8
24
P. Poizot et al. / Tetrahedron Letters 50 (2009) 822–824
Table 1
Reduction of primary iodo-alkanes at GC/Ag–Pd cathodes. The experimental condi-
tions are given in the text
1
I„RI
-Iodoalkane C
n
H
2n+1
–
R–R
(%)
R(ÀH)
R(H)
(%)
Passed charge
À1
(%)
(F mol
)
n = 4
n = 6
n = 8
n = 10
n = 12
n = 16
n = 18
Ph(CH
98
98
98
97
99
27
33
99
—
—
—
—
—
42
39
1
—
—
—
1
1.1
1.05
1.1
1.1
1
1.05
1.15
1.15
1.0
27
28
—
2 3
) I
Table 1 displays the main results obtained with a large palette
of primary alkyl iodides. It is quite noticeable that all RIs character-
ized by a linear chain no longer than C12 led almost exclusively to
homodimers. Thus, the GC/Ag–Pd electrodes offer a quite better
selectivity than massive Ag–Pd cathodes as recently reported3
since amounts found for R(H) and R(ÀH) were minor, a least with
alkyl chains no longer than C12 . It is also worth observing that with
very long chains (C16 and C18), the yield of dimers collapses and the
reaction is no longer selective. It is expected that the coverage of
the GC/Ag–Pd electrode by long chain alkyl radicals (building of a
self-organized layer) prevents the catalytic reaction from occur-
ring. In other words, long chain dimers are fairly insoluble and
could block the capacity of the electrode as well.
Figure 2. Voltammetric responses of a solution of 1-iodobutane (concentration:
À1
1
0 mmol L ) at different solid electrodes using 0.1 M TBABF
4
in DMF as electrolyte.
2
À1
Apparent surface areas: 0.8 mm . Scan rate: 50 mV s . (A) At glassy carbon; (B) at
glassy carbon electrode covered by a galvanostatic deposit of silver (Q = 1.2 Â
À3
À2
1
0
2
C mm ); (C) GC/Ag electrode treated by a solution of PdCl during 15 s; (D) GC/
4
Ag electrode treated by a solution of PdSO during 10 min (see text).
The one-electron process obtained at these new electrodes has
been additionally verified thanks to ESR process. As a matter of
fact, electrolyses of RIs achieved in the presence of N-tert-butyl-
a-phenylnitrone (TBPN) in excess (see Fig. 3) led to a monoelec-
tronic overall process with the appearance of a sole six-ray para-
magnetic signal assigned to a nitroxide radical (trapping of n-
octyl radical by TBPN). Thus, with 1-iodooctane, the spectrum
gives the following parameters: g = 2.0066, a
= 2.85 G.
N
= 14.69 G and
4. Conclusions
a
H
The use of carbon electrodes modified by a very thin layer of an
alloy Ag–Pd leads to spectacular results in the field of the reductive
homocoupling of primary alkyl iodides. At least with chains no
longer than C12 the coupling reaction is quasi quantitative. The
simple way to produce these new cathodes, their low cost as well
as their electrochemical stability allow considering this new elec-
trochemical synthesis approach as extremely valuable for the for-
mation of paraffins possessing an even number of carbon atoms.
The method is worth being improved by the concept of many kinds
of carbons now used as cathodic materials, in particular porous 3D
materials.
3
.2. Preparative electrolyses
Macroelectrolyses of primary RI compounds were potentiostat-
ically achieved (À1.2 > E > À1.4 V vs. SCE) under argon atmosphere
from two-compartment cells (volume of catholyte ranging from 8
to 25 mL). Quantities of electrolyzed RI were between 0.05 and
À3
0
.5 Â 10 mol with efficient areas of used GC/Ag–Pd electrodes
2
of about 5 cm . After total completion of electrolyses (one-electron
process occured), the catholyte solution was mixed with water and
then extracted with diethylether. The extract was twice rinsed
with water and dried over magnesium sulfate. Systematically the
mass of dry extracts corresponded to about 80 % of the theoretical
value estimated for an exclusive formation of hydrocarbons.
Acknowledgment
The authors are grateful to Michèle Nelson (LRCS) for helpful
assistance.
References and notes
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2896.
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.