Copper-Catalyzed [2+2+2] Modular Synthesis of Multisubstituted Pyridines: Alkenylation of Nitriles with Vinyliodonium Salts

Article abstract of DOI:10.1002/anie.201700696

A [2+2+2] modular synthesis of multisubstituted pyridines, with excellent regioselectivity, has been realized by copper catalysisand involves three distinct components: vinyliodonium salts, nitriles, and alkynes. The reactions proceeded with the facile formation of an aza-butadienylium intermediate by alkenylation of the nitrile with a vinyliodonium salt. Moreover, the alkynes in the reaction were extended to alkenes, which are an advantage of expense and relative scarceness of alkynes.

Full text of DOI:10.1002/anie.201700696

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201700696
German Edition: DOI: 10.1002/ange.201700696
Heterocycles
Copper-Catalyzed [2+2+2] Modular Synthesis of Multisubstituted
Pyridines: Alkenylation of Nitriles with Vinyliodonium Salts
Jinyu Sheng+, Yong Wang+, Xiang Su, Ru He, and Chao Chen*
Dedicated to Professor Gerhard Erker on the occasion of his 70th birthday
Abstract: A [2+2+2] modular synthesis of multisubstituted
pyridines, with excellent regioselectivity, has been realized by
copper catalysisand involves three distinct components: vinyl-
iodonium salts, nitriles, and alkynes. The reactions proceeded
with the facile formation of an aza-butadienylium intermediate
by alkenylation of the nitrile with a vinyliodonium salt.
Moreover, the alkynes in the reaction were extended to alkenes,
which are an advantage of expense and relative scarceness of
alkynes.
new approaches to pyridines from common building blocks
bearing more diversified substituents. To this end, we would
like to report a novel modular method leading to heavily
substituted pyridines from three simple components. The
method is of wide scope, requires an inexpensive copper
catalyst, and demonstrates excellent regioselectivity
(Scheme 1). The new modular synthesis starts with vinyl-
iodonium salts, nitriles, and alkynes of different electronic
properties, and proceeds through the facile formation of an
aza-butadienylium intermediate by the selective alkenylation
of nitriles with vinyliodonium salts,[9] and it rapidly yields
pyridines with excellent regioselectivity.
As one of the most prevalent heterocyclic backbones,
pyridines are present in biosystems, natural products, phar-
maceuticals, reagents for organocatalysis, ligands of metal
catalysis, as well as functional materials.[1] Most substituted
pyridines have to be produced synthetically because of the
lack of natural sources.[2] Thus, classic methodologies for the
construction of functionalized pyridine rings have been
developed from substrates bearing functional groups, and
involve either carbonyl condensation or cycloaddition reac-
tions.[2,3] Moreover, modern metal-mediated cross-coupling
chemistry is widely appreciated as it directly installs substitu-
ents on heterocycles and provides chemists with vast synthetic
options.[4] However, the ever-growing demand for rapid
access of heavily substituted pyridines calls for even more
efficient synthetic methods, and in response to this demand is
the recent construction of pyridines by multicomponent
reactions (MCRs).[5] Among these MCRs, transition-metal
catalyzed [2+2+2] cycloadditions for pyridine synthesis are
attractive because of the simplicity, flexibility, and availability
of starting materials.[6] It is known that most of the catalytic
[2+2+2] cycloaddition reactions have to employ noble
metals, including Ru, Rh, Co, Au etc., and some suffer
additionally from low regioselectivity because of the different
orientations of the alkyne. Nevertheless, both problems have
been alleviated with recent advances by using more specific
substrates or stoichiometric amounts of metal promoters.[2b,7]
Additionally, there are only a few methods to synthesize
pyridines by copper catalysis, and they employ limited
substrates.[8] Hence the attraction to the development of
Scheme 1. A[2+2+2] three-component process for pyridine synthesis.
Recently, we demonstrated that arylation of nitriles with
diaryliodonium salts produced N-aryl nitrilium intermediates,
and some special N-aryl nitrilium salts could be used to
prepare a series of heterocycles.[10] Inspired by these findings,
we further envisioned the alkenylation of nitriles with vinyl-
iodoniym salts to generate aza-butadienylium intermediates,
which should be trapped readily in situ by alkynes to produce
pyridines. For this purpose, in preliminary experiments,
three different kinds of vinyliodonium salts[11] [PhCH
=
CHI+ArOTfÀ (1Ar) where Ar= Ph, o-tol, and mesityl] were
reacted with benzonitrile (2a) and 1-phenyl-1-butyne (3a)
using previously developed reaction conditions.[10] To our
delight, all vinyliodonium salts gave the desired pyridine 4aaa
in moderate to good yields (Scheme 2). However, the reaction
of 1Ph also yielded a significant amount of the quinoline 5a,
thus indicating poor chemoselectivity of the vinyl-phenyl
iodonium salt (Scheme 2a). The chemoselectivity could not
be improved with an increased amount of 1Ph. Consequently,
[*] J. Sheng,[+] Y. Wang,[+] X. Su, R. He, Prof. Dr. C. Chen
Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical
Biology (Ministry of Education), Department of Chemistry, Tsinghua
University, Beijing 100084 (China)
E-mail: chenchao01@mails.tsinghua.edu.cn
[+] These authors contributed equally to this work.
1o-tol (1a) was examined with 2a and 3a, and excitingly, 4aaa
was obtained in 85% yield and the quinoline 5b was present
in only 1% yield (Scheme 2b). The yield of 4aaa was further
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201700696.
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+
À
=
Scheme 2. Screening the aryl group of PhCH CHI ArOTf (1Ar) for the
selective transformation of vinyl, versus aryl groups, into the pyridine
product 4aaa. DCE=1,2-dichloroethane, Tf=trifluoromethanesulfonyl.
Scheme 3. The scope with respect to the nitriles and alkynes for
pyridine synthesis.
raised to 90% with 2 equivalents of 1a. As expected, the
reaction of 1Mes supplied 4aaa as the sole product, albeit in
lower yield (80%; Scheme 2c). Hence, 1a was chosen as the
model vinyliodnium component to react with 2a and 3a. With
further optimization of the copper salts, solvents, and
substrates ratios, reaction conditions were discovered to
deliver 4aaa in 92% yield (for details see Table 1 in the
Supporting Information).
(Scheme 4). In this situation, the reaction presumably gave, at
first, dihydropyridines, which underwent dehydrogenation
spontaneously.[8,12] An exception was benzothiophene, as the
product 4abo was obtained by a de-arylation process. Starting
with indene, 4abp was formed in 80% yield as determined by
GC, and it was oxidized to give 4abp’ in 20% yield during
isolation. The possibility of incorporating alkenes is highly
important because alkenes (3j–l) are less expensive than the
corresponding alkynes. Additionally, in the case of 3m–p, the
corresponding cyclic alkynes are typically inaccessible.
Further investigation with respect to the scope of vinyl-
iodonium triflates was conducted (Scheme 5).[11] It was
exciting to find that aryl (1b–d), alkyl (1e–h) and ester (1i)
substituents on the vinyl motifs all gave the desired products.
Styrene was also tested with 1e and 2a, and formed the
Under the optimized reaction conditions, the scope with
respect to the nitrile and alkyne substrates was studied. The
results are summarized in Scheme 3. Firstly, a variety of
nitriles (2) were evaluated in the reaction to synthesize
pyridines with a range of substituents at the 2-position,
including substituted phenyl, naphthyl, thienyl, and alkyl
groups (4aaa–aia). 3-Methoxypropanenitrile and cyclo-
propyl-carbonitrile also worked well to give 4ajh and 4akh,
respectively, with the sensitive substituents remaining intact.
Next, a range of alkynes were examined to modify the
substituents at the 3- and 4-positions of the pyridines. As
shown in Scheme 3, reactions using either terminal or internal
alkynes bearing aryl, alkyl, halogen, and ester groups
proceeded smoothly. The use of terminal alkynes, including
phenyl acetylene, 1-heptyne, and 1-cyclohexenyl acetylene,
afforded the pyridines 4abc–abe with the substituents allo-
cated to the 4-position. The employment of aryl-alkyl
acetylene produced 4-aryl-3-alkyl pyridines in high yields
and phenylpropiolate (3 f) afforded the 3-ester-substituted
pyridine 4abf in 50% yield. Furthermore, when (bromoethy-
nyl)benzene (3g) was used, 4abg was formed exclusively.
Apparently, this synthetic method gave pyridines with great
regioselectivity when asymmetric alkynes were used. The high
regioselectivity is attributed to the electrophilic process of the
annulation reaction.[10]
Aside from acetylenes, alkenes also worked in the
reaction with 1a and nitriles to form the pyridines 4abj–abp
Scheme 4. The scope with respect to the alkenes for pyridine syn-
thesis.
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III [10,15]
À
electrophilic species, Ar Cu .
Hence, in this reaction,
vinyliodonium salts may react with CuBr to afford vinyl-CuIII
species (Scheme 7). Presumably, the reaction of 1a with CuBr
generates the vinyl-CuIII A rather than tol-CuIII B because of
the ortho effect.[16] Possibly, A is easily coordinated by 2 to
Scheme 5. The scope with respect to the vinyliodonium triflates for
pyridine synthesis.
desired product 4eac. Surprisingly, both thiophene-and
indene-substituted motifs opened access to the fused fully
substituted pyridines 4jaa and 4kbm, respectively.
Pyridines fused with carbocycles and heterocycles are
privileged scaffolds in various bioactive natural products
(especially in alkaloids) and synthetic therapeutic agents.[13] If
alkynes were tethered with nitriles (such as w-cyano-1-
alkynes), polycyclic pyridines would be produced with our
reaction.[14] In fact, 1a reacted with a range of w-cyano-1-
alkynes to give fused pyridines (Scheme 6). The process is
capable of direct synthesis of chloro- and bromopyrdines,
which are useful building blocks.
As mentioned, the reactions are accomplished with
iodonium salts, nitriles, and alkynes, as the three components,
with CuBr as the catalyst. It is precedented that diary-
liodonium salts can be affected by copper salts to generate
Scheme 7. Proposed mechanism of the three-component process for
pyridine synthesis.
afford C, which undergoes elimination of CuBr to afford the
aza-butadienyl cation D. The cation D is highly electrophilic
and is attacked by acetylene in either a concerted cyclo-
addition step to give intermediate E, or through an ionic step
to the give intermediates F and G. Both intermediates could
lose a proton to give the pyridine product 4. This series of
cationic procedures ensures the excellent regioselectivity for
asymmetric alkynes.
When acetylene is replaced by an alkene, D reacts with
the alkene to give the dihydropyridine H, which is capable of
losing dihydrogen in the reaction (Scheme 7). The dihydro-
pyridine 6 could be trapped in some cases. For instance, when
norbornene (3q) was reacted with 1a and 2b, the product 7q
was isolated in 42% yield, and was formed presumably from
the cycloaddition reaction of the dihydropyridine Iq with
another norbornene.[17]
To demonstrate the value of this new method, the
bromopyridine 4abg was prepared from 4 millimoles of 2b,
and 0.86 grams of the desired product were obtained (59%
yield; Scheme 8). Further derivatization of 4abg was readily
realized. For example, upon reaction with bis(pinacolato)di-
boron, Br was converted into a boronic acid pinacol ester
(8a), with [Pd(dppf)Cl2] as the catalyst. Moreover, the Suzuki
reaction of 4abg with anisyl boronic acid afforded the
Scheme 6. The use of w-cyano-1-alkynes in the synthesis of fused
pyridines.
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Acknowledgements
This work was supported by National Natural Science
Foundation of China (21372138 and 21672120), The National
Key Research and Development Program of China
(2016YFB0401400), and the Fok Ying Tong Education
Foundation of China (Grant No. 151014).
Conflict of interest
The authors declare no conflict of interest.
Keywords: alkynes · copper · heterocycles ·
multicomponent reactions · synthetic methods
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Manuscript received: January 19, 2017
Revised: February 28, 2017
Final Article published: && &&, &&&&
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Heterocycles
Three to one! A [2+2+2] modular syn-
thesis of multisubstituted pyridines with
excellent regioselectivity has been real-
ized by copper catalysis and involves
three distinct components: vinyliodo-
nium salts, nitriles, and alkynes. The
reactions proceeded with the facile for-
mation of an aza-butadienylium inter-
mediate by alkenylation of the nitrile with
a vinyliodonium salt.
J. Sheng, Y. Wang, X. Su, R. He,
C. Chen*
&&&& — &&&&
Copper-Catalyzed [2+2+2] Modular
Synthesis of Multisubstituted Pyridines:
Alkenylation of Nitriles with
Vinyliodonium Salts
6
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