Electrochemical allylation of carbonyl compounds in aqueous electrolyte catalyzed by zinc

Article abstract of DOI:10.1039/b922897g

A highly efficient electroallylation of carbonyl compounds in aqueous electrolyte in a divided cell with a catalytic amount of zinc consumption is reported.

Full text of DOI:10.1039/b922897g

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Electrochemical allylation of carbonyl compounds in aqueous electrolyte
catalyzed by zincw
Jing-Mei Huang* and Hai-Rui Ren
Received (in Cambridge, UK) 2nd November 2009, Accepted 18th December 2009
First published as an Advance Article on the web 25th January 2010
DOI: 10.1039/b922897g
A highly efficient electroallylation of carbonyl compounds in
aqueous electrolyte in a divided cell with a catalytic amount of
zinc consumption is reported.
of LiClO4 in anolyte on the electroallylation reaction. It was
found that a 0.275 M LiClO4 solution produced a good yield
at 93% and fairly low Zn consumption (25 mol%) at a passed
charge of 3.5 FmolÀ1. Further increase of the concentration of
LiClO4 resulted in a decrease of the yield (entry 10, Table 1).
We also investigated the effect of currency density, and it
showed that increase of the currency led to a slightly higher
of Zn consumption (entry 11, Table 1). Other cathodes
(In, Sn, Al and Pt) showed lower yields and higher metal
consumption (entries 12, 13, 14, and 15, Table 1). Hence, a
best reaction condition is illustrated in entry 9, Table 1.
Subsequently, a variety of aldehydes were examined to
generate the allylation products using the optimized condition.
Table 2 summarizes the details of the results. Both aliphatic
and aromatic aldehydes could be converted to the corresponding
homoallylic alcohols in good to excellent yields. The
functionalities including methoxyl, chloro, bromo, hydroxyl
and carboxylic acid were tolerated by this mild method. It is
noteworthy that the glyoxylic acid can be used directly to give
a-hydroxyl acid (entry 10, Table 2). The regioselectivity of
reaction was also investigated by replacing allyl bromide with
crotyl bromide, which showed that g-substitution product was
obtained exclusively and the ratio of anti : syn was 50 : 50
(entry 12, Table 2). In the case of acetophenone, the rate of
the reaction was much slower than that of aldehydes and it
took 7 FmolÀ1 for the completion of the reaction, with a
higher Zn consumption at 35 mol% (entry 13, Table 2).
Further studies were carried out in order to disclose the
reactions occurred in the cathode and anode chambers.
Firstly, under the standard reaction conditions, the pH in
both chambers was monitored and it was observed that the pH
of the solvent in anode chamber dropped to 1.0, and that of
cathode chamber dropped to 2.0 during the process of the
reaction. Cyclic voltammogram of anolyte showed an anodic
wave.8 Thus, it was confirmed that in the anode chamber,
protons were produced by the electrolysis of water and the
in situ generated protons migrated into cathode chamber.
Hence, it suggested an acid promoted allylation pathway.
When no power applied to the cell, only 8% of allylated
product was observed in cathode chamber after 4 h. And when
the cathode was replaced with a Pt foil, 10% of homoallylic
alcohol was obtained (entry 15, Table 1) with 50% of pinacol
and 30% benzyl alcohol. Pre-adding 25 mol% of Zn powder
into the cathode chamber with a Pt cathode, resulted in longer
reaction time (6 h) and lower yield at 65%. Thus, the Zn cathode
likely plays an important role in the electroallylation process.
Furthermore, we have run the reaction without carbonyl
compounds, and hexadiene was observed by GC-MS analysis.
On the other hand, pinacol (69%) and benzyl alcohol (20%)
Carbonyl allylation is a highly important synthetic trans-
formation in organic and pharmaceutical chemistry, because
the homoallylic alcohols are versatile intermediates in the
preparation of materials, natural products, and bioactive
compounds.1 So far numerous methods have been reported to
be effective in metal mediated aqueous allylations,2 however,
most of them suffer from limitation with stoichiometric
amounts of metal consumption, or catalytic amounts of the
metal combination with another stoichiometric reducing
agent, and consequently produce large amounts of salt by-
products. Electrochemical methods in organic synthesis are
another approach to green chemistry.3,4 We have been
interested in electroallylation of carbonyl compounds in water
and reported a reaction in aqueous ammonia.5 As a sacrificial
zinc anode was applied, the disadvantage of this method was
using stoichiometric amounts of zinc. Recently, efforts have
been extended to the catalytic methods in our group. Here we
report firstly an efficient electroallylation in aqueous media
with catalytic amount of zinc consumed.
First, we attempt the reaction in an undivided cell and a
quasi-divided cell6 with a Pt anode and a Zn cathode at a
constant current of 30 mA. However, both methods were far
less desirable (yields less than 20%). Next, the electroallylation
was conducted in a divided cell. The cathode and anode
chambers were separated by a Nafions551 cation-exchange
membrane7 and the aldehyde and allyl bromide were added
into the cathode chamber. Investigations were then focused on
the optimizations on the anolyte and catholyte.
When 0.1 N H2SO4 was used as electrolyte (entry 1,
Table 1), the yield was 83% with a zinc consumption at
84 mol%. In alkaline solutions (4.5 M NH3ÁH2O)5 and neutral
(0.1 M LiClO4) solutions (entries 2 and 3), the zinc consumption
was decreased to 30–40 mol%, but the yields were much lower.
Then, we used 0.1 M LiClO4 solution as anolyte, and ammonium
chloride solution as catholyte, the yield and zinc consumption
were 46% and 32 mol%, respectively. Adding THF to the
catholyte increased the yield sharply (entries 5, 6 and 7,
Table 1). We then studied the influence of the concentration
School of Chemistry and Chemical Engineering,
South China University of Technology, 510640, Guangzhou, China.
E-mail: chehjm@scut.edu.cn; Fax: +86 020 8711 0622;
Tel: +86 020 8711 0695
w Electronic supplementary information (ESI) available: Experimental
procedure, cyclic voltammetry, XRD, XPS and NMR analyses data.
See DOI: 10.1039/b922897g
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Table 1 Optimization conditions on the electrochemical allylation of benzaldehydea
Entry
Anolyte
Catholyte
Cathode
Yield (%)b
Metal Consumption (mol%)c
1
2
3
4
5
6
7
8
0.1 N H2SO4
4.5 M NH3 H2O
0.1 N H2SO4
4.5 M NH3ÁH2O
Zn
Zn
Zn
Zn
Zn
Zn
Zn
Zn
Zn
Zn
Zn
Al
83
68
44
46
63
68
93
93
93
86
93
Trace
88
90
10
84
40
35
32
37
35
48
30
25
24
30
—
84
50
—
.
0.1 M LiClO4
0.1 M LiClO4
0.1 M LiClO4
0.1 M LiClO4
0.1 M LiClO4
0.25 M LiClO4
0.275 M LiClO4
0.3 M LiClO4
0.275 M LiClO4
0.275 M LiClO4
0.275 M LiClO4
0.275 M LiClO4
0.275 M LiClO4
0.1 M LiClO4
0.1 M NH4Cl
LiClO4 (0.1 M) : THF = 4 : 1
NH4Cl (0.1 M) : THF = 4 : 1
NH4Cl (0.1 M) : THF = 1 : 1
NH4Cl (0.1 M) : THF = 1 : 1
NH4Cl (0.1 M) : THF = 1 : 1
NH4Cl (0.1 M) : THF = 1 : 1
NH4Cl (0.1 M) : THF = 1 : 1
NH4Cl (0.1 M) : THF = 1 : 1
NH4Cl (0.1 M) : THF = 1 : 1
NH4Cl (0.1 M) : THF = 1 : 1
NH4Cl (0.1 M) : THF = 1 : 1
9
10
11d
12
13
14
15
Sn
In
Pt
a
Standard condition: benzaldehyde (1 mmol), allyl bromide (2 mmol) in cathode chamber, constant current (30 mA), a zinc foil cathode
b
c
(20 Â 15 Â 0.15 mm) and a platinum foil anode (15 Â 10 Â 0.1 mm), rt. Isolated yield. Based on 1 mmol of benzaldehyde. The cathode was
treated with simple rinse by water and ethyl acetate after the reaction and the amount of metal consumption during the electrolysis was obtained by
d
weighing the metal foil before and after the electrolysis. At a constant current of 45 mA.
Cyclic voltammetry study was applied to monitor the
process of reaction too. The solution in cathode chamber
was studied at 1, 1.5, 2, 2.5, 3 h, and the CV curves showed
reductive waves of Zn(II) for all the solutions.9 It demonstrated
that fresh Zn(II) species is produced continuously during the
reaction.
Table 2 Electrosynthesis of homoallylic alcohols from various
carbonyl compounds catalyzed by Zn in aqueous solutiona
The surface of used Zn electrode remains good and is even,
compact and in dark grey color.10 The surface composition of
the used zinc electrode was studied by XRD and XPS analysis,11
and it revealed that only Zn (0) existed on the surface without
any Zn(II), Cl, Br, N and Li species.
Entry
1
R1
R2
R3
3
Yield(%)b
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
1g
1h
1i
ph
H
H
H
H
H
H
H
H
H
H
H
H
CH3
H
H
H
H
H
H
H
H
H
H
H
CH3
H
3a
3b
3c
3d
3e
3f
3g
3h
3i
93
91
51
95
82
98
82
58
86
58
96
96d
91
phCH2CH2
phCHQCH
4-MeO-C6H4
2-OH-C6H4
3-Me-C6H4
4-Cl-C6H4
2-furyl
n-hexy
HO2C
2-Br–C6H4
ph
ph
Hence we propose that the transformation is initiated by the
reaction of the allyl bromide with the zinc on the surface of the
cathode, to generate the allylzinc species, which is responsible
for the allylation of the benzaldehyde to form the homoallylic
alcohol. Meanwhile, the Zn2+ salts are susceptible to direct
electrochemical reduction to produce zero valent zinc, which
react smoothly with allyl bromide to regenerate allylzinc.
Fig. 1 shows the simplified reaction process.
9
10
11
12c
13
1j
3j
3k
3l
1k
1a
1l
While another mechanism could not be neglected, in which
the direct electroreduction of aldehyde and allyl bromide on
the cathode to produce radical intermediates (1a and 2a) was
involved (Scheme 1).12 From the above experimental results, it
was hypothesized that the zinc mediated allylation reaction
predominates in the two mechanisms.13
3m
a
Carbonyl compound (1 mmol), allyl bromide (2 mmol), constant
current (30 mA), anolyte (10 mL 0.275M LiClO4), catholyte
(NH4Cl (0.1 M) : THF = 5 mL: 5 mL), platinum electrode (1.5 cm2),
zinc electrode (3 cm2), rt. Isolated yield. Allyl bromide was
b
c
d
replaced with crotyl bromide. Anti:syn = 50 : 50 (determined by
1H NMR analysis).
It is noteworthy that the zinc foil as a cathode could be
reused after the reaction was completed. The allylation
product was still obtained in a good yield (90%) even though
the cathode was reused in sixth cycle.14 It demonstrated that
the reactivity of the cathode was not significantly dropped,
although it lost a catalytic amount of weight for each cycle.
In conclusion, a Zn catalyzed and efficient electrosynthesis
of homoallylic alcohols from allylic bromides and aldehydes in
aqueous solution is achieved in a divided cell. It compares
favourably with previous procedure: Zn cathode can be reused
were collected without allyl bromide. Finally, when it was
electrolyzed in the absence of any reactant, we found that the
pH for the catholyte and the weight of Zn cathode remained
same after 3.5 h. It can be explained that all the protons
migrated from anode chamber were electroreduced right away
on the cathode when there was no Zn(II) species to compete
with protons for the electroreduction in the cathode solution.
ꢀc
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Fig.
1
Reaction pathway for the electrochemical allylation of
aldehyde in aqueous media.
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Scheme 1 Reaction pathway for the electrochemical allylation of
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´ ´ ´
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´
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7 The yield was very low when the chamber was separated by
sintered glass. For the cation-exchange membrane, Nafions 324
and Nafions 966 did not give good yields also. In these cases,
migration of the organic starting material to the anode chamber
was observed.
with a catalytic amount of Zn consumption for each cycle
instead of a sacrificial Zn anode. Studies have shown that the
fast in situ reduction of Zn2+ on the Zn cathode to regenerate
fresh and active Zn(0) for the next cycle of allylation reaction,
constitutes a key step for the catalytic C–C bond transformation
in aqueous media. Hence, the environmental friendliness and
high efficiency render this methodology an attractive alternative
for the synthesis of homoallylic alcohols. Further investigation
to determine the mechanism of this reaction and to expand its
scope is underway in our laboratory.
We express appreciation to Prof. Ye Jian-Shan and
Prof. Zhang Wei-De for valuable discussion on the
electrochemical process. We are grateful to the National
Natural Science Foundation of China (Grant 20972055).
8 See ESIw for cyclic voltammograms.
9 See ESI for cyclic voltammogramsw and discussion on the results.
10 In contrast, an experiment of the electrodepositon of 1.5 mmol
ZnSO4 in a solution of NH4Cl (0.1 M) and THF (5 mL : 5 mL) in
cathode chamber produced a layer of loosely packed Zn particles
on the Zn electrode. And the electrodeposited Zn particles can be
easily scraped from the Zn electrode.
11 See supporting information for the spectra of XRD and XPS.
12 CV studies were carried out on the aldehyde alone in the solvent-
supporting electrolyte system and the bromide alone in the same
system. The reductive peak potential for allylbromide is
about À0.9 V, and that of benzaldehyde is about À1.1 V. For
the details of CV experiments and the results, please refrer to ESIw.
13 M. C. C. Areias, L. W. Bieber, M. Navarro and F. B. Diniz,
J. Electroanal. Chem., 2003, 558, 125.
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14 See ESI (a table) for the yields and Zn consumption for each
cycle.
ꢀc
This journal is The Royal Society of Chemistry 2010
2288 | Chem. Commun., 2010, 46, 2286–2288