A new non-metal heterogeneous catalyst for the activation of hydrogen peroxide: A perfluorinated ketone attached to silica for oxidation of aromatic amines and alkenes

Article abstract of DOI:10.1039/b100421m

A silane functionalized by octafluoroacetophenone was polymerized by the sol-gel method to form an insoluble silicate with perfluoroketone pendants; the silicate was used as a heterogeneous catalyst for the activation of aqueous hydrogen peroxide and the oxidation of aromatic amines and alkenes.

Full text of DOI:10.1039/b100421m

A new non-metal heterogeneous catalyst for the activation of hydrogen
peroxide: a perfluorinated ketone attached to silica for oxidation of aromatic
amines and alkenes
Karine Neimann and Ronny Neumann*
Department of Organic Chemistry, The Weizmann Institute of Science, Rehovot, Israel 76100.
E-mail: ronny.neumann@weizmann.ac.il
Received (in Cambridge, UK) 10th January 2001, Accepted 31st January 2001
First published as an Advance Article on the web 20th February 2001
A silane functionalized by octafluoroacetophenone was
polymerized by the sol–gel method to form an insoluble
silicate with perfluoroketone pendants; the silicate was used
as a heterogeneous catalyst for the activation of aqueous
hydrogen peroxide and the oxidation of aromatic amines
and alkenes.
The preparation of heterogeneous catalysts for the activation of
hydrogen peroxide and their use for oxidation of organic
substrates has been the subject of rather intense research efforts
over the last 10–15 years. Almost exclusively this effort has
Scheme 2
been concentrated on the incorporation of metal-based catalysts
onto or into inert matrices. This has been achieved either by
substitution of active metals (titanium, vanadium etc.) into
and derivatives to the corresponding N-oxides, showed that the
catalyst was quite effective for this reaction, Table 1. The yields
were generally high for some representative substrates such as
pyridine, 2- and 4-methylpyridine, quinoline, and 4-methyl-
quinoline; N-oxides were the only products. Interestingly,
8-hydroxyquinoline was less reactive, presumably because of
steric considerations, and the electron-poor 2,6-dichloropyr-
idine was unreactive as expected. The oxidation of aniline and
its derivatives catalysed by the perfluoroketone–silicate gave
somewhat more surprising results, Table 2. Aniline, alkyl-
1
2
molecular sieves or amorphous silica, or by encapsulation or
3
attachment of organometallic catalysts onto supports. There
are only a few organic compounds capable of activating
hydrogen peroxide. Thus, under basic conditions hydrogen
peroxide can be used for the epoxidation of electron deficient
4
alkenes and this has led to guanidine functionalized silica for
5
such reactions. A perfluorinated ketone, hexafluoroacetone,
also is a known compound capable of reacting with hydrogen
peroxide to yield an a-hydroperoxyperfluoro alcohol inter-
mediate capable of reacting with nucleophilic substrates to give
oxygenated products,6 Scheme 1. The high toxicity and
volatility of hexafluoroacetone has prevented the wide use of
this catalytic system. Recently, a high molecular weight
perfluoroketone with less volatility has been suggested as a
suitable improvement for hexafluoroacetone in similar oxida-
tion reactions.7
Table 1 Oxidation of pyridine derivatives with 60% H
perfluoroketone–silicate
2
O
2
catalysed by the
Substrate
Product
Yield
(mol%)
Pyridine
Pyridine 1-oxide
99
84
93
87
83
20
0
2
4
-Methylpyridine
-Methylpyridine
2-Methylpyridine 1-oxide
4-Methylpyridine 1-oxide
Quinoline 1-oxide
Quinoline
4
8
2
-Methylquinoline
-Hydroxyquinoline
,6-Dichloropyridine
4-Methylquinoline 1-oxide
8-Hydroxyquinoline 1-oxide
2,6-Dichloropyridine 1-oxide
Reaction conditions: 0.8 mmol substrate, 20 mg (0.05 mmmol ketone)
perfluoro–silicate, 1 mL acetonitrile, 3 mmol 60% H O , 24 h, 80 °C. Yields
2 2
were computed by GC analysis.
Table 2 Oxidation of aniline derivatives with 60% H
perfluoroketone–silicate
O
2 2
catalysed by the
Scheme 1
Yield
(mol%)
Now we present a silica immobilized perfluoroketone as an
effective heterogeneous recyclable catalyst for the oxidation of
aromatic amines such as pyridine and its derivatives and aniline
and its derivatives in addition to alkenes, using aqueous
hydrogen peroxide as oxygen donor. The strategy for the
preparation of the perfluoroketone non-metal silicate catalyst is
presented in Scheme 2. First, the ketone moiety of octa-
fluoroacetophenone was protected by formation of an imine
with n-butylamine. The protected perfluoroketone was then
attached to an aminosilane by nucleophilic substitution and the
insoluble silicate was prepared by co-polymerization with
tetraethoxysilane by the sol–gel method. The final heteroge-
neous catalyst was then obtained by removal of the n-
butylamine protecting group.†
Substrate
Aniline
Product
Diphenyldiazene 1-oxide
100
3
2
-Methylaniline
-Ethylaniline
Bis(3-methylphenyl)diazene 1-oxide 100
Bis(2-ethylphenyl)diazene 1-oxide 100
Bis(3-trifluorophenyl)diazene
3-Trifluoromethylaniline
1-oxide
100
3-Fluoroaniline
Bis(3-Fluorophenyl)diazene 1-oxide 100
3
2
4
2
4
-Nitroaniline
1,3-Dinitrobenzene
100
100
100
—
-Methyl-4-hydroxyaniline 4-Nitro-5-methylphenol
-Hydroxyaniline
-Nitroaniline
4-Nitrophenol
None
None
-methoxy-2-nitroaniline
—
Reaction conditions: 0.8 mmol substrate, 20 mg (0.05 mmmol ketone)
perfluoro–silicate, 1 mL acetonitrile, 3 mmol 60% H O , 12 h, 80 °C. Yields
2 2
The perfluoroketone–silicate catalyst was tested for activity
with several substrate types.‡ First, the oxidation of pyridine
were computed by GC analysis.
DOI: 10.1039/b100421m
Chem. Commun., 2001, 487–488
This journal is © The Royal Society of Chemistry 2001
487

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substituted aniline and halogen-substituted aniline derivatives
all yielded the dimeric azoxy compounds as sole products in
quantitative yields. On the other hand aniline with electron
donating hydroxy substitution or electron withdrawing nitro
substitution yielded the corresponding nitro derivatives, pro-
vided the nitro was not ortho to the amino substituent. In this
latter case there was no reaction, possibly related to intra-
molecular hydrogen bonding between the amino and nitro
substituents. For the other cases, we have as yet no satisfactory
explanation for partial oxidation and the selective formation of
azoxy compounds on the one hand for aniline, alkylated anilines
and halogenated anilines and full oxidation to nitro derivatives
in the case of 3-nitroaniline and 4-hydroxyaniline.
The catalytic activity of the perfluoroketone–silicate was also
tested for oxidation of alkenes, Table 3. For less nucleophilic
substrates such as oct-1-ene and oct-2-ene, activity was low
although the initial epoxide product formed was stable under the
reaction conditions and epoxides were obtained selectively.
More nucleophilic substrates such cyclohexene, 1-methylcyclo-
hexene and 2,3-dimethylbut-2-ene were much more reactive
and high conversions were obtained. However, selectivity to the
epoxide was very low due presumably to acid catalysed
formation of diols or pinacol rearrangement. Interestingly,
primary allylic alcohols reacted to yield mostly aldehyde as the
(0.25 mmmol ketone) perfluoro–silicate and 15 mmol 60%
were mixed in 5 mL acetonitrile at 80 °C for 18 h. More
2 2
H O
than 98% of the aniline reacted to diphenyldiazene 1-oxide. The
mixture was filtered and the catalyst washed twice with
dichloromethane, dried and reused in an additional reaction.
Over a period of five reaction cycles, as described above, no
significant loss of activity was observed and yields of
diphenyldiazene 1-oxide remained > 95%.
Notes and references
†
Octafluoroacetophenone (6 mmol, 1 g) was reacted with n-butylamine (6
mmol, 0.57 mL) in 50 mL dry toluene for 5 h at 60 °C in a 250 mL flask.
After this time 3-aminopropyl(trimethoxy)silane (6 mmol, 1.43 mL) was
added and the solution was heated and stirred under reflux while the reaction
was monitored by 19F NMR (CDCl
). The starting compound (Shiff base)
has peaks at 275.7 ppm (s, b position, 3F), 2161.6 ppm (d, ortho position,
F), 2138.2 ppm (m, meta position, 2F) and 2153.2 ppm (d, para position,
3
2
1F), whereas the product has peaks at 275.7 ppm (s, b position, 3F),
2160.3 ppm (d, ortho position, 2F) and 2141.5 ppm (m, meta position, 2F).
After one week the reaction was complete and tetraethoxysilane (9 mmol, 2
mL), water (3 mL), dibutyltin dilaurate (0.09 mL, 0.15 mmol) in 100 mL of
ethanol were added; the solution was then heated at 60 °C for 12 h. After this
time, the reaction mixture was transferred into a beaker and the solvent(s)
was allowed to evaporate at rt until a yellow–brown solid was obtained.
After grinding the silicate to a course powder, the solid was treated by
Soxlet extraction with diethyl ether followed by dichloromethane. The
product using 30% H
.8 mmol (Z)-hex-2-en-1-ol was reacted with 2 mmol 30%
in 1 mL EtOAc at 80 °C for 24 h to yield 94% hex-
2 2
O ; no epoxidation was observed. Thus,
0
2 2
silicate was then treated with 100 mL 30% H O under reflux for 16 h to
2 2
H O
remove the protecting group (verified by IR spectroscopy—carbonyl peak
21
2
-enal.
at 1640 cm ). The white powder that was obtained was filtered and washed
consecutively by water, acetonitrile and finally with dichloromethane. After
drying under vacuum at 60 °C overnight, the silicate was used in the
catalytic studies.
Finally, the stability and activity of the catalyst was tested in
a multi-recycle experiment. Thus, 4 mmol aniline, 100 mg
‡
The reaction ingredients as noted in the tables were placed in 5 mL vials;
Table 3 Oxidation of alkenes with 60% H
fluoroketone–silicate
2
O
2
catalysed by the per-
the vials were closed and placed in an oil bath at 80 ± 2 °C and the contents
stirred magnetically for the noted time period. After the reaction was
completed the organic phase was extracted with dichloromethane (5 mL)
and analyzed by GC and GC-MS using a 30 m 5% phenylmethylsilicone
capillary column (0.32 mm id, 0.25 mm coating).
Conversion
(mol%)
Substrate
Products (Yield (mol%))
Cyclooctene
Oct-2-ene
Oct-1-ene
Cyclooctene oxide (100)
Oct-2-ene oxide (100)
Oct-1-ene oxide (100)
Cyclohexane-1,2-diol (80)
Cyclohex-2-en-1-ol (15)
Other (5)
78
16
6
1 I. W. C. E. Arends, R. A. Sheldon, M. Wallau and U. Schuchardt, Angew.
Chem., Int. Ed. Engl., 1997, 36, 1143.
2 R. Hutter, T. Mallat, D. Dutoit and A. Baiker, Topics Catal., 1996, 3, 421;
R. Hutter, T. Mallat and A. Baiker, J. Catal., 1995, 153, 165; R. Hutter,
T. Mallat and A. Baiker, J. Catal., 1995, 153, 177; R. Neumann and M.
Levin-Elad, Appl. Catal. A, 1995, 122, 85; R. Neumann and M. Levin-
Elad, J. Catal., 1997, 166, 206.
3 R. A. Sheldon and I. W. C. E. Arends, Catal. Today, 1998, 41, 387.
4 R. D. Temple, J. Org. Chem., 1970, 35, 1275.
5 Y. V. Subba-Rao, D. E. de Vos and P. A. Jacobs, Angew. Chem., Int. Ed.
Engl., 1997, 36, 2661.
Cyclohexene
61
1
2
-Methylcyclohexene 1-Methylcyclohexane-1,2-diol (86) 100
-Methylcyclohexanone (10)
2
Other (4)
,3-Dimethylbut-2-ene 2,3-Dimethylbut-2-ene oxide (16) 100
2
2
,3-Dimethylbutane-2,3-diol (21)
,3-Dimethylbutan-2-one (63)
6 R. P. Heggs and B. Ganem, J. Am. Chem. Soc., 1979, 101, 2346; A. J.
Biloski, R. P. Heggs and B. Ganem, Synthesis, 1980, 810; P. A.
Ganespure and W. Adam, Synthesis, 1996, 179
Reaction conditions: 0.8 mmol substrate, 20 mg (0.05 mmmol ketone)
perfluoro–silicate, 1 mL acetonitrile, 3 mmol 60% H O , 24 h, 80 °C. Yields
2 2
were computed by GC analysis.
7
M. C. A. van Vliet, I. W. C. E. Arends and R. A. Sheldon, Chem.
Commun., 1999, 263.
488
Chem. Commun., 2001, 487–488