Supramolecular Photocatalysis by Utilizing the Host–Guest Charge-Transfer Interaction: Visible-Light-Induced Generation of Triplet Anthracenes for [4+2] Cycloaddition Reactions

Article abstract of DOI:10.1002/anie.201916732

Supramolecular photocatalysis via charge-transfer excitation of a host–guest complex was developed by use of the macrocyclic boronic ester [2+2]_BTH-F containing highly electron-deficient difluorobenzothiadiazole moieties. In the presence of a c

Full text of DOI:10.1002/anie.201916732

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Accepted Article
Title: Supramolecular Photocatalysis Utilizing Host-Guest Charge-
Transfer Interaction: Visible Light-Induced Generation of Triplet
Anthracenes for [4+2] Cycloaddition Reactions
Authors: Tatsuhiro Uchikura, Mari Oshima, Minami Kawasaki, Kohei
Takahashi, and Nobuharu Iwasawa
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201916732
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10.1002/anie.201916732
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Supramolecular Photocatalysis Utilizing Host-Guest Charge-
Transfer Interaction: Visible Light-Induced Generation of Triplet
Anthracenes for [4+2] Cycloaddition Reactions
Tatsuhiro Uchikura,[a,b] Mari Oshima,[a] Minami Kawasaki,[a] Kohei Takahashi,[a] and Nobuharu
Iwasawa*[a]
Abstract:
A
supramolecular photocatalysis via charge-transfer
induced generation of not the cation radical but the triplet state
of anthracenes via charge-transfer excitation of a host-guest
complex derived from [2+2]BTH-F (Figure 1). Subsequent [4+2]
cycloaddition with dienes and alkenes and specific activation of
the guest molecule are also demonstrated.
excitation of a host-guest complex was developed by use of the
macrocyclic boronic ester [2+2]BTH-F containing highly electron-
deficient difluorobenzothiadiazole moieties. In the presence of a
catalytic amount of [2+2]BTH-F, triplet excited state of anthracene was
generated
from
charge-transfer
excited
state
of
*
anthracene@[2+2]BTH-F by visible light irradiation, and cycloaddition
of the excited anthracene with several dienes and alkenes
proceeded in a [4+2] manner in high yields.
• –
*
hν
• +
donor-acceptor
interaction
triplet excited state
Supramolecular catalysis has attracted considerable attention
due to the unique reactivities utilizing characteristic properties of
the host molecules.[1] Although there are many reports of
catalytic acceleration of reactions by inclusion of guest
substrates in host molecules, supramolecular photocatalysis has
not been explored extensively and is mainly limited to two types,
that is, i) direct excitation of included guest molecules, which
would lead to specific reactions inside the hosts, and ii) use of
host molecules bearing photo-sensitizing moieties, which would
induce electron transfer or energy transfer reactions with guest
molecules.[2-4]
Charge-transfer interaction between aromatic molecules,
which often has absorption in visible light region, is frequently
observed in host-guest complexes.[5] However, to the best of our
knowledge, there is no obvious example of utilization of the
charge-transfer excitation within discrete host-guest complexes
for catalytic carbon-carbon bond formations.[6]
radical ion pair
F
F
O
O
O
B
B
B
O
N
N
S
guest
F
F
O
B
O
O
O
N
N
S
[2+2]BTH-F
Figure 1. Schematic representation of the concept.
When anthracene was mixed with [2+2]BTH-F in
dichloromethane, the solution turned yellow and new
absorption band was observed at 400-500 nm by UV-Vis spectra
(Figure 2a, b). Furthermore, according to the DFT calculation of
the molecular orbital of anthracene@[2+2]BTH-F, HOMO was
a
Previously, we have reported the electrophilic activation of
aromatic aldehydes via donor-acceptor interaction of aromatic
(a)
F
F
O
O
O
B
B
B
B
O
molecules by use of the macrocyclic boronic ester [2+2]BTH-F
,
N
N
S
[2+2]BTH-F
which has highly electron-deficient difluorobenzothiadiazole
moieties.[7] We expected that visible light-irradiation of [2+2]BTH-F
including an appropriate electron-rich aromatic guest molecucle
would induce charge-tranfer excitation to realize supramolecular
photocatalytic reactions. In this paper, we report a successful
realization of this approach, which has enabled the visible light-
F
F
CH2Cl2
O
O
O
O
N
N
S
Ka = 97 M–1
Yellow
(b)
(c)
0.5
0.4
0.3
0.2
0.1
0
[2+2] only
Ant only
[a]
Dr. T. Uchikura, M. Oshima, M. Kawasaki, Dr. K. Takahashi, Prof. N.
Iwasawa
Ant+[2+2]
Department of Chemistry, Tokyo Institute of Technology
O-okayama, Meguro-ku, Tokyo 152-8551 (Japan)
E-mail: niwasawa@chem.titech.ac.jp
[b]
Dr. T. Uchikura
300
400
500
600
Present address: Department of Chemistry, Faculty of Science,
Gakushuin University, Mejiro, Toshima-ku, Tokyo 171-8588 (Japan)
wavelength / nm
Figure 2. Properties of anthracene@[2+2]BTH-F. (a) Color change via inclusion.
(b) UV-Vis spectra of anthracene@[2+2]BTH-F in CH2Cl2. (c) Frontier orbitals of
anthracene@[2+2]BTH-F; top: HOMO, bottom: LUMO (M062X/6-31G).
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201916732
Angewandte Chemie International Edition
COMMUNICATION
distributed on anthracene and LUMO was distributed on the
benzothiadiazole ring of [2+2]BTH-F (Figure 2c). Therefore,
irradiation of 400-500 nm visible light would induce the charge-
[a] NMR yield. Performed with 1 (9 µmol), 2a (180 µmol), and additives in
CH2Cl2 (0.5 mL). [b] Based on the amount of anthracene used.
transfer from anthracene to [2+2]BTH-F
.
We then investigated the photocycloaddition reaction of
anthracene (1) with 1,3-cyclohexadiene (2a)[8] in the presence of
[2+2]BTH-F under irradiation of visible light (450 nm) (Table 1). A
dichloromethane solution of anthracene with 20 equivalents of
1,3-cyclohexadiene was irradiated with visible light in the
presence of 0.1 equivalent of [2+2]BTH-F. Interestingly, [4+2]
cycloadduct 3a of anthracene with 2a was obtained almost
quantitatively accompanied by a trace amount of anthracene
[4+4] dimer 5,[9] and other cycloadducts such as [4+4]
cycloadduct 4a of anthracene with 2a and self-dimers of 2a were
not obtained at all (Table 1, entry 1).[10]
Then we examined the generality of alkenes (Table 2). The
dienes such as 2,3-dimethyl-1,3-butadiene (2b) and isoprene
(2c) reacted efficiently in a [4+2] cycloaddition manner. Ethyl
acrylate (2d) as an electron-deficient alkene was also applicable
and gave the product in good yield although it required much
longer irradiation. Styrene derivatives (2f, 2h-2l) were also
employable. In particular, electron-poor styrenes reacted faster.
p-Methoxystyrene (2g) gave the product in low yield due to the
competitive polymerization. A simple alkene (2e) was not
applicable to this reaction.
As control experiments, several reactions were examined.
First, in the absence of [2+2]BTH-F or in the presence of
monomeric pinacol ester of difluorobenzothiadiazolediboronic
acid 6, which didn’t form the charge-transfer complex with
anthracene, the cycloaddition didn’t proceed at all by the
irradiation with 450 nm light (entries 2,3). Thus, the host
structure of [2+2]BTH-F was necessary for the reaction. Next,
competitive experiment by naphthalene (Ka = 103 M–1), which
has almost the same association constant with anthracene (Ka =
97 M–1), was examined, and addition of 5 equivalents of
naphthalene retarded the reaction considerably (entry 4). Thus,
inclusion of anthracene into [2+2]BTH-F was found to be essential
for the photocycloaddition reaction.
Table 2. Generality of alkenes.[a]
0.05 equiv. [2+2]BTH-F
R1 R2
R1
hν (450 nm)
+
R2
CH2Cl2, rt
1
2
20 equiv.
3
C4H9
2e
COOEt
2a
2b
2c
2d
quant (12 h)
95% (36 h)
91% (6 h)[b]
81% (139 h)
0% (24 h)
Table 1. Photocycloaddition of anthracene (1) and 1,3-cyclohexadiene
(2a).[a]
F
F
X = H (2f):
93% (30 h)
17% (20 h)[c]
97% (36 h)
88% (30 h)
95% (30 h)
96% (11 h)
OMe (2g):
Cl (2h):
F
F
F
Br (2i):
X
tBu (2j):
CF3 (2k):
2l
additive
1
3a
4a
99% (4 h)
hν (450 nm)
+
CH2Cl2, rt, 6 h
[a] Isolated yield. Performed with 1 (90 µmol), alkenes (1.8 mmol), and
[2+2]BTH-F (4.5 µmol) in CH2Cl2 (0.7 mL). [b] Two positional isomers were
obtained in 7 : 3 ratio. [c] NMR yield.
5
2a
20 equiv.
Next, generality of the anthracene derivatives was
investigated (Table 3). The reaction proceeded as well with
Yield
4a
several substituted anthracenes which are included in [2+2]BTH-F
,
3a
entry
1
additive
5[b]
but the reactions were slower as the association constants of
anthracene derivatives became smaller (2-tert-butylanthracene
(7): 80% for 14 h, Ka = 71 M–1; 2,6-di-tert-butylanthracene (8):
82% for 42 h, Ka = 23 M–1). Furthermore, the photocycloaddition
didn’t proceed using 9-phenylanthracene (9), which was not
included in [2+2]BTH-F as judged by ITC measurement. This
correlation is consistent with the assumption that the included
guest molecule was selectively activated (Table 1).[11]
0.1 equiv. [2+2]BTH-F
>95%
0%
0%
1%
none
2
0%
0%
0%
0%
S
N
N
pinB
Bpin
3
0%
F
F
6
0.2 equiv.
Table 3. Generality of anthracene derivatives.[a]
0.1 equiv. [2+2]BTH-F
4
47%
0%
0%
5.0 equiv.
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10.1002/anie.201916732
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COMMUNICATION
excited species of anthracene which favored [4+2] cycloaddition
pathway was selectively generated in the presence of [2+2]BTH-
[8]
It is assumed that the former is the singlet state[8a,b] and the
0.05 equiv. [2+2]BTH-F
hν (450 nm)
.
F
+
latter is the triplet state of anthracene.
Ant Derivative
CH2Cl2, rt
R
2a
[4+2]
20 equiv.
1
hν (365 nm)
+
Ant derivative
(association const.)
yield
(time)
Ant derivative
(association const.)
yield
(time)
CH2Cl2, rt, 6 h
3a
14%
4a
53%
5
18%
tBu
2a
20 equiv.
quant
(12 h)
87%[c]
(42 h)
tBu
1
8
(97 M–1
)
Scheme 2. Photocycloaddition of anthracene (1) with 1,3-cyclohexadiene (2a)
by irradiation of 365 nm UV lamp.
(23 M–1
)
Ph
tBu
80%[b]
(14 h)
0%
(6 h)
To confirm the generation of the triplet state of anthracene,
the quenching experiment was carried out. Indeed, the reaction
was completely inhibited in the presence of 5.0 equivalents of
7
9
(71 M–1
)
(~0 M–1
)
cyclooctatetraene, which is known as
a triplet quencher
[a] Isolated yield. Performed with anthracene derivatives (90 µmol), 2a
(1.8 mmol), and [2+2]BTH-F (4.5 µmol) in CH2Cl2 (0.7 mL). [b] Obtained as
Inseparable 3 isomers. [c] Obtained as inseparable 2 isomers.
(Scheme 3A). Furthermore, under oxygen, the cycloadduct 10 of
anthracene and oxygen was obtained in good yield in the
presence of [2+2]BTH-F under visible light irradiation (Scheme 3B).
These results confirmed that triplet state of anthracene was
actually generated as the active species in the [2+2]BTH-F
catalyzed reaction.[16]
We next carried out several examinations to clarify the
nature of the generated excited state of anthracene. There are
three possible active species, that is, the cation radical directly
released from the CT excited complex,[12] and the singlet and the
triplet excited states which are known to undergo
photocycloaddition reactions.[8,9]
A
5.0 equiv.
0.1 equiv. [2+2]BTH-F
+
Firstly, the reaction was examined using tris(4-
bromophenyl)ammoniumyl hexachloroantimonate as catalyst to
examine the possibility of the cation radical [4+2] cycloaddition
reaction.[13] Treatment of a mixture of anthracene and 1,3-
cyclohexadiene (2a) with the ammoniumyl salt turned out to give
only dimers of 2a[14] with moderate recovery of anthracene and
none of the [4+2] cycloadduct 3a was obtained. As the oxidation
potential of anthracene is lower than 2a,[15] this result suggests
that even when the radical cation of anthracene was generated,
the [4+2]-cycloaddition did not proceed. Thus, it is not likely that
the reactive species of the [2+2]BTH-F-catalyzed reaction is the
cation radical of anthracene generated by excitation of the CT
complex (Scheme 1).
hν (450 nm)
1
3a
20 equiv.
CH2Cl2
rt, 6 h
4a
0%
B
0.1 equiv. [2+2]BTH-F
hν (450 nm)
O
O
CH2Cl2, rt
under 1 atm O2
1
10
80%
Scheme 3. Addition of triplet quenchers.
To clarify the characteristic properties of the present
[2+2]BTH-F-catalyzed reaction, the reaction was compared using
iridium catalyst I and II, which have recently attracted attention
as a triplet-triplet energy transfer catalyst in several reactions
(Table 4).[17] When iridium catalysts I and II were employed
instead of [2+2]BTH-F, the same [4+2] cycloadduct 3a between
anthracene and 1,3-cyclohexadiene (2a) was obtained as a
major product together with a small amount of anthracene [4+4]
dimer 5,[18] accompanied by a large amount of dimers of 2a. This
result also suggested that the triplet state of anthracene is
generated as an active species of the cycloaddition in the
presence of [2+2]BTH-F. More importantly, [2+2]BTH-F could excite
anthracene selectively and only [4+2] cross-adduct 3a was
obtained in high yield, while both 1 and 2a were excited by
energy transfer from iridium catalysts, and a large amount of
1,3-cyclohexadiene dimers were obtained.[19] It should be noted
that the triplet excited energy of Ir catalyst II is somewhat lower
1 equiv. (4-BrC6H4)3N
dimers of 2a: 25%a based on 2a
recovery of 1: 40%a
+
CH2Cl2, rt
a NMR yield.
1
2a
20 equiv.
Scheme 1. Ammoniumyl salt-mediated reaction.
Next, direct excitation of anthracene with UV light was
examined. When
cyclohexadiene was irradiated with 365 nm UV light without
[2+2]BTH-F anthracene [4+4] dimer and [4+4] cross
a
mixture of anthracene and 1,3-
,
5
cycloadducts 4a were obtained as major products, along with
[4+2] cycloadduct 3a in only 14% yield (Scheme 2). This result
suggests that while the direct excitation using UV light generated
a species which favored [4+4] cycloaddition pathway, a different
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10.1002/anie.201916732
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COMMUNICATION
than that of 2a (ET of catalyst II = 49.2 kcal/mol; ET of 2a = ~52
kcal/mol; ET of 1 = ~42 kcal/mol), but still most of 2a was
intersystem crossing
anthracene@[2+2]BTH-F
to
give
the
triplet state
of
,
from which the triplet state of
dimerized under the present conditions, while [2+2]BTH-F
catalyzed reaction gave none of dimerized products of 2a. Thus,
selective excitation of anthracene was achieved in the presence
-
anthracene is released into the solution (Scheme 4). Generation
of the triplet state of guest molecules by CT-excitation of host-
guest complexes has been reported in several cases,[20] but to
our knowledge, this is the first example of successful application
to the supramolecular catalysis for carbon-carbon bond
formation.
of 2a by using [2+2]BTH-F
.
Table 4. Photocycloaddition of anthracene (1) and cyclohexadiene (2a).[a]
a)ꢀ
0.1 equiv. catalyst
1
3a
4a
HSOMOꢀ
HSOMOꢀ
LSOMOꢀ
LSOMOꢀ
hν (450 nm)
+
CH2Cl2, rt, 4-6 h
b)ꢀ
5
2a
20 equiv.
yield[b]
HSOMOꢀ
LSOMOꢀ
2a
entry
catalyst
[2+2]BTH-F
3a
4a
0%
0%
0%
5
dimers
Figure 3. Molecular orbitals of the triplet state of a) anthracene@[2+2]BTH-F
and b) anthracene (M062X/6-31G).
1
2
3
>95%
85%
89%
1%
4%
7%
0%
Ir complex I
Ir complex II
92%
88%
• –
hν
• +
[a] NMR yield. Performed with 1 (9 µmol), 2a (180 µmol), and catalyst (0.9
µmol) in CH2Cl2 (0.5 mL). [b] For 3a, 4a, and 5, the yield is based on the
amount of anthracene used. For 2a dimers, the yield is based on 1,3-
cyclohexadiene used.
Charge-transfer
complex
Charge-separated
singlet state
CF3
Charge
recombination
F
+
ISC
+
N
Ir
N
tBu
N
Ir
N
tBu
N
N
F
F
N
N
tBu
PF6
tBu
PF6
F
CF3
Triplet state
*
II Ir(ppy)2(dtbbpy)PF6
I Ir[dF(CF3)ppy]2(dtbbpy)PF6
[2+2]BTH-F
Triplet state
To obtain information on the pathway for the formation of the
triplet state of anthracene, we have carried out DFT analyses of
the triplet state of anthracene itself and that of
anthracene@[2+2]BTH-F (Figure 3). The initial structure of
anthracene@[2+2]BTH-F was constructed based on the X-ray
Scheme 4. Proposed mechanism of the [4+2] cycloaddition in the presence of
[2+2]BTH-F
.
In summary, we have developed a supramolecular photo-
catalysis which can generate the triplet state of guest molecules
via charge-transfer excitation. By using this catalyst,
photocycloaddition of anthracene derivatives with several dienes
and alkenes proceeded smoothly by visible light irradiation.
Control experiments revealed that inclusion of the guest
data of 2-naphthaldehyde@[2+2]BTH-F.
[7] As shown in Scheme 4,
LSOMO of anthracene@[2+2]BTH-F spreads over the anthracene
moiety and its phase is nearly the same with LSOMO of
anthracene itself. HSOMO of anthracene@[2+2]BTH-F spreads
between anthracene and one of the benzothiadiazole moieties,
and the appearance of the HSOMO over the anthracene moiety
in [2+2]BTH-F is close to that of anthracene itself. These analyses
suggest that the triplet state of anthracene@[2+2]BTH-F has a
character close to the triplet state of anthracene without much
perturbation from the [2+2]BTH-F moiety. Thus, with visible light
irradiation, anthracene@[2+2]BTH-F undergoes CT excitation to
generate a charge-separated singlet state, which undergoes
molecule
was
crucial
for this
reaction, and
the
photocycloaddition proceeded only with guest molecules that
can be included. Utilization of chage-transfer interacton between
host and guest molecules for generation of reactive species
would be a promising method for realization of a new type of
selective reactions.
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Samanta, J. F. Stoddart, R. M. Young, M. R. Wasielewski, Chem. Sci.
2019, 10, 4282-4292.
Acknowledgements
[6]
For related studies, see: (a) K. Ohara, Y. Inokuma, M. Fujita, Angew.
Chem. Int. Ed. 2010, 49, 5507-5509; Angew. Chem. 2010, 122, 5639-
5641. (b) L. Zhao, J. Wei, J. Zhang, C. He, C. Duan, Angew. Chem. Int.
Ed. 2017, 56, 15284-15288; Angew. Chem. 2017, 129, 15486-15490.
(c) F. Burg, T. Bach, J. Org. Chem. 2019, 84, 8815-8836. (d) A. Das, I.
Mandal, R. Venkatramani, J. Dasgupta, Sci. Adv. 2019, 5, eaav4806.
T. Uchikura, K. Ono, K. Takahashi, N. Iwasawa, Angew. Chem. Int. Ed.
2018, 57, 2130-2133; Angew. Chem. 2018, 130, 2152-2155.
This work was supported by JSPS KAKENHI Grant Number
15H05800. The authors thank Dr. Gaku Fukuhara for helpful
discussions on this subject.
Keywords: Supramolecular catalyst• Charge-transfer interaction
[7]
[8]
• Anthracene • Photocatalysis
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[2]
[3]
For review, see: (a) Supramolecular Photochemistry: Controlling
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[10] By using 1 equivalent of cyclohexadiene, [4+2] cycloaddition product 3a
was obtained in low yield (37%) and anthracene [4+4] dimer 5 was
obtained in 12% yield. For details, see Supporting information, Table
S2.
[11] The CT absorption band in UV-Vis spectra appeared only when guest
molecules were included and the absorbance increased as the
association constant of anthracene derivatives became larger (Figure
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S2, Supporting information). The reaction of
problem by using Ir catalysts.
9 proceeded without
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[15] Oxidation potentials of anthracene: E = 1.1 ~ 1.3 V vs SCE; 1,3-
cyclohexadiene: E = 1.4 ~ 1.6 V vs SCE.
[16] Additionally, the fluorescence quenching of anthracene@[2+2]BTH-F was
not observed by the addition of 2a. Hence, the charge-tranfer excitation
state did not directly react with 2a (Figure S4, Supporitng information).
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[18] A small amount of the [4+4] dimer 5 was thought to be produced via
triplet-triplet annihilation to generate singlet anthracene. See ref. [9].
[19] In this reaction, the dimer of 1,3-cyclohexadiene was obtained as a
mixture of 4 isomers as reported in the literature. The ammoniumyl-
catalysed dimerization reaction of cyclohexadiene was reported to
afford only two isomers,[13] which also supported that the present
reaction proceeded via the triplet state of anthracene. See Supporting
information, Tables S4 and S5 and the following references. (a) D.
Valentine, N. J. Turro, Jr., G. S. Hammond, J. Am. Chem. Soc. 1964,
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10.1002/anie.201916732
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86, 5202-5208. (b) M. Mella, E. Fasani, A. Albini, Tetrahedron 1991, 47,
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[20] There are several reports for generation of the triplet state of guest
molecules via charge-transfer excitation of host-guest complexes. For
examples, see: (a) S. Hitosugi, K. Ohkubo, R. Iizuka, Y. Kawashima, K.
Nakamura, S. Sato, H. Kono, S. Fukuzumi, H. Isobe, Org. Lett., 2014,
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
COMMUNICATION
Triplet state generation via charge-transfer excitation
New type of supramolecular
photocatalyst: A supramolecular
Tatsuhiro Uchikura, Mari Oshima,
Minami Kawasaki, Kohei Takahashi,
Nobuharu Iwasawa*
hν
• –
photocatalysis via charge-transfer
excitation of a host-guest complex
was realized by use of macrocyclic
boronic ester [2+2]BTH-F containing
highly electron deficient
difluorobenzothiadiazole moieties.
Triplet excited state of anthracene
was generated by visible light
irradiation, and cycloaddition with
several dienes and alkenes
• +
donor-acceptor
interaction
Page No. – Page No.
radical ion pair
Supramolecular Photocatalysis
Utilizing Host-Guest Charge-Transfer
Interaction: Visible Light-Induced
Generation of Triplet Anthracenes for
[4+2] Cycloaddition Reactions
*
hν
triplet excited state
F
F
O
O
O
O
B
B
B
N
N
S
F
F
O
O
O
B
O
N
N
S
proceeded in high yields.
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