Direct asymmetric dearomatization of pyridines and pyrazines by iridium-catalyzed allylic amination reactions

Article abstract of DOI:10.1002/anie.201404286

The first iridium-catalyzed intramolecular asymmetric allylic dearomatization reaction of pyridines and pyrazines has been realized. 2,3-Dihydroindolizine and 6,7-dihydropyrrolo[1,2-a]pyrazine derivatives were obtained with excellent yields and enantioselectivity. This methodology features dearomatization by direct N-allylic alkylation of pyridines or pyrazines under mild reaction conditions. Getting 'rid' of aromaticity: An iridium-catalyzed intramolecular asymmetric allylic dearomatization reaction converts pyridines and pyrazines (not shown) into 2,3-dihydroindolizine and 6,7-dihydropyrrolo[1,2- a]pyrazine derivatives with excellent yields and enantioselectivities. The dearomatization proceeds by direct N-allylic alkylation of pyridines or pyrazines under mild reaction conditions.

Full text of DOI:10.1002/anie.201404286

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Angewandte
Communications
DOI: 10.1002/anie.201404286
Asymmetric Catalysis
Direct Asymmetric Dearomatization of Pyridines and Pyrazines by
Iridium-Catalyzed Allylic Amination Reactions**
Ze-Peng Yang, Qing-Feng Wu, and Shu-Li You*
Dedicated to Professor Changtao Qian on the occasion of his 80th birthday
Abstract: The first iridium-catalyzed intramolecular asym-
metric allylic dearomatization reaction of pyridines and
pyrazines has been realized. 2,3-Dihydroindolizine and 6,7-
dihydropyrrolo[1,2-a]pyrazine derivatives were obtained with
excellent yields and enantioselectivity. This methodology
features dearomatization by direct N-allylic alkylation of
pyridines or pyrazines under mild reaction conditions.
A
romatic compounds have found wide application as basic
Scheme 1. Proposed allylic dearomatization reaction of pyridine.
chemical feedstocks in polymers, paints, cosmetics, pharma-
ceuticals, and many others. Dearomatization reactions of
these readily available and cheap starting materials could
produce a variety of fused, bridged, and spiro ring structures
in a very straightforward manner.[1] Therefore the dearoma-
tization process is now one of the most important trans-
formations of aromatic compounds and widely applied in the
synthesis of functional molecules. Despite significant efforts
devoted to the development of dearomatization reactions and
their subsequent transformations, it has only been recently
that direct catalytic asymmetric dearomatization reactions
have been achieved.[1b,c] We recently found that transition-
metal-catalyzed allylic substitution reactions were compatible
with asymmetric dearomatization processes for various aro-
matics such as indoles, pyrroles, and phenols.[2,3] However,
suitable aromatic compounds are limited to electron-rich
nucleophiles, and electron-deficient aromatic compounds,
such as pyridines and pyrazines, have not been explored yet.
To date, asymmetric dearomatization reactions of pyri-
dine derivatives have focused on asymmetric hydrogena-
tions,[4] transfer hydrogenations,[5] and Reissert-type reac-
tions.[6] In most cases, preactivation by N-acylation or
alkylation to generate highly electrophilic pyridinium inter-
mediates is required. The removal of the protecting group
from the nitrogen atom is always essential at a later stage. In
the course of our research program on the development of
catalytic asymmetric dearomatization reactions,[7] we became
interested in exploring pyridine derivatives as an N nucleo-
phile in allylic substitution reactions, thus providing the
corresponding dearomatized products, which are usually
found as a core structure in numerous alkaloids and
biologically active compounds.[8,9] As shown in Scheme 1,
the acidic Ha could be easily deprotonated, thus affording an
electron-rich intermediate. The intramolecular design would
favor the N atom as a nucleophile in the allylic substitution
reaction, thus leading to the dearomatized product. This
approach was found to work well under iridium catalysis, and
we herein report such a direct asymmetric allylic dearoma-
tization reaction of pyridines and pyrazines.
At the outset, we utilized a well-developed catalytic
iridium system including [{Ir(cod)Cl}2] and the phosphorami-
dite L1 (Table 1) as the catalyst.[10] In the presence of 2 mol%
of [{Ir(cod)Cl}2] and 4 mol% of L1, reaction of 1a in THF at
room temperature for 6 hours gave the dearomatized product
2a in 90% yield with 96% ee (entry 1). Several chiral ligands
were then examined, and the Alexakis ligand L2[11] was found
to be the optimal ligand, thus affording 2a in 99% yield and
98% ee (entry 2). Other screened ligands (L3, L4 and L5)
could also catalyze the reaction with excellent enantioselec-
tivity, but in decreased yields (entries 3–5). Varying the
solvents (1,4-dioxane, CH2Cl2, Et2O and cyclohexane) influ-
enced the outcome considerably. The reaction in THF gave
the best result, thus affording 2a in 99% yield and 98% ee
(entry 2).
[*] Z.-P. Yang, Dr. Q.-F. Wu, Prof. Dr. S.-L. You
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
345 Lingling Lu, Shanghai 200032 (China)
E-mail: slyou@sioc.ac.cn
Homepage: http://shuliyou.sioc.ac.cn/
Under the optimized reaction conditions with the iridium
catalyst, formed in situ from [{Ir(cod)Cl}2] and L2, various 2-
pyridyl allylic carbonates were tested to examine the general-
ity of the reaction. The results are summarized in Table 2.
Reactions of the allylic carbonate substrates bearing
a CO2Me or CO2Et group on the linking atom both gave
the dearomatized products, thus forming the indolizines 2 in
good yields with excellent ee values (entries 1 and 2).
[**] We thank the National Basic Research Program of China (973
Program 2010CB833300), NSFC (21025209, 21121062, 21332009),
and the Chinese Academy of Sciences for generous financial
support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201404286.
6986
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Table 1: Optimization of the reaction conditions.[a]
Table 2: The reaction substrate scope of pyridine derivatives.[a]
Entry
1
t
[h]
2
Yield
[%][b]
ee
[%][c]
1
2
3
4
1a
1b
1c
1d
4
4
2
4
2a
2b
2c
2d
99
93
95
93
98
97
95
95
Entry
Ligand
Solvent
t [h]
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
9
L1
L2
L3
L4
L5
L2
L2
L2
L2
THF
THF
THF
THF
6
4
6
12
12
5
12
12
12
90
99
88
81
88
78
82
44
52
96
98
93
96
95
98
83
91
89
THF
1,4-dioxane
CH2Cl2
Et2O
cyclohexane
[a] Reaction conditions: [{Ir(cod)Cl}2]/L/1a=0.02:0.04:1.0; 0.2 mmol of
1a in solvent (2.0 mL), RT. Catalyst was prepared by nPrNH2 activation.
[b] Yield of isolated product. [c] Determined by HPLC analysis. cod=1,5-
cyclooctadiene, THF=tetrahydrofuran.
Substrates bearing either an electron-donating group (5-
MeO, 4-Me, 5-Me, 5-Ph; entries 3–6) or electron-withdrawing
group (5-Cl, 5-Br; entries 7 and 8) on the pyridine core all led
to the corresponding products in excellent yields and
ee values. When the substrates 1i and 1j, bearing 4-Cl and
4-Br, respectively, were tested, the reaction proceeded
smoothly to afford the dearomatized products with full
conversion. However, the products were not stable during
the purification process. After a Diels–Alder reaction (see
below) with diethyl acetylenedicarboxylate in situ, the
enantioselectivity of the products was determined to be
99% ee (entries 9 and 10).[12] The structure of the product was
confirmed unambiguously by an X-ray crystallographic
analysis of a single crystal of 2 f.[12] Next, various groups
(R’) on the linking atom were tested. Substrates bearing an
Ac (entry 11), Bz (entry 12), and p-Br-C6H4CO (entry 13)
groups all gave the desired amination products in good to
excellent yields and excellent enantioselectivity (82–94%
yield, 91–99% ee). The substrate 1n, having a PhSO2CH
linker, was also tolerated and afforded the corresponding
product in 82% yield and 83% ee (entry 14). Notably, when
the dimethyl malonate substrate 1o was used, the desired
product 2o was unstable during the purification. Therefore
Pt/C-mediated hydrogenation was carried out after the
dearomatization reaction, and 2o’ was obtained in 92%
yield and 89% ee (entry 15).
5
6
7
8
1e (R=Me)
1 f (R=Ph)
1g (R=Cl)
1h (R=Br)
4
4
4
3
2e
2 f
2g
2h
90
99
94
98
96
98
98
96
9
1i
1j
4
4
2i
2j
>95[d]
99[e]
10
>95[d]
99[e]
11[f]
1k
12
2k
82
93
12[f]
1l
3
2l
94
99
Interestingly, under slightly modified reaction conditions,
as shown in Table 3, pyrazine derivatives were also found to
be suitable for allylic dearomatization reactions. Various
substituted pyrazine-linked allylic carbonates were tested to
examine the substrate scope. The results are summarized in
Table 3. To our delight, all the substrates, having either
various electron-withdrawing groups on the linking carbon
atom or substituents on the pyrazine core, underwent the
asymmetric allylic dearomatization reaction smoothly. Their
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Table 3: The reaction substrate scope of pyrazine derivatives.[a]
Table 2: (Continued)
Entry
1
t
[h]
2
Yield
[%][b]
ee
[%][c]
Entry
3
t
[h]
4
Yield
[%][b]
ee
[%][c]
13
1m
1n
5
2m
2n
90
82
91
83
1
2
3
3a (R’=CO2Et)
3b (R’=CO2tBu)
3c (R’=COPh)
18
17
12
4a
4b
4c
88
68
95
94
92
97
14[f]
12
4
5
3d
3e
36
40
4d
4e
75
75
96
97
15[g]
1o
0.2
2o’
92[h]
89
[a] Reaction conditions: [{Ir(cod)Cl}2]/L2/1=0.02:0.04:1.0; 0.2 mmol of
1 in THF (2.0 mL), RT. Catalyst was prepared by nPrNH2 activation.
[b] Yield of isolated product. [c] Determined by HPLC analysis. [d] Con-
version determined by 1H NMR analysis of the crude reaction mixture.
[e] Determined after Diels–Alder reaction with diethyl acetylenedicar-
boxylate.[12] [f] Reaction conducted at 508C. [g] Used 0.2 equiv of DABCO
as the base and hydrogenation was carried out after dearomatization
reaction.[12] [h] The d.r. value of 2o’ is 5:1.
6
7
3 f
1.5 4 f
64
71
91
86
corresponding 6,7-dihydropyrrolo[1,2-a]pyrazine derivatives
were obtained in 64–95% yield and 86–97% ee. These results
show that the dearomatization strategy by direct N-allylic
alkylation is general for nitrogen-containing heteroarenes.
Furthermore, the dearomatized products obtained here
could be applied to the Diels–Alder reaction for the synthesis
of the multifunctionalized bridged ring compounds. As shown
in Scheme 2, in the presence of 1.2 equivalents of diethyl
acetylenedicarboxylate as the dienophile, various Diels–
Alder products (5) could be obtained in good yields with
excellent d.r. values. In all cases, the products were obtained
without the loss of the enantiomeric purity.
3g
40
4g
8
3h
36
4h
91
92
[a] Reaction conditions: [{Ir(cod)Cl}2]/L4/3=0.02:0.04:1.0; 0.2 mmol of
3 in THF (2.0 mL), RT. Catalyst was prepared by nPrNH2 activation.
[b] Yield of the isolated product. [c] Determined by HPLC analysis.
In summary, the first iridium-catalyzed intramolecular
asymmetric allylic dearomatization reaction of pyridines and
pyrazines has been realized. Under extremely mild reaction
conditions, the dearomatized 2,3-dihydroindolizine and 6,7-
dihydropyrrolo[1,2-a]pyrazine derivatives were obtained in
excellent yields and enantioselectivity. The enantioenriched
dihydroindolizine products could be subjected to Diels–Alder
reactions with diethyl acetylenedicarboxylate as the dieno-
phile. Another feature of the current dearomatization reac-
tion is the fact that the dearomatization reaction proceeds by
the direct N-allylic alkylation of pyridines and pyrazines.
Work on the extension of the reaction scope and development
of more efficient catalytic systems are currently underway in
our laboratory.
Received: April 14, 2014
Scheme 2. Diels–Alder reactions with diethyl acetylenedicarboxylate.
Published online: May 26, 2014
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