Preferential Photoreaction in a Porous Crystal, Metal-Macrocycle Framework: PdII-Mediated Olefin Migration over [2+2] Cycloaddition

Article abstract of DOI:10.1021/jacs.8b08534

A nanosized confined space with well-defined functional surfaces has great potential to control the efficiency and selectivity of catalytic reactions. Herein we report that a 1,6-diene, which normally forms an intramolecular [2+2] cycloadduct under photoirradiation, preferentially undergoes a photoinduced olefin migration in a porous crystal, metal-macrocycle framework (MMF), and alternatively [2+2] cycloaddition is completely inhibited in the confined space. A plausible reaction mechanism for olefin migration triggered by the photoinduced dissociation of the Pd-Cl bond is suggested based on UV-vis diffuse reflectance spectroscopy, single-crystal XRD, and MS-CASPT2 calculation. The substrate scope of the photoinduced olefin migration in MMF was also examined using substituted allylbenzene derivatives.

Full text of DOI:10.1021/jacs.8b08534

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Article
A Preferential Photoreaction in a Porous Crystal, Metal-Macrocycle
II
Framework (MMF): Pd-Mediated Olefin Migration over [2+2] Cycloaddition
Hirotaka Yonezawa, Shohei Tashiro, Takafumi Shiraogawa, Masahiro
Ehara, Rintaro Shimada, Takeaki Ozawa, and Mitsuhiko Shionoya
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 08 Nov 2018
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A Preferential Photoreaction in a Porous Crystal, Metal–Macrocycle
Framework (MMF): PdII-Mediated Olefin Migration over [2+2] Cy-
cloaddition
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Hirotaka Yonezawa,1 Shohei Tashiro,1 Takafumi Shiraogawa,2 Masahiro Ehara,2 Rintaro Shi-
mada,1 Takeaki Ozawa,1 and Mitsuhiko Shionoya1*
1 Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo
113-0033, Japan
2 Institute for Molecular Science and SOKENDAI, Myodaiji, Okazaki, Aichi 444-8585, Japan
ABSTRACT: A nano-sized confined space with well-defined functional surfaces has great potential to control the efficien-
cy and selectivity of catalytic reactions. Herein we report that a 1,6-diene, which normally forms an intramolecular [2+2]
cycloadduct under photo-irradiation, preferentially undergoes a photo-induced olefin migration in a porous crystal, met-
al–macrocycle framework (MMF), and alternatively [2+2] cycloaddition is completely inhibited in the confined space. A
plausible reaction mechanism for olefin migration triggered by the photo-induced dissociation of the Pd-Cl bond is sug-
gested based on UV-vis diffuse reflectance spectroscopy, single-crystal XRD, and MS-CASPT2 calculation. The substrate
scope of the photo-induced olefin migration in MMF was also examined using substituted allylbenzene derivatives.
clusively produced an intramolecular [2+2] cycloadduct
INTRODUCTION
(1a) in solution under a similar photo-irradiation condi-
Nano-sized confined spaces often provide an excellent
of the MMF channels with PdII centers on the reactivity of
platform for highly efficient and selective reactions, as
tion (Figure 1). We discuss the effects of the inner surfaces
typified by enzymes involved in preferential catalytic re-
actions in biological systems, and also as seen in a signifi-
cant number of reports on in-space reactions using syn-
thetic nano-spaces of molecular cages1 and porous solids2
such as zeolites, metal-organic frameworks (MOFs), and
covalent-organic frameworks. Molecular reactivity is gen-
erally sensitive to a change in chemical surrounding. For
instance, a porous coordination network was found to
completely suppress the polymerization of an olefin and
alternatively facilitate another oxidation reaction.3a An-
other notable example is that encapsulation of a molecule
in MOF brings the reactivity from nucleophilic to electro-
philic.3b Thus, synthetic, confined nano-spaces potentially
play roles in inhibiting a usual reaction pathway and acti-
vating an alternative reaction pathway.
diene 1 by UV-vis diffuse reflectance spectroscopy, single-
crystal XRD, and MS-CASPT2 calculations as well as a
model reaction using a simpler olefin compound 3.
In this context, attention is currently focused on photo-
reactions in confined spaces,4a-c for instance, related to
photo-activation of inert coordination compounds,4d sta-
bilization of photo-labile molecules,4e and regio- and ste-
reo-selective photoreactions.4f,g From another perspective,
if a photoreaction is inhibited and an alternative reaction
is photo-induced in a confined space, it would be possible
to switch photoreaction pathways even under the same
photo-irradiation conditions. Herein, we report that a
photo-induced olefin migration of a 1,6-diene 1 to produce
1b is preferentially promoted in a porous crystal, metal–
macrocycle framework (MMF),5 whereas the diene 1 ex-
Figure 1. Two photoreaction pathways of 1 for [2+2] cycload-
dition in solution and olefin migration in MMF.
Recently, we have reported a porous crystal, metal–
macrocycle framework (MMF), formed from four struc-
tural isomers of trinuclear PdII complexes of a macrocyclic
hexamine L.5a This porous crystal has one-dimensional
channels with a 1.4 × 1.9 nm2 dimension with five enanti
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Figure 2. (a) Self-assembly of four structural isomers of Pd3LCl6, (P)-syn, (M)-syn, (P)-anti, and (M)-anti, into MMF with a sin-
gle-crystalline channel. (b) A partial MMF channel structure with readily accessible Pd centers as highlighted in red circles. (c)
Site-selective uptake of 1 (39% occupancy). The side chain with a terminal olefin of 1 was highly disordered in the channel. MMF:
stick and ball model, 1: CPK model. C: gray, N: blue, O: red, Cl: green, Pd: yellow. The enlarged figure (right) shows two hydro-
gen bonds formed between 1 and two PdII centers.
omeric pairs of molecular binding sites (Figure 2a,b). The
structural features allowed for site-selective arrangement
of guest molecules via hydrogen bonding,5b,c in-situ X-ray
snapshot observation of molecular arrangement process-
es,5d and size-specific reactions with acid catalysts immo-
bilized on the channel surface.5f In this study, we focused
on the exposed and therefore readily accessible PdII cen-
ters (Figure 2b) to clarify the mechanism of the PdII-
mediated photoreactions.
sion (Table 1, Entry 1). In contrast, this olefin migration
did not proceed in the dark even at 70 °C (Entry 2), indi-
cating that the olefin migration of 1 was photo-induced in
the MMF channels. In addition, it is worthy noted that
the crystallinity of MMF was maintained to some extent
even after photoirradiation for 5 h as proven by XRD
analyses (Figures S6 and S7).
Table 1. Photoreactions of 1 in various reaction media
RESULTS AND DISCUSSION
Preferential Photoreaction in MMF. 1,6-Diene 1 was
initially examined for a photochemical intramolecular
[2+2] cycloaddition reaction. As previously reported,6 the
diene 1 quantitatively underwent [2+2] cycloaddition in
CD3CN to exclusively generate a desired cycloadduct 1a
(Table 1, Entry 3). Then, the encapsulation of 1 into MMF
channels was carried out by soaking MMF crystals in a
solution of 1 in CH3CN (6 mM) and left to stand for sever-
al days at 20 °C. The single-crystal X-ray diffraction
(SXRD) analysis of a resulting crystal demonstrated that 1
was site-selectively adsorbed via two hydrogen bonds to
the bottom corners of the channel with 39% occupancy
(Figure 2c). The number of encapsulated molecules was
estimated to be 0.5 per a unit-space after crystal digestion
in DCl-DMSO-d6 by NMR spectroscopy (Figure S2). This
result is comparable to that of the SXRD analysis (39%
occupancy for the two binding sites per a unit-space),
which suggests that most of encapsulated molecules were
adsorbed to the inner surfaces of MMF channels.
Conversionb
Entry
1
Conditiona
Time
5 h
(1, 1a, 1b,c %)
in MMFd
in MMFe
78 nd 22
2
5 h
>99 nd nd
without photo-
irradiation
in CD3CNf
3
3 min
3 min
nd 87h nd
nd 82h nd
in CD3CNf
with suspended MMF
4
60
min
in CD3CN (frozen)g
in CD3OD (not frozen) g
molecular crystal of 1d
5
6
7
93 6h nd
nd 91h nd
98 2 nd
10 min
60
min
aPhoto-irradiation with a 300 W Xenon lamp (280–630
nm) at room temperature. The conversion rates were esti-
mated based on the ratios of 1, 1a, and 1b (Entries 1, 2, and 7)
or on the comparison with the signal of 1,1,2,2-
tetrachloroethane as the internal standard (Entries 3–6). cThe
E/Z ratios of the products were over 10. dThe sample was
warmed up to 70 °C due to the photo-irradiation. eThe reac-
b
Next, an MMF crystal including 1 obtained as above was
picked up, covered with fluorolube® to prevent solvent
evaporation, and then irradiated with a 300 W Xenon
lamp (280–630 nm). Under this condition, no [2+2] cy-
cloadducts were detected, but olefin migration exclusively
proceeded to produce an internal olefin 1b in 22% conver-
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tions were conducted at room temperature and at 70 °C.
fThe reaction was conducted at room temperature. gThe reac-
tion was conducted at –78 °C. The rest of the compounds
was decomposed. nd = not detected.
Light source
(nm)
280–630c
450 ± 10c,e
no irradiationc
280–630d
450 ± 10c,e
Time
(h)
Conversiona
Entry
Medium
(3, 3a,b %)
1
2
3
4
5
6
7
8
h
1
in CD3CN
2
2
70g nd
>99 nd
>99 nd
nd >99
40 60
2
3
4
5
6
7
In all the control experiments with
H
trinuclear PdII complexes [Pd3LCl6] or
each component of MMF (L or
PdCl2(CH3CN)2 only) instead of MMF
72
3
N
O
in MMF
2
O
15
15
2
crystals, diene
1 afforded a photo-
no irradiationc
no irradiationf
>99 nd
9
induced [2+2] cycloadduct 1a only (Scheme S2). Moreover,
MMF crystals without soaking treatment suspended in
solution had no effects on the photo-induced [2+2] cy-
cloaddition (Entry 4). This marked difference in the reac-
tion pathway under photo-irradiation suggests that the
olefin migration proceeds only in the confined space of
MMF. For comparison, compound 2, which does not un-
dergo olefin migration due to the presence of two methyl
groups at the allylic position and is expected to readily
cyclize by the Thorpe-Ingold effect, was incorporated into
MMF crystals (containing 1.2 molecules per a unit-space;
Figures S11 and S12) and examined for the photoreaction.
The result showed that no reactions occurred in the MMF
channels (Figure S13), whereas compound 2 produced a
[2+2] cycloadduct in 83% conversion in solution (Scheme
S5).
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aThe conversion rates were estimated based on the com-
parison with the signal of 1,1,2,2-tetrachloroethane as the
internal standard (Entries 1–3), or based on the ratios of 3
b
and 3a (Entries 4–7). The E/Z ratios of the products were over
10. cPhoto-irradiation was carried out with a 300 W Xenon
lamp (280–630 nm) at room temperature. Photo-irradiation
d
was carried out with a 300 W Xenon lamp (280–630 nm). The
sample was warmed up to 70 °C due to the photo-irradiation.
f
eA band-path filter was used (HWHM = 10 nm). The reaction
was conducted at 65 °C. gApproximately 30% of 3 was decom-
posed. nd = not detected.
Photo-induced Olefin Migration in MMF. To con-
firm if the reaction is promoted by direct photo-excitation
of the olefin moiety, the photoreactions with a simple
olefin 3, which does not undergo [2+2] cycloaddition,
were examined. The photoreaction of 3 in solution with-
out MMF showed decomposition, and did not produce an
olefin-migrated product 3a at all (Table 2, Entry 1). On the
other hand, the reaction of 3 in MMF crystals (containing
3.4 molecules of 3 per a unit-space; Figures S19 and S20)
under photo-irradiation (280–630 nm) produced 3a quan-
titatively (Entry 4). The same is true of photo-irradiation
at 450 ± 10 nm (Entries 2 and 5) although the reaction rate
was significantly slowed down in MMF. Importantly, this
indicates that the olefin migration reaction in MMF was
not promoted by the photo-excitation of olefin itself. As a
matter of course, no reactions proceeded without photo-
irradiation at room temperature (Entries 3 and 6), and
only the trace amount of 3a was detected when heated at
65 °C without irradiation (Entry 7). Therefore, the photo-
irradiation was considered to affect the reaction in a way
different from that by direct excitation of olefins.
Inhibition of [2+2] Cycloaddition in MMF. One pos-
sible reason why the photo-induced [2+2] cycloaddition
with 1 or 2 was inhibited in MMF is a photo-shielding
effect by the MMF framework. Azobenzene encapsulated
in MMF underwent isomerization with lower efficiency
(23% conversion in 5 h; Figure S17) than a photoreaction
in solution (99% conversion in 2 h; Scheme S7) under the
same photo-irradiation condition, suggesting the photo-
shielding in the applied area being irradiated is likely to
be effective to some extent for suppressing the excitation
of 1 or 2 in MMF. Another possible reason is the limited
mobility of the substrates in the confined space. In par-
ticular, the space-specific, internal close solvation of their
side chain moiety may inhibit the flexible conformational
changes required for forming the transition state of the
intramolecular cycloaddition (Figures S37–S39). The pho-
to-induced [2+2] cycloaddition of 1 without MMF smooth-
ly proceeded in CD3CN at room temperature (Table 1,
Entry 3) or in CD3OD even at temperature as low as –
78 °C (Table 1, Entry 6). However, the reaction extremely
slowed down in frozen CD3CN at –78 °C and also in the
crystal state at room temperature (Table 1, Entries 5 and
7). Taken all together, the decrease in the mobility of sol-
vent molecules in MMF channels appears to restrict the
conformational changes of 1 required for the intramolecu-
lar [2+2] cycloaddition.
Figure 3. (a) A DRUV-Vis spectrum of MMF. (b) A calculated
spectrum of a model complex (MS-CASPT2). (c) The chemi-
cal structure and the Pd-Cl σ* orbital of a model complex.
Table 2. Comparison between photoreactions of 3 in
solution and in MMF
The mechanism of olefin migration reaction was then
theoretically examined with a focus on the photochemical
reactivity of the Pd centers, as it is well known that the
olefin migration is promoted by acids, bases, photo-
irradiation, or metal catalysts.7 Olefin migration reactions
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are, in general, initiated by photo-excitation of olefin
Page 4 of 7
lylbenzene in MMF to examine the substrate scope of the
photo-induced olefin reaction (Table 3). The substrates
were first encapsulated by soaking MMF crystals in a
CH3CN solution containing an olefin substrate, and then
the resulting crystals were collected and irradiated with a
Xenon lamp for 1 h. As a result, all the substrates 4–9 were
converted into internal olefins 4a–9a in moderate conver-
sion yields (41–82%, Table 3). For instance, the position of
the electron-donating methoxy group had only a little
effect on the reactivity (Table 3, Entries 1–3), and electron-
withdrawing substituents such as bromo and trifluorome-
thyl groups were also acceptable (Table 3, Entries 4, 5).
Moreover, the migration reaction took place even on mul-
tiply functionalized substrates 8 and 9 although the rate
of conversion slightly decreased (Table 3, Entries 6, 7).
These results indicate that the photo-induced olefin mi-
gration in MMF can be applied to allylbenzene derivatives
regardless of the kind of substituents on the aromatic ring.
1
2
3
4
5
6
7
8
moieties at a wavelength shorter than 260 nm (over 300
kJ/mol) and proceed via [1,3]-sigmatropic rearrangement.
Based on an assumption that Pd-Cl bonds become labile
by photo-irradiation to facilitate ligand exchange and
thereby activation of substrates on the Pd centers on the
inner surface of MMF,8 the reaction mechanism was in-
vestigated both experimentally and theoretically. The
diffuse reflectance UV-vis (DRUV-Vis) spectrum of MMF
crystals showed broad absorption around 290 and 390 nm
(Figure 3a). Multi-state complete-active-space second-
order perturbation theory (MS-CASPT2) calculations9
with a model complex (PdCl2 complex of N,N’-diethyl-1,2-
phenylenediamine) suggested electron transitions at 295
and 457 nm, which basically agreed with the experimental
data (Figure 3b). These absorption bands were thus as-
signed as transitions to a Pd-Cl σ* orbital of the Pd com-
plex, which indicates a significant relationship with Pd-Cl
cleavage (Figure 3c).
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It is well known that some of 2-propenylbenzene (al-
lylbenzene) and 1-propenylbenzene derivatives are useful
for several applications such as fragrance industry and
pharmaceutical chemistry. For instance, compounds 3
and 8 are naturally occurring compounds and named es-
tragole and safrole, respectively, and their isomers, ane-
thole (3a) and isosafrole (8a), have been studied for test-
ing their biological activities. In addition, anethole (3a)
and isosafrole (8a) are also known to be key intermedi-
ates for preparing important bioactive compounds and
pharmaceuticals.7a Therefore the development of a new
method for the olefin migration reaction of allylbenzene
derivatives must be a promising way to expand their po-
tential applications.
Single-crystal XRD analysis was proven to be available
for the direct observation of the Pd-Cl perturbation after
photo-irradiation. As a result, the electron density of Cl
atoms on the ceiling of MMF channels was significantly
decreased after photo-irradiation for 2 h at –180 °C (Fig-
ure 4). This observation suggests that the Pd-Cl bond was
partially cleaved, presumably followed by ligand exchange,
under photo-irradiation. In addition, Raman spectroscopy
also supported the photo-activation of the Pd-Cl bond
(Figure S40).10 Thus, it was concluded based on these re-
sults that the Pd centers on the inner surfaces of MMF
channels were activated by photo-dissociation of the Pd-
Cl bond to promote the olefin migration reaction. This
was significantly more effective than heating only (Table 2,
Entries 4 and 7). The activated Pd centers should be labile
and capable of binding to the terminal double bond of a
substrate, and then olefin migration would take place by
one of the known transition-metal mediated mechanisms
of olefin migration such as alkyl and allyl mechanisms.7a
On the other hand, the in situ generation of active metal
sites demonstrated here is also consistent with recent
application of labile or unsaturated coordination sites in
MOFs to catalysis, gas storage, and separation.11
Table 3. Photochemical reactivity of substituted al-
lylbenzene derivatives in MMF
Figure 4. (a) Crystal structure of a unit-space of MMF. (b)
2D electron density maps of a PdCl2 center which is a
macrocyclic component on the ceiling sites, before and after
the photo-irradiation. Positive electron density around the
PdCl2 center is represented as a contour map in green solid
lines, and the border between positive and negative electron
densities in blue dotted lines.
Substrate Scope of Photo-induced Olefin Migration.
Finally, we examined the photoreaction of several other
olefins 4–9 with a substituent on the benzene ring of al-
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aPhoto-irradiation was carried out with a 300 W Xenon
lamp (280–630 nm) at room temperature, and the irradiation
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c
ucts were over 10 in each case. The conversion was estimat-
ed based on the ratios of substrate and product.
CONCLUSION
In this study, photo-induced olefin migration reaction
of 1,6-diene which preferentially takes place in the MMF
channels instead of intramolecular [2+2] cycloaddition
expected from the reactivity in solution was well charac-
terized, and the Pd-mediated mechanism triggered by the
photo-activation of PdCl2 centers on the inner surfaces of
MMF was established experimentally and theoretically.
The reaction preference in the confined space of MMF
turned out to be due to the significant changes in the
chemical surroundings, the molecular mobility and the
local concentration effects on the reaction efficiency at
the activated Pd centers as well as the photo-shielding
effect by MMF. The activation-inhibition strategy in a
confined space demonstrated here is therefore expected
to open new opportunities for finding a hidden photore-
action pathway as well as for developing precisely con-
trolled catalytic photoreactions.12 Moreover, such unex-
pected molecular behaviors in a confined space with a
well-defined surface structure would provide an im-
portant clue to space-specific, efficient and selective reac-
tions, as realized in natural enzymatic systems, which
have high potential for metal-catalyzed reactions such as
bond activation, cross-coupling, and redox reactions.
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on
the ACS Publications website. Experimental procedures
including characterization of compounds.
AUTHOR INFORMATION
Corresponding Author
shionoya@chem.s.u-tokyo.ac.jp
(8) (a) Dilorenzo, K. K.; Gilbert, R. M.; Hoggard, P. E. J. Coord.
Chem. 2010, 63, 558. (b) Perumareddi, J. R.; Adamson, A. W. J.
Phys. Chem. 1968, 72, 414.
ORCID
Shohei Tashiro: 0000-0002-4706-3581
Masahiro Ehara: 0000-0002-2185-0077
Takeaki Ozawa: 0000-0002-3198-4853
Mitsuhiko Shionoya: 0000-0002-9572-4620
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J.; Wolinski, K. J. Phys. Chem. 1990, 94, 5483. (b) Andersson, K.;
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Finley, J.; Malmqvist, P.-Å.; Roos, B. O.; Serrano-Andrés, L. Chem.
Phys. Lett. 1998, 288, 299.
Notes
(10) Wang, H.; Shen, S.; Wang, L.; Zheng, X. J. Phys. Chem. B
2010, 114, 16847.
No competing financial interests have been declared.
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ACKNOWLEDGMENTS
This research was supported by JSPS KAKENHI Grant
numbers JP26248016 and JP16H06509 (Coordination
Asymmetry) to M.S. and JP15H05478 and JP18H04502
(Soft Crystals) to S.T. We are grateful to Prof. E. Nakamu-
ra, Dr. K. Harano, and Mr. H. Nishioka and to NETZSCH
Japan K. K. for DSC analysis.
(12) The catalytic use of MMF for this photoreaction is of con-
cern to this work, so that the reusability of MMF was preliminar-
ily tested. So far we have not observed significant reactivity of
MMF yet for the photo-induced olefin migration of 3 in the se-
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cond cycle probably due to the collapse of pore entrances. The
details of the catalytic properties and the mechanism of this
photoreaction will be reported elsewhere.
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