A recyclable electrochemical allylation in water

Article abstract of DOI:10.1021/ol050483h

(Chemical Equation Presented) To develop environmentally benign processes for C-C bond formation, electrochemistry is applied in a tin-mediated allylation reaction in water. In this electrochemical process, the corresponding homoallylic alcohols are prepa

Full text of DOI:10.1021/ol050483h

ORGANIC
LETTERS
2005
Vol. 7, No. 10
1903-1905
A Recyclable Electrochemical Allylation
in Water
Zhenggen Zha, Ailing Hui, Yuqing Zhou, Qian Miao,*,† Zhiyong Wang,* and
Hanchang Zhang
Hefei National Laboratory for Physical Science at Microscale and Department of
Chemistry, UniVersity of Science and Technology of China,
Hefei Anhui, 230026, China
zwang3@ustc.edu.cn
Received March 6, 2005
ABSTRACT
To develop environmentally benign processes for C−C bond formation, electrochemistry is applied in a tin-mediated allylation reaction in
water. In this electrochemical process, the corresponding homoallylic alcohols are prepared in excellent yields, while both tin salt and water
can be recycled and electrode materials are not consumed.
Organic reactions in water have been widely studied in order
to minimize the use of organic solvents, many of which are
flammable, toxic, or carcinogenic.1 Electrochemical methods
in organic synthesis are another approach to green chemistry2
because they can fundamentally eliminate the waste treatment
and disposal of used redox reagents.3 Obviously, a combina-
tion of aqueous media and electrochemical conversion can
utilize both advantages and provide a more efficient approach
to environmentally benign processes. Metal-mediated allyla-
tion is a well-known example of organic reactions in aqueous
media.4,1a However, excessive metal is used and the corre-
sponding salt is generated as waste.
On the other hand, it has been reported that low-valent
metal can be regenerated electrochemically in situ to mediate
allylation reactions in organic solvents.5 To develop envi-
ronmentally benign processes for C-C bond formation, we
applied electrochemistry in the tin-mediated allylation reac-
tion in water. In this electrochemical process, both tin salt
and water can be easily recycled and electrode materials are
not consumed.5,6 This study is detailed in the following text.
As shown in Scheme 1, our approach to the recyclable
electrochemical process for tin-mediated allylation was based
on the idea of using the cathode as a reducing reagent for
tin(II or IV) salts. The metallic tin generated on the cathode
reacts with allyl bromide to generate allyltin bromide and
diallyltin dibromide, which can mediate the allylation reac-
tion of aldehyde.7,4a A system of benzaldehyde, allyl bromide,
† Current address: Department of Chemistry, Columbia University, New
York, NY 10027.
(1) (a) Li, C. J. Chem. ReV. 1993, 93, 2023. (b) Li, C. J.; Chan, T. H.
Organic Reactions in Aqueous Media; John Wiley & Sons: New York,
1997. (c) Lindstro¨m, U. M. Chem. ReV. 2002, 102, 2751.
(2) (a) Anastas, P. T.; Kirchhoff, M. M. Acc. Chem. Res. 2002, 35, 686.
(b) Anastas, P. T.; Warner, J. C. Green Chemistry: Theory and Practice;
Oxford University Press: New York, 1997.
(3) (a) Hilt, G. Angew. Chem., Int. Ed. 2003, 42, 1720. (b) Seka, S.;
Burize, O.; Ne´de´lec, J. Y.; Pe´richon, J. Chem. Eur. J. 2002, 8, 2534. (c)
Gomes, P.; Gosmini, C.; Pe´richon, J. J. Org. Chem. 2003, 68, 1142. (d)
Grigg, R.; Putnikovic, B.; Urch, C. J. Tetrahedron Lett. 1997, 38, 6307.
(e) Hilt, G. Angew. Chem., Int. Ed. 2002, 41, 3586. (f) Nematollahi, D.;
Rahchamani, R. A. J. Electroanal. Chem. 2002, 520, 145.
(4) For reviews on metal-mediated reactions in aqueous media, see: (a)
Li, C. J. Tetrahedron 1996, 52, 5643. (b) Lubineau, A.; Auge, J.; Queneau,
Y. Synthesis 1994, 741. (c) Li, C. J. Acc. Chem. Res. 2002, 35, 533.
(5) (a) Hilt, G.; Smolko, K. I. Angew. Chem., Int. Ed. 2001, 40, 3399.
(b) Hilt, G.; Smolko, K. I.; Waloch, C. Tetrahedron Lett. 2002, 43, 1437.
(6) In electrochemical organic reactions, metal electrodes are often
consumed sacrificially. For examples, see: (a) Durandetti, M.; Ne´de´lec, J.
Y.; Pe´richon, J. Org. Lett. 2001, 3, 2073.
10.1021/ol050483h CCC: $30.25
© 2005 American Chemical Society
Published on Web 04/09/2005

measured by using cyclic voltammetry. After careful tests,
an electrolytic potential of 2.0 v was found to be the most
effective for this electrochemical allylation in water. When
the electrolytic potential is below 1.5 v, no allylation product
is yielded. When it is higher than 2.5 v, byproducts result
from reduction and coupling reaction of benzaldehyde. It is
noticed that this electrochemical allylation is also sensitive
to the concentration of SnCl2. If the concentration of SnCl2
is lower than 1 M, no allylation product occurs at an
electrolytic potential of 2.0 v. On the other hand, a higher
concentration of SnCl2 results in byproducts. This can be
explained by considering the correlation between the reduc-
tion potential and the concentration of Sn(II), which is ruled
by the Nernst equation.
A variety of aldehydes were tested in this electrochemical
allylation reaction. A typical procedure is described as
follows. A mixture of benzaldehyde (5.0 mmol), allyl
bromide (8 mmol) and stannous chloride (10 mmol) in 10
mL of water was stirred in a round-bottom flask cell equipped
with a pair of graphite electrodes at room temperature. The
Scheme 1. Proposed Mechanism for an Electrochemical
Process for Tin-Mediated Allylation of Aldehydes in Watera
a In this electrochemical process, Sn2+ and Sn4+ (from spent
Sn2+) were reduced at the cathode, while Sn2+ and Br- were
oxidized at the anode. The generated Sn0 mediated the allylation,
and the generated Br2 was hydrolyzed in water.
and SnCl2 was first tested by using cyclic voltammetry. As
shown in trace c (green) in Figure 1, two cathodic peaks
Table 1. In Situ Tin-Mediated Allylations of Aldehydes in
Water under Electrochemical Regeneration of SnCl2 Mediator
Figure 1. Cyclic voltammograms of the solution in water recorded
at a scan rate of 50 mV s-1 and room temperature with two glassy
carbon electrodes (3.0 mm diameter). Sodium bromide is applied
as a supporting electrolyte: (a) 5 mM SnCl2, 10 mM NaBr, 2.5
mM benzaldehyde, and 4 mM allyl bromide; (b) 10 mM NaBr only;
(c) 5 mM SnCl2 and 10 mM NaBr.
(C1 and C2, corresponding to the transformations of Sn(IV)
to Sn(II) and of Sn(II) to Sn(0), respectively) and one anodic
peak (A1, corresponding to the transformation of Sn(II) to
Sn(IV)) are recorded for a solution of SnCl2 (5 mM) and
NaBr (10 mM, as a supporting electrolyte) in water. When
benzaldehyde (2.5 mM) and allyl bromide (4 mM) were
added, similar peaks were recorded (shown in trace a (black)
in Figure 1). Then, the electrolytic potential was determined
on the basis of the reduction potential of Sn(II), which was
(7) (a) Nokami, J.; Otera, J.; Sudo, T.; Okawawa, R. Organometallics
1983, 2, 191. (b) Chan, T. H.; Yang, Y.; Li, C. J. J. Org. Chem. 1999, 64,
4452. (c) Tan, X. H.; Hou, Y. Q.; Huang, C.; Liu, L.; Guo, Q. X.
Tetrahedron 2004, 60, 6129. (d) Zha, Z.; Qiao, S.; Jiang, J.; Wang, Y.;
Miao, Q.; Wang, Z. Tetrahedron 2005, 61, 2521.
a Concentration of SnCl2 was 0.5 M. b In company with trace byproducts.
c Allyl bromide was replaced with crotyl bromide.
1904
Org. Lett., Vol. 7, No. 10, 2005

suspension was electrolyzed at a constant potential (2.0 v)
and stirred alternately until benzaldehyde was completely
consumed. Then, the reaction mixture was extracted with
diethyl ether, and common workup afforded the correspond-
ing homoallylic alcohol almost quantitatively without further
purification. The results are summarized in Table 1. Both
aromatic (entries 1-7 and 9 in Table 1) and aliphatic (entry
8 in Table 1) aldehydes can be allylated in this reaction.
The acetal group is not hydrolyzed under these conditions
(entry 2, Table 1). The regioselectivity of reaction was also
investigated by replacing allyl bromide with crotyl bromide.
As indicated in entry 9 of Table 1, the reaction between
benzaldehyde and crotyl bromide affords the γ-adduct as the
major product.
The reuse of mediator (SnCl2) can be easily achieved in
this electrochemical process because metallic tin is regener-
ated on the cathode by reducing tin(II or IV) salts, which
result from the allylation reaction. To reuse the mediator,
the reaction mixture is extracted with diethyl ether three
times, and the remaining aqueous layer is used in the next
cycle without further purification. As shown in Table 2, the
allylation product is still obtained in a good yield (86%) even
though the mediator is reused in the fifth cycle.
Table 2. Yield for Allylation of Benzaldehyde in Subsequent
Cycles with the Mediator (SnCl2) Reused Electrochemically
entry
mediator (SnCl2)
yield (%)
1
2
3
4
5
1st cyclea
2nd cycleb
3rd cycleb
4th cycleb
5th cycleb
100
99
95
85
86
a Performed with 5.0 mmol of benzaldehyde and 8 mmol of allyl bromide
in this cycle. b Performed with 2.5 mmol of benzaldehyde and 4 mmol of
allyl bromide in this cycle.
Further research is in progress in our laboratory to utilize
this electrochemical allylation reaction in wide synthetic
applications.
Acknowledgment. The authors are grateful to the Na-
tional Natural Science Foundation of China (Grants 50073021
and 20472078) and the National Natural Science Foundation
of Anhui Province (Grant 01046301) for their support. Great
thanks are owed to Dr. Dongfang Meng for helpful sugges-
tion.
In conclusion, the study above puts forth a new approach
to an environmentally benign process for metal-mediated
allylation reactions by combining electrochemical conversion
and aqueous media. In this electrochemical process, tin is
generated in situ by electrolysis and mediates allylation
reaction of aldehydes in water. Both tin salt and water can
be easily reused, and electrode materials are not consumed.
Supporting Information Available: Details for electro-
chemical reactions and characterization of products. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OL050483H
Org. Lett., Vol. 7, No. 10, 2005
1905