Iron-catalyzed synthesis of benzoxazoles by oxidative coupling/cyclization of phenol derivatives with benzoyl aldehyde oximes

Article abstract of DOI:10.1039/c7cc04965j

An iron-catalyzed oxidative coupling/cyclization reaction for the synthesis of benzoxazoles at room temperature is reported. This reaction was enabled by an inexpensive iron(iii) catalyst by treating readily available phenol derivatives with benzoyl aldehyde oximes. Mechanistic studies show that benzoyl aldehyde oxime is not only used as a substrate, but also serves as an ancillary ligand to support the iron salt in the promotion of the transformation.

Full text of DOI:10.1039/c7cc04965j

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Iron-Catalyzed Synthesis of Benzoxazoles by Oxidative
Coupling/Cyclization of Phenol Derivatives with Benzoyl Aldehyde
Oximes
Received 00th January 20xx,
Accepted 00th January 20xx
Sen Gao,a Liming Gao,b Hong Meng,a Meiming Luo*,a and Xiaoming Zeng*,a,c
DOI: 10.1039/x0xx00000x
www.rsc.org/
It was reported an iron-catalyzed oxidative coupling/cyclization
reaction for the synthesis of benzoxazoles at room temperature.
This reaction was enabled by an inexpensive iron(III) catalyst by
treating readily available phenol derivatives with benzoyl
aldehyde oximes. Mechanistic studies show that benzoyl aldehyde
oxime is not only used as substrate, but also serves as ancillary
ligand to support iron salt in the promotion of the transformation.
Functional benzoxazole derivatives are appealing motifs found
in a wide range of bioactive and pharmacologically interesting
molecules such as antimycobacterial pseudopteroxazole, HIV
reverse transcriptase inhibitor L-697,661, estrogen receptor-β
agonist ERB-041, anticancer agent of NSC-693638, and even in
fluorescent materials (Figure 1).1,2 Traditional methodologies
that are used to construct benzoxazole scaffolds include the
annulation of ortho-halide-substituted N-phenylamides
(Scheme 1, path a).3,4 The reactions using 2-aminophenols and
2-halide-phenols as precursors provide alternative strategies in
the preparation of benzoxazole compounds (paths b and c).5,6
Scheme 1 Methodologies for the construction of benzoxazole motifs.
In contrast, simple and greater availability of phenols has
rarely been employed as building blocks to construct
benzoxazole frameworks. In the presence of oxidants, phenols
can transform to cyclohexa-3,5-diene-1,2-diones via copper
catalysis, which react with primary amines allowing for the
formation of benzoxazole motifs (path d).7–10 Herein, we
report an efficient iron-catalyzed oxidative coupling/cyclization
reaction of phenol derivatives with benzoyl aldehyde oximes
(path e).11 This protocol was enabled by simple and
inexpensive iron(III) catalyst, and provides an operationally
simple method to form benzoxazole derivatives at room
temperature.
At the outset, we envisioned that the treatment of phenols
with transition metal salt may give the radical species
A in the
presence of oxidant,12 which allows for undergoing a process
of coupling/cyclization with aldehyde oximes in the formation
of benzoxazole scaffolds (Scheme 2). To test our hypothesis, 2-
naphthol (1a) was chosen as precursor to treat with benzoyl
aldehyde oxime (2a) (Table 1). In the presence of copper salts
Figure 1 Biologically interesting molecules containing benzoxazole scaffolds.
a.Key Laboratory of Green Chemistry & Technology of Ministry of Education, College
of Chemistry, Sichuan University, 29 Wangjiang Road, Chengdu 610064, P. R.
China. E-mail: luomm@scu.edu.cn; zengxiaoming@scu.edu.cn
b.Institute for the Control of Agrochemicals of Sichuan Province Chengdu, Sichuan,
610041, P. R. China
c. Frontier Institute of Science and Technology, Xi’an Jiaotong University, Xi’an
710054, P. R. China
Electronic Supplementary Information (ESI) available: details of any supplementary
information available should be included here. See DOI: 10.1039/x0xx00000x
Scheme 2 Hypothesis for the oxidative coupling/cyclization reaction of phenols
with aldehyde oximes.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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bis(oxazole) compound 3k was successful via two oxidative
coupling/cyclization of naphthalene-2,3-diol. The reaction with
substituted phenols proceeded effectively, giving benzoxazole
Table 1 Optimization of reaction conditionsa
compounds 3l
substituents in benzaldehyde O-benzoyl oxime has no
influence on the conversion (3r 3af). Functional groups such
–q in moderate to good yields. Variation of the
–
Entry
Metal salt
(mol %)
2a
(equiv)
Solvent
Time (h)
3a (%)b
as fluoride, trifluoromethyl, chloride, bromide and hydroxyl
are well tolerated by the reaction system. Furthermore,
alkenyl, thienyl, furyl and alkyl substituents can be facilely
1
CuCl (10)
2
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
DCM
8
trace
trace
20
2
CuCl2 (10)
FeCl2 (10)
FeCl3 (10)
Fe(NO3)3 (10)
Fe2(SO4)3 (10)
Fe(ClO4)3 (10)
FeCl3 (10)
FeCl3 (10)
FeCl3 (10)
FeCl3 (10)
FeCl3 (10)
FeCl3 (30)
FeCl3 (30)
FeCl3 (10)
FeCl3 (10)
none
2
8
incorporated into the C2 position of oxazole scaffolds (3ah
3an).
–
3
2
8
4
2
8
35
5
2
8
25
Table 2 Iron-catalyzed synthesis of substituted benzoxazole derivativesa
6
2
8
17
7
8c
2
8
8
2
8
30
nde
9d
10
11
12
13
14f
15g
16h
17
18
19
20
2
8
3
8
45
3.5
3.5
3.5
3.5
2
8
51
12
12
11
8
67
78
91
30
2
8
28
2
8
nd
FeCl3 (10)
FeCl3 (10)
FeCl3 (10)
2
8
30
2
THF
8
trace
2
DMF
8
trace
aThe reaction was performed on 0.4 mmol scale at room temperature using
2a under the atmosphere of argon. bIsolated yields are given. cIn the presence
of 1 equivalent of trifluoroacetic acid. dIn the presence of 1 equivalent of
triethylamine. eNot detected. fThe reaction was performed under dioxygen.
g2b was employed. h2c was employed.
(10 mol %), only trace amount of the benzoxazole derivative
3a was formed (entries 1 and 2). Compared with other iron
salts, the use of iron(III) chloride in the reaction gave relatively
good result, leading to 3a in 35% yield (entry 4). The use of
trifluoroacetic acid as additive cannot promote the
transformation (entry 8). While the reaction was completely
inhibited in the presence of triethylamine triethylamine (entry
9). The increase of the amount of FeCl3 and 2a with an
extended reaction time resulted in high conversion of 1a
(entry 13). Excellent yield (91%) of 3a was obtained when
performing the reaction under the atmosphere of dioxygen
(entry 14). The replacement of benzyl group by acetyl and
ethoxycarbonyl in aldehyde oxime precursors led to relatively
low conversions (entries 15 and 16). Further studies showed
that the reaction media had an effect on the transformation,
the best result was obtained in toluene (entries 18–20). Noted
that the transformation proceeded in high regioselectivity, and
aReaction Conditions:
atm), toluene, rt. b1.6 mmol of
1
(0.4 mmol),
2
2
(1.4 mmol), FeCl3 (0.12 mmol), O2 (1
was employed.
was employed. c2 mmol of
2
the regioisomeric isoxazole product
cases.
4
was not formed in these
To gain mechanistic insight into the possible reaction
pathway, the influence of iron(III) chloride, benzoyl aldehyde
With the optimized conditions in hand, we next explored oxime, and dioxygen on the oxidative transformation was then
the scope of the iron-catalyzed reaction for the preparation of probed by controlled oxidation of 2-
structurally diverse benzoxazole derivatives. As shown in Table (benzylideneamino)phenol.13 In the absence of FeCl3 or 2a, it
2, the incorporation of electron-donating substituents such as was found that the desired benzoxazole product 3p was not
alkyl and alkoxy into the scaffolds of 2-naphthols has no effect formed, suggesting that iron salt and benzoyl aldehyde oxime
on the transformation, giving the annulated oxazole products play important roles in promoting the oxidation of 2-
3b
deficient 2-naphthols led to relatively low conversions (3e
Interestingly, the application of this method in the synthesis of atmosphere of argon rather than dioxygen, relatively low yield
–
3d in good yields. In contrast, the reactions with electron- (benzylideneamino)phenol
5
to 2-phenylbenzo[d]oxazole
–
3g). (Scheme 3).14 When performing the reaction under the
2 | J. Name., 2012, 00, 1-3
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Based on the preliminary mechanistic results, we proposed
that benzoyl aldehyde oxime-ligated iron complex can be
initially formed, which chelates with phenol in the formation
of active iron species
(Scheme 4).12,15 Subsequently, a single-
B
FeCl3
2a/O2
OH
N
O
O
Ph
Ph
C
rt, 12 h
N
H
N
Ph
electron transfer (SET) may proceed to give the cyclohexa-2,4-
dien-1-one radical species D16 with cleavage of N-O bond while
the release of benzoic acid, followed by attacking the imino
oxidation
3p
5
6
FeCl3
2a
Atmosphere
Yield (3p)
moiety17 to give the intermediate
the iron scaffold, (benzylideneamino)phenol
afforded by isomerization, which is proposed to be in
E
. After the dissociation of
can be
none
3.5 equiv
none
O2 (1 atm)
O2 (1 atm)
Argon
nd
(5)
30 mol%
30 mol%
30 mol%
30 mol%
nd
5e-5h,13,14
3.5 equiv
1.5 equiv
1.5 equiv
73%
51%
79%
equilibrium with benzoxazoline (
6)
and converts into
Argon
benzoxazole (3p) in the presence of iron salt and 2a
.
In summary, we have developed a cost-effective iron-
catalyzed synthetic protocol for the buildup of functionalized
benzoxazole molecules. This reaction was enabled by an
inexpensive iron(III) salt using simple and widely accessible
O2 (1 atm)
Scheme 3 Studying the influence of FeCl3, 2a and molecular dioxygen on the
transformation of benzoxazoline to benzoxazole.
of 3p was obtained (51% versus 79%). This result suggests that phenol derivatives as precursors to treat with benzoyl
dioxygen accelerates the oxidative conversion. We observed aldehyde oximes, and can be performed at room temperature
that FeCl3 did not dissolve in toluene at room temperature to form structurally diverse benzoxazole derivatives as single
even in the presence of phenols. However, upon addition of regioisomers in moderate to excellent yields. Detailed
oxime ester, the black precipitate of FeCl3 was dissolved mechanistic studies and application of this methodology in
immediately in toluene to afford a deep brown-red solution, preparation of functional molecules are underway in our
suggesting that a coordinated complex B might be formed. laboratory.
Moreover, in situ analysis of the reaction mixture of FeCl3 with
2a in toluene by electrospray ionization high-resolution mass
Acknowledgements
spectrum (ESI HRMS) technique, a major ion peak at 350.9528
m/z in the spectrum that corresponds to the [
–Cl]+ species
B
We gratefully acknowledge the National Natural Science
Foundation of China (Nos. 21321061, 21202128, 21572175
21072134, and J1103315/J0104) for financial support, and the
Comprehensive Training Platform of Specialized Laboratory,
College of Chemistry, Sichuan University, for NMR and HRMS
measurements.
(calculated m/z of 350.9516) was observed, indicating that
benzoyl aldehyde oxime may serve as ancillary ligand to
chelate with iron salt in forming catalytically active species to
promote the transformation (Scheme 4).
Ph
O
N
Cl
OH
Cl
Fe Cl
O
H
O
H
PhCOOH
Notes and references
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Fe
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