Polyethylene glycol as a non-ionic liquid solvent for polyoxometalate
catalyzed aerobic oxidation
Adina Haimov and Ronny Neumann*
Department of Organic Chemistry, Weizmann Institute of Science, Rehovot, Israel, 76100.
E-mail: Ronny.Neumann@weizmann.ac.il; Fax: 972 8 9344142; Tel: 972 8 9343354
Received (in Cambridge, UK) 16th January 2002, Accepted 7th March 2002
First published as an Advance Article on the web 21st March 2002
Table 1 Oxidation of benzylic alcohols catalyzed by H5PV2Mo10O40 in
PEG
The H5PV2Mo10O40 polyoxometalate in a polyethylene
glycol solvent was effective for a series of aerobic oxidation
reactions including oxydehydrogenation of alcohols and
cyclic dienes, oxidation of sulfides and the Wacker reaction;
the solvent-catalyst phase can be recovered and recycled.
Substrate
Product (selectivity, mol%d)
Benzyl alcohol
Benzyl alcohola
Benzaldehyde (100)
Benzaldehyde (100)
The search for alternative reaction media to replace volatile and
often toxic solvents commonly used in organic synthetic
procedures is an important objective of significant environ-
mental consequence. While the use of water as solvent is
probably the most desirable approach, this is often not possible
due to the hydrophobic nature of the reactants and the sensitivity
of many catalysts to aqueous conditions. Despite this there are
some examples of aqueous/organic biphasic catalysis in alcohol
oxidation, carbonylation, hydrogenation, and C–C coupling
reactions.1 Other alternatives include (a) the use of supercritical
fluid media2 that have the advantage of facile solvent removal
and easy recycle but require high pressure. (b) Fluorous based
systems3 have the advantage of being highly hydrophobic and
the solvents are probably innocuous but have the disadvantage
of being volatile and expensive systems. (c) Ionic liquids4 have
a particularly useful set of properties, being non-volatile and
virtually insoluble in water and alkanes but readily dissolving
many transition metal catalysts. They are, however, very
expensive. In this communication we describe the use of a
simple and widely available polymer, polyethylene glycol
(PEG) and optionally its derivatives as non-toxic, inexpensive,
non-ionic liquid solvents of low volatility. Specifically, the PEG
is used as solvent for known polyoxometalate catalyzed aerobic
oxidations.5 With H5PV2Mo10O40, it is possible to directly
oxidize benzylic alcohols to aldehydes,6 to dehydrogenate
cyclic dienes to the corresponding aromatic derivatives,7 to
oxidize sulfides to sulfoxides and sulfones,8 and to use the
polyoxometalate as co-catalyst in palladium catalyzed Wacker
oxidations.9
4-Methylbenzyl alcohol
4-Bromobenzyl alcohol
4-Nitrobenzyl alcohol
1-Phenylethanol
4-Methylbenzaldehyde (100)
4-Bromobenzadehyde (100)
4-Nitrobenzaldehyde (100)
Styrene (66), acetophenone (26),
benzaldehyde (8)
Benzaldehyde (80), benzylformate (4),
dibenzylether (16)
Benzaldehyde (50), benzylformate (25),
dibenzylether (25)
4-Methylbenzyl alcoholb
4-Methylbenzyl alcoholc
4-Nitrobenzyl alcohola
4-Nitrobenzyl alcoholb
4-Nitrobenzaldehyde (100)
4-Nitrobenzaldehyde (100)
Reaction conditions: 0.01 mmol H5PV2Mo10O40, 0.5 mmol benzylic
alcohol, 0.5 ml PEG-200, O2 – 2 atm, 100 °C, 16 h. Conversions were
quantitative.a PEG-400 instead of PEG-200. b Polyethyleneglycol dimethy-
lether-250. c Polyethyleneglycol dicarboxylic acid—250. d mol% given
product of all products.
pathway. PEG-400 was similarly effective compared to PEG-
200. It is possible, but not advantageous to use other PEG
derivatives, namely the dimethyl ether and the dicarboxylic acid
derivatives as solvents. With these derivatives there is a
tendency to form some benzyl formate esters and the acid
catalyzed dimerization to the dibenzyl ether is more prominent.
Also, the solubility of H5PV2Mo10O40 in PEG–dimethyl ether is
more limited.
In order to prove that the use of polyethylene glycol as
solvent is also practical, it must be conveniently recycled with
minimal loss and decomposition. Since polyethylene glycol is
immiscible with aliphatic hydrocarbons, the desired product, in
this case benzaldehyde, may be extracted with compounds such
as cyclohexane, and the retained polyoxometalate–PEG phase
may be reused. Such a solvent recycle was carried out on a 50
mL scale (1.0 mmol H5PV2Mo10O40, 50 mmol benzylic
alcohol, 50 ml PEG-200). The solvent phase was recycled with
no loss of reactivity for three cycles, although a weight loss of
~ 5% was observed from cycle to cycle due to the degradation
of polyethylene glycol. Alternatively, the product may be
removed by vacuum distillation and the catalyst–polyethylene
glycol phase can be reused. Such a procedure was similarly
effective. The stability of the PEG under the reaction conditions
( ~ 100 °C, high Brønsted acidity and molecular oxygen) was
studied also by analysis of the reaction mixture by GC-MS in
search of decomposition products. Practically the only such
product (2–5 mol%) observed was dioxolane. From this one
may infer that 1,2-ethanediol and formaldehyde are the major
decomposition products. By GC-MS and 1H NMR, no oxidation
of the terminal alcohol moiety of PEG to an aldehyde moiety
was observed. This is probably due to the low reactivity of
primary aliphatic alcohols in such reaction systems.6
Initially the concept of using polyethylene glycol as solvent
was tested using the oxidation of benzylic alcohols to
benzaldehydes, eqn. 1, as an exemplary reaction, Table 1.
(1)
Thus, by dissolving the H5PV2Mo10O40 polyoxometalate
catalyst and the benzylic alcohol substrate in PEG-200 in a glass
pressure tube under 2 atm oxygen the originally orange
(H5PVV2Mo10O40—oxidized form) polyoxometalate solution
turned green-blue (H7PVIV2Mo10O40—reduced form) upon
heating and initiation of the reaction.† Upon the conversion of
all the substrate the polyoxometalate was reoxidized to the
initial orange solution. Analysis of the reaction mixture by GC
and GC-MS revealed that for PEG-200, primary benzylic
alcohols yielded only benzaldehydes as reaction products. For a
secondary alcohol, 1-phenylethanol, selectivity towards oxida-
tion was low. Here, acid catalyzed dehydration to styrene (some
of which was oxidized to benzaldehyde) was the major reaction
The utility of polyethylene glycol as a non-ionic liquid
solvent of low volatility was further tested in other reactions
catalyzed by H5PV2Mo10O40. First, in the case of oxy-
876
CHEM. COMMUN., 2002, 876–877
This journal is © The Royal Society of Chemistry 2002