The selective catalytic oxidation of terminal alcohols: A novel four-component system with MTO as catalyst

Article abstract of DOI:10.1016/S0022-328X(99)00013-3

A four-component system (H₂O₂, MTO, HBr, TEMPO) in acetic acid catalyzes the selective oxidation of terminal alcohols to the corresponding aldehydes with excellent selectivity and yield. The system allows the oxidation of alcohols with hydrogen peroxide as oxidants either selectively to aldehydes or to the corresponding acids, depending on the reaction parameters. The new technique is especially applicable to the oxidation of carbohydrates.

Full text of DOI:10.1016/S0022-328X(99)00013-3

Journal of Organometallic Chemistry 579 (1999) 404–407
Priority communication
The selective catalytic oxidation of terminal alcohols: a novel
four-component system with MTO as catalystꢀ
Wolfgang A. Herrmann a,*, Jochen P. Zoller a, Richard W. Fischer b
a Anorganisch-chemisches Institut der Technischen Uni6ersita¨t Mu¨nchen, Lichtenbergstraße 4, D-85747 Garching, Germany
b Celanese GmbH, Ruhrchemie Plant, Research and De6elopment, D-46128 Oberhausen, Germany
Received 24 December 1998
Dedicated to Professor Dietmar Seyferth on the occasion of his 70th birthday
Abstract
A four-component system (H2O2, MTO, HBr, TEMPO) in acetic acid catalyzes the selective oxidation of terminal alcohols to
the corresponding aldehydes with excellent selectivity and yield. The system allows the oxidation of alcohols with hydrogen
peroxide as oxidants either selectively to aldehydes or to the corresponding acids, depending on the reaction parameters. The new
technique is especially applicable to the oxidation of carbohydrates. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Methyltrioxorhenium; TEMPO; HBr; Oxidation; Carbohydrates; Starch; Catalysis; Renewable resources
Recently it has been reported that MTO also catalyzes
the conversion of alcohols to aldehydes in the presence
1. Introduction
of hydrogen peroxide [12,13]. It is known that the
The development of new catalytic methods for the
oxidation of primary and secondary alcohols can be
selective oxidation of alcohols applying simple and
accelerated upon addition of bromide as a co-catalyst
cheap oxidants like hydrogen peroxide is still challeng-
[13]. We now report an improved, highly selective oxi-
ing. Usually, strong oxidizing agents (KMnO4, MnO2,
dation of terminal alcohols to aldehydes using a novel
SeO2, etc.) in stoichiometric amounts are necessary to
four-component catalytic system consisting of H2O2,
perform this reaction [1]. Since the first reports on the
catalytic applications of organorhenium(VII) com-
MTO, HBr and TEMPO. Acetic acid is used as solvent.
pounds have appeared in 1990 [2–8], a growing number
of publications and patents have shown that methyltri-
oxorhenium(VII) (MTO, CH3ReO3) exhibits a high cat-
alytic activity in a variety of organic reactions [9–11].
2. Results and discussion
Parallel to our studies, Espenson et al. observed an
MTO/HBr-catalyzed oxidation of alcohols by hydrogen
peroxide [13]. They found that the addition of HBr
accelerates the catalytic oxidation of different alcohols
to aldehydes or ketones. The addition of HBr decreases
the reaction time by a factor of 1000 [13]. This is in
agreement with our own results. A disadvantage of this
ꢀ Multiple bonds between transition metals and main-group ele-
ments Part 175; preceding communication: W.A. Herrmann, J. Frid-
gen, G. Lobmeier, M. Spiegler, New J. Chem. 23 (1999) 5.
* Corresponding author. Tel.: +49-89-2891-3080; fax: +49-89-
2891-3473.
E-mail address: lit@arthur.anorg.chemie.tu-muenchen.de (W.A.
Herrmann)
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(99)00013-3

W.A. Herrmann et al. / Journal of Organometallic Chemistry 579 (1999) 404–407
405
Table 1
catalytic system, however, is its selectivity failure in the
MTO/HBr/TEMPO catalyzed oxidation of terminal alcohols
oxidation of terminal alcohols like benzyl alcohol. The
use of the stoichiometric amount as well as an excess of
hydrogen peroxide leads to a mixture of the corre-
sponding aldehyde and the acid [13].
It is well known [14–16] that the selective oxidation
of primary alcohols in the presence of secondary ones is
preferred when the stable organic nitroxyl radical
2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO, 1) is
used as a mediator. The nitrosonium ion 2 is the
intermediately observed oxidizing species; it becomes
reduced to the hydroxylamine 3 during the oxidation
process while it regenerates to 1 during further reaction
[15].
Substrate
Time (min) Conversion (%)a Selectivity (%)a
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol 120
4-Isopropyl
benzyl
alcohol
4-Isopropyl
benzyl
alcohol
4-Isopropyl
benzyl
1
15
51
70
81
62
\99
\99
\99
\99
1
15
78
\99
\99
120
\90
alcohol
a Determined by GC-MS.
Several research groups dealt with mechanistic inves-
tigations of the TEMPO-mediated oxidation of alco-
hols. In this context, Semmelhack and coworkers
excluded a radical mechanism as well as a direct hy-
dride abstraction for this type of reaction [19]. Further-
more, van Bekkum et al. discussed the different
mechanisms in the TEMPO-catalyzed oxidation of al-
cohols under alkaline as well as under acidic reaction
conditions [14]. However, only differences in the oxida-
tion rate of secondary alcohols but not of primary
alcohols are found. The mechanistic details of the
TEMPO-catalyzed oxidation of primary alcohols,
which are proposed in the literature [15,18], are agree-
able with our mechanistic understandings. A simplified
catalytic cycle of the novel H2O2/MTO/HBr/TEMPO
system is shown in Fig. 1.
Van Bekkum and Besemer found that the C6-pri-
mary alcohol group in carbohydrates can be oxidized
selectively by the in situ generation of the nitrosonium
ion 2 using hypochlorite as oxidant and the bromide/
hypobromide co-catalyst in water [15,16]. An analogous
system was applied to monosaccharide by Flitsch et al.
[17].
We now found that the oxidizing nitrosonium agent
2 is also generated with H2O2 and HBr in the presence
of methyltrioxorhenium (MTO) as catalyst. In our
studies we first used benzyl alcohol and its derivatives
as a model system. The results will be transferred to
aliphatic alcohols, especially to carbohydrate deriva-
tives as technically interesting substrates.
We found that all four components of the catalyst
system are essential to reach the reported results. With-
out MTO, hydrogen peroxide does not activate bro-
mide or TEMPO efficiently (low conversion of benzyl
alcohol even after 2 days). This means that MTO is the
‘endorsement catalyst’ for the oxidation of bromide to
hypobromite with hydrogen peroxide as oxidant. If
TEMPO is not applied, the system H2O2/MTO/HBr
gives both the aldehyde and the carboxylic acid unselec-
tively. The oxidation of aromatic aldehydes with the
system MTO/H2O2 generates the corresponding benzoic
2.1. Oxidation of benzyl alcohol deri6ates
In a typical run, 0.52 ml (5 mmol) of benzyl alcohol,
37 mg of MTO (3 mol%) and 23 mg of TEMPO (3
mol%) are dissolved in 5 ml of acetic acid and treated
with 0.1 ml (10 mol%) of a solution of HBr (30 wt.%)
in CH3COOH. The addition of 2 ml (25 mmol) of H2O2
(30%) starts the oxidation process. The catalytic results
for two model systems on different time scales are
summarized in Table 1. The selective oxidative forma-
tion of aromatic aldehydes, without unselective further
oxidation to carboxylic acid, is of high interest for the
efficient production of fine chemicals.
Table 2
MTO/HBr-catalyzed oxidation of terminal alcohols
The application of the four-component system
(H2O2, MTO, HBr, TEMPO) in this reaction reduces
the conversion time (Table 1) significantly as compared
to the analogous reaction without TEMPO (Table 2).
In contrast to the oxidation process without TEMPO,
where benzoic acid is formed as a by-product (Table 2),
we observed a selectivity of \99% and no formation of
the corresponding acid (Table 1).
Substrate
Time (min) Conversion (%)a Selectivity (%)a
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol 120
1
2
59
63
15
ꢀ80b
ꢀ80b
a Determined by GC-MS.
b ꢀ20% formation of the corresponding acid (benzoic acid).

406
W.A. Herrmann et al. / Journal of Organometallic Chemistry 579 (1999) 404–407
Fig. 1. Proposed cascade-type mechanism of H2O2/MTO/HBr/TEMPO catalyzed oxidation [15,18].
Fig. 2. Partial oxidation (with respect to the content of hydroxymethyl groups at position C6) of the C6-group by the three-component system
H2O2/MTO/HBr.
acid derivatives. However, in the bromide- and
TEMPO-free system the selectivity is low and the for-
mation of phenols as by-products was observed (MTO-
catalyzed ‘Dakin reaction’ [20]). The selectivity of the
oxidation of aromatic aldehydes is also dependent on
the electronic character of the substituents of the aro-
matic system (92% formation of o-nitro benzoic acid
starting from the corresponding aldehyde) [21]. The
system we found permits the oxidation of aromatic as
well as aliphatic alcohols with hydrogen peroxide either
to the corresponding aldehydes with very high selectiv-
ity, or to the corresponding acid depending on the
applied reaction parameters (hydrogen peroxide
amount, presence of TEMPO, reaction time).
with concomitant gas evolution over almost 2 h. After
that the sample was prepared for 13C-NMR by removal
of the solvents in vacuo. The NMR-spectroscopic mea-
surements were carried out in methanol-d4/D2O (60/40).
As expected the NMR data showed a partial oxidation
of the C6-group (Fig. 2). The characteristic signal for
the carboxylic C6-group [15] was monitored at l=
176.5 ppm. Signals due to the corresponding aldehyde-
groups were not seen.
In agreement with the mechanistic proposals of Es-
penson et al. [13] for the oxidation of alcohols with
MTO/H2O2 and HBr as a co-catalyst, we suggest the
mechanistic considerations shown in Fig. 3. Like van
Bekkum et al. [15] we believe that further formation of
hypobromite in the presence of excess hydrogen perox-
ide during the oxidation process takes place. We believe
this is a reasonable explanation for the observation that
no aldehyde, but only the preferred carboxylic acid is
formed.
2.2. Oxidation of carbohydrates
From an industrial point of view, the complete oxi-
dation of the C6-hydroxymethyl-group of potato starch
yielding the corresponding carboxylic acid would be
preferred since such biopolymers can be used in a
variety of applications, e.g. as superabsorbing agents.
Thus, the task was to generate carboxylated starch
exclusively. Therefore, to achieve extensive oxidation of
the hydroxymethyl groups of the starch molecules to
carboxylic acid units, we used the three-component
system (H2O2, MTO, HBr) without TEMPO; further
oxidation of the intermediate aldehyde was thus ex-
pected to occur.
The water soluble starch used contains 27% amylose
and 73% amylopectin. In a typical run, 700 mg of
potato starch and 12.5 mg of MTO (0.05 mmol) were
dissolved in 5 ml of acetic acid and treated with 0.05 ml
of a solution of HBr (30 wt.%) in CH3COOH. Addition
of 2 ml (25 mmol) of H2O2 (30%) started the reaction
3. Conclusions
The four-component system MTO, H2O2, HBr and
TEMPO in acetic acid is efficient for the selective
catalytic oxidation of terminal alcohols. An important
advantage—apart from the accessibility, the mild con-
ditions, and the short reaction times—is the amazing
highly selective formation of aldehydes. We succeeded
in the desired substitution of bleach (NaOCl) as pri-
mary oxidant by the application of environmental
friendly hydrogen peroxide in BrO−/TEMPO-mediated
oxidation reactions. Furthermore, the catalytic system
(MTO, H2O2, HBr) without TEMPO allows oxidation
of the C6-atom to the corresponding carboxylic acid of

W.A. Herrmann et al. / Journal of Organometallic Chemistry 579 (1999) 404–407
407
Fig. 3. Proposed simplified mechanism of the starch oxidation by H2O2/MTO/HBr.
[6] W.A. Herrmann, R.W. Fischer, D.W. Marz, Angew. Chem. Int.
Ed. Engl. 30 (1991) 1638.
[7] W.A. Herrmann, M. Wang, Angew. Chem. 103 (1991) 1709.
[8] W.A. Herrmann, M. Wang, Angew. Chem. Int. Ed. Engl. 30
(1991) 1641.
carbohydrates as demonstrated for starch. The MTO-
based oxidation of terminal alcohol groups in carbohy-
drates opens perspectives for broadly desired industrial
applications in this area.
Currently we are investigating the potential of this
novel catalyst/co-catalyst system, especially in the field
of the selective oxidation of carbohydrates and related
substrates derived from renewable resources.
[9] W.A. Herrmann, F.E. Ku¨hn, Acc. Chem. Res. 30 (1997) 169.
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Acknowledgements
The authors are grateful to their coworkers J.-O.
Barth and M. Hoffmann for support. This work was
generously supported by the ‘Bayerischer Forschun-
gsverbund Katalyse’ and the Bavarian Ministry of Nu-
trition, Agriculture and Forestry (grants for J.P.
Zoller).
[15] H. van Bekkum, A.C. Besemer, Carbohydrates in Europe, Car-
bohydrate Research Foundation, November 1995, p. 16.
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