Zinc-Catalyzed Asymmetric Hydrosilylation of Cyclic Imines: Synthesis of Chiral 2-Aryl-Substituted Pyrrolidines as Pharmaceutical Building Blocks

Article abstract of DOI:10.1002/adsc.202001043

The first successful enantioselective hydrosilylation of cyclic imines promoted by a chiral zinc complex is reported. In situ generated zinc-ProPhenol complex with silane afforded pharmaceutically relevant enantioenriched 2-aryl-substituted pyrrolidines in high yields and with excellent enantioselectivities (up to 99% ee). The synthetic utility of presented methodology is demonstrated in an efficient synthesis of the corresponding chiral cyclic amines, being pharmaceutical drug precursors to the Aticaprant and Larotrectinib. (Figure presented.).

Full text of DOI:10.1002/adsc.202001043

Title: Zinc-Catalyzed Asymmetric Hydrosilylation of Cyclic Imines:
Synthesis of Chiral 2-Aryl-Substituted Pyrrolidines as
Pharmaceutical Building Blocks
Authors: Izabela Węglarz, Karol Michalak, and Jacek Mlynarski
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10.1002/adsc.202001043
Advanced Synthesis & Catalysis
Very Important Publication
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Zinc-Catalyzed Asymmetric Hydrosilylation of Cyclic Imines:
Synthesis of Chiral 2-Aryl-Substituted Pyrrolidines as
Pharmaceutical Building Blocks
Izabela Węglarz, Karol Michalak, Jacek Mlynarski*
Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland;
Fax: (+48)-22-632-66-81; phone: (+48)-22-632-2322; e-mail: jacek.mlynarski@gmail.com (homepage:
www.jacekmlynarski.pl)
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Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract. The first successful enantioselective
hydrosilylation of cyclic imines promoted by a chiral zinc
complex is reported. In situ generated zinc-ProPhenol
complex with silane afforded pharmaceutically relevant
enantioenriched 2-aryl-substituted pyrrolidines in high
yields and with excellent enantioselectivities (up to 99%
ee). The synthetic utility of presented methodology is
demonstrated in an efficient synthesis of the
corresponding chiral cyclic amines, being pharmaceutical
drug precursors to the Aticaprant and Larotrectinib.
attractive strategy. The advantages of hydrosilylation
reactions include its simplicity combined with mild
conditions and employing readily available and safe
hydrosilanes. In contrast to well-established
asymmetric hydrosilylation of acyclic imines
promoted by numerous metal-based complexes,[8] the
arsenal of known catalysts for hydrosilylation of cyclic
imines is restricted to few examples of titanium
complexes and platinum-group metals.[9] Therefore,
the development of efficient and inexpensive catalysts
for the hydrosilylation of prochiral cyclic imines is still
very challenging and highly desirable goal.
Keywords: Asymmetric synthesis; Zinc; Reduction;
Hydrosilylation; Cyclic imines
Chiral 2-substituted pyrrolidines are important
building blocks due to their ubiquitous structural
motifs found in many natural products,[1]
pharmaceutical compounds[2] and drug candidates.[3] In
particular, 2-aryl-substituted pyrrolidine derivatives
are pharmaceutically relevant molecules used in the
synthesis of drugs e.g. Aticaprant (-opioid receptor
antagonist effective in the treatment of depression
disorder and anxiety) or Larotrectinib (a highly
selective inhibitor of neurotrophin receptor kinase
recently approved for treating tumors harboring
neurotrophin receptor (NTRK) gene fusions) (Scheme
1).[3] Broad application scope in both pharmaceutical
and agrochemical industries stimulates intensive
studies, and it is of central importance in organic
synthesis.[1-3]
In the past decades, numerous catalytic
methodologies for the construction of optically pure
cyclic amines have been developed,[4] mostly rely on
bio-,[5] organo-[6] and transition-metal catalysis.[7]
Despite these achievements,[4-7] the alternative
synthesis of chiral amines via the enantioselective
hydrosilylation of cyclic imines appears especially
Scheme 1. Biologically active compounds and
pharmaceutical drugs containing 2-aryl-substituted
pyrrolidine fragments.
A major breakthrough in the enantioselective
hydrosilylation of cyclic imines was reported by
Buchwald in 1996.[9c] The use of chiral titanocene-
complex proceeded highly enantioselectively, however,
the proposed catalyst required pre-activation and the
substrate scope was mainly limited to a few simple
cyclic imines.[9b,9c,10] Following this achievement, the
more efficient catalysts were showed by Uemura and
Hidai.[9a] Application of ruthenium(II)- and iridium(II)-
1
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10.1002/adsc.202001043
Advanced Synthesis & Catalysis
oxazolinyl-ferrocenylphosphine-complexes provided reaction efficiency. To our delight, higher silane
optically pure 2-substituted pyrrolidine derivatives loading (4 equiv.) from the initially used amount of
with good enantioselectivities (up to 88% ee), whereas (EtO)2MeSiH resulted in the formation of desired
replacing substrates to six-membered imines was amine 2a in 98% yield and with a high level of
completely ineffective.[9a] Recently, Speed and co- enantioselectivity in the range of 88% (Table 1, entry
workers presented an alternative method to this 10 vs. 5). The use of the (R,R)-ProPhenol enantiomer
approach by asymmetric hydroboration using chiral of ligand L1 also promoted the reduction without any
phosphenium cations.[4a] Although authors tested loss of both reactivity and selectivity (Table 1, entry
several hydrosilanes, the highest yields and 11).
enantioselectivites (72–94% ee) of the corresponding
chiral pyrrolidines and piperidines were obtained in hydrosilylation, further investigations were carried out
case of the use of HB(pin) as a reducing agent.[4a]
including the effects of catalyst loading, time and
To improve the stereoselectivity of the
Despite the progress achieved in enantioselective reaction temperature (Table 2). Based on the initial
hydrosilylation of prochiral cyclic imines, some areas screening performed at room temperature, cooling
such as: difficult catalyst preparation, limited
availability, high toxicity, and the cost of noble metal
complexes, indicate the need for the development of
more sustainable reaction conditions. Replacement of
platinum-group metals is recently one of the great
challenges in organic chemistry, because their usage
represents substantial limitations, especially in the
synthesis of bioactive compounds.[1-3,9] A promising
alternative, still unexplored, is to use in the reduction
of cyclic imines, an environmentally benign and easily
accessible chiral zinc complexes. Inspired by our
previous experience in zinc-catalyzed asymmetric
down to 4 C increased the ee up to 95% of the
corresponding product 2a in case of 5 mol% of the
catalyst, albeit the efficiency suffered (Table 2, entries
3-4). Although the prolongation of the reaction time
had only little effect on the conversion of substrate, the
highly promising attempt with chiral zinc-complex
encouraged us to pursue further optimizations.
A satisfactory yield up to 97% was obtained without
any loss of ee by employing 10 mol% of the catalyst
and prolonged the reaction time up to 48 h (Table 2,
entry 7).
hydrosilylation
of
ketones[8b,11]
and
acylic
imines,[8a,8b,11] in this paper, we present the first Table 1. Initial screening of silanes and solvents in
asymmetric hydrosilylation of cyclic imines.[a]
example of a highly efficient and enantioselective
hydrosilylation of a wide variety of 2-aryl-substituted
pyrrolines promoted by a Trost-type zinc complex.[12]
Presented concept requires application of the
commercially available ProPhenol ligand and is
superior to previously published protocols in terms of
reaction efficiency and enantioselectivity.
Yield
ee
Our initial aim was to identify the best-suited zinc
complex in terms of both reaction conversion and
enantioselectivity. After many trials, we found that the
readily available chiral Zn-ProPhenol complex is the
best catalyst candidate for the hydrosilylation of
2-phenyl-1-pyrroline (1a) as a model substrate.
Encouraged by the promising results, a series of silanes
as reducing agents and various solvents were tested to
improve the reaction outcome (Table 1). Data collected
in Table 1 indicates that the application of
(EtO)2MeSiH as a hydrogen source in reduction had a
significant impact on the reaction selectivity (Table 1,
entry 5). From among the tested solvents, only
tetrahydrofuran (THF) proved to be the best choice
with respect to efficiency and enantioselectivity up to
66% ee of the newly formed product (2a) (Table 1,
entry 5). Considering further improvement of reaction
enantioselectivity, the optimal amount of employed
hydrosilane was evaluated in the presence of 5 mol%
of the catalyst (Table 1). We found that the increased
values of equivalents of this particular hydrosilane
played a crucial role in asymmetric induction and
Entry
Silane
Equiv. Solvent
(%)[b] (%)[c]
1
2
3
4
5
6
7
8
9
(EtO)3SiH
PhSiH3
PhMe2SiH
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.4
4.0
4.0
4.5
THF
THF
THF
THF
THF
MeCN
Toluene
DCM
THF
THF
THF
73
49
trace
67
80
30
97
92
87
98
45
10

64
66
59
54
55
74
88
88 (S)
86
PMHS
(EtO)2MeSiH
(EtO)2MeSiH
(EtO)2MeSiH
(EtO)2MeSiH
(EtO)2MeSiH
(EtO)2MeSiH
(EtO)2MeSiH
(EtO)2MeSiH
10
11[d]
12
98
98
THF
[a] Reactions were carried out by stirring a solution of ZnEt2
(10 mol%, 15 wt. % in toluene) with (S,S)-ProPhenol
L1-ligand (5 mol%) in THF (1 mL) at room temperature for
24 h, containing DEMS (1.5-2.3 mmol) and cyclic imine
(1a) (0.5 mmol) dissolved in THF (0.2 mL) unless otherwise
[b]
stated.
Isolated yield after silica gel chromatography.
[c]
[d]
Determined by chiral HPLC analysis.
Reaction
performed with (R,R)-ProPhenol L1. The absolute
2
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10.1002/adsc.202001043
Advanced Synthesis & Catalysis
stereochemistry of product (2a) was determined by Scheme 2. Asymmetric zinc-catalyzed hydrosilylation of
comparison of optical rotation with literature values.
various 2-aryl-substituted pyrrolidines.[a,b,c]
Table 2. Asymmetric hydrosilylation of cyclic imines under
various conditions. [a]
In the range of substrate studies, a variety of cyclic
imines participated successfully in the asymmetric
reduction and gave access to optically pure
pyrrolidines with high yields and excellent
enantioselectivities. These compounds are mostly
pivotal components in pharmaceutical drugs or drug
candidates. The investigations revealed that the
electronic properties and the position of the substituent
attached to the phenyl ring have a decisive influence,
mainly on the reaction enantioselectivity. Cyclic
imines possessing electron-donating groups (2b–2i)
gave products with a high level of enantioselectivity
(80-99%), whereas replacing by the substrates bearing
electron-withdrawing groups (2j–2l) decreased the
selectivity in each case. Interestingly, cyclic amines
containing electron-donating groups placed in the
ortho position were formed with higher ee (95-99%),
while the meta- and para-substituted imines provided
slightly lower enantioselectivities up to 90% ee.
Moreover, the presence of alkyl substituents in the
ortho position attached to the phenyl ring afforded the
desired products 2b–2d with very good yields and the
highest enantioselective control up to 99% ee.
In contrast to the above-mentioned substrates, 2-aryl
pyrroline halogen derivatives resulted in a formation
of the corresponding products 2j–2l with good yields
and the same level of enantioselectivity up to
70%, regardless of the substituent positions. The
replacement of the aryl substituent by the heterocyclic
groups led to slightly lower selectivity (79% ee) for
cyclic amine 2o containing thiophene rings. Similar
investigation made for the reduction of furan-based
pyrroline 2p was unsuccessful, probably due to
additional coordination of the Lewis acid by the
heteroatom. Application of elaborated methodology to
the six-membered cyclic imines was also not
successful and led to racemic amines, unfortunately.
T
(C)
Time
(h)
Cat.
Yield
ee
Entry
(mol%) (%)[b]
(%)[c]
1
2
3
4
5
rt
4
4
4
4
4
4
4
24
24
60
72
24
36
48
48
5
5
5
98
trace
42
53
45
80
97
88
88

94
95
95
95
95
82
5
10
10
10
10
6
7
8[d]
[a] Reactions were carried out by stirring a solution of ZnEt2
(2x mol%, 15 wt. % in toluene) with (S,S)-ProPhenol L1
(x mol%) in THF (1 mL) at the specified time and
temperature, containing DEMS (2.0 mmol, 4.0 equiv.) and
cyclic imine 1a (0.5 mmol) dissolved in THF (0.2 mL)
[b]
unless otherwise stated.
chromatography.
Isolated yield after silica gel
[c]
Determined by chiral HPLC analysis.
[d] Reaction was carried out in 0.5 mL of THF.
At the established optimal conditions, the scope of
the zinc-catalyzed enantioselective hydrosilylation
with respect to the 2-aryl-substituted pyrroline
substrates was examined, as presented in Scheme 2.
[a] Reactions were performed with ZnEt2 (20 mol%, 15 wt. %
in toluene) and (S,S)-ProPhenol L1 (10 mol%) in THF
(1 mL) at 4 C for 48 h, containing DEMS (2.0 mmol) and
specified imines 1a-p (0.5 mmol) dissolved in THF (0.2 mL).
[b] Isolated yield after silica gel chromatography. [c] Ee values
were determined by chiral HPLC analysis.
Scheme 3. Synthesis of pharmaceutical drug precursors via
zinc-catalyzed enantioselective reduction of cyclic imines.
3
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10.1002/adsc.202001043
Advanced Synthesis & Catalysis
References
Finally, we demonstrated the utility of the
developed methodology and the potential for the
further functionalization of chiral cyclic amines in the
synthesis of biologically active molecular skeletons
(Scheme 3). Zinc-catalyzed asymmetric hydrosily-
lation of the corresponding cyclic imines under
elaborated conditions provided 2-aryl pyrrolidines
(2q–2r) with high yields and enantioselectivity up to
94% ee (for 2q). Asymmetric hydrosilylation of
difluoro- substituted Larotrectinib precursor was
however less selective (68% ee). Improvement of the
purity of product 2r to 80% ee was achieved by
additional recrystallization with D-malic acid.4a Thus,
the afforded cyclic amines 2q–2r may represent
valuable pharmaceutical key intermediates for drugs
such as: Aticaprant and Larotrectinib.
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for enantioselective hydrosilylation of cyclic imines
promoted by the chiral zinc catalyst. This is also the
first successful application of zinc-based catalyst in a
stereoselective reduction of cyclic imines, which
provides new insight into further extent of the synthetic
scope. The elaborated system is based on cost-effective
and easily accessible zinc-complex prepared from
commercially available ProPhenol ligand enantiomers.
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access to a diverse range of enantioenriched 2-aryl-
substituted pyrrolidines with very good yields and
excellent enantioselectivities. The chiral products
accessible using this methodology are versatile
scaffolds for further functionalization, and can be
rapidly transformed into biologically active
compounds or pharmaceutical drugs.
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Experimental Section
General procedure for Zinc-Catalyzed Asymmetric
Hydrosilylation of Cyclic Imines
Under an argon atmosphere at room temperature ZnEt2
solution (15 wt. % in toluene, 0.1 mL, 0.1 mmol) and (S,S)-
ProPhenol L1 (32 mg, 0.05 mmol) were dissolved in
anhydrous THF (1 mL) and stirred for 60 min. (EtO)2MeSiH
(0.32 mL, 2.0 mmol, 4.0 equiv.) was then added and the
mixture was cooled down to 4 °C, and stirred for additional
5 min. The corresponding cyclic imine (0.5 mmol, 1.0
equiv.) was dissolved in anhydrous THF (0.2 mL) and
subsequently added to the reaction. The resulting mixture
was stirred at 4 °C for 48 h. After this time, the reaction
mixture was directly poured onto a silica gel column and
eluted with a mixture of ethyl acetate-methanol (10:1 + 1%
Et3N, v/v). The ee values were determined by HPLC method.
[6] For
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Acknowledgements
Financial support from the Polish National Science Centre (OPUS
Grant No. NCN 2017/27/B/ST5/01111) is gratefully acknowledged.
4
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10.1002/adsc.202001043
Advanced Synthesis & Catalysis
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5
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Advanced Synthesis & Catalysis
COMMUNICATION
Zinc-Catalyzed Asymmetric Hydrosilylation of
Cyclic Imines: Synthesis of Chiral 2-Aryl-
Substituted Pyrrolidines as Pharmaceutical
Building Blocks
Adv. Synth. Catal. Year, Volume, Page – Page
Izabela Węglarz, Karol Michalak, Jacek
Mlynarski*
6
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