Alcohols oxidation with hydrogen peroxide promoted by TPAP-doped ormosils

Article abstract of DOI:10.1016/j.tetlet.2004.08.020

Organically modified silica gels doped with TPAP (tetra-n-propylammonium perruthenate) are effective catalysts for the oxidation of alcohols by hydrogen peroxide at room temperature, provided that the oxidant H₂O ₂ solution is added slowly. The effect of the surface catalyst polarity is the opposite of that found in aerobic alcohols oxidation and is consistent with the polar nature of the primary oxidant.

Full text of DOI:10.1016/j.tetlet.2004.08.020

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 7283–7286
Alcohols oxidation with hydrogen peroxide promoted by
TPAP-doped ormosils
a,
Sandro Campestrini, Massimo Carraro, Rosaria Ciriminna,
a
b
*
b
Mario Pagliaro and Umberto Tonellato
a
a
Universit a` di Padova, Dipartimento di Scienze Chimiche; CNR, Istituto per la Tecnologia delle Membrane,
Via Marzolo 1, 35131 Padova, Italy
b
CNR, Istituto per lo Studio dei Materiali Nanostrutturati, Via Ugo La Malfa 153, 90146 Palermo, Italy
Received 21 June 2004;revised 3 August 2004;accepted 4 August 2004
Available online 23 August 2004
Abstract—Organically modified silica gels doped with TPAP (tetra-n-propylammonium perruthenate) are effective catalysts for the
oxidation of alcohols by hydrogen peroxide at room temperature, provided that the oxidant H O solution is added slowly. The
2
2
effect of the surface catalyst polarity is the opposite of that found in aerobic alcohols oxidation and is consistent with the polar nat-
ure of the primary oxidant.
ꢀ
2004 Elsevier Ltd. All rights reserved.
Alcohols oxidation yielding ketones and aldehydes is a
chemical transformation of primary industrial impor-
tance in fine chemistry, carbonyl compounds being pre-
cursors of a variety of valuable fine chemicals including
reaction temperatures are required. Ruthenium, for in-
stance, is probably the most promising metal in promot-
ing aerobic oxidation of alcohols;in particular, the
species tetra-n-propylammonium perruthenate (TPAP)
1
10
drugs, vitamins and fragrances. Societal demand for
either dissolved in solution or immobilised on suitable
1
1
fundamental chemical transformations that are not
harmful to the environment is pressing, and an economi-
cally and environmentally viable replacement of tradi-
tional alcohols oxidation, still carried out by industry
with stoichiometric amounts of toxic and hazardous
supports is a very efficient and selective catalyst for the
aerial oxidation of primary, secondary, benzyl and alkyl
1
2
alcohols in nonaqueous media, including dense-phase
1
3
carbon dioxide. Most functional groups, such as car-
bon–carbon double bond, epoxy, indole, acetal, etc., re-
main unaffected by TPAP and high yields of carbonyls
2
metal oxidants such as dichromate and permanganate,
remains a scientific challenge for chemists both in indus-
try and academia.
1
2,14
are isolated.
However, even though molecular oxy-
3
gen is the most environmentally and economically con-
venient primary oxidant, its use in association with
platinum-group-metals species and in particular with
TPAP suffers the disadvantage of requiring high reac-
Obviously, the utilization of oxygen and even better of
air as stoichiometric oxidant for the selective oxidation
of organics is the final, ideal goal of prolonged research
efforts and a number of catalytic procedures for the aero-
tion temperatures (T P 80ꢁC) in order to reoxidise the
À
spent catalyst (formally RuO ) to the native species.
2
4
5
6
7
bic alcohols oxidation employing Ru, Pd, Cu, Co,
8
9
V, Os derivatives have been reported in the last few
years. Most of these methods, however, suffer from
some drawback, and in most cases either relatively large
amounts of expensive catalysts are needed, or the stoi-
chiometric consumption of a ÔsacrificialÕ reactant or high
On the other hand, besides oxygen a few primary oxi-
15
2
À À
dants, such as S
and N-methylmorpholine N-oxide in organic solvent
2
O8 and BrO3 in aqueous media
1
6
at room temperature, have been reported in associa-
tion with TPAP for oxidizing alcohols catalytically;
and, to the best of our knowledge, no reports on the uti-
lization of hydrogen peroxide with TPAP appeared in
the literature in spite of the fact that H O is a clean
2
Keywords: Oxidation;Hydrogen peroxide;Peroxides;Sol–gel.
*
2
Corresponding author. Tel.: +39 049 827 5255;fax: +39 049 827
239;e-mail: sandro.campestrini@unipd.it
and relatively cheap oxidant, which can now be effi-
ciently produced in situ by direct reaction of O with
5
2
0
040-4039/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.08.020

7284
S. Campestrini et al. / Tetrahedron Letters 45 (2004) 7283–7286
1
7
H over a gold catalyst. In this letter we describe the
2
Table 1. Benzaldehyde and benzoic acid yields as function of H
À2
2
O
2
addition velocity (3.0mL of H
2
O
2
4.7 · 10 M in ether) in the
oxidation of benzyl alcohol (0.05mmol in 3.0mL of ether) in the
first use of H O in association with entrapped TPAP
2
in the catalytic oxidation of alcohols at room
temperature.
2
presence of TPAP-ormosils (0.005mmol) at 25ꢁC
Run
H
2
O
2
Vel.
add. (mL/h) conversion (%) yield (%)
Benzyl alcohol Benzaldehyde Benzoic acid
yield (%)
We recently reported an efficient method for alcohols
oxidation by molecular oxygen catalysed by TPAP-
a
1
2
3
4
5
6
7
ꢀ180
0.30
0.15
0.08
0.04
0.03
4.0
41.2
45.5
68.8
95.0
99.8
85.3
4.0
41.2
45.5
67.6
81.8
26.6
85.3
—
—
1
3
doped ormosils in scCO2. These solid catalysts are pre-
pared by sol–gel entrapment of TPAP in silica glasses
obtained using methyltrimethoxysylane (MTMS) and
tetramethylorthosilicate (TMOS) as precursors of the
final xerogel. Catalysts with various surface hydrophob-
icity were obtained by varying the percent of methyl-
modified precursor utilised in the synthesis of the xero-
gels (i.e. 0%, 25%, 75% and 100% of MTMS) whereas
catalytic doped glasses with different pores size and
entrapment modality of TPAP were also obtained by
varying the process employed for the xerogels prepara-
tion, that is, the amount of water and the presence of
—
1.2
13.2
73.2
—
b
0.03
a
2 2
The H O solution was added in a single blow.
˚
In the presence of activated 3A molecular sieves.
b
shows the marked effect of varying the velocity of hydro-
gen peroxide addition on the conversion of the alcohol.
NaF. The results obtained in scCO indicate that the
2
Indeed the quick addition of the H O solution to the
2 2
best performing catalysts require both a high degree of
surface hydrophobicity as well as high amounts of water
in the sol–gel polycondensation process.
reaction mixture containing the substrate and the cata-
lyst leads to a very low alcohol conversion (3–4%, Table
1), hydrogen peroxide decomposition being the major
reaction pathway. Conversely, when the H O solution
1
3
2
2
In order to assess the possibility of exploiting TPAP-
based catalysts in alcohols oxidation by H O , we
decided to carry out some preliminary experiments uti-
lising a mixture of the above mentioned TPAP-doped
ormosils (catalytic load of 0.05mmol TPAP/g ormosil)
is slowly added by means of a syringe-pump, alcohol
conversions increase up to becoming almost
quantitative.
2
2
These results suggest a different kinetic order in hydro-
gen peroxide relative to the two processes involved,
recycled upon oxidation experiments in scCO . The oxi-
2
dations were conducted in a 10mL jacketed reactor thermo-
namely the oxidation of the alcohol and H O decompo-
2 2
stated at 25ꢁC by suspending 100mg of ormosil
sition. In particular, the kinetic order for hydrogen per-
oxide in its decomposition catalysed by ruthenium
derivatives should be higher compared to that of alcohol
oxidation, so that a steeper, more pronounced depend-
ence of the decomposition rate on the addition of the
oxygen donor is observed in the former process. When
the concentration of hydrogen peroxide is kept relatively
low during the whole process, the alcohol is smoothly
(
(
0.005mmol) in a 3mL ether solution of benzyl alcohol
0.050mmol) and n-dodecane (as internal standard,
0
H O in ether (0.14mmol in 3.0mL, by titration with
.022mmol) kept under stirring. A solution of dry
2
2
acidic KMnO ) was thus added by means of a syringe-
4
pump at various time intervals. After completion of
the oxidant addition, the reaction mixture was analysed
by GLC to determine the alcohol conversion along with
benzaldehyde and benzoic acid yields. Figure 1 clearly
oxidised and decomposition of H
minimised.
O to water and O
2 2
2
Even the selectivity of the oxidative process is affected
by the pace of hydrogen peroxide addition as shown
by the data of Table 1. Hence, when alcohol conversions
are lower than 50%, benzaldehyde is the only product
detected (runs 1–3). At conversion higher than 50%,
the alcohol overoxidation becomes significant and ben-
zoic acid is detected along with benzaldehyde (runs 4–
1
1
10
00
9
0
0
0
0
0
0
0
8
7
6
5
4
3
6
). However, the overoxidation can be completely elim-
inated (run 7) by sequestering the water formed along
with the aldehyde in the first oxidative step.
Once assessed the TPAP capability of acting as catalyst
in alcohol oxidation by H O , we prepared various
2
2
ormosils by sol–gel method, co-polymerising TMSO
with MTMS at various ratios in the presence of TPAP
in order to verify the effect of surface ormosils hydro-
phobicity on the catalyst activity. Thus, a catalyst pre-
pared with 0% MTMS is essentially a complete
inorganic glass whereas, a catalyst prepared with 100%
MTMS is an organically modified glass bearing one
methyl group each silicon atom. The results concerning
0
.00
0.05
0.10
0.15
0.20
0.25
0.30
H O Adding speed (mL/hours)
2
2
Figure 1. Benzyl alcohol conversion (0.05mmol in 3.0mL ether) as
À2
function of the velocity of H
2
O
2
addition (3.0mL of H
2
O
2
4.7 · 10
M
in ether) in the presence of TPAP-doped ormosils (0.005mmol) at
5ꢁC.
2

S. Campestrini et al. / Tetrahedron Letters 45 (2004) 7283–7286
7285
Table 2. Alcohol conversion, aldehyde and acid yields as function of
catalyst methylation in the oxidation of benzyl alcohol (0.05mmol in
ated ormosils (entries 9–11) lead to some alcohol overoxi-
dation, thus suggesting that their drying power is
comparable to that of molecular sieves. When TPAP-
doped silica is the catalyst, a decrease in the speed of
À2
3.0mL of ether) with hydrogen peroxide (3.0mL of H
in ether added at the velocity of 0.15mL/h) catalysed by TPAP
2
O
2
4.7 · 10
M
(
0.0025mmol) encapsulated in various MTMS/TMOS mixed gels, in
˚
H O addition affords higher alcohol conversion (run
2
2
the presence of 3A molecular sieves, at 25ꢁC
13 compared to run 8);in this case, however, the excess
Run MTMS Benzyl alcohol Benzaldehyde Benzoic acid
conversion (%) yield (%)
of hydrogen peroxide employed yields a lower benzalde-
hyde selectivity.
(%)
yield (%)
8
9
1
1
1
1
1
1
0
25
50
75
93.4
90.7
82.5
73.1
49.3
99.0
94.3
51.2
86.2
82.5
63.5
68.5
49.3
72.7
94.3
51.2
7.2
8.2
19
4.6
0
The speed of H O addition has a much lower effect
2
2
0
1
2
3
4
5
when the TPAP-doped fully methylated silica is the cat-
alyst, the selectivity to carbonyl remaining excellent (run
15 compared to run 12). Finally, an almost quantitative
alcohol conversion and an excellent aldehyde selectivity
100
0
a
b
a
26.3
0
0
can be obtained with TPAP-doped unmodified SiO by
2
100
0
simply reducing the hydrogen peroxide excess (run 14).
Results in Table 3 prove also the generality of the meth-
od. Contrary to crystalline catalytic materials such as
a
À2
3
.0mL of H
2
O
2
4.7 · 10 M in ether added at the velocity of
0
.04mL/h.
b
À2
3
.0mL of H
2
O
2
2.3 · 10 M in ether added at the velocity of
1
9a
19b
zeolites (including zeolite X
and MCM-41
doped
0
.15mL/h.
with perruthenate, which are intrinsically shape selec-
tive), ormosils are amorphous materials showing a dis-
tribution of inner porosities, which allows their
application to the conversion of largely different sub-
strates including, in this case, long-chain aliphatic alco-
hols such as octanol.
the activity of these TPAP doped gels in the oxidation of
benzyl alcohol by H O are summarised in Table 2:
2
2
Entries 8–12 in Table 2 clearly show that the catalyst
efficiency largely depends on the ormosil surface HL bal-
ance. In particular, less hydrophobic the surface, more
active is the catalyst: an outcome related to the polar
nature of hydrogen peroxide, which prevents its diffu-
sion through an hydrophobic surface. Accordingly, in
the aerobic alcohols oxidation mediated by these ormo-
sils, where oxygen freely diffuses into the catalyst frame-
The catalyst could be reused: when the oxidation of benzyl
alcohol was repeated twice utilising the same mixture
of catalyst and molecular sieves, and simply after filter-
ing and washing the powder (runs 17–18), benzaldehyde
was produced in lesser but yields compared to the first
run. We ascribe the decreasing in aldehyde yields and
the slight increasing in acid formation to the impossibil-
ity of separate and recover completely all the catalyst
from the crushed and inactivated molecular sieves.
1
1,13
work, a reverse reactivity order is observed.
Interestingly, the selectivity of the process also depends
on the catalyst hydrophobicity and only the fully methyl-
ated ormosil (run 12) completely suppresses overoxi-
dation. A possible rationale for this behaviour, which
has been observed in other catalytic applications of
Similarly to aerobic TPAP mediated oxidations, pri-
mary alcohol functionality revealed to be more reactive
than secondary alcohols both in the case of alkyl and
benzyl alcohols. Remarkably, however, alkyl alcohols
are only slightly less reactive than benzyl alcohols;while
the lower selectivity measured in the oxidation of pri-
mary alkyl alcohol, in its turn, might be related to a
greater extent of hydratation in the case of the aliphatic
aldehyde compared with that of the aromatic aldehyde.
The oxidation of the allylic alcohol geraniol (run 22)
1
8
sol–gels, is that the highly hydrophobic permethylated
catalyst spills out the water formed during alcohol oxi-
dation, thus preventing aldehyde hydration and conse-
quently also the overoxidation to benzoic acid.
Despite the presence of molecular sieves, both the
unmodified doped SiO (entry 8) and the partially methyl-
2
Table 3. Alcohol conversions and corresponding carbonyl compounds yields in the oxidation of various alcohols (0.05mmol in 3.0mL of ether) with
À2
hydrogen peroxide (3.0mL of H
2
O
2
2.3 · 10 M in ether added at the velocity of 0.15mL/h) catalysed by TPAP (0.0025mmol) encapsulated in pure
˚
silica (0% MTMS) in the presence of 3A molecular sieves, at 25ꢁC
Entry
Substrate
Alcohol conversion (%)
Carbonyl compound yield (%)
Carboxylic acid yield (%)
16
17
18
19
20
21
22
23
24
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
94.3
70.2
65.4
63.3
71.6
58.9
62.9
40.0
20.0
94.3
69.7
63.9
63.0
41.7
58.9
25.5
11.3
20.0
0
a
0.5
1.5
0
b
(±)-1-Phenylethanol
1-Octanol
29.9
0
(±)-2-Octanol
Geraniol
37.4
28.7
0
Furfuryl alcohol
Borneol
a
First catalyst recycling.
Second catalyst recycling.
b

7286
S. Campestrini et al. / Tetrahedron Letters 45 (2004) 7283–7286
proceeds with fair yield and involves only the alcoholic
function. In fact, neither the 2,3-allylic or 6,7-olefinic
carbon–carbon double bonds undergo oxidation. Fur-
furyl alcohol too (run 23) leads to a fair conversion
being the corresponding acid the major product under
these experimental conditions. A more sterically hin-
dered secondary alcohol than 2-octanol, such as borneol
4. Yamaguchi, K.;Mizuno, N. Chem. Eur. J. 2003, 9,
353–4361.
. (a) Steinhoff, B. A.;Stahl, S. S. Org. Lett. 2002, 4, 4179;
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(
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. Sharma, V. B.;Jain, S. L.;Sain, B. Tetrahedron Lett. 2003,
(
run 24) gives only moderate substrate conversion with
4
4, 383.
an excellent selectivity for the corresponding ketone. It
should be noted that these reactions have not been
optimised. Higher conversions for high molecular
weight alcohols and higher selectivities for aldehydes
in primary alcohols oxidation should be expected by
tuning opportunely the catalyst hydrophobicity and
the amount of hydrogen peroxide utilised as well as
the velocity of peroxide addition.
8. Maeda, Y.;Kakiuchi, N.;Matsumura, S.;Nishimura, T.;
Kawamura, T.;Uemura, S. J. Org. Chem. 2002, 67, 6718.
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M.;Militzer, H. C.;Beller, M. Tetrahedron Lett. 2001, 42,
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1
5
In conclusion, we have discovered that TPAP-doped
organically modified silica gels are effective catalysts
for the oxidation of alcohols by hydrogen peroxide at
room temperature, provided that the oxidant solution
is added slowly. The effect of surface catalyst hydro-
phobicity is opposite of that found in aerobic alcohols
oxidation and is consistent with the polar nature of
the H O primary oxidant. Considering the ease of
2. Hasan, M.;Musawir, M.;Davey, P. N.;Kozhevnikov, I.
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7
exploiting hydrogen peroxide formed in situ and the
unique advantages of commercial sol–gel catalytic mate-
2
0
rials, these findings might open the way to the intro-
duction of environmentally friendly and cost-effective
alcohols oxidation.
1
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