Strong Br?nsted acid promoted asymmetric hydrogenation of isoquinolines and quinolines catalyzed by a Rh-thiourea chiral phosphine complex: Via anion binding

Article abstract of DOI:10.1039/c5sc04712a

Rhodium catalyzed asymmetric hydrogenation of both isoquinolines and quinolines provides a new method to synthesize chiral tetrahydroisoquinolines and tetrahydroquinolines. By introducing strong Br?nsted acid HCl, anion binding between the substrate and t

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Catalyzed by a Rh/bisphosphine-thiourea(L1) complex, isoquinolines and quinolines are
hydrogenated with high conversions and high enantioselectivities.

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Strong Brønsted acid promoted asymmetric hydrogenation of
isoquinolines and quinolines catalyzed by a Rh-thiourea chiral
phosphine complex via anion binding†
Received 00th January 20xx,
Accepted 00th January 20xx
Jialin Wen a, Renchang Tan a, Shaodong Liu a, Qingyang Zhao *b,and Xumu Zhang*a
DOI: 10.1039/x0xx00000x
Rhodium catalyzed asymmetric hydrogenation of both isoquinolines and quinolines provides a new method to synthesize
chiral tetrahydroisoquinolines and tetrahydroquinolines. By introducing strong Brønsted acid HCl, anion binding between
the substrate and the ligand was established to achieve high reactivity and high enantioselectivity (up to 99% conversion
and 99% ee). NMR study suggests an anion binding between the catalyst and the substrate. Deuterium labeling
www.rsc.org/
experiments
reveal
a
plausible
reaction
pathway.
N-heteroaromatics, especially isoquinolines, remains
a
Introduction
challenging task. 7 Although quinolines have been successfully
hydrogenated in several cases, 8 there were only few examples
of isoquinolines. Zhou’s group used iridium-bisphosphine
catalyst to obtain high enantioselectivity, but this
transformation needs activation by chloroformate or addition
As one of the important non-covalent interactions in nature,
especially in biological system, anion binding has been
1
demonstrated as
a powerful tool in organocatalysis.
Triggered by the pioneering work done by Jacobsen and other
groups, many successful asymmetric catalytic systems
involving anion binding have been developed in recent years. 2
On the other hand, utilizing the secondary interaction has
been one of the fundamental ideas in catalyst design, which
combines more catalytic centers (transition metal catalysis,
organocatalysis and enzyme catalysis) in single molecules to
provide an efficient chiral environment. Inspired by this
strategy, we recently developed a novel ferrocene-thiourea
chiral bisphosphine ligand (L1). Through a covalent linker,
cooperation of transition metal catalysis and organocatalysis
could be achieved. This strategy has been successfully applied
9
of BCDMH. By introducing chiral phosphoric acid, transfer
hydrogenation of isoquinoline was achieved by Zhou in 2014
and N-protected 1,2-dihydroisoquinolines were synthesized
with moderate enantioselectivities.10 In 2013, Mashima’s
group synthesized chiral THIQ using a dinuclear iridium(III)-
bisphosphine complex, but the substrate scope is still limited
to aromatic or bulky substituents (schemie 1). 11 Isoquinolines
with less hindered alkyl substituents on the prochiral carbons
are still challenging. Most of these catalysis cases for N-
heteroaromatics are performed with ruthenium, iridium and
palladium complexes. 6 Rhodium was rarely reported in direct
Zhou, 2006 and 2012
Cl
in rhodium-catalyzed asymmetric hydrogenation of
3,4,5
ClCO2Bn
Ir Cat.
N
N
OBn
N
OBn
nitroalkenes and unprotected iminiums.
Control
H2
R
R
O
experiments implied that the covalent linker was essential for
high activities and enantioselectivities. 4,5
R
O
COOR
COOR
NH
R'
Ir Cat., H2
R'
Both reauire activating
reagents
Tetrahydroquinolines (THQs) and tetrahydroisoquinolines
(THIQs) are an important family of biologically active
N
I2 or BCDMH
Mashima, 2013
molecules, including natural alkaloids and important
6
pharmaceutical products.
Among various synthetic
R'
R'
*
[{Ir(H)(L)}2)(μ-X)3]X
works with high ee
for aromatic or bulky substituents
approaches to synthesizing chiral THQs and THIQs, direct
hydrogenation, with high atom economy, relatively simple
procedure and easy work-up, deserves special attention. To
our best knowledge, however, asymmetric hydrogenation of
NH
NHCl
*
R
H2
R
This work
R
R2
R2
*
CF3
R
NH
NHCl
*
R1
S
R1
Rh/L1
up to 99% ee
F3C
N
H
N
H
H2
Ph2P Fe
R
R
(S,R)-L1
*
N
Ph2P
N
HCl
R1
R1
H
up to 99% ee
Scheme 1. Asymmetric hydrogenation of isoquinolines
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asymmetric hydrogenation of N-heteroaromatics with high small changes in conversion and enantioselectivity (table 1,
turnovers and ee’s. 7 This is in sharp contrast to the broad entry 6 vs entry 7).
application of Rh complex in asymmetric hydrogenation of
olefins, enamides and imines. 12
Counterion effects were also investigated. Bromide or
iodide anion do not bring significant influence on the
As continuing research on the ferrocene-based thiourea conversion or enantioselectivity, while fluoride anion
chiral phosphine system, we herein report a highly reactive dramatically inhibits this reaction (For more details, see
and enantioselective example of rhodium/bisphosphine- supporting information, Section 6).
catalyzed asymmetric hydrogenation of isoquinolines and Table 2, Substrate scope for isoquinolines. a
quinolines. In this catalysis system, a secondary interaction
between the catalyst and the substrates was believed to occur
via anion binding of ion-pair intermediates. We envision that
the introduction of a strong Brønsted acid such as HCl, brings
13
dual benefits: (1) activating the aromatic ring,
(2)
establishing a salt bridge between the catalyst and the
substrates.
Result and discussion
Table 1, Optimization of condition. a
Entry
Metal
Solvent
conversionb
eec
a
reaction condition:
1 (0.2 mmol) in 1.2 ml solvent, 1/[Rh(COD)Cl]2/L1 ratio =
100/0.5/1; yield was determined with isolated THIQ products; ee was determined by
GC or HPLC with a chiral stationary phase; eperformed in Dioxane/iPrOH = 2:1(v/v) at
60 oC under 60 atm H2 with 2% catalyst.
1
2
3
4
5
6d
7d
8
a
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)I]2
[Ir(COD)Cl]2
i-PrOH
>99%
85%
70%
30%
92%
90%
90%
99%
93%
83%
MeOH
Then we applied this method to synthesize various chiral
THIQs. Different alkyl substituents at 3-position or benzo ring
do not show obvious changes in yields and enantioselectivities.
Aryl substituent, such as phenyl group, on 3- position leads to
full conversion and moderate enantioselectivity (76% ee). In
addition to 3-alkyl isoquinolines, 1-methylisouinoline was also
hydrogenated with high enantioselectivity (Table 2).
DCM
>99%
95%
Dioxane
Dioxane/i-PrOH=2:1
DCM/i-PrOH=2:1
DCM/i-PrOH=2:1
Dioxane/i-PrOH=2:1
>99%
>99%
99%
Table 3, Substrate scope for quinolines. a
>99%
Reaction condition: 1a (0.1 mmol) in 1.0 ml solvent, 1/[Rh(COD)Cl]2/L1
ratio=100/0.5/1, 40 atm H2, 40 oC, 24 h; bconversion was determined by 1H NMR
analysis, no by product was observed; cee was determined by GC with
stationary phase; dperformed at 25 oC.
a chiral
Our study was initiated with 3-methylisoquinoline
hydrochloride as a model substrate. The Rh catalyst was
prepared in situ by mixing the metal precursor [Rh(COD)Cl]2
4,5
with (S,R)-L1. In accordance with our previous reports, this
catalysis system is solvent-dependent. After screening of alkyl
halide, alcohols and ethers, we found that under 40 atm H2
pressure at 40 oC, dichloromethane gives the best result (>99%
conversion and 92% ee were observed with 1% catalyst
loading). In addition to using single solvents, solvent pairs were
also tested. The mixture of dichloromethane and isopropanol
(2:1, v/v) at 25 oC gives the optimal condition with 99% ee and
full conversion. Iridium complex [Ir(COD)Cl]2/L1 also shows
high activity with, but lower enantioselectivity than its Rh
counterpart (table 1, entry 5 vs entry 8). Halide effect, which
causes considerable difference in some cases,14 was evaluated
as well. Using [Rh(COD)I]2 as a metal precursor resulted in
a
reaction condition: 3 (0.2 mmol) in 1.2 ml solvent, 3/[Rh(COD)Cl]2/L1 ratio =
100/0.5/1; yield was determined with isolated THQ products; ee was determined by GC
or HPLC with a chiral stationary phase.
2 | J. Name., 2012, 00, 1-3
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As far as we know, few catalysis systems can be applied in triphenylphosphine can hardly catalyzed the hydrogenation
asymmetric hydrogenation of both isoquinolines and reaction. On the other hand, the mixture of thiourea molecule
quinolines with both high turn-overs and excellent and bisphosphine-Ugi’s amine L8 failed to show catalytic
enantioselectivities. 7, 8 Then a question merged to us: can this activity. These results
(L1 vs L7/PPh3 and L8/thiourea)
synthetic protocol be used to synthesize chiral THQs with high demonstrate the importance of a covalent incorporation of
ee? By employing the optimized condition (DCM/iPrOH=2:1, bisphosphine moiety and thiourea. The idea of secondary
v/v) in the case asymmetric hydrogenation of isoquinolines, offers an alternative strategy for asymmetric hydrogenation.
we found that the enantioselectivity was dramatically Table 4. Ligand evaluation and control experiment.
increased to 99% ee5. This observation, in return, suggests that
this thiourea-Rh-bisphosphine catalysis system is highly
solvent-dependent. Substrate scope of asymmetric
hydrogenation of quinolines is broad. High enantioselectivities
have been obtained with various substituents (Table 3).
The potential application of this synthetic protocol was
estimated: we have scaled up this asymmetric hydrogenation
reaction into gram scale. No significant sign of changes in
conversion or enantioselectivity was observed with 0.5%
catalyst loading. 15 In addition, the role of strong Brønsted acid
HCl is essential in this chemical transformation. Without HCl,
no hydrogenation product was observed. 16
L1+1a
L1
a
reaction condition:
1 (0.1 mmol) in 0.6 ml solvent, 1/[Rh(COD)Cl]2/ligand ratio =
100/0.5/1; conversion was determined by 1H NMR analysis; ee was determined by GC
with a chiral stationary phase.
In order to obtain insight of this transformation, deuterium
Figure 1. NMR study for the interaction of L1 and 1a.
labeling
experiments
was
conducted.
First,
3-
To gain evidence of a secondary interaction of L1 with
chloride anion of substrates, we mixed L1 and 1a (3 eq.) in
CDCl3. The 1HNMR showed similar obvious changes of chemical
shifts (Figure 1). The original thiourea N-H peaks (hidden in the
aromatic peaks within 7.3~7.0 ppm) shift downfield to 9.10
and 7.62 (marked with red circles in Figure 1). In addition, the
three protons on the 3,5-bis(trifluoromethyl)phenyl group also
shift downfield a little by 0.19 ppm (marked by arrow in Figure
1). These changes of chemical shifts, consistent with the case
methylisoquinoline hydrochloride was reduced by deuterium
gas in methanol (Scheme 2, Eq. 1). D atoms are added only at
1- and 3- position respectively. This indicates that the
hydrogen atom at 4- position of the product comes from the
methanol
solvent.
Second,
3-methylisoquinoline
deuterochloride was reduced with hydrogen gas in deuterated
methanol (Scheme 2, Eq. 2). Hydrogen atoms are added at 1-
and 3- position. The original H at 4- position, however, was
exchanged with D, resulting two D atoms at 4- position. This
indicates that D atoms at 4- position come from the Methanol-
d4 solvent. A tautomerization is probably responsible for this
H-D exchange. Interestingly, H-D exchange also occurs on the
methyl group (53% original H was replaced by D). This
observation, in return, proves the existence of
tautomerization. Based on this results, we proposed a possible
path in this transformation (Scheme 2). After addition of a
hydrogen at 1- position, the substrate is partially reduced,
leading to conjugate enamine, which could not be further
reduced in this rhodium-thiourea-bisphosphine catalytic
system. After a tautomerization follows, giving an iminiums
intermediate, which could be easily hydrogenated. 5 Another
tautomer, a terminal enamine is less preferred than its
conjugate counterpart, because the latter is more stable. This
explains why H atoms on methyl group was partially replaced
5
of L1 with Cl- from TBAC, suggest the anion binding between
the ligand and the substrate.
We synthesized a series of analogues of L1 and conducted
control experiments to evaluate the collaboration manner of
each unit of L1 (Table 4). Urea bisphosphine ligand L2 only
gives trace product, and this sharp contrast suggests the
crucial role of thiourea moiety in L1. Compared to H (L3) and
methyl (L4), more electron-withdrawing trifluoromethyl group
at 3- and 5- position on the phenyl ring increases the
enantioselectivity, which is probably due to the stronger
acidity of N-H proton on the thiourea. After N-methylation of
the less acidic thiourea N-H proton, enantioselectivity results
in a minor decrease. This observation reveals that the more
acidic thiourea N-H proton contributes mostly in anion binding
with chloride. Furthermore, monophosphine ligand L6 and the
mixture of ferrocene-thiourea compound L7 with
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by D (but H at 4- position is almost replaced by D). (For details, hydrogenation of (iso)quinolinium salts, further study is
please see supporting information.)
needed in the future.
95% H
H
H
H
CH3
CH3
D
Rh-L1(2%), D2, CH3OH
(1)
71% D
NH
NH
Cl
then basic work-up
D
H
H
H
99% H
76% D
95% D
57% D
D
D
CH3
Cl
CH(D)3
H
Rh-L1(2%), H2, CD3OD
99% H
(2)
ND
NH
then basic work-up
H
H
H
99% H
H
H
H
H
H
H
H
H
H
H
H
CH3
CH3
CH3
[Rh-H]
NH
NH
NH
NH
H
H
H
H
conjugate enamine
more stable
Reactive iminium
terminal enamine
less stable
[Rh-H]
then basic work-up
H
H
CH3
H
NH
H
H
Scheme 2. Deuterium labeling experiment and proposed
transformation path.
Recent reports revealed an outer-sphere mechanism for
bisphosphine-transition metal catalyzed hydrogenation. In
2014, Zhou reported a series of detailed studies on palladium
catalyzed asymmetric hydrogenation of protonated indoles.
After careful evaluation, hydride transfer from a Pd-H complex
to an iminium intermediate was proposed. (Scheme 3, Eq. 1)17
In 2015, Mashima’s group proposed a mechanism on iridium-
catalyzed asymmetric hydrogenation of isoquinoline hydrogen
halide salts. Strong experimental evidence was presented to
favor an outer-sphere mechanism, in which hydrogen bonding
Scheme 3. Outer-sphere mechanism in asymmetric
hydrogenation of N-heteroaromatics.
Conclusions
In summary, we report a successful example of Rh-catalyzed
asymmetric hydrogenation of both isoquinolines and
quinolines. Compared with previous catalysis systems, this
method has broader substrates scope. The strong Brønsted
acid HCl activates the N-heteroaromatic substrates and
introduces anion binding into the catalysis system, which
played an important role in this transformation. Deuterium
labeling experiments revealed an enamine-iminium
tautomerization equilibrium after the first hydride transfer
step.
between Ir-Cl complex and N-H proton was proposed. (Scheme
18
3, Eq. 2)
Both of these mechanism studies above were
focused on hydrogenation of the halide salts of aromatic
compounds. Similarly, the nature of protonated (iso)quinolines
suggests that it seems not possible for the substrates to
coordinate to the rhodium complex in this case, no matter in a
σ-bonding or
a π-bonding manner. A hydride transfer
mechanism is more possible in this catalytic case, instead of
the traditional inner-sphere mechanism that involves the
direct coordination of unsaturated bonds to the metal. Based
on previous reports and our deuterium labeling experiments,
we propose a catalytic cycle to explain a plausible outer-
sphere mechanism (scheme 3). After oxidative addition of H2, a
dihydride rhodium (III) complex is formed. The chloride anion
of the isoquinolinium salt will bind with the hydrogen of
thiourea, forming a Rh dichloro-dihydride anionic complex.
The equatorial hydride of the active rhodium complex will be
transferred to the isoquinolinium due to strong trans effect of
a phosphine. After this 1,2- addition, a tautomerization step
will follow, giving an iminium intermediate. The iminium,
instead of an enamine, will undergo the insertion of hydride
from a rhodium dihydride complex. The chirality of the
product is originated in this step. Another molecule of
hydrogen will react and form the active rhodium dihydride
species, finishing the catalytic cycle. In order to approve the
mechanism of this rhodium catalyzed asymmetric
Acknowledgements
We are grateful for Mr. Weijie Chen’s helpful discussion and
Mr. Yang Gao’s technical supports.
Notes and references
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15 99% yield and 98% ee was obtain in asymmetric
hydrogenation of 5 mmol 2-methylisoquinolinium chloride
with 0.5% catalyst.
16 100% recovery of starting material.
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