Selective oxidation of aromatic primary alcohols to aldehydes using molybdenum acetylide oxo-peroxo complex as catalyst

Article abstract of DOI:10.1016/j.tetlet.2009.03.178

Selective oxidation of various aromatic alcohols to aldehydes has been carried out with very high conversion (90%) and selectivity (90%) for aldehydes using cyclopentadienyl molybdenum acetylide complex, CpMo(CO)₃(C{triple bond, long}CPh) (1) as catalyst and hydrogen peroxide as environmentally benign oxidant. Water-soluble Mo acetylide oxo-peroxo species is formed in situ after reaction of 1 with aqueous hydrogen peroxide during the course of reaction as catalytically active species. Interestingly even though the catalyst is homogeneous it could be recycled very easily by separating the products in organic phase and catalyst in aqueous phase using separating funnel. Even after five recycles no appreciable loss in alcohol conversion and aldehyde selectivity was observed.

Full text of DOI:10.1016/j.tetlet.2009.03.178

Tetrahedron Letters 50 (2009) 2885–2888
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Tetrahedron Letters
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Selective oxidation of aromatic primary alcohols to aldehydes using
molybdenum acetylide oxo-peroxo complex as catalyst
Ankush V. Biradar, Mohan K. Dongare, Shubhangi B. Umbarkar *
Catalysis Division, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411 008, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Selective oxidation of various aromatic alcohols to aldehydes has been carried out with very high conver-
sion (90%) and selectivity (90%) for aldehydes using cyclopentadienyl molybdenum acetylide complex,
CpMo(CO) (C„CPh) (1) as catalyst and hydrogen peroxide as environmentally benign oxidant. Water-
3
soluble Mo acetylide oxo-peroxo species is formed in situ after reaction of 1 with aqueous hydrogen per-
oxide during the course of reaction as catalytically active species. Interestingly even though the catalyst is
homogeneous it could be recycled very easily by separating the products in organic phase and catalyst in
aqueous phase using separating funnel. Even after five recycles no appreciable loss in alcohol conversion
and aldehyde selectivity was observed.
Received 15 January 2009
Revised 19 March 2009
Accepted 26 March 2009
Available online 28 March 2009
Keywords:
Oxo-peroxo
Molybdenum complex
Primary aromatic alcohol
Oxidation
Ó 2009 Elsevier Ltd. All rights reserved.
Homogenous catalyst
Hydrogen peroxide
Oxidation of alcohols to aldehydes and ketones is one of the
most important transformations in organic synthesis.1 In particu-
lar; the oxidation of primary alcohols to aldehydes is important
since they find wide applications as intermediates in fine chemi-
cals particularly for perfume industry.2 Traditionally, the oxida-
tion of alcohols is carried out using stoichiometric inorganic
oxidants such as permanganate,4 bromate,5 or Cr(VI) based re-
fered with immobilized hydrogen phosphate in aqueous acetoni-
trile containing KBr and sodium hypochlorite, in one pot, for
oxidation of alcohols to carboxylic acids, with excellent yields
(>90%) of acids. Among the early transition-metal complexes,
molybdenum(VI) complexes have functional as well as structural
similarity with molybdo-enzymes and have the ability to catalyze
,3
17
a variety of oxidation reactions. Various molybdenum organome-
tallic complexes and oxides are known to be very good homoge-
6
agents which generates large amount of heavy metal waste. Sev-
eral transition metal-based homogeneous systems such as
neous and heterogeneous catalysts for the oxidation/epoxidation
7
8
9
10
11
18,19
palladium, ruthenium, manganese, tungsten, rhenium, cop-
reactions.
Molybdenum carbonyl complexes with different li-
1
2,13
14
20
21
per,
and iron have also been reported. However, mixtures
gands such as halides, N-containing ligands, and cyclopentadi-
5
5 5
g -C R
; R = H, Me, and Ph22 are mainly used for epoxidation
of the organic substrates, products, solvents, and molecular oxygen
are well known for being explosion hazards in many cases. Also,
some of the oxidation reactions have to be performed under severe
conditions, such as high temperature and high oxygen pressure. In
many cases the reactions are carried out in environmentally unde-
sired solvents, typically chlorinated hydrocarbons. Due to these
limitations, there is significant interest in the development of envi-
ronmentally benign and safe catalytic route for oxidation of alco-
hols using dioxygen or hydrogen peroxide as an oxidant and
preferably without solvent.15 Ley et al. have reported oxidation
of alcoholic group in azadirachtin derivative to carbonyl group
enyl;
of a variety of olefins. Recently, we have used molybdenum acety-
5
3 5 5
lide complex, CpMo(CO) (C„CPh); Cp = g -C H (1) as an efficient
oxidation catalyst for cis-dihydroxylation of various olefins as well
as for selective N-oxidation of amines to nitrosocompounds.23 In
continuation of our efforts to investigate the catalytic oxidation
of a variety of organic substrates, oxidation of aromatic alcohols
has been studied using catalyst 1 and hydrogen peroxide as an oxi-
dant. To our knowledge there are no reports in the literature on the
use of the above-mentioned complex for oxidation of alcohol. Here
we report the use of complex 1 for selective oxidation of aromatic
primary alcohols to aldehydes.
1
6
using Dess–Martin periodinane as stoichiometric reagent. They
have also used a mixture of immobilized oxidizing reagents such
as polystyrene-supported TEMPO, resin-bound chlorite and buf-
3
Molybdenum acetylide complex, CpMo(CO) (–C„CPh) was pre-
2
4
pared as per literature method. The oxidant used was hydrogen
peroxide and hence to confirm the stability of the catalyst and to
study the nature of catalytically active species formed under the
*
Corresponding author. Tel.: +91 20 25902044; fax: +91 20 25902633.
E-mail address: sb.umbarkar@ncl.res.in (S.B. Umbarkar).
2 2
reaction conditions, catalyst 1 was treated with H O and was
0
040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.03.178

2
886
A. V. Biradar et al. / Tetrahedron Letters 50 (2009) 2885–2888
characterized by FTIR. On treatment of the complex 1 with 30%
aqueous hydrogen peroxide, corresponding oxo-peroxo Mo(VI)
acetylide complex is formed after loss of carbonyl ligands (Scheme
Table 1
a
Benzyl alcohol oxidation in various solvents
Entry
Solvent
Conv (%)
Selectivity (%)
Aldehyde Acid
TONb
2
3
1). This in situ generated Mo-oxo-peroxo acetylide complex was
used for oxidation of aromatic primary alcohols with hydrogen
peroxide as an oxidant.
1
2
3
4
5
No solventc
Acetonitrile
t-Butanol
Methanol
Chlorobenzene
86
92
8
395.6
38.5
18.1
8
45.2
22.6
17.4
23.8
85.2
80.5
45.9
86.9
14.8
19.5
54.1
13.1
Benzyl alcohol was first examined as a model substrate with
2
H O
2
(30%) as oxidant. Considering the solubility of the molybde-
20.7
num-oxo-peroxo species in water, the oxidation reaction was ini-
tially carried out without additional solvent in the presence of a
a
2 2
Reaction conditions: benzyl alcohol—0.01 mole; 30% H O —0.02 mole, solvent—
2
5
catalytic amount of 1, (Scheme 2, Table 1). As 30% hydrogen per-
oxide is used as oxidant the remaining 70% water acts as a solvent.
The reaction mixture was allowed to reflux at 80 °C in the presence
of 1 mol % catalyst for 8 h. The progress of the reaction was moni-
tored by GC. A high conversion of 86% with very high selectivity
10 g, catalyst 1—0.1 mmol; time 8 h; temp 80 °C.
b
TON for aldehyde = moles of aldehyde formed per mole of catalyst.
c
2 2
Benzyl alcohol 0.05 mol, 30% H O 0.1 mol.
(
92%) for aldehyde was obtained (Table 1, entry 1). Very high turn
Table 2
Effect of temperature on benzyl alcohol oxidation
a
over number (TON = moles of benzaldehyde formed per mole of
the catalyst) of 396 was obtained for benzaldehyde in one cycle.
To improve the conversion and selectivity, various other solvents
were tried and to our surprise, the conversions and selectivity were
considerably lower (entries 2–5) compared to those obtained with-
out additional solvent.
The influence of temperature on benzyl alcohol conversion and
product selectivity was examined (Table 2). When reaction tem-
perature was increased gradually from rt to 80 °C, the rate of the
reaction increased. Even up to 40 °C, no reaction took place and
at 60 °C only 40% conversion was obtained. At 80 °C highest con-
version (86%) was obtained after 8 h.
Entry
Temperature (°C)
Conv (%)
Selectivity (%)
Aldehyde
Acid
1
2
3
4
RT
40
60
80
0
0
40
86
0
0
100
92
0
0
0
8
a
Reaction conditions: benzyl alcohol—0.05 mole; 30% H
—0.1 mmol; time 8 h.
2
O
2
—0.1 mole, catalyst
1
One major advantage with catalyst 1 is the recyclability of the
catalyst in spite of being homogeneous complex. After addition
of hydrogen peroxide to complex 1, the in situ formed oxo-peroxo
Mo acetylide complex is soluble in aqueous hydrogen peroxide and
hence forms miscible phase with alcohol. As the reaction proceeds
and the product aldehydes are formed, it forms separate organic
phase and hence the catalyst recycle becomes very easy for this
system. After completion of the reaction the organic phase (alde-
hyde) was easily separated using separating funnel and the fresh
charge of alcohol was added for the next recycle. The catalyst
was recycled efficiently for five cycles (Table 3). The conversion
and aldehyde selectivity did not decrease significantly even after
five recycles. Hence very easy recycle of this homogeneous catalyst
is one of the major achievements in this work.
Table 3
Catalyst-recycle studies
a
Entry
Run
Conv (%)
Selectivity (%)
Aldehyde
Acid
1
2
3
4
5
6
0
1
2
3
4
5
86
92
93
93
92
92
92
8
7
7
8
8
8
85.8
85.3
84.9
84.6
84.2
a
2 2
Reaction conditions: benzyl alcohol—0.05 mole; 30% H O —0.1 mole, catalyst
1—0.1 mmol; time 8 h.
After optimization of the reaction parameters, wider applicabil-
ity of catalyst 1 was tested for oxidation of a variety of substituted
aromatic primary alcohols and the results are summarized in Table
Lower conversion (60%) was obtained for benzyl alcohol with
electron-withdrawing substituents such as p-nitro, p-chloro, and
2,4-dicholoro benzyl alcohols (entries 5–7). Additionally to test
the oxidation of alcohol in presence of other functional groups such
as double bond, cinnamyl alcohol oxidation was tested (entry 9). In
cinnamyl alcohol, alcohol group was selectively oxidized without
oxidation of double bond with 70% conversion and 75% selectivity
for aldehyde. The lower selectivity for aldehyde may be due to
presence of double bond that makes further oxidation of aldehyde
to acid more favorable. When non-aromatic alcohols such as cyclo-
hexanemethanol and cyclohexenemethanol (entries 10 and 11)
were oxidized, 82% conversion with more than 85% selectivity for
aldehyde was obtained. However when aliphatic alcohols such as
n-propanol, n-butanol, and n-octanol were oxidized using catalyst
4. In case of p-methyl benzyl alcohol best results were obtained
with 90% conversion and 90% selectivity for aldehyde. Activated
and electron-rich alcohols such as p-methyl benzyl alcohol (entry
2
) and p-methoxy benzyl alcohol (entry 3) gave high conversion
and selectivity for aldehyde compounds.
H O ,R.T.
2
2
Mo
Ph -
Mo
O 1
Ph
3CO
OC
O
O
a
CO
CO
1
1
, no reaction took place.
Uemira et al. described the palladium-catalyzed oxidation of
Scheme 1. In situ generation of oxo-peroxo Mo(VI) acetylide complex.
alcohols to aldehydes and ketones using a 5 mol % Pd(OAc)
2
, pyri-
dine and 3 Å molecular sieves (MS3A) under atmospheric pressure
of oxygen with 100% conversion and 92% selectivity. This system
has given very high efficiency though using high loading of expen-
O
CHO
7
OH Cat
sive Pd catalyst and needs an additive.
OH
+
Sheldon et al. have used various copper salts in combination
with different N-containing ligands, TEMPO, and a base as co-cat-
alyst, for oxidation of a series of primary alcohols to aldehydes
oxidant
Scheme 2. Oxidation of aromatic alcohol.

A. V. Biradar et al. / Tetrahedron Letters 50 (2009) 2885–2888
2887
Table 4
system involving co-catalyst, base, expensive reagent TEMPO along
with an oxidant whereas the present study involves a simple reac-
tion system with catalyst and hydrogen peroxide as oxidant apart
from substrate alcohol. The TON in the present system is very high;
more than 10-fold (262–396) compared to those in both the copper
systems (ꢀ25). Minisci et al. have reported oxidation of various
benzylic alcohols to aldehydes and ketones using molecular oxy-
gen as oxidizing agent and N-hydroxyphthalimide (NHPI) and
Co(II) salts as catalytic system at room temperature with very high
conversion (75–100%) and selectivity (>90%) for aldehydes as well
as oxidation of primary and secondary alcohols (benzylic and ali-
phatic) to aldehydes or ketones using Mn(II)–Co(II) or Mn(II)–
Cu(II) nitrates in combination with TEMPO in acetic acid solvent
a
Oxidation of different alcohols
Entry
Substrate
% conv
Selectivity (%)
TON
Yield (%)
Aldehyde
Acid
1
2
3
OH
86
90
90
92
8
396
391
392
79
78
90
OH
87
90
13
10
OH
OH
OH
2
6,27
using air or oxygen as oxidant with >95% yield.
MeO
MeO
Hou et al. described the use of Fe-coordinated polymers for oxi-
dation of alcohols and the result showed excellent selectivity,
though the recycle of the catalyst is not very efficient. Martın
et al. have carried out aerobic oxidation of secondary and benzylic
alcohols with very high efficiency using binary catalyst system
4
83
85
15
352
71
3 3 3
Fe(NO ) –FeBr under air at room temperature with good yield
OMe
1
4
(
75%).
Mo-oxo-peroxo complex with quinone as ligand has been used
by Bhattacharya et al. for oxidation of alcohols as well as sulfides
with varying TONs ranging from 84 to 1780 for various alcohols
using oxygen, hydrogen peroxide, or mixture of oxygen and H O
2 2
5
6
7
8
9
60
65
78
72
70
82
82
88
91
90
82
75
88
85
12
9
264
296
343
295
262
361
348
53
62
70
59
52
72
68
O N
2
as oxidant. Though very small amount of the catalyst is used in this
OH
19
paper, no recycle is reported for the catalytic system.
The mechanism for the oxidation of various alcohols to alde-
Cl
Cl
hydes using catalyst 1 has been proposed (Scheme 3) by compari-
2
8,29
son with the literature reports.
complex 1 on treatment with H
oxo complex which was well characterized before.
Molybdenum acetylide
OH
Cl
10
18
25
12
15
2 2
O forms corresponding oxo-per-
2
3a
When alco-
hol molecule approaches the catalyst the Mo-peroxo ring opens
by abstracting alcohol –OH proton by peroxo moiety. The carbon
proton abstraction followed by liberation of water molecule and
aldehyde leads to the formation of Mo-dioxo complex which after
OH
2 2
further treatment with H O regenerates the Mo-oxo-peroxo
OH
species.
The results show that molybdenum acetylide complex 1 is very
efficient catalyst for the oxidation of a variety of primary aromatic
alcohols to aldehydes with maximum selectivity. It can very well
tolerate electron-rich as well as electron-withdrawing substituents
on aromatic ring. Water soluble nature of the catalytically active
species, oxo-peroxo molybdenum acetylide moiety and formation
of organic phase of the product (aldehyde) have enabled very easy
and efficient recycle of this homogeneous catalyst even up to five
OH
1
0
OH
1
1
a
Reaction conditions: benzyl alcohol—0.05 mole; 30% H
—0.01 mmol; time 8 h.
2 2
O —0.1 mole, catalyst
1
Cp
Cp
Mo C
Cp
2
PhCH OH
H
2
O
2
VI
O
Mo C
C
Ph
C
Ph
O
Mo C C Ph
O
O
CO
using air as oxidant under very mild reaction conditions. In case of
benzyl acohol 100% conversion was obtained in 2.5 h whereas 61%
conversion was obtained in 24 h for octanol. Even for 100% alcohol
conversion the TON obtained in this case was ꢀ20. Ragauskas
et al. have also used copper catalyst for oxidation of primary as
well as secondary alcohols. A multi-component system consisting
CO
O
O
CO
H
O
C
Ph
H
1
2
H
2 2
H O
Cp
of Cu(ClO
4
)
2
, TEMPO, TMDP as well as DABCO has been used for
Cp
O
Mo C
O
C Ph
room temperature oxidation of alcohols to carbonyl compounds
using oxygen as oxidant with good to excellent yields. The conver-
sions ranged from 98% to 11% for various alcohols with corre-
sponding reaction times varying from 2 to 24 h, respectively. The
maximum TON obtained for this system was 25. This catalyst sys-
tem was recycled three times; however during the recycle 8 to 17%
decrease in the conversion was observed due to leaching of copper,
O
Mo C
O
C
Ph
O
Ph
C
O
H
H
H
PhC
H
O
2
H O
2
4,13
hence it is not a very efficient recyclable catalytic system.
Both
the copper-catalyzed alcohol oxidations need a multi-component
Scheme 3. Possible mechanism of alcohol oxidation.

2
888
A. V. Biradar et al. / Tetrahedron Letters 50 (2009) 2885–2888
recycles without appreciable loss in conversion and aldehyde
selectivity. This system has a number of advantages compared to
the literature reports, such as simple catalyst system, high TON
as well as recyclability of the catalyst.
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Acknowledgments
S.B.U. acknowledges the financial support from the Department
of Science and Technology, and A.V.B. acknowledges the Council of
Scientific and Industrial Research for a fellowship.
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3
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4
5
.
.
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3
À1
was confirmed using FTIR spectroscopy. m(CO)
,
1940, 2031 cm ; m(C„C),
13
À1
1
2102 cm . NMR in CDCl : d ( H); (s, 5H, Cp), 7.15–7.38 (m, 5H, Ph); d ( C);
3
92.93 (Cp), 87.85. 126.01, 127.0, 129.39, 130.8, 130.87 (C„CPh); 222.4, 238.78
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2
50 mL round-bottomed flask. The reaction mixture was vigorously stirred
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6890 Gas Chromatograph equipped with a HP-5 dimethyl polysiloxane (60 m
length, 0.25 mm internal diameter, 0.25 lm film thickness) with flame
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Network MSD.
3
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