Cation-triggered switchable asymmetric catalysis with chiral aza-crownphos

Article abstract of DOI:10.1002/anie.201411593

An aza-crown ether, modified phosphoramidite ligand, has been designed and synthesized. The ON/OFF reversible switch of catalytic activity for its rhodium catalyst was thoroughly investigated in the asymmetric hydrogenation of dehydroamino acid esters modulated by host-guest interactions. In the OFF state, the catalyst is almost inactive (less than 1% conversion) because of the formation of an intermolecular sandwich complex by two aza-crown ether moities and the cationic rhodium metal center. In using alkali-metal-cations as the trigger, the catalytic activity was turned ON and consequently resulted in full conversions and excellent enantioselectivities (up to 98% ee).

Full text of DOI:10.1002/anie.201411593

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Chemie
DOI: 10.1002/anie.201411593
Asymmetric Catalysis
Cation-Triggered Switchable Asymmetric Catalysis with Chiral Aza-
CrownPhos**
Guang-Hui Ouyang, Yan-Mei He, Yong Li, Jun-Feng Xiang, and Qing-Hua Fan*
Abstract: An aza-crown ether, modified phosphoramidite
ligand, has been designed and synthesized. The ON/OFF
reversible switch of catalytic activity for its rhodium catalyst
was thoroughly investigated in the asymmetric hydrogenation
of dehydroamino acid esters modulated by host–guest inter-
actions. In the OFF state, the catalyst is almost inactive (less
than 1% conversion) because of the formation of an inter-
molecular sandwich complex by two aza-crown ether moities
and the cationic rhodium metal center. In using alkali-metal-
cations as the trigger, the catalytic activity was turned ON and
consequently resulted in full conversions and excellent enan-
tioselectivities (up to 98% ee).
catalyze asymmetric Michael addition reactions. To the best
of our knowledge, the application of switchable chiral
transition-metal catalysts having a clear cut between the
ON and OFF states for asymmetric catalysis has not been
demonstrated.[2f,o]
Herein we describe a new kind of switchable catalyst for
transition-metal-catalyzed asymmetric catalysis which is trig-
gered by external stimuli, that is, host–guest interactions.
Transition-metal complexes containing a bidentate chiral
ligand or two monodentate chiral ligands have proven to be
powerful in numerous asymmetric catalytic reactions.[3]
A
number of them have a cationic metal center, which might be
able to associate with electron-rich macrocyclic hosts.[4] We
envisaged that if crown ether units are fixed on the proper
position of the ligand, they may associate with the cationic
metal center through cation–macrocycle interactions, thus
leading to the formation of an intermolecular sandwich
complex.[5] This host–guest interaction may thus block the
catalytic center from substrates (Figure 1; state I). In contrast,
S
witchable catalysis has gained increasing attention because
of its potential to trigger or modulate catalytic activity and/or
selectivity in situ with spatiotemporal control.[1] Nature can
efficiently control an enzyme reaction rate through a variety
of trigger-induced effects. Inspired by biocatalytic systems,
a variety of synthetic switchable-catalysis systems,[2] such as
photoswitchable catalysts,[2a–d] allosteric catalysts,[2e–h] molec-
ular-machine-based catalysts,[2a] self-locking catalysts[2i,j] and
so on,[2k–o] have been established over the past several years.
Their catalytic activity and even stereoselectivity can be
reversibly switched by external stimuli such as light, pH, ions,
small molecules, etc. Despite great progress made in this field,
only few examples have been reported for switchable
asymmetric catalysis.[2a,f,l,o] In 2011, Feringa and co-workers
described a chiral molecular motor-based bifunctional orga-
nocatalyst for asymmetric Michael addition. It was found that
light or heat could turn “ON” or “OFF” the catalytic activity,
and also change the stereochemical outcome. Most recently,
Figure 1. Schematic representation of switchable catalysis triggered by
host–guest interactions.
a
rotaxane-based switchable chiral organocatalyst was
designed by the group of Leigh, and the secondary dibenzyl
amine moiety was masked or exposed in response to acid/base
regulations. This chiral rotaxane was successfully applied to
adding proper alkali metal cations, which have stronger
complexation with the crown ether, will decompose the
original complexes to release the catalytic centers (Figure 1;
state II). Furthermore, switching back to state I can also be
realized by removing the alkali-metal cations with crypt-
ands,[6] which have stronger binding affinities for alkali metal
cations.
To exemplify our strategy for switchable asymmetric
catalysis governed by host–guest interactions,[7] a new mono-
dentate chiral phosphoramidite ligand [(S)-Aza-CrownPhos;
Scheme 1], modified by an aza-crown ether, was designed and
synthesized for this study (for details, see Scheme S1 in the
Supporting Information).[8,9] With the ligand in hand, we then
[*] G.-H. Ouyang, Y.-M. He, Y. Li, J.-F. Xiang, Prof. Dr. Q.-H. Fan
Beijing National Laboratory for Molecular Sciences, CAS Key
Laboratory of Molecular Recognition and Function, Institute of
Chemistry, Chinese Academy of Sciences (CAS)
Beijing 100190 (P.R. China)
and
Collaborative Innovation Center of Chemical Science and Engi-
neering, Tianjin 300072 (P.R. China)
E-mail: fanqh@iccas.ac.cn
[**] Financial support from the National Basic Research Program of
China (973 Program No 2011CB808600) and the National Natural
Science Foundation of China (No 21232008 and No 21373231) are
gratefully acknowledged.
synthesized
the
precatalyst
[Rh(Aza-CrownPhos)2-
(NBD)]BF4 from two equivalents of (S)-Aza-CrownPhos
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201411593.
and one equivalent of Rh(NBD)2BF4. The 1H NMR study of
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is formed (state I). Furthermore, the result of
ESI HR-MS confirms the formation of complex
[Rh(Aza-CrownPhos)2]+ (see Figure S2).
By adding two equivalents of NaBArF
(BArF = [3,5-(CF3)2C6H3]4B) into the above
[Rh(Aza-CrownPhos)2]BF4 system (state I),
[Rh(Aza-CrownPhos·Na+)2]BF4 (state II) was
formed spontaneously because of the stronger
association ability of Na+ with aza-crown ethers.
1H and 31P NMR studies showed the disassoci-
ation of Rh+ from the aza-crown ether moieties,
thus leading to the decomposition of the
sandwich structure (for details see Figures S4
and S5). In addition, the peak at m/z 434.44221
in the ESI HR-MS spectrum strongly supports
the formation of [Rh(Aza-CrownPhos·Na)2]3+
with the experimental isotopic distribution
neatly matching the calculated one (see Fig-
ure S3). The Job plot analysis of state I gave
a host/guest molar ratio of 1:2 for Na+, thereby
indicating that Na+ cations were bound to the
aza-crown ethers (see Figure S7 in the Support-
ing Information). Thus, the switching from state
Scheme 1. Coordination structures of Rh/Aza-CrownPhos complexes in different states.
the precatalyst (Scheme 2Ab) showed that the signals of
NCH2 protons on Aza-CrownPhos shifted downfield because
of the coordination of phosphorus ligand to rhodium. How-
ever, no obvious chemical shift changes were observed for the
OCH2 signals, thus revealing that the coordinated rhodium
cation in the precatalyst was not bound to the aza-crown ether
ring.[10] The 31P NMR spectroscopic analysis (Scheme 2B b)
showed two broad absorption peaks, which indicates that the
precatalyst has two structural isomers which differ in the
relative orientation of the two coordinated monodentate Aza-
CrownPhos ligands. The two isomers interconverted at room
temperature and showed broad absorptions both in 1H NMR
and 31P NMR spectra.[11a]
Subsequently, the precatalyst in CD2Cl2 was stirred under
hydrogen gas for 1 hour to remove norbornadiene.[11a–d] As
shown in Scheme 2Ac, the 1H NMR spectrum showed an
obvious signal splitting and broadening in the region d = 2.8–
4.5 ppm, and are ascribed to the protons of aza-crown ether
units. This result suggests a strong complexation between the
aza-crown ether ring and cationic rhodium.[5] More impres-
sively, the signals of the aromatic protons were separated into
12 sets with some of them shifted upfield. By following 1H-1H
1
COSY and H-1H NOESY NMR investigations, the signals
can be assigned (for details, see Figure S1) to the 12 different
protons on the naphthalene unit, thus implying that the two
naphthalene units are magnetically equivalent. Meanwhile,
the obvious upfield shifts of the aromatic proton signals
suggests that there is strong p–p stacking between the two
naphthalene units.[12] In addition, only one doublet splitting
signal was shown in the 31P NMR spectrum (d = 144.3 ppm,
Scheme 2. 1H NMR spectra (A; 600 MHz, CD2Cl2, 298 K, 10 mm) and
31P NMR spectra (B; 500 MHz, CD2Cl2, 298 K, 50 mm) of a) Aza-
CrownPhos; b) [Rh(Aza-CrownPhos)2(NBD)]BF4; c) [Rh(Aza-Crown-
Phos)2]BF4; d) [Rh(Aza-CrownPhos·Na+)2]BF4; e) [Rh(Aza-Crown-
Phos)2]BF4 regenerated from d) by adding 2 equivalents of cryptand-
[2,2,2]. NBD=norbornadiene, NBE=norbornene.
J
PRh = 319 Hz; Scheme 2Bc), and indicates that the two Aza-
CrownPhos units in this complex are equal with respect to the
chemical environment. From all these results, it is naturally
concluded that both aza-crown ether units participate in the
association with the rhodium cation, and a sandwich structure
2
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I to state II is achieved by the
triggering of alkali metal cations.
Switching from state II to state I
was further realized by removal of
Na+ cations from their aza-crown
ether complexes. Cryptand[2,2,2] is
a better host molecule for Na+ as
compared with aza-18-crown-6.[6]
1
The H and 31P NMR spectra indi-
cate that the complex [Rh(Aza-
CrownPhos)2]+ (state I) was regen-
erated (Scheme 2e versus Sche-
me 2c) upon the addition of
2 equivalents of the cyptand[2,2,2].
In addition, a strong peak for the
[cryptand·Na]+
complex
was
detected by ESI HR-MS (see Sec-
tion S4.4). Therefore, all the results
obtained demonstrate that the cat-
alyst transformation cycle between
state I and state II can be success-
fully achieved by the modulation
through host-guest interactions.
Having established the efficient
and reversible transformations of
the rhodium catalytic system, we
then applied this switchable rho-
dium catalyst to the asymmetric
hydrogenation of methyl a-acetam-
Scheme 3. Control of the reversible switching in the asymmetric hydrogenation of 1a by using the
Rh/Aza-CrownPhos catalyst.
ido cinnamate (1a; Scheme 3). As expected, the Rh/Aza-
CrownPhos complex in state I is almost inactive, thus giving
less than 1% conversion in 4 hours. In contrast, in state II, full
conversion with 96% ee was obtained under the same
reaction conditions. As shown in Scheme 3, two cycles of
ON/OFF catalysis were achieved by sequentially adding Na+
cations and cyptand[2,2,2] as additives (for details see Section
S5).[13]
Table 1: Switchable asymmetric hydrogenation of dehydroamino acid
esters by using Rh/Aza-CrownPhos catalyst.[a]
Entry
Substrate
ON state
ON state
OFF state
Conv. [%][b]
ee [%][b]
Conv. [%][b]
To further understand this cation-triggered asymmetric
hydrogenation, the effects of different alkali metal cations
were investigated. A great increase of enantioselectivity is
observed for all cations tested, while the catalytic activity
varied extremely. It was found that faster rates and higher
conversions were obtained for smaller alkali metal cations,
Li+ and Na+, in comparison to those using K+ and Cs+. The
Job plot analyses of state I gave a binding ratio of 1:1 for both
K+ and Cs+, thus differing from that of Na+. These results
show the importance of the matching between cation and aza-
crown ether moiety to the efficacy of supramolecular
modulation through cation–macrocycle interactions (for
details see Figure S6 in the Supporting Information).[6,7,14]
Finally, we investigated the asymmetric hydrogenation of
a variety of dehydroamino acid esters by using this switchable
catalytic system (Table 1). By using NaBArF as a trigger, all
the hydrogenation reactions proceeded smoothly with com-
plete conversions. Excellent ee values (92–98% ee) were
obtained, and are comparable to the reported ones.[9c,e]
Higher ee values were provided by ortho-substituted sub-
strates in comparison to those for meta- and para-substituted
ones. However, in the absence of NaBArF, the catalyst stayed
1
2
3
4
5
6
7
8
1a (R=H)
100
100
100
100
100
100
100
100
98
98
95
95
96
95
95
92
<1
<1
<1
<1
<1
<1
<1
<1
1b (R=2-OMe)
1c (R=3-OMe)
1d (R=4-OMe)
1e (R=2-Br)
1 f (R=3-Cl)
1g (R=4-F)
1h (R=4-Me)
[a] Reaction conditions: substrate (0.50 mmol), (S)-Aza-CrownPhos
(1.0ꢀ10À3 mmol), [Rh(NBD)2]BF4 (0.5ꢀ10À3 mmol), NaBArF
(1.0ꢀ10À3 mmol), 2 mL CH2Cl2, 15 atm H2, 4 h, RT. [b] Conversions and
ee values were determined by GC using a Varian Chirasil-l-Val column
CP7495 (25 mꢀ0.25 mm).
in its inactive state, and conversions of less than 1% were
observed. Therefore, this switchable catalytic system has
shown a remarkable difference in activity between its ON and
OFF state for asymmetric hydrogenation (with an ON/OFF
ratio > 100).
In summary, we have synthesized a new kind of phos-
phoramidite ligand modified by an aza-crown ether. The
catalytic activity of its rhodium complex can be switched
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between the ON and OFF state through Na+-triggered
modulation using host–guest interactions. In the ON state,
100% conversion and excellent enantioselectivity (up to 98%
ee) are obtained in the asymmetric hydrogenation of dehy-
droamino acid esters. In contrast, in the OFF state, the
catalyst is almost inactive, thus giving less than 1% con-
version for all screened substrates. NMR spectroscopy and
HR-MS studies and a set of control experiments indicate that
the reversible cation–macrocycle interactions are responsible
for this interesting activity-switching cycle (ON/OFF ratio
> 100). Further applications of this kind of chiral switchable
catalysts in other catalytic reactions are currently in progress.
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Received: December 1, 2014
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Keywords: asymmetric catalysis · cations · crown compounds ·
host-guest systems · hydrogenation
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Asymmetric Catalysis
G.-H. Ouyang, Y.-M. He, Y. Li, J.-F. Xiang,
Q.-H. Fan*
&&&& — &&&&
Cation-Triggered Switchable Asymmetric
Catalysis with Chiral Aza-CrownPhos
Quick on the trigger: A new kind of
and “OFF” in the asymmetric hydroge-
phosphoramidite ligand, modified by an
aza-crown ether, has been designed and
synthesized. The activity of its rhodium
catalyst can be reversibly switched “ON”
nation of dehydroamino acid esters by
modulations derived from host–guest
interactions.
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