Copper-Catalyzed Synthesis of Polysubstituted Pyrroles through [3+1+1] Cycloaddition Reaction of Nitrones and Isocyanides

Article abstract of DOI:10.1021/acs.orglett.8b00798

An efficient, copper-catalyzed [3+1+1] cycloaddition reaction was developed for the expedient synthesis of pharmacologically interesting polysubstituted pyrroles from easily available nitrones and α-acidic isocyanides. The given approach features a new mo

Full text of DOI:10.1021/acs.orglett.8b00798

Letter
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Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Copper-Catalyzed Synthesis of Polysubstituted Pyrroles through
[3+1+1] Cycloaddition Reaction of Nitrones and Isocyanides
Zhuang Tian,†,§ Jiaojiao Xu,†,§ Bingxin Liu,† Qitao Tan,*,† and Bin Xu*,†,‡
†Department of Chemistry, Innovative Drug Research Center, School of Materials Science and Engineering, Shanghai University,
Shanghai 200444, China
‡State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
Shanghai 200032, China
S
* Supporting Information
ABSTRACT: An efficient, copper-catalyzed [3+1+1] cycloaddition
reaction was developed for the expedient synthesis of pharmacolog-
ically interesting polysubstituted pyrroles from easily available
nitrones and α-acidic isocyanides. The given approach features a
new mode of cycloaddition between nitrones and isocyanides with
wide substrate scope, good functional group tolerance, and operational simplicity. The operando infrared spectroscopy was used
for the characterization of reaction intermediates.
Scheme 1. Cycloaddition of Nitrones with Isocyanides
socyanides have attracted tremendous attention in organic
Isynthesis due to their unique reactivity toward electrophiles,
nucleophiles, and radicals.1 Meanwhile, isocyanides also act as
“CN” sources2 or ligands for transitional metals1c,3 in many
transformations. The [3+2] cycloaddition reaction of α-acidic
isocyanides with multiple bonds has provided a wide range of
nitrogen-containing heterocycles, such as pyrroles,4 pyrrolines,5
oxazoles,6 and imidazoles.7 As a typical α-acidic isocyanide
bearing both an isocyanide group and an acidic α-carbon
fragment, isocyanoacetates occur as one of the most useful
building blocks with respect to their exceptionally rich synthetic
possibilities.8
Nitrones, readily available dipoles for 1,3-dipolar cyclo-
additions, have been widely utilized for the construction of
numerous nitrogen-containing heterocycles with activated
alkenes or alkynes.9,10 However, few reports have been
documented on the reactions of nitrones with isocyanides. In
2012, Soeta and co-workers demonstrated the chlorosilane-
promoted addition reaction of isocyanides to 3,4-dihydroisoqui-
noline N-oxides, affording 1,2,3,4-tetrahydroisoquinoline-1-
carboxylamides in moderate to high yields.11 Several cyclo-
addition reactions of nitrones with isocyanides were also reported
(Scheme 1a). For example, Zeeh, Lorke, and Zhu have
independently reported the synthesis of four-membered 4-
imino-1,2-oxazetidines by a [3+1] cycloaddition of isocyanides
to nitrones with internal octet stabilization, respectively.12 In a
[3+2] cycloaddition manner, nitrones react with palladium-
bound isocyanides to provide the carbene complexes.13 In
addition, α-acidicisocyanideswerereportedtoreactwithnitrones
throughaproposed[3+3]cycloadditiontogivefive-membered2-
imidazolidinones.14 As a part of our interest in exploring
isocyanides in the synthesis of heterocycles,2a,b,3a,e,f,15 we herein
report a copper-catalyzed, unprecedented [3+1+1] cycloaddition
of nitrones and isocyanoacetates for the expedient synthesis of
polysubstituted pyrroles, whereby three sequential C−C bonds
are formed (Scheme 1b). The prepared polysubstituted pyrroles
are prevalent five-membered heterocycles, which are the basic
constituents of natural products, pharmaceuticals, functional
materials, and agrochemicals.16 To our knowledge, the given
approach features a new mode of cycloaddition between nitrones
and isocyanides in the presence of a copper catalyst.
We started our investigation by exploring the reaction of (Z)-
N-benzylideneaniline oxide 1a with isocyanide 2a in the presence
of copper acetate monohydrate and KHCO3 in dioxane at 80 °C
under a nitrogen atmosphere. Intriguingly, trisubstituted pyrrole
3aa was isolated in 62% yield. The identity of 3aa was determined
by spectral analysis and further confirmed by X-ray crystallo-
graphic analysis.17 A screening of Cu(II) catalysts showed that
Cu(OAc)2·H2O was the most effective catalyst (Table 1, entries
1−4). Subsequently, examination of different solvents revealed
Received: March 12, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b00798
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Letter
a
Table 1. Optimization of the Reaction Conditions
b
entry
Cu source
solvent
base
atmos
temp (°C)
yield (%)
1
2
Cu(OAc)2·H2O
CuCl2
dioxane
dioxane
dioxane
dioxane
KHCO3
KHCO3
KHCO3
KHCO3
KHCO3
KHCO3
KHCO3
KHCO3
KHCO3
KHCO3
K3PO4
NaOAc
DBU
N2
N2
N2
N2
N2
N2
N2
N2
N2
N2
N2
N2
N2
N2
N2
N2
N2
N2
N2
O2
air
N2
N2
N2
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
70
90
80
80
80
80
80
62
/
3
Cu(acac)2
/
4
CuBr2
/
5
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
/
CH3CN
PhCN
toluene
DCE
60
64
41
72
52
76
55
44
61
80
86
38
48
76
73
71
47
6
7
8
9
DMF
NMP
NMP
NMP
NMP
NMP
NMP
NMP
NMP
NMP
NMP
NMP
NMP
NMP
NMP
NMP
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Cs2CO3
CsOAc
CsOAc
/
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
CsOAc
CsOAc
CsOAc
CsOAc
CsOAc
CsOAc
CsOAc
c
46
d
69
e
48
a
b
c
Reaction conditions: 1a (0.3 mmol), 2a (0.9 mmol), [Cu] (0.09 mmol), base (0.6 mmol), solvent (1 mL), 20 h. Isolated yields. 10 mol %
d
e
Cu(OAc)2·H2O was used. 20 mol % Cu(OAc)2·H2O was used. (Z)-N-Benzylidenemethanamine oxide was used instead of 1a.
that the reaction in N-methyl-pyrrolidone (NMP) afforded the
best result (entries 5−10). After an extensive screening of the
bases (entries 10−15), CsOAc was found to be the best choice,
affording the pyrrole 3aa in 86% yield (entry 15). The synergetic
effect of the copper catalyst and base was justified, as the absence
ofeither led toasignificantdecrease of theyields (entries16−17).
The temperature, atmosphere, and loadings of the catalyst also
havedramatic effectson theyields (entries 18−23). When (Z)-N-
benzylidenemethanamine oxide was used instead of 1a, the
desired pyrrole product was obtained in only 48% yield (entry
24), indicating the crucial substituent effect of the nitrones.
With the optimized conditions in hand, we next examined the
substrate scope of nitrones and isocyanides. As shown in Scheme
2, the reaction showed good functional group tolerance. Aryl
nitrones bearing either electron-withdrawing or electron-
donating groups in the para position were smoothly converted
to the corresponding products in moderate to excellent yields
(3ba−3ha). The ortho-, meta-substituted, or disubstituted aryl
nitrones also afforded the desired products in good yields (3ia−
3na). The present protocol is not only compatible with simple
benzene-based substrates: nitrones with naphthyl (3oa and 3pa)
orheteroaromaticsubstituentsbearingindolyl(3qa), thienyl(3ra
and 3sa), and furyl (3ta) groups afforded the corresponding
products in moderate to good yields. Notably, a variety of alkyl
nitrones all gave pyrroles in moderate to good yields (3ua−3wa).
The ester groups of isocyanoacetates were also investigated with
the final pyrrole products obtained smoothly (3ab, 3vb, 3ac).
However, attempts to use methyl 2-isocyanopropanoate18 as
substrate failed under the same conditions, which gave a
complicated mixture and was hard to identify.
The prepared polysubstituted pyrroles are important starting
materials for the synthesis of biologically active compounds and
organic functional materials.16 As an example, postfunctionaliza-
tion of pyrrole 3aa was performed to afford pyrrolo[1,2-a]-
quinoxalines 5 in 52% yield through CuI/L-proline-catalyzed
intermolecular coupling with 2-iodotrifluoroacetanilide 4,19
followed by hydrolysis and a spontaneous intramolecular
cyclization reaction (Scheme 3). The cyclic product 5 was a key
intermediate for the synthesis of nonpeptide inhibitors of platelet
aggregation.20
To define the possible intermediates and pathway, several
controlexperiments werecarried out, asshownin Scheme4. Only
a trace amount of the desired product 3aa was obtained when
benzaldehyde was used instead of nitrone 1a under the standard
reaction conditions (Scheme 4a). The reason for the low yield of
3aa may be due to the formation of other byproducts through
different pathways.21 This result suggested that the aldehyde was
not the key intermediate in this reaction, and the use of nitrone as
substrate is superior to aldehyde for the cycloaddition reaction.
However, ahighyieldcouldbeachievedwhenvinylisocyanide622
was used instead of nitrone 1a (Scheme 4b), which indicated that
it might be a possible intermediate for this reaction. Furthermore,
the addition of 2,2,6,6-tetramethyl-1-piperidinyl-oxy (TEMPO)
or 1,4-dinitrobenzene as radical scavengers had no significant
effect on this reaction (Scheme 4c,d), which might rule out a
radical pathway.
B
DOI: 10.1021/acs.orglett.8b00798
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
nism.23 We contemplated that the nitrone fragment may be
eliminated as nitrosobenzene during the reaction. To probe the
progress of this cascade transformation further, we investigated
this reaction using operando IR to monitor the possible existence
ofnitrosobenzene. Duringthestudy, thecharacteristic absorption
of nitrosobenzene at 1505 cm−1 was selected for observation to
avoid interferences from the reaction system. Finally, the in situ
generation of nitrosobenzene was proved by the operando
infrared spectroscopy (Figures S1−S3, see the SI for more
details), indicating that the reaction may undergo the release of
nitrosobenzene.
a b
,
Scheme 2. Substrate Scope of Pyrroles
Based on the above preliminary results, a plausible mechanism
for this copper-catalyzed [3+1+1] cycloaddition is proposed in
Scheme 5. We envisaged that C−H metalation of isocyanide 2a
would occur to generate the intermediate α-cuprio-isocyanide A
in the presence of Cu(OAc)2·H2O and a base.22b,24 Then, a
nucleophilic addition between nitrone 1a and intermediate A
took place to produce the copper intermediate B.22b Intermediate
B was presumably converted into a more stable vinyl isocyanide
intermediate C22 through an elimination process with the
expulsion of nitrosobenzene and the regeneration of the copper
catalyst. Finally, the olefinic intermediate C underwent cyclo-
addition with the second molecule of isocyanide 2a to yield
pyrrole 3aa via intermediates D and E.4a
Scheme 5. Plausible Mechanism
a
Reaction conditions: 1 (0.3 mmol), 2 (0.9 mmol), Cu(OAc)2·H2O
(0.09 mmol), CsOAc (0.6 mmol), and NMP (1.0 mL), 80 °C,
b
nitrogen atmosphere. Isolated yield.
Scheme 3. Synthetic Application of the Product
In summary, we have developed a novel, copper-catalyzed
[3+1+1] cycloaddition reaction for the expedient synthesis of
polysubstituted pyrroles from easily available nitrones and
isocyanoacetates with operational simplicity, mild reaction
conditions, good functional group tolerance, and wide substrate
scope. Operando infrared spectroscopy was used for the
characterization of reaction intermediates. This approach
represents a unique synthetic strategy of nitrones and isocyanides
through the formation of three sequential C−C bonds and offers
an alternative route for the preparation of polysubstituted
pyrroles. Further insight into the mechanism, reaction scope,
and biological applications is under investigation in our
laboratory.
a
Scheme 4. Mechanistic Studies
ASSOCIATED CONTENT
* Supporting Information
■
S
TheSupportingInformationisavailablefreeofchargeontheACS
Publications website at DOI: 10.1021/acs.orglett.8b00798.
a
Experimental procedures and characterization data for all
compoundsandX-raystructuresofcompounds3aa(PDF)
Condition A: Cu(OAc)2·H2O (30 mol %), CsOAc (2.0 equiv), NMP,
80 °C, nitrogen atmosphere, 20 h.
Accession Codes
The operando infrared spectroscopy has been used as an
effective method for the characterization of reaction intermedi-
ates, which provides direct evidence for the reaction mecha-
CCDC 1813419 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
C
DOI: 10.1021/acs.orglett.8b00798
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: xubin@shu.edu.cn.
*E-mail: qttan@shu.edu.cn.
ORCID
Qitao Tan: 0000-0002-6220-651X
Bin Xu: 0000-0002-9251-6930
Author Contributions
§Z.T. and J.X. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We thank the National Natural Science Foundation of China
(Nos. 21672136 and 21672140) for financial support. The
authors thank Prof. Hongmei Deng (Laboratory for Micro-
structures, SHU) for NMR spectroscopic measurements and Dr.
Mingchun Gao (School of Materials Science and Engineering,
SHU) for his helpful discussions.
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