Asymmetric Hydrogenation of Dibenzo[ c,e]azepine Derivatives with Chiral Cationic Ruthenium Diamine Catalysts

Article abstract of DOI:10.1021/acs.orglett.9b01859

An efficient Ru-catalyzed asymmetric hydrogenation of dibenzo[c,e]azepines is reported. A series of seven-membered cyclic amines were obtained with moderate to excellent enantioselectivity. The catalyst counteranion played an important role in achieving high-level chiral induction. Moreover, a one-pot synthesis of chiral 6,7-dihydro-5H-dibenz[c,e]azepines via two-step reductive amination was also developed.

Full text of DOI:10.1021/acs.orglett.9b01859

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Asymmetric Hydrogenation of Dibenzo[c,e]azepine Derivatives with
Chiral Cationic Ruthenium Diamine Catalysts
Shanshan Zhang, Fei Chen,* Yan-Mei He, and Qing-Hua Fan*
Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Molecular Recognition and Function, Institute of
Chemistry, Chinese Academy of Sciences (CAS), and University of Chinese Academy of Sciences, Beijing 100190, P. R. China
S
* Supporting Information
ABSTRACT: An efficient Ru-catalyzed asymmetric hydrogenation of dibenzo[c,e]azepines is reported. A series of seven-
membered cyclic amines were obtained with moderate to excellent enantioselectivity. The catalyst counteranion played an
important role in achieving high-level chiral induction. Moreover, a one-pot synthesis of chiral 6,7-dihydro-5H-
dibenz[c,e]azepines via two-step reductive amination was also developed.
hiral N-heterocycles are ubiquitous structural units found
Cin various biologically active substances like natural
products and pharmaceutical and agrochemical compounds.1
Transition metal-catalyzed asymmetric hydrogenation (AH) of
cyclic imines and enamines represents a versatile method for
synthesizing these chiral heterocycles.2 To date, hydrogenation
of various five- and six-membered cyclic imines has been
realized with high enantiomeric excesses.2d However, there
have been few successful reports of AH of seven-membered
cyclic imines (azepines).3−5 In 1994, Buchwald first reported
the Ti-catalyzed AH of 7-phenyl-3,4,5,6-tetrahydro-2H-azepine
with 98% ee.3a Since then, several iridium and ruthenium
complexes have been employed in the AH of similar seven-
membered cyclic imines.3b−e Recently, some successful
examples have been reported in the AH of some benzo-fused
seven-membered cyclic imines for the preparation of chiral
benzazepines and benzodiazepines (Figure 1a,b).4,5 For
example, Zhou and co-workers reported that some benzo-
fused seven-membered cyclic imines like dibenzo[b,f ][1,4]-
oxazepines,4a benzodiazepines,4b and dibenzo[b,f ][1,4]-
thiazepines4c were efficiently hydrogenated by using iridium
Figure 1. Representative examples of AH of benzo-fused azepines.
diphosphine catalysts (Figure 1a). Following these leading
works, a few Ir, Pd, and Rh complexes of chiral phosphines
s.3e,5a,b,7 It was found that a series of benzodiazepines were
have been applied to the AH of such seven-membered cyclic
imines (Figure 1a).4d−h
hydrogenated smoothly with excellent enantioselectivity and
diastereoselectivity.5a,b Intrigued by these results and on the
basis of our continuing interest in synthesizing chiral cyclic
amines via enantioselective hydrogenation, we hope to expand
Recently, the ruthenium/diamine complexes6 and iridium/
P,N ligand complexes have been demonstrated to be effective
for the AH of seven-membered cyclic imines (Figure 1b) by
our group.5a−e In particular, the chiral ruthenium catalysts
proved to be very effective in the AH of not only cyclic and
acyclic imines but also a broad range of N-heteroaromatic-
Received: May 29, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01859
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
the substrate scope to dibenzo[c,e]azepines (Figure 1c). The
resulting chiral products, 6,7-dihydro-5H-dibenz[c,e]azepines,
are one kind of seven-membered cyclic amines, which contain
a biaryl motif and possess both central chirality and switchable
axial chirality.8 Moreover, these chiral heterocycles have been
attracting a great deal of attention because of the potential
biological activities9 and promising applications in organo-
catalysis.10 Great efforts have been made to construct this kind
of framework by using chiral-pool starting materials or an
auxiliary strategy.8,11 However, only a few catalytic asymmetric
strategies have been reported.12,13 Very recently, Turner and
co-workers reported the synthesis of chiral 6,7-dihydrodibenz-
[c,e]azepines through the reductase-catalyzed asymmetric
reduction of the corresponding imines.12a Herein, we disclose
the details of the AH of dibenzo[c,e]azepines with Ru/diamine
catalysts (Figure 1c), including one-pot preparation of these
chiral amines via direct asymmetric reductive amination
(DARA).13
without (Boc)2O (entry 7), possibly due to the catalyst
poisoning effect of the resulting amine.3e,7a Subsequently, the
screening of organic solvents (entries 6 and 8−10) showed
toluene was optimal (entry 8). Then, various Ru catalysts were
tested (entries 11−13), and (R,R)-6f gave the best results
(entry 13). Finally, the effect of hydrogen pressure and
reaction temperature was also studied (see the Supporting
Information and Table S1), and the highest enantioselectivity
was obtained under 30 atm of H2 at 25 °C (entry 14).
The substrate scope was then studied under the optimized
reaction conditions described above (Table 1, entry 14).
Generally, various 7-alkyl-substituted dibenzo[c,e]azepines
were hydrogenated smoothly, providing the chiral cyclic
amines with high ee values (Table 2, 81−96% ee). The
Table 2. Asymmetric Hydrogenation of 7-Alkyl-Substituted
a
Dibenzo[c,e]azepines
In our initial study, 7-methyl dibenzo[c,e]azepine 1a was
subjected to hydrogenation as a model substrate with (Boc)2O
in dichloromethane (DCM) under 50 atm of H2 and 50 °C.
We first examined the counteranion effect of the catalyst
(Table 1, entries 1−6). (R,R)-3f bearing a noncoordinating
BArF anion led to the highest selectivity (entry 6). Notably, a
lower activity and a lower enantioselectivity were observed
Table 1. Optimizing the Conditions for Asymmetric
a
Hydrogenation
b
c
entry
catalyst
anion [X]−
solvent
conversion (%)
ee (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
(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)-4f
(R,R)-5f
(R,R)-6f
(R,R)-6f
OMs
OTf
BF4
DCM
DCM
DCM
DCM
DCM
DCM
DCM
toluene
Et2O
iPrOH
toluene
toluene
toluene
toluene
>99
>99
>99
>99
>99
>99
68
>99
>99
>99
>99
>99
>99
>99
3
12
26
50
59
79
58
83
47
34
82
73
91
94
a
Reaction conditions: 1a−j (0.1 mmol) in toluene (1 mL), (R,R)-6f
(2.0 mol %), (Boc)2O (1.1 equiv), H2 (30 atm), stirred at 25 °C for
14 h. Isolated yields. The enantiomeric excesses were determined by
HPLC, and the absolute configuration of 2l was determined by
PF6
SbF6
BArF
BArF
BArF
BArF
BArF
BArF
BArF
BArF
BArF
comparison of its optical rotation with literature data.8 (R,R)-6f
b
c
d
(10.0 mol %). (R,R)-6f (5.0 mol %).
enantioselectivity was observed to be more sensitive to the
electronic properties than position of the substituents on the
phenyl rings. For example, substrates bearing a methyl
substituent at the 2, 3, 4, or 9 position (1b, 1c, 1d, or 1h,
respectively) exhibited similar enantioselectivity. However, an
electron-withdrawing substituent F at position 3 of the phenyl
ring (1f) decreased the enantioselectivity obviously. Notably,
replacement of the methyl group with an ethyl group (1k)
resulted in a quite low reactivity and a quite low
enantioselectivity. Interestingly, hydrogenation of 5,7-dimeth-
yl-substituted dibenzo[c,e]azepine (1l) proceeded smoothly
with 5.0 mol % (R,R)-6f, giving the trans isomer with 76% ee
and a ratio of trans-2l to cis-2l of approximately 1.3:1.
e
14
a
Reaction conditions: 1a (0.1 mmol) in solvent (1.0 mL), (R,R)-Ru
catalyst (2.0 mol %), (Boc)2O (1.1 equiv), H2 (50 atm), stirred at 50
b
°C for 22 h. The conversions were determined by 1H NMR
c
spectroscopy of the crude reaction mixture. The enantiomeric
excesses were determined by HPLC with a chiral IC column, and the
absolute configuration of 2a was determined by comparison of its
optical rotation with literature data.8 Without (Boc)2O, and free
d
e
amine 9 was obtained. H2 (30 atm), stirred at 25 °C.
B
DOI: 10.1021/acs.orglett.9b01859
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
For the more difficult 7-phenyl-substituted substrate (1m),
hydrogenation could not occur under the optimized reaction
conditions described above. To further improve the reactivity
and enantioselectivity, we rescreened the reaction conditions
(Table 3, entries 1−4, and Table S2). Better enantioselectivity
Scheme 1. One-Pot Synthesis of Chiral 6,7-Dihydro-5H-
dibenz[c,e]azepines via Two-Step Reductive Amination
a
Table 3. Asymmetric Hydrogenation of 7-Aryl-Substituted
a
Dibenzo[c,e]azepines
b
c
entry
catalyst
solvent
R1/R2/R3
yield (%)
ee (%)
1
2
3
4
5
6
7
8
(R,R)-3f
(R,R)-3f
(R,R)-3f
(R,R)-6f
(R,R)-5f
(R,R)-5f
(R,R)-5f
(R,R)-5f
(R,R)-5f
(R,R)-5f
(R,R)-5f
toluene
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
H/H/H (1m)
H/H/H (1m)
H/H/H (1m)
H/H/H (1m)
H/H/H (1m)
H/Me/H (1n)
H/Cl/H (1o)
H/H/Me (1p)
H/H/OMe (1q)
H/H/F (1r)
17
38
>99
>99
10 (+)
40 (+)
40 (+)
4 (−)
80 (+)
68 (+)
77 (+)
81 (+)
60 (+)
77 (+)
70 (+)
a
Reaction conditions: 7 (0.1 mmol) in toluene (1 mL), RNH2 (0.1
mmol), (R,R)-4b (5.0 mol %), 4 Å molecular sieves (100 mg), TfOH
(2.5 mol %), H2 (50 atm), stirred at 50 °C for 22 h. Isolated yields are
given. The enantiomeric excesses were determined by HPLC.
d
97
d
91
d
98
conditions for other aromatic amines. A higher ee value was
observed when p-toluidine was used (83% ee). The use of
dibenzylamine resulted in a much lower reactivity and a much
lower enantioselectivity (47% ee).
d
94
d
9
10
11
87
d
97
d
OMe/H/H (1s)
87
Finally, a scale-up preparation of 5-methyl-6,7-dihydro-5H-
dibenz[c,e]azepine was developed (Scheme 2). Reaction of 1a
a
Reaction conditions: substrates 1m−s (0.1 mmol) in solvent (1.0
mL), (R,R)-Ru catalyst (10.0 mol %), (Boc)2O (1.1 equiv), H2 (50
atm), stirred at 50 °C for 22 h, except for entries 1 and 2, for which a
b
Ru catalyst (5.0 mol %) was used. The yields were determined by 1H
Scheme 2. Scale-Up Preparation of (R)-5-Methyl-6,7-
dihydro-5H-dibenz[c,e]azepine
c
NMR spectroscopy of the crude reaction mixture. The enantiomeric
excesses were determined by HPLC. Isolated yields.
d
was achieved in Et2O instead of toluene, and full conversion
could not be obtained until 10.0 mol % catalyst was used
(entry 3). Notably, an obvious electronic effect of the N-
sulfonate substituent on the catalyst was observed (entries 3−
5). (R,R)-5f bearing a strong electron-withdrawing CF3
substituent led to a significant increase in enantioselectivity
(entry 5). Then, several 5-aryl-substituted dibenzo[c,e]azepines
were studied, affording the chiral cyclic amines in good yields
with moderate enantioselectivities (entries 5−11).
DARA has been considered to be one of the most efficient
and economical approaches for directly synthesizing chiral
acyclic and cyclic amines.14 To date, only a few transition
metal-catalyzed intramolecular DARAs have been reported for
the construction of chiral N-heterocycles,15 including seven-
membered N-heterocycles.13,15a−c DARA in which a secondary
amine participates is still a formidable challenge.16,17 Most
recently, an efficient cascade AH of quinolines/intramolecular
reductive amination for preparing benzo-fused quinolizidines
and indolizidines was developed by our group.17 Thus, we
envisioned that one-pot preparation of chiral 6,7-dihydro-5H-
dibenz[c,e]azepines via two-step intermolecular and intra-
molecular cascade reductive amination (RA) could be realized
(Scheme 1).
was carried out on a scale of 414 mg, affording chiral amine
(aR,5R)-2a in 98% yield and 93% ee. After a deprotection/
neutralization sequence with formic acid and NaHCO3, free
amine (aS,5R)-9 with an identical enantioselectivity could be
obtained. Notably, after N-deprotection, a resulting switch in
axial conformation occurs by the center-axis relay effect.8
In summary, we described the Ru-catalyzed enantioselective
hydrogenation of dibenzo[c,e]azepines for the first time. It was
found that the counteranion of the catalyst was critically
important for high-level chiral induction, and good to excellent
enantioselectivities (≤96% ee) were achieved. Moreover, a
one-pot methodology for syntheses of chiral 6,7-dihydro-5H-
dibenz[c,e]azepines via a two-step reductive amination was
developed. These new protocols thus provide easy ways
leading to chiral 6,7-dihydro-5H-dibenz[c,e]azepines, which are
attractive substructures in biological and synthetic chemistry.
We then chose 2′-acetyl[1,1′-biphenyl]-2-carbaldehyde 7 as
the model substrate with aniline for our DARA study. After a
survey of reaction conditions (Table S3), chiral tertiary amine
8a was obtained in 91% yield and 77% ee via two-step DARA
by using catalyst (R,R)-4b (5.0 mol %). This transformation
was also performed smoothly under otherwise identical
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b01859.
■
S
C
DOI: 10.1021/acs.orglett.9b01859
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Experimental procedures, synthesis of the starting
Letter
Fan, Q.-H. Org. Chem. Front. 2014, 1, 952. (c) Wang, T.; Zhuo, L.-G.;
Li, Z.; Chen, F.; Ding, Z.; He, Y.; Fan, Q.-H.; Xiang, J.; Yu, Z.-X.;
Chan, A. S. C. J. Am. Chem. Soc. 2011, 133, 9878. (d) Qin, J.; Chen,
F.; Ding, Z.; He, Y.-M.; Xu, L.; Fan, Q.-H. Org. Lett. 2011, 13, 6568.
(e) Zhang, J.; Chen, F.; He, Y.-M.; Fan, Q.-H. Angew. Chem., Int. Ed.
2015, 54, 4622. (f) Yang, Z.; Chen, F.; He, Y.; Yang, N.; Fan, Q.-H.
Angew. Chem., Int. Ed. 2016, 55, 13863.
materials, and compound characterization data (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: fanqh@iccas.ac.cn.
*E-mail: chenfei211@iccas.ac.cn.
(8) Pira, S. L.; Wallace, T. W.; Graham, J. P. Org. Lett. 2009, 11,
1663.
ORCID
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Brinkø, A.; Larsen, M. T.; Sinning, S.; Williams, D. C.; Jensen, H. H.
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Y. M.; Zheng, J.; Aslanian, R. G.; McCormick, K. D.; Zeng, Q. WO
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2011, 133, 18130. (b) Bulman Page, P. C.; Bartlett, J. C.; Chan, Y.;
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Qing-Hua Fan: 0000-0001-9675-2239
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the National Natural Science Foundation of
China (21672218, 21790332, and 21521002), the Youth
Innovation Promotion Association CAS (2017046), and CAS
(QYZDJSSW-SLH023) for financial support.
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DOI: 10.1021/acs.orglett.9b01859
Org. Lett. XXXX, XXX, XXX−XXX