Ruthenium-catalyzed asymmetric transfer hydrogenation of 1-aryl-substituted dihydroisoquinolines: Access to valuable chiral 1-aryl-tetrahydroisoquinoline scaffolds

Article abstract of DOI:10.1002/anie.201301134

Give me an H! Give me another H! The first general and highly enantioselective Ru-catalyzed transfer hydrogenation of a wide range of 1-aryl-substituted 1,2,3,4-dihydroisoquinolines is described. This atom-economic reaction proceeds under mild conditions,

Full text of DOI:10.1002/anie.201301134

Angewandte
Chemie
DOI: 10.1002/anie.201301134
Asymmetric Transfer Hydrogenation
Ruthenium-Catalyzed Asymmetric Transfer Hydrogenation of 1-Aryl-
Substituted Dihydroisoquinolines: Access to Valuable Chiral 1-Aryl-
Tetrahydroisoquinoline Scaffolds**
Zi Wu, Marc Perez, Michelangelo Scalone,* Tahar Ayad,* and Virginie Ratovelomanana-Vidal*
The frequent occurrence of chiral 1-substituted-1,2,3,4-tetra-
hydroisoquinoline (THIQ) ring systems in a large number of
alkaloids possessing a broad spectrum of biological and
pharmaceutical properties has led to significant increasing
interest in their synthesis.[1] To date, most of the traditional
synthetic approaches are based on procedures employing
chiral building blocks, auxiliaries, or reagents.[2] Thus, with
particular emphasis on economic and ecologically valuable
processes, much effort has been directed toward the develop-
ment of catalytic enantioselective transformations to access
enantiomerically pure 1-substituted-THIQ frameworks with
a high level of selectivity.[3] Among them, the asymmetric
hydrogenation[4] and asymmetric transfer hydrogenation
dium complexes have been recently reported by Zhang
et al.[7d] and our group,[7e] with enantioselectivities higher than
90%. However, these two catalytic systems provide only
moderate enantioselectivity for sterically hindered 1-(2’-
substituted-aryl)-3,4-DHIQs. Therefore, the development of
highly enantioselective methods that allow rapid and efficient
access to the valuable 1-aryl-tetrahydroisoquinoline scaffold
remains highly desirable. As part of our ongoing research
program toward the use of metal-catalyzed asymmetric
reduction for the synthesis of biologically relevant targets,[8]
and taking in account the scarce examples of ATH of aryl-
substituted-dihydroquinoline derivatives, we report herein
a general and highly enantioselective Ru-catalyzed transfer
hydrogenation of 1-aryl-substituted-1,2,3,4-DHIQs under
mild conditions leading to the corresponding THIQ deriva-
tives with a broad substrate scope and enantioselectivities of
up to 99%.
We first examined the ATH of 1-phenyl-3,4-dihydro-6,7-
dimethoxyisoquinoline 1a in dichloromethane at 308C for
16 h using an azeotropic 5:2 formic acid and triethylamine
mixture as the hydrogen source in the presence of platinum
group metal-based N-(p-toluenesulfonyl)-1,2-diphenylethyle-
nediamine (TsDPEN) complexes (1 mol%). The results
summarized in Table 1 clearly show that the stereochemical
outcome of the reaction is dramatically affected by the
structure of the complexes used. The reduction of 1a under
the aforementioned conditions using either Cp*RhIII-
TsDPEN (3a) or Cp*IrIII-TsDPEN (3b) as catalysts pro-
ceeded with full conversion, but afforded only racemic
product 2a (Table 1, entries 1 and 2). RuII-TsDPEN com-
plexes[9] 3c–g were also tested in this transformation.
Although complexes 3c–f exhibited very poor catalytic
activity, catalyst 3g, which bears benzene as its h6-arene,
gave encouraging results in terms of both selectivity and
reactivity, providing 1-phenyl-1,2,3,4-tetrahydroquinoline 2a
with an enantiomeric ratio of 87.5:12.5 and complete con-
version (Table 1, entries 3–6 vs. 7).
(ATH)[5]
of
1-substituted-3,4-dihydroisoquinolines
(DHIQs)[6] are powerful methods because they have an
intrinsic operational efficiency and are highly atom econom-
ical. However, despite the significant advances produced in
these areas over the last two decades, only relatively few
catalyst systems operating with high selectivity have been
reported so far in the literature for the reduction of 1-alkyl-
3,4-DHIQs.[6h,i,7d] Furthermore, and to the best of our knowl-
edge, the asymmetric reduction of 1-aryl-substituted-3,4-
DHIQs has only been sporadically described and still
continues to be a challenge in the field of asymmetric
hydrogenation.[7] To date, most of the existing catalytic
systems are restricted to 1-phenyl-3,4-DHIQ as a model
substrate and provide low to moderate catalytic efficiency. As
far as the ATH of 1-aryl-substituted-3,4-DHIQs is concerned,
only very few examples have been described. In 1999, Vedejs
et al.[7c] reported the RuII-catalyzed ATH of 1-aryl-substi-
tuted-3,4-DHIQ substrates. Although high enantioselectivity
was achieved (up to 98.7%), the method only tolerates
a narrow range of ortho substituents, such as o-Br, o-NO2, and
o-N(R)SO2Ar, with low to reasonable yields of 1–76%.
Asymmetric hydrogenation of 1-aryl-3,4-DHIQs using iri-
[*] Z. Wu, M. Perez, Dr. T. Ayad, Dr. V. Ratovelomanana-Vidal
Laboratoire Charles Friedel, ENSCP Chimie ParisTech
11, rue Pierre et Marie Curie, 75231 Paris Cedex 05 (France)
E-mail: tahar-ayad@chimie-paristech.fr
Further examinations focused on the solvent and temper-
ature effects. Although this asymmetric transfer hydrogena-
tion proceeded with full conversion in all solvents tested, the
best selectivity was obtained with iPrOH (Table 1, entry 14;
e.r. = 91:9). Further, a significant drop in enantioselectivity
was observed when the reaction was carried out at 508C,
whereas lowering the temperature from 308C to 08C led to
a dramatic decrease in reactivity, resulting in only 30%
conversion while maintaining good selectivity, providing 2a in
90.5:9.5 e.r. (Table 1, entries 15 and 16).
virginie-vidal@chimie-paristech.fr
Dr. M. Scalone
Process Research and Development, CoE Catalysis, F. Hoffmann-La
Roche AG, Bldg. 62/413, 4070 Basel (Switzerland)
E-mail: michelangelo.scalone@roche.com
[**] Z.W. and M.P. are grateful to the Ministꢀre de l’Education Nationale
et de la Recherche for financial support (2010–2013).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201301134.
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Table 1: Screening of PGM-based TsDPEN complexes.[a]
show that the enantioselectivity of the reaction gradually
increases when the bulkiness of the substituents at the ortho
position increases. These results may be a consequence of the
steric effects significantly breaking the coplanarity of the
[10]
=
1-phenyl ring with the prochiral C N double bond in the
transition state, thereby leading to high levels of enantiofacial
discrimination. The data in Table 2 also indicates that
substituents on the benzene ring of the isoquinoline moiety
have a marked effect on the reactivity, as well as on the
stereochemical outcome of the reaction (Table 2, entries 17–
26). More specifically, we found that the transfer hydro-
genation of imines 1q–t, which bear no substituent (R1 = H),
gave consistently lower enantioselectivity and conversion
(and consequently lower yield) than those obtained with the
corresponding 6,7-dimethoxy analogues 1j, 1l, 1n, and 1p,
even with an extended reaction time (Table 2, entries 10, 12,
14, and 16 vs. 17–20). Furthermore, when only one methoxy
group is present at the 5-, 6-, or 7-position of the benzene ring
of the dihydroisoquinoline core, the reaction outcome is less
affected when using substrates that bear electron-withdraw-
ing groups (Table 2, entries 14 and 15 vs. 21, 22, 25, and 26)
than substrates bearing electron-donating groups (Table 2,
entries 8 and 9 vs. 23 and 24), giving the desired products 2u–z
in high yields (86–90%) and with 97.5:2.5–98:2 e.r. and 96:4
e.r., respectively.
Entry
Catalyst
Solvent
Conv. [%][b]
e.r. [%][c]
1
2
3
4
5
6
7
8
3a
3b
3c
3d
3e
3 f
3g
3g
3g
3g
3g
3g
3g
3g
3g
3g
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
toluene
THF
100
100
36
13
9
0
0
0
n.d.
n.d.
n.d.
87.5:12.5
88.5:11.5
85:15
82.5:17.5
74:26
72.5:27.5
83:17
91:9
15
100
100
100
100
100
100
100
100
100
30
9
10
11
12
13
14
15[d]
16[e]
dioxane
DMF
CH3CN
MeOH
iPrOH
iPrOH
iPrOH
86:14
90.5:9.5
[a] Reactions were conducted at 308C using substrate 1 (1 mmol) and
catalyst (1 mol%) for 16 h. [b] Determined by H NMR analysis of the
1
crude product. [c] Determined by chiral stationary phase supercritical
fluid chromatography (CSP-SFC). The absolute configuration of the
product was assigned by comparison with literature data. [d] Reaction
performed at 508C. [e] Reaction performed at 08C. n.d.=not deter-
mined.
Finally, to showcase the practicality of the process, a gram-
scale synthesis of compound I, a potent noncompetitive
AMPA receptor antagonist currently being investigated in
phase III trials as an antiepileptic agent,[1d,g] was performed
(Scheme 1). Under the optimized reaction conditions, DHIQ
A series of 1-aryl-dihydroisoquinolines 1a–z was sub-
jected to ATH under the optimized reaction conditions
(Table 2). The reaction proceeded well in all cases, giving the
corresponding THIQ derivatives 2a–z in good to high yields
(72–97%) and good to excellent enantiomeric ratios
(90.5:9.5–99.5:0.5 e.r.). The stereochemical outcome of ATH
appears to be sensitive to both the nature and the position of
the substituents on the aromatic rings. Methyl, bromo, and
chloro substituents at the meta position of the 1-phenyl group
of 6,7-dimethoxy substituted isoquinolines 1b–d gave similar
enantioselectivities as those obtained with compound 1a, but
products 2b–d were isolated in significantly better yields
(Table 2, entries 1–4). Moving substituents to the para
position caused a marked increase in catalytic activity,
giving 2e–g in excellent yields (91–94%) and high enantio-
meric ratios of 93.5:6.5–95.5:4.5 (Table 2, entries 5–7). Even
more impressive results were obtained with 6,7-dimethoxy
substituted isoquinoline substrates 1h–p, having either elec-
tron-donating or electron-withdrawing groups at the ortho
position of the 1-phenyl group. These gave the desired
hydrogenated products 2h–p in high to excellent yields (83–
97%) and with consistently excellent enantioselectivities
(Table 2, entries 8–16, 98:2–99.5:0.5 e.r.). These results clearly
Scheme 1. Synthesis of AMPA receptor antagonist (R)-I.
1g was reduced to give 2g in 93% yield with a 93.5:6.5 e.r.
Subsequent acetylation, followed by a single recrystallization
furnished the desired compound I, which was isolated with up
to 99:1 e.r and in 80% yield.
In conclusion, we report a highly enantioselective Ru-
catalyzed transfer hydrogenation with a broad substrate scope
of readily available 1-aryl-substituted 3,4-dihydroisoquinoline
derivatives. This method offers several advantages, including
mild reaction conditions, low catalyst loading, and operational
simplicity, which makes it a useful and attractive method for
the synthesis of the valuable 1-aryl-tetrahydroisoquinoline
scaffold with excellent enantioselectivities (up to 99.5:0.5 e.r.)
and in high yields of isolated products. Moreover, the
synthetic utility of this method for the preparation of the
potent noncompetitive AMPA receptor antagonist I was also
demonstrated on a gram scale. Application to the synthesis of
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Table 2: ATH of 1-aryl substituted isoquinoline derivatives.[a]
Entry
Substrate
Yield
[%][b]
e.r.
Entry
Substrate
Yield
[%][b]
e.r.
[%][c]
[%][c]
1
Ar
R1
1
Ar
R1
1
2
3
1a
6,7-di-MeO
82
90
90
91:9 (+)
92:8 (+)
91:9 (+)
14
15
16
1n
6,7-di-MeO
93
91
92
99:1 (À)
1b
1c
6,7-di-MeO
6,7-di-MeO
1o
1p
6,7-diMeO
6,7-diMeO
99:1 (À)
99.5:0.5 (À)
4
1d
6,7-di-MeO
93
91.5:8.5 (+)
17[d]
1q
H
75
95:5 (+)
5
6
7
8
9
1e
1 f
1g
1h
1i
6,7-di-MeO
6,7-di-MeO
6,7-di-MeO
6,7-di-MeO
6,7-di-MeO
92
91
94
86
84
95.5:4.5 (+)
93.5:6.5 (À)
93.5:6.5 (À)
99:1 (+)
18[d]
19[d]
20[d]
21
1r
1s
1t
H
74
73
72
87
90
95:5 (À)
96:4 (À)
96:4 (À)
97.5:2.5 (À)
98:2 (À)
H
H
1u
1v
5-MeO
6-MeO
98:2 (+)
22
10
1j
6,7-di-MeO
83
99:1 (À)
23
1w
7-MeO
90
96:4 (+)
11
12
13
1k
1l
6,7-di-MeO
6,7-di-MeO
6,7-di-MeO
85
86
97
98:2 (+)
99:1 (+)
98:2 (+)
24
25
26
1x
1y
1z
7-MeO
7-MeO
7-MeO
86
87
89
96:4 (+)
98:2 (À)
1m
97.5:2.5 (À)
[a] Reactions were conducted at 308C using substrate 1 (1 mmol) and catalyst (1 mol%) in iPrOH for 16 h. [b] Yield of isolated product.
[c] Determined by chiral stationary phase supercritical fluid chromatography (CSP-SFC). The absolute configuration of the product is assigned by
comparison of the rotation sign with literature data. [d] 80% conversion was obtained after a reaction time of 40 h.
other pharmaceutically relevant targets is currently underway
in our laboratory and will be reported in due course.
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Received: February 7, 2013
Published online: March 26, 2013
De Sarro, G. Ferreri, S. Quartarone, E. Russo, A. Constanti, A.
Keywords: asymmetric catalysis · enantioselectivity ·
hydrogen transfer · isoquinolines · ruthenium
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