Tungstoborates as highly active catalysts for cycloalkane oxygenation using hydrogen peroxide

Article abstract of DOI:10.1055/s-2003-41421

Keggin-type tungstoborates [BW₁₂O₄₀]^5-, H_x[BW₁₁O₃₉]^9x)- and [BM(H ₂O)W₁₁O₃₉]^6-, M = Fe^III, Mn^III or Ru^III proved to be highly active catalysts for the H₂O₂ oxidation of cyclooctane and cyclohexane to the corresponding ketone, alcohol and alkyl hydroperoxide derivatives. High turnover numbers and high selectivity for the hydroperoxide were observed.

Full text of DOI:10.1055/s-2003-41421

LETTER
1643
Tungstoborates as Highly Active Catalysts for Cycloalkane Oxygenation Using
Hydrogen Peroxide
a,b
a,b
a
a
T
I
ungstob
s
orates
a
s
a
H
ighly Act
b
ive Catalyst se l C. M. S. Santos, M. Salete S. Balula, Mário M. Q. Simões, M. Graça P. M. S. Neves,
a
a,b
José A. S. Cavaleiro, Ana M. V. Cavaleiro*
a
Departamento de Química, Universidade de Aveiro, 3810-193 Aveiro, Portugal
b
CICECO, Universidade de Aveiro, 3810-193 Aveiro, Portugal
Fax +351(234)370084; E-mail: ana@dq.ua.pt
Received 15 July 2003
O
OH
OOH
5
–
9–x)–
Abstract: Keggin-type tungstoborates [BW O ] , H [BW O ]
1
2
40
x
III
11 39
6
–
III
III
and [BM(H O)W O ] , M = Fe , Mn or Ru proved to be
highly active catalysts for the H O oxidation of cyclooctane and
2
11 39
+
+
2
2
cyclohexane to the corresponding ketone, alcohol and alkyl hydro-
peroxide derivatives. High turnover numbers and high selectivity
for the hydroperoxide were observed.
O
OH
OOH
Keywords: polyoxometalates, cycloalkanes, oxidation, hydrogen
peroxide, tungstoborates
+
+
Scheme 1
The catalytic transformation of saturated hydrocarbons
into oxygenated derivatives is a very important target both
in industry and in academic research. The obtained prod-
ucts are currently valuable intermediates for organic syn-
only been reported by us in a recent publication dealing
with the oxidation of some monoterpenes.22
thesis and many systems have been investigated over the In the present study we present the results obtained for the
1
–3
last few decades. In particular, the selective oxidation cyclooctane and cyclohexane oxidation with hydrogen
of cyclohexane under mild conditions to ketone/alcohol peroxide in the presence of tetrabutylammonium salts of
4
mixtures is a topic of great interest. In industrial condi- Keggin-type tungstoborates. In the reaction conditions
23
tions, the total conversion of cyclohexane has normally here reported, the ketone, the alcohol and the hydroper-
been limited to 3–4% in order to achieve the required se- oxide were obtained as the only products for each sub-
4
,5
lectivity.
strate (Scheme 1). The ketone and hydroperoxide
derivatives together were always more than 80% of the
obtained products. Conditions were found for which high
selectivity for the hydroperoxide was obtained.
Hydrogen peroxide is readily available and is considered
to be environmentally innocuous. It has been shown that
metal-catalyzed oxidation of alkanes with H O may af-
6
,7
2
2
ford considerable amounts of the corresponding alkyl hy- The following salts were used as catalysts:
3
,8–10
droperoxides.
catalytic studies of alkane oxidations in the presence of (BW ) and TBA H BM(H O)W O ·nH O (BM), M =
Similar result was observed in our TBA H[BW O ]·nH O (BW ), TBA H BW O ·nH O
4 12 40 2 12 5 4 11 39 2
1
1
4
2
2
11 39
2
4
– 11,12
the Keggin-type anion PFe(H O)W O
.
Formation Fe, Mn, Ru, n = 1–3. The synthesis of BW , BW and
2
11 39
12 11
The synthetic
of intermediate alkyl hydroperoxides may be explored in BMn have been reported previously.2
2,24
several ways, as they may be selectively decomposed to procedure described for BMn was adapted for the prepa-
5
,13
25
ketone(aldehyde)/alcohol mixtures
corresponding alcohols.
or reduced to the ration of BFe and BRu.
Reports on the use of Keggin-type polyoxotungstates as
1
4,15
catalysts in alkane oxidation have been published,
in-
1
1,16–18
cluding studies with H O as oxidant.
To our knowl-
2
2
edge, there are no previous reports on the oxidative
catalysis of alkanes in the presence of Keggin-type tung-
stoborates. So far, catalytic studies using tungstoborates
1
9–22
deal mainly with epoxidation reactions.
The com-
bined use of tungstoborates and hydrogen peroxide has
Figure 1 Selectivity for cyclooctyl hydroperoxide formation with 2
Synlett 2003, No. 11, Print: 02 09 2003. Web: 25 08 2003.
Art Id.1437-2096,E;2003,0,11,1643,1646,ftx,en;D15703ST.pdf.
DOI: 10.1055/s-2003-41421
mmol of H O after 12 hours (9 hours for BFe) or 9.8 mmol of H O
2
2
2
2
after 9 hours (6 hours for BFe) in the cyclooctane oxidation reactions.
©
Georg Thieme Verlag Stuttgart · New York

1
644
I. C. M. S. Santos et al.
LETTER
Table 1 Oxidation of Cyclooctane with Hydrogen Peroxide Catalyzed by Tungstoborates at 80 ºCa
Selectivity (%)b
C H OH
Catalyst
Time (h)
H O (mmol) Conversion
Turnover
numberc
Used H O
C H O
C H OOH
2
2
2
2
8
14
8
15
8
15
b
(
%)
(mmol)
1.78
5.4
BW12
BW12
BW11
BW11
BFe
12
9
2
78
59
95
63
99
95
88
92
63
38
0
520
393
633
420
660
633
587
613
420
253
–
58
10
44
28
28
21
71
32
77
64
–
10
0
32
90
50
72
65
77
19
68
9
9.8
2
12
9
1.98
7.9
6
9.8
2
0
9
1.95
8.6
7
BFe
6
9.8
2
2
BMn
BMn
BRu
12
9
1.92
5.8
10
0
9.8
2
12
9
1.99
7.9
14
0
BRu
9.8
2
36
–
No catalyst
12
9
–
–
No catalyst
9.8
0
–
–
–
–
–
a
Reaction conditions: catalyst, 1.5 mmol; solvent, CH CN (1.5 mL); cyclooctane, 1 mmol.
Based on the gas chromatographic peak areas.
Mol of total products/mol of catalyst used.
3
b
c
Table 2 Oxidation of Cyclohexane with Hydrogen Peroxide Catalyzed by TBA H BFe(H O)W O ·H O
4
2
2
11 39
2
Selectivity (%)b
C H OH
Conditionsa
Conversion
Turnover
Numberc
Used H O
(mmol)
C H O
C H OOH
2
2
6
10
6
11
6
11
b
(
%)
BFe1
BFe2
87
99
98
580
1.97
49
32
26
19
15
17
32
53
57
660
3.59
BFe3
1307
3.86
a
BFe1 reaction conditions: 1 mmol of cyclohexane, 1.5 mmol of catalyst, 2 mmol of H O , 1.5 mL of CH CN, after 12 h at reflux; BFe2 reaction
2
2
3
conditions: 1 mmol of cyclohexane, 1.5 mmol of catalyst, 4 mmol of H O , 1.5 mL of CH CN, after 6 h at reflux; BFe3 reaction conditions:
2
2
3
2
mmol of cyclohexane, 1.5 mmol of catalyst, 4 mmol of H O , 1.5 mL of CH CN, after 12 h at reflux.
2 2 3
b
Based on gas chromatographic peak areas.
Mol of total products/mol of catalyst used.
c
The results obtained for the cyclooctane catalytic oxida- with 2 mmol and 9.8 mmol of H O indicated that in most
2
2
tion, including selectivity and turnover numbers, are sum- cases an improvement in the selectivity for cyclooctyl hy-
marized in Table 1. BFe gave rise to very high droperoxide occurred when 9.8 mmol of oxidant were
percentages of conversion. In fact, after 9 hours of reac- used (Figure 1). It was also observed that in the presence
2
6
tion, 99% of cyclooctane conversion was obtained with a of I , a well known radical scavenger, no oxidation of
2
H O /substrate ratio of 2, reaching 660 turnovers and 65% cyclooctane took place, which was an indication that the
2
2
of selectivity for cyclooctyl hydroperoxide. In the same oxidation of cyclooctane is probably a radical process.
reaction conditions BW originated 95% of cyclooctane
27
1
1
The efficiency of the usage of H O did depend on the
2
2
conversion, reaching 633 turnovers, with 50% and 44%
selectivity for the hydroperoxide and the ketone, respec-
tively. After 9 hours of reaction and in large excess of
H O , BW originated only cyclooctanone and cyclooctyl
hydroperoxide, with an excellent selectivity of 90% for
the hydroperoxide. The comparison of the results obtained
concentration of H O . With BFe, at low H O concentra-
2
2
2
2
tions (2 mmol), only 2.7% of the initial hydrogen peroxide
was present after 9 h of reaction (Figure 2) and the effi-
ciency was approximately 65%. At higher H O con-
2
2
12
2
2
centrations (9.8 mmol) the efficiency was lower, since
Synlett 2003, No. 11, 1643–1646 ISSN 1234-567-89 © Thieme Stuttgart · New York

LETTER
Tungstoborates as Highly Active Catalysts
1645
after 6 h only 12% of the initial H O was present (14% Acknowledgment
2
2
efficiency). Therefore, the only advantage of using a
higher concentration of H O was the higher selectivity
observed for the formation of cyclooctyl hydroperoxide.
Thanks are due to the University of Aveiro, Fundação para a Ciên-
cia e a Tecnologia (FCT) and FEDER (POCTI/QUI/38377/2001)
for funding. I.C.M.S. Santos and M.S.S. Balula are also grateful to
FCT/FEDER for their PhD grants.
2
2
It is worth mentioning that these results for the oxidation
of cyclooctane seem very promising. Recent reports, in
which the same products were referred, but using other
kind of catalysts, presented much lower turnover num-
References
8
,9
(1) Sheldon, R. A.; Kochi, J. K. Metal-Catalyzed Oxidations of
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2) Activation and Functionalization of Alkanes; Hill, C. L., Ed.;
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3) Shilov, A. E.; Shul’pin, G. B. Activation and Catalytic
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(
4) Schuchart, U.; Cardoso, D.; Sercheli, R.; Pereira, R.; Cruz,
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6) Catalytic Oxidations with Hydrogen Peroxide as Oxidant;
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(
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(
9) Shul’pin, G. B. J. Chem. Research, Synop. 2002, 351.
Figure 2 Time course of cyclooctane oxidation in the presence of
BFe with 2 mmol of H O [(!) conversion (") H O decomposition]
(
(
10) Shul’pin, G. B.; Süss-Fink, G.; Smith, J. R. L. Tetrahedron
999, 55, 5345.
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2
2
2
2
1
and 9.8 mmol of H O [(!) conversion (#) H O decomposition].
2
2
2
2
Only the results obtained with BFe catalyst, which gave
the best conversions for cyclooctane oxidation (95% and
(
9
9%), are reported for the oxidation of cyclohexane in
three different reaction conditions (BFe1, BFe2, BFe3,
see Table 2). The total conversion and the selectivity for
the cyclohexyl hydroperoxide were higher in the case of
BFe3, giving rise to high turnover numbers.
1999, 13, 93.
(
13) Hansen, C. B.; Agterberg, F. P. W.; van Eijndhoven, A. M.
C.; Drenth, W. J. Mol. Catal. A: Chem. 1995, 102, 117.
14) Neumann, R. Prog. Inorg. Chem. 1998, 47, 317.
15) Hill, C. L.; Prosser-McCartha, C. M. Coord. Chem. Rev.
(
(
These reactions were accompanied by good (higher than
4
6
0% ) H O oxidation efficiency under BFe1 (61%) and
2
2
1995, 143, 407.
BFe3 (65%) conditions. Although a recent study on the
oxidation of cyclohexane reported higher turnover num-
(
16) Hill, C. L. In Activation and Functionalization of Alkanes;
Hill, C. L., Ed.; Wiley: New York, 1989, Chap. 8.
1
0
bers (3300), the H O efficiency was much lower (30%).
(17) Mizuno, N.; Kiyoto, I.; Nozaki, C.; Misono, M. J. Catal.
999, 181, 171.
18) Süss-Fink, G.; Gonzalez, L.; Shul’pin, G. B. Appl. Catal., A
001, 217, 111.
19) Zhang, X.; Pope, M. T. J. Mol. Catal. A: Chem. 1996, 114,
01.
(20) Duncan, D. C.; Chambers, R. C.; Hecht, E.; Hill, C. L. J. Am.
Chem. Soc. 1995, 117, 681.
(21) Aubry, C.; Chottard, G.; Platzer, N.; Brégeault, J. M.;
Thouvenot, R.; Chauveau, F.; Huet, C.; Ledon, H. Inorg.
Chem. 1991, 30, 4409.
22) Santos, I. C. M. S.; Simões, M. M. Q.; Pereira, M. M. M. S.;
Martins, R. R. L.; Neves, M. G. P. M. S.; Cavaleiro, J. A. S.;
Cavaleiro, A. M. V. J. Mol. Catal. A: Chem. 2003, 195, 253.
23) The reactions were typically carried out by heating a solution
of 1 mmol of the cycloalkane and 1.5 mmol of the catalyst in
2
2
1
With polyoxometalates, the best result reported was 741
turnovers for the oxidation of cyclohexane with tert-butyl
hydroperoxide when a sandwich type polyoxometalate
(
(
2
2
8
was used as catalyst. With an almost total conversion of
the substrate, a turnover number of 1307 and a H O effi-
2
2
2
ciency of 65%, the results shown here are one of the best
for the oxidation of cyclohexane.
In conclusion, the Keggin-type tungstoborates were used
for the first time as efficient catalysts for the oxidation of
cycloalkanes under environmentally clean conditions. Us-
ing TBA H BFe(H O)W O ·H O, high conversions
(
4
2
2
11 39
2
(
were obtained accompanied by high turnover numbers
and high H O efficiency. As far as we know, these results
2
2
1.5 mL of acetonitrile at 80 ºC. The oxidant used was 30%
seem to indicate that we are in the presence of one of the
most promising systems known for the oxidation of al-
kanes. Further studies on other alkanes and other polyox-
ometalates are currently in progress in our laboratory.
aqueous H O . Aliquots were withdrawn from the reaction
2
2
mixture and injected directly into a GC-MS (fused silica
Supelco capillary column, SPB-5, with 30 m × 0.25 mm i.d.;
0.25 mm film thickness). The percentages of each compound
in the reaction mixture were estimated directly from the
corresponding chromatographic peak areas.
Synlett 2003, No. 11, 1643–1646 ISSN 1234-567-89 © Thieme Stuttgart · New York

1
646
I. C. M. S. Santos et al.
LETTER
–
1
(
24) Tézé, A.; Michelon, M.; Hervé, G. Inorg. Chem. 1997, 36,
05.
25) For BFe an aqueous solution of BW11 was added to a
1.48; hydration H O, 0.95; IR (cm ): 998 (m), 950 (vs), 902
2
5
(vs), 825 (vs), 743 (s, sh), 530 (m). m = 1.99 mB.
(26) Barton, D. H. R.; Launay, F.; Le Gloahec, V. N.; Li, T.;
Smith, F. Tetrahedron Lett. 1997, 38, 8491.
(
solution of Fe(NO ) ·9H O (order of addition reversed in
3
3
2
comparison with BMn). TBA H BFe(H O)W O ·H O
(27) The decomposition of H O during the reaction was
4
2
2
11 39
2
2
2
Yield, 91%. Anal. Found: W, 55.0; Fe, 1.48; C, 20.53; H,
.09, N, 1.56; hydration H O, 0.51. Calcd: W, 54.4; Fe, 1.50;
monitored by titration of aliquots with ceric sulphate. At the
end of the reactions, the total of unused H O and
4
2
2
2
–
1
C, 20.60; H, 4.03, N, 1.50; hydration H O, 0.48; IR (cm ):
hydroperoxide produced was estimated by titration with
ceric sulphate. From the yields of hydroperoxide (and
cyclohexanol and cyclohexanone) determined by gas
chromatography, the amount of H O used in the reaction
2
997 (m), 956(vs), 900(vs), 825(vs), 757 (s, sh), 528 (m). For
preparation of BRu RuCl ·H O was used.
3
2
TBA H BRu(H O)W O ·2H O Yield, 89%. Anal. Found:
4
2
2
11 39
2
2
2
W, 54.2; Ru, 2.56; C, 20.58; H, 4.11, N, 1.72; hydration
H O, 1.02. Calcd: W, 53.5; Ru, 2.69; C, 20.30; H, 4.02, N,
2
could be determined.
(28) Neumann, R.; Khenkin, A. M. Inorg. Chem. 1995, 34, 5753.
Synlett 2003, No. 11, 1643–1646 ISSN 1234-567-89 © Thieme Stuttgart · New York