New efficient aerobic oxidation of some alcohols with dioxygen catalysed by N-hydroxyphtalimide with vanadium co-catalysts

Article abstract of DOI:10.1039/b313417m

New efficient vanadium co-catalysts have been developed for the oxidation of some alcohols with O₂ catalysed by N-hydroxyphthalimide (NHPI). Various alcohols (primary and secondary) were selectively oxidized by O ₂ under mild conditions in the presence of a catalytic amount of NHPI as a radical-producing agent combined with small amounts of vanadium complexes with or without the addition of a simple salt (e.g. LiCl) or base (e.g. pyridine).

Full text of DOI:10.1039/b313417m

New efficient aerobic oxidation of some alcohols with dioxygen catalysed
by N-hydroxyphtalimide with vanadium co-catalysts
Pawe´l J. Figiel, Jaros´law M. Sobczak* and Józef J. Zió´lkowski
Faculty of Chemistry, University of Wroc´law, 14 F. Joliot-Curie, 50-383 Wroclaw, Poland.
E-mail: js@wchuwr.chem.uni.wroc.pl; Fax: 48 71 3282348; Tel: 48 71 3757386
Received (in Cambridge, UK) 23rd October 2003, Accepted 31st October 2003
First published as an Advance Article on the web 27th November 2003
New efficient vanadium co-catalysts have been developed for
the oxidation of some alcohols with O2 catalysed by N-
hydroxyphthalimide (NHPI). Various alcohols (primary and
secondary) were selectively oxidized by O2 under mild condi-
tions in the presence of a catalytic amount of NHPI as a radical-
producing agent combined with small amounts of vanadium
complexes with or without the addition of a simple salt (e.g.
LiCl) or base (e.g. pyridine).
positive catalytic activity of VO(acac)2 towards the oxidation of
adamantane with NHPI has been reported.10 On the other hand, it is
well known that in many solvents VO(acac)2 undergoes rather slow
air oxidation to vanadium(
) species.11 However, this process
V
proceeds rapidly after the addition of H2O2 and is accompanied by
the appearance of a characteristic heather colour of the solution.12
Indeed, the addition of stoichiometric amounts of H2O2 or t-
BuOOH to the mixture of well purified components containing
VO(acac)2 caused the transient appearance of a heather colour but
did not generate a catalytically active form of the vanadium
complex. Only just after the addition to the vanadium of
stoichiometric amounts of amines, pyridine and its derivatives,
tetra-alkylammonium chlorides and/or lithium chloride does the
system become highly catalytically active (Table 1). Such a high
activity allowed us to decrease the NHPI concentration in further
experiments from 10% to 5%. Among a number of tetraalk-
ylammonium halogenides, the best activity was found for tetra-
ethyl- and tetrabutylammonium chloride (entry 1). The addition of
4-methylpyridine (entry 4) was the most effective among the
different pyridine derivatives used. A particular effect was
observed for lithium chloride (entries 2, 3). Each of these
compounds interacts with the vanadium complex VO(acac)2 of a
nearly square pyramidal structure in its six-coordinate site to form
distorted octahedral complexes. To prove this hypothesis we
prepared six-coordinated vanadium complexes, (e.g. VO(acac)2Cl,
VO(acac)2py), and these compounds were found to be effective co-
catalysts without any additional additives (entries 5, 6).
Recently a great number of interesting aerobic oxidation reactions
of organic compounds have been achieved by using N-hydroxy-
phthalimide (NHPI), which serves as a radical-producing catalyst.1
The catalytic system is especially attractive because of the mild
conditions and high selectivity of the reactions. Among a number of
co-catalysts used, complexes of Co and Mn,1 as well as acet-
aldehyde,2 acids, ammonium salts,3 NO2,4 and other species, have
shown a positive effect on catalytic activity. During our studies on
influence of different transition metal complexes (mainly acet-
ylacetonates) as co-catalysts we have observed interesting effect in
the cyclohexanol oxidation reaction in acetonitrile (at 75 °C and 1
atm O2) catalysed by a system composed of NHPI, VO(acac)2 and
some additives (vide infra).
VO(acac)2 is well known as a catalyst for oxidation reactions of
unsaturated hydrocarbons,5 hydroquinones,6 and propargylic alco-
hols7 using dioxygen as oxidant. Vanadyl tetradentate Schiff base
9
complexes8 and VOCl3 are also catalysts in aerobic selective
oxidation of olefins and a-hydroxycarbonyls, respectively. Never-
theless, under the reaction conditions described above (with
purified reagents), VO(acac)2 does not catalyse the cyclohexanol
oxidation reaction with dioxygen and shows hardly any influence
on the course of reactions with the participation of NHPI, although
ESR and 51V NMR studies of the reaction mixtures show that V
(IV) is rapidly oxidized to V (
V
). Therefore V ( ) complexes should
V
also demonstrate catalytic activity as co-catalysts. This assumption
was proved when VO(OPr)3 or [Bu4N]VO3 (entries 7, 9) were used
Table 1 Aerobic oxidation of alcohols catalysed by NHPI combined with various vanadium (IV, V
) co-catalysta
Entry
Alcohol
Co-catalyst
Conversionb (%)
Time/h
Selectivityb (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
Cyclohexanol
Cyclohexanol
Cyclohexanolc
Cyclohexanol
Cyclohexanol
Cyclohexanol
Cyclohexanol
Cyclohexanol
Cyclohexanol
Cyclohexanol
1-Hexanol
VO(acac)2 + Bu4NCl
VO(acac)2 + LiCl
VO(acac)2 + LiCl
VO(acac)2 + 4-Me–py
VO(acac)2Cl
90.3
81.7
87.4
84.7
89.2
82.7
11.2
80.1
72.5
74.4
100
18
2
66.4
100
100
21
1.5
18
18
2
73.9
60.1
80.2
> 99
89.0
89.0
85.8
96.6d
100
VO(acac)2py
VO(OPr)3
VO(OPr)3 + LiCl
[Bu4N]VO3
[Bu4N]VO3 + LiCl
VO(acac)2 + LiCl
[Bu4N]VO3
1
1.5
1.5
22
2
2-Pentanol
2-Pentanol
73.6
96.4
VO(acac)2+LiCl
20
> 99
a Substrate (5 mmol) was reacting with O2 in the presence of NHPI (0.25 mmol, 5 mol %) and vanadium co-catalyst (0.01 mmol, 0.2 mol %) at 75 °C.
b Conversion of alcohols and reaction selectivity to ketones were calculated from HPLC analysis. c At 50 °C. d Selectivity to hexanoic acid.
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as co-catalysts, although for VO(OPr)3 the addition of LiCl was
necessary (entry 8). Among a number of catalytic systems based on
NHPI and vanadium complexes, the most attractive was that
composed of VO(acac)2–LiCl and [Bu4N]VO3 (i.e. entries 8 and 9).
For these two catalysts the oxidation of other alcohols (1-hexanol
and 2-pentanol) was studied (entries 11–13). Recently, a new
highly active and selective catalytic system for alcohol oxidation
with dioxygen, based on NHPI (10 mol %), Co(OAc)2 (0.5 mol %)
and meta-chlorobenzoic acid (MCBA) (5 mol %), has been
reported.13,14 Ishii et al.13 reported oxidation of different alcohols at
room temperature with high conversions (from 75 to > 99%) and
yields (47–98%) after 15–20 hours. Minisci et al.14 demonstrated
unusual selectivity of that system in the oxidation of benzyl
alcohols to benzaldehydes with a selectivity of from 91 to 99% (at
alcohol conversions of from 75 to 100%) at room temperature after
1–4 hours. We compared our catalytic system (with carefully
purified reactants, however) at reaction conditions similar to those
reported in papers13 and14 except for temperature, which in our case
was 30 °C. We studied the oxidation reaction of cyclohexanol (3
mmol) in EtOAc (5 ml) and benzyl alcohol (3 mmol) in MeCN (15
ml) catalysed by NHPI (10 mol %) with [Bu4N]VO3 (0.3 mol %)
and additionally NHPI (10 mol %) with Co(OAc)2·4H2O (0.5 mol
%) and MCBA (5 mol %). The results presented in Fig. 1 are
completely different from those reported in.13 and 14 Generally, for
the oxidation of cyclohexanol, a catalytic system based on NHPI–
Co(OAc)2–MCBA shows a lower conversion of alcohol than one
based on our NHPI–[Bu4N]VO3 system. In the case of benzyl
alcohol oxidation reaction, quite long induction times were
observed for both catalytic reactions (165 and 172 min for cobalt-
and vanadium-based systems, respectively). It is worth noting that
for the cobalt-based system very rapid oxidation to benzoic acid is
observed after ca. 1086 min. Such a reaction course may suggests
that during the first stage of the reaction, when slower absorption is
observed (1.26 3 1023 mmol O2/min), only alcohol-to-aldehyde
oxidation occurs whereas after ca. 1100 min very fast oxidation of
aldehyde to benzoic acid takes place (0.15 mmol O2/min). This
reaction was confirmed in an additional experiment in which
benzaldehyde oxidation was carried out at the same reaction
conditions and found to be very fast (0.11 mmol O2/min) (Fig. 1).
However, in a sample taken from the reaction mixture after 991 min
of benzyl alcohol oxidation the following amounts were found:
benzyl alcohol (0.54 mmol), benzaldehyde (1.82 mmol), benzoic
acid (0.65 mmol). These results may suggest simultaneous
oxidation of both the substrate and the intermediate product —
benzaldehyde.
This work was partially supported by the Polish State Committee
for Scientific Research (KBN), grant 4 T09A 164 25.
Chromatographic analysis from Marek Hojniak (GC-MS Labo-
ratory, Faculty of Chemistry) is kindly acknowledged.
Notes and references
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Fig. 1 Aerobic oxidation of benzyl alcohol in MeCN (8,-), cyclohexanol
in EtOAc (2,5) catalysed by NHPI–Co(OAc)2–MCBA (solid symbols)
and NHPI–Bu4NVO3 system (open symbols), respectively, and benzalde-
hyde in MeCN (+) catalysed by NHPI–Co(OAc)2–MCBA at 30 °C (see text
for reagent concentrations).
14 F. Minisci, C. Punta, F. Recupero, F. Fontana and G. F. Pedulli, Chem.
Commun., 2002, 688.
C h e m . C o m m u n . , 2 0 0 4 , 2 4 4 – 2 4 5
245