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K. B. Kokoh, E. M. Belgsir / Tetrahedron Letters 43 (2002) 229–231
As anticipated, during the positive sweep of the poten-
tial, bright platinum is weakly active towards the oxida-
tion of 1 (Fig. 2, dashed line). In contrast, when Pb2+
ions are under potentially deposited onto the electrode,
the electrochemical behaviour of 1 is dominated by a
wide oxidation wave from 0.6 to 1.5 V with a maximum
at 0.92 V (Fig. 2, solid line). The catalytic effect is
caused by the upd-lead submonolayers, which change
the local work function and provide new sites. As a
result, the increasing of the oxidation rate suggests a
weaker adsorption of the intermediates.
Our initial attempts focused on potentiostatic electroly-
ses. Efforts to perform electrolyses at constant potential
between 0.6 and 1.1 V were met with frustration as the
decrease of the corresponding current densities were
found to be connected with electrode deactivation. Pro-
grammed potential electrolyses could avoid these prob-
lematic processes. This possibility consists of applying a
sequence of three plateaus of potential to the working
electrode (Fig. 3).
Figure 2. Effect of upd-lead on the electroreactivity of 1.
Voltammograms recorded at 50 mV s−1 in 70 mL of 0.1 mol
L−1 NaOH: (- - -) in the absence of Pb2+; (—) in the presence
of 5×10−6 mol L−1 Pb(ClO4)2.
The adatom layer is deposited at 0.4 V (Eads). Oxida-
tion of 1 is performed at 0.9 V (Eox) until the current
density reaches its minimum then a reactivation process
is achieved during a short pulse at more positive poten-
tial (Edes=1.9 V).
During electrolyses† the reaction mixture composition
was followed by HPLC‡ (Fig. 4). The system is
described by consecutive first-order reactions where
5-(hydroxymethyl)-2-furaldehyde (3) is
a
primary
product. 2 and 5-hydroxymethyl furoic acid (4) are
secondary products. The rate formation of 2 seems to
be proportional to the concentration of 3, which indi-
cate that 2 is not formed directly, but via intermediate
3. At the end of the electrolysis 80 and 63% chemical
and faradaic yields were obtained, respectively.
Figure 3. Potential program used during electrolysis.
A final issue that must be addressed is the effect of the
value of the oxidation plateau (0.8; 0.9 or 1.0 V). The
results collected in Table 1 show that the best conver-
sion yields are obtained at 0.9 V. At higher potential
traces of 2,5-dicarboxylic acid (5) are obtained. We thus
conclude that the electrode activity is dependant of the
superficial concentration of lead adatoms. In fact, the
submonolayer concentration decreases from 0.7 to 1.1
† 0.448 g of 1 were dissolved in aqueous supporting electrolyte (70 mL
of 0.1 mol L−1 NaOH, 5×10−6 mol L−1 Pb(ClO4)2). The solution
was electrolysed at room temperature (21°C). After the electrolysis,
the analyte was treated by a strongly acidic cation-exchange resin
(10 mL, 17 mequiv. of Amberlite 200). The clear aqueous solution
was then lyophilised without any further purification. The chemical
yields were also estimated from 1H NMR which corresponded to
those in the literature.
‡ Analysis of the composition of the reaction mixture was carried out
by HPLC which consisted of a pump (PU 980, Jasco) and two
detectors settled online (UV 975 and RI 1530, Jasco). The partition
was performed on a RP-18 column (Lichrosorb, Merck, water/ACN
85/15+TFA 0.2%, 0.6 mL min−1) and the quantitative analyses were
carried out using the so-called ‘external standard’ method.
Figure 4. Concentration profile of the reactant and products
during the programmed potential electrolysis of 1 at 0.9/V
(RHE); (ꢀ) 2,5-bis-hydroxymethylfuran; (ꢁ) hydroxymethyl
furfural; (ꢂ) furan-2,5-dicarbaldehyde; (ꢃ) 5-hydroxy-
methylfurane-2-carboxylic acid.
Kokoh
