Asymmetric Hydrogenation of 2-Aryl-5,6-dihydropyrazine Derivatives with Chiral Cationic Ruthenium Diamine Catalysts

Article abstract of DOI:10.1002/cjoc.201400422

The first asymmetric hydrogenation of unfunctionalized 2-substituted and 2,3-disubstituted 5,6-dihydropyrazines catalyzed by chiral cationic Ru-diamine complex (R,R)-1a was developed, affording chiral piperazine derivatives with good enantioselectivities (up to 89% ee).

Full text of DOI:10.1002/cjoc.201400422

COMMUNICATION
DOI: 10.1002/cjoc.201400422
Asymmetric Hydrogenation of 2-Aryl-5,6-dihydropyrazine
Derivatives with Chiral Cationic Ruthenium Diamine Catalysts†
Yong Li,a,b Yanmei He,a Fei Chen,a and Qinghua Fan*,a,c
a Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Molecular Recognition and Function,
Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190, China
b Department of Chemistry, Henan Institute of Education, Zhengzhou, Henan 450046, China
c Collaborative Innovation Center of Chemical Science and Engineering, Tianjin, China
The first asymmetric hydrogenation of unfunctionalized 2-substituted and 2,3-disubstituted 5,6-dihydropyra-
zines catalyzed by chiral cationic Ru-diamine complex (R,R)-1a was developed, affording chiral piperazine deriva-
tives with good enantioselectivities (up to 89% ee).
Keywords asymmetric hydrogenation, ruthenium, chiral diamines, piperazines
O
Introduction
F
Optically active chiral piperazines are an important
class of building blocks for the asymmetric synthesis of
biologically active and/or naturally occurring com-
pounds which are important for pharmaceutical and ag-
rochemical industries.[1] Some important representative
examples include lomefloxacin, mirtazapine, k-receptor
agonist, dragmacidin and piperazinomycin (Figure 1).[1]
Furthermore, some chiral piperazine derivatives have
proven to be useful chiral ligands in asymmetric cataly-
sis.[2] To date, many methods have been developed to
prepare such chiral heterocyclic compounds, mainly
including chiral pool synthesis[3] and resolution of the
corresponding racemic compounds.[4] Although the
asymmetric hydrogenation offers a more straightfor-
ward and environmentally benign route to these opti-
cally active compounds,[5] much less attention has been
directed toward the transition-metal-catalyzed reduction
of pyrazines or partially reduced pyrazines.[6] For ex-
ample, Fuchs reported the asymmetric hydrogenation of
pyrazinecarboxylic acid derivatives catalyzed by
[Rh(COD)Cl]2/diphosphine with ee values up to 78%.[6b]
Rossen and co-workers found that the Rh-BINAP com-
plex was an efficient catalyst for the hydrogenation of
2-amide functionalized tetrahydropyrazines to give
piperazines with up to 99% ee.[6c,6d] To the best of our
knowledge, there are no reports on asymmetric hydro-
genation of dihydropyrazine derivatives.
H3C
HN
N
N
N
N
COOH
F
CH2CH3
N
CH3
Mirtazapine
Lomefloxacin
Cl
O
N
N COOCH3
Cl
N
k-receptor agonist
OH
NH
CH3
N
O
H
H
N
Br
Br
N
R
N
HN
Piperazinomycin
Dragmacidin A: R = H
Dragmacidin B: R = CH3
Figure 1 Selected bioactive compounds containing piperazine
scaffold.
hydrogenation of different types of N-containing het-
eroaromatic compounds[8] and various imines.[9] Most
recently, the effectiveness of these Ru-catalysts was
further proved in the asymmetric hydrogenation of a
class of benzo-cyclic diimines, 2,4-disubstituted-3H-
1,5-benzodiazepines, with excellent diastereoselectivity
and excellent enantioselectivity.[10] Encouraged by these
results and as a continuation of our ongoing endeavor to
Recently, we have demonstrated that the cationic
Ru- and Ir-complexes of chiral mono-tosylated dia-
mines[7] are very efficient catalysts for the asymmetric
*
E-mail: fanqh@iccas.ac.cn; Tel.: 0086-010-62554472; Fax: 0086-010-62554449
Received June 27, 2014; accepted August 22, 2014; published online September 29, 2014.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201400422 or from the author.
Dedicated to Professor Chengye Yuan and Professor Li-Xin Dai on the occasion of their 90th birthdays.
†
Chin. J. Chem. 2014, 32, 991—994
© 2014 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
991

Li et al.
COMMUNICATION
prepare enantiomerically pure N-containing heterocyclic
compounds, herein, we report the first example of
highly enantioselective hydrogenation of a range of
readily available 2-aryl substituted 5,6-dihydropyrazine
derivatives using chiral cationic Ru-diamine catalysts,
thus providing an easy access to chiral 2-aryl substituted
piperazine derivatives.
in DCM with 2.2 equiv. (Boc)2O as additive. To our
delight, the reaction proceeded smoothly, affording the
piperazine product (6a) in 64% isolated yield with 81%
ee (Table 1, Entry 1). Based on this promising result,
other catalysts as shown in Figure 2 were then tested,
and the results are listed in Table 1.
Table 1 Catalyst screening and optimization of the reaction
conditionsa
Boc
η6-arene
N
N
N
Ru
Ru
N
catalyst, (Boc)2O
H2, 40 oC, solvent
*
X
SO2R
Ph
Ms
TfO
N
N
N
H2N
H
Boc
Ph
H3C
OCH3
OCH3
Ph
Ph
6a
5a
(R,R)-1a - 1h
(R,R)-2
Entry Catalyst
Solvent
(R,R)-1a DCM
(R,R)-1b DCM
H2/atm Yieldb/% eec/%
1
2
40
40
40
40
40
40
40
40
40
40
40
40
40
40
40
40
64
46
58
60
55
58
65
42
49
32
95
62
46
52
63
35
31
70
62
61
44
0
81
81
77
79
21
76
81
77
6
+
Ru
Ir
BArF
Ts
3
(R,R)-1c DCM
(R,R)-1d DCM
(R,R)-1e DCM
Ms
TfO
N
H2N
N
H2N
Ph
4
5
Ph
(R,R)-4
(R,R)-3
6
(R,R)-1f
DCM
(R,R)-1a: R = CH3, η 6-arene = p-cymene, X = OTf;
(R,R)-1b: R = CH3, η 6-arene = benzene, X = OTf;
7
(R,R)-1g DCM
(R,R)-1h DCM
8
(R,R)-1c: R = CH3, η 6-arene = 1,3,5-trimethylbenzene, X = OTf;
(R,R)-1d: R = 4-CH3C6H4, η 6-arene = p-cymene, X = OTf;
(R,R)-1e: R = 4-CH3C6H4, η 6-arene = hexamethylbenzene, X = OTf;
(R,R)-1f: R = 4-CF3C6H4, η 6-arene = p-cymene, X = OTf;
(R,R)-1g: R = CH3, η 6-arene = p-cymene, X = PF6;
9
(R,R)-2
(R,R)-3
(R,R)-4
DCM
DCM
DCM
10
11
12
13
14
15
16
17
18
19
20
21d
22e
23f
24
0
(R,R)-1h: R = CH3, η 6-arene = p-cymene, X = BArF.
(R,R)-1a DCE
(R,R)-1a Toluene
(R,R)-1a THF
(R,R)-1a EA
81
80
69
60
52
69
77
85
86
86
0
Figure 2 The catalysts used for the asymmetric hydrogenation
of 2-aryl-5,6-dihydropyrazines.
Experimental
(R,R)-1a MeOH
(R,R)-1a CF3CH2OH 40
Typical procedure for asymmetric hydrogenation
(R,R)-1a DCM
(R,R)-1a DCM
(R,R)-1a DCM
(R,R)-1a DCM
(R,R)-1a DCM
(R,R)-1a DCM
70
20
10
10
10
10
In a 50 mL glass-lined stainless steel reactor with a
magnetic stirring bar was charged with Ru-catalyst[8b]
(0.008 mmol), substrate (0.2 mmol), 0.44 mmol
(Boc)2O and solvent (2 mL). The hydrogenation was
processed under 10 atm H2 at 40 ℃ for 24 h. After
carefully releasing the hydrogen, the mixture was con-
centrated. The residue was purified by column chroma-
tography on silica eluted with petroleum ether/DCM
(1/1, V/V) to give the pure product. The enantiomeric
excess was determined by HPLC with a chiral AD-H
column.
54
89
a
Reaction conditions: 0.2 mmol substrate in 2 mL solvent, 4
mol% catalyst, 0.44 mmol (Boc)2O, 40 ℃, 24 h. b Isolated yields.
c Determined by HPLC with a chiral AD-H column. d Hydrogena-
tion at 25 ℃. e Without the additive of (Boc)2O. f 2 mol% catalyst
was used.
Results and Discussion
To evaluate the chiral Ru-diamine catalysts and op-
timize the reaction conditions, the hydrogenation of
2-(4-methoxyphenyl)-5,6-dihydropyrazine (5a) was
selected as the standard reaction. The initial hydrogena-
tion was carried out in the presence of 4 mol% (R,R)-1a
Generally, the catalytic performance was signifi-
cantly affected by both the substituent groups of the
η6-arene ligand and the N-sulfonate substituents (Table
1, Entries 1－6). Introduction of more alkyl substituents
into the η6-arene of the Ru catalyst led to significant
992
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© 2014 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2014, 32, 991—994

Asymmetric Hydrogenation of 2-Aryl-5,6-dihydropyrazine Derivatives
decrease in both yield and enantioselectivity (Table 1,
Entry 2 vs. Entry 3, Entry 4 vs. Entry 5). Introducing
electron-withdrawing N-sulfonate substituents into
catalyst, for example (R,R)-1f, resulted in slightly lower
enantioselectivity and reactivity (Table 1, Entries 6 vs.
Entry 4). Moreover, it was observed that the weakly
coordinating counteranion of Ru-complex had an obvi-
ous effect on enantioselectivity (Table 1, Entries 7 and
8). It is worth noting that the introduction of a methyl
group into the primary amine of the diamine ligand in
catalyst (R,R)-2 greatly deteriorated the enantioselectiv-
ity of hydrogenation (Table 1, Entry 9). Replacement of
the diamine ligand MsDpen with MsCydn led to much
lower enantioselectivity and reactivity (Table 1, Entry
10). In addition, it is notable that the Ir-complex (R,R)-4
exhibited the highest reactivity. The reduction of 5a was
very clean with quantitative isolated yield. Unfortu-
nately, the product was found to be racemic (Table 1,
Entry 11). Thus, the Ru-complex (R,R)-1a was chosen
as the optimal catalyst.
Table 2 Asymmetric hydrogenation of 2-aryl-5,6-dihydro-
pyrazinesa
Boc
N
N
(R,R)-1a, (Boc)2O
H2 (10 atm), 40 oC, DCM
*
N
Ar
N
Ar
5a - 5h
Boc
6a - 6h
Entry Ar/Substrate
Yieldb,d/% eec,d/%
1
2
3
4
5
6
7
8
4-OCH3-Ph (5a)
Ph (5b)
61 (54)
70 (49)
68 (48)
64 (42)
36 (42)
67 (47)
70 (50)
63
86 (89) (＋)
79 (83) (＋)
80 (82) (＋)
84 (88) (＋)
60 (88) (＋)
74 (80) (＋)
75 (82) (＋)
75 (＋)
3-OCH3-Ph (5c)
3,4-(OCH3)2-Ph (5d)
2-Nap (5e)
4-Cl-Ph (5f)
4-Br-Ph (5g)
4-CF3-Ph (5h)
With the Ru-complex (R,R)-1a as the catalyst, the
reaction conditions were then optimized. It was found
that the low-polarity solvents, especially the chlorinated
solvents dichloromethane and dichloroethane, were
suitable for obtaining high enantioselectivity (Table 1,
Entries 1, 12－13). Decreased enantioselectivities were
obtained in THF and EA, albeit with similar yields (Ta-
ble 1, Entries 14－15). Moreover, low reactivities and
enantioselectivities were also observed in alcoholic sol-
vents (Table 1, Entries 16－17). Subsequently, the ef-
fect of hydrogen pressure and temperature were also
tested. The results showed that slightly higher enantio-
selectivities could be obtained when the reaction was
performed under lower hydrogen pressure (Table 1, En-
tries 18－20). Reducing the reaction temperature was
favorable to obtain higher enantioselectivity, but a sig-
nificant decrease in reactivity was also observed (Table
1, Entry 21). In addition, the additive of (Boc)2O was
indispensable to the hydrogenation. When the reaction
was carried out in the absence of (Boc)2O, no hydro-
genation product was found at all (Table 1, Entry 22).
According to our previous studies,[9a,9b] the addition of
(Boc)2O was to prevent deactivation of the catalyst,
which might be poisoned by the amine product as well
as the by-product due to the hydrolysis of substrate,
through in situ protection of the corresponding amine
poisons.[11] Notably, when the amount of catalyst
(R,R)-1a was reduced from 4 mol% to 2 mol%, the
highest enantioselectivity (89% ee) was obtained (Table
1, Entry 23). However, futher decreasing the catalyst
loading to 1.0 mol% resulted in much low yield (18%
yield and 89% ee). Considering both reactivity and en-
antioselectivity, the optimal reaction conditions were
thus established: 0.2 mmol substrate in 2 mL DCM, 4
mol% catalyst, 0.44 mmol (Boc)2O, 10 atm H2, 40 ℃,
24 h.
a Reaction conditions: 0.2 mmol substrate in 2 mL DCM, 4 mol%
catalyst, 0.44 mmol (Boc)2O, 10 atm H2, stirred at 40 ℃ for 24
b
c
h. Isolated yields. Determined by HPLC with a chiral AD-H
d
column. The data in brackets were obtained with 2 mol% cata-
lyst.
Ru-catalyzed asymmetric hydrogenation of 2-aryl-5,6-
dihydropyrazines (5a－5h). Generally, all the substrates
could be efficiently hydrogenated to afford the corre-
sponding chiral 2-arylpiperazine derivatives with good
yields and enantioselectivities (Table 2). The electronic
characteristic of substituents in the substrate has a sig-
nificant influence on the enantioselectivity and reactiv-
ity. The substrates containing an electron-donating
group gave higher enantioselectivities (Table 2, Entries
1, 3, 4). It was observed that the best results were ob-
tained for the substrates bearing an electron-donating
methoxyl group on the para position of the phenyl ring
(Table 2, Entry 1 and Entry 4). In contrast, hydrogena-
tion of substrates with electron-withdrawing groups of-
fered much lower enantioselectivities (Table 2, Entries 6
－8). For the 2-naphthyl-substituted substrate 5e, the
hydrogenation could be accomplished only with 60% ee
in 36% yield. When the dosage of Ru-MsDpen catalyst
was reduced from 4 mol% to 2 mol%, improved enanti-
oselectivities were achieved, albeit the efficacy of the
catalyst decreased obviously. Enantiomeric excesses up
to 89% were afforded for the substrates screened in the
presence of 2 mol% catalyst.
On the basis of our successful asymmetric hydro-
genation of 2-aryl-5,6-dihydropyrazines, a disubstituted
substrate of 2-methyl-3-phenyl-5,6-dihydropyrazine (7)
was further subjected to the hydrogenation catalyzed by
(R,R)-1a under 40 atm of H2 (Scheme 1). The hydro-
genation proceeded very well, a nearly quantitative iso-
lated yield was achieved. Notably, a high diastereose-
lectivity (cis/trans＝12) was observed, but with an ex-
tremely low enantioselectivity of 14% ee. In the subse-
With the optimal reaction conditions and catalyst
(R,R)-1a in hand, we further explored the scope of the
Chin. J. Chem. 2014, 32, 991—994
© 2014 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
993

Li et al.
COMMUNICATION
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Scheme 1 Asymmetric hydrogenation of 2,3-disubstituted di-
hydropyrazine 7
Boc
N
CH3
Ph
N
CH3
Ph
(R,R)-4, 2.2 equiv. (Boc)2O
*
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H2 (40 atm), 40 oC, DCM
*
N
N
Boc
7
8
d.r. (cis/trans) = 16a
>95% yield, 29% eeb
a Determined by 1H NMR spectroscopy;
b Determined by HPLC with a chiral AD-H column.
quent survey of different Ru-catalysts, no obvious en-
hancement of enantioselectivity was observed. When
Ir-TsDpen (R,R)-4 (Figure 2) was used as the catalyst,
the reaction furnished the hydrogenation product with
the highest enantioselectivity of 29% ee.
Conclusions
In summary, we have developed the first asymmetric
hydrogenation of a range of 2-aryl-5,6-dihydropyrazines
catalyzed by the chiral cationic Ru-diamine catalysts.
Moderate to good isolated yields and high enantioselec-
tivities (up to 89% ee) were achieved. Further work will
be directed toward expanding the substrate scope of
2,3-disubstituted-5,6-dihydropyrazines and the mecha-
nism study of this reaction.
Acknowledgement
We thank the National Natural Science Foundation
of China (Grant No. 21232008), National Basic Re-
search Program of China (973 Program, 2010CB-
833300), Foundation for University Youth Key Teacher
of Henan Province (2013GGJS-207) and the Chinese
Academy of Sciences (CMS-PY-201303) for financial
supports.
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Chin. J. Chem. 2014, 32, 991—994