Carbene-catalyzed LUMO activation of alkyne esters for access to functional pyridines

Article abstract of DOI:10.1039/c7cc08039e

A carbene-catalyzed LUMO activation of α,β-unsaturated alkyne esters is reported. This catalytic process allows for effective reactions of alkyne esters with enamides to synthesize functional pyridines via simple protocols. A previously unexplored unsaturated alkyne acyl azolium intermediate is involved in the key step of the reaction.

Full text of DOI:10.1039/c7cc08039e

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Carbene-Catalyzed LUMO Activation of Alkyne Esters for Access to
Functional Pyridines
Chengli Mou,†a Jichang Wu,†a Zhijian Huang,†b Jun Sun,a Zhichao Jin,*a and Yonggui Robin Chi*a,b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A carbene-catalyzed LUMO activation of α,β-unsaturated alkyne
esters is reported. This catalytic process allows for effective
reactions of alkyne esters with enamides to synthesize functional
a) Examples of pyridineligandsin transitionmetal catalysis.
A
R
B
pyridines via simple protocols.
A
previously unexplored
Ph
Ar
unsaturated alkyne acyl azolium intermediate is involved in the
key step of the reaction.
N
Ir
N
N
N
H
H
N
Pyridines are common scaffolds in both natural and synthetic
molecules with broad utilities.1 For example, various pyridine
and bipyridine derivatives have been extensively studied as
ligands for transition metal catalysts (Figure 1a).2 Numerous
methods, typically complementary to each other, have been
developed for the synthesis of pyridine derivatives.3 We’re
interested in exploring metal-free N-heterocyclic carbene
(abbreviated as NHC or carbene) catalysis for new activations
and/or effective organic synthesis. Carbene organic catalysis
has a long history of studies4 and many new interests are re-
generated in recent years.5 The most widely studied key
intermediates involved in NHC catalysis include Breslow
intermediates6 derived from aldehydes, and acyl azolium ester
intermediates7 derived from carboxylic esters (Figure 1b). The
Ru
N
Cl
PPh2
Ph2P
b) Key intermediatesin NHC-catalyzedreactions.
OH
O
O
N
N
N
R
R
N
R
N
X
N
X
X
alkyne ester-derived
acyl azolium intermediate
Breslow
intermediate
Acyl azolium
intermediate
(unexplored)
c) This work: NHC-catalyzedalkyneester activationand pyridinesynthesis.
(seeSI for a complete pathway)
X
N
NHC
O
N
NHC-bound intermediates derived from the related α,β
-
unsaturated aldehydes8 and esters9 bearing a carbon-carbon
double bond next to the carbonyl group have also been
studied.
+
functional
pyridines
OAr
N
R
NHTs
3
1
Ts
2
R
O
isomerize
(60 oC)
Here we disclose a catalytic generation and reaction of
unsaturated alkyne ester-derived azolium ester intermediate
NHC
bearing
a carbon-carbon triple bond (Figure 1b). This
O
previously unexplored intermediate could likely lead to unique
reaction and reaction cascade due to the presence of a carbon-
carbon triple bond. For example, carbene-catalyzed reactions
lactam
formation
NTs
N
2
Ts
N
R
R
N
X
I
NHC
N
O
R
O
N
3'
X
Figure 1. Functional pyridines and our carbene-catalyzed
synthetic method.
between alkyne esters and enamides can be realized for quick
access to hydroxyl pyridines (Figure 1c). Specifically, addition
of an NHC catalyst to the ester moiety of unsaturated alkyne
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Journal Name
ester
Nucleophilic conjugated addition10 or 1,2-addition/Claisen product of 3a in 83% isolated yield when carrying out the
rearrangement11 of enamide
with intermediate affords reaction in DMF using NHC as the catalyst and K3PO4 as the
1
leads to the corresponding acyl azolium intermediate
I.
the pyridine product (entries 7 to 10). At last, we could get the
2
I
B
intermediate II that undergoes further reactions to eventually base (entry 9).
form unsaturated lactam adduct 3’ (see SI for complete
pathway) with a generation of the NHC catalyst. Under a Table 2. Substrate scope.a
slightly elevated temperature in the same reaction mixture,
adduct 3’ goes through an isomerization to form hydroxyl
pyridine
3 3)
as the product.12 The hydroxyl pyridine products (
from our reactions can be readily transformed to useful
bypyridines13 and other pyridine-derived amine functional
molecules.14
Ph
Ph
Ph
Table 1. Condition optimization.a
N
N
N
Ts
Ts
Cl
Ts
O
O
O
H3C
F3C
3b, 65%
3c, 84%
3d, 80%
Ph
Ph
Ph
N
N
O
N
Ts
Ts
O
Ts
H3C
O
Br
CH3
3g, 67%
3e, 81%
3f, 48%
CH3
OCH3
H3C
Ph
Br
N
N
N
N
Entry
NHC
A
Base
NaHCO3
NaHCO3
NaHCO3
Na2CO3
K3PO3
Solvent
THF
Yield [%]b
25
Ts
Ts
Ts
Ts
Ts
Ts
O
Ph
O
Ph
O
Ph
O
O
O
1
2
3j, 87%
3k, 85%
3h, 64%
3i, 60%
Cl
F
B
THF
30
O
S
3
C
THF
20
4
B
THF
37
N
N
N
N
Ts
Ph
O
Ph
5
B
THF
40
Ts
Ts
Ts
Ph
O
Ph
Ph
O
3m, 78%
3n, 59%
3o, 73%
3l, 81%
6
B
TEA
THF
19
Ph
7
B
K3PO3
toluene
CH2Cl2
DMF
CH3CN
Trace
Trace
83
H3C
N
N
2q
8
B
K3PO3
N
Ph
N
NTs
Ts
9
B
K3PO3
H3C
Ph
2r
O
Ph
Ts
10
B
K3PO3
39
3p, 75%
3r, 65%
3q, 42%
O
a
a
Reaction conditions: 1a (0.05 mmol), 2a (0.10 mmol), NHC
b
Reaction conditions as stated in Table 1, entry 9. Yields are
(0.01 mmol), base (0.05 mmol), solvent (0.5 mL), RT, 24 h.
Yields were isolated yields after column chromatography.
isolated yields after purification by column chromatography.
With the optimized reaction conditions at hand, we then
examined the reaction scope using different substituted
3-Phenylpropynoic acid ester 1a was chosen as the model
substrate to react with enamide precursor 2a for the synthesis
of multi-substituted hydroxyl pyridine 3a. Various NHC pre-
catalysts could mediate this transformation (Table 1, entries 1
substrates
electron-withdrawing groups were well tolerated on the
benzene rings of substrate 3b to 3f). Substituents on the 4-
1 and 2 (Table 2). Both electron-donating and
1
(
to 3). Triazolium-typed NHC pre-catalyst
group gave the desired product with the best yield (entry 2).
We then decided to use catalyst for further optimizations.
B bearing one mesityl
and 3-positions of the benzene rings generally gave the
products in good yields (eg. 3b to 3e). Lower product yields
B
were observed with substrates
benzene groups (eg. 3f). The aromatic phenyl group on
substrate could be switch to an aliphatic methyl group with
1 bearing 2-substituted
Switching the base additive from NaHCO3 to K3PO3 led to an
increase on product yield (entry 5). After screening of different
solvents we found that solvents with higher polarities could
give the desired products in better yields, while solvents with
lower polarities such as toluene and CH2Cl2 could hardly afford
1
the substituted pyridine product afforded in 67% yield (3g).
Various substituents with different electronic properties could
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be installed on the benzene rings of the imide substrate
corresponding products afforded in moderate to good yields activation of
3h to 3m). The substituted benzene rings on substrate could activation generates a previously unexplored unsaturated
also be replaced with both hetero aromatic groups (3n to 3o azolium ester intermediate bearing a reactive carbon-carbon
2
with
In summary, we have developed an NHC-catalyzed
α,β
-unsaturated alkyne esters. This catalytic
(
2
)
and even simple aliphatic groups (3p), and the tri-substituted triple bond. Under mild conditions, the catalytic reactions of
pyridine products could be afforded in moderate to good alkyne esters and enamides effectively afford pyridine
yields. To our delight, 2-substituted imide substrate (2q) and derivatives with important utilities. For example, our products
the cyclic imide substrate (2r) could also react smoothly with can be transformed to bi-pyridines and related functional
3-phenylpropynoic acid ester 1a through this process. Both of molecules that can be used as ligands in transition metal
the tetra-substituted pyridine product
(
3q
)
and the catalysis. The alkyne ester-derived azolium ester intermediate
dihydrobenzoquinoline product (3r) could be afforded in contains carbon-carbon triple bond and shall exhibit rich
moderate yield under our current catalytic conditions. chemical reactivities. We therefore expect that further study
Switching the Ts protecting group on imine substrate
2 to a Ms of this intermediate shall lead a valuable set of reactions such
(Methanesulfonyl) group did not afford the desired product. as cascade reactions for quick access to sophisticated
Our attempt to prepare imine substrates with a Tf (Triflate) functional molecules.
unit as the protecting group was unsuccessful.
The O-tosyl group on product 3a could be switched to a
Conflicts of interest
good leaving group to afford
Product could be coupled with another 2-substituted
pyridine compound to produce the bi-pyridine molecule
5
in excellent yield over 2 steps.15
5
There are no conflicts to declare.
6
7
(Figure 2).16 Molecules containing the bi-pyridine structures
have been frequently used as ligands in various transition
Acknowledgements
metal catalytic reactions (Figure 1a, A
).13
We acknowledge financial support by the National Natural
Science Foundation of China (No. 21472028), National Key
Technologies R&D Program (No. 2014BAD23B01), “Thousand
Talent Plan”, The 10 Talent Plan (Shicengci) of Guizhou
Province, Guizhou Province Returned Oversea Student Science
and Technology Activity Program, Guizhou University (China)
and Singapore National Research Foundation (NRF-NRFI2016-
06), the Ministry of Education of Singapore (MOE2013-T2-2-
003; MOE2016-T2-1-032; RG108/16), A*STAR Individual
Research Grant (A1783c0008), and Nanyang Technological
University.
Notes and references
Figure 2. Construction of bipyridine ligand from product 3a
.
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