Highly Enantioselective Direct Synthesis of Endocyclic Vicinal Diamines through Chiral Ru(diamine)-Catalyzed Hydrogenation of 2,2′-Bisquinoline Derivatives

Article abstract of DOI:10.1002/anie.201608181

An asymmetric hydrogenation of 2,2′-bisquinoline and bisquinoxaline derivatives, catalyzed by chiral cationic ruthenium diamine complexes, was developed. A broad range of chiral endocyclic vicinal diamines were obtained in high yields with excellent diastereo- and enantioselectivity (up to 93:7 dl/meso and >99 % ee). These chiral diamines could be easily transformed into a new class of chiral N-heterocyclic carbenes (NHCs), which are important but difficult to access.

Full text of DOI:10.1002/anie.201608181

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201608181
German Edition: DOI: 10.1002/ange.201608181
Asymmetric Catalysis
Highly Enantioselective Direct Synthesis of Endocyclic Vicinal
Diamines through Chiral Ru(diamine)-Catalyzed Hydrogenation of
2,2’-Bisquinoline Derivatives
Wenpeng Ma+, Jianwei Zhang+, Cong Xu, Fei Chen, Yan-Mei He, and Qing-Hua Fan*
Abstract: An asymmetric hydrogenation of 2,2’-bisquinoline
and bisquinoxaline derivatives, catalyzed by chiral cationic
ruthenium diamine complexes, was developed. A broad range
of chiral endocyclic vicinal diamines were obtained in high
yields with excellent diastereo- and enantioselectivity (up to
93:7 dl/meso and > 99% ee). These chiral diamines could be
easily transformed into a new class of chiral N-heterocyclic
carbenes (NHCs), which are important but difficult to access.
with excellent enantioselectivity. Expectedly, direct asym-
metric reduction of the readily available aromatic bishetero-
cycles (e.g., 2,2’-bisquinolines) is undoubtedly the most
convenient strategy for obtaining chiral rigid endocyclic
diamines. However, asymmetric reduction of such bisheter-
ocyclic compounds, particularly adjacent bisheteroaromatic
substrates, has been less exploited and remains a challenging
task. In comparison with the most studied monoheterocyles,
many kinds of bisheterocycles are known to coordinate more
strongly to transition metals[11] and thus inhibit catalysis. In
addition, it is more challenging to realize high diastereo- and
enantioselectivity. To date, only few examples have been
reported,[3a,c,4b,f,6b,10] and most of them involve heterogeneous
catalysis to give racemic products.[3a,4b,f] For example, only two
reports concerning the reduction of 2,2’-bisquinoline could be
found. In 2007, Blechert et al. reported the hydrogenation of
2,2’-bisquinoline by using PtO2 as the heterogeneous cata-
lyst.[3a] Most recently, Xiao et al. demonstrated the first
homogeneous hydrogenation of 2,2’-bisquinoline catalyzed
by a cyclometallated Ir complex.[3c] In both cases, however,
the meso isomer was obtained as the main product
(Scheme 1).
Over the past decades, the chemistry of chiral vicinal
diamines and their derivatives has attracted a great deal of
interest because they are key substructures in biologically
active natural products, pharmaceuticals, and chiral cata-
lysts.[1] To date, a number of chiral vicinal diamines have been
reported and successfully used as metal ligands or organo-
catalysts in asymmetric catalysis.[1c–l] In the pursuit of more
efficient chiral catalysts, the design and synthesis of rigid
chiral cyclic diamines is one of the frequently used strategies
in asymmetric catalysis. Although many efficient methods
have been developed for the preparation of chiral vicinal
diamines,[1c–e,2] the practical synthesis of enantiomerically
pure endocyclic vicinal diamines, such as 2,2’-bis(1,2,3,4-
tetrahydroquinoline),[3] 1,1’-bis(1,2,3,4-tetrahydroisoquino-
line),[4] bisindoline, bisisoindoline,[5] and 2,2’-bispiperidine[4f,6]
derivatives is still a serious challenge. To the best of our
knowledge, the direct catalytic enantioselective synthesis of
such rigid diamines has not yet been achieved.
Transition-metal-catalyzed asymmetric hydrogenation
(AH) has been established as one of the most powerful
tools for the preparation of a wide range of enantiomerically
pure compounds in organic synthesis.[7] In particular, the AH
of heteroaromatic compounds has recently been an important
topic in asymmetric catalysis.[8,9] A number of N-containing
heteroaromatic substrates, including quinolines,[9a–e] quinox-
alines,[9f–i] isoquinolines,[9j–m] indoles,[9n–q] pyrroles,[9r,s] pyridi-
nes,[9t–v] indolizidines,[9w] pyrimidines,[9x] phenanthrolines,[10a,b]
and naphthyridines,[10c,d] have been successfully hydrogenated
Scheme 1. Hydrogenation of 2,2’-bisquinoline.
[*] W. Ma,[+] J. Zhang,[+] C. Xu, F. Chen, Y.-M. He, Prof. Dr. Q.-H. Fan
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)
Most recently, we found that the Ru complexes of chiral
vicinal diamines, which are often used as catalysts for
asymmetric transfer hydrogenation,[1h,i,12] are highly efficient
in the AH of N-containing heteroaromatic compounds and
give excellent enantioselectivity.[9b,c,10b–d] For example, a wide
range of quinolines, including 2-aryl quinolines, could be
effectively hydrogenated with high reactivity and enantiose-
lectivity.[9c] In particular, this catalytic system has been
demonstrated to be highly effective in the AH of challenging
and
Collaborative Innovation Center of Chemical Science
and Engineering, Tianjin 300072 (P.R. China)
E-mail: fanqh@iccas.ac.cn
[+] These authors contributed equally to this work.
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201608181.
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1,10-phenanthrolines.[10b] Inspired by these results and given
the importance of chiral endocyclic vicinal diamines, we
attempted to apply this kind of cationic Ru(diamine) catalyst
to the AH of bisquinoline and bisquinoxaline derivatives.
In our initial study, the commercially available 2,2’-
bisquinoline (1a) was chosen as the model substrate. Pleas-
ingly, the reaction proceeded smoothly under 50 atm of H2 at
208C in methanol to give the desired product in 80%
conversion with a 70:30 dl/meso ratio and > 99% ee
(Table 1, entry 1). To optimize the reaction conditions, the
Table 1: Optimization of reaction conditions for the AH of bisquinoline
1a.[a]
Entry
Solvent
Catalyst
Conv. [%][b]
dl/meso[b]
ee [%][c]
1
2
3
4
5
6
7
8
9
MeOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
(R,R)-3a
(R,R)-3a
(R,R)-3b
(S,S)-3c
(R,R)-3d
(R,R)-3e
(R,R)-3 f
(R,R)-3g
(R,R)-3h
80
>95
>95
70
70:30
90:10
90:10
86:14
89:11
70:30
93:7
>99
>99
>99
>99
99
95
>95
>95
>95
87
90
>99
>99
>99
90:10
65:35
[a] Reaction conditions: Substrate 1a (0.1 mmol, 25.8 mg) in solvent
(1.0 mL), Ru catalyst (2.0 mol%), H2 (50 atm), stirred at RT for 18 h.
[b] The conversions and ratio of dl/meso were determined by 1H NMR
spectroscopy of the crude reaction mixtures. [c] The enantiomeric
excesses were determined by HPLC with an AD-H chiral-phase column.
Scheme 2. Scope of the AH of 2,2’-bisquinoline and 2,2’-bisquinoxaline
derivatives. General conditions: Substrate (0.1 mmol) in iPrOH
(1.0 mL), (R,R)-3 f (2.0 mol%), H2 (50 atm), stirred at RT for 18 h.
Yields of isolated product are given. The ee values were determined by
HPLC with a chiral stationary phase. [a] Stirred for 36 h. [b] Stirred at
508C for 24 h. [c] Stirred at 508C for 48 h. [d] Total yield of reduced
products. [e] (R,R)-3 f (5.0 mol%) in 1,2-dichloroethane (2.0 mL),
stirred at 508C for 48 h. [f] (R,R)-3 f (3.0 mol%) in dichloromethane.
[g] (R,R)-3b (1.0 mol%), 2 h. [h] (R,R)-3b (2.0 mol%), 10 h.
effect of solvent, hydrogen pressure, reaction temperature,
and catalyst on the AH of 1a was investigated. The results are
shown in Table 1 and Tables S1,S2 of the Supporting Infor-
mation. Notably, the hydrogenation proceeded well in several
organic solvents with good diastereoselectivity and excellent
enantioselectivity, and isopropanol was found to give the best
result (entry 2). Moreover, the enantioselectivity was insensi-
tive to hydrogen pressure and temperature, while low
reactivity and diastereoselectivity were observed under low
pressure (Table S2, entry 5). Then, various Ru complexes
were further tested, and (R,R)-3 f turned out to be optimal
(entry 7).
Under the optimized reaction conditions, a variety of 2,2’-
bisquinoline derivatives were investigated. As shown in
Scheme 2, all these reactions proceeded smoothly to produce
the desired diamines with unprecedented enantioselectivity in
most cases. It was found that the hydrogenation was
insensitive to steric hindrance from alkyl side chain at the
6,6’-, 7,7’-, or 5,5’-positions of 2,2’-bisquinoline. However,
introduction of alkyl groups into the 8,8’-positions of 2,2’-
bisquinoline (1n and 1o) resulted in a prominent decrease in
diastereoselectivity. In addition, unsymmetric 2,2’-bisquino-
lines (1p–s) and disubstituted 2,2’-bisquinoline (1t) could also
be hydrogenated with good to excellent enantioselectivity.
Notably, substrate 1 f, which bears methoxy groups at the 6,6’-
positions, showed markedly low reactivity and diastereose-
lectivity, and substrates bearing CF3 (1g, 1r, and 1s) resulted
in low reactivity, diastereoselectivity, and enantioselectivity.
Interestingly, hydrogenation of substrates bearing methyl
groups at the 3,3’-positions (1u and 1v) proceeded smoothly
2
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to give the chiral product with fairly good enantioselectivity
between the h6-arene ligand in the Ru complex and the
and meso product, respectively.
fused phenyl ring (Figure S1 in the Supporting Information).
To demonstrate the potential of this new method, these
chiral vicinal diamines were employed for the preparation of
chiral benzimidazolium salts, a precursor of chiral NHC
ligands. As shown in Scheme 3, AH of 1a was carried out on
Furthermore, it is desirable to apply this catalytic system
to other bisheterocycles. We demonstrated this through the
AH of more challenging 2,2’-biquinoxalines (1w, 1x). After
screening of the reaction conditions, (Table S3), both sub-
strates could be reduced smoothly with good diastereose-
lectivity and excellent enantioselectivity. Notably, the
unsymmetric 2-(quinolin-2-yl)quinoxaline (1y) could also
be hydrogenated, albeit with much low enantioselectivity.
In addition, hydrogenation of 2,2’-bisquinoline N-oxides
(1a’, 1l’) proceeded smoothly. Only the quinoline ring was
reduced, with quite good enantioselectivity, and could be
easily transformed to 1,2,3,4-tetrahydro-2,2’-bisquinolines
(Scheme S3 in the Supporting Information).
To gain more information on the catalytic process,
bisquinoline 1a was hydrogenated with 1.0 mol% (R,R)-
3b in CH2Cl2 under low hydrogen pressure. After 5 hours,
41% conversion was observed, and the partially reduced
product (5a) was obtained in 28% yield with 84% ee
[Eq. (1)]. This result indicates that the two quinoline rings
Scheme 3. Synthesis of chiral benzimidazolium tetrafluoroborates.
a gram scale to give (2S,2’S)-2a in 89% yield of isolated
product with > 99% ee. Subsequent treatment of (2S,2’S)-2a
with triethyl orthoformate and NH4BF4 provided the key
benzimidazolium tetrafluoroborate (2S,2’S)-4a in quantita-
tive yield, providing a facile and practical approach to a new
class of rigid NHC ligands, which might be useful for
asymmetric catalysis.[3a,b,4f–h,14] Similarly, several other benzi-
midazolium salts were synthesized in good yields.
In conclusion, we have developed a highly efficient AH of
2,2’-bisquinoline and 2,2’-bisquinoxaline derivatives that
makes use of chiral cationic ruthenium diamine catalysts
and proceeds with very good diastereoselectivity and an
unprecedented level of enantioselectivity. This new method
thus provides a practical and facile approach to the synthesis
of optically pure chiral endocyclic vicinal diamines, which can
be used as chiral ligands for developing new catalysts. In
particular, these chiral vicinal diamines can be easily derived
to give a new class of chiral NHC ligands, which are difficult to
access through other methods. We believe that this study will
stimulate future work on applications of these new chiral
vicinal diamines and NHC ligands in asymmetric catalysis.
are hydrogenated step sequentially and the first quinoline
ring is reduced faster than the second one. Then, the reaction
intermediate (+)-5a (99% ee after recrystallization) was
hydrogenated by using both enantiomers of catalyst 3b. In
both cases, full conversion was observed, and (S,S)-2a and
meso-2a were obtained as the sole products, respectively
[Eq. (2) and (3)]. This result suggests that the generation of
Acknowledgements
We thank the National Natural Science Foundation of China
(Nos. 21232008, 21473216 and 21521002) and CAS for
financial support.
Keywords: 2,2’-bisquinolines · asymmetric catalysis ·
homogeneous catalysis · hydrogenation · vicinal diamines
the second chiral center is completely controlled by the
chirality of catalyst, and the diastereoselectivity was unusually
high (> 20:1). In addition, the absolute configuration of 2a
was determined to be 2S,2S’ based on single-crystal X-ray
analysis of N-tosyl-1,1’,2,2’,3,3’,4,4’-octahydro-2,2’-biquino-
line (Scheme S1).[13] Based on this result and our previous
studies on the AH of quinoline, a cyclic 10-membered
transition state for the AH of 1a is proposed, and the
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4
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Angew. Chem. Int. Ed. 2016, 55, 1 – 6
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Angewandte
Communications
Chemie
2009, 28, 1233; g) A. Kannenberg, D. Rost, S. Eibauer, S. Tiede,
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Received: August 22, 2016
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Asymmetric Catalysis
W. Ma, J. Zhang, C. Xu, F. Chen, Y.-M. He,
Q.-H. Fan*
&&&& — &&&&
Highly Enantioselective Direct Synthesis
of Endocyclic Vicinal Diamines through
Chiral Ru(diamine)-Catalyzed
Hydrogenation of 2,2’-Bisquinoline
Derivatives
Diamines are forever: Chiral cationic
ruthenium diamine complexes were used
to catalyze the asymmetric hydrogenation
of bisquinoline and bisquinoxaline deriv-
atives. A broad range of chiral endocyclic
vicinal diamines were obtained in high
yields with excellent diastereoselectivity
and enantioselectivity. Moreover, these
products could be easily transformed into
a new class of chiral N-heterocyclic
carbenes.
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!