6994
A. M. Costero et al. / Tetrahedron Letters 48 (2007) 6992–6995
absence of an appropriate leaving group. However, the
thiourea present in the solution firstly allows the
electron transfer to the carbonyl carbon and secondly
generates a good leaving group.
The reaction has been extended to different aromatic
carboxylates (2-nitro-, 4-nitro-, 2-methoxy benzoates)
with the same results. Until now the reactions have
been carried out under substoichiometric conditions
but as the ligand seems to be recovered unchanged
after the reduction reaction, studies directed to know
the lowest amount of thiourea needed are being carried
out.
Figure 4. CV at Pt electrode for 4,40-dimethoxybenzyl (1.0 mM) in
0.10 M Bu4NPF6/DMSO. Potential scan rate 100 mV/s.
NO2
NO2
In conclusion it is possible to affirm that ligands 1–3 cat-
alyze the cathodic reduction of aromatic carboxylates to
give the corresponding dicarbonyl compounds.
O
O
Ar
+ 1e
O
Ar
1/2
Ar
O
Acknowledgements
N
N
S
H
N
N
S
H
O
H
H
We thank the Spanish Ministerio de Ciencia y Tecnolo-
gia for financial support (Project CTQ2006-15456-02).
GMRM also acknowledges the Spanish Government
for a PhD grant. We also thank de SCSIE of the Univer-
sity of Valencia.
Ar
O
R
R
Scheme 1.
ure 2 can be interpreted in terms of a two step process in
which the irreversible reduction of the receptor-4-meth-
oxybenzoate adduct (process C3) first occurs. This pro-
cess yields 4,4-dimethoxybenzyl which experiences a
subsequent reversible reduction (C4) strongly over-
lapped with the process C1 for the reduction of the
receptor.
Supplementary data
CV’s of TBA 4-methoxybenzoate and 4-methoxyben-
zoic acid. UV spectra (before and after coulometry) of
4-methoxybenzoate, 4-methoxybenzoic acid and ligand
1
1. H NMR and IR spectra of 4,40-dimethoxybenzyl.
The voltammetry of the ligand plus anion solutions
could be interpreted on assuming that the anion coordi-
nates to the ligand via hydrogen bond formation
between the thiourea groups of the ligand and the
carboxylate oxygens of the anion, as depicted in Scheme
1.5 The overall reduction process C3 can be represented
as:
CV’s of 2 and 2 + 5 equiv TBA 4-methoxybenzoate in
0.1 M Bu4NP6/DMSO. CV of 3 + 5 equiv of TMA 4-
methoxybenzoate in 0.1 M Bu4NP6/DMSO. CV’s in
0.1 M Bu4NP6/DMSO of 1, 1 + 0.2 equiv TBA 4-meth-
oxybenzoate, 1 + 1 equiv TBA 4-methoxybenzoate and
1 + 5 equiv TBA 4-methoxybenzoate. Supplementary
data associated with this article can be found, in the on-
RCOOꢀ þ Hþ þ eꢀ ! 1=2 RCOCOR þ OHꢀ ð1Þ
This can formally be obtained as a combination of the
following series of processes involving the ligand, repre-
sented here as LH2:
References and notes
1. For reviews on organocatalysts, see: (a) Dalko, P. I.;
Moisan, L. Angew. Chem., Int. Ed. 2001, 40, 3726–3748; (b)
Dalko, P. I.; Moisan, L. Angew. Chem., Int. Ed. 2004, 43,
5138–5175; (c) Special issue on Asymmetric Organocatal-
ysis Acc. Chem. Res. 2004, 37, 487–631; (d) Takemoto, Y.
Org. Biomol. Chem. 2005, 3, 4299–4306; (e) Seayad, J.; List,
B. Org. Biomol. Chem. 2005, 3, 719–724.
2. Schreiner, P. R. Chem. Soc. Rev. 2003, 32, 289–296.
3. (a) Kelly, T. R.; Kim, M. K. J. Am. Chem. Soc. 1994, 116,
7072–7080; (b) Schmidtchen, F. P.; Berger, M. Chem. Rev.
1997, 97, 1609–1646; (c) Linton, B. R.; Goodman, M. S.;
Hamilton, A. D. Chem. Eur. J. 2000, 6, 2449–2455; (d)
Connon, S. J. Chem. Eur. J. 2006, 12, 5419–5427.
RCOOꢀ þ LH2 ! fLH ꢁ ꢁ ꢁ RCOOHgꢀ
ð2Þ
fLH ꢁ ꢁ ꢁ RCOOHgꢀ þ eꢀ ! 1=2 RCOCOR
þ LHꢀ þ OHꢀ
ð3Þ
ð4Þ
LHꢀ þ Hþ ! LH2
Benzyl generation through cathodic reduction has been
reported in the literature for acid chlorides, esters and
other related compounds.8 In these reactions a C-cen-
tred radical gives rise to benzyl derivative through a
dimerization reaction. The described mechanism
involves the releasing of the corresponding leaving
group (chloride, alcoxy, etc.). This type of reaction has
never been observed with carboxylates due to the
4. (a) Nosal-Wiercinska, A.; Dalmata, G. Electroanalysis
2002, 14, 1275–1280; (b) Lyle, F. R. U.S. Patent
5,973,257, 1985; Chem. Abstr. 1985, 65, 2870.
´
5. Costero, A. M.; Gavina, P.; Rodrıguez-Muniz, G.; Gil, S.
˜
˜
Tetrahedron 2006, 62, 8571–8577.