LETTER
▌1235
letter
Electrochemical Deoxygenation of Primary Alcohols
Electrochemical Deoxygenation of Primary Alcohols
Kevin Lam, István E. Markó*
Département de Chimie, Bâtiment Lavoisier, Université catholique de Louvain, Place Louis Pasteur 1, 1348 Louvain-la-Neuve, Belgium
Fax +32(10)472788; E-mail: istvan.marko@uclouvain.be
Received: 18.01.2012; Accepted after revision: 24.02.2012
would involve the electrolysis of the desired alcohol, in
Abstract: Direct electrolysis of primary alcohols, in the presence of
the presence of an excess of methyl toluate, in a solution
methyl toluate, leads smoothly to the formation of the correspond-
containing a tetrabutylammonium electrolyte (Figure 1).
ing deoxygenated product in high yield.
Key words: deoxygenation, electrochemical, esters, reduction, free
radicals
ROH
4
The conversion of a hydroxyl group into the correspond-
ing alkane is often a delicate process. For years, the Bar-
ton–McCombie reaction has been considered as the acme
1
of such transformation. Unfortunately, this reaction re-
C
quires the prior derivatization of the alcohol into a heat-
+
RO– NBu4
2
and light-sensitive xanthate and often suffers from the
5
O
A
T
H
O
D
E
use of toxic reducing agents such as tin hydride. Whilst
this drawback can be partially overcome by the use of less
Me
O
3
4
toxic systems, such as borane/air or phosphites, this re-
5
action is such as difficult to perform on multigrams scale.
+
Recently, our laboratory has been involved in the mono-
electronic reduction of aromatic esters. Our investigations
MeO–
NBu4
O
6
in that field prompted us to develop a chemical and
R
7
O
electrochemical deoxygenation reaction using aromatic
O
esters as benign radical precursors (Scheme 1).
Me
6
O
O
monoelectronic
reduction
RH + RO–
R
+
O
R
RH
3
NBu4
5
1
2
3
Scheme 1 Deoxygenation of aromatic esters
Figure 1 Electrotransesterification–Deoxygenation
Compared to most of the classical chemical deoxygen-
ation reactions, the electrochemical reduction method that
we developed has the advantage of avoiding the use of
toxic reagents or co-solvents. Moreover, it employs the
cheapest source of electrons: the electric current itself.
Whilst this process proved to be efficient for the deoxy-
In the first step, the alcohol 4 would be deprotonated at the
cathode to generate hydrogen gas and a highly nucleophil-
1
2
ic tetrabutylammonium alkoxide 5. This alkoxide would
then undergo transesterification with methyl toluate, lead-
ing to the in situ formation of 6, the corresponding toluate
of the starting alcohol 4. Finally, toluate 6 could be elec-
troreduced into the corresponding alkane 3. Alternatively,
8
genation of secondary and tertiary derivatives, primary
esters usually gave poor yields of deoxygenated product
6
could be deprotected leading back to the initial tetrabu-
9
when subjected to the reductive conditions. Previous
tylammonium alkoxide 5. Indeed, we have shown previ-
ously that the reduction of aromatic esters, in the presence
of a proton source, such as an alcohol, leads to their che-
works by Nakajima, who developed an electrochemical
1
0
benzoylation protocol and by Utley, who reported on an
electrochemical transesterification–reduction system for
oxalate esters,11 prompted us to consider a similar se-
quence for the deoxygenation of alcohols without the need
to esterify them prior to electrolysis. The basic principle
6
,7
moselective deprotection.
In this case, such a side reaction will be of minor impor-
tance since the alkoxide can undergo a new transesterifi-
cation–deoxygenation sequence. This cycle can then be
repeated until full conversion is reached.
SYNLETT 2012, 23, 1235–1239
Advanced online publication: 26.04.2012
Since some of the methyl toluate will also be directly elec-
trolyzed before being able to serve as a transesterification
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0031-1290778; Art ID: ST-2012-D0052-L
Georg Thieme Verlag Stuttgart · New York
©