Tetrahedron Letters
Reduction of aldehydes catalyzed by oxo-rhenium(V) complexes
containing heterocyclic ligands
a
a
b
b
a,
⇑
Joana R. Bernando , Pedro R. Florindo , Mariusz Wolff , Barbara Machura , Ana C. Fernandes
a
Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal
Department of Crystallography, Institute of Chemistry, University of Silesia, 9th Szkolna St., 40-006 Katowice, Poland
b
a r t i c l e i n f o
a b s t r a c t
Article history:
This work describes the catalytic activity of several oxo-rhenium complexes containing the heterocyclic
ligands 2-(2-hydroxy-5-methylphenyl)benzotriazole (Hhmpbta), 2-(2-hydroxyphenyl)benzothiazole
Received 1 September 2014
Revised 20 November 2014
Accepted 26 November 2014
Available online 3 December 2014
(Hhpbt), 2-(2-hydroxyphenyl)benzoxazole (Hhpbo), 2-(2-hydroxyphenyl)-1H-benzimidazole (Hhpbi),
isoquinoline-1-carboxylic acid (iqcH), and 4-methoxy-2-quinolinecarboxylic acid (mqcH) in the reduc-
tion of 4-nitrobenzaldehyde using phenylsilane as reducing agent. In general, all of the catalysts tested
gave good to excellent yields of the 4-nitrobenzyl alcohol. Although, the best result was obtained with
the catalytic system PhSiH /[ReOBr (hmpbta)(PPh )] (5 mol %). This system was also applied to the
3 2 3
reduction of a large variety of aldehydes, producing the corresponding primary alcohols in good to
excellent yields and good chemoselectivity.
Keywords:
Oxo-rhenium complexes
Reduction
Aldehydes
Silanes
Ó 2014 Elsevier Ltd. All rights reserved.
The selective reduction of carbonyl groups is an important
transformation in chemistry and represents a challenge in both
chemical and pharmaceutical industries. In particular, benzylic
alcohols occupy an important position in organic synthesis as tar-
get molecules of biological interest and synthetic intermediates.
For these reasons, the development of new methodologies for the
chemoselective reduction of carbonyl groups remains a challenge
in organic synthesis.
reductions,12 in this work we explore the catalytic activity of
several oxo-rhenium complexes with general formula [ReOX (L)
(PPh )](X = Cl, Br), containing different heterocyclic ligands
(Fig. 1), namely, 2-(2-hydroxy-5-methylphenyl)benzotriazole
2
3
1
3
14
(Hhmpbta), 2-(2-hydroxyphenyl)benzothiazole (Hhpbt), 2-(2-
1
5
hydroxyphenyl)benzoxazole (Hhpbo), and 2-(2-hy-droxyphenyl)-
1
6
1H-benzimidazole (Hhpbi),
isoquinoline-1-carboxylic acid
1
7
17
(iqcH), and 4-methoxy-2-quinolinecarboxylic acid (mqcH) in
the reduction of carbonyl compounds using silanes as reducing
agents.
In 2003 Toste and co-workers have demonstrated that the high
valent oxo-rhenium complex ReIO (PPh ) was an excellent
2 3 2
catalyst for the hydrosilylation of aldehydes and ketones to the
corresponding silyl ethers using silanes as reducing agents.
To compare the catalytic activity of these oxo-rhenium com-
plexes, the reduction of the test substrate 4-nitrobenzaldehyde
1
Since then, several studies were reported in the literature for the
hydrosilylation and for the reduction of the carbonyl group to
the corresponding alcohols catalyzed by several oxo-rhenium and
oxo-molybdenum complexes.2
3
was performed with PhSiH (2 mmol) in reflux of THF under air
atmosphere, followed by deprotection of the resulting silyl ether
with TBAF, affording the 4-nitrobenzyl alcohol (Table 1). In general,
all of the oxo-rhenium complexes tested were efficient and pro-
duced good to excellent yields of 4-nitrobenzyl alcohol. Neverthe-
less, the catalysts containing the ligand Hhmpbta were the most
efficient (Table 1, entries 1 and 5).
–9
In our previous works, we have demonstrated that several oxo-
rhenium complexes containing the heterocyclic ligands Hhmpbta,
Hhpbt, Hhpbo, and Hhpbi were excellent catalysts for the direct
reductive amination of aldehydes, producing secondary and
The best result was obtained with the catalyst [ReOBr
2
(hmpbta)
10
tertiary amines in good to excellent yields and also for the deox-
ygenation of a large variety of sulfoxides using silanes and boranes
(PPh )] (5 mol %), affording the 4-nitrobenzyl alcohol after 13 min
3
in 93% yield at reflux of THF (Table 1, entry 1). Similar reduction
performed at room temperature produced the alcohol in 78% yield
after 24 h (Table 1, entry 2). Using only 3 mol % or 2 mol % of this
oxo-rhenium complex, the 4-nitrobenzyl alcohol was obtained in
1
1
as reducing agents.
In continuation of our studies on the use of oxo-molybdenum
and oxo-rhenium complexes as excellent catalysts for organic
7
8% and 76% yields, respectively (Table 1, entries 3- and 4). The
reduction carried out in the presence of [ReOCl (hmpbta)(PPh )]
5 mol %) also gave the alcohol in excellent yields (91%) after 1 h
2
3
⇑
(
040-4039/Ó 2014 Elsevier Ltd. All rights reserved.
0