Sonoelectroreduction of metallic salts: A new method for the production of reactive metallic powders for organometallic reactions and its application in organozinc chemistry

Article abstract of DOI:10.1002/(sici)1099-0690(199911)1999:11<2845::aid-ejoc2845>3.0.co;2-r

The reactivity of ultrafine zinc powders, produced by a new electrochemical method coupled to the use of ultrasound, has been evaluated for various organometallic reactions such as the Reformatsky reaction, Barbier allylation, alkyne reduction, reductive dehalogenation and p- nitroester cleavage. The results show that zinc powders produced by sonoelectrochemistry are highly reactive. Particle-size effects, reactivity of zinc-copper alloys and metal-waste recycling procedures have also been studied. The production of highly reactive zinc powder by sonoelectrochemical methods appears to be an interesting alternative to conventional methods used in organozinc chemistry and may be especially important in medium- or large- scale processes.

Full text of DOI:10.1002/(sici)1099-0690(199911)1999:11<2845::aid-ejoc2845>3.0.co;2-r

FULL PAPER
Sonoelectroreduction of Metallic Salts: A New Method for the Production of
Reactive Metallic Powders for Organometallic Reactions and Its Application
in Organozinc Chemistry
Alex Durant,[a] Jean Luc Delplancke,[a] Valery Libert,*[a] and Jacques Reisse*[a]
Keywords: Sonoelectrochemistry / Ultrasound reaction / Zinc / Barbier reaction / Reformatsky reaction
The reactivity of ultrafine zinc powders, produced by a new
electrochemical method coupled to the use of ultrasound, has
been evaluated for various organometallic reactions such as
the Reformatsky reaction, Barbier allylation, alkyne
reduction, reductive dehalogenation and p-nitroester
cleavage. The results show that zinc powders produced by
sonoelectrochemistry are highly reactive. Particle-size
effects, reactivity of zinc-copper alloys and metal-waste
recycling procedures have also been studied. The production
of highly reactive zinc powder by sonoelectrochemical
methods appears to be an interesting alternative to
conventional methods used in organozinc chemistry and may
be especially important in medium- or large-scale processes.
Metallic zinc as required in organometallic chemistry is
generally an expensive reagent because it is obtained from
a costly reduction procedure. One of the best, but most ex-
pensive, procedures is the reduction of a dry zinc salt by
metallic lithium or metallic potassium in an anhydrous or-
ganic solvent. The highly reactive zinc thus obtained has
been extensively used in the literature, and the names of
Rieke and his co-workers are definitively linked to the prep-
aration of these active metals by reduction of salts with
group I metals in organic solvents.[1,3,7]
It is probably difficult to obtain more active metals and
it could be concluded that an alternative method is not
needed. Although this conclusion is perhaps true for the
synthetic organic chemist working in a research laboratory,
environmental constraints must be taken into account for
larger scale processes. Organic solvents and the use of met-
allic lithium as a reducing agent are certainly rather less
environmentally friendly than water and electrons. In other
words, active zinc obtained by chemical reduction of zinc
salts in nonaqueous media does not fulfill the criteria of
“green chemistry”.
The only reduction of a zinc salt which does not gener-
ate a by-product is direct electrochemical reduction. This
was the starting point for our efforts to develop a new
method able to give metallic zinc (or other metals or al-
loys) directly. Good reagents in organometallic chemistry
must be of high purity and fine granularity in order to
obtain the highest possible surface/volume ratio. The me-
tal obtained by our electrochemical method possesses both
of these characteristics; this metal is produced electro-
chemically and ultrasound is used to expel the newly
formed zinc crystals from the surface of the electrode and
so prevent their further growth. The new method is of low
cost with an absence of by-products, and allows the pos-
sibility of recycling the zinc salt obtained at the end of
the reaction.
Introduction
The formation of organometallic compounds by the reac-
tion of organic substrates with finely divided metal powders
is a powerful tool for the synthetic chemist. Direct reaction
with zero-valent metal is the only viable synthetic method
for the production of most of these compounds because,
for a large number of functional groups (e.g. ketone, ester,
aldehyde, nitrile, epoxide), preparation by transmetallation
starting from organomagnesium or organolithium reagents
is not possible. However, the low reactivity of the majority
of commercial zero-valent metals requires physical or
chemical activation through a nonclassical preparation
method.[1Ϫ5] Adsorption of the freshly prepared zero-valent
metal on graphite[4] or titanium oxide[6] are excellent acti-
vation methods although this implies the initial chemical
reduction of metal salts to prepare the zero-valent metal.
Of the numerous metals used in organometallic chemistry,
zinc plays a particularly important role due to its low cost
with respect to many other metals, its versatility and also
to the fact that it can be used with water as solvent.[8Ϫ14]
The present paper describes a new method for the prep-
aration of highly active, finely divided zinc metal. The ad-
vantages of this new method are its low cost, the use of
water as a solvent, its versatility and, most important, the
possibility it offers for recycling the zinc salt obtained at
the end of the reduction step of the organic substrate. In
organozinc chemistry, the zinc salt obtained at the end of
the reaction is always considered to be a by-product which
is removed from the reaction and then either discarded or, in
industry, stockpiled and eventually recycled for other uses.
[a]
´
Universite Libre de Bruxelles,
50 avenue F.D. Roosevelt (CP165), B1050 Brussels, Belgium
Fax: (internat.) ϩ 32-2/650-3605
E-mail: jreisse@ulb.ac.be
[b]
Lilly Development Centre,
´
11 rue Granbonpre, B1348 Mont-Saint-Guibert, Belgium
Eur. J. Org. Chem. 1999, 2845Ϫ2851
WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999
1434Ϫ193X/99/1111Ϫ2845 $ 17.50ϩ.50/0
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A. Durant, J. L. Delplancke, V. Libert, J. Reisse
FULL PAPER
Results and Discussion
The Sonoelectroreduction of Zinc Salts
It is well-known that the electroreduction of a zinc salt
(or other metallic salts) on a cathode leads to metal depo-
sition. Electrometallurgy is indeed the classical method for
zinc preparation. If the current density is very high, the nu-
cleation rate increases with respect to the growing rate and
small crystals may be obtained instead of large deposits.
The originality of the electroreduction we have devel-
oped[16][17] is essentially due to the use of out-of-phase pul-
ses of high-density current and high-intensity pulsed press-
ure waves. A titanium-rod cathode forms part of an immer-
sion horn activated by two piezoelectric ceramics and there-
fore also acts as an ultrasound emitter. During the electrical
pulse, a large number of small metal nuclei are produced.
The subsequent 20 kHz high-intensity ultrasound pulse ex-
pels the nuclei from the sonocathode, preventing the growth
of the metal particles on the cathode surface. Furthermore,
the ultrasound pulses clean the cathode surface (avoiding
its passivation) and fully replenish the diffusion layer with
metal cations by stirring the solution.[18] The metal particles
so obtained are thus in a finely divided state with high sur-
face areas and have a high degree of chemical purity. Conse-
quently, this zinc powder is expected to exhibit a high de-
gree of reactivity towards organic substrates. On the other
hand, it must be noted that the sonoelectrochemical process
is safe and environmentally friendly (the sonoelectroreduc-
tion is performed in an aqueous medium and requires only
metal salts). It is cheap and easily reproducible (all param-
eters can be controlled and electricity is the cheapest re-
agent for the organic chemist). Another interesting feature
of the pulsed sonoelectrochemical technique is its ability to
control the size of the metal particles by modifying the elec-
trochemical and ultrasound parameters.[17] All other par-
ameters being constant, the shorter the electrical pulse dur-
ation, the smaller is the mean diameter of the metal nuclei
and nanopowders are produced. High resolution trans-
mission electron microscopy of zinc powders produced from
a pure zinc sulfate bath with a current density of 2000 A/
Figure 1. High resolution transmission electron microscopy of zinc
nanopowder produced from a pure zinc sulphate bath
of Zn2ϩ and Cu2ϩ in the electrolyte. Following this pro-
cedure, we obtained two Zn/Cu alloys one containing about
60% zinc to 40% copper and the other about 90% zinc to
10% copper. Finally, it must be observed that, per se, the
sonoelectroreduction of metal salts is a very good way of
minimising metal waste by recycling. Indeed, it is the metal
cations resulting from the zero-valent metal consumed in
organometallic reactions that are the starting material in
ultrafine metal-powder production by sonoelectroreduc-
tion. This advantage of our new method will be illustrated
at the end of this paper.
Examples of the Use of the Zinc Powder Obtained by
Sonoelectroreduction in Organometallic Chemistry
The examples which will be shown below correspond to
m2 during pulses of 100 ms is shown in Figure 1. The par- well-known reactions leading to products already described.
ticle size distribution curve reveals a mean diameter of Our aim is only to show that, in many different cases, the
about 50 nm together with larger agglomerates.
zinc powder we obtain is comparable with the best active
The electrical yield of zinc electrodeposition is always less zinc available on the market and we are therefore confident
than 100%. Hydrogen evolution takes place in agreement that what can be exemplified with known reactions reac-
with thermodynamic studies.[19] Due to the strong agitation tants and products would be also true for reactants and
promoted by the ultrasound, oxygen degassing of the elec- reactions that have not yet been studied. In all cases the
trolyte is very effective. This dissolved oxygen is replaced by yields which are given correspond to a quantitative analysis
dissolved hydrogen generated during each electrochemical by NMR spectroscopy (and in many cases HPLC quantitat-
pulse which seems to prevent oxidation of the freshly pre- ive analysis) of the crude reaction mixture after elimination
pared metal particles.
of the extraction solvent. In all cases, the reaction mixture
On the other hand, it is also possible to produce by sono- was clean, containing the starting reactant(s) and the prod-
electrochemistry zincϪcopper alloys that are in a finely di- uct(s) with less than 2% of noncharacterised contaminants.
vided state. Indeed, the sonoelectroreduction of mixtures of More details are given in the Experimental Section. The
Zn2ϩ and Cu2ϩ leads to fine Zn/Cu alloy powders with reactions used to demonstrate the efficiency or our new
good electrical yields. The composition of the alloy pro- method for reactive zinc preparation were chosen in such
duced by sonoelectrochemistry depends on the relative ratio a way as to have a large variety of different experimental
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Eur. J. Org. Chem. 1999, 2845Ϫ2851

Sonoelectroreduction of Metallic Salts
FULL PAPER
conditions, including reactions taking place in water and in Table 2. Barbier allylation in THF with sonoelectroproduced zinc
organic solvents.
Reformatsky Reaction
The Reformatsky reaction of benzaldehyde with ethyl
bromoacetate has been extensively studied in the litera-
ture.[20] Various activated zinc species have been used for
this reaction. It is therefore possible to compare the reactiv-
ity of the zinc powder produced by our sonoelectrochemical
method with others. From the results reported in Table 1,
it is obvious that the zinc powder produced by sonoelectro-
chemistry is one of the most reactive forms known and is
almost as reactive as Rieke zinc.
Table 1. Comparison of the reactivity of zinc powder produced
sonoelectrochemically with other zinc species in the Reformatsky
reaction
Table 3. Effect of the reaction medium and the nature of the halide
for the allylation of benzaldehyde in the presence of sonoelectro-
produced zinc
Barbier Reaction
We have previously reported that zinc powder pro-
duced sonoelectrochemically exhibits a high degree of reac-
tivity in the allylation of benzaldehyde in an aqueous me- rate-determining step seems to be the electron transfer from
dium.[16] This reaction was also performed in an organic zinc to the organic halide. On the other hand, classical or-
solvent and can be efficiently extended to less-reactive sub- ganometallic reactions in aprotic solvents strongly depend
strates than benzaldehyde such as aliphatic ketones, acyl on chelation and are therefore very sensitive to the nature
chlorides or even alkynes (Table 2).
of the halogen atom.
The reaction with benzoyl chloride does not stop after
Reactivity differences in THF and water as solvents are
the substitution of the chlorine atom since the more reactive also observed when various bromo-derivatives reacting with
carbonyl function is immediately involved in a subsequent benzaldehyde are compared (Table 4). In THF, the mecha-
reaction with allyl bromide. On the other hand, allyl chlo- nism probably implies the formation of a true CϪZn bond
ride, which is usually a poor reagent in organozinc chemis- and shows the classically poor reactivity of alkyl bromide
try compared to allyl bromide, reacts with benzaldehyde in with respect to allyl bromide or ethyl bromoacetate. In
the presence of zinc powder produced by sonoelectrochemi- water, allyl bromide is the only derivative of the series which
stry, even in an aprotic solvent (Table 3).
can react in a significant way.
However, the behaviour is different in an aqueous me-
The lack of reactivity of alkyl bromides in THF or water
dium. In this latter case, the reaction proceeds via a Љone- has already been described in the literature[13] with pure
electron transferЉ mechanism.[2] Similar reactivities can zinc as reagent. In this case, the superiority of Rieke zinc
therefore be predicted for chloride and bromide since the for carbonyl compounds alkylation is obvious.[7]
Eur. J. Org. Chem. 1999, 2845Ϫ2851
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A. Durant, J. L. Delplancke, V. Libert, J. Reisse
FULL PAPER
Table 4. Comparison of the reactivity of some bromide reagents in order to optimize the rate/stereoselectivity balance of the
aqueous and aprotic media for the Barbier reaction with benzalde-
hyde (reaction time: 1 hour)
reduction. In this context, distilled water seems to be a good
compromise. The reaction can be easily extended to simple
aromatic and aliphatic alkynes (Table 6). The aqueous re-
duction with sonoelectrochemically produced zinc powder
is accordingly a general, safe, easy, smooth and chemoselec-
tive method to reduce triple bonds to double bonds with
high stereoselectivity.
Table 6. Reduction of aromatic and aliphatic alkynes with zinc
powder produced by sonoelectroreduction
Alkyne Reduction
The chemo- and stereoselective reduction of α-hydroxy
alkynes has been reported in the literature.[22] We per-
formed these reductions in order to compare once again
Reactivity of Zinc؊Copper Alloys
the reactivity as well as the chemo- and stereoselectivity of
The sonoelectroreduction of salt mixtures is a simple
sonoelectrochemical zinc with those of highly active Rieke method of preparing metal alloys at room temperature.
zinc. Zinc powder produced by the new method reduces α- Since sonoelectroreduction can be used to prepare alloys
hydroxy alkynes with a degree of efficiency, a chemospec- of various compositions we decided to test the relationship
ificity and a stereoselectivity similar to Rieke zinc (Table 5). between the copperϪzinc content of the alloys and their
reactivity in a very typical Barbier reaction (Table 7). Since
Table 5. Reduction of α-hydroxy alkynes with zinc powder produ-
copper is ineffective in this reaction, it is normal to observe
ced by sonoelectroreduction: reactivity, and chemo- and stereose-
a significant rate decrease as soon as the amount of copper
becomes appreciable. More interesting is the acceptable re-
activity decrease observed with a 90/10 zincϪcopper alloy
and the possibility of seeing whether the decrease in reactiv-
ity is associated with an increase in selectivity. This was
tested in the case of two different reactions, namely a re-
ductive dehalogenation[23] (which can give mono- and dide-
halogenated products) and a competitive allylation starting
from a 1/1 mixture of an aldehyde and a ketone (Table 8).
The rate decrease, acceptable in all cases, is associated with
a selectivity increase, but the change is not sufficient to be
of great importance in organic synthesis.
lectivity
Table 7. Reactivity of sonoelectroproduced zinc-copper alloys and
pure metals in the allylation of benzaldehyde
Furthermore, it appears that the use of aqueous systems
increases the reaction rates. Nevertheless, the resulting α-
hydroxy alkenes can isomerize in acidic aqueous systems.
Reaction conditions must therefore be carefully chosen in
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Sonoelectroreduction of Metallic Salts
FULL PAPER
Table 8. Effect of the sonoelectroproduced alloy composition for
reductive dehalogenation and benzaldehyde allylation
dation. In order to solve the metal waste recycling problem
we were obliged to apply an alternative procedure. The p-
NO2-benzyl ester in THF was added to a zinc powder sus-
pension previously obtained by the sonoelectroreduction of
an aqueous electrolyte (NH4Cl 2  and ZnCl2 0.24 ).
After being stirred for 30 minutes, the reaction mixture was
extracted twice with dichloromethane and the aqueous
phase directly re-used in subsequent zinc powder sonoelec-
troproduction and organometallic reaction steps. In this
way, we performed six quantitative ester cleavages with the
same electrolyte and without any significant loss of zinc cat-
ions (<15%, determined by colorimetric titration of Zn2ϩ
by dithizone).
Conclusions
The sonoelectrochemical reduction of metal salts in water
appears to be an interesting alternative to conventional
methods for reactions in organozinc chemistry. Indeed, zinc
powders produced by sonoelectrochemistry are generally
highly reactive. Furthermore, the process is safe, cheap and
easy to control, and it is possible to recycle the metal salts
produced during the reaction. As we have previously
shown, the sonoelectrochemical method described here is
not limited to the production of zinc. Moreover, it can be
performed in solvents other than water. Many other metals,
oxides and even semi-conductors can be produced as very
fine powders and with a very high degree of purity by this
technique.[25] Considering the importance of heterogeneous
liquidϪsolid reactions in organic synthesis where the solid
can be a reactant (as illustrated in this paper) or a catalyst,
the method described here may have many other appli-
cations in synthetic organic chemistry. Applications in ma-
terial sciences are also under study in our laboratories.
Metal Waste Recycling
As discussed in the introduction, the recycling of zinc
salts at the end of the reduction of the organic substrate is
a major challenge and chemists, even those working in re-
search laboratories, are more and more concerned by this
problem. Obviously, the method described here for zinc
preparation provides the opportunity for recycling the zinc
salts generated by organometallic reactions using an electric
current as the only source of electrons. Recycling metal wa-
ste can be considered in two different ways. On the one
hand, it might be possible to re-use the cationic zinc species
obtained after the organometallic reaction for the sonoelec-
trolytic production of the zinc powder as a distinct step.
However, on the other hand, it would be more interesting
to establish an in situ catalytic recycled system. Thus, we
decided to test this stategy on the cleavage of p-nitroesters
with zinc.[24]
We found first that it was possible to perform the in situ
ester cleavage of cbz-p-NO2-benzylphenylalanine. This
corresponds to the sonoelectroproduction of the zinc pow-
der in the presence of the p-NO2-benzyl ester, which is elec-
trochemically inert in the electrolysis conditions in the ab-
sence of zinc salts but which is fully cleaved after 90 minutes
of electrolysis in the presence of zinc salts.
Experimental Section
Sonoelectrochemical Process for the Production of Ultrafine Zinc
Powder: The sonoelectrochemical device has been described pre-
viously.[16] In a typical procedure a mixture of zinc chloride (8 g)
and ammonium chloride (supporting electrolyte, 26 g) in water
(250 mL) was subjected to a pulsed electric current (current den-
sity ϭ 8000 A/m2; pulse duration ϭ 300 ms) and pulsed pressure
waves (ultrasound intensity above the cavitation threshold; pulse
duration ϭ 100 ms followed by a dead time of 200 ms) for one
hour with the two irradiation periods out of phase. The resulting
Nevertheless, we did not succeed in performing the pro-
cess catalytically. Furthermore, the electrolyte solution did
not resist for a long time, probably due to a THF degra- suspension was filtered under argon on a 0.1 µm Millipore filter,
Eur. J. Org. Chem. 1999, 2845Ϫ2851 2849

A. Durant, J. L. Delplancke, V. Libert, J. Reisse
FULL PAPER
washed with water, rinsed thoroughly with anhydrous THF and
dried under an argon flux, affording 613 mg of finely divided zinc
powder (electrical yield ϭ 67%). The electrolysis could be extended
for longer periods in order to produce larger amounts of zinc pow-
der. High-resolution transmission electron microscopy pictures of
the obtained zinc powder showed that most of the metal particles
were submicronic (around 500 nm) (see Figure 1).
with HCl, extracted twice with ether and dried over MgSO4. Con-
centration in vacuo of the organic phases gave a mixture of the
dehalogenated products which was analysed by 1H NMR spec-
troscopy and HPLC.
Procedure for the Competitive Allylation of Benzaldehyde and 5-
Nonanone: Benzaldehyde (8 mmol), 5-nonanone (8 mmol) and allyl
bromide (1210 mg, 10 mmol) in THF (10 mL) were added to 15
mmol of the metal species in 10 mL THF under a nitrogen atmos-
phere at room temperature. The resulting mixture was stirred for
one hour, diluted with water and extracted twice with dichloro-
methane. Concentration in vacuo of the organic phases gave the
crude reaction mixture which was analysed by 1H NMR spec-
troscopy.
Sonoelectrochemical Process for the Production of Zinc؊Copper Al-
loys: The sonoelectroreduction (electrical and ultrasonic param-
eters described in the typical procedure reported above) of an elec-
trolyte containing CuSO4·5H2O (6 g/L); ZnCl2 (33 g/L) and NH4Cl
(107 g/L) gave a Zn/Cu alloy for which the composition, deter-
mined by Energy Dispersive X-ray analysis, was about 60% of zinc
and 40% of copper (electrical yield ϭ 92%). In a similar manner, a
ZnϪCu alloy containing about 90% of zinc and 10% of copper
was obtained by changing the CuSO4·5H2O concentration (2 g/L;
electrical yield ϭ 71%). The alloy powders were shown to be true
solid solutions and not simple mixtures of zinc and copper or cop-
per cementation on the zinc particles by X-ray diffraction.
Procedure for the In Situ Ester Cleavage of Compound 3: A mixture
of the ester (2 mmol) in THF/H2O (60 mL), and (NH4)2SO4 (0.32
) and ZnSO4·7H2O (0.22 ) (1/1) was subjected to pulsed elec-
trolysis (parameters described above in the sonoelectrochemical
process; platinum counter-electrode). The evolution of the reaction
was followed by HPLC.
Procedure for the Reformatsky Reaction of Benzaldehyde: Benzal-
dehyde (848 mg, 8 mmol) and ethyl bromoacetate (1670 mg, 10
mmol) in 10 mL THF were added to zinc powder (982 mg, 15
mmol) in 10 mL THF under a nitrogen atmosphere at room tem-
perature. The resulting mixture was stirred for one hour, diluted in Acknowledgments
water and extracted twice with dichloromethane. The concentration
in vacuo of the organic phases gave 1600 mg of an oil (100% reac-
tion yield based on an NMR spectroscopic analysis {at 250 MHz}
of the relative integrations of the aldehydic proton for benzaldehyde
and the proton on the carbon bearing the hydroxy group for the
hydroxy ester). Flash chromatography (CH2Cl2/silica gel) of the
crude mixture gave 1428 mg of the expected product (92% iso-
lated yield).
One of the authors (J.R.) would like to thank the COST Chemistry
Program (D-6 Action) for its support. J.L.D. is grateful to the
Fonds National de la Recherche Scientifique of Belgium for finan-
cial support.
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powder (982 mg, 15 mmol) in THF (10 mL) under a nitrogen at-
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Sonoelectroreduction of Metallic Salts
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