Photoswitchable Chiral Phase Transfer Catalyst

Article abstract of DOI:10.1021/acscatal.1c00057

Azo-crown ether-based photoswitching chiral phase transfer catalysts have been developed to control the catalytic activity by photoirradiation. Azobenzene binaphthyl crown ether (ABCE) can switch its reactivity and selectivity through structural transform

Full text of DOI:10.1021/acscatal.1c00057

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Letter
Photoswitchable Chiral Phase Transfer Catalyst
Masaru Kondo,* Kento Nakamura, Chandu G. Krishnan, Shinobu Takizawa, Tsukasa Abe,
and Hiroaki Sasai*
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ABSTRACT: Azo-crown ether-based photoswitching chiral phase transfer catalysts have been developed to control the catalytic
activity by photoirradiation. Azobenzene binaphthyl crown ether (ABCE) can switch its reactivity and selectivity through structural
transformation of the crown ether moiety induced by E/Z photoisomerization of azobenzene. (Z)-ABCE promoted enantioselective
alkylation of the glycine Schiff base to afford chiral amino acid derivatives in good yields with high enantiomer ratios. In contrast,
(E)-ABCE hindered the reaction progress under the same conditions.
KEYWORDS: photoresponsive chiral molecule, crown ether, azobenzene, enantioselective organocatalysis, phase transfer catalyst
photoswitchable catalyst, involving a catalytically active
Asite and a photoresponsive unit, can be used to iteratively
modulate catalytic activity through reversible switching of
steric and electronic properties with photoisomerization using
light as a clean external stimulus. Because photoswitchable
catalysts are unique, many efforts have been made to develop
photoswitchable organo- and organometallic catalysts.1 Photo-
switchable chiral catalysts allow the switching of reactivity and
enantioselectivity under photoirradiation.2 It is crucial to
introduce a photoresponsive unit to the chiral skeleton and
appropriately position each unit to achieve a high selectivity
and significant photomodulation of the reaction outcome. For
highly sophisticated catalytic systems, multistep preparation
and expensive reagents are often required. Among the
photoresponsive units, azobenzene, an easily available and
well-known photochromic molecule, rapidly and reversibly
switches the E/Z isomer under exposure to UV or visible light,
resulting in significant changes in its dipole moment and
conformation.3 Azo-crown ether,4 consisting of azobenzenes
and crown ethers, is one of the most important photo-
switchable supramolecules. Shinkai et al. developed various
elegant azo-crown ethers such as the butterfly crown ether4c
and macrocycle units containing azobenzene and crown
ether,4d which can switch their geometry, extraction, and
coordination ability with alkali metals through the photo-
isomerization of azobenzene units (Figure 1A). As a pioneering
report on photocontrol of the catalytic activity of an azo-crown
ether, in 2003, Cacciapaglia et al. reported the ethanolysis of
anilides using a butterfly crown ether.5a Inspired by these
reports on the unique properties and catalytic activities of azo-
crown ethers, we focused on azo-crown ether as a photo-
responsive unit. Despite many examples of azobenzene-
containing achiral catalysts,1j,5 there is only one report on
chiral photoswitchable catalysts employing azobenzene (salen
complexes for asymmetric sulfoxidation).2l
In this work, to introduce the functionalities of both the azo-
crown ether and a chiral unit to a catalytic molecule, a chiral
crown ether consisting of BINOL and an azobenzene
framework was designed (Figure 1B). We envisioned that an
azobenzene binaphthyl crown ether (ABCE) with an
appropriate ring structure can control the reactivity and
enantioselectivity as a chiral phase transfer catalyst (PTC)6
through geometry modulation attributed to the reversible
photoisomerization of azobenzene. To verify our catalyst, we
demonstrated light-controlled reactivity of the enantioselective
reaction,2c,h which is still less explored compared to the effect
of photocontrol over the stereochemical outcome.
Received: January 6, 2021
Revised: January 21, 2021
Published: January 26, 2021
https://dx.doi.org/10.1021/acscatal.1c00057
© 2021 American Chemical Society
ACS Catal. 2021, 11, 1863−1867
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Letter
a
Scheme 1. Synthesis of ABCE 4
Figure 1. (A) Previously reported azo-crown ether: early reports4c,d
and catalytic application.5a (B) This work: ABCE as a photo-
switchable chiral PTC.
a
Reaction conditions: (a) 2-(2-iodoethoxy)ethanol, 2-iodoethanol, or
2-(2-(2-iodoethoxy) ethoxy)ethanol (3.0 equiv), K2CO3 (4.0 equiv),
60 °C, DMF; (b) TsCl (2.5 equiv), Et3N (4.0 equiv), DMAP (20 mol
%), 0 °C to rt, DCM; (c) 2,2′- or 3,3′-dihydroxy azobenzene (1.05
equiv), Cs2CO3 (3.3 equiv), dibenzo-18-crown-6 (33 mol %), 80 °C,
DMF.
We commenced our study by establishing a general
procedure and a library of ABCE 4. ABCEs 4a−4f were
synthesized from BINOL or 3,3′-disubstituted BINOL 1 in
three steps: (a) alkylation and (b) tosylation, followed by (c) a
ring-closing reaction with the corresponding dihydroxyazo-
benzene (Scheme 1). Then, we performed photoisomerization
of the representative ABCE 4a by UV−vis spectroscopy as
shown in Figure 2A,B. As expected, UV irradiation at 365 nm
rapidly decreased the π → π* band with a slight increase of the
n → π* transition for the formation of the Z isomer of 4a.7
Visible light irradiation at 405 nm reversibly led to the E
isomer of 4a. Moreover, we quantified the photostationary
with low enantioselectivities (52:48 and 46:54 er) (entries 5
and 6). The reaction catalyzed by 4d bearing phenyl groups at
the 3,3′-position of the binaphthyl unit exhibited a lower yield
and er (entries 7 and 8). When 3,3′-diiodinated ABCE 4e was
used, the switching reaction rate was moderate (67% yield vs
29% yield), probably owing to the weak coordination of the
iodo group with potassium cations (entries 9 and 10).12 In
addition, the enantioselectivity of 7a dropped from 81:19 to
64:36 er without photoirradiation. When we employed ABCE
4f derived from 3,3′-dihydroxyazobenzene, the desired product
was obtained in 42% and 26% yields with no enantioselectivity
under UV and dark conditions, respectively (entries 11 and
12).13 In the absence of PTC 4, a trace amount of the desired
product was detected (entries 13 and 14).
Under the reaction conditions shown in Table 1, we
examined the photomodulating reactivity of enantioselective
alkylation using a variety of benzyl bromides 6 in the presence
of ABCE 4a (Table 2). Other para-substituted substrates 6b
(Ar = 4-BrC6H4) and 6c (Ar = 4-MeC6H4) were tolerable,
providing the corresponding alkylated products 7b and 7c in
61% and 54% yields with 90:10 and 86:14 er, respectively,
under UV irradiation (entries 3 and 5). In contrast, non-UV
irradiation significantly suppressed these transformations,
recovering the corresponding substrates (entries 4 and 6).
Likewise, when we used meta-substituted benzyl bromides 6d
and 6e (6d: Ar = 3-ClC6H4; 6e: Ar = 3-BrC6H4) as a substrate,
these reactions accelerated to afford the corresponding
products 7d and 7e in higher yields than those obtained
under the dark condition (7d: 77% yield and 86:14 er; 7e: 69%
1
states of 4a by H NMR analysis in toluene-d8. Before UV
irradiation, the E/Z ratio was calculated to be 84:16 on the
basis of the azobenzene-associated proton signals (Figure 2C).
UV irradiation (5 min, 365 nm) promoted photoisomerization
from the E to Z isomer, leading to E/Z = 12:88 (Figure 2D).
Reversible photoisomerization from Z to E was observed, and
E/Z = 68/32 was obtained under exposure to visible light at
405 nm (Figure 2E).8,9
To investigate the catalytic activity of the photoswitchable
chiral PTC with or without UV, we applied ABCE 4 to the
enantioselective alkylation reaction of glycine Schiff base 5a
with 4-chlorobenzyl bromide 6a in toluene, affording unnatural
amino acid precursor 7a under the modified reaction
conditions reported previously (Table 1).6d,h Gratifyingly,
ABCE 4a (1 mol %) switched reactivity, affording the desired
product 7a in 75% yield, 92:8 er under UV irradiation during
the reaction, and 11% yield, 92:8 er in the absence of UV
(entries 1 and 2).10,11 Using shorter-chained ABCE 4b, 7a was
obtained in 45% yield and 58:42 er at 365 nm, and the reaction
hardly proceeded without UV irradiation (entries 3 and 4).
The use of longer-chained ABCE 4c afforded higher yields and
the opposite enantiomer of 7a under UV irradiation, albeit
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Table 2. Evaluation of Photo-Controlled Reactivity of
Enantioselective Alkylation of 5a with 6 Using ABCE 4a
a
d
entry
6
Ar
light
UV
time (h) yield (%)
er
b
1
2
3
4
5
6
7
8
6a
4-ClC6H4
6
7
6
5
7
4
4
75
11
61
92:8
92:8
90:10
c
b
6b
6c
6d
6e
6f
4-BrC6H4
4-MeC6H4
3-ClC6H4
3-BrC6H4
2-MeC6H4
C6H5
UV
UV
UV
UV
UV
UV
c
c
trace
b
54
86:14
trace
b
77
86:14
90:10
86:14
c
10
b
9
69
c
c
c
10
11
12
13
14
trace
b
71
85:15
80:20
trace
b
6g
63
trace
a
Reaction conditions: 5a (0.2 mmol), 6 (0.6 mmol), KOH (1.0
mmol), and ABCE 4a (1 mol %) in toluene (0.5 mL) at 0 °C.
b
c
Isolated yield. 1,3,5-Trimethoxybenzene was used as an internal
d
standard. Enantiomeric ratio was determined by HPLC analysis.
afford the corresponding product 7f in 71% yield and 85:15 er
(entry 11). Sterically less bulky benzyl bromide 6g was
converted into the alkylated product 7g in 63% yield and 80:20
er (entry 13). In the absence of UV, trace amounts of products
7f and 7g were detected by TLC monitoring and crude H
NMR analysis (entries 12 and 14).
Figure 2. Changes in the UV−vis spectra (in CHCl3, 100 μM) and
1H NMR spectra (in toluene-d8, 8 mM) of 4a upon photoirradiation.
1
a
Table 1. Screening of ABCE 4
To gain further insights into the control over the reactivity
of the enantioselective alkylation of 5a with 6a under
photoirradiation, kinetic studies were performed using photo-
responsive ABCE 4a (Figure 3). While UV-treated 4a
b
c
entry
ABCE (wavelength)
E/Z ratio
yield (%)
er
1
2
3
4
5
6
7
8
4a (365 nm)
4a (no light)
4b (365 nm)
4b (no light)
4c (365 nm)
4c (no light)
4d (365 nm)
4d (no light)
4e (365 nm)
4e (no light)
4f (365 nm)
4f (no light)
− (365 nm)
− (no light)
12:88
84:16
12:88
91:9
15:85
88:12
13:87
93:7
75 (75)
11
45
trace
59
80
65
trace
67
29
42
92:8
92:8
58:42
52:48
46:54
66:34
Figure 3. Reaction profile of 5a and 6a promoted by (A) 1 mol % of
ABCE 4a (4 mM) (orange: in the dark; blue: under photoirradiation
at 365 nm); (B) photoacceleration of the reaction using ABCE 4a (4
mM) under the dark followed by UV (365 nm) irradiation.
9
11:89
92:8
6:94
81:19
64:36
50:50
50:50
10
11
12
13
14
85:15
26
trace
trace
indicated a higher reactivity over nonphotoirradiated con-
ditions (7a: 75% vs 11% yield) (Figure 3A), no acceleration
was observed when using 18-crown-6 as a nonphotoresponsive
crown ether under photoirradiation (see Figure S7).
Considering these results, we can rule out the possibility of
thermal- and photoactivation of substrates by LED irradiation.
The reaction rate would increase due to the geometry
transformation of the crown ether ring on 4a induced by
photoisomerization of azobenzene.14 The Z isomer of 4a
would be highly suitable for the alkylation reaction of 5a with
6a over the E isomer. Next, acceleration and deceleration of
a
Reaction conditions: 5a (0.2 mmol), 6a (0.6 mmol), KOH (1.0
b
mmol), and ABCE 4 (1 mol %) in toluene (0.5 mL) at 0 °C. 1,3,5-
Trimethoxybenzene was used as an internal standard. Isolated yield is
shown in parentheses. Enantiomeric ratio was determined by HPLC
analysis (DAICEL Chiralpak IE).
c
yield and 86:14 er) (entries 7−10). Using 2-methyl-benzyl
bromide 6f, the UV-mediated reaction smoothly proceeded to
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Letter
the enantioselective alkylation of 5a with 6a were examined in
the presence of 4a by switching wavelengths during the
reaction process. It is noted that the reaction was accelerated
by switching the E to Z isomer by UV irradiation after 1 h in
the dark (7a: 68% vs 11% yield) (Figure 3B). Furthermore,
visible light irradiation at 405 nm moderately decelerated the
alkylation within the course of the reaction after 1 h under UV
light (7a: 58% vs 71% yield)15,16 (see Figure S8).
and JST CREST Grant Number JPMJCR20R1, Iketani Science
and Technology Foundation, Research Foundation for Opto-
Science and Technology, Daiichi Sankyo Foundation of Life
Science, Hoansha Foundation. We also acknowledge the
Artificial Intelligence Research Center of ISIR and the
technical staff of the Comprehensive Analysis Center of
ISIR, Osaka University (Japan).
In conclusion, we demonstrated chiral BINOL-derived azo-
crown ethers control the reactivity for the enantioselective
alkylation reaction of glycine Schiff base with benzyl bromides
as a photoswitchable PTC with and without UV irradiation.
The alkylation reaction was accelerated and decelerated by in
situ photoisomerization of the catalyst at different wavelengths.
Further investigations of the mechanism of switching reactivity
in more detail and applications of the photoswitchable PTCs
to modulate stereoselectivity are being carried out in our
group.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acscatal.1c00057.
■
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(PDF)
AUTHOR INFORMATION
Corresponding Authors
■
Masaru Kondo − The Institute of Scientific and Industrial
Research, Osaka University, Ibaraki-shi, Osaka 567-0047,
Japan; orcid.org/0000-0001-6299-5718;
Email: mkondo@sanken.osaka-u.ac.jp
Hiroaki Sasai − The Institute of Scientific and Industrial
Research, Osaka University, Ibaraki-shi, Osaka 567-0047,
Japan; orcid.org/0000-0002-7221-488X; Email: sasai@
sanken.osaka-u.ac.jp
Authors
Kento Nakamura − The Institute of Scientific and Industrial
Research, Osaka University, Ibaraki-shi, Osaka 567-0047,
Japan
Chandu G. Krishnan − The Institute of Scientific and
Industrial Research, Osaka University, Ibaraki-shi, Osaka
567-0047, Japan
Shinobu Takizawa − The Institute of Scientific and Industrial
Research, Osaka University, Ibaraki-shi, Osaka 567-0047,
Japan; orcid.org/0000-0002-9668-1888
Tsukasa Abe − The Institute of Scientific and Industrial
Research, Osaka University, Ibaraki-shi, Osaka 567-0047,
Japan; orcid.org/0000-0002-2400-3797
́
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Complete contact information is available at:
https://pubs.acs.org/10.1021/acscatal.1c00057
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
The authors gratefully acknowledge the generous support from
JSPS KAKENHI Grant Numbers JP18K14221 in Early-Career
Scientists and JP18KK0154 in Promotion of Joint Interna-
tional Research (Fostering Joint International Research(B))
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(k) Pizzolato, S. F.; Stacko, P.; Kistemaker, J. C. M.; van Leeuwen, T.;
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+
calculated for C40H36KN2O6 : 679.2205; observed: 679.2206) (see
Figure S2).
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of 5a and 6a. The reaction using 4g and 4h afforded the desired
product 7a in 62% and 77% yields with 70:30 and 51:49 er under UV
irradiation, respectively, while the reaction was significantly sup-
pressed to provide a trace amount of the product without
photoirradiation.
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(14) We presumed that the different reaction rate between the E and
Z isomers of 4a was probably caused by the different extraction ability
of solid KOH. The Z isomer of 4a can efficiently form a catalytic
active species owing to the stronger extraction ability of the potassium
cation over the E isomer. This hypothesis would be supported by
DFT studies (see Figure S9). However, we cannot rule out the
possibility that the steric difference between both the isomers
modulated the reactivity of the nucleophilic substitution reaction
process.
(15) The reaction was not completely turned off due to the
moderate photoisomerization from the Z to E isomer of 4a at 405 nm.
Although other wavelengths of LED and solvents were examined, they
were ineffective for the photoisomerization from the Z to E isomer of
4a (see Tables S1 and S2). Functionalization on the azobenzene
moiety for the stabilization of the E isomer or the destabilization of
the Z isomer would sharply control the reactivity under UV and
visible light.
̀
Planes, L.; Rodríguez-Esrich, C.; Pericas, M. A. Photoswitchable
Thioureas for the External Manipulation of Catalytic Activity. Org.
Lett. 2014, 16, 1704−1707. (f) Arif, T.; Cazorla, C.; Bogliotti, N.;
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Saleh, N.; Blanchard, F.; Gandon, V.; Metivier, R.; Xie, J.; Voituriez,
A.; Marinetti, A. Bimetallic Gold(I) Complexes of Photoswitchable
Phosphines: Synthesis and Uses in Cooperative Catalysis. Catal. Sci.
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(6) For selected reports on chiral PTC, see: (a) Ooi, T.; Maruoka,
K. Recent Advances in Asymmetric Phase-Transfer Catalysis. Angew.
Chem., Int. Ed. 2007, 46, 4222−4266. (b) Shirakawa, S.; Maruoka, K.
Recent Developments in Asymmetric Phase-Transfer Reactions.
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Kumatabara, Y.; Shirakawa, S. A New Generation of Chiral Phase-
Transfer Catalysts. Org. Biomol. Chem. 2016, 14, 5367−5376. (d) Ooi,
T.; Uematsu, Y.; Maruoaka, K. Evaluation of the Efficiency of the
Chiral Quaternary Ammonium Salt β-Np-NAS-Br in the Organic-
Aqueous Phase-Transfer Alkylation of a Protected Glycine Derivative.
Adv. Synth. Catal. 2002, 344, 288−291. (e) Schettini, R.; Sicignano,
M.; De Riccardis, F.; Izzo, I.; Della Sala, G. Macrocyclic Hosts in
Asymmetric Phase-Transfer Catalyzed Reactions. Synthesis 2018, 50,
4777−4795 For selected publications on the chiral crown ether
catalyzed asymmetric reaction, see:. (f) Cram, D. J.; Sogah, G. D. Y. J.
Chiral Crown Complexes Catalyse Michael Addition Reactions to
Give Adducts in High Optical Yields. J. Chem. Soc., Chem. Commun.
1981, 625−628. (g) Akiyama, T.; Hara, M.; Fuchibe, K.; Sakamoto,
S.; Yamaguchi, K. Synthesis of a Novel Crown Ether Derived from
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(16) The range of er for product 7a was between 90:10 and 92:8
after photoacceleration and deceleration.
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ACS Catal. 2021, 11, 1863−1867