Kolbe electrosynthesis of 1,2-di(bicyclo[2.2.1]heptan-2-yl)ethane and 1,2-diphenylethane

Article abstract of DOI:10.1080/00397911003668588

Kolbe electrosynthesis, at low current densities, of 1,2-diphenylethane and 1,2-di(bicyclo[2.2.1]heptan-2-yl)ethane with 77 and 72% yields, respectively, is described.

Full text of DOI:10.1080/00397911003668588

This article was downloaded by: [University of Arizona]
On: 04 June 2014, At: 14:39
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered
office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Synthetic Communications: An
International Journal for Rapid
Communication of Synthetic Organic
Chemistry
Publication details, including instructions for authors and
subscription information:
http://www.tandfonline.com/loi/lsyc20
Kolbe Electrosynthesis of 1,2-
Di(bicyclo[2.2.1]heptan-2-yl)ethane and
1,2-Diphenylethane
António J. Mendonça a , Massuquinini Inês b , Ana P. Esteves c , Dina
I. Mendonça d & Maria J. Medeiros c
a CICS-UBI–Centro de Investigação em Ciências da Saúde e
Departamento de Química , Universidade da Beira Interior , Covilhã
, Portugal
b Departamento de Química , Universidade Agostinho Neto ,
Luanda , Angola
c Centro de Química , Universidade do Minho , Braga , Portugal
d Departamento de Química , Universidade da Beira Interior ,
Covilhã , Portugal
Published online: 25 Feb 2011.
To cite this article: António J. Mendonça , Massuquinini Inês , Ana P. Esteves , Dina I. Mendonça &
Maria J. Medeiros (2011) Kolbe Electrosynthesis of 1,2-Di(bicyclo[2.2.1]heptan-2-yl)ethane and 1,2-
Diphenylethane, Synthetic Communications: An International Journal for Rapid Communication of
Synthetic Organic Chemistry, 41:6, 820-825, DOI: 10.1080/00397911003668588
To link to this article: http://dx.doi.org/10.1080/00397911003668588
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the information (the
“Content”) contained in the publications on our platform. However, Taylor & Francis,
our agents, and our licensors make no representations or warranties whatsoever as to
the accuracy, completeness, or suitability for any purpose of the Content. Any opinions
and views expressed in this publication are the opinions and views of the authors,
and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content
should not be relied upon and should be independently verified with primary sources
of information. Taylor and Francis shall not be liable for any losses, actions, claims,
proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or

howsoever caused arising directly or indirectly in connection with, in relation to or arising
out of the use of the Content.
This article may be used for research, teaching, and private study purposes. Any
substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,
systematic supply, or distribution in any form to anyone is expressly forbidden. Terms &
Conditions of access and use can be found at http://www.tandfonline.com/page/terms-
and-conditions

Synthetic Communications1, 41: 820–825, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911003668588
KOLBE ELECTROSYNTHESIS OF
1,2-DI(BICYCLO[2.2.1]HEPTAN-2-YL)ETHANE
AND 1,2-DIPHENYLETHANE
1
Antonio J. Mendonc¸a, Massuquinini Ines, Ana P. Esteves,
2
3
´
ˆ
Dina I. Mendonc¸a,4 and Maria J. Medeiros3
1
ˆ
CICS-UBI–Centro de Investigac¸a˜o em Ciencias da Saude e Departamento
de Quımica, Universidade da Beira Interior, Covilha˜, Portugal
´
´
2
´
Departamento de Quımica, Universidade Agostinho Neto, Luanda, Angola
Centro de Quımica, Universidade do Minho, Braga, Portugal
3
4
´
´
Departamento de Quımica, Universidade da Beira Interior, Covilha˜,
Portugal
GRAPHICAL ABSTRACT
Abstract Kolbe electrosynthesis, at low current densities, of 1,2-diphenylethane and
1,2-di(bicyclo[2.2.1]heptan-2-yl)ethane with 77 and 72% yields, respectively, is described.
Keywords Dimerization; Kolbe electrosynthesis; oxidation; radical reactions
INTRODUCTION
The Kolbe synthesis[1] is considered to be the first electro-organic reaction of
interest in the synthesis of natural products.[2] Recently, this reaction has been
carried out by photochemistry,[3] sonoelectrosynthesis using a biphasic system as
Received November 9, 2009.
Address correspondence to Antonio J. Mendonc¸a, Departamento de Quımica, Universidade da
´
Beira Interior, R. Marques Avila e Bolama, 6201-001 Covilha˜, Portugal. E-mail: mendonca@ubi.pt
´
´
ˆ
820

1,2-DI(BICYCLO[2.2.1]HEPTAN-2-YL)ETHANE SYNTHESIS
821
solvent,[4,5] solid polymer electrolyte (SPE) composite electrode,[6] and solid-phase
organic synthesis.[7–9]
The Kolbe reaction consists of the oxidative decarboxylation of a carboxylic
acid with the formation of a radical intermediate that can react in coupling reactions.
Depending on the experimental conditions, the anodic decarboxylation of carboxy-
lates, generated from carboxylic acids, can afford radicals (Kolbe electrolysis) or car-
bocations (non-Kolbe electrolysis)[10] at the anode, in a selective fashion. The
radicals can be used in homo- and heterocoupling or in inter- and intramolecular
addition reactions to double bonds.[11] The selectivity of the Kolbe synthesis is
strongly dependent on the chemical structure of carboxylic acid(s) and on the experi-
mental conditions.[10] The Kolbe reaction mechanism involves an initial electron
transfer from the carboxylates at the anode, giving rise to a radical that can decar-
boxylate, and the radicals combine between them, producing the dimer.[12] If a cation
is formed, it may react in a non-Kolbe reaction, namely by substitution, dispropor-
tionation, C-C bond cleavage or rearrangement, affording compounds such as alco-
hols, ethers, esters, and amides.[13] The best conditions to obtain the dimer are high
current density, 0.25 V–1.0 A=cm2, platinum electrodes, neutral or slightly acidic
aqueous methanol as solvent, and temperatures less than 50 ꢀC.[10]
In this work, we describe the electrosynthesis of 1,2-diphenylethane and
1,2-di(bicyclo[2.2.1]heptan-2-yl)ethane at low current density. The synthesis of 1,2-
diphenylethane at high current density was described previously.[14]
RESULTS AND DISCUSSION
Electrosynthesis of 1,2-Diphenylethane
A solution of phenylacetic acid 1 0.8 M was electrolyzed at platinum electrodes
using a current density of 15 mA ꢁ cmꢂ2 in MeOH=Py, in the presence of 10% of
sodium methoxide (NaOMe), which was used to neutralize the carboxylic acid at
a controlled temperature of 19 ꢀC (Scheme 1). The expected product, 1,2-dipheny-
lethane 2, was obtained in 2.5% yield (Table 1, entry 1). To improve the dimer yield,
we studied the influence of the concentration of NaOMe. We found that an increase
Scheme 1. Kolbe electrosynthesis of 1,2-diphenylethane.

822
A. J. MENDONC¸ A ET AL.
Table 1. Electrosynthesis of 1,2-diphenylethane by electrolysis of phenylacetic acid (0.8 M) at platinum
electrodes and at constant current density (15 mA ꢁ cmꢂ2) in MeOH=Py (4:1)
Product 2 (%)
NaOMe
(%)
Electrodes
(cm2)
Time
(h)
Temperature
(ꢀC)
Charge
(C)
Entry
na
gm
gc
1b
2b
3c
10
50
10
1.1 ꢃ 1.1
1.1 ꢃ 1.1
2.5 ꢃ 2.5
7.00
7.47
9.37
5
5
915
976
0.5
0.5
1.6
2.5
0.4
77
3.4
0.4
29
19
6325
aNumber of electrons involved per molecule of phenylacetic acid.
bTotal volume of electrolyte solution was 25 cm3.
cTotal volume of electrolyte solution was 50 cm3.
Note. gm ¼ [obtained mass of dimer (g) = theoretical mass of dimer (g)] ꢃ 100; gc ¼ [moles of product
obtained by a charge c = theoretical moles of product obtained by a charge c] ꢃ 100.
on the concentration from 10% to 50% led to a decrease in the yield of 2 to 0.4%
(Table 1, entry 2). The influence of the electrode’s size was also explored, and an
increase from 1.1 ꢃ 1.1 cm2 to 2.5 ꢃ 2.5 cm2 in the presence of 10% NaOMe led to
a mass yield (gm) of 77% with a current efficiency (gc) of 29% (Table 1, entry 3).
A higher current efficiency (55%) using a current density of 128 mA ꢁ cmꢂ2 was
reported.[13] The dimer 2 was identified by nuclear magnetic resonance spectra (1H
and 13C), and its melting point is in good agreement with that reported.[14] The num-
ber of electrons obtained in the formation of the dimer was 1.6 eꢂ instead of 1. This
is because of the formation of by-products from non-Kolbe reactions, observed as
high-polarity compounds in thin-layer chromatography (TLC).
Electrosynthesis of 1,2-Di(bicyclo[2.2.1]heptan-2-yl)ethane
The electrosynthesis of 1,2-di(bicyclo[2.2.1]heptan-2-yl)ethane 4 (Scheme 2)
starting with 0.8 M 2-norbornaneacetic acid 3 was also carried out at constant cur-
rent with a density of 15 mA ꢁ cmꢂ2 in MeOH=Py (4:1) in the presence of 10% of
sodium methoxide, at a constant temperature (19 ꢀC) at platinum electrodes
(2.5 ꢃ 2.5 cm2). The number of electrons involved in the reaction per molecule of
2-norbornaneacetic acid was equal to 2. Again, as in the electrosynthesis of 1,2-
diphenylethane, this value is a result of the formation of by-products by non-Kolbe
Scheme 2. Kolbe electrosynthesis of 1,2-di(bicyclo[2.2.1]heptan-2-yl)ethane.

1,2-DI(BICYCLO[2.2.1]HEPTAN-2-YL)ETHANE SYNTHESIS
823
Scheme 3. Partial sequence of fragmentation for compound 4.
reactions, observed as polar compounds in TLC. The expected product, 1,2-
di(bicyclo[2.2.1]heptan-2-yl)ethane (4), was obtained in a mass yield of 72% and a
1
1
current efficiency of 71%. H NMR, 13C NMR, H–1H correlation spectroscopy
(COSY), heteronuclear single quantum coherence (HSQC), heteronuclear multiple-
bond correlation (HMBC), and mass spectrometry (MS) data (Scheme 3) were
consistent with the dimer 4 structure.
CONCLUSIONS
These results demonstrate that Kolbe synthesis can be carried out under experi-
mental conditions that are not favorable to dimer formation. In fact, it is reported[10]
that high current densities are needed to obtain the desired dimer. However, the
dimers, 1,2-diphenylethane and 1,2-di(bicyclo[2.2.1]heptan-2-yl)ethane, were
obtained from the electro-oxidation of phenylacetic and 2-norbornaneacetic acids,
respectively, in yields better than 70% at low current density and at room tempera-
ture. Thus, we have developed a ‘‘green’’ methodology that enables the reaction to be
carried out under mild conditions, operating at room temperature using low current
densities. Moreover, the product, 1,2-di(bicyclo[2.2.1]heptan-2-yl)ethane, is new, and
this work describes it for the first time.
EXPERIMENTAL
Phenylacetic acid (Merck, >99%), 2-norbornaneacetic acid (2-(bicyclo[2.2.1]
heptan-2-yl)acetic acid) (Sigma-Aldrich, >98%), pyridine (Sigma-Aldrich, >99%),
sodium methoxide (Fluka, ꢄ97%), and methanol (Sigma-Aldrich, >98%) were used
as received. TLC was carried out on Merck silica-gel 60 F254 TLC plates. Melting
points were measured on a Buchi melting-point B-540 apparatus. Electron impact
¨
mass spectrometry (EIMS) was carried out on a VG Autospec M and recorded at

824
A. J. MENDONC¸ A ET AL.
1
70 eV; H and 13C NMR spectra were recorded on a Bruker AC 250P spectrometer
or a Bruker Avance II, at 600 MHz (1H NMR) and 150.9 MHz (13C NMR) in
CDCl3. Chemical shifts are given in parts per million (ppm) and were measured
versus residual peak of the solvent, 7.26 ppm for 1H and 77.0 ppm for 13C.
Two-dimensional experiments (1H–1H COSY, HSQC, HMBC) were performed with
standard Bruker software and were carried out for complete assignment of proton
and carbon signals in the NMR spectra.
Constant-Current Electrolysis
Electrolyses were carried out in a single-compartment glass cell with two Pt foil
electrodes separated by 5 mm, under magnetic stirring at a constant current of
15 mA=cm2 using a dc power supply (Good Will Instruments GSP-3030D). Electro-
lyte solution volumes were 25 cm3 or 50 cm3, using 1.1 ꢃ 1.1 cm2 or 2.5 ꢃ 2.5 cm2 elec-
trodes, respectively. The carboxylic acid was dissolved in MeOH=Py (4:1) and
partially neutralized with NaOMe. The electrolysis was carried out at room tempera-
ture, until the pH of electrolyte solution was approximately 8. During the reaction,
every 30 min, the polarity inversion was performed to avoid electrode passivation.
After the electrolysis, the reaction mixture was extracted with chloroform
(3 ꢃ 20 cm3) and washed with water. The combined organic extracts were dried
(Na2SO4) and concentrated. The crude residue was submitted to column chromato-
graphy over silica gel G (230–400 mesh) using n-hexane=ethyl acetate gradients of
increasing polarity as solvent.
1,2-Diphenylethane (2)
White crystals; mp 50.1–51.3 ꢀC; IR (nujol) 3086, 3064, 3031 (t arC-H), 2960,
2927, 2856 (t C-H), 1947, 1881, 1810, 1775 (comb monosubst.), 1599, 1487 (t
arC-C), 1146, 1027, 968 (dip arC-H), 909, 752, 699 (doop) cmꢂ1 1H NMR
;
(250 MHz, CDCl3), d 2.95 (s, 4H, H-1 and H-2), 7.15–7.24 (m, 7H, H aromatic),
7.28–7.35 (m, 3H, H-aromatic); 13C NMR (63 MHz, CDCl3), d 38.1 (C-1 and
C-2), 126.4 (C-40 and C-400), 128.6 (C-20, C-60, C-200 and C-600), 128.9 (C-30, C-50,
C-300 and C-500), 141.9 (C-10 and C-100).
1,2-Di(Bicyclo[2.2.1]heptan-2-yl)ethane (4)
White crystals; mp 98.3–99.2 ꢀC; IR (nujol) 2952, 2933, 2873, 2854 (t C-H),
1
1455 (d CH2) cmꢂ1; H NMR (600 MHz, CDCl3), d 1.17–1.21 and 0.97–1.04 (m,
4H, H-60 and H-600), 1.23–1.28 (m, 2H, H-20 and H-200), 1.24–1.28 (m, 4H, H-1
and H-2), 1.34–1.41 and 0.93–1.00 (m, 4H, H-70 and H-700), 1.39–1.46 and
1.04–1.10 (m, 4H, H-30 and H-300), 1.42–1.46 and 1.08–1.12 (m, 4H, H-40 and
H-400), 1.89–1.94 (m, 2H, H-10, H-10), 2.11–2.16 (m, 2H, H-50, H-500); 13C NMR
(150.9 MHz, CDCl3), d 28.9 (C-30 and C-300), 30.2 (C-40 and C-400), 35.3 (C-60 and
C-600), 35.3 and 35.4 (C-1 and C-2), 36.6 (C-50 and C-500), 38.4 (C-70 and C-700),
41.2 (C-10 and C-100), 42.5 (C-20 and C-200); EIMS (%) m=z 218 (5) [M]þ, 190 (11),
135 (8), 121 (10), 108 (15), 95 (100).

1,2-DI(BICYCLO[2.2.1]HEPTAN-2-YL)ETHANE SYNTHESIS
825
ACKNOWLEDGMENTS
One of the authors (M. I.) gratefully acknowledges a GRICES PhD scholar-
ship and INABE (Angola) for financial support.
REFERENCES
1. Kyriacou, D. Modern Electroorganic Chemistry; Springer: Berlin, 1994.
2. Weedon, B. C. L. In Advances in Organic Chemistry; R. A. Raphael, E. C. Taylor, and
H. Wynberg (Eds.); Interscience: New York, 1960.
3. Kraeutler, B.; Bard, A. J. Heterogeneous photocatalytic synthesis of methane from
acetic-acid—New Kolbe reaction pathway. J. Am. Chem. Soc. 1978, 100, 2239–2240.
4. Behrend, O.; Ax, K.; Schubert, H. Influence of continuous phase viscosity on emulsifi-
cation by ultrasound. Ultrason. Sonochem. 2000, 7, 77–85.
5. Wadhawan, J. D.; Del Campo, F. J.; Compton, R. G.; Foord, J. S.; Marken, F.; Bull, S.
D.; Davies, S. G.; Walton, D. J.; Ryley, S. Emulsion electrosynthesis in the presence of
power ultrasound biphasic Kolbe coupling processes at platinum and boron-doped dia-
mond electrodes. J. Electroanal. Chem. 2001, 507, 135–143.
6. Ogumi, Z.; Yamashita, H.; Nishio, K.; Takehara, Z.; Yoshizawa, S. Application of the
solid polymer electrolyte (SPE) method to organic electrochemistry, 3:Kolbe-type reac-
tions on PT-SPE. Electrochim. Acta 1983, 28, 1687–1693.
7. Tajima, T.; Fuchigami, T. Environmentally friendly electroorganic synthesis using
solid-supported acids and bases. J. Syn. Org. Chem. Jpn. 2006, 64, 406–414.
8. Kurihara, H.; Tajima, T.; Fuchigami, T. Mixed Kolbe electrolysis using solid-supported
bases. J. Electrochem. 2006, 74, 615–617.
9. Kurihara, H.; Fuchigami, T.; Tajima, T. Kolbe carbon–carbon coupling electrosynthesis
using solid-supported bases. J. Org. Chem. 2008, 73, 6888–6890.
10. Torii, S.; Tanaka, H. In Organic Electrochemistry; H. Lund and O. Hammerich (Eds.);
Marcel Dekker: New York, 2001.
11. Scha¨fer, H. J. In Radicals in Organic Synthesis; P. Renaud and M. P. Sibi (Eds.), Wiley–
VCH: Weinheim, 2001.
12. Brown, A. C.; Walker, J. Elektrolytische Synthese zweibasischer Sa¨uren. Justus Liebigs
Ann. Chem. 1891, 261, 107–128.
13. Utley, J. H. P. In Techniques of Electroorganic Synthesis; N. L. Weinberg (Ed.); Wiley:
New York, 1974.
14. Linstead, R. P.; Shephard, B. R.; Weedon, B. C. L. Anodic syntheses, 7: Electrolyses of
mono-phenylacetic, di-phenylacetic, and tri-phenylacetic acids in non-aqueous solutions.
J. Chem. Soc. 1952, 3624–3627.