Iridium-catalyzed asymmetric hydrogenation of pyrrolo[1,2- a ]pyrazinium salts

Article abstract of DOI:10.1021/ol5013313

Highly enantioselective iridium-catalyzed hydrogenation of pyrrolo[1,2-a]pyrazinium salts has been achieved, providing a direct access to chiral 1,2,3,4-tetrahydropyrrolo[1,2-a]pyrazine derivatives with up to 95% ee. The key feature of the reaction is the addition of cesium carbonate, which increases the conversion and prohibits the racemization pathway of products.

Full text of DOI:10.1021/ol5013313

Letter
pubs.acs.org/OrgLett
Iridium-Catalyzed Asymmetric Hydrogenation of
Pyrrolo[1,2‑a]pyrazinium Salts
Wen-Xue Huang,† Chang-Bin Yu,† Lei Shi,† and Yong-Gui Zhou*,†,‡
†State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian
116023, China
‡State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345
Lingling Lu, Shanghai 200032, China
S
* Supporting Information
ABSTRACT: Highly enantioselective iridium-catalyzed hydro-
genation of pyrrolo[1,2-a]pyrazinium salts has been achieved,
providing a direct access to chiral 1,2,3,4-tetrahydropyrrolo[1,2-
a]pyrazine derivatives with up to 95% ee. The key feature of the
reaction is the addition of cesium carbonate, which increases the
conversion and prohibits the racemization pathway of products.
1,2,3,4-Tetrahydropyrrolo[1,2-a]pyrazines are considered to be
an important class of heterocycles for their potential
antihypersensitive, antiarrhythmic, antiamnesic, antihypoxic,
psychotropic, and aldose reductase inhibition activities.1
Especially, chiral 1,2,3,4-tetrahydropyrrolo[1,2-a]pyrazines
have been found in several natural products such as Hanishin
and Longamide B (Figure 1).2 The chiral skeleton is also
used. Only a few reports have been published, starting from
partially saturated 3,4-dihydropyrrolo[1,2-a]pyrazines.6
During the past decades, asymmetric hydrogenation of
aromatic and heteroaromatic compounds has emerged as a
powerful strategy for the synthesis of saturated or partially
saturated cyclic molecules.7 Many aromatic substrates such as
quinolines,8 isoquinolines,9 quinoxalines,10 pyridines,11 in-
doles,12 pyrroles,13 furans,14 imidazoles,15 thiophenes,16 and
carbocyclic aromatic rings17 have been successfully hydro-
genated with excellent enantioselectivities. The finding of a
robust catalyst and proper activation of inactive aromatic
compounds are key points for the successful implementation of
the reaction. Substrate activation is a quite efficient and
generally used strategy. With proper activators, such as
Brønsted acids,18 chloroformates,9a or benzyl bromides,9c,11f
the aromaticity of substrates can be partially destroyed, which
facilitates hydrogenation.
Figure 1. Some natural products and bioactive molecules containing
the 1,2,3,4-tetrahydropyrrolo[1,2-a]pyrazine skeleton.
Pyrrolo[1,2-a]pyrazines are bicyclic aromatics with one
nitrogen atom in the bridgehead position of the fused pyrazine
and pyrrole rings. Similar to pyridine and isoquinoline, both the
substrate and the hydrogenated product possess a strong
coordination ability. Once partially hydrogenated, the reserved
pyrrole ring is relatively sensitive to acid, which further
increases the difficulty in selective reduction of these substrates.
We envisaged that the activation of pyrrolo[1,2-a]pyrazine
with benzyl bromide would efficiently reduce the coordination
ability of the substrate and improve the reactivity for
hydrogenation (Scheme 1). Herein, we disclose an iridium-
catalyzed asymmetric hydrogenation of pyrrolo[1,2-a]pyra-
zinium salts, providing the chiral 1,2,3,4-tetrahydropyrrolo-
[1,2-a]pyrazines with up to 95% ee.
present in bioactive compounds such as 7-(1,2,3,4-tetrahydro-
pyrrolo[1,2-a]pyrazin-7-yl)quinolone, which has exhibited
potent in vivo efficacy in a murine lethal systemic infection
model.3 The position of the methyl group on the tetrahydro-
pyrazine ring as well as the S-configuration is important for this
efficacy.
Although many efforts have been devoted to the synthesis of
chiral 1,2,3,4-tetrahydropyrrolo[1,2-a]pyrazines, most of these
materials are synthesized via chiral starting materials or
auxiliaries,4 and there are quite a few methods for the catalytic
asymmetric preparation of these intriguing heterocycles.5 Thus,
the development of a simple and flexible asymmetric synthetic
method is highly desirable. According to retrosynthesis, the
asymmetric hydrogenation of easily available pyrrolo[1,2-
a]pyrazines is one of the most straightforward to access
optically active 1,2,3,4-tetrahydropyrrolo[1,2-a]pyrazines. How-
ever, to the best of our knowledge, this method has hardly been
We began our studies with 2-benzyl-1-phenylpyrrolo[1,2-
a]pyrazin-2-ium bromide 2a as the model substrate using
Received: May 9, 2014
Published: June 9, 2014
© 2014 American Chemical Society
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Organic Letters
Letter
Scheme 1. Synthesis of Chiral 1,2,3,4-
Tetrahydropyrrolo[1,2-a]pyrazine via Asymmetric
Hydrogenation
Scheme 2. A Proposed Hydrogenation Mechanism and the
Racemization Pathway of 3a under the Acidic Condition
[Ir(COD)Cl]2/(S,S,Rax)-C3*-TunePhos as the catalyst (Table
1). Initial exploration of reaction conditions revealed the
a
Table 1. Optimization of the Reaction Conditions
b
c
entry
solvent
ligand
base (equiv)
conv (%)
ee (%)
1
CH2Cl2
toluene
THF
EtOAc
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L2
L3
L4
L1
−
−
−
−
28
14
8
58
64
55
3
2
According to literature reports, molecules with a similar
structure to 3a are easy to racemize on silica gel.4a We
speculated that the low ee value was caused by the partial
racemization of 3a under the acidic conditions through
enamine/iminium isomerization (Scheme 2). To verify this
assumption, the product 3a was mixed with 1.0 equiv of HCl
(aq) and stirred at room temperature (eq 1 in Scheme 2). The
ee of 3a dropped from 90% to 79% in 12 h. Upon further
stirring, the ee dropped to 64%. Based on these results, we
decided to neutralize part of the HBr to prevent the substantial
racemization of the product. To our surprise, upon adding 0.8
equiv of triethylamine (TEA), full conversion was realized, but
the ee value was quite low (entry 5).19 This was possibly due to
the coordination of the organic base to the catalyst. When the
inorganic base was added, the reaction proceeded well with
increased enantioselectivity. A screen of several kinds of
inorganic bases showed that Cs2CO3 was the best choice,
giving full conversion with 87% ee (entries 6−10).
3
40
4
26
5
TEA (0.8)
NaOH (0.8)
NaHCO3 (0.8)
Na2CO3 (0.4)
K2CO3 (0.4)
Cs2CO3 (0.4)
Cs2CO3 (0.5)
Cs2CO3 (0.6)
Cs2CO3 (0.6)
Cs2CO3 (0.6)
Cs2CO3 (0.6)
Cs2CO3 (0.6)
>95
>95
>95
>95
>95
>95
>95
>95
>95
>95
>95
>95
6
52
76
76
85
87
88
90
87
86
79
92
7
8
9
10
11
12
13
14
15
d
16
a
Conditions: 2a (0.20 mmol), [Ir(COD)Cl]2 (1.5 mol %), (S,S,Rax)-
C3*-TunePhos (3.3 mol %), H2 (600 psi), solvent (3.0 mL), 20 °C, 24
h; TEA = triethylamine. Determined by H NMR. Determined by
HPLC with chiral column. H2 (400 psi).
b
c
1
d
To our delight, when excess Cs2CO3 was added to the
reaction system, better enantioselectivities were obtained with
no detection of the 1,2-dihydropyrrolo[1,2-a]pyrazine inter-
mediate (entries 11−12). Because of the relatively poor
solubility of Cs2CO3 in THF, the neutralization rate may be
slower than the rate of HBr generated. So the reaction
proceeded well in the presence of excess Cs2CO3. Other
commercially available chiral diphosphine ligands were also
tested under the same conditions, but no ligand gave a better
result than the initially used (S,S,Rax)-C3*-TunePhos (entries
13−15). A decrease of the hydrogen pressure had a positive
effect on enantioselectivity (92% ee, entry 16). Thus, the
optimized conditions were established as [Ir(COD)Cl]2/
(S,S,Rax)-C3*-TunePhos/Cs2CO3/THF.
With the optimized reaction conditions in hand, the scope of
iridium-catalyzed asymmetric hydrogenation of pyrrolo[1,2-
a]pyrazinium salts was explored (Table 2). In general, the
transformations performed well with moderate to excellent
yields under the standard conditions. The position and
electronic effect of substituents on the phenyl ring had a
great influence on the enantioselectivity. Substrates bearing
eletron-donating groups on the 3-position gave much higher ee
importance of the solvent. THF gave a much higher yield than
other solvents, but only moderate enantioselectivity was
obtained (entries 1−4).
Considering the structural similarity of pyrrolo[1,2-a]pyra-
zine with isoquinoline, we proposed the following reaction
pathway based on our previous work.9c As shown in Scheme 2,
the substrates first undergo a 1,2-hydride addition to give a 1,2-
dihydropyrrolo[1,2-a]pyrazine intermediate A, which would
isomerize to an iminium salt B in the presence of in situ
generated HBr. Subsequent hydrogenation of the iminium salt
gives the product. The in situ generated HBr is important for
the reaction to proceed forward.
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Organic Letters
Letter
Finally, to demonstrate the practicality of our approach, we
directed our efforts to the removal of the benzyl group (Scheme
3). When 3a was subjected to 150 psi of H2 in the presence of
Table 2. Scope of the Asymmetric Hydrogenation of
Pyrrolo[1,2-a]pyrazinium Salts
a
Scheme 3. Removal of the Benzyl Group
b
c
entry
R1, R2
Ph, H
yield (%)
ee (%)
1
2
3
4
97 (3a)
91 (3b)
90 (3c)
93 (3d)
91 (3e)
93 (3f)
96 (3g)
94 (3h)
97 (3i)
97 (3j)
94 (3k)
82 (3l)
95 (3m)
74 (3n)
97 (3o)
93 (3p)
85 (3q)
92 (R)
89 (−)
93 (−)
80 (−)
82 (−)
87 (−)
94 (−)
87 (−)
92 (−)
95 (−)
94 (−)
80 (−)
88 (−)
92 (−)
60 (−)
73 (−)
75 (−)
3-MeC6H4, H
3-MeOC6H4, H
4-MeC6H4, H
4-tBuC6H4, H
4-FC6H4, H
3-ClC6H4, H
4-ClC6H4, H
3,5-F2C6H3, H
4-CF3C6H4, H
4-NCC6H4, H
4-EtO2CC6H4, H
4-PhC6H4, H
2-naphthyl, H
Me, H
5% Pd/C, (Boc)2O, and DIPEA in ethanol for 2 days, the
desired product 4 was obtained in 62% yield with a slight drop
of the ee value. The main side product arose from the opening
of the piperazine ring (see the Supporting Information).
In conclusion, we have successfully realized the asymmetric
hydrogenation of pyrrolo[1,2-a]pyrazinium salts with an
iridium catalyst. This method provides a direct access to chiral
1,2,3,4-tetrahydropyrrolo[1,2-a]pyrazine derivatives with high
to excellent ee values. The key feature of the reaction is the
addition of cesium carbonate, which increases the conversion
and prohibits the racemization pathway of products. Further
efforts to illuminate the mechanism and apply this strategy to
other challenging heteroaromatic compounds are ongoing in
our laboratory.
d
5
6
7
8
9
10
11
12
d
13
14
e
15
16
17
Ph, 6-Me
Ph, 6-Ph
a
Conditions: 2 (0.20 mmol), [Ir(COD)Cl]2 (1.5 mol %), (S,S,Rax)-
C3*-TunePhos (3.3 mol %), Cs2CO3 (0.12 mmol), H2 (400 psi), THF
ASSOCIATED CONTENT
b
c
■
(3.0 mL), 20 °C, 24 h. Isolated yield. Determined by HPLC.
S
* Supporting Information
d
e
Cs2CO3 (0.10 mmol) was used, 36 h. L5 was used as the ligand.
Experimental procedures, characterization data, X-ray struc-
tures, data for the determination of enantiomeric excess, and
NMR spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.
values than those on the 4-position (entries 2−5). However, for
electron-withdrawing groups, generally high ee values were
obtained (entries 6−10). Notably, a substrate with a 4-CF3
group gave the best enantioselectivity (95% ee, entry 10). The
reaction system was also compatible with a cyano group and
gave excellent ee (94% ee; entry 11), but for the ester group, a
lower 80% ee was obtained (entry 12). For the biphenyl and
naphthyl substituted substrates, good enantioselectivities were
maintained (entries 13−14). 1-Methyl-pyrrolo[1,2-a]pyrazine
was a suitable reaction partner, but only moderate enantiose-
lectivity was obtained (60% ee, entry 15). Substrates with
substituents on the pyrrole ring were also tested, providing the
corresponding products with good yields and moderate ee
values (entries 16−17).
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: ygzhou@dicp.ac.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for the financial support from the National
Natural Science Foundation of China (21125208 and
21032003) and the National Basic Research Program of
China (2010CB833300).
After a single recrystallization from dichloromethane and n-
hexane, the product 3a could be obtained in 81% yield with up
to >99% ee. The absolute configuration of product 3a was
determined to be R based on single-crystal X-ray diffraction
analysis (Figure 2).20
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