Aerobic oxidation of alcohols and alkylaromatics with dioxygen catalysed by N-hydroxyphthalimide with vanadium co-catalysts

Article abstract of DOI:10.1039/b702443f

[Bu₄N]VO₃ or a combination of vanadium complexes with chloride salts are attractive co-catalysts for oxidation reactions of organic compounds with dioxygen catalysed by N-hydroxyphthalimide. The role of the chloride additive relates to the formation of more labile six-coordinate vanadium complexes. The same effect is also observed with amines. Two catalytic systems based on NHPI/VO(acac)₂ with LiCl or [Bu₄N]Cl and NHPI/[Bu₄N]VO₃ are most effective in oxidation reactions of primary and secondary alcohols, but oxidation of alkylaromatics proceeds with conversion rates of from 14.4% in the case of toluene to 67.0% in the case of ethylbenzene and with different selectivities, dependent on the structure of the substrate. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.

Full text of DOI:10.1039/b702443f

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Aerobic oxidation of alcohols and alkylaromatics with dioxygen
catalysed by N-hydroxyphthalimide with vanadium co-catalysts
P. J. Figiel and J. M. Sobczak*
Received (in Durham, UK) 15th February 2007, Accepted 14th May 2007
First published as an Advance Article on the web 11th June 2007
DOI: 10.1039/b702443f
[Bu4N]VO3 or a combination of vanadium complexes with chloride salts are attractive co-catalysts
for oxidation reactions of organic compounds with dioxygen catalysed by N-hydroxyphthalimide.
The role of the chloride additive relates to the formation of more labile six-coordinate vanadium
complexes. The same effect is also observed with amines. Two catalytic systems based on NHPI/
VO(acac)2 with LiCl or [Bu4N]Cl and NHPI/[Bu4N]VO3 are most effective in oxidation reactions
of primary and secondary alcohols, but oxidation of alkylaromatics proceeds with conversion
rates of from 14.4% in the case of toluene to 67.0% in the case of ethylbenzene and with different
selectivities, dependent on the structure of the substrate.
N-Hydroxyphthalimide (Sigma), toluene, ethylbenzene,
cumene, ortho-, meta- and para-xylene, (POCh), mesitylene
(Koch-light), 2-pentanol (Schuchard), isopropanol (J. T.
Baker), 1-hexanol and benzyl alcohol (Reachim), 1-decene,
quaternary ammonium salts (Fluka), triethylamine (Ubi-
chem), 4-methyl and 4-cyano-pyridine (Riedel), VO(OiPr)3
(Aldrich), were used as received. Pyridine (Ubichem) was dried
over NaOH.
1
Introduction
Selective catalytic oxidation using dioxygen is one of the most
attractive transformations in organic synthesis.1–4 In recent
years, there has been a noticeable increase of interest in
oxidation reactions with nitroxyl radicals used as catalysts.
Generally, it is possible to divide these reactions into two
groups: those in which the catalysts are persistent nitroxyl
radicals, such as 2,20,6,60-tetramethylpiperidine-N-oxyl deri-
vatives (TEMPO),3–5 and those with non-persistent radicals,
e.g. phthalimide-N-oxyl (PINO) derivatives,2,3,6 or saccharin-
N-oxyl (SINO) derivatives,7 generated in situ from suitable
N-hydroxy derivatives. The great attraction of above men-
tioned reactions is application of different co-catalysts, most
often transition metal complexes (regarding TEMPO, see, refs.
3 and 8–10 regarding PINO, developed especially by the Ishii
group2,6 and others10–17). The catalytic systems are particu-
larly attractive because of the mild conditions and high
selectivity of the reactions.
VO(acac)2 was recrystalized from CHCl3, VO(acac)2py,19
18
22
[Bu4N]VO3 ꢀ H2O,20 VO(acac)2Cl,21 and [Bu4N]3H3V10O28
were prepared according to the published methods.
LiCl was obtained in reaction of Li2CO3 (BDH) with a
solution of hydrochloric acid (POCh).
2.2 Procedure for catalytic oxidation
The standard conditions for oxidation reactions of organic
substrates, were as follows: organic substrate 5 mmol, NHPI
0.25 mmol (0.0408 g), co-catalyst 0.025 mmol, and acetonitrile
to top up the volume to 10 cm3 at atmospheric pressure of O2.
The reactions were carried out at constant volume in a
thermostated glass-reactor equipped with a magnetic stirring
bar and connected to computer-controlled automatic gas
absorption measuring apparatus. The substrate, acetonitrile
and NHPI were added to the reactor under an oxygen atmo-
sphere. Next, a fixed amount of the co-catalyst was introduced
in one portion, immediately after which stirring was started
and the progress of the absorbed dioxygen was measured. The
mmole of oxygen that reacted was calculated from the volume,
pressure and temperature data using the ideal gas equation.
In a recent preliminary report, we described new efficient
aerobic oxidation of some alcohols with dioxygen catalysed by
N-hydroxyphthalimide combined with vanadium co-cata-
lysts.15
In this paper, investigations of the oxidation reaction of
organic compounds by dioxygen, catalysed by NHPI in com-
bination with vanadium complexes and some additives, are
presented.
2
Experimental
2.3 Product analysis by chromatography
2.1 Materials
Product analysis was performed on a Hewlett–Packard HP
5890 II series gas chromatograph equipped with a flame
ionization detector and an HP 5971A mass selective detector.
An HP-5 25-m-long capillary column (25 ꢁ 0.2 ꢁ 0.33 mm)
was used in both cases. In some experiments an HP 1090 II
Liquid Chromatograph with an ODS Hypersil column (10 cm
long and 2.1 mm in diameter) and equipped with an HP 1047A
RI detector was used.
Acetonitrile (ROTH) was distilled with CaH2 under a nitrogen
atmosphere (MeCN for HPLC from J. T. BAKER was used
without additional purification).
Cyclohexanol (POCh) was distilled under a nitrogen atmo-
sphere.
Faculty of Chemistry, University of Wroc!aw, 14 F. Joliot-Curie str,
50-383 Wroc!aw, Poland. E-mail: js@wchuwr.pl; Fax: 48 71 3282348
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3
Results and discussion
VO(acac)2 is well known as a catalyst for oxidation reactions
of unsaturated hydrocarbons,23 hydroquinones,24 and pro-
pargylic alcohols,25 using dioxygen as oxidant. Vanadyl tetra-
dentate Schiff base complexes26 and VOCl327 are also catalysts
in aerobic selective oxidation of olefins and a-hydroxycarbo-
nyls, respectively. Nevertheless, under the reaction conditions
described in the experimental part, VO(acac)2 does not cata-
lyse the cyclohexanol oxidation reaction with dioxygen and
shows hardly any influence on the course of reactions with the
participation of NHPI (Fig. 1), although positive catalytic
activity of VO(acac)2 towards the oxidation of adamantane
with NHPI has been reported.28
We found that VO(acac)2 combined with pyridine hydro-
chloride and NHPI gives an effective catalytic system for
oxidation reactions. This catalytic system is particularly
attractive compared with other metal complex co-catalysts, it
is characterised by a shortest induction time and the highest
rates of the oxidation reactions (Fig. 1).
Fig. 2 shows the influence of some ammonium salts com-
bined with VO(acac)2 on the oxidation reaction of cyclohexa-
nol catalysed by NHPI. It presents a considerable influence of
the anion of tetrabutylammonium salts on the rate of the
reaction, the induction time and dioxygen uptake. The influ-
ence of the quaternary ammonium cation is not as significant
as the influence of the anion.
Fig. 2 Aerobic oxidation of cyclohexanol (5 mmol) in MeCN (9.5 ml)
at 75 1C, catalysed by NHPI (0.25 mmol) and VO(acac)2 (0.01 mmol)
with ammonium salts (0.01 mmol): (,) [Me4N]Cl; (n) [Bu4N]Cl; (’)
[Bu4N]HSO4; (B) [Bu4N]Br (0.03 mmol); (K) [Bu4N]I; (J) [Et4N]Cl;
(&) [Et3NH]Cl.
The highest reaction rate was observed for tetrabutyl-
ammonium chloride (0.059 mmol O2 minꢃ1) and the lowest
for [Bu4N]HSO4 (0.0011 mmol O2 minꢃ1).
For halogen ions, the reaction rate decreases in the order
Cl 4 Br 4 I.
The longest induction time is observed for iodide salt (ca.
730 min) and [Bu4N]HSO4 (516 min). Fast reactions with
[Et3NH]Cl, [Bu4N]Cl and [Bu4N]Br show lower induction
times: 4, 38 and 54 min, respectively.
The above presented results led to the conclusion that the
active form of the co-catalyst forms faster in systems where
chloride or bromide salts are used. This prompted us to study
the influence of other chloride additives combined with
VO(acac)2 on the course of the oxidation reaction of cyclo-
hexanol catalysed by NHPI (Table 1).
Table 1 Oxidation of cyclohexanol with O2 catalysed by the system:
NHPI + VO(acac)2 + chloride additivea
Chloride
Entry additive
Induction Vmax mmol Conversionb Selectivityb
time/min O2/min
(%)
[%]
1
2
3
4
5
[Bu4N]Cl
LiCl
NH4Cl
HCl
VO(acac)2Cld 265
38
3.7
6.1
328
0.059
0.034
0.036
0.0004
0.035
79,5c
93.8
91.8
B10
89.2
88.5c
87.2
66.4
37.7
60.1
a
Reagents and conditions: cyclohexanol (5 mmol), MeCN (9.5 ml), NHPI
Fig. 1 Aerobic oxidation of cyclohexanol (5 mmol) in MeCN (9.5 ml)
at 75 1C, catalysed by NHPI (0.5 mmol) with metal acetylacetonates
(0.02 mmol): (B) none; (K) VO(acac)2 + py ꢀ HCl (1 : 1); (,)
VO(acac)2; (J) TiO(acac)2; (n) Cr(acac)3; (&) Co(acac)3; (+)
Al(acac)3; (’) Ni(acac)2.
(0.25 mmol), VO(acac)2 (0.01 mmol) and chloride additive (0.03 mmol),
b
75 1C, 1 atm. O2, 18 h. Conversion of cyclohexanol and selectivity to cyclo-
c
d
hexanone were calculated from HPLC analysis. Data after 2 h. Without
VO(acac)2.
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Fig. 5 Effect of basicity of amines in oxidation reaction of cyclohexanol
catalysed by NHPI with combination of VO(acac)2 and amine (Vmax
were calculated from Fig. 4 and pKb values were taken from ref. 18).
Fig. 3 Effect of the molar ratio of LiCl to VO(acac)2 (A) and the co-
catalyst concentration (B) on the oxidation reaction of cyclohexanol
catalysed by NHPI.
similar activity (except induction time) as in reactions where a
combination of VO(acac)2 and chloride salt was used.
For [Bu4N]Cl, NH4Cl and LiCl (entries 1–3) very effective
catalytic systems were obtained and the dioxygen absorption
curves were almost the same. Unexpectedly, HCl (entry 4),
which is on its own an effective co-catalyst in oxidation
reactions of alcohols catalysed by NHPI,29 shows very low
absorption of dioxygen in combination with VO(acac)2. This
result was attributed to the formation of oxovanadium chlor-
ides, for example VOCl2 and/or VOCl3, which are inactive as
co-catalysts in reactions catalysed by NHPI (VOCl2 and
VOCl3 were re-examined in separate experiments).
The above results indicate that the combination of
VO(acac)2 and LiCl is the most active co-catalyst for oxidation
of cyclohexanol with dioxygen catalysed by NHPI.
The effect of the LiCl : VO(acac)2 molar ratio and co-
catalyst concentration on the oxidation reaction of cyclo-
hexanol catalysed by NHPI is presented in Fig. 3.
The rate of O2 absorption reaches its maximum at a LiCl :
VO(acac)2 molar ratio equal to 1 : 1, as shown in Fig. 3(A).
Likewise, the maximum rate of the oxidation reaction of
However, complex VO(acac)2Cl (entry 5) used as a co-catalyst
in oxidation reaction of cyclohexanol catalysed by NHPI shows
Fig. 6 Aerobic oxidation of cyclohexanol (5 mmol) in MeCN (9.5 ml)
at 75 1C, catalysed by NHPI (0.25 mmol) and vanadium complexes
(0.01 mmol) without any additive (open symbols) and with LiCl
(0.01 mmol) (solid symbols): (K, J) [Bu4N]3H3V10O28; (E, B)
Bu4NVO3 ꢀ H2O; (’, &) VO(OiPr)3.
Fig. 4 Aerobic oxidation of cyclohexanol (5 mmol) in MeCN (9.5 ml)
at 75 1C, catalysed by NHPI (0.25 mmol) and VO(acac)2 (0.01 mmol)
with amines (0.01 mmol): (J) 4-Me-py; (n) Et3N; (B) py; (K)
4-CN-py) or [VO(acac)2(py)] (0.01 mmol) (&).
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Fig. 8 Aerobic oxidation of alkylaromatics (5 mmol) in MeCN at
Fig. 7 Aerobic oxidation of alcohols (5 mmol) in MeCN at 75 1C,
catalysed by NHPI (0.25 mmol) with VO(acac)2 with LiCl (each
0.01 mmol) (open symbols) or with [Bu4N]VO3 (0.01 mmol) (solid
symbols): (’, &) 1-hexanol; (K, J) 2-pentanol; (B) 2-propanol.
75 1C, catalysed by NHPI (0.25 mmol) with VO(acac)2 with LiCl (each
0.01 mmol) (open symbols) or with [Bu4N]VO3 (0.01 mmol) (solid
symbols): (K, J) cumene; (’, &) ethylbenzene; (m) m-xylene; (.)
o-xylene; (n) p-xylene.
cyclohexanol (Vmax = 0.02 mmol O2 minꢃ1) was observed for
the molar ratio [Bu4N]Cl : VO(acac)2 = 1 : 1.
hexanol catalysed by NHPI; however, such complexes are
known as catalysts of oxidation reactions.26,32
Fig. 3(B) presents the maximum rates of dioxygen absorp-
tion for various concentrations of the co-catalyst, at a ratio of
LiCl to VO(acac)2 equal to 1 : 1. The optimum concentration
Using above discussed results we decided to check influence
of other, more labile vanadium complexes on the catalytic
activity of NHPI (Fig. 6).
of the co-catalyst is between 0.7 and 2 mmol dmꢃ3
.
As can be seen from Fig. 6, oxidation reactions of cyclo-
hexanol, catalysed by NHPI and vanadium complexes (with-
out chloride salt or amine additives), proceed in most cases
rather ineffectively, like reactions in which the co-catalyst is
only VO(acac)2. [Bu4N]VO3 is an exception: it is a very active
co-catalyst without any other additives.
The role of the chloride additive probably is important
for the formation a more labile six-coordinate complex
VO(acac)2Cl. The same effect should also be observed with
amines because [VO(acac)2A], where A—amines, are well
known vanadium complexes.30 Fig. 4 presents the course of
oxidation reaction of cyclohexanol catalysed with NHPI and a
combination of vanadyl acetylacetonate with some amines.
It should be noted that amines in combination with NHPI
(without VO(acac)2) show an inhibiting effect. This can ex-
plain the lower catalytic activity of the system based on NHPI
+ VO(acac)2 + py, compared to NHPI and the complex
[VO(acac)2(py)]. Generally, the activity of the catalytic system
depends on the basicity of the amines (Fig. 5); the more basic
the amine, the more labile the vanadium complex and the
more effective the co-catalyst (for example, for the V–Oa bond
between the vanadyl ion and the acetylacetonate ligand, the
corresponding V–Oa stretching force constant was estimated
at 2.88 and 0.51 N cmꢃ1 for VO(acac)2 and VO(acac)2(py),
respectively).31
Among catalytic systems in which LiCl was used in addition
to NHPI and a vanadium complex, the highest activity was
observed for the VO(OiPr)3 complex (Fig. 6). Other vanadium
complexes were also examined, mainly with ligands such as
Schiff bases; however, they were ineffective as co-catalysts
(also with the addition of LiCl or [Bu4N]Cl) in the oxidation
reaction of cyclohexanol, although some of these complexes
are known as catalysts of the oxidation reaction.26,32
Two catalytic systems based on NHPI/VO(acac)2 with LiCl
or [Bu4N]Cl and NHPI/[Bu4N]VO3 were chosen for further
studies. The results of the investigation of the catalytic activity
of these catalytic systems in oxidation reactions of alcohols and
alkylaromatics are presented in Fig. 7 and 8 and in Table 2.
It can be seen that both catalytic systems are very effective in
oxidation reactions of primary and secondary alcohols, and
the observed dissimilarities concern the induction times, which
It is possible to explain the lack of effect of vanadyl
complexes with Schiff bases in aerobic oxidation of cyclo-
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Table 2 Oxidation of some organic compounds with O2 catalysed by NHPI/VO(acac)2/LiCl or by NHPI/VO(acac)2/[Bu4N]Cl
(values in parenthesis)a
Substrate
O2 uptake/mmol
Substrate conversion (%)
100 (100)
Products, selectivityb (%)
1-Hexanol
5.80 (5.20)
2.41 (2.48)
2-Pentanol
96.4 (97.2)
(44.4)
o-Xylene
(3.39)
Ethylbenzene
2.84 (3.35)
56.2 (67)
Toluene
Cumene
1.03
1.98
14.4
46.8
Mesitylene
0.89
18.2
a
Reagents and conditions: substrate (5 mmol), MeCN (9.5 ml), NHPI (0.25 mmol), VO(acac)2 (0.01 mmol) and chloride additive
b
(0.03 mmol), 75 1C, 1 atm. O2, 20 h. Conversion of substrates and selectivity to determined products were calculated based on GC
analysis.
are longer for catalytic systems with vanadyl acetylacetonate
(Fig. 7).
[Bu4N]VO3, which do not require any additives) or complexes
such as VO(acac)2, which require the addition of chloride salt
(LiCl or [R4N]Cl) or amine, weakening the vanadium–acetyl-
acetonate bond.
Larger differences in the catalytic activity of both catalytic
systems can be noticed in oxidation reactions of alkylaro-
matics, in which tetrabutylammonium vanadate ([Bu4N]VO3)
is a low-activity co-catalyst (Fig. 8).
Results of spectroscopic studies of NHPI–vanadium com-
pounds catalytic system are under preparation and will be
published.
The results presented in Table 2 confirm that these catalytic
systems are most effective in oxidation reactions of primary
and secondary alcohols, whereas oxidation of alkylaromatics
proceeds with conversion rates ranging from 14.4% for to-
luene to 67.0% for ethylbenzene and with different selectiv-
ities, dependent on the structure of the substrate.
Acknowledgements
The authors wish to express their gratitude to Professor Jozef
´
It is possible, on the basis of the obtained results, to draw
the conclusion that active co-catalysts are complexes of vana-
dium with a labile coordinating sphere (such as VO(OR)3 and
J. Zi"kowski (Faculty of Chemistry, University of Wroc"aw)
for his helpful comments. The financial support with the Grant
PBZ-KBN-118/T09/2004
is
gratefully
acknowledged.
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Chromatographic analysis from Marek Hojniak (GC-MS
Laboratory, Faculty of Chemistry) is kindly acknowledged.
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