Exploiting the Reactivity of Isocyanide: Coupling Reaction between Isocyanide and Toluene Derivatives Using the Isocyano Group as an N1 Synthon

Article abstract of DOI:10.1021/acs.orglett.6b01928

An unusual oxidative coupling reaction of isocyanide and toluene derivatives using tetrabutylammonium iodide (TBAI) as a catalyst is disclosed. The experimental results and mechanistic study show that the isocyano group acts formally as an N1 synthon during the transformation, thus expanding the reactivity profile of isocyanide.

Full text of DOI:10.1021/acs.orglett.6b01928

Letter
pubs.acs.org/OrgLett
Exploiting the Reactivity of Isocyanide: Coupling Reaction between
Isocyanide and Toluene Derivatives Using the Isocyano Group as an
N1 Synthon
Zhiqiang Liu,†,§ Xinglu Zhang,†,§ Jianxiong Li,† Feng Li,† Chunju Li,† Xueshun Jia,† and Jian Li*,†,‡
†Department of Chemistry, Innovative Drug Research Center, Shanghai University, 99 Shangda Road, Shanghai 200444, P. R. China
‡Shanghai Key Laboratory of High Temperature Superconductors, Shanghai University, Shanghai 200444, P. R. China
S
* Supporting Information
ABSTRACT: An unusual oxidative coupling reaction of isocyanide
and toluene derivatives using tetrabutylammonium iodide (TBAI) as
a catalyst is disclosed. The experimental results and mechanistic study
show that the isocyano group acts formally as an N1 synthon during
the transformation, thus expanding the reactivity profile of
isocyanide.
in isocyanide chemistry have been devoted to developing the use
ver since the Passerini and Ugi reactions were reported,
Eisocyanides have found wide application in a variety of of isocyanides as carbenes, making them highly valuable and
versatile C1 building blocks.8,9 However, to our knowledge,
examples aimed at exploring other novel reactivity profiles of the
carbon−carbon and carbon−heteroatom bond-forming reac-
tions.1 From a mechanistic standpoint, the divalent carbon atom
isocyano group are still rare. Zhu and co-workers have disclosed a
multicatalytic reaction of isocyanides and propargylamines to
afford imidazole derivatives (Scheme 1, eq 2).10 In this case, the
isocyano group serves as a polarized triple bond rather than a
carbene, thereby greatly expanding the reactivity mode of
isocyanide. Similar reactivity was also exhibited in the silver-
catalyzed synthesis of imidazole from simple alkyne and
isocyanide.11 Xu and co-workers recently reported a very
interesting cross-cycloaddition of two different isocyanides to
synthesize pyrrolo[3,4-b]indoles (Scheme 1, eq 3).12 According
to their experimental outcomes and DFT calculations, the
unusual heterodimerization of two isocyanides was believed to be
the key step of the whole transformation. Additionally, another
interesting reactivity of isocyanide allowed the isocyano group to
be used as a cyano source when sterically hindered tert-butyl
isocyanide was used as a reaction partner.13 Although much
progress has been made in the past decades, exploiting new
reactivity patterns of the isocyano group continues to be a
challenging goal for organic chemists.
of isocyanides has pronounced nucleophilicity, which makes
them particularly important reaction partners in multicompo-
nent reactions (IMCRs).2,3 In particular, the classical carbene-
like reactivity of isocyanide has also been well-exemplified by
many Lewis acid-catalyzed isocyanide insertion reactions
(Scheme 1, eq 1).4,5 Recently, transition-metal-catalyzed
isocyanide insertion reactions (also known as imidoylative
reactions) have emerged as a powerful tool in organic
synthesis,6,7 in which the reactivity of isocyanide as a carbon
monoxide equivalent has been fully demonstrated. A careful
literature survey revealed that most of the reported achievements
Scheme 1. Versatile Reactivity Profiles of the Isocyano Group
In the past years, we have been particularly interested in
isocyanide chemistry and have focused our attention on the
development of isocyanide-based novel transformations.14 Thus,
a series of novel strategies have been developed for the
construction of structurally unusual drug-like heterocycles
using isocyanides as versatile building blocks. Most recently,
we also reported that selective double isocyanide insertion
reactions could furnish indole-fused polycyclic skeletons in an
efficient manner.14a On the other hand, toluene and its
derivatives have been recognized as simple and valuable starting
Received: July 3, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b01928
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Organic Letters
Letter
materials in organic synthesis because of their easy availability
and versatile reactivity.15 It is therefore not surprising that many
new reactions have been reported using arylmethanes as
significant reaction components.16 As a continuation of our
previous research,14,17 herein we report the coupling reaction of
toluene derivatives and isocyanides using the isocyano group as
an N1 synthon. To the best of our knowledge, no such examples
have been reported previously.
Scheme 2. Scope of the Reaction with Respect to the
Arylmethane Substrate 1
a b
,
We initially investigated the coupling reaction with model
substrates toluene (1a) and tert-butyl isocyanide (2a) using
TBHP as the oxidant. During our early runs, no reaction
occurred upon treatment of the mixture in DCE solution with a
catalytic amount of CuI or I2 (Table 1, entries 1 and 2).
a
Table 1. Optimization of the Reaction Conditions
b
entry
catalyst
CuI
solvent
oxidant
yield (%)
1
2
DCE
DCE
DCE
DCE
DCE
DCE
MeCN
EtOAc
PhCl
DMSO
THF
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
H2O2
0
0
I2
3
CuBr2
CuCl2
Cu(OAc)2
TBAI
TBAI
TBAI
TBAI
TBAI
TBAI
TBAI
TBAI
TBAI
TBAI
−
40
30
<5
72
45
<10
<5
35
20
57
0
4
5
6
7
8
9
10
11
12
13
14
15
16
a
Reaction conditions: 1 (1.5 mmol), 2a (0.5 mmol), TBAI (20 mol
%), TBHP (6.0 equiv), and DCE (3 mL) at 80 °C in a sealed tube.
Yields of products after silica gel chromatography are shown.
PhCF3
DCE
DCE
DCE
b
K2S2O8
DTBP
TBHP
0
0
derivatives 1 having different electron-withdrawing and electron-
donating substituents on the aromatic ring were first employed to
react with 2a under the standard conditions (3a−j). The
experimental outcome revealed that a wide range of substituents
in arylmethanes 1, including halide, alkyl, methoxy, and nitro
groups at the ortho, meta, and para positions of the aromatic ring,
were well-tolerated, affording the desired compounds 3.
Representative results are summarized in Scheme 2, and all
new compounds were characterized by 1H and 13C NMR
spectroscopy and HRMS (see the Supporting Information for
details). Remarkably, p- and m-xylene were proven to be good
reaction components, affording the corresponding products 3g
and 3h. It is also worth noting that the methoxy group seemed to
favor the formation of 3 (3i and 3j), whereas the presence of a
nitro group in substrate 1 dramatically decreased the yield (3k).
After a broad scope of simple benzene-based substrates 1 were
examined, reactions with arylmethanes 1 containing naphthyl
and pyridyl groups were conducted. The naphthyl groups were
proven to be compatible in the present reaction (3l and 3m), but
no reaction took place when a pyridyl group was present in
substrate 1.
0
a
Unless otherwise noted, all reactions were carried out with 1a (1.5
mmol), 2a (0.5 mmol), catalyst (20 mol %), oxidant (6.0 equiv), and
solvent (3 mL) at 80 °C in a sealed tube. Yields of products after
silica gel chromatography.
b
Pleasingly, employing CuBr2 as catalyst essentially gave rise to
compound 3a in 40% yield (entry 3). The 13C NMR spectrum of
compound 3a clearly showed that only one carbonyl carbon was
present, which indicated that one carbon was missing during the
reaction. Encouraged by this unusual result, we briefly screened
the reaction parameters, including the catalyst, the solvent, and
the oxidant, to optimize the reaction performance. Of the
catalysts examined, TBHP showed the best catalytic activity
toward the formation of 3a (entry 6), whereas the use of CuCl2
and Cu(OAc)2 only led to decreased yields (entries 4 and 5). The
following experimental results showed that the solvent also had a
significant impact on this reaction. Lower yields were observed
when reactions were performed in other solvents such as
CH3CN, EtOAc, chlorobenzene, and THF (entries 7−11). In
addition, benzotrifluoride as an eco-friendly solvent was also
proven to be compatible (entry 12). Notably, the replacement of
TBHP with other oxidants, including H2O2, K2S2O8, and DTBP,
failed to generate 3a (entries 13−15).
To further explore the utility of the present reaction, the
substituents of isocyanides 2 were then varied. As shown in
Scheme 3, a series of aliphatic and aromatic isocyanides 2 were
subjected to the optimal conditions. Gratifyingly, almost all of the
reactions proceeded smoothly to produce the products 4a−g.
Unfortunately, no reaction occurred when sterically hindered
After the optimal conditions were established, we focused our
attention on investigating the scope and limitations of the
present coupling reaction. As shown in Scheme 2, toluene
B
DOI: 10.1021/acs.orglett.6b01928
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Organic Letters
Letter
5). Finally, an isotope-labeling experiment was also performed.
The reaction of toluene-[methyl-13C1] (1a′) and 2a proceeded
readily to give the amide-[carbonyl-13C1] product 3a′ (Scheme 4,
eq 6), which clearly indicated that the carbon of the isocyano
group was eliminated during the reaction.
Scheme 3. Scope of the Reaction with Respect to the
Isocyanide Substrate 2
a b
,
The mechanism of this unusual oxidative coupling reaction has
not been unequivocally established, but one reasonable
mechanistic process is proposed to explain the formation of
product 3. As shown in Scheme 5, the beginning of this reaction
Scheme 5. Proposed Mechanism
a
Reaction conditions: 1 (1.5 mmol), 2 (0.5 mmol), TBAI (20 mol %),
b
TBHP (6.0 equiv), and DCE (3 mL) at 80 °C in a sealed tube. Yields
of product after silica gel chromatography are shown.
1,1,3,3-tetramethylbutyl isocyanide and less reactive 4-nitro-
phenyl isocyanide were used. Subsequent experiments showed
that 2-naphthyl isocyanide (2k) worked well to furnish the
corresponding product 4h with good performance. Furthermore,
the methyl groups in 2,6-dimethylphenyl isocyanide and 2,4,6-
trimethylphenyl isocyanide remained intact during the oxidative
coupling (4i and 4j), thus significantly featuring the chemical
selectivity and substrate scope.
To gain further insight into the aforementioned coupling
reaction, three mechanistic experiments were conducted. First,
the radical scavenger TEMPO was added to the preformed
solution of 1a and 2a under the optimal conditions and was
found to completely inhibit the present reaction (Scheme 4, eq
4). To our delight, two significant radical intermediates were also
captured.18 To find out the real reaction intermediate, the
reaction of benzaldehyde (5) with 2a was also carried out. In this
case, 3a was isolated in 82% yield under the optimal conditions,
indicated that 5 is a possible reaction intermediate (Scheme 4, eq
involves the formation of tert-butoxyl and/or tert-butylhydroper-
oxide radicals from the catalytic cycle of TBAI and TBHP.19
Subsequently, toluene 1 is oxidated to benzoyl radical B20 via
intermediate benzaldehyde A. This step can also explain the
formation of benzil as a byproduct. On the other hand, the
reaction between isocyanide and TBHP in the presence of TBAI
leads to the formation of reactive formamido radical C, which is
believed to be the key step. The resultant intermediate C then
eliminates carbon monoxide to generate radical D.21 Finally,
radical coupling of the two radicals B and D essentially yields the
product 3.
In conclusion, we have developed a novel and interesting
coupling reaction between arylmethanes and isocyanides. In this
reaction, the reactivity of the isocyano group as an N1 synthon
other than a carbene is unprecedented, offering a new
opportunity to enrich isocyanide chemistry. A broad range of
toluene derivatives and isocyanides readily participated in this
coupling reaction. As a result, the present reaction has potential
to be further applied in organic synthesis.
Scheme 4. Preliminary Mechanistic Studies
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b01928.
Experimental procedures, full characterization of all
1
compounds, spectral data, and H and 13C NMR spectra
for all products (PDF)
C
DOI: 10.1021/acs.orglett.6b01928
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Organic Letters
Letter
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AUTHOR INFORMATION
■
Corresponding Author
*lijian@shu.edu.cn
Author Contributions
§Z.L. and X.Z. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Natural Science Foundation of China
(21272148 and 21472121) for financial support. This work was
also partly supported by the Shanghai Key Laboratory of High
Temperature Superconductors (14DZ2260700). We thank Dr.
H. Deng (Laboratory for Microstructures, Shanghai University)
for NMR measurements.
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