Highly Diastereo- And Enantioselective Ir-Catalyzed Hydrogenation of 2,3-Disubstituted Quinolines with Structurally Fine-Tuned Phosphine-Phosphoramidite Ligands

Article abstract of DOI:10.1021/acs.orglett.9b03925

A highly diastereo- and enantioselective Ir-catalyzed hydrogenation of unfunctionalized 2,3-disubstituted quinolines, especially 3-alkyl-2-arylquinolines, has been realized. The success of this hydrogenation is ascribed to the use of a structurally fine-tuned chiral phosphine-phosphoramidite ligand with a (S_a)-3,3′-dimethyl H₈-naphthyl moiety and (R_c)-1-phenylethylamine backbone. The hydrogenation displayed broad functional group tolerance, thus furnishing a wide range of optically active 2,3-disubstituted tetrahydroquinolines in up to 96% ee and with perfect cis-diastereoselectivity.

Full text of DOI:10.1021/acs.orglett.9b03925

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Highly Diastereo- and Enantioselective Ir-Catalyzed Hydrogenation
of 2,3-Disubstituted Quinolines with Structurally Fine-Tuned
Phosphine−Phosphoramidite Ligands
†
,‡
,†
Xin-Hu Hu and Xiang-Ping Hu*
†
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
University of Chinese Academy of Sciences, Beijing 100049, China
‡
*
S Supporting Information
ABSTRACT: A highly diastereo- and enantioselective Ir-catalyzed hydrogenation of unfunctionalized 2,3-disubstituted
quinolines, especially 3-alkyl-2-arylquinolines, has been realized. The success of this hydrogenation is ascribed to the use of a
structurally fine-tuned chiral phosphine−phosphoramidite ligand with a (S )-3,3′-dimethyl H -naphthyl moiety and (R )-1-
a
8
c
phenylethylamine backbone. The hydrogenation displayed broad functional group tolerance, thus furnishing a wide range of
optically active 2,3-disubstituted tetrahydroquinolines in up to 96% ee and with perfect cis-diastereoselectivity.
ptically active tetrahydroquinoline derivatives are
important molecular scaffolds for many natural products,
3-alkyl-2-arylquinolines in terms of enantioselectivity. In 2015,
Du reported a metal-free hydrogenation of 3-alkyl-2-arylquino-
lines using 5 mol % of chiral diene derived borane catalyst,
O
as well as useful building blocks for the synthesis of
1
8b
pharmaceuticals and bioactive compounds. Due to its
providing the hydrogenation products in up to 80% ee.
inherent efficiency and atom economy, catalytic asymmetric
hydrogenation of quinolines should be one of the most direct
and convenient approaches to chiral tetrahydroquinoline
derivatives. Indeed, in the past decade, many successful
examples have been reported based on various transition-
Therefore, the development of a new and efficient catalytic
system for highly diastereo- and enantioselective hydro-
genation of unfunctionalized 2,3-disubstituted quinolines,
especially 3-alkyl-2-arylquinolines, remains a highly desirable
and challenging task.
2
−8
metal catalytic systems and organocatalysts.
In comparison
In the past decades, unsymmetrical hybrid chiral phos-
phine−phosphoramidite ligands have emerged as a novel and
promising ligand class which exhibited unusual reactivities and
stereoselectivities toward many challenging asymmetric cata-
with much success in the hydrogenation of monosubstituted
quinolines, asymmetric catalytic hydrogenation of 2,3-disub-
stituted quinolines represents a more difficult task due to the
requisite diastereocontrol in the concurrent construction of
two vicinal stereogenic centers. Zhou and some other groups
have realized the catalytic asymmetric hydrogenation of various
functionalized 2,3-disubstituted quinolines in high diastereo-
9
lytic reactions (Figure 1). In particular, we have recently
demonstrated the high efficiency of chiral 1-phenylethylamine-
derived phosphine−phosphoramidite ligands in the Ir-
catalyzed enantioselective hydrogenation of sterically hindered
5
a
5b,c
5d
and enantioselectivity, with a NO , phthaloyl,
CF3,
10
2
5
e
5f,g
5h
N-arylimines. We therefore envisioned that this kind of
NHTs, ester, or SCF3 functionality at the 3-position and
an aryl or alkyl group at the 2-position of quinoline. However,
the catalytic asymmetric hydrogenation of unfunctionalized
ligand may be efficient for the Ir-catalyzed asymmetric
hydrogenation of 3-alkyl-2-arylquinolines. As a result, herein
we describe a highly diastereo- and enantioselective Ir-
catalyzed hydrogenation of 3-alkyl-2-arylquinolines by employ-
ing a structurally fine-tuned chiral phosphine-phosphoramidite
2
,3-disubstituted quinolines remains a huge challenge. In 2009,
Zhou et al. examined the [Ir(COD)Cl] /MeO-BIPHEP/I
2
2
system for the hydrogenation of 2,3-dialkylquinoline with up
6
ligand with a 3,3′-dimethyl H -binaphthyl moiety derived from
to 86% ee and high diastereoselectivity. Fan and Yu reported a
8
Ru-catalyzed asymmetric hydrogenation of 2,3-dialkylquino-
lines in up to 98% ee but with low diastereoselectivity.
7
Received: November 4, 2019
However, both of the above catalytic systems were inefficient
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b03925
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
phosphoramidite ligand (R ,R )-L1 for this Ir-catalyzed
c
a
asymmetric hydrogenation because of its demonstrated
reactivity and enantioselectivity in various catalytic asymmetric
10,11
hydrogenations.
Unfortunately, the hydrogenation led to
very low enantioselectivity, although full conversion was
achieved (entry 1). Interestingly, a substantial increase in the
enantioselectivity to 52% ee was observed by replacing (R ,R )-
c
a
L1 with its diastereomer (R ,S )-L2a (entry 2), which proved
c
a
to be unmatched in the Ir-catalyzed hydrogenation of N-
10
arylimines. It seems that the central chirality of ligand
determined the absolute configuration of the product, as
(
R ,R )-L1 and (R ,S )-L2a gave the product with same
c a c a
configuration (entries 1 and 2). I additive was unnecessary
2
to this hydrogenation, as also observed by Zhou et al. in the Ir-
1
2
catalyzed hydrogenation of some N-heterocycles. An attempt
to improve the hydrogenation performance by screening the
different solvents proved to be unsuccessful, and only
moderate enantioselectivity was obtained in all cases (entries
Figure 1. Unsymmetrical hybrid chiral phosphine−phosphoramidite
ligands for catalytic asymmetric hydrogenation.
2
−7). Among various solvents, 1,4-dioxane was the best choice
in terms of reactivity and enantioselectivity (entry 7). A further
optimization of ligand structure with (R ,S )- or (S ,R )-
c
a
c
a
chiral 1-phenylethylamine, in which a wide range of optically
active 2,3-disubstituted tetrahydroquinolines could be obtained
in high yields and with up to 96% ee and perfect cis-
diastereoselectivity. Interestingly, the matched configuration in
this ligand was found to be R-central and S-axial chiralities,
different with a (R,R)-configuration observed in the Ir-
catalyzed asymmetric hydrogenation of N-arylimines.
absolute configuration was then performed in order to enhance
the enantioselective induction of ligand. After extensive studies
on the ligand modification, we disclosed that either the
introduction of the substituent into 3,3′-positions of
binaphthyl moiety or replacing it with the H -binaphthyl
8
framework is favorable to improve the enantioselectivity
(
entries 8−10). In particular, a fine-tuned ligand (R ,S )-L3b
c
a
We started our investigation with 3-methyl-2-phenylquino-
line 1a as a model substrate, and the hydrogenation was
bearing a 3,3′-dimethyl H -binaphthyl backbone led to 3-
8
methyl-2-phenyl-1,2,3,4-tetrahedroquinoline 2a in full con-
version, perfect cis-selectivity, and with 85% ee (entry 11).
When the hydrogenation was performed at room temperature,
the enantioselectivity could be significantly improved to 93%
ee with full conversion maintained (entry 13). The efficiency
of the present catalytic system was further demonstrated by
performing the hydrogenation at the catalyst loading as low as
performed at 1 mol % of catalyst loading under a H pressure
2
of 10 bar (Table 1). Initially, we examined chiral phosphine−
a
Table 1. Optimization of Reaction Conditions
0
.2 mol %, in which full conversion and 92% ee was achieved
(
entry 14). Furthermore, the hydrogenation performed at
lower substrate concentration increased the enantioselectivity
to 94% ee (entry 15).
b
c
entry
L*
solvent
DCE
DCE
DCE
DCM
toluene
benzene
THF
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
T (°C)
conv (%)
ee (%)
With the optimal [Ir(COD)]Cl] /(R ,S )-L3b catalytic
2
c
a
1
2
3
L1
80
80
80
80
80
80
80
80
80
80
80
80
80
25
25
25
>95
>95
>95
>95
90
6 (2S,3R)
52 (2S,3R)
30 (2S,3R)
49 (2S,3R)
50 (2S,3R)
52 (2S,3R)
35 (2S,3R)
58 (2S,3R)
78 (2S,3R)
79 (2S,3R)
63 (2R,3S)
85 (2S,3R)
77 (2S,3R)
93 (2S,3R)
92 (2S,3R)
94 (2S,3R)
system, we then examined the scope of 2,3-disubstituted
quinolines, and the results are summarized in Table 2. The
hydrogenation proceeded in cis-diastereoselectivity and gave
the hydrogenation products in >20:1 dr in all cases. Initially,
various 2-aryl-3-methylquinolines were submitted to hydro-
genation, and good to excellent performance was achieved in
all cases. The results indicated that the substitution pattern on
the phenyl ring showed some influence on the enantioselec-
tivity.
Thus, ortho-substituted 1b led to lower enantioselectivity in
comparison with its meta- and para-analogues (1c and 1d).
The electronic property of the para-substituent on the phenyl
ring hardly affected the hydrogenation outcome, and all
substrates exhibited high yields and excellent enantioselectiv-
ities. 2-Naphthyl substrate 1i also worked well, leading to the
hydrogenation product 2i in 95% yield and with 93% ee.
Heteroaromatic substituents were not well tolerated. By
increasing the catalyst loadings to 5 mol %, 1j and 1k could
be completely hydrogenated in 89% ee and 95% ee,
respectively. Different alkyl substituents at the 3-position of
quinolines were next examined. Both ethyl and propyl groups
were well tolerated, and good performance was achieved for
L2a
L2a
L2a
L2a
L2a
L2a
L2a
L2b
L2c
L3a
L3b
L3c
L3b
L3b
L3b
d
4
5
6
7
8
9
87
>95
>95
>95
>95
>95
>95
>95
>95
>95
>95
1
1
1
1
1
1
1
0
1
2
3
4
5
6
e
f
a
Reaction conditions: 1a (0.4 mmol), [Ir(COD)Cl] (0.5 mol %), L*
1.2 mol %), solvent (2 mL), 24 h, unless otherwise noted.
Conversion and dr were determined by H NMR spectroscopy. In
all cases, dr >20:1 was obtained. Determined by HPLC analysis by
2
(
b
1
c
d
e
using a chiral stationary phase. I (5 mol %) was added. S/C = 500.
2
f
The use of 4 mL of 1,4-dioxane.
B
DOI: 10.1021/acs.orglett.9b03925
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Table 2. Ir-Catalyzed Asymmetric Hydrogenation of 2-
Alkyl-3-arylquinolines
97% yield and with 83% ee. The substituent on the phenyl
moiety of quinolines displayed some effect on the reactivity
and enantioselectivity. Thus, 5-Me-substituted substrate 1o led
to incomplete hydrogenation even at an elevated catalyst
loading although excellent enantioselectivity was maintained.
In comparison, the substrates 1p−r bearing a methyl group at
the 6-, 7-, or 8-position gave high yields and excellent
enantioselectivities. The electronic property of the substituent
at 6-position was somewhat sensitive to the hydrogenation. 6-
Methoxy-substituted substrate 1s led to 88% ee, obviously
lower than its F, Cl, or Br analogues 1t−v. The absolute
configuration of the hydrogenation product is unambiguously
confirmed by X-ray structure analysis of 2w, to which a
a
(
2S,3R)-configuration was assigned. The aryl group at 2-
position seems to be critical to this hydrogenation, as all 2-alkyl
substrates led to low to moderate enantioselectivities (2y,z).
Unfortunately, the present catalytic system did not tolerate
2
,3,4-trisubstituted substrate (2x−3).
In conclusion, we have reported a highly diastereo- and
enantioselective Ir-catalyzed hydrogenation of 3-alkyl-2-
arylquinolines. The hydrogenation features broad substrate
scope, no additive, and mild condition, thus generating a
variety of optically active cis-3-alkyl-2-aryl-1,2,3,4-tetrahedron-
quinolines in high yield and enantioselectivity even at low
catalyst loadings of 0.2 mol %. A structurally fine-tuned chiral
phosphine−phosphoramidite ligand with an (S )-3,3′-dimethyl
a
H -binaphthyl moiety and an (R )-1-phenylethylamine back-
8
c
bone should be critical to the success of this hydrogenation.
The observation of the (R ,S )-diastereomer of ligand as the
c
a
matched configuration is opposed to that in the Ir-catalyzed
asymmetric hydrogenation of N-arylimines with an (R ,R )-
c
a
matched configuration. Further development and application
of this hydrogenation and further modification and application
of chiral phosphine−phosphoramidite ligands are still under-
way.
ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.9b03925.
Experimental details and characterization data (PDF)
Accession Codes
CCDC 1954871 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
■
*
E-mail: xiangping@dicp.ac.cn.
a
Reaction conditions: 1 (0.4 mmol), [Ir(COD)Cl] (0.5 mol %), L3b
2
ORCID
(
1.2 mol %), 1,4-dioxane (4 mL), 10 bar of H , 25 °C, 24 h, unless
2
otherwise noted. In all cases, dr >20:1. Isolated yield was provided,
Xiang-Ping Hu: 0000-0002-8214-1338
Notes
and ee was determined by HPLC analysis by using a chiral stationary
phase. [Ir(COD)Cl] (2.5 mol %), L3b (6 mol %). 1,4-Dioxane/
toluene (1 mL/3 mL), 0 °C. I (5 mol %) was added.
b
c
2
d
The authors declare no competing financial interest.
2
ACKNOWLEDGMENTS
Financial support from the National Natural Science
Foundation of China (21572226 and 21772196), Dalian
■
both substrates 1l and 1m. Cyclic substrate 1n was also
suitable for the hydrogenation, giving octahydroacridine 2n in
C
DOI: 10.1021/acs.orglett.9b03925
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Science and Technology Innovation Project (2018J12GX054),
and Dalian Institute of Chemical Physics (DICP 1201901) is
gratefully acknowledged.
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DOI: 10.1021/acs.orglett.9b03925
Org. Lett. XXXX, XXX, XXX−XXX