Synthesis of oxazoles by silver catalysed oxidative decarboxylation-cyclization of α-oxocarboxylates and isocyanides

Article abstract of DOI:10.1039/c5cc02253c

A silver catalysed synthesis of oxazoles by the oxidative decarboxylation-cyclization of α-oxocarboxylates and isocyanides was developed. This method provided a novel strategy to construct oxazole rings compared to traditional methods. Mechanistic investigations such as operando IR, EPR and radical inhibition experiments were carefully done and confirmed the acyl cation and Ag(ii) as the intermediates in this transformation, and the involvement of a radical decarboxylative process.

Full text of DOI:10.1039/c5cc02253c

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Synthesis of oxazoles by silver catalysed oxidative
decarboxylationꢀcyclization of αꢀoxocarboxylates and
isocyanides
Cite this: DOI: 10.1039/x0xx00000x
Yiyang Ma,a Zhiyuan Yan,a Changliang Bian,a Ke Li,a Xiaowen Zhang,a Mengfan
Received 00th January 2015,
Accepted 00th January 2015
Wang,a Xinlong Gaoa, Heng Zhanga* and Aiwen Lei a, b
*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A
silver catalysed synthesis of oxazoles by oxidative
O
O
N
O
decarboxylationꢀcyclization of
αꢀoxocarboxylates and
Cl
Me
N
O
N
N
N
isocyanides was developed. This method provided a novel
strategy to construct oxazole rings comparing to traditional
methods. The mechanistic investigations such as operando IR,
EPR and radical inhibiting experiment were carefully done
and confirmed the acyl cation and Ag(II) as the intermediate
in this transformation and a radical decarboxylative process
is related.
OMe
O
H
H
O
N
N
N
N
NH2
H
N
S
S
CF3
O
Cl
Pimprinaphine
Phosphodiesterase 4
Bistratamide C
H
R'
C
R
N
N
Fiveꢀmembered heterocycle containing nitrogen atom is one of the
most prevailing compounds1, which widely exists in numerous
natural products, biologically active alkaloids, pharmaceuticals, and
agrochemicals2. In addition, these key cores are also the basic
constituents of organic dyes and material molecules3. Various efforts
in recent years have focused on the development of improved
methods for the synthesis of these heterocycles4. Recently, Bi’s
group and our group has developed a novel pyrrole synthesis from
the [2+3] cycloaddition of terminal alkynes and isocyanides by silver
catalysis5. This efficient and highly regioselective protocol intrigued
us to apply this method for synthesis of other fiveꢀmembered
heterocycles synthesis. For example, when the carbon atom at 3ꢀ
position of pyrrole was replaced by an oxygen atom, there comes out
another widely applied aromatic heterocycleꢀoxazole. The unique
structural nature of oxazole moiety endows its derivatives to exhibit
various bioactivities and some special properties, and thus results in
their wide potential application in medicinal and agrochemical field
as well as materials sciences and so on (Scheme 1)6. Therefore, the
synthetic methods of oxazole ring have been receiving increasing
attention, and a large amount of effort has been directed towards its
syntheses7. In fact, similar to pyrrole synthesis by terminal alkynes
and isocyanides, from the retrosynthetic analysis, oxazoles can also
be synthesized from acyl compounds and isocyanides. Though there
are several examples using acyl halogens, esters and amides as
electrophiles with isocyanides to oxazoles8, using nucleophilic acyl
reagents is still rare. Thus, the development of a new oxidative
cyclization for the synthesis of oxazoles from nucleophilic acyl
reagents is still highly attractive (Scheme 1).
+
CH
R'
C
C
C
N
H
R
O
Electrophile
Nucleophile
R'
X
R'
O
R
+
R'
C
O
N
R
acyl resource
O
R'
COOH
This work
Scheme 1. (A) Representative oxazoles moiety in functional materials
and bioactive molecule; (B) Strategy to synthesis of oxazoles from
retrosynthetic analysis.
Transition metal catalysed decarboxylative coupling have
received much attention for their applications to construct CꢀC and
Cꢀhetero bonds in recent years9. Among various resources of
carboxylic acids, αꢀoxocarboxylic acids have emerged as a novel
class of acyl surrogate after releasing
a
CO2 molecular10.
Theoretically, there are three types of acyl reagents from
decarboxylation of αꢀoxocarboxylates: acyl anion, acyl radical and
acyl cation11. In 2008, Gooßen first demonstrated a Pdꢀcatalyzed
direct decarboxylative acylation of aryl bromides with αꢀ
oxocarboxylates as acyl anion to afford diaryl ketones12. Recently,
You and coꢀworkers reported a transition metal free decarboxylation
of αꢀoxocarboxylates with C(sp3)ꢀX13. In addition, it has also been
reported that oxidative decarboxylation of αꢀoxocarboxylic acids via
the acyl radical intermediates, which led to acylation of aromatic and
heterocyclic ring in recent years10b, 14. In spite of the significant
progress offered by these reactions from acyl anions and acyl
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radicals, the oxidative decarboxylation of αꢀoxocarboxylic acids or cmꢀ1 and benzoylꢀacylium ion at 2345 cmꢀ1, we could assign the
component A to be the benzoyl cation ꢀꢀ [PhꢀC≡O+] respectively.
salts via acyl cations is still rare. Based on our continuing interests in
oxidative decarboxylation of αꢀoxocarboxylic acids15, we present our
progressive development for the oxidative decarboxylationꢀ
cyclization of αꢀoxocarboxylates and isocyanides.
The combination of Ag2CO3, 1,10ꢀphen and K2S2O8 in DMF at
80 C gave the corresponding oxazole (3a) with the best result for
o
the
oxidative
decarboxylationꢀcyclization
of
potassium
0.08
0.04
0.00
oxophenylacetate (1a) and ethyl 2ꢀisocyanoacetate (2a) (Table 1,
entry 1). The choice of catalyst was crucial for this reaction. When
Ag2CO3 was replaced by AgOAc or AgNO3, the yield decreased
respectively (entry 2 and 3). The usage of Cu(OAc)2 as catalyst
decreased the yield dramatically (entry 4). In the absence of catalyst,
essentially trace amount of product was obtained (entry 5). Without
1,10ꢀphen as additive, the yield dropped to 78% (entry 6). As to
oxidants, K2S2O8 gave the highest yield for this oxidative reaction.
Na2S2O8 and O2 showed less or no efficiency in terms of chemical
yield (entry 7 and 8). Changing the solvent from DMF to MeCN or
DMSO, relatively lower yield of the corresponding oxazole product
was obtained (entry 9 and 10).
2360 2350 2340 2330
Figure 1. (A) The absorbance of component A in 3DꢀKinetic profile; (B)
Spectra of the component A (green curve) and standard sample (benzoylꢀ
acylium salt, red curve).
Table 1. Agꢀcatalyzed oxidative cyclization of potassium oxophenylacetate
(1a) and Isocyanides (2a): effects of reaction parameters.a
To confirm Ag(II) intermediate in this oxidative reaction17, the
electron paramagnetic resonance (EPR) was applied to study it. As
shown in Figure 2, no EPR signal was observed when Ag2CO3 was
o
tested alone in DMF, 80 C, N2 for 2 hours (red color). However,
when the Ag2CO3 and K2S2O8 were mixed the same condition, a
strong EPR signal could been detected (green color). According to
relative literatures, we may ascribe this obvious signal to Ag(II)
species18. Therefore, this oxidative decarboxylation referred a Ag(I)ꢀ
Ag(II) catalytic circle.
Ag2CO3
Ag2CO3+K2S2O8
a
Standard reaction conditions: 1a (0.25 mmol), 2a (0.50 mmol), Ag2CO3
(0.025 mmol), 1,10ꢀphen (0.125 mmol), K2S2O8 (0.75 mmol), DMF (1.0 mL),
b
in N2, 80 oC, 12 h. The yield was determined by GC, calibrated using
biphenyl as internal standard. c Yield in the parentheses is isolated yield.
3250
3300
3350
3400
3450
X [G]
In order to investigate the mechanism of this silver catalysed
decarboxylation of αꢀoxocarboxylates, the reaction between
potassium oxophenylacetate (1a) and ethyl 2ꢀisocyanoacetate (2a)
with Ag2CO3 and K2S2O8 was monitored by operando IR. As can be
seen from the kinetic profiles of relative absorbance in Figure 1(A),
when mixing 1a with Ag2CO3 and K2S2O8 in DMSO, we found a
new peak component A (at 2342 cmꢀ1) increased very quickly. While
2a was added to the solution, the absorbance of 2a started to
decreased. This result suggested that the component A might be the
reactive intermediate in this transformation. We noticed that the
band of 2300ꢀ2400 cmꢀ1 usually could be attributed to the
absorbance of C≡C or C≡O. Therefore, component A which has an
infrared absorption at the 2342 cmꢀ1 might be assigned as benzoylꢀ
acylium ion. In order to confirm our assumption, the reaction
between benzoyl chloride and one equivalent of AlCl3 were
performed in DCE and monitored by operando IR. A weak band at
2345 cm−1 was observed which was reported as the characteristic
infrared absorption of benzoylꢀacylium salt ꢀꢀ [PhꢀC≡O+]AlCl4
(Figure 1B)16. Comparing the absorbance of component A at 2342
Figure 2. Evidence of Ag(II) species in EPR spectra.
Moreover, radical scavengers, such as TEMPO and BHT, were
employed in the reaction to confirm whether this reaction related to
radical process (see Supporting Information, Figure S4). As a result,
the reactions was profoundly suppressed, which could indicate that
this transformation involved radical intermediates. We proposed the
decarboxylation of oxocarboxylates could be a radical process19:
oxocarboxylates firstly experienced a single electron transfer (SET)
to be oxidized to oxocarboxylic radical, then it decarboxylate and
release CO2 to give the acyl radical and subsequently a second SET
process happened to form the acyl cation10b, 11 which was confirmed
by operando IR.
On the basis of the results described above, a proposed reaction
pathway is depicted in Scheme 3. In the presence of Ag2CO3 and
K2S2O8, the αꢀoxocarboxylates experienced
decarboxylative process to generate the benzoylꢀacylium ion I and II,
which are two kind resonance structures. Meanwhile, silver salts
could also coordinate to the isocyano group and activate isocyanides
a fast oxidative
2 | J. Name., 2012, 00, 1-3
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to form III according to previous literatures20. Therefore, we believe
that the silver catalyst played the dual roles in this reaction, which
participated in the decarboxylation of αꢀoxocarboxylates as well as
activated the isocyanide. Subsequently, the cycloaddition between
benzoyl cation and III would afford the intermediate IV. And finally,
protonation and tautomerization gives the oxazole product 3a.
isocyanoacetate also gave the products in good yields (3l). It is
noteworthy that aliphatic αꢀoxocarboxylates were compatible in this
reaction as well albeit gave lower yields (3m and 3n). Heterocyclic
αꢀoxocarboxylates were found to be favored under this catalytic
system to afford the corresponding products (3o). In addition, αꢀ
oxocarboxylates with naphthyl group (3p) also participated in this
decarboxylative process with a high reactivity.
Conclusions
In summary, we have developed a novel silver catalysed
oxidative decarboxylationꢀcyclization of αꢀoxocarboxylates and
isocyanides to construct oxazoles. In this reaction, a variety of
substituted αꢀoxocarboxylates were wellꢀtolerated. The
mechanistic investigations including operando IR, EPR and
radical inhibit experiments supported our proposed mechanism
well. From these mechanistic experiments, acyl cation was
observed utilizing in-situ IR and Ag(II) was also monitored
through EPR. Finally, this reaction provided an efficient
pathway to synthesize oxazoles and showed a potential
application prospect.
Scheme 3. Proposed mechanism.
Table 2. Substrates scope of the cyclization of αꢀoxocarboxylates 1 and
isocyanides 2a
This work was supported by the 973 Program (2012CB725302)
, the National Natural Science Foundation of China (21390400,
21025206, 21272180, and 21302148), and the Research Fund f
or the Doctoral Program of Higher Education of China (201201
41130002) and the Ministry of Science and Technology of Chin
a (2012YQ120060). The Program of Introducing Talents of Dis
cipline to Universities of China (111 Program) is also appreciat
ed.
Notes and references
a College of Chemistry and Molecular Sciences, the Institute for Advanced
Studies (IAS), Wuhan University, Wuhan, 430072, P. R. China. E-mail:
aiwenlei@whu.edu.cn; Tel: (+86)-27-68754672;
b
National Research Center for Carbohydrate Synthesis, Jiangxi Normal
University, Nanchang, 330022, P. R. China
†
Electronic Supplementary Information (ESI) available: See
DOI: 10.1039/c000000x/
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