Ruthenium-Catalyzed Enantioselective Hydrogenation of Phenanthridine Derivatives

Article abstract of DOI:10.1021/acs.orglett.7b00419

The first asymmetric hydrogenation of phenanthridines catalyzed by chiral cationic ruthenium diamine complexes has been developed with up to 92% ee and full conversions. The choice of the counteranion of the catalyst was found to be critical for achieving high enantioselectivity. In addition, the obtained 5,6-dihydrophenanthridine could be used as a chiral hydride donor for organocatalytic asymmetric transfer hydrogenation.

Full text of DOI:10.1021/acs.orglett.7b00419

Letter
pubs.acs.org/OrgLett
Ruthenium-Catalyzed Enantioselective Hydrogenation of
Phenanthridine Derivatives
Zhusheng Yang,† Fei Chen,* Shuxin Zhang, Yanmei He, Nianfa Yang,* and Qing-Hua Fan*
,‡
,‡
‡
‡
,†
,‡
†Key Laboratory of Environmentally Friendly Chemistry of the Ministry of Education, College of Chemistry, Xiangtan University,
Xiangtan, Hunan 411105, P. R. China
‡
CAS Key Laboratory of Molecular Recognition and Function, Institute of Chemistry, Chinese Academy of Sciences (CAS),
University of Chinese Academy of Sciences, Beijing 100190, P. R. China
*
S Supporting Information
ABSTRACT: The first asymmetric hydrogenation of phenanthridines catalyzed by chiral cationic ruthenium diamine complexes
has been developed with up to 92% ee and full conversions. The choice of the counteranion of the catalyst was found to be
critical for achieving high enantioselectivity. In addition, the obtained 5,6-dihydrophenanthridine could be used as a chiral
hydride donor for organocatalytic asymmetric transfer hydrogenation.
he 5,6-dihydrophenanthridines are important structural
However, to the best of our knowledge, the asymmetric
hydrogenation of phenanthridines has not been documented
yet. This is probably due to the following two facts: (1) the
T
units in natural products and biologically active
1
6
molecules, such as those outlined in Figure 1. To date, a
asymmetric hydrogenation of heteroarenes consisting of more
than two fused aromatic rings is still difficult because of the
possibility of dehydronative aromatization of the reduced
6b,7
products,
and (2) the strong coordinative nitrogen atom of
the substrate and product can easily poison the metal catalyst.
Most recently, we found that the cationic ruthenium complexes
8
of chiral monotosylated diamine are highly effective catalysts
Figure 1. Selected biologically active compounds.
for asymmetric hydrogenation of various N-containing
heteroarenes. In particular, this catalytic system has been
9
demonstrated to be highly efficient for the asymmetric
hydrogenation of challenging polycyclic 1,10-phenanthrolines.
9e
number of methods have been developed for the preparation of
2
6
-substituted 5,6-dihydrophenanthridine derivatives. In addi-
Encouraged by these results, we report herein a ruthenium-
catalyzed asymmetric hydrogenation of phenanthridines with
high enantioselectivity for the first time.
In the initial experiment, 6-methylphenanthridine 1a was
chosen to be hydrogenated with (R,R)-3a in THF under 50 atm
tion, recent research has shown that the chirality of 6-
substituted 5,6-dihydrophenanthridines plays a very important
role in determining their bioactivity. However, few catalytic
asymmetric approaches for obtaining such chiral compounds
1e
1
e,3
were reported.
enantioselective synthesis of a wide range of 6-aryl-substituted
,6-dihydrophenanthridine derivatives through retro-carbopal-
ladation of chiral o-bromobenzylamines with excellent ee
Most recently, Lautens reported the
of H (Table 1). It was found that 1a could be hydrogenated at
2
5
0 °C, giving the desired product in full conversion but with
5
only 5% ee (Table 1, entry 1). Encouraged by this initial result
10
3
a
and our previous reports, the counteranion effect of catalyst
values.
was first examined. Interestingly, the enantioselectivity was
influenced by the counteranions significantly (entries 1−6).
The highest enantioselectivity was achieved by using catalyst
In the past decade, asymmetric hydrogenation of hetero-
aromatic compounds has become one of the most straightfor-
ward paths toward the synthesis of optically active compounds
4
(R,R)-3f with a bipolPO anion, albeit with a lower yield.
2
with chiral heterocyclic skeletons. For example, since the
Subsequently, several conventional organic solvents were
pioneering work on asymmetric hydrogenation of quinolines
5
a
reported by Zhou, numerous iridium complexes containing
chiral phosphorus ligands have proven to be effective in the5
asymmetric hydrogenation of quinolines and isoquinolines.
Received: February 10, 2017
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b00419
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Table 1. Optimization of Conditions for Asymmetric
Hydrogenation
Table 2. Asymmetric Hydrogenation of 6-Substituted
Phenanthridines Catalyzed by (R,R)-6f
a
a
1
2
3
b
c
entry
R /R /R
yield (%)
ee (%)
1
2
3
4
5
6
7
8
9
CH /H/H
90 (2a)
91 (2b)
89 (2c)
88 (2d)
92 (2e)
89 (2f)
87 (2g)
86 (2h)
88 (2i)
84 (2j)
90 (2k)
92 (2l)
91 (2m)
90 (2n)
86 (2o)
92 (2p)
NR
89 (−)
77 (−)
75 (−)
86 (−)
91 (−)
91 (R)
90 (−)
89 (−)
91 (−)
89 (−)
92 (−)
89 (−)
92 (−)
92 (−)
89 (−)
90 (−)
3
CH /H/F
3
CH /H/CH3
3
CH /F/H
3
CH /CH /H
3
3
anion [X]−
b
c
CH
3
/OCH /H
3
entry
catalyst
solvent
THF
conv (%) ee (%)
Et/OCH /H
3
1
2
3
4
5
6
7
8
9
(R,R)-3a
(R,R)-3b
(R,R)-3c
(R,R)-3d
(R,R)-3e
(R,R)-3f
(R,R)-3f
(R,R)-3f
(R,R)-3f
(R,R)-3f
(R,R)-3f
(R,R)-4f
(R,R)-5f
(R,R)-6f
(R,R)-4g
(R,R)-4h
(R,R)-4i
(R,R)-6f
OTf
>99
>99
>99
>99
>99
85
5
7
n-Pr/OCH /H
3
BF4
THF
n-pentyl/OCH /H
3
PF6
THF
19
37
10
66
4
10
11
12
13
14
15
16
17
C H CH /OCH /H
6
5
2
3
SbF6
THF
4-FC H CH /OCH /H
6
4
2
3
BArF
THF
4-OCH C H CH /OCH /H
3
6
4
2
3
bipolPO2
bipolPO2
bipolPO2
bipolPO2
bipolPO2
bipolPO2
THF
4-CH C H CH /OCH /H
3
6
4
2
3
MeOH
92
3-CH C H CH /OCH /H
3
6
4
2
3
DCM
92
7
2-CH C H CH /OCH /H
3
6
4
2
3
toluene
91
56
65
71
86
56
89
75
87
80
89
−CHCHC H /OCH /H
6
5
3
1
1
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
8
DME
85
C H /H/H
6
5
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
94
a
Reaction conditions: 1 (0.2 mmol) in 1,4-dioxane (1 mL), (R,R)-6f
1.0 mol %), H (70 atm), stirred at 50 °C for 24 h. Isolated yield.
The enantiomeric excesses were determined by chiral HPLC analysis.
The absolute configuration of 2f was determined to be R based on
single-crystal X-ray analysis of the corresponding tosylated derivative
10 (Scheme S1, Supporting Information).
bipolPO2
88
b
(
2
bipolPO2
72
c
bipolPO2
92
(PhO) PO
95
2
2
(R)-binolPO2
88
(S)-binolPO2
93
d
bipolPO2
>99
position than the electronic property of the substituents. When
substrates bearing methyl and fluoro groups at the 2-position
were used, low enantioselectivity was observed (entries 2 and
). The highest ee value was achieved in the hydrogenation of
f bearing a methoxyl group at the 8-position. Then, a variety
of 6-substituted phenanthridine derivatives bearing 8-methoxyl
groups were hydrogenated. Generally, 6-alkyl-substituted 8-
methoxyphenanthridines (1f−i) were hydrogenated smoothly
regardless of the length of the chain (entries 6−9). Moreover,
the electronic properties of the substituents at the para, meta,
even ortho position of the phenyl ring had no apparent effect on
enantioselectivity (1k−o). Interestingly, hydrogenation of 6-
styryl-substituted phenanthridine 1p proceeded smoothly
under identical conditions, and the conjugated double bond
was also hydrogenated. For the 6-phenyl-substituted substrate
(1q), hydrogenation could not take place.
a
Reaction conditions: 1a (0.2 mmol) in solvent (1 mL), (R,R)-Ru (1.0
mol %), H2 (50 atm), stirred at 50 °C for 24 h. DCM =
dichloromethane; DME = ethylene glycol dimethyl ether. The
conversions were determined by H NMR spectroscopy of the crude
reaction mixture. The enantiomeric excesses were determined by
HPLC with a chiral OD-H column. H (70 atm), stirred at 50 °C for
b
3
1
1
c
d
2
2
4 h.
investigated (entries 6−11). It was found that ether solvents,
such as THF, DME, and 1,4-dioxane, gave high enantiose-
lectivities. To further improve the enantioselectivity, a variety of
catalysts were screened in 1,4-dioxane (entries 11−15), and
R,R)-6f was found to be optimal in terms of both reactivity
and enantioselectivity (entry 14). Notably, the chirality of the
(
counteranion slightly influenced the enantioselectivity (entries
1
6 and 17). In addition, the enantioselectivity of the reaction
To explore the potential application of this synthetic
protocol in the practical synthesis of chiral 5,6-dihydrophenan-
thridines, a gram-scale experiment was carried out (Scheme 1a).
The asymmetric hydrogenation of 1a performed on a gram
scale (1.0 g) afforded the chiral (R)-2a in 94% yield with 88%
ee. After a single recrystallization from DCM and petroleum
ether (PE), enantiomerically pure (R)-2a with 99% ee could be
obtained.
was found to be insensitive to hydrogen pressure, and full
conversion was obtained under 70 atm of H (entry 18).
However, increasing the reaction temperature resulted in a
remarkable decrease in enantioselectivity (see the Supporting
Information, Table S1).
Under the optimal reaction conditions (Table 1, entry 18), a
variety of phenanthridine derivatives were efficiently hydro-
genated with good to excellent enantioselectivities (Table 2).
Considering the possible effect of substituents on the
enantioselectivity, we first investigated the asymmetric hydro-
genation of several 6-methyl-substituted phenanthridine
derivatives bearing different groups (Table 2, entries 1−6). It
was found that the enantioselectivity was more sensitive to the
2
Considering that chiral 5,6-dihydrophenanthridines have the
7
,11
potential to serve as a new kind of chiral NAD(P)H models,
we chose the organocatalytic transfer hydrogenation of 3-
phenyl-2H-1,4-benzoxazine as the model reaction (Scheme 1b).
It was found that the reaction proceeded smoothly in the
presence of 10 mol % of racemic phosphate acid and 2 equiv of
B
DOI: 10.1021/acs.orglett.7b00419
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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Scheme 1. Scale-up Synthesis of Chiral 5,6-
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2% yield with 91% ee.
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hydrogenation of 6-substituted phenanthridines by using
phosphine-free chiral cationic ruthenium diamine catalysts
with up to 92% ee. The counteranion was found to be critically
important for the high enantioselectivity. This new protocol
thus provides an easy path for the preparation of optically pure
(4) For recent reviews on asymmetric hydrogenation of hetero-
5
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biologically active molecules and pharmaceuticals.
ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b00419.
(5) For selected examples of asymmetric hydrogenation of quinolines
and isoquinolines, see: (a) Wang, W.-B.; Lu, S.-M.; Yang, P.-Y.; Han,
X.-W.; Zhou, Y.-G. J. Am. Chem. Soc. 2003, 125, 10536. (b) Lu, S.-M.;
Wang, Y.-Q.; Han, X.-W.; Zhou, Y.-G. Angew. Chem., Int. Ed. 2006, 45,
Experimental procedures, synthesis of the starting
materials, and compound characterization data (PDF)
X-ray data for derivative of chiral compound 2f (CIF)
2
260. (c) Xu, L.; Lam, K. H.; Ji, J.; Wu, J.; Fan, Q.-H.; Lo, W.-H.;
Chan, A. S. C. Chem. Commun. 2005, 1390. (d) Wang, Z.-J.; Deng, G.-
J.; Li, Y.; He, Y.-M.; Tang, W.-J.; Fan, Q.-H. Org. Lett. 2007, 9, 1243.
AUTHOR INFORMATION
Corresponding Authors
(e) Shi, L.; Ye, Z.-S.; Cao, L.-L.; Guo, R.-N.; Hu, Y.; Zhou, Y.-G.
■
Angew. Chem., Int. Ed. 2012, 51, 8286. (f) Ye, Z.-S.; Guo, R.-N.; Cai,
X.-F.; Chen, M.-W.; Shi, L.; Zhou, Y.-G. Angew. Chem., Int. Ed. 2013,
*E-mail: chenfei211@iccas.ac.cn.
*E-mail: nfyang@xtu.etu.cn.
*E-mail: fanqh@iccas.ac.cn.
5
2, 3685. (g) Iimuro, A.; Yamaji, K.; Kandula, S.; Nagano, T.; Kita, Y.;
Mashima, K. Angew. Chem., Int. Ed. 2013, 52, 2046. (h) Wen, J.; Tan,
R.; Liu, S.; Zhao, Q.; Zhang, X. Chem. Sci. 2016, 7, 3047. (i) Shi, L.; Ji,
Y.; Huang, W.; Zhou, Y.-G. Huaxue Xuebao 2014, 72, 820. (j) Zhang,
ORCID
Q.; Lu
6) Few examples of hydrogenation and transfer hydrogenation of
unsubstituted phenanthridine were reported. For recent examples, see:
a) Chen, F.; Surkus, A.-E.; He, L.; Pohl, M.-M.; Radnik, J.; Topf, C.;
Junge, K.; Beller, M. J. Am. Chem. Soc. 2015, 137, 11718.
b) Hernandez-Juarez, M.; Lopez-Serrano, J.; Lara, P.; Morales-
Ceron, J. P.; Vaquero, M.; Alvarez, E.; Salazar, V.; Suarez, A. Chem. -
Eur. J. 2015, 21, 7540. (c) Xuan, Q.; Song, Q. Org. Lett. 2016, 18,
250. (d) Mai, V. H.; Nikonov, G. I. Organometallics 2016, 35, 943.
7) (a) Chen, Q.-A.; Gao, K.; Duan, Y.; Ye, Z.-S.; Shi, L.; Yang, Y.;
̈
, J.; Luo, S. Huaxue Xuebao 2016, 74, 61.
Qing-Hua Fan: 0000-0001-9675-2239
Notes
(
(
The authors declare no competing financial interest.
(
́
́
́
ACKNOWLEDGMENTS
We thank the National Natural Science Foundation of China
Nos. 21672218, 21232008, and 21521002) and CAS
QYZDJSSW-SLH023) for financial support.
■
(
(
́
́
́
4
(
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DOI: 10.1021/acs.orglett.7b00419
Org. Lett. XXXX, XXX, XXX−XXX