Electrochemical allylation of aldehydes in a solvent-free cavity cell with a graphite powder cathode

Article abstract of DOI:10.1039/c0gc00947d

Mixtures of carbonyl compounds and allyl halides, adsorbed on a graphite powder cathode, were reduced under aqueous anolyte to give high yields of homoallylic alcohols. Catalytic amounts of silver or tetrabutylammonium tetrafluoroborate improved the yields significantly. Inexpensive allyl chloride gave superior results when compared to allyl bromide.

Full text of DOI:10.1039/c0gc00947d

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COMMUNICATION
Electrochemical allylation of aldehydes in a solvent-free cavity cell with a
graphite powder cathode
Ronny F. M. de Souza, Madalena C. C. Areias, Lothar W. Bieber and Marcelo Navarro*
Received 23rd December 2010, Accepted 14th February 2011
DOI: 10.1039/c0gc00947d
Mixtures of carbonyl compounds and allyl halides, ad-
sorbed on a graphite powder cathode, were reduced under
aqueous anolyte to give high yields of homoallylic alco-
hols. Catalytic amounts of silver or tetrabutylammonium
tetrafluoroborate improved the yields significantly. Inex-
pensive allyl chloride gave superior results when compared
to allyl bromide.
the selective reductive dimerization of benzyl halides or their
coupling with benzaldehyde.13 Here we describe the application
of this simple electrochemical method to promote the allylation
of aldehydes in high yield (Scheme 1).
The allylation of carbonyl compounds has been extensively
studied due to the utility of homoallylic alcohols, which are
versatile building blocks in the synthesis of molecules of greater
complexity.1 Among the synthetic protocols developed recently,
the aqueous Barbier reaction has special interest: metals such as
zinc, tin, indium and magnesium are used successfully in the ally-
lation reaction of carbonyl compounds and other electrophiles
in aqueous media.2 However, excess of metal, halide and co-
solvents is required in most cases, causing important economical
and environmental drawbacks. In addition, the preference for
allyl bromide instead of chloride makes the process even more
expensive and polluting. An alternative to these problems is the
use of electrochemical systems, which can be controlled through
adjustment of current or potential, and are classified as a
green approach.3 Nevertheless, few studies address the allylation
reaction of carbonyl compounds in electrochemical systems.
Most studies are limited to anhydrous solvents,4 concentrated
solutions of ammonium salts,5 heavy metal cathodes,6 sacrificial
Scheme 1
Explorative voltammetry of allyl chloride (1a) and bromide
(1b) using the same electrolytic cell with very slow scan rates12,13
indicated reduction at potentials between -1.0 and -1.4 V (vs.
Ag/AgCl, KCl(sat’d)) for both halides producing propene gas
(3) and 1,5-hexadiene (4). When equimolar mixtures of the
halides 1a and 1b with benzaldehyde (2a) were electrolyzed at
a constant potential of -1.0 V, the same reduction products 3
and 4 were obtained, without any trace of the expected addition
product 5a. A more encouraging result of 8% of 5a (Table 1,
entry 1) was obtained with 1a using graphite powder doped
with 2% of silver13 (method A†). Optimization of the applied
potential and stoichiometry of this system (Table 1, entries 2–9)
led to a nearly quantitative yield of addition product 5a, when
1a and 2a in a 3 : 1 ratio were electrolyzed at -1.4 V at room
temperature (Table 1, entry 9).
Although silver-doped graphite can be easily recovered and
reused several times without loss of efficiency,13 a completely
metal-free alternative was developed adding 3% of tetrabutylam-
monium tetrafluoroborate to undoped graphite powder (method
B†). Using this cathode material with 1a and 2a, similar or even
superior yields were observed (Table 2, entries 1–3). Method
B was clearly more efficient at all potentials applied, when
1b was used instead of 1a (Table 2, entries 4–6). As almost
all experiments with 2a gave the highest yields at -1.4 V, the
anodes,7 use of metal catalysts8,9 and micro-flow systems.10
A
large amount of supporting electrolyte makes the isolation of
the product difficult.
An initial approach to overcome these problems has been
achieved recently in the related Reformatsky reaction, by
adsorbing the pure reagents on a graphite fibre cathode, in
an aqueous electrolyte.11 Optimization of these principles led
to the construction of a new electrochemical cell which uses
graphite powder in the cathode cavity combined with an
inert anode in aqueous electrolyte, resulting in high yields
of b-hydroxyesters.12 This methodology was also applied in
Departamento de Qu´ımica Fundamental, CCEN, Universidade Federal
de Pernambuco, av. Prof. Luis Freire S/N, 50740-901, Recife, Brazil.
E-mail: navarro@ufpe.br; Fax: +55 81 2126 8442; Tel: +55 81 2126
7460
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Table 1 Experimental optimization of the electrochemical allylation
reactiona (1a + 2a), using graphite powder doped with Ag
(Table 2, entries 13 and 14). By contrast, cinnamaldehyde (2f)
was effectively allylated only with 1a and method A, giving
selectively a-addition (Table 2, entries 15 and 16).
The relative superiority of this combination was also evi-
denced by the results with the saturated aldehyde (2g), although
the yield of 5g was unsatisfactory, probably due to competitive
aldol-type reactions (Table 2, entries 17 and 18). Even lower
yields were observed with acetophenone (2h) in all experiments,
but activation of the ketone by the trifluormethyl group in (2i)
provided an excellent yield with 1a by method A (entries 19–22).
In analogy to the observations with benzyl halides, a car-
banionic mechanism can be proposed for this novel electro-
chemical addition reaction, which is strongly supported by
the pronounced dependence of the electrophilic power of the
carbonyl substrate. However, further studies will be necessary to
corroborate this interpretation.
Entry
Reagent ratio (1a:2a)
E/V b
Yield of 5a (%) c
1
2
3
4
5
6
7
8
9
1 : 1
1 : 1
1 : 1
2 : 1
2 : 1
2 : 1
3 : 1
3 : 1
3 : 1
-1.0
-1.2
-1.4
-1.0
-1.2
-1.4
-1.0
-1.2
-1.4
8
30
14
57
62
24
91
88
95
a General reaction conditions: 0.2–0.6 mmol 1a and 0.2 mmol 2a
using the method A†; each reaction was continued until transfer of
the theoretical charge or until the current reached -1.0 mA at room
temperature. b vs. Ag/AgCl, KCl(sat’d). c Yields were determined by GC
analysis taking 2 as the limiting reagent.
From the preparative point of view, the results described
here are comparable or superior to conventional organometallic
methods, including Barbier-type reactions, at least in the case
of aromatic aldehydes. In relation to the challenges of green
chemistry, electrochemical allylation on a graphite cathode has
an unprecedented combination of advantages:
∑ No expensive or hazardous organometallic compounds are
required.
∑ The less expensive and less polluting allyl chloride gives
better yields than bromide.
∑ No organic solvent or sacrificial electrode is used in the
reaction.
∑ Inert atmosphere, purging and stirring are unnecessary.
∑ Reagents and products are confined to the cathode material
and are easily isolated.
following experiments with other carbonyl compounds were
all performed at this potential comparing both methods, A
and B. The highly electrophilic pentafluorbenzaldehyde (2b)
produced nearly quantitative yields of addition with both halides
and both methods (Table 2, entries 7 and 8), whereas the less
activated 4-fluoro-substituted aldehyde (2c) showed significantly
better results with 1a by method A and B (Table 2, entries 9
and 10). The same effect was even more pronounced with 2-
methylbenzaldehyde (2d), a sterically and electronically deacti-
vated substrate (Table 2, entries 11 and 12). 2-Naphthaldehyde
(2e) also demonstrated the superiority of 1a for both methods
Table 2 Electrochemical reactiona of allyl halides with aldehydes and
ketones
∑ High yields and faradic efficiencies are achieved.
∑ The catalysts are environmentally benign or easily recovered.
∑ The progress of the reaction can be electrochemically
followed.
Yield of product
5a–i (%) c
Entry Carbonyl compound Halide E/V b Method A Method B
1
2
3
4
5
6
7
8
2a
2a
2a
2a
2a
2a
2b
2b
2c
2c
2d
2d
2e
2e
2f
1a
1a
1a
1b
1b
1b
1a
1b
1a
1b
1a
1b
1a
1b
1a
1b
1a
1b
1a
1b
1a
1b
-1.0
-1.2
-1.4
-1.0
-1.2
-1.4
-1.4
-1.4
-1.4
-1.4
-1.4
-1.4
-1.4
-1.4
-1.4
-1.4
-1.4
-1.4
-1.4
-1.4
-1.4
-1.4
91d
88d
95d
13
36
22
>99
>99
>99
40
85
Trace
97
13
87
67
28
16
6
Trace
95
53
89
92
98
33
89
84
>99
91
>99
42
89
4
96
57
21
27
9
Conclusions
In summary, we demonstrate here the synthetic utility of a
simple graphite powder cell for the allylation of aldehydes. The
procedure eliminates organic solvents and sacrificial anodes
of traditional electrosynthetic methods. It is experimentally
simple, has high chemical and faradic efficiencies, and low
environmental impact.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
Acknowledgements
The authors wish to thank granting authorities CNPq and
CAPES for financial support and fellowship of Ronny
F. M. Souza.
2f
2g
2g
2h
2h
2i
13
7
Trace
75
79
Notes and references
2i
† The general design of the cavity cell used for the allylation of
aldehydes was a home-made three-electrode undivided cell described
in a previous works.12,13 The cathodic compartment was prepared by
compressing 150 mg of graphite powder with 5 mg of catalyst (Method
A: silver nitrate powder), or 5 mg of supporting electrolyte (Method B:
tetrabutylammonium tetrafluoroborate) under 1.90 kg cm-2 for 10 min.
0.6 mmol of 1a or 1b were premixed with 0.2 mmol of 2a–i without
solvent at 0 ◦C, and the cold mixture was added dropwise on the
a General reaction conditions: 0.6 mmol 1a or 1b and 0.2 mmol
2a–i using two methods, method A and method B†; each reaction was
continued until transfer of the theoretical charge or until the current
reached -1.0 mA at room temperature. b vs. Ag/AgCl, KCl(sat’d).
c Yields were determined by NMR 1H analysis using trimethoxybenzene
as the internal reference. d Data from Table 1.
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graphite powder of the cathodic compartment at 25 ◦C, which was
covered with filter paper. The anodic compartment was always filled
with 0.1 M KCl (1a) or KBr (1b) solution. During the electrolysis an
eventual gas evolution can occur in the cathodic compartment causing
an ohmic drop, which can be minimized by using the reference electrode
to press the filter paper covering the cathode cavity. After the passage
of the theoretical charge, the aqueous electrolyte was decanted and the
cathode cavity was extracted with 2 mL of CDCl3 containing 0.01 mmol
of 1,3,5-trimethoxybenzene as internal quantitative standard. The crude
extracts were analyzed by 1H NMR (Unity-Plus-300) and GC–MS (QP
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1120 | Green Chem., 2011, 13, 1118–1120
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