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M. Nechab et al. / Journal of Organometallic Chemistry 691 (2006) 1809–1813
devoted to the electrochemical activation of heteroaro-
matic compounds we decided to reinvestigate this proce-
dure and envisaged a stepwise approach towards such
compounds. Thus, we now report herein a convenient syn-
thesis of various alkyl- and aryldiphenylphosphines start-
ing from chlorodiphenylphosphine and organic halides,
and involving the electrochemical preparation of magne-
sium chloride diphenylphosphanide as the key step.
reduction potential (ca. À2 V versus SCE). This formal
ECE mechanism is depicted in Scheme 1.
The difficulty to reduce Ph2P–PPh2 into the correspond-
ing phosphanide Ph2PÀ, as attested by such a reduction
potential, could be confirmed in a chemical experiment
involving chlorodiphenylphosphine and magnesium pow-
der. Indeed, the very exothermic reaction gave rise to the
exclusive formation of Ph2P–PPh2 which proved to be very
sensitive to moisture but could be nevertheless detected
using GC/MS. Such reduction potentials are also consis-
tent with previous results which revealed that one-step
electroreductive couplings of chlorodiphenylphosphine
and benzyl halides are inefficient. This behavior can be
attributed to the competitive reduction of Ph2P–PPh2 and
these halides which can occur near À2 V (versus SCE).
Such results might also be predicable with aryl iodide
and some carbonyl- or nitrile-bearing benzenes which are
also reducible compounds. Thus, the use of a two-step pro-
cedure was essential to circumvent such drawbacks. Fol-
lowing typical experimental conditions [18], alkyl halides
and aryl fluorides were then exposed to the electrogener-
ated phosphanide and allowed to react for few hours at
room temperature.
In the laboratory, it was previously shown that one-step
coupling reactions occur upon electroreduction of a mix-
ture of chlorodiphenylphosphine and alkyl halides using
a consumable magnesium anode [16]. The scope of the
reaction was somewhat limited by the obligation to use ali-
phatic haloalcanes to avoid electrochemical side-reactions.
This behaviour was confirmed within the framework of a
preliminary study in which mixtures of benzyl halide and
chlorodiphenylphosphine were exposed to electroreduction
using a sacrificial magnesium anode. Indeed, only a small
amount of the wanted benzyldiphenylphosphine was
detected in reaction mixtures while toluene and bibenzyl
were formed in large amounts. Then, in order to extend
the scope of the reaction, we decided to set up a stepwise
protocol in which the first step would be the electroreduc-
tion of chlorodiphenylphosphine. Thus, our first goal was
to find the most simple and efficient conditions to achieve
the electrochemical activation of chlorodiphenylphosphine
as its Grignard-related form, magnesium chloride diph-
enylphosphanide. Independently, it was previously shown
in the laboratory that the use of a sacrificial magnesium
electrode process in dimethylformamide can allow the con-
version of aromatic amines into convenient nucleophiles
for the displacement of aliphatic halides [17]. Thus, it was
postulated that related reaction conditions could be
adapted to the activation of chlorodiphenylphosphine
and indeed the hypothesis was confirmed on the base of
preliminary electrochemical reductions which revealed the
formation of nucleophilic species in the reaction medium
upon reaction with benzyl iodide. Using this test-reagent,
it was found that no phosphanide was present in the reac-
tion medium until a charge corresponding to ca. 1 F/mol
charge was passed while a charge corresponding to ca.
2.5 F/mol (e.g. 2500 C for 10 mmol of Ph2PCl) was the
optimum value for the completion of the electrolyses.
During the course of the electrolyses, which were carried
out at a constant current intensity of 0.2 A, the cathodic
potential was monitored. It was remarked an evolution
from ca. À1.4 V vs. SCE to ca. À2 V versus SCE after a
charge corresponding to ca. 1 F/mol was passed. This
observation is consistent with the electroreduction of
Ph2PCl into Ph2PMgCl (at ca. À1.4 V versus SCE) which
fast react with remaining Ph2PCl to provide the dimeric
compound Ph2P–PPh2 which is further reduced at a lower
Results are presented in Table 1.
Yields of alkyl- and aryldiphenylphosphines [20] are gen-
erally good, ranging from 35% to 85%. It can be noted that
yields can be improved by using a slight overstoechiometric
amount of chlorodiphenylphosphine at the electrolysis step.
This behaviour can be clearly illustrated with comparison of
entries 3 and 4 of Table 1 or 9 and 14 of Table 1 for which
yields were improved from 38% to 83% or from 50% to
84%, respectively. The residual water which can be present
in the solvent might be responsible of the consumption of
a part of chlorodiphenylphosphine thus requiring the use
of a slight excess of this starting compound.
It is noteworthy that ortho-but also meta-substituted
aryl fluorides react efficiently, best results being neverthe-
less obtained with electron-withdrawing substituents
located at the ortho position. These results are consistent
with a SNAr-type reaction mechanism in which aryl fluo-
rides are also known to react more efficiently than aryl
chlorides. This could be obviously confirmed by an exper-
iment (entry 11 of Table 1) which showed that in the pres-
ence of both fluoride and chloride connected to the
phenyl moiety, only the fluoride is displaced by the
phosphanide.
On the other hand, we decided to test the scope of the
reaction and some preliminary experiments involving chlo-
rodiphenylphosphine and aryl bromides were conducted in
this goal. It could be observed that such couplings do not
proceed in standard conditions described below [18] but
require heating in the presence of a metal-transition cata-
e-, Mg anode
i = 0.2 A
Ph2PCl
e-, Mg anode
2 Ph2PMgCl
Ph2PMgCl
Ph2P PPh2
Ph2PCl
i = 0.2 A
Scheme 1.