Aerobic, chemoselective oxidation of alcohols to carbonyl compounds catalyzed by a DABCO-copper complex under mild conditions

Article abstract of DOI:10.1002/adsc.200700213

A DABCO-copper(I) chloride complex (5 mol%) together with TEMPO (5 mol%) in nitromethane as solvent has been used as an efficient catalytic system for the selective oxidation of benzylic and allylic alcohols into the corresponding carbonyl compounds at room temperature where molecular oxygen acts as an ultimate, stoichiometric oxidant and water is the only by-product. The solidstate structure determination of the DABCOcopper complex shows that the copper is in the + II oxidation state with trigonal bipyramidal geometry and exists in a linear polymeric structure due to strong hydrogen bonding.

Full text of DOI:10.1002/adsc.200700213

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DOI: 10.1002/adsc.200700213
Aerobic, Chemoselective Oxidation of Alcohols to Carbonyl
Compounds Catalyzed by a DABCO-Copper Complex under
Mild Conditions
a
a
a,
Sreedevi Mannam, S. Kumar Alamsetti, and G. Sekar *
a
Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India
Phone: (+91)-44-2257-4229; fax: (+91)-44-2257-4202; e-mail: gsekar@iitm.ac.in
Received: April 27, 2007
th
This paper is dedicated to Prof. Dr. Dr. h.c. Lutz F. Tietze on the occasion of his 65 birthday.
Abstract: A DABCO-copper(I) chloride complex
5 mol%) together with TEMPO (5 mol%) in ni-
Amongthe above-mentioned metal salts, Cu is
(
more attractive as it is a biomimetic functional model
of the mononuclear copper enzyme galactose ox-
tromethane as solvent has been used as an efficient
catalytic system for the selective oxidation of ben-
zylic and allylic alcohols into the correspondingcar-
bonyl compounds at room temperature where mo-
lecular oxygen acts as an ultimate, stoichiometric
oxidant and water is the only by-product. The solid-
state structure determination of the DABCO-
copper complex shows that the copper is in the +II
oxidation state with trigonal bipyramidal geometry
and exists in a linear polymeric structure due to
strong hydrogen bonding.
[
11]
idase. As copper salt alone is an inefficient catalyst,
a variety of ligands is used with copper salts to in-
[
12]
crease the catalytic activity. Unfortunately, most of
these Cu complexes need higher temperature and an
[
13]
external base for efficient oxidation.
In this com-
munication, we report an oxidation reaction that can
be performed under very mild conditions (room tem-
perature) in the presence of commercially available,
inexpensive 1,4-diazabicyclo[2.2.2]octane (DABCO)
A
H
R
U
G
as ligand with CuCl without use of an external base,
where molecular oxygen acts as an ultimate, stoichio-
metric oxidant and water is the only by-product
(Scheme 1).
Keywords: aerobic oxidation; alcohols; aldehydes;
copper catalyst; diamine ligand; ketones
To find the optimal conditions for the alcohol oxi-
dation, 4-methoxybenzyl alcohol was dehydrogenated
under various conditions and the results are summar-
ized in Table 1. Initially, commercially available nitro-
The selective oxidation of alcohols is a fundamental gen-containing ligands such as 1,8-diazabicyclo-
[
1]
transformation in organic synthesis. In particular,
A
H
R
U
G
A
H
R
U
G
the conversion of primary alcohols to aldehydes is 5-ene (DBN), DABCO and (À)-sparteine were used
crucial in the total synthesis of natural products and with CuCl and the DABCO-CuCl complex provided
[
2]
fine chemicals such as fragrances or food additives.
the highest yield for the oxidized product (entry 3). A
A variety of reagents has been developed to achieve variety of copper salts was screened with DABCO in
this oxidation process which is testimony for the im-
portance of this reaction. Unfortunately, these re-
agents turned out to involve stoichiometric or super-
stoichiometric amounts of toxic or hazardous oxidiz-
[
3]
inga ge nts such as CrO , KMnO , MnO , SeO , etc.
3
4
2
2
So a catalytic oxidation process is thus extremely
useful from economic and environmental points of
view, and those employingmolecular oxy ge n or air as
stoichiometric oxidant are particularly attractive. In
the last decade several transition metal salts such as
[4]
[5]
[6]
[7]
[8]
[9]
[10]
Cu, Co, V, Pd, Ru, Rh, and Mo
have
been used as catalyst for the oxidation of alcohols to-
gether with molecular oxygen.
Scheme 1. DABCO-Cu complex-catalyzed oxidation of alco-
hols.
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Sreedevi Mannam et al.
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Table 1. Optimizingthe oxidation of 4 -methoxybenzyl alco- faster for the benzylic primary alcohols and allylic pri-
hol.
mary alcohols but it took only a slightly longer time
for benzylic secondary alcohols. In the case of aliphat-
ic alcohol the reaction takes an even longer time
(
entry 16).
The DABCO-Cu catalytic system can be used for
selective oxidation of one alcohol over another
Table 3). When a mixture of 4-methoxybenzyl alco-
(
hol and 1-phenylethanol (1 mmol each) was reacted
with molecular oxygen in the presence of 5 mol%
catalytic system for 2 h, 4-methoxybenzyl alcohol was
fully consumed (85% isolated yield) and only 9% of
acetophenone was isolated from the 1-phenylethanol
oxidation (Table 3; entry 1). Similarly, when a mixture
of 4-methoxybenzyl alcohol and trans-cinnamyl alco-
hol (1 mmol each) was oxidized, 4-methoxybenzyl al-
cohol was selectively oxidized (entry 2). The cinnamyl
alcohol was fully converted to correspondingalde-
hyde in the presence of 1-phenylethanol whereas the
secondary alcohol was fully recovered as unreacted
material (entry 3). Similarly, the competingreaction
between 4-methoxybenzyl alcohol and 1-octanol
showed complete consumption (87% isolated yield)
for 4-methoxybenzyl alcohol but 1-octanol was left
intact (entry 4). Interestingly, when both the primary
and secondary benzylic alcohols are present in the
same molecule, only the primary alcohol was selec-
[
[
[
a]
b]
c]
Isolated yield.
Without K CO .
2
3
Without TEMPO.
toluene at 1008C and DABCO-CuCl turned out to be tively oxidized and the secondary alcohol was left
the best Cu complex in terms of reaction rate and intact and the reaction produced the corresponding
conversion (entry 3). Then the reaction was carried aldehyde with 81% yield (entry 5). The reactivity of
out in several organic solvents and toluene became the alcohols is as follows: benzylic primary alcohols
the first choice of solvent (entry 17).
> allylic primary alcohols > benzylic secondary alco-
There was almost no reaction when the DABCO- hols > aliphatic alcohols. These results clearly dem-
CuCl complex was not used (entry 19). Without onstrate the excellent selectivity of the DABCO-CuCl
DABCO the reaction gave only 9% yield (entry 20). complex as an efficient catalyst in the aerobic oxida-
So it is clear that the CuCl alone is not an active cata- tion of alcohols providingconsiderable advanta ge s for
lyst and DABCO is needed to increase its catalytic ac- synthetic organic chemistry.
tivity. Similarly, the oxidation reaction did not provide
If we consider the DABCO-Cu complex, both the
any oxidized product when the reaction was carried nitrogen atoms of one DABCO molecule cannot coor-
out without TEMPO (2,2,6,6-tetramethyl-1-piperidi- dinate with one copper atom since the lone pair of
nyloxyl free radical) (entry 18). Although the exact electrons of both the nitrogens are 1808 away from
role of TEMPO is unclear, we assume that the each other. So we assume that the DABCO-Cu com-
TEMPO might act as a hydrogen acceptor during the plex may be a dimer or an oligomer where two
catalytic cycle. In general, the Cu complex needs an copper atoms are linked through a DABCO molecule
[
14]
external base for better activity. In contrast to this ob- as bridge. The X-ray single crystal analysis of the
servation, the yield was increased from 84% to 89% DABCO-Cu complex [(DABCO) CuCl ·HCl] shows
2
2
when K CO was not used (entry 17 vs. 3). We assume that copper exists in the Cu(II) oxidation state and
2
3
that the DABCO might have dual roles: as a base to that two DABCO molecules are coordinated to
deprotonate the hydroxy group and as an N-donor Cu(II) using one nitrogen atom of each (Figure 1).
ligand to CuCl.
The complex crystallizes in a rhombohedral crystal
To probe the efficiency of the DABCO-CuCl cata- system with space group R32 and with unit cell di-
lyst, a series of alcohols was oxidized to the corre- mensions a=10.6777(2) Š, b=10.6777(2) Š, c=
spondingcarbonyl compounds under the optimized
12.0487(2) Š, a=908, b=908, g=1208. The Cu(II)
condition and the results are summarized in Table 2. center has a perfect trigonal bipyramidal geometry
It is clear that all the primary alcohols have been se- (TBP). Copper and three chlorine atoms lie in one
lectively oxidized to aldehydes and over-oxidized car- plane. The ClÀCuÀCl bond angle is 1208, the ClÀCuÀ
boxylic acids were not observed. The reaction was N bond angle is 908, and the NÀCuÀN bond angle is
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Aerobic, Chemoselective Oxidation of Alcohols to Carbonyl Compounds
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[a]
Table 2. Oxidation of alcohols usingthe DABCO-CuCl complex.
[
a]
In all the cases, the primary alcohols were selectively oxidized to correspondingaldehydes and over oxidized carboxylic
acids were not observed. All the products gave satisfactory spectral data.
Isolated yield.
[
[
b]
c]
10 mol% of catalyst was used.
[a]
Table 3. Selective oxidation of alcohols by the DABCO-CuCl complex in toluene.
[
[
a]
b]
1
mmol each of both the alcohols were treated with 5 mol% of DABCO-CuCl, 5 mol% of TEMPO in toluene at 1008C.
Isolated yield.
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Sreedevi Mannam et al.
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crystal packingin the form of a linear polymer
(
Figure 2). The hydrogen bond length between N···H
is 1.76 Š which is shorter than the typical hydrogen
bond length of 2.4 Š. Because of this strong hydrogen
bondingthe Cu-DABCO complex is stable.
Since the DABCO-Cu complex exists in linear
polymeric structure due to strongintermolecular hy-
drogen bonding, we thought that if we break the in-
termolecular hydrogen bond by a polar solvent, the
DABCO-Cu complex may exist in monomeric form
and its catalytic activity will be enhanced, thus the re-
action may be carried out at room temperature. Also,
in monomeric form one nitrogen atom of the
DABCO-Cu complex (the nitrogen which is not pro-
Figure 1. Solid-state structure determination of the
DABCO-Cu complex (30% probability; CCDC No.:
633455).
1
808. The three CuÀCl distances are equivalent with a tonated) will be free from hydrogen bonding and will
bond distance of 2.377 Š. The CuÀN bond distance is be able to act as internal base to deprotonate the al-
2
.120 Š. Out of the two DABCO molecules attached cohol molecule. To check this assumption, we tried
to copper, one nitrogen is with tetrahedral geometry the reaction usingpolar solvents such as CH 3CN,
due to protonation and the other nitrogen is having DMSO, DMF, 1,4-dioxane and nitromethane and the
pyramidal geometry and a proton of one DABCO results are summarized in Table 4. Surprisingly, as per
molecule is linked to the nitrogen of the other our prediction the reaction took place at room tem-
DABCO through a strong H-bond resulting in the perature in polar solvents. The reaction was faster in
Figure 2. Linear polymeric structure of the DABCO-Cu complex due to strong intermolecular hydrogen bonding.
Table 4. DABCO-CuCl-catalyzed oxidation of alcohols at room temperature in nitromethane.
[
[
[
[
a]
b]
c]
d]
Isolated yield.
Without CuCl and TEMPO.
Without TEMPO.
Without DABCO.
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Aerobic, Chemoselective Oxidation of Alcohols to Carbonyl Compounds
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Figure 3. Proposed catalytic cycle for the oxidation of alcohols usingDABCO-CuCl-TEMPO complex.
tion was done by silica gel (100–200 mesh) column chroma-
tography and hexane and ethyl acetate mixture as eluent.
Cu
TEMPO, p-methoxybenzyl alcohol, 3,4,5-trimethoxybenzyl
alcohol, cinnamyl alcohol, 1-phenylethanol, furfuryl alcohol,
DMF and gave 84% yield in just 2 h. Nitromethane
gave the highest isolated yield of 93% in 5 h at room
temperature. Highest yield and easy work-up proce-
dure in nitromethane encouraged us to expand its
A
H
R
U
G
2
scope with other alcohols. In all the cases, the reaction and p-nitrobenzyl alcohol were obtained from Aldrich
took place at room temperature in shorter time with chemicals. Benzyl alcohol, o-chlorobenzyl alcohol, p-chloro-
excellent yields (entries 9–14). The procedure is very benzyl alcohol, and n-octanol were obtained from SRL
chemicals, India. o-Hydroxybenzyl alcohol, o-nitrobenzyl al-
cohol, m-nitrobenzyl alcohol, benzhydrol, (4-chlorophenyl)-
simple, and it has to be noted that the reaction is per-
formed directly under oxygen atmosphere by using O2
A
H
R
U
G
A
T
E
N
(phenyl)methanol, 1-
balloon and bubblingof oxy ge n is not necessary.
Based on our experimental results and the solid
state structure determination of DABCO-Cu com-
plex, we are proposingthe catalytic cycle as shown in
(
AHCTREUNG
spondingcarbonyl compounds by reduction usingLiAlH or
4
[15]
Figure 3. However, studies on the full scope of the
NaBH . 1-(4-(Hydroxymethyl)phenyl)propan-1-ol was syn-
4
DABCO-Cu complex as catalyst in alcohol oxidation, thesized from terephthalic acid. All the products were fully
1
13
detailed mechanistic studies, increasingthe catalystꢁs
characterized by H and C NMR, IR, and mass spectrosco-
activity towards aliphatic alcohol oxidation and the py.
chiral version of this oxidation (non-enzymatic kinetic
resolution) usingchiral Cu complex are under prog-
ress.
In conclusion, we have developed a new catalytic
system (5 mol% of DABCO-CuCl-TEMPO in nitro-
methane) for the aerobic, selective oxidation of alco-
hols to correspondingcarbonyl compounds with ex-
cellent yield under very mild conditions. In all the
cases, the primary benzylic and allylic alcohols were added and stirred for 5 h at room temperature under O at-
Typical Experimental Procedure in Nitromethane at
Room Temperature
A mixture of CuCl (4.95 mg, 0.05 mmol) and DABCO
5.7 mg, 0.05 mmol) in 2 mL of nitromethane was stirred at
room temperature for 10 min, TEMPO (7.82 mg,
.05 mmol) was added to the reaction mixture. After stirring
for 5 min 4-methoxybenzyl alcohol (138 mg, 1 mmol) was
(
0
2
selectively oxidized to correspondingaldehydes and
mosphere (usingballoon). After completion of reaction (fol-
over oxidized products carboxlylic acids were not ob- lowed by TLC), the reaction mixture was concentrated and
the resultingresidue was directly purified by silica eg l
served. Solid state structure determination of
DABCO-Cu complex shows that the Cu is in the
Cu(II) oxidation state with TBP geometry and exists
in a linear polymeric structure due to stronghydro ge n
bonding. We have increased the catalytic activity of
DABCO-Cu complex by usingnitromethane, and
thus we could accomplish the oxidation reaction at
room temperature with very good yield.
column chromatography (hexane-ethyl acetate) to obtain
correspondingpure aldehyde; yield: 126.5 mg(93%); R =
f
0
.69 (20% ethyl acetate in hexane); IR (neat): n=1690 cm
À1
1
;
H NMR (CDCl , 400 MHz): d=9.79 (s, 1H), 7.7 (d, J=
3
13
8
.65 Hz; 2H), 6.9 (d, J=8.64 Hz; 2H), 3.8 (s, 3H); C NMR
(
CDCl3, 100 MHz): d=190.7, 164.6, 131.9, 130, 114.3, 55.5;
+
HR-MS: m/z=137.0602, calcd mass: 137.0603 (M ).
Typical Experimental Procedure in Toluene at 1008C
A mixture of CuCl (4.95 mg, 0.05 mmol) and DABCO
Experimental Section
(
5.7 mg, 0.05 mmol) in 2 mL toluene was stirred at room
temperature for 10 min, then TEMPO (7.82 mg, 0.05 mmol)
was added to the reaction mixture. After stirringfor 5 min,
4-methoxybenzyl alcohol (138 mg, 1 mmol) was added and
General Remarks
All oxidation reactions were performed under oxygen at-
mosphere usingan oxy ge n balloon. All the solvents (used in
Table 1) were obtained from Merck, India and dried by
Vogelꢁs procedure. Reactions were monitored by TLC
plates (silica gel 60 F254, obtained from Merck) usingappro-
priate mixture of ethyl acetate and hexane. Product purifica-
then heated to 1008C under an O atmosphere (usingO 2
2
balloon) for two hours. After coolingto room temperature,
the reaction mixture was diluted with ethyl acetate, washed
with dilute HCl followed by water. The organic layer was
dried over sodium sulfate, concentrated and the resulting
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Sreedevi Mannam et al.
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residue was purified by silica gel column chromatography
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(
(
hexane-ethyl acetate) to give the aldehyde; yield: 121 mg
89%).
Synthesis of the DABCO-Cu Complex
[
[
8] a) H. Egami, S. Onitsuka, T. Katsuki, Tetrahedron Lett.
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2
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(
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was added. The resultingsolution was sli gh tly warmed to
give clear solution. The reaction mixture was filtered
through Whatman filter paper. The filtrate was kept at
room temperature for slow evaporation which gave slightly
green-colored needle-like crystals whose solid state structure
determination shows that the copper is in the Cu(II) oxida-
tion state with the molecular formula [(DAB-
CO) CuCl ·HCl].
5
7
9] J. Martin, C. Martin, M. Faraj, M. J. Bregeault, Nouv.
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[
[
[
2
004, 6, 4821–4824.
2
2
11] Y. Wang, J. L. DuBois, B. Hedman, K. O. Hodgson,
Crystallographic data: C H Cl CuN , M=395.25, rhom-
1
2
25
3
4
T. D. P. Stack, Science 1998, 279, 537–540.
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(
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Acknowledgements
We thank DST, New Delhi (SERC Fast Track Research Proj-
ect No.: SR/FTP/CS-19/2004) for the financial support. S. M.
and S. K. A. thank CSIR for Junior Research Fellowship.
1
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2258
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ꢀ 2007 Wiley-VCH VerlagGmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2007, 349, 2253 – 2258