2
28
Chemistry Letters Vol.36, No.2 (2007)
Polycyclic Aromatic Compounds-mediated Electrochemical Reduction
of Alkyl Mesylates
ꢀ
Hisanori Senboku, Megumi Takahashi, Tsuyoshi Fukuhara, and Shoji Hara
Laboratory of Organic Reaction, Division of Chemical Process Engineering, Graduate School of Engineering,
Hokkaido University, Sapporo 060-8628
(Received October 30, 2006; CL-061275; E-mail: senboku@eng.hokudai.ac.jp)
Electrochemical reduction of alkyl mesylates was success-
anode gave a 35:65 mixture of dodecane 2a and 1-dodecanol.
Therefore, we tried the electrolysis in the presence of polycyclic
aromatic compounds as an electron-transfer mediator. Other
than benzene and toluene, we found naphthalene, phenanthrene,
and m-terphenyl were effective for yielding 2a selectively.
Better results were obtained when the electrolysis of 1a was
carried out in the presence of biphenyl which exclusively gave
2a in 62% yield under the reaction conditions described in eq 1
fully carried out by using an undivided cell equipped with a Pt
cathode and an Mg anode in the presence of biphenyl and
t-BuOH. The reaction could proceed efficiently under mild con-
ditions to give the corresponding alkanes in moderate to good
yields. This procedure could also be applicable to chemoselec-
tive reduction of mesylates having functional groups such as
epoxide, olefin, acetal, hydroxy, or cyano groups.
(
Conditions A). Biphenyl could be recovered quantitatively
after the reaction and the recovered biphenyl could also be
reused as a mediator.
Reduction of alcohols to the corresponding alkanes is an im-
portant transformation in organic synthesis.1 One of simple and
useful methods involves conversion of hydroxy groups to the
corresponding alkyl sulfonates, such as tosylates or mesylates,
followed by reduction with hydride. As a hydride reagent,
LiAlH4 or LiBEt3H is frequently used for this aim. Their high
reactivity, however, sometimes causes serious troubles such as
over-reduction of other functional groups in the substrates.
Therefore, chemoselective and widely applicable reduction of
alkyl sulfonates under mild conditions would be still desirable
although several alternative methods for the reduction of alkyl
–3
Mg
Pt
10 OMs
H
(1)
10
0
.1M Bu4NBF4 - DMF
2
1
a
20 mA/cm , 3 F/mol, 0 °C
2a 62%
4
5,6
biphenyl (4 equiv.)
During continuous study on scope and limitation of the pres-
ent reaction using functionalized substrates, it was found that
this reaction system was also applicable to the substrate having
1
6
a hydroxy group (See below). Then, we tried the electrolysis
in the presence of a proton source and the results are summarized
in Table 1. As the best result, electrochemical reduction of
mesylate 1a in the presence of biphenyl (4 equiv.) and t-BuOH
7
–13
tosylates or mesylates have been developed.
hand, electrochemical process can reduce various organic com-
On the other
1
4
pounds under neutral and mild conditions. While electrochem-
ical reduction of alkyl tosylates predominantly gave the corre-
sponding alcohols,15 a similar reduction of alkyl mesylates gave
(
10 equiv.) with 8 F/mol of electricity gave 2a in 74% yield
(Entry 7 in Table 1) (Conditions B). It is also noted that in this
case, biphenyl could also be recovered in quantitative yields.
Electrochemical reduction of various alkyl mesylates 1 was
carried out under ‘‘Conditions A (in the presence of biphenyl
with 3 F/mol of electricity) and B (in the presence of biphenyl
the corresponding alkanes. Thus, electrochemical reduction of
alkyl mesylates could take place efficiently by using a divided
cell equipped with a lead cathode and a platinum anode to give
1
2
the corresponding alkanes in good yields. It was also reported
that mesylates of ꢀ-hydroxy esters could be reduced to the cor-
responding esters by indirect electrochemical reduction using
1
7
and t-BuOH with 8 F/mol of electricity),’’ and the results are
summarized in Table 2. The present reaction was also effective
1
3
Table 1. Effect of an additivea
diphenyl diselenide in a divided cell. Although both electro-
chemical reactions could proceed under neutral and mild condi-
tions to give the products in good yields, use of unfavorable met-
al as an electrode and a complicated divided cell could not be
satisfactory from the view points of environmentally friendly or-
ganic synthesis. We tried electrochemical reduction by using a
simple and easy-handled undivided cell equipped with a plati-
num cathode and a magnesium anode, and found that electro-
chemical reduction of alkyl mesylates could proceed efficiently
in the presence of polycyclic aromatic compound such as naph-
thalene and biphenyl as an electron-transfer mediator to give
the corresponding alkanes in moderate to good yields. In this
communication, we wish to report polycyclic aromatic com-
pounds-mediated electrochemical reduction of alkyl mesylates
using an undivided cell.
Electricity Yieldb Recovered 1ab
Additive
(
Entry
ꢁ
1
equiv.)
/F mol
/%
/%
1
2
3
4
5
6
7
8
9
none
5
5
5
5
5
5
8
8
8
8
59
62
64
61
57
56
74
66
37
61
0
0
trace
17
20
28
0
trace
42
15
t-BuOH (2)
t-BuOH (4)
t-BuOH (10)
t-BuOH (20)
t-BuOH (50)
t-BuOH (10)
i-PrOH (10)
EtOH (10)
c
10
H2O (10)
a
2
A constant current electrolysis (20 mA/cm ) of 1a (1 mmol)
in DMF (16 mL) containing 0.1 M Bu4NBF4 was carried out
in the presence of biphenyl (4 equiv.) at 0 C. Isolated yield.
A constant current electrolysis of a DMF solution of dodesyl
mesylate 1a containing 0.1 M Bu4NBF4 using an undivided cell
equipped with a platinum plate cathode and a magnesium rod
ꢂ
b
c
1-Dodecanol was obtained in 4% yield.
Copyright Ó 2007 The Chemical Society of Japan