Metal-free iodine(iii)-promoted synthesis of 2,5-diaryloxazoles

Article abstract of DOI:10.1039/c8ob00401c

A nonmetal-catalyzed oxidative cyclization to achieve 2,5-disubstituted oxazoles from inexpensive and readily available substituted chalcone, (diacetoxyiodo)benzene (PIDA) and ammonium acetate (NH₄OAc) at room temperature is described. The reaction forms a variety of 2,5-diaryloxazoles in good to excellent yields with broad substrate scope under mild conditions without the requirement of ligands and additional bases.

Full text of DOI:10.1039/c8ob00401c

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Takeharu Haino et al.
Solvent-induced emission of organogels based on tris(phenylisoxazolyl)
benzene
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Metal-Free Iodine(III)-Promoted Synthesis of 2,5-Diaryloxazoles
Xueying Yanga, Xin Guoa, Mingda Qina, Xinglong Yuana, Huanwang Jinga,*and Baohua Chena,b
*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A
nonmetal-catalyzed oxidative cyclization to achieve 2,5- The methods to synthesize the disubstituted oxazoles are
disubstituted oxazoles from inexpensive and readily available typically classified into intramolecular cyclization of amide
substituted chalcone, (diacetoxyiodo)benzene (PIDA) and precursors9, cycloaddition-type reaction using azides, amines
ammonium acetate (NH4OAc) at room temperature is described. or nitriles10,11, oxidation of oxazolines12, and coupling reaction
13
The reaction formed a variety of 2,5-diaryloxazoles in good to of
prefunctionalized
oxazoles
under
metal-
excellent yields with broad substrate scope under mild conditions catalyzed/mediated conditions9a-g,10,12a-d,13 or under metal-free
without the requirement of ligands and additional bases.
conditions, for which iodine reagents/catalysts are often
used.9h-j,11 However, high temperature, low atom-economical
transformation and transition-metals which are relatively
expensive and toxic often be considered in these reactions.
The oxazole motif is a significant class of five-membered
heterocycles in natural products1 which plays an important
role in both the improvement of life and the development of
industry.1e,2 Moreover, substituted oxazoles can be utilized in
natural products and medicinal chemistry,3 fluorescent dyes,4
and polymers5 and also as ligands for transition-metal
catalysis.6 In particular, 2,5-disubstituted and 2,4,5-
trisubstituted oxazoles are considered as the indispensable
synthetic intermediates7 and active pharmacophores3b,4a,8 in a
wide variety of nature product (Fig. 1). Accordingly, more
Therefore, developing
a more general, eco-friendly and
versatile method for the synthesis of the oxazole scaffold is
urgently desirable.
Chalcone is a readily available starting material thanks to its
either commercially sources or through laboratory
condensation of acetophenone and aldehyde. Cheng and
coworkers14 reported a copper-catalyzed oxidative cyclization
of benzylamine and chalcone via carbonecarbon double bond
Chengs' work
O
NH2
a
CuBr2, additive
O
+
K2CO3, solvent, 110 o
C
N
Mehdi Adibs' work
O
NH4OAc, AcOH
b
100 oC, solvent-free, 4 h
N
Our work
Figure 1. Biologically active molecules containing the oxazole structural motif
.
O
O
NH4OAc, PhI(OAc)2
solvent, rt
attention was paid to the synthesis of substituted oxazoles due
to their unique properties in the past decades.
c
N
Scheme 1. Synthetic approaches to N-heterocyclic aromatic compounds from
chalcone
cleavage to form 2,5-diaryl oxazole (Scheme 1, a). Mehdi Adibs’
group15 introduced a new facile synthesis of pyridines by
heating a mixture of chalcone and NH4OAc in the presence of a
catalytic amount of AcOH at 100°C (Scheme 1, b). Recently,
Saito’s9h,i,11a,b and Xiong’s9j groups reported metal-free
synthesis of oxazoles via the iodine-mediated/catalyzed
intramolecular cyclization or cycloaddition-type reaction at
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Table 1. Optimization of the Reaction Conditionsa
metal-free conditions and room temperature based on the
iodine(III)-promoted in considerable yield (Scheme 1, c).
We commenced our investigation using chalcone as the
model system to identify reaction conditions for this metal-
free intramolecular cyclization. We tested various nitrogen
sources, oxidants, additives, and solvents for this reaction and
the experiments were summarized in Table 1. We initiated our
study by examining the reaction of chalcone 1a, NH4OAc as
nitrogen source, and PIDA in DMSO under room temperature.
When the reaction was run without any additive, the desired
2a was observed in 13% yield (Table 1, entry 1). Then,
we found solvent played an important role in this one-pot
annulation, and much lower yields were obtained when the
reactions were carried out in toluene, chlorobenzene (Table 1,
entries 3-4). The reaction did not proceed at all in the presence
of DMF (Table 1, entry 2). To our delight, 67% yield was
obtained when TFE was used as the solvent (Table 1, entry 6),
while DCM also enhanced the reaction yield to 46% (Table 1,
entry 5). Because the yield was highly improved by using
fluorous solvent, we then used HFIP as solvent. To our surprise,
the yield increased to 86%.This result encouraged us to screen
a series of nitrogen sources for this transformation. When we
substituted NH4Cl and (NH4)2CO3 for NH4OAc, the reaction
yields decreased to 42% and 49%. Interestingly, the desired
product was generated in 86 % yield when increasing oxidant
loading of PIDA to 4 equiv (Table 1, entry 16). In contrast, this
reaction did not work well under too much oxidant loading of
5 equiv (Table 1, entry 17). Taking the temperature into
consideration, we attempted to start the reaction under 40 °C.
However, the desired product slightly decreased. On the basis
of these results, the optimized reaction conditions of 10 equiv
of NH4OAc and 4 equiv of PIDA in 0.5 mL HFIP at room
temperature were used for further investigation.
Entry
1
Nitrogen
source
Oxidant
(equiv)
PIDA
Additive
(equiv )
-
Solvent
(equiv)
DMSO
Yieldb
(%)
NH4OAc
13
2
NH4OAc
NH4OAc
NH4OAc
NH4OAc
NH4OAc
NH4OAc
NH4Cl
PIDA
PIDA
PIDA
PIDA
PIDA
PIDA
PIDA
-
DMF
ND
3
-
toluene trace
4
-
PhCI
DCM
TFE
trace
46
5
-
6
-
67
7
-
HFIP
HFIP
HFIP
HFIP
HFIP
HFIP
HFIP
71
8
-
42
9
(NH4)2CO3 PIDA
m-CPBA
-
49
10
11
12
13f
14g
15
16C
17d
18e
NH4OAc
NH4OAc
NH4OAc
NH4OAc
NH4OAc
NH4OAc
NH4OAc
NH4OAc
NH4OAc
PhI
ND
nd
ND
54
CH3COOOH PhI
TBHP
PIDA
PIDA
PIFA
PIDA
PIDA
PIDA
I2
Under the optimized conditions, the chalcone substrate
scope of the method was investigated (Table 2). Chalcones
containing both electron-donating and electron-withdrawing
substituents underwent cyclization to afford the
corresponding 2,5-diaryloxazoles in trace-to-excellent yields
(Table 2, 2a-2x). Chalcone derivatives with electron-donating
(4-Me, 4-OMe) groups on the aromatic ring proceeded in this
reaction smoothly to afford the desired products in good to
excellent yields. Generally, electron-withdrawing substituents
such as −NO2, −CN had deleterious effect on this reacꢀon, they
did not furnish the desired oxazoles. To our surprise, the −CF3
as electron-withdrawing substituent afforded the desired
product 2u in good yield. Halogen substituted chalcone
derivatives were also well tolerated in this annulation (Table 2,
2b-2d, 2j-2n), which provides the possibility to allow reactions,
respectively. Interestingly, 3-(4-fluorophenyl)-1-phenylprop-2-
en-1-one gave the corresponding product in 89% yield (Table 2,
K2CO3
CH3COOH HFIP
57
-
-
-
-
HFIP
HFIP
HFIP
HFIP
65
86
74
82
aReaction conditions: 1a (0.2 mmol), nitrogen source (2 mmol, 10 equiv), additive
(20 mol%), oxidant (0.6 mmol, 3 equiv), HFIP(0.5 mL) at room temperature for 5
h. bIsolated yield. cOxidant (0.8 mmol, 4 equiv). dOxidant (1.0 mmol, 5 equiv). eAt
40 °C. fAdditive (0.1 mmol, 1 equiv). gAdditive (0.1 mmol, 1 equiv). ND = not
detected
room temperature. However, the formation of oxazoles from
chalcone is not achieved under metal-free and mild conditions.
As our efforts on using chalcone and hypervalent iodine
reagent to construct oxazole motif heterocycles, herein, we
describe a general, simple, one-pot, and atom economical
approach for 2,5-diaryloxazoles formation from readily
available chalcone. The reaction could be realized under
2j).
In addition, the reactions with chalcone bearing
substituents at the para-, meta-, and ortho-positions worked
smoothly to give the corresponding 2,5-diaryloxazoles (Table 2,
2c, 2h-2i). Under the modified reaction conditions, moderate
to good yields were generally obtained when a trisubstituted
chalcone was employed (Table 2, 2w-2x). Unfortunately, 4-
2 | J. Name., 2012, 00, 1-3
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To gain more insight into the mechanism of this process,
several control experiments were investigated. Firstly, we
added 2.0 equiv of TEMPO to confirm whether this reaction
went through a radical reaction pathway (Scheme 2, A). The
reaction was no significant decrease in yield which indicated
phenylbut-3-en-2-one failed to undergo the desired reaction
(Table 2, 2v).
Table 2. Substrate Scope for Oxazole Synthesisa,b
that
a radical process may not be involved in this
transformation. Subsequently, we tested the reaction from
acetophenone and aldehyde, α-bromoketones and
benzylamine derivatives. Unfortunately, no desired product
was detected (Scheme 2, B,C) which indicated the reaction
started from chalcone. Moreover, when we used PhIO instead
of PhI(OAc)2 the desired 2a was observed in 55% yield,
indicating that hypervalent iodine(III) reagents played an essential
part in the reaction (Scheme 2, D).
O
TEMPO 2 equiv
O
A
B
standrand conditions
N
1a
2a
, 74%
O
O
standrand conditions
O
N
H
+
2a
, ND
O
Br
PhI(OAc)2 (4 equiv)
HFIP
NH2
O
+
O
C
D
N
2a, ND
PhIO (4 equiv)
O
NH4OAc (10 equiv), HFIP
N
1a
2a
, 55%
Scheme 2. Investigation into the reaction mechanism
aReaction scale: 1 (0.2 mmol), PhI(OAc)2 (0.8 mmol), NH4OAc (2 mmol) in HFIP
(0.5 mL) at room temperature for 5 h
bYield of isolated product
(4-Me, 4-OMe) groups on the aromatic ring proceeded in this
reaction smoothly to afford the desired products in good to
excellent yields. Generally, electron-withdrawing substituents
such as −NO2, −CN had deleterious effect on this reacꢀon, they
did not furnish the desired oxazoles. To our surprise, the −CF3
as electron-withdrawing substituent afforded the desired
product 2u in good yield. Halogen substituted chalcone
derivatives were also well tolerated in this annulation (Table 2,
2b-2d, 2j-2n), which provides the possibility to allow reactions,
respectively. Interestingly, 3-(4-fluorophenyl)-1-phenylprop-2-
en-1-one gave the corresponding product in 89% yield (Table 2,
2j).
In addition, the reactions with chalcone bearing
substituents at the para-, meta-, and ortho-positions worked
smoothly to give the corresponding 2,5-diaryloxazoles (Table 2,
2c, 2h-2i). Under the modified reaction conditions, moderate
to good yields were generally obtained when a trisubstituted
chalcone was employed (Table 2, 2w-2x). Unfortunately, 4-
Scheme 3. Possible mechanism
On the basis of aforementioned results and previous reports,
16 a plausible reaction mechanism was proposed in the Scheme
3 to explain the formation of product
2
. Initially, the chalcone
phenylbut-3-en-2-one failed to undergo the desired reaction compound
1
the present of NH4OAc through the release of two equivalent
react with PIDA to form the iodo compound A
in
(Table 2, 2v).
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of HOAc.17 Following is an intramolecular cyclization of the
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compound
PhI and AcOH. Then, the intramoleular
molecule of PIDA to form intermediate
of AcOH.16e
would be furnished by the intermediate
A
to form the immediate
B
, with the elimination of
react with another
by losing a molecule
7
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driving force for the ring-opening of intermediate
One of the resonance structures of intermediate
about the cyclization to intermediate , which lead to desired
product after deprotonation.
C
to D 18
can bring
.
D
E
2
In summary, we have developed an efficient room
temperature oxidative cyclization of chalcone to generate 2,5-
disubstituted oxazoles by using PIDA as oxidant. The reactions
complete smoothly and the corresponding substituted oxazole
products are obtained in good to excellent yields. Functional
groups such as halogen and trifluoromethyl were well
tolerated under the optimized reaction conditions. Our
strategy is markedly simpler with highly atom-economical
transformation and does not require any transition metal
catalyst which is harmful to the environment. Further detailed
investigations of the reaction mechanism and synthetic
applications remain in progress in our laboratory.
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Conflicts of interest
There are no conflicts to declare
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We gratefully acknowledge the National Natural Science
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DOI: 10.1039/C8OB00401C