Asymmetric Hydrogenation of Cyclic Imines of Benzoazepines and Benzodiazepines with Chiral, Cationic Ruthenium–Diamine Catalysts

Article abstract of DOI:10.1002/ejoc.201700236

A method for the highly enantioselective hydrogenation of diverse seven-membered N-containing heterocycles, including 2,4-diaryl-3H-benzo[b]azepines, racemic 2,2,4-trisubstituted 2,3-dihydrobenzo[b][1,4]diazepines, and 4-substituted 1H-benzo[b][1,4]diazep

Full text of DOI:10.1002/ejoc.201700236

A Journal of
Accepted Article
Title: Asymmetric Hydrogenation of Cyclic Imines of Benzoazepines
and Benzodiazepines with Chiral Cationic Ruthenium Diamine
Catalysts
Authors: Zhusheng Yang, Ziyuan Ding, Fei Chen, Yanmei He, Nianfa
Yang, and Qing-Hua Fan
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201700236
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10.1002/ejoc.201700236
European Journal of Organic Chemistry
COMMUNICATION
Asymmetric Hydrogenation of Cyclic Imines of Benzoazepines
and Benzodiazepines with Chiral Cationic Ruthenium Diamine
Catalysts
Zhusheng Yang,‡[a,b] Ziyuan Ding,‡[b] Fei Chen,*[b] Yan-Mei He,[b] Nianfa Yang*[a] and Qing-Hua Fan*[b]
Abstract: Highly enantioselective hydrogenation of diverse seven-
membered N-containing heterocycles, including 2,4-diaryl-3H-
asymmetric induction in the hydrogenation of 2,4-diaryl
substituted 3H-1,5-benzodiazepines.[6a] Encouraged by these
results, we hope to expand the substrate scope from 2,4-
disubstituted 3H-1,5-benzodiazepines to other seven-membered
cyclic imines. Herein, we disclose the details of the asymmetric
hydrogenation of 2,4-diaryl-3H-benzo[b]azepines, racemic 2,2,4-
benzo[b]azepines,
racemic
2,2,4-trisubstituted
and 4-substituted
2,3-
dihydrobenzo[b][1,4]diazepines
1H-
benzo[b][1,4]diazepin-2(3H)-ones, using chiral cationic ruthenium
diamine catalysts, has been developed. Reversal of
enantioselectivity was observed for all these substrates by simply
changing the achiral counteranion of the catalyst. This method thus
provides a facile access to both enantiomers of these optically pure
seven-membered heterocycles.
trisubstituted
2,3-dihydrobenzo[b][1,4]diazepines
and
4-
substituted 1H-benzo[b][1,4]diazepin-2(3H)-ones catalysed by
Ru-MsDPEN complexes with good to excellent enantioselectivity.
Introduction
Seven-membered N-containing heterocycles[1] such as
benzoazepines,[2] benzodiazepines[3] and benzodiazepinones[4]
are versatile pharmacophores in medicinal chemistry. As
exemplified in Figure 1, OPC-51803 has proven to be an
efficient non-peptide agonist for the AVP V2 receptor.[2b]
Compound (R)-A represents a new class of hepatitis Cvirus
(HCV) NS5B polymerase inhibitors.[3a] Moreover, compound (S)-
B acts as a potent Caspase-1 (ICE) Inhibitor.[4b] Therefore, a
number of methods have been developed for obtaining such
type of heterocycles. Among them, however, direct catalytic
asymmetric synthesis of optically pure benzoazepines and
benzodiazepines remains less investigated.[5-8]
Figure 1. Selected biologically active seven-membered N-containing
heterocycles.
Results and Discussion
The
asymmetric
hydrogenation
of
2,4-diaryl-3H-
benzo[b]azepines containing C=C and C=N double bonds was
first studied. We chose 2,4-diphenyl-3H-1-benzo[b]azepine (2a)
as the standard substrate (Table 1). It was found that 2a could
be hydrogenated with (R,R)-1a in dichloromethane (DCM),
giving the desired product in full conversion but with only 4% ee
(entry 1). Based on our previous finding that the counteranion
affected the outcome in asymmetric hydrogenation,[6a,8a,12a-d] we
then examined a series of more weakly coordinating anions,
Transition-metal-catalyzed asymmetric hydrogenation of
imines have enabled a straightforward and powerful approach to
optically active amines.[9] Over the past two decades, a variety of
chiral metal catalysts, including Rh, Ru, Ir and Pd complexes of
chiral phosphorus ligands, have been successfully employed in
the asymmetric hydrogenation of cyclic imines or enamines, and
N-heteroarenes.[10] Most recently, the phosphine-free cationic
ruthenium complexes of chiral mono-sulfonated diamines[11]
have proved to be very efficient catalysts for asymmetric
hydrogenation of a broad range of acyclic and cyclic imines by
us and others.[6a,8a,12] Particularly, we found that this catalytic
system was highly enantioselective for the asymmetric
hydrogenation of a broad range of 1,5-benzodiazepines, and the
choice of achiral counteranions determined the sense of
-
-
-
including BF4 , PF6 , SbF6 , and BArF- (tetrakis(3,5-
bistrifluoromethylphenyl)borate). Interestingly, a distinct increase
in enantioselectivity was observed (entries 2–5), and the catalyst
(R,R)-1e bearing a BArF- anion gave the best result (80% ee,
entry 5). Notably, the use of the ruthenium catalyst (R,R)-1f
bearing a coordinative phosphate anion gave the chiral product
with opposite configuration (87% ee, entry 6).
Subsequently, the effects of solvent, hydrogen pressure, and
temperature on the catalytic performance of this reaction were
investigated. It was found that aprotic solvents such as DCM,
toluene and THF were suitable for this catalytic system (entries
6-8), and DCM turned out to be optimal in terms of both
reactivity and enantioselectivity (entry 6). However, when the
reaction was carried out in MeOH, the product with opposite
configuration was obtained but with a lower ee value (entry 9). In
addition, the enantioselectivity of the reaction was insensitive to
hydrogen pressure and temperature (entries 10-13). Notably,
when the catalyst loading of (R,R)-1f was decreased to 0.2
mol %, full conversion and 85% ee were still observed (entry 15).
In the case of (R,R)-1e,[13] a decrease in catalyst loading (0.2
[a]
[b]
Key Laboratory of Environmentally Friendly Chemistry of the
Ministry of Education, College of Chemistry, Xiangtan University
Xiangtan, Hunan 411105, (P. R. China)
E-mail: nfyang@xtu.edu.cn
CAS Key Laboratory of Molecular Recognition and Function, CAS
Research/Education Center for Excellence in Molecular Sciences,
Institute of Chemistry, Chinese Academy of Sciences (CAS), and
University of Chinese Academy of Sciences
Beijing 100190 (P. R. China)
E-mail: fanqh@iccas.ac.cn; chenfei211@iccas.ac.cn
Equal contribution to this work
[‡]
Supporting information for this article is given via a link at the end of
the document. ((Please delete this text if not appropriate))
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10.1002/ejoc.201700236
European Journal of Organic Chemistry
COMMUNICATION
mol %) resulted in lower enantioselectivity (61% ee, entry 24 in
Table S1 in Supporting Information). Therefore, both
enantiomers of 3a could be obtained with good enantioselectivity
by using the same enantiomer of the ligand but with different
achiral counteranions (entries 5 and 6).
contrast, the catalyst (R,R)-1e bearing BArF- anion provided the
opposite enantiomers 3a–i with quite good enantioselectivity (up
to 87% ee). Notably, the hydrogenation of substrate 2g bearing
a CF3 substituent catalysed by (R,R)-1e gave much lower
enantioselectivity (50% ee, entry 7).
Table 1. Optimization of the reaction conditions for the asymmetric
hydrogenation of 2a[a]
Table 2. Scope of 2,4-diaryl-3H-benzo[b]azepines and reversal of
enantioselectivity[a]
Entry
Substrate (Ar)
Ee (%)[b] for
Ee (%)[b] for
(R,R)-1f
(R,R)-1e
Entry
Anion (X-)
OTf-
Solvent
H2 (atm);
Conv.
(%)[b]
Ee (%)[c,d]
1
2
3
4
5
6
7
8
9
Ph (2a)
87 (-)
94 (-)
92 (-)
84 (-)
89 (S)
88 (-)
83 (-)
93 (-)
95 (-)
80 (+)
83 (+)
82 (+)
85 (+)
80 (R)
80 (+)
50 (+)
86 (+)
85 (+)
Temp. (oC)
4-MeO-Ph (2b)
4- Me-Ph (2c)
4-F-Ph (2d)
4-Cl-Ph (2e)
4-Br-Ph (2f)
4-CF3-Ph (2g)
3-tolyl (2h)
1
DCM
DCM
DCM
DCM
DCM
DCM
toluene
THF
50 ; 40
50 ; 40
50 ; 40
50 ; 40
50 ; 40
50 ; 40
50 ; 40
50 ; 40
50 ; 40
50 ; 25
50 ; 60
20 ; 40
80 ; 25
50 ; 40
50 ; 40
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
4 (+)
-
2
BF4
35 (+)
74 (+)
72 (+)
80 (+)
87 (-)
85 (-)
87 (-)
61 (+)
87 (-)
86 (-)
86 (-)
87 (-)
87 (-)
85 (-)
-
3
PF6
-
4
SbF6
5
BArF-
-
6
Ph2PO4
-
7
Ph2PO4
2-naphthyl (2i)
-
8
Ph2PO4
[a] Reaction conditions: substrate 2 (0.2 mmol) in DCM (2 mL), (R,R)-1f (1.0
mol %) or (R,R)-1e (2.0 mol %), 50 atm of H2, stirred at 40 oC for 10-24 h. [b]
Full conversions were observed, which were determined by 1H NMR
spectroscopy of the crude reaction mixtures. The enantiomeric excesses were
determined by chiral HPLC analysis. The absolute configuration of (-)-3e was
determined to be S based on single-crystal X-ray analysis.[14]
-
9
Ph2PO4
MeOH
DCM
DCM
DCM
DCM
DCM
DCM
-
10
11
12
13
14[e]
15[f]
Ph2PO4
-
Ph2PO4
-
Ph2PO4
-
Ph2PO4
Encouraged by these results obtained above and in the
asymmetric hydrogenation of 2,4-diaryl substituted 3H-1,5-
benzodiazepines,[6a] we attempted the dynamic kinetic resolution
-
Ph2PO4
-
Ph2PO4
of
racemic
2,2,4-trisubstituted
2,3-
dihydrobenzo[b][1,4]diazepines by asymmetric hydrogenation.
Recently, Gong and co-workers reported the dynamic kinetic
transfer hydrogenation reaction of such type of substrates by
using phosphoric acids as catalysts and Hantzsch ester as
hydride source.[6e] In our study, 2-methyl-2,4-diphenyl-2,3-
dihydrobenzo[b][1,4]diazepine 4a was chosen as the standard
substrate for optimization of reaction conditions. Gratifyingly, the
hydrogenation proceeded smoothly in DCM in the presence of
2.0 mol% (R,R)-1 bearing different counteranions. As shown in
Table 3, in all cases, the cis-isomers was obtained as the main
products but with lower enantioselectivity (entries 1-6).
[a] Reaction conditions: 2a (0.1 mmol) in solvent (1 mL), catalyst (R,R)-1 (2.0
mol %), 15 h. [b] The conversions were determined by 1H NMR spectroscopy
of the crude reaction mixtures. [c] The enantiomeric excesses were
determined by chiral HPLC with a chiral OD-H column. [d] Optical rotations
were measured with PerkinElmer 341 polarimeter. [e] With 1.0 mol % catalyst.
[f] With 0.2 mol % catalyst.
With optimized reaction conditions in hand, the generality of
the achiral counteranion induced reversal of enantioselectivity
was investigated by using
a variety of 2,4-diaryl-3H-1-
-
Interestingly, the catalysts bearing BArF-, SbF6 , and PF6- anions
benzo[b]azepine derivatives. As shown in Table 2, all reactions
proceeded smoothly to give full conversions, regardless of the
steric and electronic properties of the substrates. The use of the
Ru/Ph2PO4 catalyst (R,R)-1f gave the desired products 3a–i in
good to excellent enantioselectivity (83% to 95% ee). In
gave the product of opposite configuration to that obtained when
-
-
using the catalysts bearing BF4 , OTf-, and Ph2PO4 anions. The
best result was achieved with Ru/Ph2PO4 catalyst (R,R)-1f,
affording the chiral amine isomers with 2/3 dr, 99% ee and 87%
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10.1002/ejoc.201700236
European Journal of Organic Chemistry
COMMUNICATION
ee for the minor and major diastereomers, respectively (entry 6).
Notably, good stereoselectivity was also observed in toluene
and THF (entries 7-8), but opposite configuration and lower ee
value for cis-product was obtained in MeOH (entry 9).
Scheme
1.
Racemization
pathway
of
2,2,4-trisubstituted
2,3-
dihydrobenzo[b][1,4]diazepines.
Table 3. Optimization of reaction conditions of asymmetric hydrogenation of
racemic 4a[a]
Table 4. Asymmetric hydrogenation of racemic 2,2,4-trisubstituted 2,3-
dihydrobenzo[b][1,4]diazepines catalyzed by (R,R)-1f [a]
Entry
Anion
BArF-
Solvent
Dr [b]
(trans/cis)
Ee (%)[c,d]
(trans/cis)
1
2
3
4
5
6
7
8
9
DCM
DCM
DCM
DCM
DCM
DCM
toluene
THF
1/3
1/4
1/4
1/5
1/4
2/3
2/3
1/2
1/7
88 (-) / 48 (+)
71 (-) / 25 (+)
69 (-) / 16 (+)
25 (+) / 4 (-)
68 (+) / 22 (-)
99 (+) / 87 (-)
99 (+) / 66 (-)
99 (+) / 85 (-)
92 (-) / 18 (+)
Entry
Ar
Dr[b] (trans/cis)
Ee (%)[c] (trans/cis)
99 (+)/87 (-)
-
SbF6
1
2
3
4
Ph (4a)
2/3
2/3
2/3
1/3
-
PF6
4-Me-Ph (4b)
4-Br-Ph (4c)
3-Br-Ph (4d)
99 (+)/82 (-)
-
BF4
99 (2S,4S)/76 (2R,4S)
99 (+)/54 (-)
OTf-
-
-
-
-
Ph2PO4
Ph2PO4
Ph2PO4
Ph2PO4
[a] Reaction conditions: substrate 4a-d (0.1 mmol) in DCM (1 mL), catalyst
(R,R)-1f (1.0 mol %), stirred at 50 atm H2, 40 oC, 10 h. [b] Full conversions
were obtained in all reactions, and d.r. (trans/cis) were determined by 1H NMR
spectroscopy of the crude reaction mixture. [c] The enantiomeric excesses
were determined by chiral HPLC with a chiral AD-H column.
MeOH
[a] Reaction conditions: 4a (0.1 mmol) in solvent (1 mL), catalyst (R,R)-1
(2.0 mol %), 50 atm of H2, stirred at 40 oC for 10 h. [b] Full conversions
were obtained in all cases, and diastereoselectivity (trans/cis) were
determined by 1H NMR spectroscopy of the crude reaction mixture. [c] The
enantiomeric excesses were determined by chiral HPLC with a chiral AD-H
column. [d] The absolute configurations were determined by comparing with
the data reported in reference.[6e]
To further extend the scope of this catalytic system in the
asymmetric hydrogenation of seven-membered N-containing
heterocycles, the less studied substrates of 4-substituted 1H-
benzo[b][1,4]diazepin-2(3H)-ones[7] were investigated. Similar
counteranion effect on catalytic performance was observed
(Table S3 in SI). Catalyst (R,R)-1e was found to be optimal in
terms of reactivity and enantioselectivity, providing 7a in full
conversion with 70% ee in R-configuration. The hydrogenation
of other two 4-aryl substituted 1,5-benzodiazepinones 6b and 6c
with (R,R)-1e afforded the reduced (R)-products with moderate
enantioselectivity (Scheme 1). In contrast, the Ru/Ph2PO4
catalyst gave the chiral product with opposite configuration and
with much lower reactivity and enantioselectivity (32%
conversion and 59% ee, entry 6, Table S3, SI). Notably, this
ruthenium catalytic system was found to be inactive in the
Several
racemic
2,2,4-trisubstituted
2,3-
dihydrobenzo[b][1,4]diazepines were then investigated under the
optimized reaction conditions (Table 3, entry 6). As shown in
Table 4, all reactions proceeded smoothly to give products with
good to excellent ee values but moderate diastereoselectivity.
Generally, both the diastereo- and enantioselectivity did not
depend on the electronic characteristic of the substituent, but
was influenced by the position of the same substituent on the
asymmetric
hydrogenation
of
4-methyl-1,5-dihydro-2H-
phenyl ring (entries
2 and 3 vs entry 4). The minor
benzo[b][1,4]diazepin-2-one 6d.
diastereomers were obtained with higher enantioselectivity (99%
ee) than the major ones (54–87% ee).
The absolute configuration of the products was assigned by
single-crystal X-ray analysis. The X-ray structure of trans-isomer
(+)-5c indicate the S configuration at both C2 and C4 positions
(Scheme S3 in Supporting Information).[14] The absolute
configuration of cis-isomer (-)-5c was determined to be (2R,4S)
by comparing with the data reported in reference.[6e] The
racemization might occur via the sequential retro-Mannich and
Mannich reactions (Scheme 1), similar to the mechanism
reported by Gong.[6e]
Conclusions
In conclusion, we have developed a general method for the
enantioselective hydrogenation of diverse seven-membered
cyclic imines, including 2,4-diaryl-3H-benzo[b]azepines, racemic
2,2,4-trisubstituted 2,3-dihydrobenzo[b][1,4]diazepines and 4-
substituted 1H-benzo[b][1,4]diazepin-2(3H)-ones, with chiral
cationic
ruthenium
diamine
catalysts.
Reversal
of
enantioselectivity was observed for all these imines by using the
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10.1002/ejoc.201700236
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COMMUNICATION
[3]
a) O. Nyanguile, F. Pauwels, W. Van den Broeck, C. W. Boutton, L.
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334, 3.
same enantiomer of the ligand but with different achiral
counteranions. This new method thus provides a facile approach
to the synthesis of optically pure chiral seven-membered N-
containing heterocycles, which are of great importance for
pharmaceutical applications.
[4]
a) B. E. Evans, K. E. Rittle, M. G. Bock, R. M. DiPardo, R. M. Freidinger,
W. L. Whitter, G. F. Lundell, D. F. Veber, P. S. Anderson, R. S. L.
Chang, V. J. Lotti, D. J. Cerino, T. B. Chen, P. J. Kling, K. A. Kunkel, J.
P. Springer, J. Hirshfield, J. Med. Chem. 1988, 31, 2235; b) D. J.
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Oshitari, H. Takahashi, H. Natsugari, Chem. Eur. J. 2012, 18, 1572.
Only few examples of asymmetric synthesis of benzoazepines were
reported. See: a) Y. S. Park, E. K. Yum, A. Basu, P. Beak, Org. Lett.
2006, 8, 2667; b) H. He, W.-B. Liu, L.-X. Dai, S.-L. You, Angew. Chem.
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Zhou, Org. Lett. 2016, 18, 5920.
[5]
[6]
Scheme
2.
Asymmetric
hydrogenation
of
4-substituted
1H-
benzo[b][1,4]diazepin-2(3H)-ones 6a-d catalyzed by (R,R)-1e.
For selected examples of asymmetric synthesis of benzodiazepines,
see: a) Z.-Y. Ding, F. Chen, J. Qin, Y.-M. He Q.-H. Fan, Angew. Chem.
Int. Ed. 2012, 51, 5706; b) B. Ma, Z. Ding, J. Liu, Y. He, Q.-H. Fan,
Chem. Asian J. 2013, 8, 1101; c) K. Gao, B. Wu, C.-B. Yu, Q.-A. Chen,
Z.-S. Ye, Y.-G. Zhou, Org. Lett. 2012, 14, 3890; d) V. Dragan, J. C.
McWilliams, R. Miller, K. Sutherland, J. L. Dillon, M. K. O’Brien, Org.
Lett. 2013, 15, 2942; e) Z. -Y. Han, H. Xiao, L. -Z. Gong, Bioorg. Med.
Chem. Lett. 2009, 19, 3729; f) K. Horiguchi, E. Yamamoto, K. Saito, M.
Yamanaka, T. Akiyama, Chem. Eur. J. 2016, 22, 8078; g) N. A.
Strotman, C. A. Baxter,; K. M. J. Brands, E. Cleator, S. W. Krska, R. A.
Reamer, D. J. Wallace, T. J. Wright, J. Am. Chem. Soc. 2011, 133,
8362; h) P. Roszkowski, J. K. Maurin, Z. Czarnocki, Synthesis 2012, 44,
241; i) X. Fu, J. Feng, Z. Dong, L. Lin, X. Liu, X. Feng, Eur. J. Org.
Chem. 2011, 5233; j) Y. Wang, M.-S. Tu, F. Shi, S.-J. Tu, Adv. Synth.
Catal. 2014, 356, 2009.
Experimental Section
Typical procedure for asymmetric hydrogenation of 2,4-diaryl-3H-
benzo[b]azepines: A 50 mL glass-lined stainless-steel reactor equipped
with a magnetic stirrer bar was charged with Ru-catalyst (R,R)-1f (1.6 mg,
1.0 mol %) or (R,R)-1e (5.6 mg, 2.0 mol %), corresponding substrates 2
(0.2 mmol) in DCM (2 mL) under nitrogen atmosphere in a glove box.
The autoclave was closed, and the final pressure of the hydrogen gas
was adjusted to 50 atm after purging the autoclave with hydrogen gas
several times. The reaction mixture was stirred at 40 oC for 10-24 h. Then
the hydrogen gas was carefully released and the conversion was
determined by 1H NMR. The reaction mixture was filtered through a short
pad of silica (triethylamine/petroleum, 5/95, v/v) to give the pure products.
The enantiomeric excess of the product was determined by HPLC with a
chiral column.
[7]
[8]
Only few examples of asymmetric synthesis of benzodiazepinones
were reported. See: a) M. Rueping, E. Merino, R. M. Koenigs, Adv.
Synth. Catal. 2010, 352, 2629; b) R. Borrmann, R. M. Koenigs, J. Zoller,
M. Rueping, Synthesis 2017, 49, 310; c) X. Chen, Y. Zheng, C. Shu, W.
Yuan, B. Liu, X. Zhang, J. Org. Chem. 2011, 76, 9109.
Acknowledgements
For selected recent examples on asymmetric synthesis of other seven-
membered N-containing heterocycles, see: a) F. Chen, Z. Ding, J. Qin,
T. Wang, Y.-M. He, Q.-H. Fan, Org. Lett. 2011, 13, 4348; b) K. Gao, C.-
B. Yu, W. Li, Y.-G. Zhou, X. Zhang, Chem. Commun. 2011, 47, 7845;
c) R.-N. Guo, K. Gao, Z.-S. Ye, L. Shi, Y. Li, Y.-G. Zhou, Pure Appl.
Chem. 2013, 85, 843; d) J. Wang, Tetrahedron Lett. 2013, 54, 5956; e)
Li, W.; Schlepphorst, C.; Daniliuc, C.; Glorius, F. Angew. Chem. Int. Ed.
2016, 55, 3300; f) B. Balakrishna, A. Bauzá, A. Frontera, A. Vidal-
Ferran, Chem. Eur. J. 2016, 22, 10607; g) D. J. Cowan, J. L. Collins, M.
B. Mitchell, J. A. Ray, P. W. Sutton, A. A. Sarjeant, E. E. Boros, J. Org.
Chem. 2013, 78, 12726.
We thank the National Natural Science Foundation of China
(Nos. 21672218, 21232008 and 21521002) and CAS
(QYZDJSSW-SLH023) for financial support.
Keywords: asymmetric catalysis • diamine ligand •
hydrogenation • benzoazepine • benzodiazepine
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10.1002/ejoc.201700236
European Journal of Organic Chemistry
COMMUNICATION
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10.1002/ejoc.201700236
European Journal of Organic Chemistry
COMMUNICATION
COMMUNICATION
The title hydrogenation was achieved
in the presence of phosphine-free
chiral cationic ruthenium diamine
complexes, providing a facile access
to various chiral seven-membered N-
containing heterocycles. Reversal of
enantioselectivity was observed by
Asymmetric Hydrogenation*
Zhusheng Yang, Ziyuan Ding, Fei
Chen,* Yan-Mei He, Nianfa Yang* and
Qing-Hua Fan*
Page No. – Page No.
Title
simply
changing
the
achiral
Asymmetric Hydrogenation of Cyclic
counteranion of the catalyst.
Imines
of
Benzoazepines
and
Benzodiazepines with Chiral Cationic
Ruthenium Diamine Catalysts
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