An N-heterocyclic carbene iridium catalyst with metal-centered chirality for enantioselective transfer hydrogenation of imines

Article abstract of DOI:10.1039/c7cc04691j

A cyclometalating N-heterocyclic carbene iridium complex featuring metal-centered chirality was designed and used for the asymmetric transfer hydrogenation (ATH) of imines. Four strongly σ-donating carbon-based substituents (2 carbenes and 2 phenyl moieti

Full text of DOI:10.1039/c7cc04691j

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An N-Heterocyclic Carbene Iridium Catalyst with Metal-Centered
Chirality for Enantioselective Transfer Hydrogenation of Imines
Yanjun Li,a Meng Lei,a Wei Yuan,a Eric Meggersa,b and Lei Gong*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/chemcomm
A cyclometalating N-heterocyclic carbene iridium complex ATH reaction of imines by a chiral metal carbene catalyst has not
featuring metal-centered chirality was designed and applied been reported thus far. Recently, our groups developed one class of
to the asymmetric transfer hydrogenation (ATH) of imines. chiral-only-at-metal Lewis acid catalysts in which the asymmetric
Four strongly σ-donating carbon-based substituents (2 induction occurs directly from the metal to the substrates and
carbenes and 2 phenyl moieties), a chirality transfer directly provides powerful stereocontrol for
from the stereogenic metal center to the C=N bond of transformations.11,12
a
range of organic
W
e were wondering if utilizing such a strategy
substrates, as well as a restriction of catalyst deactivation by to assemble NHC ligands around a central metal would lead to the
steric demanding substituents, render the new complex one discovery of highly active and enantioselective catalysts for ATH of
of the most efficient catalysts for ATH of cyclic N- imines (Fig. 1a). The use of chelating carbene ligands with strong σ-
sulfonylimines (down to 0.01 mol% cat., 24 examples, 94– donor ability is presumably not only beneficial for enhancing
98% ee).
reactivity of the metal hydride intermediates during the catalysis,13
but also capable of stabilizing the configuration of the catalyst as
Transition-metal-catalyzed asymmetric transfer hydrogenation (ATH) suggested by a recent study.14
of imines provides straightforward access to chiral amines and has
attracted great attention owing to the simplified operation,
sustainability and possible industrial applications.1,2 Such reactions
have been enabled by Ru, Rh, Ir or Fe-based catalysts, often in
combination with a chiral diamine ligand. However, in contrast to
the reduction of ketones or olefins, they are much less effective
regarding the substrate scope, catalyst loading and
stereoselectivity.3 The drawbacks of imine ATH reactions include: 1)
catalyst poisoning through metal coordination of imines or amines;
2) intrinsic reactivity of imines toward hydrolysis, formation of
(hemi)aminals with amines or N,O-acetals with alchols; 3) decline of
enantioselectivities raised by the Z/E isomerization of acyclic imines;
4) an often complex reaction mechanism.4–7 Consequently, the
design of new catalysts with different structures for ATH of imines
Fig. 1 Catalyst design.
remains a challenging task, while at the same time is of great
importance for fundamental studies and practical applications.8
N-heterocyclic carbene (NHC) complexes have been established
as potentially highly useful catalysts in transfer hydrogenations.9
However, much less success has been achieved when applied in an
asymmetric fashion.10 To the best of our knowledge, the catalytic
We have demonstrated that a co-catalyst system consisting of a
chiral-at-metal complex and a pyrazole additive was effective for
the enantioselective transfer hydrogenation of acetophenones and
some other aromatic ketones.11f However, only very sluggish
catalytic outcomes were achieved when applied to the reduction of
imines. The potential issue of catalyst deactivation, combined with
Crabtree’s observation that the complex [Ir(bis-NHC)(OAc)I2]
exhibited prominent activity for hydride transfer,15 gave rise to our
a.College of Chemistry and Chemical Engineering, Xiamen University, Xiamen
361005, People’s Republic of China. Email: gongl@xmu.edu.cn
b.Philipps-Universität Marburg, Hans-Meerwein-Strasse 4, 35043 Marburg,
Germany
Electronic Supplementary Information (ESI) available: Experimental details and
analytical data. See DOI: 10.1039/x0xx00000x
consideration to develop chiral iridium catalysts based on
chelating carbene scaffold.16 After primary experiments on ligand
a
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screening, we selected L1a–b as the carbene precursors17 for the presence of 0.5 mol% Δ-IrC1a in THF/H2O (1:1) at 50 °C, 5a was
construction of propeller-type complexes with an exclusive metal- provided in 64% conversion and with 93% ee in 6 h (entry 1). The
centered chirality (Fig. 1a). In theory, six possible propeller-type reaction was significantly dependent on the solvents (see more
geometries (Δ-I, Δ-II, Δ-III and their enantiomers) exist for this type details in the ESI), of which the best rate and enantioselectivity
of complexes (Fig. 1b), while the C2-symmetric Δ-I and Λ-I are the were achieved in DMF/H2O (2:1, entry 2). The catalyst still exhibited
most interesting ones in view that the steric demanding tert-butyl great efficiency and stereocontrol at lower loadings (entries 3–6).
groups locating at the top and bottom of catalytic active sites would For example, in the presence of only 0.05 mol% of Δ-IrC1a and at
be beneficial for chirality transfer and restriction of amines/imines 60 °C, 5a was obtained in 91% conversion and with 98% ee within 5
coordination. Therefore, it is critical to control both the relative and h (entry 4). A further reduction to 0.02 or 0.01 mol% led to a
absolute stereochemistry during the synthesis.
somewhat decrease of rate and enantioselectivity (entries 5, 6). As
The chiral catalysts were prepared through a straightforward a comparison, a methoxy derivative complex Δ-IrC1b and previously
route as demonstrated for ꢀ-IrC1a in Scheme 1. Accordingly, developed Lewis acids ꢀ-IrO, ꢀ-IrS11a,b were tested in the catalysis.
Imidazolium salt L1a synthesized by a one-step procedure was ꢀ-IrS was significantly less beneficial (entry 4 vs. entry 7), while ꢀ-
reacted with [Ir(μ-Cl)(COD)]2 at 60 °C for 24 h to afford dimeric IrO only provide trace amounts of product (entries 8, 9). Replacing
complex rac-1a (Scheme 1). As the key step of diastereoselective ammonium formate by sodium formate or HCO2H/Et3N (5:2) as the
synthesis, the cyclometalation was significantly temperature- reductant led to sluggish transformation (entries 10, 11), suggesting
dependant. Above 70 °C, the undesired isomers were also obtained. that the ammonium ion might play an important role. To confirm
Subsequently, rac-1a was converted to the racemic catalyst rac- this assumption, we monitored the reaction under the optimal
IrC1a in 94% yield by treatment with AgPF6 in MeCN at room conditions without addition of the substrate. A new species was
temperature for 10 h. A crystal structure confirmed its geometry of successfully isolated and identified to be a derivative complex with
two carbene carbons locating at the positions trans to each other.18 two coordinated ammonia ꢀ-6 (Scheme 2), which provided almost
Finally, an enantiopure version of the catalyst, namely Δ-IrC1a, was identical activity in the reaction of 4a→5a (entry 12). In addition,
obtained with > 99% ee through an auxiliary-mediated method.19
complex ꢀ-6 would slowly decompose when left under vacuum,
most likely by affording a 16-electron mono-ammonia complex
according to HRMS analysis and a recent work by Nolan et. al.24
Table 1 Optimization of reaction conditions.a
catalyst
(mol%)
T
t
yield
(%)b
64
ee
(%)c
93
99
97
98
94
90
92
n.d.
94
15
67
94
entry
solvent
(°C) (h)
1
2
-IrC1a(0.5)
-IrC1a(0.5)
THF/H2O(1:1)
DMF/H2O(2:1)
DMF/H2O(2:1)
DMF/H2O(2:1)
DMF/H2O(2:1)
DMF/H2O(2:1)
DMF/H2O(2:1)
DMF/H2O(2:1)
DMF/H2O(2:1)
DMF/H2O(2:1)
DMF/H2O(2:1)
DMF/H2O(2:1)
50
50
50
60
60
60
60
60
60
60
60
60
6
2.5
11
5
ꢀ
ꢀ
ꢀ
ꢀ
93
3
-IrC1a(0.05)
90
4
-IrC1a(0.05)
91
5
ꢀ-IrC1a(0.02)
-IrC1a(0.01)
12
17
5
84
6
82
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
7
-IrC1b(0.05)
70
Scheme 1 Synthetic route to the non-racemic catalyst ꢀ-IrC1a.
8
-IrO(0.05)
5
trace
17
With the desired catalyst in hand, we next examined its
properties regarding ATH of imines. Cyclic N-sulfonylimines were
chosen as the substrates, considering that: 1) enantioselective
reduction of cyclic N-sulfonylimines is the most straightforward
strategy to obtain chiral sultams, which are important synthetic
targets existing in a wide range of biologically active compounds;20
2) to date, only TsDPEN-Ru-type complexes21 and a binapine-nickel
catalyst2e were reported for ATH of cyclic N-sulfonylimines, while
typically performing with narrow substrate scope and catalyst
loadings higher than 1 mol% to achieve enantioselectivities above
90%. Hence, the development of an efficient and general method
for the synthesis of optically active sultams is highly desirable.22,23
The transfer hydrogenation of 4a with ammonium formate was
investigated as a model system (Table 1). Encouragingly, in the
9
-IrS(0.05)
5
10d
11e
12
-IrC1a(0.05)
5
12
-IrC1a(0.05)
5
94
-6(0.05)
5
98
ꢀ
a Reaction conditions: 4a (0.10 mmol), HCO2NH4 (0.90 mmol) as the H
source under air atmosphere. b Yield determined by 1H NMR analysis. c Ee
d
value determined by chiral HPLC analysis. HCO2Na as the hydrogen
source. e HCO2H/Et3N (5:2) as the hydrogen source.
Based on the catalytic outcomes and observed potential
intermediates, we propose a mechanism (Scheme 2). First, ꢀ-IrC1a
undergoes ligand exchange and affords complex A or/and ꢀ-6 as
the active catalyst in the catalytic cycle. Subsequently, A or/and ꢀ-6
slowly liberate one MeCN or NH3 and generate the 16-electron
2 | J. Name., 2012, 00, 1-3
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within the fused benzene ring, and a steric more demanding
aliphatic substitutent on the imine moiety (products 5l–p) were
examined. The desired products were formed in 82–99% yields and
with 94–98% ee (Fig. 2). It is worth to mention that all the halogen
substituents were well tolerated in the reaction, providing the
opportunity for further functionalization of the chiral sultams.
Moreover, Fig. 3 reveals that catalyst ꢀ-IrC1a could remarkably
discern structural differences between the two aromatic rings in the
benzofused N-sulfonylimines with a second aryl on the imino
moiety, by affording chiral diaryl imines 5q–x in excellent yields
(95–99%) and enantioselectivities (96–98% ee).
species B (observed in HRMS), followed by a fast hydride transfer to
produce the highly reactive intermediate C. The final transfer of a
hydride and a proton to imine 4a leads to the formation of 5a and
regeneration of the active catalyst. Notably, ammonium formate
not only serves as the hydrogen source, but also provides free
ammonia to generate the real active catalyst. In addition, water
cluster might also be involved considering the strong solvent effects
in the catalysis.25 The excellent asymmetric induction is presumably
attributed to the less steric hindrance in the favored transition state
in addition to the hydrogen-bonding network (Scheme 2). This
mechanism is consistent with an observed S-configuration of the
product.22b
Fig. 3 Substrate Scope. The yield refers to the isolated yield.
Finally, we investigated the synthetic utility of this method. For
instance, the ATH product 5t could be readily converted into the
corresponding chiral diarylmethylamine 8 with 96% ee through an
N-Boc protection followed by fast treatment with naphthalene
sodium. Notably, chiral diarylmethylamines are important building
blocks in numerous bioactive compounds and very difficult to
achieve by enantioselective reduction of imines.26 Moreover, a half-
gram-scale synthesis of bioactive chiral sultam 9 with anti-HIV
activity was developed based on the ATH of cyclic N-sulfonylimine
4x and an N-methylation step. 515 mg of compound 9 was obtained
in a total yield of 96% and with 97% ee (Scheme 3).22b,27
Scheme 2 Tentatively mechanistic proposal.
Scheme 3 Synthetic utility of the method.
Conclusions
Fig. 2 Substrate Scope. The yield refers to the isolated yield.
In summary, we have developed a cyclometalating N-heterocyclic
Next,
a range of cyclic N-sulfonylimines bearing electron
carbene iridium catalyst Δ-[Ir(CycloNHC)2(MeCN)2]PF6 with exclusive
withdrawing (products 5b–i) or donating groups (products 5j, k)
metal-centered
chirality
for
enantioselective
transfer
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performance in the ATH reaction of cyclic N-sulfonylimines
regarding catalyst loading (down to 0.01 mol%), substrate scope (24
examples) and enantioselectivities (94–98% ee). During the catalytic
process, ammonium formate releases free ammonia to generate
the real active catalyst in addition to serving as the hydrogen
source.We believe that this system will not only provide new
considerations of catalyst design for ATH of imines, but also have
remarkable potential for application in a practical method to
prepare chiral sultams and diarylmethylamines.
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Acknowledgements
We gratefully acknowledge funding from the National Natural
Science Foundation of China (grant nos. 21572184 and
21472154), the Natural Science Foundation of Fujian Province
of China (grant no. 2017J06006), and the Fundamental
Research Funds for the Central Universities (grant no.
20720160027).
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ꢀ-IrC1a also exhibits high catalytic activity for the ATH reaction of other
imines, in
a
primary study, 6,7-dimethoxy-1-methyl-3,4-dihydro-
isoquinoline could be quantitatively converted into its corresponding
amine derivative with 87% ee at a catalyst loading of 0.5 mol%.
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24
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