Oxidation of N-benzylidene-2-hydroxyaniline by dioxygen catalysed by a dicobalt complex

Article abstract of DOI:10.1039/a903699g

The oxidation of N-benzylidene-2-hydroxyaniline and its related derivatives by O₂ catalysed by [Co₂L(μ-OH)] {H₃L = 2,6-bis[(2-hydroxyphenyl)iminomethyl]-4-methylphenol} in DMF at 363 K was investigated. The oxidation products were the corresponding 2-substituted benzoxazoles. However, a side reaction leading to 2-aminophenoxazine-3-one via hydrolysis of the starting compound with water produced in the oxidation, followed by the oxidation of aminophenol, was established. If a dehydrating agent such as anhydrous Na₂SO₄ or 4 A molecular sieves was present in the reaction mixture the isolated product yields were above 87%. The fact that the reaction rate of the oxidation process in the presence of a radical scavenger was similar to that without suggests that it was a non-radical process. The initial oxidation rates depended linearly on the concentration of catalyst. The rates also depended linearly on the concentration of the organic substrate N-benzylidene-2-hydroxyaniline and O₂ pressure when these variables were small ([substrate] < 0.2 M, O₂ pressure < 70 kPa). At high concentration of substrate and O₂ pressure the rate showed saturation behavior. These kinetic data could be satisfactorily accounted for by a mechanism with initial co-ordination of substrate to [Co₂L(μ-OH)], followed by the co-ordination of O₂ to the catalyst, then the rate limiting step of oxidation of substrate. The formation of the adduct between N-benzylidene-2-hydroxyaniline and catalyst at 298 K was characterized by UV-Vis spectroscopy. The variation of the initial rates among the organic substrates could be explained by stereoelectronic effects. Moreover, the influence of acetic acid which slowed the initial rate of oxidation and weak base (2,6-di-tert-butylpyridine) which had little effect on the rate could also be satisfactorily accounted for based on the acid-base properties of the proposed reaction intermediates.

Full text of DOI:10.1039/a903699g

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Oxidation of N-benzylidene-2-hydroxyaniline by dioxygen
catalysed by a dicobalt complex†
Shian Yuh Chang, Yee Hsing Cheng, Bing-Jium Uang and Cheu Pyeng Cheng*
Department of Chemistry, National Tsing Hua University, Hsin-Chu, 30043, Taiwan.
E-mail: cpcheng@faculty.nthu.edu.tw. Fax: 886-3-5721174
Received 10th May 1999, Accepted 8th June 1999
The oxidation of N-benzylidene-2-hydroxyaniline and its related derivatives by O catalysed by [Co L(µ-OH)]
2
2
{
H L = 2,6-bis[(2-hydroxyphenyl)iminomethyl]-4-methylphenol} in DMF at 363 K was investigated. The oxidation
3
products were the corresponding 2-substituted benzoxazoles. However, a side reaction leading to 2-amino-
phenoxazine-3-one via hydrolysis of the starting compound with water produced in the oxidation, followed by the
oxidation of aminophenol, was established. If a dehydrating agent such as anhydrous Na SO or 4 Å molecular sieves
2
4
was present in the reaction mixture the isolated product yields were above 87%. The fact that the reaction rate of the
oxidation process in the presence of a radical scavenger was similar to that without suggests that it was a non-radical
process. The initial oxidation rates depended linearly on the concentration of catalyst. The rates also depended
linearly on the concentration of the organic substrate N-benzylidene-2-hydroxyaniline and O pressure when these
2
variables were small ([substrate] < 0.2 M, O pressure < 70 kPa). At high concentration of substrate and O pressure
2
2
the rate showed saturation behavior. These kinetic data could be satisfactorily accounted for by a mechanism with
initial co-ordination of substrate to [Co L(µ-OH)], followed by the co-ordination of O to the catalyst, then the rate
2
2
limiting step of oxidation of substrate. The formation of the adduct between N-benzylidene-2-hydroxyaniline and
catalyst at 298 K was characterized by UV-Vis spectroscopy. The variation of the initial rates among the organic
substrates could be explained by stereoelectronic effects. Moreover, the influence of acetic acid which slowed the
initial rate of oxidation and weak base (2,6-di-tert-butylpyridine) which had little effect on the rate could also be
satisfactorily accounted for based on the acid–base properties of the proposed reaction intermediates.
Introduction
the oxidation of N-benzylidene-2-hydroxyaniline which can be
prepared by the condensation of aldehydes and 2-aminophenol.
Cobalt complexes have been widely used in catalysing the re-
actions of organic substrates with molecular oxygen.
28
The oxidants that have been used include nickel peroxide,
1
–20
When
29
30,31
32
silver oxide, lead() tetraacetate
and lead() phosphate.
dioxygen participates directly in the oxidation reaction the
catalytic reaction may involve several different intermediates,
such as cobalt–dioxygen adducts, organic free radicals, and
organic hydroperoxides. There is also another type of cobalt
catalysed dioxygen reaction in which oxygen is used only
Oxygen has also been employed as the oxidant with a copper()
33
salt as the catalyst. However, a large amount of catalyst
I
I
(
substrate, 5 mmol; Cu , 2 mmol) was required in this Cu -
catalysed reaction. It is interesting that N-alkylidene-2-
hydroxyanilines can also be cyclized to the corresponding
4
,7
indirectly. In these reactions the co-ordinated nitro group
serves as an oxygen donor to oxidize organic substrates. The
nitro group is then turned into co-ordinated nitrosyl which
can be oxidized by dioxygen to regenerate the nitro group.
The organic substrates involved in these catalytic oxidation
reactions consist of a variety of functional groups including
34
oxazole photochemically. Alternatively, the reaction between
a primary alcohol and 2-aminophenol catalysed by ruthenium
35
complexes above 473 K can also lead to benzoxazole, pre-
sumably involving the N-benzylidene-2-hydroxyaniline inter-
mediate.
Recently, we showed that the oxidation of PPh using O as
1–7
8–10
11
3
2
olefins, phenols,
secondary alcohols, substituted anil-
oxidant in DMF solution at 353 K can be catalysed by the
1
2
13,14
15
3,16
17
ines, amines,
hydrazones, ethers,
iodo-compounds,
dicobalt complex 1, [Co L(µ-OH)] {H L = 2,6-bis[(2-hydroxy-
2
2
2,18,19
2
3
36
naphthalene,2 anthracene, benzylic compounds
and
phenyl)iminomethyl]-4-methylphenol}. In solid or solution,
each cobalt() ion in 1 is expected to be co-ordinated by other
ligand(s) including that of solvent molecule(s). Though the
detailed structure of 1 is not known exactly, the co-ordination
of L to the cobalt() ions can be depicted as below based on the
structure of [Cu L(µ-OH)] in which a planar Cu N O
0
cyclohexane. To add to this broad range of organic substrates,
we report here that N-benzylidene-2-hydroxyaniline and its
related derivatives can be oxidized by dioxygen to the corre-
sponding benzoxazoles catalysed by a dicobalt complex.
Derivatives of 2-substituted benzoxazole are conventionally
prepared by the dehydrative cyclization of the amides formed
by the condensation of an aminophenol and a carboxylic
acid or its derivative.
aminophenol under CO atmosphere catalysed by palladium
2
2
2
4
37
arrangement was observed.
2
1–26
The reaction of aryl halides and an
CH3
27
compounds can also lead to the desired amides. Another
convenient method of preparing 2-substituted benzoxazole is
C
N
C
N
O
†
Supplementary data available: characterisation data. For direct
Co
Co
electronic access see http://www.rsc.org/suppdata/dt/1999/2769/, other-
wise available from BLDSC (No. SUP 57583, 2 pp.) or the RSC Library.
See Instructions for Authors, 1999, Issue 1 (http://www.rsc.org/dalton).
O
O
H
1
O
J. Chem. Soc., Dalton Trans., 1999, 2769–2776
2769

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Table 1 Yields and initial reaction rates of benzoxazole and its deriv-
a
atives IIa–IIf from O oxidation of Ia–If catalysed by [Co L(µ-OH)]
2
2
Yield(%)
d
Ϫ5
Ϫ1
Entry
Product
b
c
Rate /10 M s
1
2
3
4
5
6
IIa
IIb
IIc
IId
IIe
IIf
68
21
81
74
70
88
87
—
96
93
90
—
3.52
0.427
3.26
3.90
3.45
4.55
a
Reaction conditions: substrate, 5.0 mmol; catalyst, 50 µmol, in 10 mL
b
DMF at 363 K; PO2 90.7 kPa, V(O ) 0.11 L at 298 K. Isolated yield for
a 24 h reaction period under the conditions in footnote a. Isolated
2
c
yield for a 24 h reaction period under the reaction conditions in
d
footnote a with the addition of 1.0 g of 4 Å molecular sieves. Initial
rate is defined as d[substrate]/dt obtained from the O pressure change
2
as {dPO2 /dt}{V(O )/RT}{1/V(sol)}.
2
of oxidation is much slower than for the other compounds (Fig.
1
b). In a 24 h reaction period the O pressure decreased by 13
2
kPa only. As for If, which exists as benzothiazoline rather than
the structure depicted in eqn. (1), it can be oxidized directly by
O at 363 K (Fig. 1f). However, addition of 1 also speeds up the
2
R2
R1
R2
R1
N
X
90 °C
DMF
N
C
+ 1
/2 O2
+ H2O (1)
XH
R1
H
R2
R1
H
R2
X
X
Fig. 1 The oxygen pressure change with (᭺) and without (᭡) the pres-
ence of dicobalt catalyst 1 for the substrates Ia (a), Ib (b), Ic (c), Id (d), Ie
Ia
H
O
O
O
O
O
S
IIa
H
O
O
O
O
O
S
Ib
Ic
Id
Ie
If
5-Cl
5-Me
H
H
IIb 5-Cl
H
H
IIc 5-Me
H
(
e) and If (f). Reaction conditions: V(O ) = 0.110 L at 25 K; reaction
2
4-Cl
4-Me
H
IId
IIe
IIf
H
H
H
4-Cl
4-Me
H
mixture 5.0 mmol substrate, 0.050 mmol catalyst in 10.0 mL DMF at
63 K.
H
H
3
In this oxidation a reactive intermediate which is capable of
oxidation reaction. The oxygen pressure changes in Fig. 1(c)–
oxidizing olefins to epoxides, alcohols to carbonyl compounds,
sulfide to sulfoxide and sulfoxide to sulfone was generated.
These substrates can not be oxidized without the presence
1
(e) are very similar to that in Fig. 1(a).
The H NMR spectrum of a sample containing 0.30 mmol Ia
1
and 10 µmol 1 in 0.5 mL d -DMF solvent in the absence of O2
7
of PPh under the reported reaction conditions. We have now
3
(
removed by at least three freeze–pump–thaw cycles) indicated
that no new compound is formed in a wide temperature range
223 to 363 K) or after prolonged (1 h) heating at 363 K. After
studied the oxidation of N-benzylidene-2-hydroxyaniline by
O to benzoxazole catalysed by this dicobalt complex to extend
2
(
the range of application of this catalytic system. The dicobalt
complex was proven to be very effective and gave almost
quantitative yield for the oxidation of N-benzylidene-2-
O was introduced into the NMR tube and the reaction mixture
2
1
heated to 363 K, the H NMR spectrum revealed that there was
only one organic product, 2-phenylbenzoxazole. Furthermore,
a signal at δ 3.5 indicated the formation of water in the reaction.
Therefore, the selectivity of the oxidation reaction is close to
hydroxyaniline even in the absence of PPh . The kinetics of this
3
oxidation reaction is consistent with an initial co-ordination of
the organic substrate and O to the catalyst, followed by a rate
2
1
00% in the early stages. After the catalytic oxidation had pro-
determining step that leads to the product and water.
ceeded for 24 h chromatographic separation of the reaction
mixture gave a major product II for each reactant I. The
oxidation reactions catalysed by 1 are summarized in eqn. (1).
The yields for compounds II are summarized in Table 1. The
particularly low yield for IIb is not due to any side reaction, but
because of the inherent low reactivity of the substrate. After
24 h reaction time a large amount of Ib was still unchanged.
However, the low isolated yields for Ia–Ie in Table 1 are
probably due to the additional oxidation pathway that requires
Results and discussion
N-Benzylidene-2-hydroxyaniline Ia and its derivatives Ib–Ie in
DMF do not react or react very slowly with O directly at 363 K
2
(
Fig. 1). However in the presence of a small amount of dicobalt
catalyst 1 an appreciable O pressure drop can be observed. For
2
reactants Ia and Ic–Ie the O pressures decrease rapidly for 600
2
min, then slow down. The decrease is approximately 65 kPa.
The number of moles of N-benzylidene-2-hydroxyaniline (5.0
mmol) in the reaction mixture is slightly less than twice the
O2 consumed (2.9 mmol). This stoichiometric relationship
more O than that in eqn. (1).
2
To investigate the nature of the additional oxidation pathway
we carried out the catalytic oxidation of compound Ia in
the presence of anhydrous Na SO or 4 Å molecular sieves.
2
4
indicates that each mol of O can oxidize two mol of organic
The total amount of O consumed in the presence of the de-
2
2
substrate. Additionally, there is another oxidation reaction in
which the stoichiometry requires more oxygen. For Ib the rate
hydrating agent is less than that in its absence. Interestingly,
the initial rates are exactly the same regardless of whether the
2
770
J. Chem. Soc., Dalton Trans., 1999, 2769–2776

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dehydrating agent is present or not. Furthermore, the isolated
yield for IIa increases from 68 to 87% (Table 1). For IIc–IIe
the isolated yields are equal or better than 90%. From these
results it is obvious that water plays a key role in the additional
oxidation pathway. It is likely that the water generated in the
oxidation of Ia may cause the hydrolysis of the substrate to
benzaldehyde and 2-aminophenol. We have tested the oxidation
of benzaldehyde and 2-aminophenol in the presence of 1.
The result indicates that benzaldehyde is not oxidizable, but
2
-aminophenol can be oxidized to 2-aminophenoxazin-3-one
by consuming more than one half mol of O per mol of
2
12
reactant as reported. The details of this oxidation reaction
will be published elsewhere. Hence the additional oxidation
pathway, in the absence of dehydrating agent, is shown in
Scheme 1. In eqn. (4) the reaction yields more water which
catalyst
Ia
+
1/2 O2
IIa
+
H2O
(2)
(3)
NH2
CHO
Fig. 2 The oxygen pressure change in the oxidation of compound
H2O
+
Ia
+
Ia (0.50 M) in DMF at 363 K under various conditions: (a) no catalyst,
OH
(b) [Co L(µ-OH)] = 5.0 mM, (c) same as (b) with additional acetic
2
acid (0.10 M), (d) same as (b) with additional 2,6-di-tert-butylpyridine
(
0.10 M).
added to the reaction mixture (5.0 mmol Ia and 50 µmol 1 in
0 mL DMF) the rate of oxidation was greatly reduced (Fig. 2).
NH2
N
O
NH2
1
2
+
3/2 O2
+
3 H2O (4)
Judging from the initial rates of reaction, the presence of acetic
acid reduced the rate of the reaction by a factor of 10. The
presence of a non-co-ordinating base 2,6-di-tert-butylpyridine
induced only a small increase in the rate of the oxidation reac-
tion (Fig. 2).
OH
O
Scheme 1
causes more Ia to undergo hydrolysis. In support of this
mechanism, benzaldehyde was found chromatographically as a
product of the oxidation of Ia in the absence of dehydrating
agent.
Compound If was not tested for the presence of the
additional oxidation pathway because the isolated yield for
IIf was already high in the absence of dehydration agent.
This result suggests that the additional oxidation pathway
analogous to that in Scheme 1 is not important. This is further
The kinetics of the catalytic oxidation of compound Ia was
investigated at 363 K. In Fig. 3(a) the initial rate of oxidation
depends linearly on catalyst concentration in the range
Ϫ3
Ϫ3
0
.474 × 10 to 7.5 × 10 M at an O pressure of 90.7 kPa and
2
Ia concentration 0.5 M. The dependence of initial rate on the
concentration of Ia under the conditions of fixed initial O2
Ϫ3
pressure 90.7 kPa and [1] = 5.1 × 10 M is shown in Fig. 3(b).
At low concentration of Ia the initial rate depends linearly on
[
Ia]. However for [Ia] > 0.2 M the initial rate levels off. This
evidenced from the amount of O consumed (2.4 mmol), nearly
half that of If (5.0 mmol) oxidized at the completion of the
oxidation reaction as shown in Fig. 1(f).
2
dependence indicates that the co-ordination of Ia to the catalyst
must be involved in the reaction mechanism. In Fig. 3(c), the
initial reaction rate depends linearly on O pressure and shows
2
In the presence of dehydration agent this catalysed oxidation
method gives a yield of benzoxazole or its derivatives that is
equal or better than any other oxidation method in the
Ϫ3
slight saturation behavior at [Ia] = 0.5 M and [1] = 5.1 × 10
M. This leveling off indicates that O co-ordination to catalyst
2
2
8–33
must also be involved in the reaction mechanism. From these
literature.
Based on the isolated yields, the turnover num-
dependencies of initial rates on [1], [Ia], and P , the rate law (5)
O
2
bers of the dicobalt complex 1 for the oxidation of compound
Ia in the absence and presence of dehydrating agent are 67 and
8
7, respectively. We would point out that the C᎐N bonds in 1
rate = d[Ia]/dt = b[1][Ia]P /(1 ϩ a[Ia]P )
(5)
᎐
O
2
O
2
38
might be susceptible to hydrolysis. However, there is no sign
based on UV-Vis spectroscopy) of the occurrence of this
hydrolysis in either dry or wet DMF at 363 K over a period of
4 h. This stability against hydrolysis may be attributed to the
(
is initially proposed. According to this rate law the initial rate
depends linearly on [1], and also linearly on [Ia] and PO2 when
these two parameters are small, and a saturation behavior is
expected when either [Ia] or PO2 is large. These dependencies are
qualitatively consistent with the experimental observations.
The solid curve in Fig. 3(b) is the result of the non-linear
least-square curve-fitting analysis of the initial rates vs. [Ia]
according to the rate law (5) with the parameters b =
2
conjunction of C᎐N bonds with the phenyl rings. Since there is
no sign of decay of this catalyst, the turnover number should be
much more than 100.
᎐
When a small amount of galvinoxyl radical [galvinoxyl = 2,6-
di-tert-butyl-α-(3,5-di-tert-butyl-4-oxo-2,5-cyclohexadien-1-yl-
idene)-p-tolyloxy] and 2,6-di-tert-butyl-4-methylphenol, which
are a known radical scavenger and a chain inhibitor, were added
separately to the reaction mixture the oxidation rates were prac-
tically the same. This indicates that the reaction does not go
through a free radical chain reaction pathway. In fact, the
absence of an induction period in Fig. 1 is also consistent with
a non-radical reaction pathway. This is in contrast with the
radical mechanism when the reaction is catalysed by copper()
Ϫ3
Ϫ1
Ϫ1 Ϫ1
Ϫ1
1
.79(±0.45) × 10 kPa
M
s
and a = 0.178(±0.054) M
Ϫ1
kPa . The solid curves in Fig. 3(a) and 3(c) are calculated
from the rate law (5) with these values of a and b. It is obvious
that the fit to the data in Fig. 3(c) is unacceptably poor. To
account for the weak dependence of the initial oxidation rate
on dioxygen pressure we propose a modified rate law (6). In this
33
salt. When a small amount of acetic acid (1.0 mmol) was
rate = d[Ia]/dt = b[1][Ia]P /(1 ϩ a[Ia](1 ϩ mP )) (6)
O
2
O
2
J. Chem. Soc., Dalton Trans., 1999, 2769–2776
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Fig. 3 Kinetics of the catalysed oxidation of compound Ia in DMF at 363 K: (a) initial rate as a function of [Co L(µ-OH)], [Ia] = 0.50 M, P = 90.7
2
O
2
kPa; (b) initial rate as a function of [Ia], [Co L(µ-OH)] = 5.0 mM, P = 90.7 kPa; (c) initial rate as a function of P , [Co L(µ-OH)] = 5.0 mM,
2
O
2
O
2
2
[
Ia] = 0.50 M. The solid curves are calculated based on the rate law (5) and parameters obtained from Fig. 3(b) by non-linear least-squares
curve-fitting analysis. The dashed curves are calculated based on the rate law (6) and parameters obtained from Fig. 3(b) and 3(c) by non-linear
least-squares curve-fitting analysis.
proposed rate law both the linear dependence of the initial rate
in Scheme 2 yields a rate law (10) consistent with the experi-
mental rate law (6).
on PO2 and slight saturation can be accounted for by the new
Ϫ3
Ϫ1
Ϫ1
parameter m. With a fixed value of b = 1.79 × 10 kPa
s
M
Ϫ1
which was obtained from fitting data in Fig. 3(b) using eqn.
d[Ia]/dt = kK K [1][Ia]P /(1 ϩ K [Ia](1 ϩ K P )) (10)
1
2
O
2
1
2
O
2
(
5), a non-linear least-squares fitting to the data in Fig. 3(c)
Ϫ1
yields the parameters a = 11.7(±0.9) M and m = 4.3(±1.5) ×
1
Ϫ3
Ϫ1
0
kPa . The dashed curves in Fig. 3 are calculated based on
Using the values of a, b and m deduced from the data in
Fig. 3 according to the rate law (6), k, K and K are calculated
the rate law (6) and the a, b and m parameters given above. The
agreement with experimental data is exceedingly good.
According to the rate law (6) the mechanism shown in
Scheme 2 is proposed. In eqn. (7), the organic substrate binds to
1
2
Ϫ2 Ϫ1
Ϫ1
Ϫ3
to be 3.6(±2.1) × 10 s , 11.7(±0.9) M and 4.3(±1.5) × 10
kPa , respectively at 363 K. The magnitude of the equili-
Ϫ1
brium constants K1 and K2 provides a clue to answer the
question why O can co-ordinate to III, eqn. (8), but there is no
2
indication of the existence of the O –1 adduct in the rate law.
2
Ia
Let’s make the reasonable assumption that O co-ordinates to 1
K1
2
(7)
Co(II) Co(II)
+
+
Ia
Co(II) Co(II)
with an equilibrium constant approximately equal to K , then
2
Ϫ2
[
O –1]/[III] is 7.4 × 10 under the conditions [Ia] = 0.5 M and
2
1
III
P 2 = 100 kPa. This small value indicates that [O –1] is neg-
O 2
ligible. If O also binds to 1 with a large equilibrium constant,
O
2
Ia
Ia
O
then one would expect an extra term related to PO2 in the
denominator of eqn. (6). The absence of this term shows that
our rate law and the parameters are consistent with each other
in this regard.
To confirm the existence of complex III in eqn. (7) this
equilibrium reaction was investigated by UV-Vis spectroscopy
(Fig. 4a) at 298 K. At fixed concentration of 1, the absorbance
in the 420–500 nm range increased with the concentration of Ia
K2
Co(II) Co(II)
O2
Co(II) Co(III)
(8)
(9)
III
IV
IV
k
IIa
rds
Scheme 2
(
(
spectra 1–5). Since substrate Ia did not absorb in this region
spectrum 6), the variations in absorbance were attributed to
the dicobalt complex 1 to form adduct III with an equilibrium
the formation of III. The absorbances at 438 nm were further
analysed quantitatively. After straightforward mathematical
analysis, the relation (11) between absorbance A and concen-
constant K . Dioxygen then co-ordinates to III to form adduct
1
IV in eqn. (8) with an equilibrium constant K . The oxidation
2
state of the cobalt co-ordinated by O increases from ϩ2 to
2
Ϫ1
ϩ3 and the co-ordinated O becomes negatively charged. This
2
A = ε lC ϩ (1/2)(ε l Ϫ ε l ){(C ϩ L ϩ K ) Ϫ
1
0
III
1
0
0
1
oxidation state change is well documented both in the artificial
Ϫ1
2
1/2
3
9,40
[
(C ϩ L ϩ K ) Ϫ 4C L ] }
(11)
oxygen transport systems of cobalt complexes
and in heme
0
0
1
0
0
39,40
systems related to O transport.
In eqns. (7) and (8), simpli-
2
fied notations for 1, III and IV were used. Co-ordination of
DMF solvent to some of the cobalt ions in these species must
occur. Since the exact picture of solvent co-ordination is not
known we adopt these simplified notations for 1, III, IV and
other species in the following paragraphs just for convenience
without any implication that they represent the exact co-
ordination spheres of the cobalt centers. In eqn. (9) the co-
ordinated Ia in IV transfers two protons and two electrons to
the co-ordinated dioxygen, and transforms into product IIa.
What happens after the rate determining transfer of electrons
and protons may be very complicated. Several questions,
such as does the O–O bond break or does H O form and how
tration of Ia was obtained, where l is the cell pathlength. ε and
1
εIII are the molar absorptivities of cobalt complex 1 and III,
respectively, C and L are the initial concentrations of 1 and Ia,
0
0
respectively, and K is the equilibrium constant in eqn. (7). The
1
analysis by the non-linear least-squares fitting of ∆A
(A Ϫ ε lC ) vs. concentration of substrate Ia (Fig. 4b) yielded
1
0
3
Ϫ1
3
Ϫ1
K = 1.5(±0.3) × 10
cm from the data at 438 nm. The large standard deviation
M
and εIII Ϫ ε = 2.0(±1.4) × 10 M
1
1
Ϫ1
in K is due to the similarity of the spectra of 1 and III in
1
the region which is suitable for quantitative analysis. Further-
more, the small difference in molar absorptivity between 1
4
4
Ϫ1
Ϫ1
(1.47 × 10 ) and III (1.67(±0.14) × 10
M
cm ) prevents
2
2
the catalyst returns to its original state, may be raised. No
matter what are the answers to these questions, the mechanism
us from obtaining large values of A Ϫ ε lC0 for accurate
numerical analysis. Though there are limitations in the
1
2
772
J. Chem. Soc., Dalton Trans., 1999, 2769–2776

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effect of O binding in eqn. (8) by UV-Vis spectroscopy is not
2
unique. For example, we observed that the UV-Vis spectrum
of 1 in DMF in the 420–500 nm range was independent of
the amount of PPh added to the solution, though PPh would
3
3
co-ordinate to the cobalt ions in 1 (details will be presented
elsewhere).
EPR Spectroscopy has been widely used to characterize the
4
1
binding of O to some cobalt() complexes. No EPR signals
2
Ϫ2
were detected for a sample containing 1 (≈10 M) and Ia
≈10 M) under O which was then frozen to 77 K. This
Ϫ2
(
2
negative EPR result does not rule out the existence of IV.
This is because if IV exists one may expect some EPR signal
III
Ϫ
from the Co –O2 moiety. However, the presence of a nearby
paramagnetic cobalt() ion will render the expected EPR signal
too broad to be detected. Therefore, EPR spectroscopy can
not be used to characterize the binding of O to the dicobalt
2
complex 1. Therefore, the presence of this reasonable complex
IV is based mainly on kinetic data.
We propose two possible pathways for the transformation of
O2 and catalyst in eqn. (9). In the first, after the transfer
of protons and electrons from co-ordinated Ia, the O–O bond
is cleaved to yield water as in eqn. (12). The other oxygen is
proposed to be still co-ordinated to 1 to form an intermediate
V. This intermediate, if it exists, is likely to be the same as that
36
generated in the catalysed O oxidation of PPh . The inter-
2
3
mediate V then oxidizes another molecule of Ia to form IIa
and the catalyst returns to the original state as in eqn. (13).
k
IV
V
V
+
H2O
+
IIa
(12)
(13)
rds
Ϫ4
fast
Fig. 4 (a) UV-Vis spectra of catalyst 1 (2.68 × 10 M) and Ia ((1) 0.0,
+
Ia
Co(II) Co(II) + H2O IIa
Ϫ4
Ϫ3
Ϫ3
Ϫ3
(
2) 5.41 × 10 , (3) 1.18 × 10 , (4) 1.74 × 10 , (5) 2.50 × 10 M) in
DMF under N2 in a 0.100 cm cuvette at 298 K. Spectrum (6): Ia
Ϫ4
(
8.38 × 10 M) in DMF without 1. Inset: spectra magnified in the
According to this mechanism the k in rate law (10) should be
range 390–500 nm. (b) Analysis of the absorbance difference as a func-
tion of [Ia] according to eqn. (11) using a non-linear least-squares
curve-fitting method.
Ϫ2 Ϫ1
2
k. The value of k is 1.8(±1.0) × 10 s .
In the second proposed mechanism H O is produced and the
2
2
catalyst returns to its original state as in eqn. (14). We have
precision of K , the presence of the equilibrium relation (7)
1
k
is unequivocal. Comparing with the K value of 11.7(±0.9)
1
IV
Co(II) Co(II)
+
2
H O
2
+
IIa
(14)
Ϫ1
0
rds
M
deduced from the kinetic data at 363 K, ∆H is estimated
Ϫ1 1
to be Ϫ67.1(±2.9) kJ mol . We stated before that the H NMR
observed that the dicobalt complex catalyst 1 can function as a
spectrum of Ia and 1 in d -DMF indicates no new compound
7
very effective catalase, i.e. it converts H O into H O and O as
is formed. This seems to contradict the fact that III exists.
2
2
2
2
1
in eqn. (15).
Actually there is no contradiction because the H NMR
spectrum of 1 and III can not be detected since cobalt() ions
in 1 as well as in III are paramagnetic.
An attempt to study the equilibrium (8) by UV-Vis spec-
troscopy was also carried out. We observed that the spectrum
fast
H O2
H O ϩ ¹O
2
2
(15)
2
1
¯²
A competition experiment with an additional substrate
besides Ia can be used to distinguish these two pathways. We
chose trans-stilbene, which can’t be oxidized directly under
the reaction conditions for the oxidation of Ia, and it can be
co-oxygenated to trans-stilbene oxide with PPh3. If stilbene
can’t be co-oxidized, then the pathway with the reactive inter-
mediate V is highly unlikely. This argument is valid only when V
can oxidize Ia with a rate constant comparable or less than that
with stilbene. This condition can easily be established with a
concentration of stilbene higher than that of Ia. In view of the
expected complicated elementary reactions in the hypothetical
reaction between V and Ia, it is reasonable to assume that
of complex 1 was indistinguishable whether under O (1 atm)
2
or N . Furthermore, the spectra of 1 and Ia (0.05 M) were
2
insensitive to the presence of O . Hence we suspected that UV-
2
36
Vis spectroscopy could not furnish additional data concerning
O binding in eqn. (8). Nevertheless, we still carried out system-
2
atic investigations following the procedure used to study the
equilibrium under N in the hope that there is significant dif-
2
ference in K that may allow us to characterize the effect of
1
Ϫ4
O . From a reaction solution containing 1 (2.68 × 10 M)
and Ia (0–3.2 × 10 M) at 298 K under air we obtained
K = 1.8(±1.0) × 10 M . Under O the experimental data
yielded K = 2.9(±0.7) × 10 M . For the air and O systems,
the differences in molar absorptivity are 1.5(±0.2) × 10 and
.4(±0.1) × 10 M cm , respectively. Indeed, these K values
2
Ϫ3
3
Ϫ1
1
2
3
Ϫ1
this rate constant is smaller than that between V and PPh . We
1
2
3
3
carried out the following experiment. In a reaction flask
containing 10 mL DMF solution of 1 (50 µmol), Ia (5.0 mmol),
4 Å molecular sieves (1.0 g) and stilbene (10 mmol) under 92.0
3
Ϫ1
Ϫ1
1
1
are equal to that under nitrogen within experimental error. It is
impossible to extract any reliable information on K . Therefore,
kPa of O pressure at 363 K (conditions similar to that used
2
2
we were unable to characterize the binding of O in eqn. (8) in
in eqn. (1)), the amount of O consumed is independent of
2
2
more detail. This is probably because of the small equilibrium
the amount of stilbene (0, 1.0 and 2.0 M) present. This co-
oxygenation experiment certainly indicates that a reactive inter-
mediate which is capable of oxidizing a large excess of stilbene
constant K and the small differences in the molar absorptivities
2
of 1, III and their dioxygen adducts. This failure to observe the
J. Chem. Soc., Dalton Trans., 1999, 2769–2776
2773

View Article Online
was not produced in the oxidation of Ia, and the pathway with
ordinated hydrogen superoxide. The small dissociation constant
of H O further supports our viewpoint concerning the sig-
the formation of H O is more likely.
2
2
2
2
The structure of intermediate IV is not certain. We propose
a likely structure and its transformation in Scheme 3. The
nificance of pathway IV → VI → VIII in Scheme 3. The
question why the rate of oxidation of Ib is particularly slow can
also be answered based on Scheme 3. The electron withdrawing
chloro group in Ib reduces the nucleophilicity of the phenoxide
oxygen in VI, and disfavors the formation of VIII in the equi-
librium between VI and VIII. Furthermore, the chloro group
can also reduce the effective negative charge on the nitrogen of
VIII and impede the subsequent electron transfer. Both factors
render the rate of oxidation slow.
Co(III) Co(II)
O
Co(III) Co(II)
O
–O
Ph
Ph
HO
N
C
N
C
O–
OH
Since the substituents on benzylidenes in compounds Id
and Ie are far away from the co-ordinated nitrogen in VIII,
these were anticipated to have minimum effect on the rate
of oxidation. Indeed, this is confirmed experimentally, the rates
of oxidation of Id and Ie being similar to that of Ia. How-
ever, when IX was used as a substrate it did not oxidize to the
IV
VI
Co(III) Co(II)
O
Co(III) Co(II)
O
corresponding benzoxazole under the standard conditions that
Ϫ3
–O
H
H
[1] = 5.0 × 10 M, P 2
O
= 101 kPa at 363 K.
HO
N–
H
N
Ph
Ph
CH3
O
VII
VIII
Scheme 3
N
C
OH
CHO
reactant Ia can exist in cis and trans geometries with trans being
OH
34
more stable. The co-ordinated Ia in IV may lose a proton to
the co-ordinated superoxide to form VI. Further cyclization of
the organic moiety in VI forms VIII. Alternatively, Ia in IV may
also first cyclize to VII, then the proton of the co-ordinated
benzoxazoline may be transferred to the co-ordinated super-
oxide to form VIII. There is no direct evidence in favor of
any species as the key intermediate in Scheme 3. However,
it is known that benzoxazoline is not stable, hence VII may
play only a minor role. Also, when Ia in IV loses the phenolic
proton, the enhanced nucleophicity of phenoxide in VI should
facilitate the cyclization reaction for form VIII. This reasoning
indicates that the pathway IV → VI → VIII may be the
major transformation pathway of the organic substrate in the
initial stage. The oxazoline anion in VIII then transfers two
electrons and one proton to the co-ordinated superoxide to give
the product. In VIII the protonated superoxide may facilitate
electron transfer from the co-ordinated benzoxazoline anion on
the basis of charge considerations.
IX
The explanation is that the strong electron donating effect of
the hydroxy group in the benzylidene renders the carbon atom
unfavorable to attack by the nucleophile in the hydroxyaniline
moiety, and the corresponding species VIII are not formed. The
formation of species such as VIII is also sterically hindered.
Furthermore, the ionization of the additional phenolic proton
may also impede the formation of intermediate VI. Therefore,
no oxidation product was isolated.
This non-reactivity of compound IX may be interpreted
as indicating that the oxidation mechanism is non-radical.
Suppose the mechanism is related to the formation of a
phenoxyl radical, then one expects to detect typical oxidation
products such as derivatives of benzoquinone or diphenyl-
8–10
quinone.
These quinone products are not detected in the
oxidation of Ia–If. Therefore, a mechanism involving an un-
co-ordinated phenoxyl radical is highly unlikely in this catalytic
system.
The reason that compound If can react directly with O in the
2
absence of catalyst is probably that it exists in the form of
34
benzothiazoline which is more stable than the structure
depicted in eqn. (1). Therefore, it is reasonable to expect that the
formation of oxazoline assisted by the dicobalt catalyst is an
important aspect in this catalysed oxidation reaction. This
point has been taken into account in Scheme 3. The enhanced
oxidation rate of If in the presence of dicobalt catalyst may
be due to the formation of a species such as VIII. The co-
Conclusion
Dicobalt complex 1 was shown to be an effective catalyst in the
oxidation of derivatives of N-benzylidene-2-hydroxyaniline to
the corresponding 2-substituted benzoxazoles. These oxidation
reactions were carried out using dioxygen as oxidant and under
mild conditions. The product yields are about the best ever
reported for the preparation of 2-substituted benzoxazoles by
the oxidation processes. The high yields were achieved when a
dehydrating agent was present. Its function is to prevent
hydrolysis of the reactant. This environmentally friendly high
yield oxidation process may be of practical importance because
derivatives of benzoxazole have been patented as a useful
ordinated O H is more susceptible to accept an electron from
2
the co-ordinated benzothiazoline anion which is more electron
rich than its neutral form. Therefore, with the proposed
mechanism, the accelerated oxidation of If in the presence of
the dicobalt catalyst 1 can be rationalized.
In the presence of acetic acid the superoxide in IV may be
protonated, and the formation of VI is then hindered. Another
possible effect of the acid is that the negatively charged nitrogen
in VIII may be protonated and therefore the subsequent trans-
fer of an electron from the co-ordinated benzoxazoline to the
co-ordinated superoxide may be suppressed. Hence the rate
of oxidation is reduced. A weak base, such as 2,6-di-tert-
butylpyridine, exerts negligible effect on VI and VIII because
it can not extract the proton that has been bonded to the super-
43
component of organic light emitting diode (OLEDs). The
kinetics of this dioxygen reaction can be studied thoroughly.
The catalysed reaction undergoes a non-radical pathway
because the presence of radical scavenger had a negligible effect
on the reaction rate. The rate law indicates that the dicobalt
catalyst 1 serves the purpose of binding to the organic substrate
and dioxygen simultaneously to initiate the oxidation reaction.
There is no evidence to indicate that the co-ordinated dioxygen
undergoes O–O bond cleavage to yield water after accepting
two protons and two electrons from the co-ordinated organic
42
oxide. This is clear since the acid dissociation constant of
Ϫ6
pyridinium ion (5.9 × 10 ) is 6 orders larger than that of H O
(
2
2
Ϫ12
2.2 × 10 ) which should be a good model for the co-
2
774 J. Chem. Soc., Dalton Trans., 1999, 2769–2776

View Article Online
substrate. Instead, a more likely transformation is that hydro-
gen peroxide is produced. This is then decomposed into water
and oxygen by 1 which possesses catalase activity. The trans-
formation of the co-ordinated dioxygen induced by the co-
ordinated N-benzylidine-2-hydroxyaniline is in direct contrast
with that induced in the oxidation of PPh . In PPh oxidation,
Na SO or 4 Å molecular sieves used was 1.0 g in each run.
2 4
Products IIa, IIc–IIe were isolated by extraction instead of
column separation. After DMF solvent was removed from the
reaction mixture by distillation under reduced pressure at 313
K, diethyl ether was used to extract the benzoxazole product.
Then the product was obtained after diethyl ether was removed
and the solid dried in a vacuum oven.
3
3
after Ph P᎐O is formed, a reactive intermediate, which was not
3
formed in the oxidation of N-benzylidine-2-hydroxyaniline, is
formed as mentioned in the Introduction. The different trans-
formation pathways of co-ordinated dioxygen demonstrate the
complexity of oxidation by dioxygen. Further extension of this
work to other organic substrates is underway.
The kinetics of the oxidation of compound Ia was estab-
lished by the initial rate method. The non-linear least-squares
curve-fitting analyses of the kinetic data according to the rate
laws (5) and (6) and fitting of UV-Vis data using eqn. (11) were
accomplished by using the commercially available program
Origin, Version 5.0 (Microcal Software Inc., Northampton,
MA, USA).
Experimental
Synthesis of N-benzylidene-2-hydroxyaniline and its derivatives
Ia–If
Acknowledgements
We thank the National Science Council of Taiwan ROC for
financial support (NSC 87-2113-M-007-029).
These compounds were synthesized following the same pro-
cedure. Using N-benzylidene-2-hydroxyaniline as an example, a
3
0 mL methanol solution of 2-hydroxyaniline (0.05 mol) was
added slowly to a 15 mL methanol solution of benzaldehyde
0.05 mol). After refluxing for 3 h and subsequent cooling, the
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recrystallization using diethyl ether as solvent. The compounds
Ia–If were characterized by mass, H NMR spectra and melting
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2
36
The preparation of this catalyst follows the literature method.
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(
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J. Chem. Soc., Dalton Trans., 1999, 2769–2776
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Paper 9/03699G
2
776
J. Chem. Soc., Dalton Trans., 1999, 2769–2776