Aerobic Oxidation of Vicinal Diols Catalyzed by an Anderson-Type Polyoxometalate, [IMo6O24]5-

Article abstract of DOI:10.1002/1615-4169(200210)344:9<1017::AID-ADSC1017>3.0.CO;2-X

An Anderson-type polyoxometalate, [I^VIIMo₆O₂₄]^5-, has been used as a catalyst for the aerobic oxidation at 80°C of vicinal diols (glycols). This is the first report on the use of such a polyoxometalate as an oxi

Full text of DOI:10.1002/1615-4169(200210)344:9<1017::AID-ADSC1017>3.0.CO;2-X

FULL PAPERS
Aerobic Oxidation of Vicinal Diols Catalyzed by an
Anderson-Type Polyoxometalate, [IMo6O24]5
Alexander M. Khenkin, Ronny Neumann*
Department of Organic Chemistry, Weizmann Institute of Science, Rehovot, Israel, 76100
Fax: ( ‡ 972)-8-9344142, e-mail: Ronny.Neumann@weizmann.ac.il
Received: May 8, 2002; Accepted: July 12, 2002
Dedicated to Professor Roger A. Sheldon on the occasion of his 60th birthday.
Abstract: An Anderson-type polyoxometalate, the presence of additional acid. An abbreviated
[IVIIMo6O24]5 , has been used as a catalyst for the mechanistic study was carried out indicating that the
aerobic oxidation at 80 8C of vicinal diols (glycols). polyoxometalate oxidizes the diol to the various
This is the first report on the use of such a products even under anaerobic conditions. The
polyoxometalate as an oxidation catalyst. Reactivity reduced polyoxometalates (heteropoly blues and
and selectivity were dependent on the substrate. Thus, heteropoly browns) formed in the oxidation of the
aryl-substituted diols yielded mostly the carbon- diols are re-oxidized by the molecular oxygen.
carbon bond cleavage products, while 1,2-cyclohex-
anediol yielded cyclohexanone-2-ol and 1,2-cyclohex-
anedione. Aliphatic diols were less reactive but Keywords: carbon-carbon bond cleavage; diols; ho-
yielded carbon-carbon bond cleavage products in mogeneous catalysis; oxidation; polyoxometalates
Introduction
the use of highly acidic H3‡xPVxMo12 xO40 (x ˆ 2, 3)
Keggin-type polyoxometalate as catalyst for oxidation
Environmentally benign or ™green∫ synthetic proce- of cycloalkanones,[9] vicinal diols,[10] and a-hydroxyke-
dures are of key importance in the development of new tones.[11]
chemical processes. In the area of oxidation, there is a
In this manuscript, we report on the use of an
special and very strong incentive to replace stoichio- Anderson-type polyoxometalate, [IVIIMo6O24]5 , Fig-
metric procedures that often use toxic oxidizing agents ure 1, as catalyst for the oxidation of vicinal diols.
and result in the formation of copious amounts of
This is the first report on the use of a polyoxometalate
dangerous by-products. In the area of research consid- of such a structure as catalyst for an oxidation reaction.
ered in this paper, the oxidation of vicinal diols, the The rationale behind the use of this specific polyoxo-
classic stoichiometric oxidants are periodate[1] and lead metalate as catalyst was as follows. Since periodate is a
tetraacetate[1] for carbon-carbon bond cleavage, and
silver carbonate,[2] permanganate,[3] and also TEMPO-
sodium hypochlorite[4] for diketone formation. Some
research has been carried out using hydrogen peroxide
as oxidant for both types of transformation.[5] In general,
the activation of molecular oxygen and its use in a
selective manner remains a very challenging goal for
many catalytic transformations. Thus, an especially
attractive catalytic and alternative method would be to
oxidize vicinal diols with molecular oxygen. For vicinal
diol oxidation there have been only a few reports of
catalytic aerobic reactions using as catalysts
Ru(PPh3)3Cl2 on active carbon[6] or N-hydroxyphthali-
mide and Co(acac)3.[7] We are interested in the applica-
tion of polyoxometalates as catalysts for oxidation
reactions, especially using molecular oxygen as oxidant
or oxygen donor.[8] In the area of oxidative carbon-
carbon bond cleavage there have been a few reports on Figure 1. The Anderson-type [IMo6O24]5 polyoxometalate.
Adv. Synth. Catal. 2002, 344, No. 9
¹ 2002 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1615-4150/02/34409-1017 1021 $ 17.50+.50/0
1017

Alexander M. Khenkin, Ronny Neumann
FULL PAPERS
Table 1. Aerobic oxidation of vicinal diols catalyzed by Q3H2[IMo6O24].[a]
POMox + Substrate
POMred + Product
POMox
Substrate
OH
Time [h] Conversion [mol %]
Products [mol %]
POMred + O2
CHO
73
COOH
27
OH
Scheme 1.
15
>99
>99
HO
OH
O
CHO
88
3.5
known stoichiometric oxidant of vicinal diols, it was
hypothesized that periodate incorporated into a poly-
oxomolybdate matrix would lead to a catalytic reagent
with similar reactivity. In the course of the oxidation of
the diol, the polyoxometalate would be reduced.
However, the latter are often known to undergo re-
oxidation in the presence of molecular oxygen,
Scheme 1, enabling completion of a catalytic cycle.
12
O
O
HO
OH
2
>99
100
OH
OH
OH
OH
OH
O
OHC
OHC
5
5
>99
>99
CHO
CHO
O
O
5
3
64
41
31
56
OH
O
O
O
Results and Discussion
OH
O
O
OHC
5
>99
59
CHO
OH
OH
30
The [IMo6O24]5 polyoxometalate was prepared first as
Na5[IMo6O24] ¥ 3 H2O from sodium molybdate and
sodium periodate according to the published literature
70
17
O
O
15
OHC
CHO
O
O
OH
83
procedure.[12]
The
tetrabutylammonium
salt,
O
O
HO
100
100
15
4
27
‡
Q3H2[IMo6O24] where Q ˆ (n-Bu)4N , was prepared by
addition of a small excess of QBr to an aqueous solution
of Na5[IMo6O24] ¥ 3 H2O brought to pH ˆ 1.5 by addition
of 1 N H2SO4. Cyclic voltammetry of Q3H2[IMo6O24] in
0.1 N QBF4 in acetonitrile yielded an irreversible
oxidation potential of Ea ˆ‡ 0.93 V versus SCE. Vici-
nal diols were oxidized under 2 atm oxygen using
pressure tubes in acetonitrile as summarized in Table 1.
Depending on the substrate type various reaction
products were observed. For vicinal diols substituted
with a phenyl ring(s), products associated with carbon-
carbon bond cleavage are dominant. Multiple substitu-
OH
O
O
HO
[b]
OH
OH
>99
C5
C5
OH
15
15
0
C6
O
OH
CHO
C6
OH
[b]
>99
O
83
17
[a] Reaction conditions: 250 mM substrate, 10 mM Q3H2[IMo6O24], 1 mL CH3CN,
T = 80 °C, PO2 = 2 atm.
[b] In the presence of 250 mM CH3SO3H.
tion increases the reaction rate. For both cis- and trans- medium, respectively, yielding results similar to those
cyclohexanediol, only a small amount of carbon-carbon reported in Table 1. 10% Na5[IMo6O24] on silica was
bond cleavage is observed and the dominant products significantly less effective. The stability of the
are the cyclohexanone-2-ol and 1,2-cyclohexanedione. Q3H2[IMo6O24] polyoxometalate catalyst was surveyed
In the case of 1,2-cyclooctanediol, there is both carbon- by IR and 127I NMR spectroscopy. Thus, the IR spectrum
carbon bond cleavage and formation of the a,b- of Q3H2[IMo6O24] with peaks at 941, 913, 806, and
diketone. Furthermore, for aliphatic diols and cyclo- 682 cmÀ1 attributable to the polyanion was compared to
pentanediol, only carbon-carbon bond cleavage is the IR spectrum of the polyoxometalate recovered from
observed, coupled with significant acetal formation by the reaction solution with peaks at 940, 912, 802, and
reaction of the aldehyde product with the diol substrate. 682 cmÀ1. There was essentially no change in the IR
For improved reactivity, acidification, for example with spectrum for samples before and after the reaction.
methanesulfonic acid, of the reaction medium is to be Furthermore, the 127I NMR of the polyoxometalate
preferred.
prior to a reaction carried out in CD3CN showed a
Among the solvents tested (acetonitrile, 1,2-dichoro- singlet at 4084 ppm (Dv1/2 ˆ 250 Hz, 5 M KI in D2O as
ethane, nitroethane, acetic acid and ethanol), acetoni- external standard) versus a broader peak at 3950 ppm
trile was the preferred solvent in terms of both activity (Dv1/2 ˆ 1000 Hz) over the course of the reaction. Our
and selectivity using the oxidation of phenyl-1,2- conclusion from these IR and 127I NMR experiments is
ethanediol as model substrate. For example, under the that the structure of the Q3H2[IMo6O24] polyoxometa-
conditions given in Table 1, and acetic acid as solvent late remains unchanged during the oxidation reaction,
there was only 14% conversion and formation of mostly however, the Q3H2[IMo6O24] polyoxometalate is slightly
PhC(O)CH2OH and PhCH(OH)CHO. However, 10% reduced, probably at a molybdenum atom, leading to a
Na5[IMo6O24] supported on active carbon with toluene broadened and slightly shifted peak in the 127I NMR due
as solvent were also an effective catalyst and reaction to formation of paramagnetic molybdenum(V).
1018
Adv. Synth. Catal. 2002, 344, 1017 1021

Aerobic Oxidation of Vicinal Diols Catalyzed by an Anderson-Type Polyoxometalate
FULL PAPERS
We were also interested in the mechanism of the
reaction, a subject or question that is often overlooked
in recent catalytic examples and investigations of vicinal
diol oxidation. First, stoichiometric reactions were
carried out under anaerobic conditions (10 mM sub-
strate, 10 mM Q3H2[IMo6O24], 1 mL CH3CN, Tˆ 80 8C,
4 h), Eqs. 1 3. In all cases there was quantitative
conversion of the organic substrate. For cis-cyclohex-
anediol as substrate there was formation only of the 2
electron oxidized product cyclohexanone-2-ol. The
solution turned green indicating formation of a heter-
opoly blue polyoxometalate attributable to formation of
Figure 2. Kinetic profile of the oxidation of stereoisomers of
1,2-cyclohexanediol and 1,2-diphenyl-1,2-ethanediol. Reac-
tion conditions: 250 mM diol, 10 mM Q3H2[IMo6O24] in
acetonitrile at 80 8C under 2 atm oxygen.
molybdenum(V) centers.[13] For both 1-phenyl-1,2-
ethanediol and the meso-1,2-diphenyl-1,2-ethanediol,
there was multi-electron oxidation with formation of
both the cleavage product, benzaldehyde (also form-
aldehyde) and the dicarbonyl product, b-ketophenyl-
acetaldehyde and benzil, respectively, in similar ratios. periodic acid-catalyzed reactions.[15] The products of 1,2-
Important also was the observation that the solution cyclohexanediol are initially only cyclohexanone-2-ol,
turned brown rather than green, indicating formation of with increasing quantities of 1,2-cylohexanedione being
heteropoly browns attributable to formation of highly formed as a function of time, that is the ratio 1,2-
reduced polyoxometalate species with multiple molyb- cylohexanedione/cyclohexanone-2-ol increases as a
denum(V) and probably also molybdenum(IV) function of time. On the other hand in the hydro-
atoms.[14] A second important observation was that benzoin oxidation, the benzaldehyde/benzil ratio is ~9
a,b-diketones such as benzil and 1,2-cyclohexanedione, and more or less constant as a function of time. Since
did not react in the presence of Q3H2[IMo6O24] nor did there was effective oxidation of diols under anaerobic
simple alcohols such as cyclohexanol and benzyl conditions, radical-type mechanisms such as autooxida-
alcohol. Benzoin, PhCH(OH)C(O)Ph, was more reac- tion or other direct involvement of oxygen in the
tive than hydrobenzoin.
oxidation of the diols may be discounted. Therefore, a
preliminary but feasible mechanistic scheme to explain
the results can be proposed as follows, Scheme 2.
First, there is complexation of the polyoxometalate to
the glycol substrate (step a), which is followed by a two-
electron oxidation of the vicinal diol to a hydroxy-
ketone or hydroxy-aldehyde (step b). The hydroxy-
ketone (aldehyde) is always observed either under
anaerobic conditions or at low conversions. The associ-
ation or complexation (step a) of the diol to the
polyoxometalate seems to be critical since a simple
alcohol such as benzyl alcohol with a lower (compared
to cyclohexanediol) oxidation potential and C-H bond
energy is not oxidized by the Q3H2[IMo6O24] polyox-
ometalate. After its formation, the hydroxy-ketone
(aldehyde) may disassociate from the polyoxometalate
and in the presence of oxygen the catalyst is re-oxidized
with formation of water (step c). Alternatively, the
hydroxy-ketone (aldehyde) will be further oxidized to
yield the dicarbonyl product (steps d and e) or there will
be oxidative cleavage of the carbon-carbon bond
(step f). Dicarbonyl products, once formed, do not
lead to carbon-carbon bond cleavage. In general and
OH
O
OH
Ar
100%
5−
+ [IMo6O24
]
ꢀ1†
OH
OH
100%
O
OH
CHO
Ar
100%
CHO
5−
+ [IMo6O24
]
+
41%
59%
ꢀ2†
ꢀ3†
HO
OH
O
O
CHO
Ar
5−
+ [IMo6O24
]
100%
44%
56%
The kinetic profile of the vicinal diol oxidation reaction according to the results presented in Table 1, the specific
was also measured for cis- and trans-cyclohexanediol pathway observed, that is the relative rate of each step is
and meso- and R,S-hydrobenzoin, Figure 2.
strongly substrate dependent. Another factor to con-
Clearly cis- and trans-glycols react at similar rates. sider concerns the aerobic re-oxidation of the polyoxo-
This finding would tend to discount the possibility of a metalate. The observation that under anaerobic con-
cyclic-intermediate mechanism as was suggested for ditions (argon) heteropoly blue and heteropoly brown
Adv. Synth. Catal. 2002, 344, 1017 1021
1019

Alexander M. Khenkin, Ronny Neumann
FULL PAPERS
Formation of hydroxyketone
H
H
C
C
H
OH
OH
OH
O
O
C
5−
+
c
C
C
H
a
b
C
7−
[IMoVI4MoV2O24
]
+ 2 H+
[IMoVI6O24
]
5−
[IMoVI6O24
]
5−
[IMoVI6O24
]
+
+ H2O
C
H
1/2 O2
C
H
OH
OH
OH
Formation of diketone
via hydroxyketone
H
H
C
C
H
OH
C
O
O
OH
OH
O
e
C
C
d
a
C
5−
5−
9−
[IMoVI6O24
]
[IMoVI2MoV4O24
]
+ 4 H+
5−
[IMoVI6O24
]
+ 2 H2O
+
[IMoVI6O24
]
+
C
H
C
O2
OH
O
Carbon-carbon bond cleavage
via hydroxyketone
O
O
O
H
H
C
C
H
H
C
H
H
C
OH
OH
OH
C
5−
+
g
a
f
9−
5−
[IMoVI2MoV4O24
]
+ 2 H+
+ ]
[IMoVI6O24
[IMoVI6O24
]
5−
[IMoVI6O24
]
+ 2 H2O
2 H+, O2
C
H
OH
H
C
C
O
Scheme 2.
species are obtained, while underaerobic conditions, the Experimental Section
polyoxometalate is observed to be largely in its oxidized
form would indicate that the aerobic re-oxidation of the
polyoxometalate is relatively fast compared to the diol
Materials
Reaction substrates and solvents were obtained from com-
mercial sources and were used without further purification.
Na5[IMo6O24] ¥ 3 H2O was prepared according to the literature
procedure[12] by dissolving 13.5 g Na2MoO4 ¥ 2 H2O in 50 mL
water and heating the solution to ~95 8C. 6.2 mL of 12 N HCl
were added slowly, followed by dropwise addition of a hot
solution of sodium periodate (2 g in 10 mL water). The clear
solution was reduced to 25 mL and upon cooling, white platelet
type crystals precipitated from the solution. Q3H2[IMo6O24]
was prepared by mixing 0.75 g (0.6 mmol) Na5[IMo6O24] ¥
3 H2O in 5 mL water with 1.28 g (4 mmol) QBr dissolved in
5 mL water. 1 N H2SO4 was added until a pH ˆ 1.5 was
obtained. The resulting precipitate was filtered and washed
successively with water, absolute ethanol and ether and then
recrystallized from acetonitrile to yield very light yellow
crystals; yield: 0.75 g. Elemental analysis: found (calculated): C
31.53 (31.7), H 6.30 (6.06), N 2.73 (2.31); IR: n ˆ 941, 913, 806,
oxidation steps. In this context, it is important tonote the
following: (a) the oxidation of the diol to the hydroxy-
ketone (aldehyde) is a two-electron oxidation with
formation of two protons (step b), (b) the oxidation to
the dicarbonyl is a four-electron oxidation with forma-
tion of four protons (step ), and (c) the carbon-carbon
bond cleavage reaction is a four-electron reaction with
however formation of two protons, (step f). Since the re-
oxidation of the polyoxometalate with molecular oxy-
gen is a four-electron oxidation requiring four protons,
the absence of sufficient acid may inhibit the re-
oxidation reaction, (step g). This may explain the
positive effect of the addition of an acid on the diol
oxidation reaction.
À
1
721 and 681 cm ; UV-vis: lmax ˆ 250 nm (log e ˆ 4.18);
127I NMR (CD3CN): d ˆ 4084 ppm (KI in D2O external stand-
ard).
Conclusions
Catalytic aerobic oxidation of vicinal alcohols is possible
using an Anderson-type polyoxometalate, [IMo6O24]5
as catalyst. Reactivity and selectivity varied depending
on the substrate. Thus, aryl-substituted diols yielded
mostly the carbon-bond bond cleavage products, while
1,2-cyclohexanediol yielded cyclohexanon-2-ol and 1,2-
cyclohexanedione. Aliphatic diols were less reactive but
yielded carbon-carbon bond cleavage products in the
presence of additional acid.
General Oxidation Procedures
Reactions were carried out in 20-mL glass pressure tubes.
Typically, the tubes were loaded with the Q3H2[IMo6O24]
polyoxometalate (20 mmol), diol (500 mmol) and acetonitrile
(2 mL) and degassed by three successive ™freeze-pump-thaw∫
cycles and pressurized with O2 to 2 atm. The solution was
brought to 80 8C in a thermostatted oil bath. After the reaction
was completed the mixture was cooled. Reactions were
quantified by GLC (HP 6890) using a 30 m 5% phenylmethyl
silicone capillary column with an ID 0.32 mm and 0.25 mm
coating (Restek 5MS) and products were identified by GC-MS
(HP 5973) with the same column and conditions. Analysis was
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Adv. Synth. Catal. 2002, 344, 1017 1021

Aerobic Oxidation of Vicinal Diols Catalyzed by an Anderson-Type Polyoxometalate
FULL PAPERS
preformed on aliquots directly withdrawn from the reaction
mixture. Mass balances were verified by addition of an external
standard (n-decane).
Reactions under anaerobic conditions were carried out in
20-mL Schlenk flasks. The flasks were loaded with the
Q3H2[IMo6O24] polyoxometalate (100 mmol), diol (100 mmol)
and acetonitrile (2 mL) and degassed by three successive
™freeze-pump-thaw∫ cycles and loaded with Ar. Further
reaction and analysis was as for described for aerobic reactions.
Yamawaki, T. Ura, H. Yamada, T. Yoshida, M. Ogawa, J.
Org.Chem. 1988, 53, 3587; d) T. Oguchi, T. Ura, Y. Ishii,
M. Ogawa, Chem. Lett. 1989, 857; e) R. Neumann, A. M.
Khenkin, D. Juwiler, H. Miller, M. Gara, J. Mol. Catal.
1997, 117, 169.
[6] E. Takezawa, S. Sakaguchi, Y. Ishii, Org. Lett. 1999, 1,
715.
[7] T. Iwahama, Y. Yoshino, T. Keitoku, S. Sakguchi, Y. Ishii,
J. Org. Chem. 2000, 65, 6923.
[8] a) N. Mizuno, M. Misono, Chem. Rev. 1998, 98, 199;
b) C. L. Hill, C. M. Prosser-McCartha, Coord. Chem.
Rev. 1995, 143, 407; c) I. V. Kozhevnikov, Chem. Rev.
1998, 98, 171; d) R. Neumann, Prog. Inorg. Chem. 1998,
37, 317.
Acknowledgements
This research was supported by the Basic Research Foundation
administered by the Israeli Academy of Science and Humanities
and the Israel Ministry of Science. RN is the Rebecca and Israel
Sieff Professor of Organic Chemistry.
¬
[9] a) J.-M. Bregeault, B. El Ali, J. Mercier. J. Martin, C.
Martin, C. R. Acad. Sci., Ser II 1988, 307, 2011; b) J. M.
¬
Bregeault, F. Launay, A. Atlamsani, C. R. Acad. Sci., Ser
¬
IIc 2001, 4, 11; c) B. El Ali, J. M. Bregeault, J. Mercier. J.
Martin, C. Martin, O. Convert, Chem. Commun. 1989,
¬
825; d) A. Atlamsani, J. M. Bregeault, M. Ziyad, J. Org.
Chem. 1993, 58, 5663.
References
¬
[1] a) T. V. Lee, in Comprehensive Organic Synthesis, Vol. 7
(Eds.: B. M. Trost, I. Flemings, S. V. Ley), Pergamon,
Oxford, 1991, p. 291; b) A. H. Haines, Methods for the
Oxidation of Organic Compounds, Academic Press,
London, 1988, p. 277.
[2] M. Fetzion, M. Golfier, J. M. Louis, Chem. Commun.
1969, 1102.
[3] S. Baskaran, J. Das, S. Chandrasekaran, J. Org. Chem.
1989, 54, 5182.
[10] J. M. Bregeault, B. El Ali, J. Mercier. J. Martin, C.
Martin, C. R. Acad. Sci., Ser II 1989, 309, 459.
[11] L. El Aakel, F. Launay, A. Altamsani, J.-M. Bregeault,
Chem. Commun. 2001, 2218.
[12] A. Filowitz, R. K. C. Ho, W. G. Klemperer, W. Shum,
Inorg. Chem. 1979, 18, 93.
[13] N. Casan-Pastor, P. Gomez-Romero, G. B. Jameson,
L. C. W. Baker, J. Am. Chem. Soc. 1991, 113, 5658 and
references cited therein.
¬
[4] a) T. Inokuchi, S. Matsumoto, T. Nishiyama, S. Torii,
Synlett 1990, 57; b) U. G. Bauwell, V. S. Bridges, J. R.
Dupiche, S. L. Richards. J. M. Walter, J. Org. Chem.
1994, 59, 6338.
[14] a) K. Piepgrass, M. T. Pope, J. Am. Chem. Soc. 1989, 111.
753; b) M. H. Dickman, T. Ozeki, H. T. Evans, C. Y.
Rong, G. B. Jameson, M. T. Pope, J. Chem. Soc. Dalton
Trans. 2000, 149.
[5] a) T. Iwahama, S. Sakaguchi, Y. Nishiyama, Y. Ishii,
Tetrahedron Lett. 1995, 36, 1523; b) C. Venturello, M.
Ricci, J. Org. Chem. 1986, 51, 1599; c) Y. Ishii, K.
[15] a) G. J. Buist, C. A. Bunton, W. C. Hipperson, J. Chem.
Soc. B 1971, 2128; b) G. J. Buist, C. A. Bunton, J. H.
Miles, J. Chem. Soc. 1959, 743.
Adv. Synth. Catal. 2002, 344, 1017 1021
1021