C. Martin et al. / Tetrahedron Letters 47 (2006) 3459–3462
3461
Table 1. Electrocatalytic reduction of 1 on lead cathode in various electrolytes
Electrolyte Potential (V vs MSE) Initial concentration of 1 (M)
X
1
(%)
S
2
(%)
sF2 (%)
S
3
(%)
s
F3 (%)
n
exp
0
.5 M NaHCO
3
/Na
2
CO
3
ꢀ2.0
ꢀ1.1
0.09
.5
92
88
90
64
91
65
—
—
—
—
2.96
2.75
0
0.5 M H
2
SO
4
0.125
78
79
76
84
—
—
—
—
—
—
—
—
41
54
56
69
Qexp
F ðC0ꢀCtÞ v
14
23
26
33
2.95
2.19
2.17
2.05
0
0
1
.3
.9
.7
ꢀ
ꢁ
ꢀ
ꢁ
ꢀ
ꢁ
C0ꢀCt
C0
nFvCi
1
mi
Cit
X: yield of 1 X ð%Þ ¼
initial concentration of 1; C
ꢁ 100 ; sFi: Faradaic yield sFi ¼ Qexp ; Sit: selectivity Sit ð%Þ ¼
ꢁ
ꢁ 100 , nexp ¼
ꢂ
0
, where C is the
C0ꢀCt
t
is its concentration at the time t; Cit is the concentration of the considered reaction product; n is the number of electrons,
i
m , the stoichiometric number of the reaction product and v corresponds to the volume of solution, nexp is the experimental number of electrons.
and 69%, respectively, on lead and at ꢀ1.1 V versus
HO OH
OH
H
MSE.
H COOC
3
COOCH3
H3C
COOCH3
H C CH3
3
Moreover, up to 2 M, no reduction of 1 is observed,
which is probably due to the saturation of the lead
surface by adsorbed molecules of 1.
Methylester of lactic acid (2')
Dimethylester of 2,3-dimethyltartaric acid (3')
Figure 4.
à
Analyses of the electrolyzed solutions by HPLC
allowed to determine 2 and 3. A chemical step of esteri-
fication is necessary to separate and isolate the reaction
products. However, the aqueous electrolyte was
removed at the end of electrolysis to recover dry organic
compounds free from inorganic ions. The esterified
corresponding products were separated by flash chroma-
tography and then identified spectroscopically by GC–
In conclusion, we have shown that the electrocatalytic
reduction of 1 on Pb cathode favours the selectivity of
2
or 3 by changing the nature of the supporting electro-
lyte and by utilizing a suitable initial concentration of 1.
This electropinacolization enlarges the scope of the syn-
thesis of 3 at room temperature in aqueous medium.
§
– 1
13
k
MS–MS, ESI-MS high resolution, H and C NMR.
0
0
20,21
Compounds 2 and 3
(Fig. 4) were obtained as the
major products in different proportions according to
the suitable electrolyte. The yields of isolated products
Acknowledgements
0
2
0
(at ꢀ2.0 V vs MSE, in carbonate buffer with ca.
0
This work was supported by the ‘Minist e` re de l’Educa-
tion Nationale de la Recherche et de la Technologie’
with a doctoral fellowship for C. Martin. We would like
to thank ‘Centre R e´ gional de Mesures Physiques de
l’Ouest (C.R.M.P.O.) and in particular, Dr. Lambert
and C. Canaff, for their help in ESI-MS high resolution
studies.
.1 M of 1) and 3 (at ꢀ1.1 V vs MSE, in sulfuric acid
with ca. 1.7 M of 1) are 76% and 29%, respectively.
Other side products, which were probably issued from
the reduction of the carboxylic group of 1 were detected
in small amounts.
0
It can also be noticed that 3 exists in two different con-
0
0
figurations 3 a and 3 b (racemic and meso forms), which
were separated successfully but not yet assigned.
References and notes
à
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SO
4
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§
3 4
70 eV, CI/NH and CI/CH : 100 eV, 1200L Varian).
–
ESI high resolution: positive mode at 60 ꢁC in methanol with spray
voltage: 4 kV, ZabSpec TOF Micromass.
k 1
13
H and
C NMR: 300 MHz, CDCl
3
, WP 200 SY Bruker
spectrometer.