(2,4) = 1.9 (H-2); 7.74 d, 1 H, J(2,4) = 1.9 (H-4); 7.57 dd, 1 H,
J(6,7) = 8.2, J(6,8) = 1.5 (H-6); 7.53 vtd, 1 H, J(6,7,8) = 8.2, J(7,9) = 1.5 (H-7); 7.39 vtd, 1 H,
J(7,8,9) = 8.2, J(6,8) = 1.5 (H-8); 4.05 s, 3 H (OCH3). MS: 262 (31), 260 (100) [M+], 231 (24),
229 (75) [M+ – OCH3], 203 (12), 201 (32) [M+ – CO2CH3], 175 (22), 173 (65), 166, (16) [M+
–
CO2CH3, – Cl], 138 (46), 111 (24), 98 (19), 87 (46), 74 (24), 63 (32), 59 (15) [CO2CH+3], 50
(18), 39 (25) .
Dimethyl dibenzofuran-1,3-dicarboxylate. 1H NMR (CDCl3): 8.87 dd, 1 H, J(8,9) = 8.2, J(7,9) =
1.3 (H-9); 8.69 d, 1 H, J(2,4) = 1.3 (H-2); 8.40 d, 1 H, J(2,4) = 1.3 (H-4); 7.62 dd, 1 H, J(6,7) =
8.2, J(6,8) = 1.3 (H-6); 7.60 vtd, 1 H, J(6,7,8) = 8.2, J(7,9) = 1.3 (H-7); 7.42 vtd, 1 H, J(7,8,9) =
8.2, J(6,8) = 1.3 (H-8); 4.08 s, 3 H (OCH3).
Electrocarboxylation of Dibenzofuran (12)
Startin g m aterial 1.50 g (8.90 m m ol); cath ode m aterial zin c plate; ch arge, recovery an d cur-
ren t efficien cy could n ot be determ in ed because th e product m ixture was n ot fully an alyzed;
products (GC, relative yields): diben zofuran -1,4-dicarboxylic acid (13) 68%, 1,2-dih ydro-
diben zofuran -1,4-dicarboxylic acid (22) 20%, oth er carboxylic acids 11%.
Dimethyl 1,2,3,4-tetrahydro-3,4-methanodibenzofuran-trans-1,4-dicarboxylate (23). M.p.
113–113.5 °C (dieth yl eth er). 1H NMR (C6D6): 7.30 dd, 1 H, J(8,9) = 7.7, J(7,9) = 1.3 (H-9);
7.23 dd, 1 H, J(6,7) = 8.2, J(6,8) = 1.0 (H-6); 6.96 vtd, 1 H, J(6,7,8) = 7.7, J(7,9) = 1.0 (H-7);
6.84 vtd, 1 H, J(7,8,9) = 7.7, J(6,8) = 1.3 (H-8); 3.34 dd, 1 H, J(1,2) = 7.3, J(1,2′) = 2.4 (H-1);
2.14 dvt, 1 H, J(2,2′) = 14.6, J(1,2′,3) = 2,4 (H-2′); 1.91 dd, 1 H, J(10,10′) = 4.5, J(3,10′) = 9.4
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

Chlorinated Aromatic Compounds
869
(H-10′); 1.74 m , 1 H (H-3); 1.49 ddd, 1 H, J(6,7) = 8.0, J(2,2′) = 14.6, J(1,2) = 7.3, J(2,3) = 4.7
(H-2); 1.03 dd, 1 H J(10,10′) = 4.5, J(3,10) = 7.3 (H-10). 13C NMR (CDCl3): 173.9 (C=O),
171.4 (C=O), 154.4 (C-5a), 152.5 (C-4a), 127.2 (C-9a), 123.7 (C-8), 122.7 (C-7), 119.5 (C-6),
111.3 (C-9), 107.1 (C-9b), 52.5 (OCH3), 52.3 (OCH3), 36.2 (C-1), 27.2 (C-3), 27.1 (C-2), 22.7
(C-10), 22.2 (C-4). MS: 300 (17) [M+], 268 (53) [M+ – OCH3], 241 (24) [M+ – CO2CH3], 209
(42), 181 (100), 152 (12), 139 (3), 59 (7).
Electrocarboxylation of 2-Chlorodibenzo[1,4]dioxine (24)
Startin g m aterial 875 m g (4.00 m m ol); cath ode m aterial zin c plate; ch arge 5.8 F; recovery
94%; curren t efficien cy 17%; products (GC): 2-ch lorodiben zo[1,4]dioxin e (24) 71%,
diben zo[1,4]dioxin e 2%, diben zo[1,4]dioxin e-2-carboxylic acid (25) 21%.
Methyl dibenzo[1,4]dioxine-2-carboxylate. M.p. 104–105 °C, ref.15 m .p. 103–107.5 °C. 1H NMR
(CDCl3): 7.60 dd, 1 H, J(3,4) = 8.4, J(1,3) = 2.0 (H-3); 7.51 d, 1 H, J(1,3) = 2.0 (H-1); 6.89 m ,
4 H (H-6, H-7, H-8, H-9); 6.86 d, 1 H, J(3,4) = 8.4 (H-4); 3.90 s, 3 H (OCH3).
Electrocarboxylation of 1-Chloronaphthalene (26)
Startin g m aterial 1.30 g (8.00 m m ol); cath ode m aterial zin c plate; ch arge 3.1 F; recovery
92%; curren t efficien cy 32%; products (GC): 1-ch loron aph th alen e (26) 27%, n aph th alen e
(27) 17%, 1-n aph th oic acid (28) 37%, dih ydron aph th alen edicarboxylic acids 10%.
Methyl 1-naphthoate. MS: 186 (56) [M+], 155 (76) [M+ – OCH3], 127 (100) [M+ – CO2CH3],
115 (4), 101 (9), 87 (5), 77 (26) [C6H+5], 63 (18), 59 (3) [CO2CH+3], 51 (13), 39 (6).
Electrocarboxylation of Naphthalene (27)
Startin g m aterial 1.14 g (8.90 m m ol); cath ode m aterial zin c plate; ch arge, recovery an d cur-
ren t efficien cy could n ot be determ in ed because th e product m ixture was n ot fully an alyzed;
products (GC, relative yields): 1,4-dih ydron aph th alen e-trans-1,4-dicarboxylic acid (29) 30%,
1,2-dih ydron aph th alen e-trans-1,2-dicarboxylic acid (30) 30%, 1-n aph th oic acid (28) <5%,
oth er dih ydron aph th alen edicarboxylic acids 10%.
Dimethyl 1,4-dihydronaphthalene-trans-1,4-dicarboxylate. MS: 244 (2) [M+ – 2 H], 215 (3) [M+
–
OCH3], 214 (11) [M+ – CH3OH], 187 (22) [M+ – CO2CH3], 186 (3) [M+ – 60], 155 (30) [M+ – 91],
128 (100), [M+ – 2 CO2CH3], 59 (14) [CO2CH+3].
Dimethyl 1,2-dihydronaphthalene-trans-1,2-dicarboxylate. 1H NMR (CDCl3): 7.20–7.30 m ,
3 H, 7.09 m , 1 H, 6.54 dd, 1 H, J(3,4) = 9.6, J(2,4) = 2.2 (H-4); 6.06 dd, 1 H, J(3,4) = 9.6,
J(2,3) = 4.6 (H-3); 4.29 d 1 H, J(1,2) = 7.7 (H-1); 3.98 ddd, 1 H, J(1,2) = 7.7, J(2,3) = 4,6, J(2,4) =
2.2 (H-2); 3.75 s, 3 H (OCH3); 3.70 s, 3 H (OCH3). MS: 246 (3) [M+], 214 (8) [M+ – CH3OH],
187 (30) [M+ – CO2CH3], 186 (60), 155 (51) [M+ – 91], 128 (100) [M+ – 2 CO2CH3], 59 (19).
X-Ray Structure An alyses
Crystallograph ic data for th e structures reported in th is paper h ave been deposited with th e
Cam bridge Crystallograph ic Data Cen tre as supplem en tary publication n um ber
CCDC-143347 (m eth yl ester of 11), CCDC-138250 (dim eth yl ester of 13) an d CCDC-138251
(23). Copies of th e data can be obtain ed free of ch arge on application to CCDC, e-m ail:
deposit@ccdc.cam .ac.uk; cf. also th e ORTEP plots (Fig. 2) an d Table I. Th e structures were
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

870
Golinske, Voss, Adiwidjaja:
TABLE I
X-Ray structural data of 11-m eth yl ester, 13-dim eth yl ester an d 23
11-Meth yl ester
13-Dim eth yl ester
23
Ch em ical form ula
Form ula weigh t
Crystal system
C14H10O3
226.22
C16H12O5
284.26
C17H16O5
300.30
m on oclin ic
P21/c
triclin ic
triclin ic
Space group
P
P
1
1
Crystal color
wh ite
wh ite
wh ite
Lattice param eters, pm , °
a
= 1 633.9(2)
a = 1 028.3(1)
b = 1 060.2(1)
c = 1 301.9(1)
α = 80.62(1)
β = 73.27(1)
γ = 76.24(1)
1 313.4(2) 106
4
a = 826.2(1)
b = 910.4(1)
c = 1 125.1(1)
α = 108.08(1)
β = 91.19(1)
α = 115.12(1)
716.87(13) 106
2
b = 1 694.8(2)
c = 1 700.1(2)
α = 90.00
β = 166.800(10)
γ = 90.00
1 075.0(2) 106
4
Volum e of cell, pm 3
Z value
R factor
0.0774
0.0539
0.0744
Rw factor
0.1734
0.1480
0.1645
Goodn ess-of-fit-value
1.189
0.905
2.750
C1
C2
C9
C3
C4
C9b
C9a
O41
C8
C7
C4a
O5
C41
C5a
C6
O42
C42
FIG. 2
ORTEP plot of th e X-ray structure of 11-m eth yl ester. Th erm al ellipsoids are drawn on th e
50% probability level
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

Chlorinated Aromatic Compounds
871
solved by direct m eth ods usin g th e SIR97 program 16 an d refin ed by full-m atrix-block
least-squares on F2 usin g all data an d th e SHELXL97 program 17
.
RESULTS AND DISCUSSION
Electrocarboxylation in the Benzene Series
Electrocarboxylation of ben zen es with up to th ree ch loro substituen ts h as
been described by Perich on et al.6. Th ey h ave sh own th at h alogen ated
ben zen es can well be tran sform ed in to m on ocarboxylic acids with good to
excellen t yields. Our aim was to test th e electrocarboxylation of h igh er
ch lorin ated ben zen es because th ese are th e n otorious xen obiotics. We used
1,2,4,5-tetrach loroben zen e (1) an d h exach loroben zen e (4) as startin g m ate-
rial. On e in terestin g question was wh eth er di- or oligocarboxylic acids
would be form ed, wh ich does n ot occur in th e case of trich loroben zen e6.
Moreover, th e total exten t of deh alogen ation , eith er by h ydrode-
ch lorin ation or carboxylation , was im portan t. Th e result of th e electro-
carboxylation of 1 is sh own in Sch em e 1.
Cl
Cl
Cl
Cl
Cl
Cl
H
Cl
H
Cl
(i)
+
+
+
Cl
H
Cl
1 (1% recovered)
18%
9%
4%
2%
Cl
Cl
CO2H
Cl
Cl
CO2H
Cl
Cl
CO2H
Cl
+
+
HO2C
2, 14%
6%
3, 17%
(i) e, CO2, DMF; Mg anode, Zn cathode
SCHEME 1
Th e selectivity of th e reaction is low. In addition to a m ixture of n eutral
reduction products (33% overall yield) in cludin g ben zen e, th ree carboxylic
acids (37% in total) were form ed. Th e sam e n eutral com poun ds are form ed
if 1 is electrolyzed in m eth an ol in th e absen ce of carbon dioxide2. Un ex-
pectedly, 2,5-dich lorotereph th alic acid (3) was th e m ain acid form ed. Obvi-
ously, th e prim ary product 2,4,5-trich loroben zoic acid (2) is selectively
carboxylated in position 4 but 3 is n ot furth er reduced. No isoph th alic acid
derivatives were foun d.
Sin ce we used an aprotic solven t-supportin g electrolyte (dry DMF), th e
form ation of h ydrodech lorin ated products n eeds a com m en t. We can ex-
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

872
Golinske, Voss, Adiwidjaja:
clude traces of water an d even tetrabutylam m on ium ion s, wh ich can un -
dergo Hofm an n elim in ation , as th e source of proton s because th eir am oun t
is too sm all to accoun t for th e con siderable yields of proton ated products.
Th erefore, on ly DMF itself com es in to question as a source of proton s.
Th e electrocarboxylation of h exach loroben zen e (4) is sh own in Sch em e 2.
Th e deh alogen ation alm ost stops after th e replacem en t of on ly on e ch lo-
rin e atom . On ly sm all am oun ts of tetrach lorocarboxylic acids are foun d – a
result quite sim ilar to th e electrocarboxylation of 1. Pen tach loroben zoic
acid (5) is th e m ain product of th is reaction . Pen tach loroben zen e, th e prod-
uct of h ydrodech lorin ation , is on ly produced in trace am oun ts.
CO2H
Cl
Cl
CO2H
Cl
CO2H
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
(i)
+
+
+
CO2H
Cl
Cl
CO2H
Cl
Cl
Cl
Cl
4 (7% recovered)
5, 64%
1%
10%
8%
(i) e, CO2, DMF; Mg anode, Zn cathode
SCHEME 2
Electrocarboxylation in the Biphenyl Series
We h ave sh own earlier th at th e electroreduction of h alogen ated biph en yls
(PCB) in m eth an ol yields biph en yl (7) as expected, but also, in addition ,
h ydrogen ated products2. Th e in terestin g question was n ow, wh eth er th e
electrocarboxylation could suppress th is side reaction or n ot. Our experi-
m en ts were perform ed with 4-ch lorobiph en yl (6) an d 4,4′-dich lorobiph en yl
(9) as substrates. Th e products foun d after th e electrocarboxylation of 6 are
sh own in Sch em e 3. As expected, biph en yl-4-carboxylic acid (8) was form ed
but in a yield of on ly 26%, wh ereas 40% of a m ixture of th ree
dih ydrocarboxylic acids was also obtain ed. In addition , h ydro-
(i)
Cl
CO2H
+
6 (7% recovered)
7, 26%
8, 26%
(in Methanol: 47%)
CO2H
+
H2
(i) e, CO2, DMF; Mg anode, Zn cathode
SCHEME 3
40%
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

Chlorinated Aromatic Compounds
873
dech lorin ation to 7 took place, wh ich was, h owever, n ot furth er h ydroge-
n ated. Th e acid 8 was clearly iden tified after isolation . Its dih ydro-
derivatives could n ot be fully ch aracterized. But from th eir m ass spectra on e
could con clude th at all th e th ree sh ould be isom eric 4-ph en ylcyclo-
h exadien e-1-carboxylic acids, i.e. h ydrogen ation occurs in th e ben zen e rin g
of 8 carryin g th e carboxylic group.
Th e result of th e electrocarboxylation of 9 is sh own in Sch em e 4. Th e
acid 8 is form ed as th e m ain product wh ereas h ydrodech lorin ation to 6 an d
7 is on ly a m in or side-reaction . Again a con siderable am oun t of h ydroge-
n ated carboxylic acids of th e sam e type as in th e case of Sch em e 3 is form ed
as foun d by GC-MS an alysis. Neith er h ydrobiph en yls n or biph en yl-4,4′-di-
carboxylic acid were detectable.
e, CO2
unidentified
Cl
Cl
6
7
8
+
+
+
carboxylic acids
9 (4% recovered)
SCHEME 4
3%
6%
38%
49%
Electrocarboxylation in the Dibenzofuran Series
Direct electrolysis of h alogen ated diben zofuran s in protic solven ts pro-
duces, besides diben zofuran (12) as th e m ain product, also a relatively large
am oun t of dih ydrodiben zofuran s11,18. Th is is a crucial drawback of th is pro-
cedure because of th e toxicity of th ese products for m ost m icroorgan ism s.
To face th is disadvan tage, we h ave developed n ew procedures to avoid th e
form ation of dih ydrodiben zofuran s. Th e in direct electrolysis with m edia-
tors (e.g. Ni) h as turn ed out to be a suitable m eth od. It can be carried out in
m eth an ol as protic solven t an d leads to diben zofuran (12) in a very good
yield with out form ation of an y h ydrogen ated diben zofuran species1.
Electrocarboxylation could provide an altern ative because carboxylic acids
sh ould n ot be as toxic an d, in addition , sh ould also exh ibit an in creased
bioavailability.
Th e direct electroreduction of 2-ch lorodiben zofuran (10) is a slow process
because th e cleavage of th e Cl–C bon d requires th e h igh est activation en -
ergy in th e m on och lorodiben zofuran series. As a result, th e curren t effi-
cien cy is very low an d th e m ain product of th e direct electrolysis in protic
solven ts is 1,4-dih ydrodiben zofuran , in stead of diben zofuran 11 (12). Better
results are again ach ieved by use of a Ni m ediator1.
Electrocarboxylation of 10 at a zin c cath ode produces a m ultitude of
carboxylic acids. Stain less steel as cath ode m aterial is disadvan tageous sin ce
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

874
Golinske, Voss, Adiwidjaja:
th e turn over an d th e curren t efficien cy are low an d th e com plexity of th e
product m ixture is even m ore m arked (Sch em e 5). Surprisin gly, th e ex-
pected diben zofuran -2-carboxylic acid is n ot at all form ed. In stead, its iso-
m er, diben zofuran -4-carboxylic acid (11) can be isolated as m eth yl ester
Cl
Cl
(i)
+
+
O
O
O
O
CO2H
CO2H
11
12
steel (49% recovered)
zinc (6% recovered)
steel 1%
zinc 7%
1%
11%
3%
1%
CO2H
Cl
CO2H
+
+
O
O
CO2H
(i) e, CO2, DMF; Mg anode
CO2H
13
14
steel 7%
zinc 30%
13%
20%
SCHEME 5
O61
C62
C42
C61
C23
O62
O42
C24
C24a
C22
C21
C41
O32
O5
C6
C4
O25
C32
C29b
C5a
C9a
C4a
C31
C29a
C7
C8
C3
C2
C25a
O31
C9b
C29
C1
C26
C27
C9
C11
C28
O11
O12
C12
FIG. 3
ORTEP plot of th e X-ray structure of 13-dim eth yl ester. Two in depen den t m olecules with
sligh tly but n ot sign ifican tly differen t geom etrical param eters are located in th e asym m etric
un it. Th erm al ellipsoids are drawn on th e 50% probability level
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

Chlorinated Aromatic Compounds
875
from th e m ixture (7%). Its structure was elucidated by an alysis of th e 1H
an d 13C NMR spectra in cludin g correlation tech n iques an d even tually by
an X-ray structural an alysis (Fig. 2, Table I). Furth erm ore, it turn ed out to
be iden tical with th e electrocarboxylation product of 4-ch lorodiben zofuran
17 (see below).
Diben zofuran -1,4-dicarboxylic acid (13) was produced as th e m ain prod-
uct (30%). Also 2-ch lorodiben zofuran -1,4-dicarboxylic acid (14) an d
2-ch lorodiben zofuran m on ocarboxylic acids were foun d in con siderable
am oun ts.
We h ave proved th e structure of 13 by NMR spectroscopy an d, un equivo-
cally, by an X-ray diffraction an alysis of its dim eth yl ester (Fig. 3, Table I).
Th e form ation of 11, 13, an d 14 n eeds an explan ation . Obviously, 10 an d
th e dech lorin ated 12 can be directly substituted with carboxyl groups dur-
in g electrolysis in an overall carboxydeproton ation process. Reductive addi-
tion of two carbon dioxide m olecules to aren es un der form ation of
dih ydrodicarboxylic acids – th ough with out rearom atization – is kn own
from th e literature6,19. In order to prove th is possibility, we h ave perform ed
th e direct electrocarboxylation of 12 an d con firm ed our results (see below).
Th e electrocarboxylation of 3-ch lorodiben zofuran (15) leads to
diben zofuran -3-carboxylic acid (16) as th e m ain product ( Sch em e 6), as ex-
pected. Especially at a zin c cath ode 16 is form ed with h igh yield an d selec-
tivity. Th e by-products 12 an d 13 are form ed in on ly sm all am oun ts. Th e
application of stain less steel as cath ode leads to a con siderably sm aller over-
all yield of on ly 65%. Furth erm ore, th e product selectivity is n ot satisfac-
tory (cf. Fig. 4).
(i)
12
13
+
+
O
Cl
O
CO2H
15
16
steel (36% recovered)
zinc (3% recovered)
27%
71%
15%
3%
22%
8%
(i) e, CO2, DMF; Mg anode
SCHEME 6
As m en tion ed above, 13 is form ed by direct electrocarboxylation of 12.
Th is reaction occurs as a follow-up reaction an d is th e reason for th e form a-
tion of 13 durin g th e electrolysis of 15 an d of an y oth er ch lorin ated
diben zofuran in th e presen ce of carbon dioxide. Th is is dem on strated in
Fig. 5.
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

876
Golinske, Voss, Adiwidjaja:
Diben zofuran -4-carboxylic acid 11 was th e m ain product obtain ed by
electrocarboxylation of 4-ch lorodiben zofuran 17. Th e selectivity was rath er
h igh (yield of 11: 57%). Diben zofuran 12 an d th e dicarboxylic acid 13 as
well as several un iden tified dih ydrodiben zofuran dicarboxylic acids were
foun d as by-products (Sch em e 7).
Th e electrocarboxylation of 1,3-dich lorodiben zofuran (18) leads to a m ul-
titude of products (Sch em e 8). Neverth eless, th e reaction sh ows a certain se-
90
80
70
%
60
50
40
30
20
stainless steel
10
0
zinc
15
12
16
13
FIG. 4
Product distribution after electrocarboxylation of 15 at zin c or stain less steel cath odes (cf.
Sch em e 6)
120
Yield, %
100
15
80
60
40
20
0
16
13
12
0
900
1800
2700
3600
Charge, As
4500
FIG. 5
Course of electrocarboxylation of 15 at a Zn cath ode
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

Chlorinated Aromatic Compounds
877
(i)
dihydrodibenzofuran-
dicarboxylic acids
11
12
13
+
+
+
O
Cl
17%
17 (7% recovered)
57%
3%
16%
(i) e, CO2, DMF; Mg anode, Zn cathode
SCHEME 7
lectivity. Reaction tem peratures below 0 °C favour th e form ation of
1-ch lorodiben zofuran -3-carboxylic acid (19) as th e m ain product. However,
th ere is on ly a relatively low level of deh alogen ation (cf. Sch em e 8). An in -
crease of th e tem perature above 0 °C leads to a lower selectivity but causes a
h igh er level of deh alogen ation , i.e., th e form ation of m ore diben zo-
furan -1-carboxylic (20)20, -3-carboxylic (16) an d -1,3-dicarboxylic acids (21).
CO2H
Cl
Cl
e, CO2
+
16
+
O
Cl
O
Cl
O
CO2H
18 (5% recovered)
19, 46%
5%
11%
CO2H
CO2H
+
13
+
+
O
O
CO2H
20, 14%
21, 11%
2%
SCHEME 8
Diben zofuran (12) itself was studied because we wan ted to verify th e for-
m ation of 13 from an y ch lorodiben zofuran via 12 as an in term ediate.
Sch em e 9 sh ows th at th e electrocarboxylation of 12 leads in deed to 13 with
good selectivity an d a yield of 68%. In addition , oth er carboxylic acids are
form ed. On e crystallin e com pon en t could be isolated in 20% yield from th e
m ixture of m eth yl esters, wh ich were prepared by use of diazom eth an e. Ac-
cordin g to its NMR spectra, we assign ed a cyclopropan o structure (23) to
th is ester an d could con firm it by X-ray structural an alysis of a sin gle crys-
tal. Figure 6 sh ows an ORTEP plot of 23, wh ich also dem on strates th e trans
con figuration of th e two ester groups (cf. also Table I). Obviously, 23 is
form ed from th e dih ydrodicarboxylic acid 22 in th e reaction with diazom e-
th an e. Dih ydro derivatives such as 22 sh ould also be precursors of 13. Th eir
rearom atization occurs at th e an ode sin ce th e electrolyses are run in an un -
divided cell.
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

878
Golinske, Voss, Adiwidjaja:
CO2H
CO2H
H
CO2H
other
carboxylic
acids
H
H
(i)
+
+
O
O
O
CO2H
11%
12
13, 68%
22
CH2N2
CO2Me
H
H
H
H
O
MeO2C
(i) e, CO2, DMF; Mg anode, Zn cathode
23, 20%
SCHEME 9
Ch lorin ated diben zo[1,4]dioxin es (“dioxin e”) are perh aps th e m ost n otori-
ous en viron m en tal pollutan ts. Alth ough we h ave sh own th at m on och loro-,
dich loro- an d oligoch lorodiben zo[1,4]dioxin es can be dech lorin ated by di-
rect electroreduction in m eth an ol3, we were in terested in th eir electro-
carboxylation . We ch ose 2-ch lorodiben zo[1,4]dioxin e (24) as an exam ple.
However, a first prelim in ary experim en t was disappoin tin g. On ly a 21%
yield of diben zo[1,4]dioxin e-2-carboxylic acid (25) was ach ieved besides
traces of diben zo[1,4]dioxin e (Sch em e 10). Furth er in vestigation s to im -
prove th is result are un der way.
C7
C6
C5a
O441
O5
C8
C421
C41
C4
C4a
C9
C9a
C9b
O412
C31
O112
C1
C3
C121
C2
C11
O111
FIG. 6
ORTEP plot of th e X-ray structure of 23. Th erm al elllipsoids are drawn on th e 50% probabil-
ity level
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

Chlorinated Aromatic Compounds
879
O
O
O
Cl
O
CO2H
(i)
+
O
O
24 (71% recovered)
25, 21%
2%
(i) e, CO2, DMF; Mg anode, Zn cathode
SCHEME 10
Electrocarboxylations in the Naphthalene Series
1-Ch loron aph th alen e (26) form s a m ixture of n aph th alen e (27), dih ydro-
an d tetrah ydron aph th alen es on direct electroreduction in m eth an ol18
.
Electrocarboxylation gives a 37% yield of th e expected 1-n aph th oic acid
(28) an d a m ixture of dih ydron aph th alen edicarboxylic acids, accordin g to
MS an alysis (Sch em e 11). An iden tification of th ese com pon en ts could n ot
be ach ieved.
Cl
CO2H
(i)
dihydronaphthalene-
dicarboxylic acids
+
+
26 (27% recovered)
27, 17%
28, 37%
10%
(i) e, CO2, DMF; Mg anode, Zn cathode
SCHEME 11
Di- or tetrah ydron aph th alen es are n ot form ed. Form ation of 1,4-dih ydro-
n aph th alen e-1,4-dicarboxylic acid (29) in th e electroreduction of 27 in
DMF at a m ercury cath ode in th e presen ce of carbon dioxide h as been ob-
served19 as early as 1959. We h ave applied our con dition s to th e electro-
carboxylation of 27 an d also obtain ed 29 togeth er with its isom er 30 an d a
m ixture of oth er dih ydron aph th alen edicarboxylic acids (Sch em e 12).
H
CO2H
other
dihydronaphthalene-
dicarboxylic acids
(i)
+
28
+
CO2H
H
27
<5%
10%
29, 30%
H
CO2H
H
CO2H
H
H
CO2H
CH2N2
+
CO2H
H
H
30
31, 30%
(i) e, CO2, DMF; Mg anode, Zn cathode
SCHEME 12
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

880
Golinske, Voss, Adiwidjaja:
Financial support of the Deutsche Forschungsgemeinschaft (SFB 188), the Bundesministerium für
Bildung, Wissenschaft, Forschung und Technologie, and the Fonds der Chemischen Industrie is grate-
fully acknowledged.
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20. Note added in proof. Recently we have electrocarboxylated (at
a zinc cathode)
1-chlorodibenzofuran, which is not easily accessible, and obtained a 76% yield of
dibenzofuran-1-carboxylic acid.
Collect. Czech. Chem. Commun. (Vol. 65) (2000)