COMMUNICATION
Zn-mediated electrochemical allylation of aldehydes in aqueous
ammoniaw
Jing-Mei Huang*ab and Yi Donga
Received (in Cambridge, UK) 19th March 2009, Accepted 5th May 2009
First published as an Advance Article on the web 28th May 2009
DOI: 10.1039/b905553c
An efficient electrosynthesis of homoallylic alcohols from allylic
bromides and aldehydes in aqueous ammonia is achieved in an
undivided cell fitted with a pair of zinc electrodes.
reached 92% (entry 8, Table 1). Further increase of the
concentration of ammonia resulted in a decrease of the yield
(entries 9–11, Table 1). In aqueous 0.1 M NaOH, no allylation
was observed (entry 12, Table 1). Finally, in the acidic aqueous
0.1 M NH4Br, allylation occurred in a low yield of 36%
Synthesis of homoallylic alcohols by allylation of carbonyl
compounds is one of the most important processes in organic
synthesis due to their versatile use as important building
blocks for natural product synthesis.1 In the past two decades,
extensive research has been carried out on developing
organometallic type allylation reactions in aqueous media,
and many metals have been found to effectively mediate such
reactions.2 Among them, zinc, a relatively cheap and low toxic
material, has been demonstrated as a good reagent to mediate
this Barbier-type reaction and has been applied in industry
production. However, it generally requires physical or
chemical activation. Among the various activation methods
available, the addition of an acidic co-reagent is a commonly
applied approach. Here we report a Zn-mediated electro-
chemical allylation reaction in aqueous ammonia.3 This new
procedure, which takes place in an aqueous alkaline solution,
is efficient and a range of functional groups are stable in these
mild reaction conditions.
+
(entry 13, Table 1), which shows that NH3 and not NH4 is
promoting the reaction.
Hence, a standard reaction procedure is described as
follows. A mixture of benzaldehyde (0.5 mmol), allyl bromide
(1 mmol) in aqueous ammonia (1.5 mL of 25% (w/w) solution
was diluted to 5 mL with water (4.5 M, pH = 12.6) and was
electrolyzed at a constant current of 15 mA in a round-bottom
flask cell equipped with a pair of zinc electrodes (1.5 cm2) at
room temperature. The electrolysis was stopped when the
aldehyde was converted completely (1.5–2 h, 2F molÀ1 of
current was consumed). Usual work-up and purification
afforded the desired product.
Subsequently, a variety of aldehydes were examined to
generate the allylation products using the standard conditions.
Table 2 summarizes the details of the results. Both the
aromatic and aliphatic aldehydes can be utilized in this
methodology. For the aromatic aldehydes, homoallylic
alcohols can be prepared smoothly in good to excellent yield,
and the functionalities including methoxyl (entry 4, Table 2),
In this one-compartment electrochemical process, Zn foils
were chosen as both anode and cathode. Studies were initiated
in a neutral 0.1 M LiClO4 solution (entry 1, Table 1). Benz-
aldehyde was not consumed completely after the electrolysis
has been carried out for 2 h and only 22% yield of the desired
product was obtained. In an acidic 0.1 N H2SO4 solution, the
desired homoallylic alcohol was isolated with a higher yield of
58% (entry 2, Table 1). We then tried electrolysis in 0.25 M
NH4BF4 solution.4 Only a trace amount of the product was
detected with most of the starting material recovered (entry 3,
Table 1). When we added 25% (w/w) of ammonia into the
above solution (final concentration: 3 M), a 66% yield of the
desired product was obtained (entry 4, Table 1). A control
reaction was run in 3 M ammonia as solvent and supporting
electrolyte and the yield was increased to 80% (entry 5,
Table 1). This result demonstrated that NH4BF4 was prohibiting
the reaction to a certain extent. Optimization of the amount of
ammonia showed that at a concentration of 4.5 M, the yield
Table 1 Optimization of the reaction conditions for the electro-
chemical allylation reactions in aqueous mediaa
Entry
Conditions
Yieldb/%
1
2
3
4
5
6
7
8
0.1 M LiClO4
0.1 N H2SO4
0.25 M NH4BF4
22
58
trace
66
80
83
86
92
91
88
84
0
0.25 M NH4BF4 in 3.0 M aqueous ammonia
3.0 M aqueous ammonia
3.6 M aqueous ammonia
4.2 M aqueous ammonia
4.5 M aqueous ammonia
4.8 M aqueous ammonia
5.4 M aqueous ammonia
6.0 M aqueous ammonia
0.1 M NaOH
9
a School of Chemistry and Chemical Engineering, South China
University of Technology, Guangzhou, Guangdong, 510640, China.
E-mail: chehjm@scut.edu.cn; Fax: +86 020 8711 0622;
Tel: +86 020 8711 0695
10
11
12
13
0.1 M NH4Br
36
b Ministry of Education Key Laboratory for the Synthesis and
Application of Organic Functional Molecules, Hubei University,
Wuhan, 430062, China
a
Benzaldehyde (0.5 mmol), allyl bromide (1 mmol) in each electrolyte
was electrolyzed at a constant current of 15 mA in a round-bottom
flask cell equipped with a pair of zinc electrodes (1.5 cm2) at room
w Electronic supplementary information (ESI) available: Experimental
procedure, spectroscopy, cyclic voltammetry, scanning electron micro-
scopy, X-ray and NMR analyses data. See DOI: 10.1039/b905553c
b
temperature. Isolated yield.
ꢀc
This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 3943–3945 | 3943