Facile electrochemical transformation of diazonium salts into carboxylic acids

Article abstract of DOI:10.1016/j.tetlet.2006.09.132

The electrolyses of aryldiazonium tetrafluoroborates in dry DMF and Bu₄NHSO₄ as solvent-supporting electrolyte system, in the presence of CO₂ led to the corresponding arylcarboxylic acids in very good yields.

Full text of DOI:10.1016/j.tetlet.2006.09.132

Tetrahedron Letters 47 (2006) 8215–8216
Facile electrochemical transformation of diazonium salts
into carboxylic acids
M. Dolores Otero, Belen Batanero and Fructuoso Barba*
Department of Organic Chemistry, University of Alcala´, 28871, Alcala´ de Henares (Madrid), Spain
Received 5 July 2006; revised 19 September 2006; accepted 22 September 2006
Abstract—The electrolyses of aryldiazonium tetrafluoroborates in dry DMF and Bu4NHSO4 as solvent-supporting electrolyte sys-
tem, in the presence of CO2 led to the corresponding arylcarboxylic acids in very good yields.
Ó 2006 Elsevier Ltd. All rights reserved.
The electrochemical carboxylation of the aromatic ring
of aryl halides is a well-known process.1 The reaction
involves the cathodic cleavage of the carbon–halogen
bond to form the corresponding anion, which is added
to a bubbled carbon dioxide molecule, affording the de-
sired carboxylic acid in a good yield, particularly under
pressurized cell. However the inconvenience of this reac-
tion is the requirement of using high cathode potentials,
around À2 V (vs Ag/Ag+).
electrolysis at the second reduction potential value (be-
tween À0.5 V and À0.7 V, vs SCE) in a 3eÀ/3H+ process
allowed to get the corresponding phenyl-hydrazine.7–10
The reduction of aryldiazonium salts at Cu, Fe or Ti cath-
odes in the presence of alkenes led to arylated products.11
Aromatic compounds were also arylated by the electro-
chemical reduction of benzenediazonium tetrafluoro-
borates in aprotic solvents.12
The attachment of organic layers to conductive or semi-
conductive surfaces by cathodic reduction of diazonium
salts has been performed.13
On the other hand some aromatic hydrocarbon rings
such as naphthalene, phenanthrene, and acenaphthylene
were converted into carboxylic acids by the reductive
carboxylation on controlled potential electrolyses; how-
ever, the yields were poor.2
The preparation of arylcarboxylic acids from diazonium
salts supposes considerable advantages related to the
reduction of the C–halogen bond. Aryl anions can be
obtained under low cathode potential (À1 V, vs Ag/
Ag+) by the electrochemical reduction of these sub-
strates, allowing the presence of other substituents in
the aromatic ring of the electroactive starting material.
Additionally, versus the conventional methods, the final
hydrolysis necessary in the Sandmeyer reaction, and the
fact that it would affect other groups on the ring, is
avoided. Finally, we can mention the regiospecific char-
acter of this carboxylation process.
In the present letter a new procedure for the obtention
of aryl carboxylic acids is described by the cathodic
reduction of aryldiazonium salts in the presence of CO2.
Few articles about the employment of diazonium salts in
organic electrosynthesis have been published. Aqueous
or alcoholic media have been used in the electroreduc-
tions of these salts at a mercury cathode.3–5 Described
polarography of diazonium salts showed two reduction
waves. Preparative electrolysis at the first reduction
value (between +0.02 V and À0.1 V, vs SCE) afforded
arylmercury salts after a 1eÀ/substrate molecule process,
involving the initial formation of radicals.6 Preparative
The electrolyses of aryldiazonium tetrafluoroborates in
dry DMF and Bu4NHSO4, as the electrolyte (the cheap-
est tetraalkylammonium salt) in the presence of CO2 was
performed under potentiostatic À1.0 V (vs Ag/Ag+)
conditions.14 However biphenyl was obtained as the
major product (75%). Its formation can be explained
due to the well-known photodediazoniation of aryl-
diazonium fluoroborates by dimethylformamide via a
homolytic process15 or because aryl radicals are also
Keywords: Aryldiazonium tetrafluoroborates; Selective cathodic car-
boxylation; Electrosynthesis; Carboxylic acids.
*
Corresponding author. Tel.: +34 91 8854617; fax: +34 91 8854686;
e-mail: fructuoso.barba@uah.es
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.09.132

8216
M. D. Otero et al. / Tetrahedron Letters 47 (2006) 8215–8216
Kagaku 1982, 50, 732; Grimshaw, J. In Rodd’s Chemistry
+ 2e
Ar
N2
Ar N2
Ar
of Carbon Compounds; Sainsbury, M., Ed.; Organic
Electrochemistry; Elsevier Science, 2002; Vol. V, pp 83–
84.
Ar-COO
+ CO2
2. Yamamura, S. In Rodd’s Chemistry of Carbon Compounds;
Sainsbury, M., Ed.; Organic Electrochemistry; Elsevier
Science, 2002; Vol. V, pp 2003–2004.
3. Bravo-Diaz, C.; Gonzalez-Romero, E. Electroanalysis
2003, 15, 303.
Scheme 1.
Table 1.
4. Atkinson, E. R.; Garland, C. E.; Butler, A. F. J. Am.
Chem. Soc. 1953, 75, 983.
_
-
+
cathodic
reduction
+ N2
BF4
Ar N2
+ CO2
Ar-COO
5. Elofson, R. M. Can. J. Chem. 1958, 35, 1209.
6. Kochi, J. K. J. Am. Chem. Soc. 1955, 77, 3208.
Ar
Yield (%)
7. Ruetschi, P.; Trumpler, G. Helv. Chim. Acta 1953, 36,
¨
¨
1649.
C6H5
73
90
85
91
88
84
85
78
77
88
8. Orange, O.; Hamet-Elfakir, C.; Caullet, C. J. Electrochem.
Soc. 1981, 128, 1889.
9. Viertler, H.; Pardini, V. L.; Vargas, R. R. In The
Electrochemistry of Triple Bond—The Chemistry of Triple
Bonded Functional Groups; Patai, S., Ed.; J. Wiley and
Sons: New York, 1994; Supplement C.
10. Bravo-Diaz, C.; Gonzalez-Romero, E. Curr. Top. Elect-
rochem. 2003, 9.
11. Ganushchak, N. I.; Obushak, N. D.; Kovel’chuk, E. P.;
Trifonova, G. V. Zh Obshch. Khim 1984, 54, 2334.
12. Gadallah, F. F.; Elofson, R. M. J. Org. Chem. 1969, 34,
3335.
13. Pinson, J.; Podvorica, F. Chem. Soc. Rev. 2005, 34, 429.
14. The electroactive diazoniumtetrafluoroborates were pre-
pared according to the conventional methods (In Organic
Reactions, Krieger, R. E., Ed.; Publishing company:
Huntington, NY, 1977; Vol. V, pp 198–228).
4-MeO–C6H4
4-Et–C6H4
2-Me–C6H4
3-Me–C6H4
4-Cl–C6H4
4-Br–C6H4
4-MeCO–C6H4
4-MeOOC–C6H4
2-MeS–C6H4
generated by the reduction of the diazonium ions at the
electrode surface. The fast dimerization reaction of these
aryl radicals (a second order reaction) hinders their fur-
ther reduction to the corresponding anions.
Electrolysis of aryldiazonium tetrafluoroborates under
low concentration conditions minimizes the dimeriza-
tion process. In this case the same reduction potential
value of À1.0 V (vs Ag/Ag+) was applied, at the same
time that CO2 was bubbled into the catholyte. The solid
portion addition of the electroactive salt—for 2 h—into
the cathodic compartment allows the electroreduction
of the C–N bond to the corresponding anion, with N2
evolution. This anion is added in solution to a CO2
molecule, affording the expected carboxylate, as it is
indicated in Scheme 1.
The electrochemical reductions were performed under
potentiostatic conditions in a concentric cell with two
compartments separated by a low porosity (D4) glass frit
diaphragm and equipped with a magnetic stirrer. A
mercury pool was used as the cathode, a platinum plate
as the anode, and a saturated Ag/AgCl electrode as the
reference. The SSE (solvent-supporting-electrolyte) was
nominally anhydrous DMF containing 0.05 M tetrabutyl-
ammonium bisulfate. The electroactive substrate (3.0
mmol in 20 mL of SSE) was added during for 2 h to the
cathodic compartment to be electrolyzed at a constant
potential of À1.0 V (vs Ag/Ag+). When the reaction was
completed the cathodic solution was removed under
reduced pressure. The residue was extracted with ether/
NaOH (5%). The aqueous phase was acidified and
extraction with ether was again performed. The second
organic phase was dried over MgSO4 and concentrated by
evaporation. The resulting solid was GC chromato-
graphed and crystallized in H2O. The physical and
spectroscopical properties of the obtained acids (available
at Aldrich) were coincident with the already described in
the literature (Beilstein 9,10-IV).
This new carboxylation process provide good yields (see
Table 1) independent of the electrodonating or electro-
withdrawing nature of the substituents in the diazonium
salt aromatic ring.
The electrolyses can also be carried out by using a power
supply.16
15. Markgraf, J. H.; Chang, R.; Cort, J. R.; Duran, J. L.;
Finkelstein, M.; Gross, A. W.; Lauyne, M. H.; Moore, W.
M.; Petersen, R. C.; Ross, S. D. Tetrahedron 1997, 53,
10009.
16. In order to make this reaction easy to be applied by the
organic chemical community, the electrolyses were also
carried out (the potentiostat is expensive and not always
available) with a power supply (cheaper and affordable),
with a reference electrode connected to a high resistance
voltmeter, and a manual control of the potential value in
À1 V (vs Ag/Ag+). The obtained yields of carboxylic acids
were almost coincident with the above mentioned by using
the potentiostat.
Acknowledgments
This study was financed by the Spanish Ministry of Sci-
ence and Education CTQ2004-05394/BQU. B.B. thanks
the Spanish Ministry of Science and Technology for the
‘Ramon y Cajal’ financial support.
References and notes
1. Barba, F.; Guirado, A.; Zapata, A. Electrochim. Acta
1982, 27, 1335; Matsue, T.; Kitahara, S.; Osa, T. Denki