Polymer-Supported Chiral-at-Metal Lewis Acid Catalysts

Article abstract of DOI:10.1021/acs.organomet.7b00016

The covalent immobilization of a chiral-at-metal bis-cyclometalated iridium(III) catalyst on a solid support is reported, and its catalytic activity has been investigated. As a catalyst immobilization strategy, a catalyst precursor was tethered to polystyrene macrobeads through an ester or amide linkage and subsequently converted to the immobilized active chiral Lewis acid by treatment with a Br?nsted acid. The amide-linked catalyst displays high robustness and can be recycled multiple times without deterioration of enantioselectivity and only a gradual loss of catalytic activity. Chiral Lewis acid activity was demonstrated as an example for the enantioselective Friedel-Crafts alkylation of indole with an α,β-unsaturated 2-acyl imidazole and for the enantioselective Diels-Alder reactions of an α,β-unsaturated 2-acyl imidazole with 2,3-dihydrofuran or isoprene.

Full text of DOI:10.1021/acs.organomet.7b00016

Communication
pubs.acs.org/Organometallics
Polymer-Supported Chiral-at-Metal Lewis Acid Catalysts
Vladimir A. Larionov, Thomas Cruchter, Thomas Mietke, and Eric Meggers*
Fachbereich Chemie, Philipps-Universitat Marburg, Hans-Meerwein-Strasse 4, 35043 Marburg, Germany
̈
S
* Supporting Information
ABSTRACT: The covalent immobilization of a chiral-at-metal bis-
cyclometalated iridium(III) catalyst on a solid support is reported,
and its catalytic activity has been investigated. As a catalyst
immobilization strategy, a catalyst precursor was tethered to
polystyrene macrobeads through an ester or amide linkage and
subsequently converted to the immobilized active chiral Lewis acid by
treatment with a Brønsted acid. The amide-linked catalyst displays
high robustness and can be recycled multiple times without
deterioration of enantioselectivity and only a gradual loss of catalytic
activity. Chiral Lewis acid activity was demonstrated as an example
for the enantioselective Friedel−Crafts alkylation of indole with an α,β-unsaturated 2-acyl imidazole and for the enantioselective
Diels−Alder reactions of an α,β-unsaturated 2-acyl imidazole with 2,3-dihydrofuran or isoprene.
mmobilized chiral catalysts constitute an important repertoire
immobilization of one member of this class of chiral-at-metal
catalysts on polystyrene (Figure 1) through an ester or amide
1−3
Ifor the asymmetric synthesis of chiral compounds.
In
addition to the aspect of simplicity of catalyst recovery and
recycling, solid-supported catalysts have also found increasing
applications in continuous flow processes.4 Many straightfor-
ward methods exist for the generation of solid-supported
asymmetric catalysts.5 In the case of metal-based asymmetric
catalysts, one chiral or achiral ligand is typically immobilized and
the complete catalyst finally assembled through the addition of
metal salts or metal complex precursors. The direct immobiliza-
tion of entire chiral organometallic catalysts or related
precatalysts is less established. Several groups formed polymer-
supported chiral organometallic catalysts by copolymerization
with metal-containing monomers.6 Davies reported the
straightforward and general immobilization of chiral dirhodium
catalysts by direct coordination of the dirhodium catalysts
through one of their vacant axial coordination sites to a pyridine-
functionalized resin.7 Nomura and Richards synthesized a solid-
supported chiral palladacycle through ligand exchange with a
triphenylphosphine-funcationalized resin.8 Here, we use a
strategy in which a catalyst precursor is immobilized on
polystyrene9 through a covalent tether and subsequently
converted to the immobilized active chiral Lewis acid by
treatment with a Brønsted acid.
Figure 1. Polymer-supported chiral-only-at-metal iridium Lewis acid
catalysts synthesized and investigated in this study (Λ and Δ
enantiomers are shown).
linkage, and we demonstrate for the enantioselective Friedel−
Crafts alkylation of indole with an α,β-unsaturated 2-acyl
imidazole and for the enantioselective Diels−Alder reactions of
an α,β-unsaturated 2-acyl imidazole with 2,3-dihydrofuran or
isoprene that these immobilized catalysts retain a high catalytic
activity with excellent stereoselectivities and can be recycled
multiple times.
Synthesis and Characterization of the Resin-Bound
Catalysts. The synthesis is shown in Scheme 1. Accordingly,
IrCl3·3H2O was bis-cyclometalated with 5-tert-butyl(3-
bromophenyl)phenylbenzoxazole (1) followed by a reaction
with the chiral salicylthiazoline (S)-210a,13 to generate the two
diastereomers Λ-(S)-Ir1 and Δ-(S)-Ir1 as single enantiomers
which were resolved by regular silica gel chromatography. The
absolute metal-centered configuration was assigned by CD
spectroscopy through comparison with published nonbromi-
We have recently introduced a new class of chiral-at-metal
Lewis acid catalysts in which an iridium(III)10 or rhodium(III)11
is bis-cyclometalated with 5-tert-butyl-2-phenylbenzoxazole or
the corresponding benzothiazole derivatives in addition to two
exchange-labile acetonitriles. Despite all ligands being achiral, the
complexes can adopt a metal-centered Λ (left-handed propeller)
or Δ configuration (right-handed propeller). We and others have
shown that such complexes are excellent chiral catalysts for a
variety of transformations, ranging from enolate chemistry,
conjugate additions, and asymmetric transfer hydrogenations to
asymmetric photoredox reactions.10−12 Here, we report the
Received: January 10, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.7b00016
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Organometallics
Communication
a
Scheme 1. Synthesis of Polymer-Supported Catalysts
Figure 2. Visualization of iridium complex immobilization on
polystyrene: (a) benzoyl chloride polystyrene resin (4); (b) resin-
bound Λ-(S)-Ir4a; (c) homogeneous complex Λ-(S)-Ir2a; (d) resin-
bound catalyst Λ-IrPS1; (e) homogeneous catalyst Λ-IrRef1. All
samples are in CH2Cl2.
released salicylthiazoline (S)-2 was reisolated in 93% and 95%
yields for Λ-IrPS1 and Λ-IrPS2, respectively.10d
Enantioselective Catalysis and Catalyst Recycling. The
catalytic activities of the polymer-supported catalysts Λ-IrPS1
and Λ-IrPS2 were evaluated using the Friedel−Crafts alkylation
of indole with the α,β-unsaturated 2-acyl imidazole 5 (Table 1).
Table 1. Enantioselective Friedel−Crafts Addition of Indole
to α,β-Unsaturated 2-Acyl Imidazole 5 Catalyzed by
a
Homogeneous and Immobilized Ir(III) Catalysts
a
Abbreviations: TBS = tert-butyldimethylsilyl, Cbz = N-carboxybenzyl.
nated analogues.10a In the subsequent key step, the individual
brominated diastereomers were cross-coupled with suitable
boronic esters in order to introduce the linker unit.14,15 For
example, Λ-(S)-Ir1 was reacted with the boronic ester 3a or 3b in
the presence of Pd(OAc)2, the ligand SPhos16 (20 mol %), and
K3PO4 at 80 °C to afford the complex Λ-(S)-Ir2a (76%) or Λ-
(S)-Ir2b (77%), respectively. Next, protecting groups were
removed to provide Λ-(S)-Ir3a (TBS removal with TBAF, 90%)
containing a primary alcohol and Λ-(S)-Ir3b (Cbz removal with
H2 and Pd/C, 93%) containing a primary amine. Initially,
Merrifield resin5 with benzyl chloride as the reactive group was
attempted for application to our system but the harsh basic
conditions required for efficient attachment proved incompatible
with our Ir(III) complexes. Therefore, these complexes were
reacted with a functionalized polystyrene resin (4) (macro-
porous 200−400 μm) containing benzoyl chloride (0.5−2.0
mmol/g)17 as the reactive functional group in the presence of
Et3N to afford the polymer-supported ester-linked complex Λ-
(S)-Ir4a and the polymer-supported amide-linked complex Λ-
(S)-Ir4b. The conversion was followed by monitoring the
consumption of Λ-(S)-Ir3a or Λ-(S)-Ir3b using mesitylene as an
internal standard (see the Supporting Information for more
details). Loadings of 0.043 mmol/g were reached for the ester-
linked complex Λ-(S)-Ir4a and 0.162 mmol/g for the amide-
linked complex Λ-(S)-Ir4b. Furthermore, the immobilization
was indicated visually by a change in color of the resin (Figure 2).
Finally, the polystyrene-tethered active catalysts Λ-IrPS1 and Λ-
IrPS2 were generated by replacing the auxiliary ligand with two
acetonitriles upon treatment with trifluoromethanesulfonic acid
in MeCN/CH2Cl2 (1/1) for 2 h, filtered, washed, and dried. The
b
c
d
entry
catalyst
T (°C)
t (h) conversn (%)
ee (%)
1
2
Λ-IrO (1.0)
room temp
room temp
40
16
72
48
72
48
72
44
41
98
70
93 (R)
97 (R)
96 (R)
97 (R)
95 (R)
97 (S)
98 (R)
97 (R)
Λ-IrPS1 (1.0)
Λ-IrPS1 (3.0)
Λ-IrPS2 (1.0)
Λ-IrPS2 (3.0)
Δ-IrPS2 (1.0)
Λ-IrRef1 (1.0)
Λ-IrRef2 (1.0)
e
3
91
4
5
6
7
8
room temp
40
75
>99
76
room temp
room temp
room temp
>99
>99
a
Reaction conditions unless specified otherwise: imidazole 5 (0.15
mmol), indole (0.38 mmol), and catalyst (1.0 or 3.0 mol %) in 0.2 mL
of THF were stirred at the indicated temperature under an atmosphere
of nitrogen. Catalyst loadings provided in parentheses (mol %).
Conversion determined by H NMR. Determined by chiral HPLC
b
c
d
1
e
analysis. Increased reaction volume of 0.4 mL of THF was used in
order to immerse all of the resin.
B
DOI: 10.1021/acs.organomet.7b00016
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Communication
Interestingly, in comparison to the homogeneous catalyst Λ-
IrO,10a the immobilized catalysts Λ-IrPS1 and Λ-IrPS2 provide
higher enantioselectivities while featuring lower catalytic
activities in comparison to the homogeneous catalysts. The
slightly lower conversion observed for Λ-IrPS1 (entry 3) in
comparison to Λ-IrPS2 (entry 5) can be attributed to the lower
concentration required for immersing all of the resin Λ-IrPS1.
Overall, the somewhat reduced catalytic activity of the
immobilized catalysts in comparison to their homogeneous
counterparts can be compensated by increasing the catalyst
loading, extending the reaction time, and raising the temperature
to 40 °C (entries 1−5). As expected, the mirror-image resin-
bound catalyst Δ-IrPS2 provides almost identical results but with
opposite enantioselectivity (entry 6). Importantly, the higher
enantioselectivities observed for Λ-IrPS1, Λ-IrPS2, and Δ-
IrPS2 can be traced back to the linker itself, as demonstrated for
the homogeneous reference catalysts Λ-IrRef1 and Λ-IrRef2,
which are modified with analogous linker units (entries 7 and 8).
Finally, we investigated catalyst recycling with the polystyrene-
immobilized complexes Λ-IrPS1 and Λ-IrPS2. Table 2 reveals
Scheme 2. Enantioselective Diels−Alder Reactions of α,β-
Unsaturated 2-Acyl Imidazole 7 with 2,3-Dihydrofuran or
Isoprene Catalyzed by Homogeneous and Immobilized
Catalysts
Table 2. Recycling Experiments with Immobilized Catalysts
a
for Enantioselective Friedel−Crafts Alkylation
Λ-IrPS1
Λ-IrPS2
b
c
b
c
imidazole 7 with isoprene provided the desired product 9 with
high enantio- and diastereoselectivity. Overall, the immobilized
catalyst Λ-IrPS2 provides the same enantio- and diastereose-
lectivity in comparison to the homogeneous counterparts Λ-IrO
and Λ-IrRef2 but needs to be used at twice the catalyst loading (4
versus 2 mol %). We also demonstrated that the immobilized
catalyst Λ-IrPS2 could be recycled and used for two additional
runs without loss of catalytic activity and enantioselectivity for
both reactions.
In conclusion, we here reported the successful covalent
immobilization of a chiral-at-metal iridium Lewis acid catalyst on
a polystyrene resin through two different linkers. In particular,
the amide-linked catalyst is very robust and can be recycled
multiple times without affecting enantioselectivity for the
Friedel−Crafts reaction of indole with an α,β-unsaturated 2-
acyl imidazole and for the enantioselective Diels−Alder reactions
of an α,β-unsaturated 2-acyl imidazole with 2,3-dihydrofuran and
isoprene. A somewhat reduced catalytic activity in comparison to
the homogeneous catalyst can be compensated by increasing the
reaction temperature, reaction time, and catalyst loading.
Interestingly, it was also found that the linker itself leads to an
increase in enantioselectivity, revealing that the functionalization
of the cyclometalated ligand at the phenyl moiety provides
opportunities to affect the asymmetric induction. This work will
pave the way for using chiral-at-metal Lewis acid catalysts in flow
chemistry applications.
conversn (%)
ee (%)
96
conversn (%)
ee (%)
1st run
2nd run
3rd run
4th run
5th run
15th run
91
>99
>99
>99
>99
95
95
96
96
96
96
92
96
97
80
d
e
d
e
47 (88)
47 (80)
n.a.
96 (96)
96 (96)
n.a.
f
f
>99
96
a
Reaction conditions: imidazole 5 (0.15 mmol), indole (0.38 mmol),
and catalyst (3 mol %) in 0.2 mL (Λ-IrPS2) or 0.4 mL of THF (Λ-
IrPS1) were stirred 48 h at 40 °C under nitrogen. n.a. = not applied.
Conversion determined by H NMR. Determined by chiral HPLC
analysis. Conversion and ee after 96 h are given in parentheses.
Conversion and ee after 112 h are given in parentheses. Conversion
and ee after 72 h at 50 °C.
b
c
1
d
e
f
that Λ-IrPS1 decreases its catalytic activity for the Friedel−
Crafts reaction of indole with imidazole 5 to around 50% after
four recyclings during the fifth run. In contrast, Λ-IrPS2 retains
95% of its catalytic activity during the fifth run. This might be
attributed to the more robust amide linkage in Λ-IrPS2 in
comparison to the more hydrolytically labile ester linkage in Λ-
IrPS1. Interestingly, the catalysts can be recycled and run
multiple times (for at least 15 cycles) without affecting
enantioselectivities for the Friedel−Crafts reaction of indole
with imidazole 5. Overall, these results demonstrate that the
amide-linked catalyst Λ-IrPS2 is superior to Λ-IrPS1. Even after
10 further runs, enantioselectivity for the Friedel−Crafts reaction
of indole with imidazole 5 is unchanged at 96% ee; when the
reaction temperature in increased to 50 °C, full conversion is
reached after 72 h (see the Supporting Information for details).
Finally, we investigated the enantioselective Diels−Alder
reactions of an α,β-unsaturated 2-acyl imidazole 7 with 2,3-
dihydrofuran or isoprene catalyzed by the immobilized catalyst
Λ-IrPS2 (Scheme 2). Accordingly, the reaction of 7 with 2,3-
dihydrofuran catalyzed by 4 mol % of Λ-IrPS2 provided the
formation of the dihydropyran 8 in a yield of 86% and with
excellent enantio- (98% ee) and diastereoselectivities (endo/exo
> 50/1). Furthermore, the Diels−Alder reaction of 2-acyl
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00016.
■
S
Synthetic details, analytical data, HPLC traces, and NMR
spectra (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail for E.M.: meggers@chemie.uni-marburg.de.
■
C
DOI: 10.1021/acs.organomet.7b00016
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
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ORCID
Eric Meggers: 0000-0002-8851-7623
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work is supported by the Deutsche Forschungsgemein-
schaft (ME1805/13-1) and by a LOEWE research cluster
(SynChemBio) of the State of Hesse, Germany.
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DOI: 10.1021/acs.organomet.7b00016
Organometallics XXXX, XXX, XXX−XXX