Synthesis of pyridines by carbenoid-mediated ring opening of 2H-azirines

Article abstract of DOI:10.1002/anie.201209301

Roaming the range: The title reaction tolerates a wide range of substituents on the resulting pyridine ring using mild reaction conditions (see scheme; esp=α,α,α′,α′-tetramethyl-1,3- benzenedipropionic acid). The formation of the key intermediate is catal

Full text of DOI:10.1002/anie.201209301

Angewandte
Chemie
DOI: 10.1002/anie.201209301
Heterocycles
Synthesis of Pyridines by Carbenoid-Mediated Ring Opening of
2H-Azirines**
Nicole S. Y. Loy, Alok Singh, Xianxiu Xu, and Cheol-Min Park*
Dedicated to Professor Sunggak Kim
Pyridines are an important class of compounds because of
their prevalence in a myriad of natural products, pharma-
ceuticals, agrochemicals, and functional materials.[1] Driven
by the demands, diverse approaches to pyridine synthesis
have been developed.[2] Traditionally, they have been pre-
pared by condensation of amine and carbonyl compounds.[3]
More recently, synthesis of pyridines based on cycloaddition
and transition-metal catalysis has been reported.[4] Also,
direct functionalization of pyridine cores also allows access to
elaborate pyridine derivatives.[5] Despite these progress,
a flexible synthesis of pyridines allowing access to diverse
substitution patterns continues to draw a great deal of interest
from the synthetic community.
Strain-driven ring expansion has proven a powerful
strategy for construction of various types of carbocycles and
heterocycles.[6] This approach has been utilized in a broad
range of transformations by employing cyclopropane and
cyclobutane derivatives as the key components.[7] In this
regard, 2H-azirines have also been exploited as useful
precursors for reactive intermediates such as vinyl nitrenes
and nitrile ylides. These species have been employed in the
synthesis of various N-heterocycles[8] including indoles, pyr-
roles, isoxazoles, and pyrazolo[1,5-a]pyridines by reactions
The remarkable versatility of metal carbenoids renders
them valuable intermediates in the synthesis of various
heterocycles.[9] Given our interest in the ring-strain-driven
synthesis of heterocycles,[8a,10] we envisioned that activation of
2H-azirines, which are readily accessible through the Neber
reaction[11] in two steps from ketones may lead to the
formation of pyridines upon reaction with vinyl carbenoids
through a cascade of rearrangements as outlined in Equa-
tion (2). Thus, the reaction initiated by the formation of the
ylide A triggers ring opening of 2H-azirine to afford the 3-
azatriene B. Subsequent 6p electrocyclization leads to the
formation of the 3,4-dihydropyridine C, which could be
readily oxidized to pyridine. Herein, we describe the success-
ful development of a flexible synthesis of pyridines by formal
[3+3] cycloaddition of 2H-azirines with vinyl carbenoids
which enables the introduction of a broad range of substitu-
tion and its application in the synthesis of highly conjugated
poly-arylpyridine systems.
À
such as C H insertion and cycloaddition. However, the
synthesis of pyridines based on carbenoid-mediated ring
opening of 2H-azirines remains to be explored. Unlike the
reactions based on 2H-azirines where reactive species are
generated by intramolecular rearrangements initiated by
either heat or light [Eq. (1)], this carbenoid-mediated reac-
tion is unique in that intermolecular activation of 2H-azirine
leads to the formation of pyridines [Eq. (2)].
Our initial efforts to realize the transformation com-
menced with the reaction of 1a with the vinyl diazoacetate 2a
in the presence of [Rh2(OAc)4] in 1,2-dichloroethane (DCE)
at 908C. The reaction proceeded smoothly to give 1,4-
dihydropyridine 3aa in 46% yield. We surmise that rapid
tautomerization of the corresponding 3,4-dihydropyridine
accounts for the formation of 3aa. The structure of 3aa was
unambiguously assigned based on comprehensive NMR
experiments and the X-ray structure of the analogue 4ba.[12]
To optimize the reaction conditions, a variety of metal
complexes were screened (Table 1). Reactions with [Rh2-
(OAc)4] at different concentrations and temperatures all
provided inferior results (Table 1, entries 4 and 5). While
copper catalysts failed to give any 1,4-dihydropyridine
(Table 1, entries 1 and 2), electron-deficient dirhodium com-
plexes such as [Rh2(TFA)4], [Rh2(tfacam)4], and [Rh2(pfb)4]
gave the desired product albeit in low yield (Table 1,
[*] N. S. Y. Loy, Dr. A. Singh, Dr. X. Xu, Prof. C.-M. Park
Division of Chemistry and Biological Chemistry, School of Physical
and Mathematical Sciences, Nanyang Technological University
Singapore 637371 (Singapore)
E-mail: cmpark@ntu.edu.sg
[**] We thank Dr. Rakesh Ganguly for X-ray crystallographic analysis. We
acknowledge the Nanyang Technological University for financial
support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201209301.
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Table 1: Development of 1,4-dihydropyridine synthesis.[a]
Table 2: Scope of 2H-azirines in the pyridine synthesis.[a]
Entry
Catalyst
T [8C]
Yield [%][b]
1
2
3
Cu(OTf)2
90
90
90
90
70
90
90
90
90
90
0
0
[Cu(hfacac)2]
[Rh2(OAc)4]
[Rh2(OAc)4]
[Rh2(OAc)4]
[Rh2(TFA)4]
[Rh2(tfacam)4]
[Rh2(pfb)4]
[Rh2(Piv)4]
44
38
38
28
34
34
71
80
4[c]
5
4aa (72%)
4ba (82%)
4ca (86%)
6
7
8
9
10
[Rh2(esp)2]
[a] Reaction conditions: 1a (0.3 mmol), 2a (0.48 mmol), DCE (0.15m).
[b] Yields determined by NMR vs. standard. [c] Reaction performed in
0.05m. esp=a,a,a’,a’-tetramethyl-1,3-benzenedipropionate, pfb=per-
fluorobutyrate, Piv=pivaloate, hfacac=hexafluoroacetylacetonate,
TFA=trifluoroacetate, tfacam=trifluoroacetamide.
4da (72%)
4ea (95%)
4 fa (93%)
entries 6–8). In contrast, improved yields
were achieved by using dirhodium com-
plexes with sterically bulky ligands such as
4ga (94%)
4ha (84%)
4ia (78%)
[Rh2(Piv)4] and [Rh2(esp)2][13] (Table 1,
entries 9 and 10), thus suggesting that the
steric effect of ligands plays an important
role in this reaction. We postulate that the
steric demand of catalysts steers the for-
mation of 3-azatriene with the correct
configuration necessary for cyclization
among potentially multiple isomers. To
4ja (74%)
4ka (84%)
4la (79%)
develop a one-pot synthesis of pyridines,
[a] Reaction conditions: 1 (0.3 mmol), 2a (0.48 mmol), DCE (0.15m). The
we attempted direct oxidation of 3aa by
reported yields in parentheses are of the isolated products.
exposure to 2,3-dichloro-5,6-dicyano-1,4-
benzoquinone (DDQ) at room temperature for 15 minutes
after the completion of the reaction with [Rh2(esp)2]. We
were pleased to find that the one-pot protocol provided the
pyridine 4aa in 72% yield.
azirines proved successful to afford 6-(2-thienyl) and 6-(2-
furanyl)pyridines in 78% and 74%, respectively. Further-
more, 6-vinyl-substituted pyridines such as 4ka were readily
accessed by using 3-vinyl-2H-azirine as the reaction partner.
To examine the necessity of an ester group at the 2-position of
2H-azirine for successful transformation, the 2,3-diphenyl-
2H-azirine 1l was subjected to the optimized reaction
conditions. Gratifyingly, the substrate also smoothly reacted
to provide the pyridine 4la in 79% yield.
Next, we turned our attention to examine the scope of the
other reaction partner. Survey of vinyl diazo compounds with
various substitutions revealed that the transformation is also
generally well tolerated and provides pyridines in good yields
(Table 3). Examination of the electronic effect by substitution
of the phenyl group of 2a with electron-donating and
electron-withdrawing groups showed that the reaction effi-
ciency is marginally affected: 4-MeOC6H4 (4ab, 76%), Ph
(4aa, 72%), and 4-MeO2CC6H4 (4ac, 67%). The reaction
with 2d, bearing a sterically more demanding 2-chlorophenyl
group, led to a moderate decrease in yield, which is in contrast
to the result of the steric effect examined on 2H-azirine 1g
Encouraged by these results, we turned our attention to
the substrate scope of the reaction. To develop a synthetic
protocol which provides pyridines with a wide range of
substitution, we first surveyed various 2H-azirines by reacting
with vinyl diazoacetate 2a (Table 2). Generally, reactions
proceeded smoothly to give pyridines in good to excellent
yields. 3-Alkyl-substituted 2H-azirines reacted well, including
those with primary and secondary alkyl groups, thus providing
6-alkylpyridines (4aa–4ca). Reactions with 3-aryl-substituted
2H-azirines also proceeded smoothly to afford 6-arylpyri-
dines, and those with both electron-rich and electron-defi-
cient aryl groups gave the corresponding products in good
yields (4da–4 fa). The 2H-azirine having an ortho-chloro-
phenyl group reacted smoothly to afford the corresponding
pyridine 4ga in 94% yield, thus suggesting that steric
hinderance is tolerated well. In addition, the substrate 1h,
having a fused aryl substituent, also provided 4ha in 84%
yield. Extension of the reaction to heteroaryl-substituted 2H-
2
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Table 3: Scope of vinyl diazo compounds in pyridine synthesis.[a]
by imposing some steric hindrance. In addition, elimination of
the chloro group from the corresponding 1,4-dihydropyridine
3ag would obviate the need for a separate oxidation. To our
delight, the reaction with 2g smoothly produced 4-unsubsti-
tuted pyridine 4ag in 61% yield after spontaneous elimina-
tion of chloride from 3ag.
Also, introduction of an additional ester group on the
pyridine ring was made possible by employing the 4-
alkoxycarbonyl vinyl diazoacetate 2h, thus resulting in the
formation of the highly deactivated trialkoxycarbonylpyri-
dine 4ah in 77% yield (Scheme 2). Under the standard
4ab (76%)
4ac (67%)
4ad (55%)
4ae (80%)
4af (44%)
4ag (61%)
4ah (77%)[b,c]
4ai (65%)[b,c]
4aj (81%)
Scheme 2. Mechanistic evidence of 3-azatriene intermediacy.
reaction conditions, the reaction provided an inseparable
mixture of the expected 1,4-dihydropyridine 3ah and pyridine
4ah in 62% along with 3-azatriene 5ah in 17%. This 3-
azatriene 5ah smoothly underwent cyclization when heated at
1708C in 1,2-dichlorobenzene to give the pyridine 4ah in 76%
after spontaneous oxidation. This observation suggests that
the pyridine formation is likely to involve 6p electrocycliza-
tion of 3-azatrienes. Alternatively, 4ah was obtained in higher
yield by directly heating the mixture of 3ah, 4ah, and 5ah at
1708C when the starting materials were consumed.
4ak (72%)
4al (71%)
[a] Reaction conditions: 1a (0.3 mmol), 2 (0.48 mmol), DCE (0.15m).
The reported yields in parentheses are of the isolated products.
[b] Reaction performed in 1,2-dichlorobenzene at 908C for 6 h, then
heated at 1708C overnight. [c] Spontaneous oxidation without DDQ.
(55% versus 94%). 4-Alkyl-substituted vinyl diazoacetates
smoothly reacted to afford the corresponding pyridines 4ae
and 4af. In an attempt to access 4-unsubstitued pyridines, we
performed the reaction with the requisite unsubstituted vinyl
diazoacetate 2g’, which turned out to be sluggish and
provided a complex mixture (Scheme 1). We reasoned that
no substitution at C4 of 2g’ might cause complication in the
reaction and that introduction of a chloro group as a tempo-
rary removable substituent may help overcome the problem
Furthermore, pentasubstituted pyridines could be pre-
pared by employing 3,4-disubstituted vinyl diazoacetates.
Thus, the reaction with 2i bearing a 3-methyl substitutent
proceeded smoothly to afford 4ai in 65% yield. The indole-
substituted pyridine 4aj could be prepared in excellent yield
by employing the corresponding diazo compound. To further
expand the scope of functional groups that could be installed
on pyridines, we examined the reaction with the diazoketone
2k and diazophosphonate 2l. Gratifyingly, the reactions
provided the pyridines 4ak (72%) and 4al (71%) bearing
these useful functional groups.
Molecules with extended conjugation have found broad
applications in the area of functional materials.[14] We
envisioned that double annulation of bis-2H-azirines would
offer a powerful means for the synthesis of such polyarylpyr-
idine systems. Indeed, the annulation reaction of 6a and 6b
with vinyl diazoacetate 2a proceeded remarkably well,
providing extended aryl-heteroaryl systems 7aa and 7ba in
72% and 70%, respectively (Scheme 3).
In summary, we have developed a novel strategy of
activation of 2H-azirines with carbenoids for the synthesis of
Scheme 1. Synthesis of the 4-unsubstituted pyridine 4ag.
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Experimental Section
An oven dried Schlenk tube charged with [Rh2(esp)2] (0.02 equiv) was
purged with nitrogen, and a solution of azirine (0.3 mmol, 1 equiv) in
DCE (1 mL) was added. A solution of the freshly prepared diazo
compound (1.6 equiv) in DCE (1 mL) was added dropwise to the
suspension under nitrogen. The reaction mixture was then heated to
908C for 3–20 h. The solution was cooled to RT and treated with
DDQ (1 equiv). The suspension was stirred at RT for 15 min and
filtered through a plug of silica gel. The filtrate was concentrated in
vacuo, and the residue was purified by flash chromatography using
hexanes/ethyl acetate (4:1) as the eluent to give the desired product.
Received: November 20, 2012
Published online: && &&, &&&&
Keywords: carbenoids · electrocyclic reactions · heterocycle ·
.
ring expansion · synthetic methods
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Communications
Heterocycles
N. S. Y. Loy, A. Singh, X. Xu,
C.-M. Park*
&&& — &&&
Roaming the range: The title reaction
tolerates wide range of substituents on
the resulting pyridine ring using mild
reaction conditions (see scheme; esp=
a,a,a’,a’-tetramethyl-1,3-benzenedipro-
pionic acid). The formation of the key
intermediate is catalyst-controlled, and
subsequent cyclization and oxidation
affords pyridines in excellent yields. The
method has been used for the efficient
synthesis of polyarylpyridines.
Synthesis of Pyridines by Carbenoid-
Mediated Ring Opening of 2H-Azirines
6
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