Stereoselective Solid-State Synthesis of Substituted Cyclobutanes Assisted by Pseudorotaxane-like MOFs

Article abstract of DOI:10.1002/anie.201806076

Regioselective photodimerization of trans-4-styrylpyridine (4-spy) derivatives is performed using pseudorotaxane-like Zn-based metal organic frameworks MOFs as templates. The formation of rctt-HT (head-to-tail) dimers is achieved by confining pairs of coordinated 4-spy derivative ligands within hexagonal windows and then irradiating them with UV light. It is also possible to achieve a photodimerization reaction where two different substituted 4-spy ligands are included in such a MOF material. The ether bond formation is employed to protect the sensitive -OH group of HO-spy and the methyl group of CH₃O-spy is subsequently removed after the formation of cyclobutane derivative in the CH₃O-spy-based MOF. Introducing substituents at the 2- or 3-position of the phenyl group of 4-spy does not significantly affect the rate of the dimerization process except in the case of the strongly electron-withdrawing nitro group where the rate is significantly decreased. These results are in striking contrast to the mixtures of photoproducts and low yields obtained by untemplated photodimerization in organic solvents.

Full text of DOI:10.1002/anie.201806076

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Stereoselective Solid-State Synthesis of Substituted
Cyclobutanes Assisted by Pseudorotaxane-like MOFs
Authors: Fei-Long Hu, Yan Mi, Chen Zhu, Brendan F. Abrahams,
Pierre Braunstein, and Jian-Ping Lang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201806076
Angew. Chem. 10.1002/ange.201806076
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10.1002/anie.201806076
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Stereoselective
Solid-State
Synthesis
of
Substituted
Cyclobutanes Assisted by Pseudorotaxane-like MOFs
Fei-Long Hu,[a,b] Yan Mi,[b] Chen Zhu,[a] Brendan F. Abrahams,[c] Pierre Bruanstein,[d] and Jian-Ping
Lang*[a]
alkene groups in close proximity and alignment for reaction but
also imposes physical constraints that prevent conformational
Abstract: Regioselective photodimerization of trans-4-styrylpyridine
(4-spy) derivatives is performed using pseudorotaxane-like Zn-based
change during the lifetime of the excited states, offers the
metal organic frameworks MOFs as templates. The formation of rctt-
enticing prospect of stereoselectivity.[6] Although establishing
HT (head-to-tail) dimers is achieved by confining pairs of
such molecular environments is challenging, the development of
coordinated 4-spy derivative ligands within hexagonal windows and
host structures which can be used as well-ordered templates or
then irradiating them with UV light. It is also possible to achieve a
vessels to efficiently control the selectivity is clearly desirable.[1b,
photodimerization reaction where two different substituted 4-spy
7] Discrete supramolecular species, including cyclodextrins and
ligands are included in such a MOF material. The ether bond
calixarenes, etc, have served as hosts that offer the appropriate
formation is employed to protect the sensitive -OH group of HO-spy
confinement of olefin pairs necessary for the photodimerization
and the methyl group of CH3O-spy is subsequently removed after
process. [8]
the formation of cyclobutane derivative in the CH3O-Spy-based MOF.
The regulation of chemical reactions in the solid state by
Introducing substituents at the 2- or 3-position of the phenyl group of
supramolecular confinement has attracted considerable
4-spy does not significantly affect the rate of the dimerization
interest[1b, 7a, 7b, 9] because the rigid molecular environment of the
process except in the case of the strongly electron-withdrawing nitro
reacting molecules in the solid state limits the opportunity for
group where the rate is significantly decreased. These results are in
conformational change, thus promoting regio- and stereo-
striking contrast to the mixtures of photoproducts and low yields
selective processes.[10] For example, the photodimerization of
obtained by untemplated photodimerization in organic solvents.
olefins can proceed stereoselectively in the solid state.
According
to
Schmidt’s
criteria,
solid
state
[2+2]
Stereoselective photodimerization reactions of alkenes remain
elusive because of unfavorable interference of non-selective
background reactions.[1] The physical state of the precursor
alkene is one factor that can impact the reaction outcome.[2] For
photodimerization of olefins can occur when C=C bonds are
parallel and the separation between the alkene groups is < 4.2
Å.[11] A solid state approach of increasing importance involves
the use of metal-organic frameworks (MOFs) in which the
reacting molecules are confined within the framework voids.[8b, 12]
The design and synthesis of a MOF that holds voids capable of
example, photoirradiation of
a dilute solution of trans-4-
styrylpyridine (4-spy) affords only the cis-isomer because of an
accompanying trans-to-cis photoisomerization reaction.[3] In
contrast, photodimerization of an asymmetrical, substituted 4-
spy in a concentrated solution generally produces a mixture of
stereo- and regio-isomeric cyclobutanes, the formation of which
is governed by Woodward-Hoffman rules, electronic and steric
effects, and the thermodynamic stability of the diradical
intermediates.[4] Inspired by nature, which exploits tailored
microenvironments within enzymes to promote a variety of
biological reactions with high regio- and stereo-selectivity, it
should be possible to obtain unique stereoselectivity and
quantitative yields by preorganizing the reactants within an
appropriately structured environment.[5]
not only accommodating but also aligning an isolated pair of
[12a,13]
olefins represents a significant challenge.
One way of
exerting some control over the organization of pairs of reacting
molecules within the voids, involves tethering the olefins to the
framework via coordinate bonds. Herein, we report
pseudorotaxane-like MOF-based approach for the generation of
wide range of substituted cyclobutanes through
a
a
stereoselective photodimerization reactions.
The generation of an environment that not only brings
[a] F. L. Hu, Prof. C. Zhu, Prof. J. P. Lang
College of Chemistry, Chemical Engineering and Materials Science,
Soochow University,
Figure 1. (a) A depiction of part of the Zn(oba) network (green bonds)
No.199 RenAi Road, Suzhou 215123, Jiangsu (P. R. China).
E-mail: jplang@suda.edu.cn
[b] Y. Mi, F. L. Hu
showing btpy ligands (pink bonds) penetrating the hexagonal window. (b)
The
photoproduct
4,4'-(2,4-bis(benzo[b]thiophen-3-yl)cyclobutane-1,3-
diyl)dipyridine (btcb) enclosed in a hexagonal window.
Guangxi Key Laboratory of Chemistry and Engineering
of Forest Products, Guangxi University for Nationalities,
Nanning, 530006 (P.R. China)
The combination of Zn(II) centers with the V-shaped
dianionic ligand 4,4'-oxybis(benzoate) (oba2-), in the presence
of 4-spy (1) and its derivatives S-spy (1a-1v) (Scheme S1),
leads to the formation of neutral 2D networks [Zn(oba)(S-
spy)]2(S-spy)x (C1-C22, Supporting Information). As depicted in
Figure 1 and Figure S1, pairs of Zn(II) centers are linked by the
bridging oba2- ligands to other pairs of Zn(II) centers. Although
[c] Prof. B. F. Abrahams
School of Chemistry, University of Melbourne, Victoria 3010
(Australia)
[d] Prof. P. Braunstein
Institut de Chimie (UMR 7177 CNRS), Université de Strasbourg
4, rue Blaise Pascal - CS 90032, 67081 Strasbourg, (France)
Supporting information for this article can be found under:
http://doi.org/10.1002/anie.xxxxxx.
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10.1002/anie.201806076
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each pair is linked to four other pairs, generating a network with
the (4,4) topology, the use of a V-shaped oba2- ligand results in
the formation of a network that contains hexagonal windows.
The S-spy ligands, binding through pyridyl groups to Zn(II)
centers, are directed above and below the mean plane of the
Zn(oba) network and extend through hexagonal windows of
generation of the substituted rctt-cyclobutane in the solid product.
The network material is then placed in nitric acid, resulting in the
destruction of the network and the precipitation of H2oba which
may then be filtered and recycled (Figure S3). Upon addition of
base to the acidic solution, the substituted cyclobutane
precipitates and is isolated by filtration. The intermolecular
photodimerization of the pre-organized olefins generates head-
to-tail rctt-cyclobutanes in almost quantitative yields and with
unique stereo- and regio-selectivities.
adjacent 2D networks, giving rise to
a pseudorotaxane
arrangement. The two molecules indicated in Figure 1a with pink
bonds represent (E)-4-(2-(benzo[b]thiophen-3-yl)vinyl)pyridine
(btpy, 1p) ligands that are bound to Zn centers (Figure S2) of the
Zn(oba)(S-spy) networks (S-spyꢀMOF) situated above and
below the green network shown. The hexagonal cavities, in
which the S-spy ligands are located, have approximate
dimensions of 8 x 14 Å and possess a size and shape
comparable to those found in cyclodextrin and cucurbit[8]ril.[7d, 14]
As can be appreciated from inspection of Figure 1a, the two btpy
ligands are arranged in a head-to-tail manner and appear to be
suitably positioned for a solid state photodimerization reaction.
Crystal structure analysis of the parent S-spyꢀMOF crystals
(Table S1) allows an accurate determination of the separation
and relative orientation of the olefinic groups before photo-
irradiation. Of particular importance are three parameters d, τ
and α indicated in Scheme 2 and Table S2. The d parameter
represents the separation of the mid-points of the double bonds,
the τ parameter provides an angular indication of how close the
double bonds are to being parallel and the α parameter is
referred to the slip angle. For an ideal alignment, d should be as
small as possible and certainly less than 4.2 Å, τ and α can be
0° and 90°, respectively. In the case of the btpy ligands depicted
in Figure 1a, d = 3.844 Å, τ = 4.78° and α = 75.99°. The
photodimerization occurs with retention of single crystal
character and the substituted rctt-cyclobutane, 4,4'-(2,4-
bis(benzo[b]thiophen-3-yl)cyclobutane-1,3-diyl)dipyridine (btcb),
is apparent from a single crystal structure determination, where
it is still coordinated to the Zn centers (Figure 1b). The crystal
structure of the pure substituted cyclobutane (Figure S4)
confirms the successful generation and isolation of the intended
product.
Scheme 2. Geometrical parameters used to evaluate the potential for
photoreactivity.
The substituted cyclobutane products (2, 2a-2t) (Scheme 3)
were obtained upon UV irradiation over S-spyꢀMOF crystals at
a wavelength of 365 nm for a period of 40 h. The 1H NMR
spectrum of each of the compounds represented in Scheme 3
showed the appearance of signals at 4-6 ppm which can be
attributed to the cyclobutane rings (Figure S5). Signals
associated with the olefinic protons were absent from the
spectra. The pyridyl proton signals were shifted from 8.6 to 8.3
ppm, which is consistent with the formation of the substituted
cyclobutane.[10b, 16] Following the recovery of the dicarboxylate
ligand, the 1H NMR spectrum showed no changes for the proton
signals of oba (7.0 and 8.0 ppm), indicating that the oba2-
ligands which form the frames of the hexagonal windows are
unaffected by the photo-irradiation process (Figure S3). The
thermal gravimetric analysis showed that the as-prepared
compounds display two weight loss stages, while the photo-
irradiated samples exhibit only one weight loss stage (Figure S6).
The different weight loss stages of the compounds before and
after photo-irradiation can be attributed to the new cyclobutane
derivatives formed from the cycloaddition reaction.
Scheme 1. (top) Preorganization of spy derivatives face-to-face using the
coordination template and the [2+2] photodimerization reaction. (below) The
procedure of the synthesis of the cyclobutane derivatives.
Previously it has been reported that both 1 and (E)-4-(2-
(naphthalen-1-yl)vinyl)pyridine (4-npy, 1a) are able to undergo
[2+2] photodimerization when each is appended to Zn centers of
the Zn(oba) network.[15] This leads to the formation of rctt-1,3-
bis(4-pyridyl)-2,4-bis(phenyl)cyclobutane (HT-ppcb) from 4-spy
and rctt-1,3-bis(4-pyridyl)-2,4-bis(1-naphthyl)cyclobutane from 4-
npy (rctt refers to the stereochemistry of the cyclobutane ring,
cis, trans, trans). We have now developed a protocol for the
generation of a wide range of head-to-tail rctt-cyclobutane
molecules. The synthetic approach is summarized in Scheme 1
and involves, firstly, the solvothermal synthesis of S-spyꢀMOF.
Upon UV irradiation, the photodimerization occurs, leading to the
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substituents, such as anthacyl in (E)-4-(2-(anthracen-9-
yl)vinyl)pyridine) (4-atpy, 1r), the formation of [Zn(oba)(4-atpy)]2
was inaccessible. Presumably the hexagonal window is not
large enough to host this large substrate. It remains to be seen
whether a different choice of metal and/or auxiliary ligands could
afford a larger pore size to accommodate such a bulky ligand.
Attempts to synthesize the expected cognate product from
HO-spy ligand failed due to the fact that the -OH group usually
forms multiple hydrogen bonds in the assembly process. A
protecting-group strategy was employed to avoid the formation
of such hydrogen bonds. Thus, the methoxy derivative, (E)-4-(3-
methoxyphenyl)pyridine (3-OCH3-spy, 1t) was easily obtained by
treating (E)-4-(3-hydroxylphenyl)vinyl)pyridine (3-HO-spy, 1s)
with CH3I under mild conditions.[17] Formation of [Zn(oba)(3-
OCH3-spy)]2 (C19) followed by the above method gave 4,4'-(2,4-
bis(3-methoxyphenyl) cyclobutane-1,3-diyl)dipyridine (pocb, 2r).
The demethylation of 2r worked efficiently using HBr or BCl3,[18]
generating the final product 3,3'-(2,4-di(pyridin-4-yl)cyclobutane-
1,3-diyl)diphenol (pdcb, 2s) in 80% yield (Figure S8).
In all cases except for 2a, 2c, 2l, 2m and 2n, structural
analyses revealed d, τ and α values which indicate a favorable
arrangement with respect to the formation of the intended rctt-
cyclobutane upon photo-irradiation. In the case of 2c, 2l, 2m and
2n, it was not possible to solve the crystal structures of the
parent network compounds and thus it is not certain that an
appropriate alignment and separation of the double bonds are
present. In the known 2a,[15] the double bonds of the 4-npy
ligands are in a criss-cross arrangement (τ= 93.16°). Given that
such an arrangement would be incompatible with the formation
of the intended substituted cyclobutane, it is proposed that the
photodimerization to produce 2a is preceded by a pedal motion
rearrangement, resulting in a 180° rotation of the naphthyl group
as shown in Scheme S2.
Figure 2. The 1H NMR spectra of [Zn(oba)(2,6-Me-spy)]2 before and after UV
light irradiation.
Scheme 3. Synthesis of cyclobutane derivatives 2~2t.
The crystal structure of [Zn(oba)(2,6-Me-spy)]2 (C21, 2,6-Me-
spy = (E)-4-(2,6-dimethylphenylvinyl)pyridine, 1u) also showed a
criss-cross arrangement of the double bonds with anangle of
95.04°. On the basis of the successful formation of 2a, it was
anticipated that the conformational change involving a pedal
motion process would bring the pair of substituted olefins into an
arrangement suitable for photodimerization. However, the 1H
NMR spectrum of the irradiated C21 showed the absence of
signals in the 4-6 ppm range, thus indicating that the
photodimerization reaction had failed to occur. Close inspection
As indicated in Scheme 3, all substrates gave high yields of
photodimerization products (2~2q), even those bearing bulky
substituents, such as 3,5-dimethylphenyl (2l), benzothiophene
(2p) and indanone (2q). Interestingly, the formation of 2l did not
occur in solution (Figure S7) and its formation in the network is
presumably facilitated by the entropically favorable containment
in the hexagonal windows. In the case of even larger
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1
of the H NMR spectrum revealed that it was different to that of
bromostyryl)pyridine (3-Br-spy, 1e), and 1a/1e using the Zn(oba)
synthetic approach all gave mixtures of two homo- and one
hetero-dimers in the ratio of 1:1:1 (Figure S15).
the parent ligand 1u. In particular, the signals associated with
the olefin protons of 1u shifted from 7.53 and 6.68 ppm to 6.74
and 6.87 ppm, respectively. Such a change suggests conversion
Whilst the regioselectivity of S-spy photodimerization
reactions can be reliably controlled by templation, finding an
appropriate template for a specific reaction type is challenging.
In nature, enzymes provide highly tailored microenvironments
that can promote reactions with high regio- and stereo-
selectivity.[19] With respect to the reaction of olefins, the pre-
organization prior to UV irradiation provided by small molecule
H-bonding templates such as resorcinol (and various substituted
resorcinols) allows asymmetrical olefins to be arranged in a
of
a trans to a cis isomer upon irradiation (Figure 2).
Corresponding shifts of the signals of protons bound to the
pyridyl and methyl groups were also observed, from 8.57 to 8.37
ppm, 7.61 to 6.87 ppm and 2.33 to 2.08 ppm. The isomerization
process was clearly confirmed by the H-1H COSY spectrum of
1
the cis product (Figure S9).
The unexpected trans-cis isomerization of 1u rather than
photodimerization, prompted closer examination of the parent
coordination network and
head-to-head[1b, 20] or head-to-tail[21] orientation. Other templates
a comparison with the initial
22]
such as cucurbit[8]ril[3,
can also facilitate the regiospecific
arrangement of criss-cross olefins, depicted in Figure 3, from
which 2a was isolated. Clearly, the presence of two methyl
groups bound to the phenyl ring increases the bulkiness of the
ligand and this may prevent the necessary rotation within the
hexagonal window that is required before photodimerization can
occur. However, the successful formation of 2l and 2n from (E)-
4-(3,5-dimethylstyryl)pyridine (3,5-Me-spy, 1l) and (E)-4-(2,5-
dimethylstyryl)pyridine (2,5-Me-spy, 1n), respectively, suggests
that the position of the methyl groups on the ring is critical to the
outcome of the irradiation process. It is proposed that the
arrangement for 1u (Figure 3), in which the methyl groups flank
the link to the double bond, impedes the required pedal motion
rearrangement which is necessary for the alignment of the
double bonds. In order to further examine the steric effect of the
methyl groups, a ligand resembling 4-npy but with an additional
methyl group, (E)-4-(2-(2-methylnaphthalen-1-yl)vinyl)pyridine
(3-Me-4-npy, 1v) (Figure 3), and its corresponding complex
[Zn(oba)(3-Me-4-npy)]2 were prepared. No photodimerization
occurred for 1v (Figure S10), probably because the additional
methyl group hindered the necessary pedal motion process
required for the alignment of the double bonds.
formation of cyclobutanes. This effect is enhanced by adding
hydrochloric acid. We have found that no photodimeric products
were observed upon UV irradiation over crystalline samples of
uncoordinated (E)-2-(2-(pyridin-4-yl)vinyl)benzonitrile (2-CN-spy,
1j) or (E)-4-(2-(pyridin-4-yl)vinyl)-2,3-dihydro-1H-inden-1-one
(idpy, 1q). The failure to achieve dimerization is attributed to
poor alignment and separation of the C=C bonds of neighboring
molecules (Figures S16-S17). When a template is employed, a
parallel alignment yields head-to-head dimers while the
antiparallel orientation produces the head-to-tail cyclobutane
products. When framework voids are exploited for templating
purposes, antiparallel orientation is commonly observed
because of steric repulsion between bulky groups in the parallel
orientation.[23] The regiospecificity of the photodimerization of S-
spy ligands is governed by the balance of different types of
interactions in the solid state. The successful cross
photodimerization of 1 and 1d in the hexagonal window of
1/1dꢀMOF (C20) leads to the formation of a single heterodimer
product 2t. This result demonstrates the power of the MOF-
templation method given that in theory six possible products
may be formed in the solution reaction of 1 and 1d (Figure S18).
However, in the absence of the coordination network, no
photodimer was formed from the batch of 1/1a, 1/1b or 1/(E)-4-
(2-fluorostyryl)pyridine (2-F-spy, 1k) even at very high
concentration (Figures S19-S21). These results demonstrate
that for 1/1d the exclusive formation of the hetero-photodimer
was clearly controlled by steric factors associated with the
hexagonal window of the appropriate MOF.
rotation forbidden rotation allowed rotation forbidden
Figure 3. Representation of the possible rotation of 1u (left), 1a (middle) or 1v
(right) in the limited space of a pseudorotaxane window of each MOF.
Cross-dimerization reactions in which mixtures of S-spy
ligands were used, were also studied. In particular, pairs of
different ligands were combined in order to determine whether a
heterodimer could be generated by using the synthetic approach
successfully employed to produce the homodimers. The
combination of 1 and (E)-3-(2-(pyridin-4-yl)vinyl)benzonitrile (3-
CN-spy, 1d) as indicated in Scheme 4 led to the formation of the
heterodimer as only one single cross photodimerization product
2t (Scheme 3) was detected by LC-MS (Figures S11-S12).
Moreover, the 1H NMR and 1H-1H COSY spectra clearly showed
that nearly equivalent 1 and 1d crystallized in the same MOF
C20 (Figures S13-S14). In contrast, mixtures of 1/1a, 1/(E)-4-(3-
Scheme 4. Cross photodimerization between 1 and 1d.
The time-dependent 1H NMR spectra were recorded to
examine the kinetics of the photodimerization reaction (Figure
4).[24] The significant difference among the reaction rates of S-
spyꢀMOF (S = H, -Me, -Et, -CN, -Br, -F, -CHO, -OCH3, -aceto, -
NO2) was attributed to the very strong electron withdrawing 3-
nitro group of (E)-4-(3-nitrostyryl)pyridine (3-NO2-spy, 1g) in C8.
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Another reason for this was probably due to the relatively strong
intermolecular H-bonding interactions between the HC=CH
group of 3-NO2-spy and O atom of the NO2 group of one nearby
3-NO2-spy (C20-H20···O2 (x, 0.5-y, -0.5+z), 2.48Å) or O atom of
oba2- (C19-H19···O10 (x, 0.5-y, 0.5+z), 2.48 Å) in C8 (Figure
S24) Interestingly, the presence of weakly donating alkyl, weakly
withdrawing halo or strongly withdrawing cyano substituents at
the 2- or 3-position had virtually no influence on the reaction rate.
The influence of substituents on the rate of solid state
photodimerization reaction has rarely been reported.[25]
The authors declare no conflict of interest.
Keywords: MOFs • regioselectivity • photodimerization • solid-
state organic synthesis • preorganization
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Figure 4. The percentage of S-spy ligand in C1, C3, C4, C5, C6, C7, C8, C9,
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In summary, a series of rctt-HT substituted cyclobutanes
have been synthesized in a stereoselective, photodimerization
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Acknowledgements
The authors thank the financial supports from the National
Natural Science Foundation of China (Grant Nos. 21531006,
21701035 and 21773163) and the Priority Academic Program
Development of Jiangsu Higher Education Institutions.
Conflict of interest
This article is protected by copyright. All rights reserved.

10.1002/anie.201806076
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents (Please choose one layout)
COMMUNICATION
Pairs of trans-4-styrylpyridine or its
derivatives are oriented in a parallel
and head-to-tail manner for the solid-
Fei-Long Hu, Yan Mi, Chen Zhu,
Brendan F. Abrahams, Pierre Braunstein
and Jian-Ping Lang*
state photodimerization within
a
Page 1. – Page 5.
pseudorotaxane-like MOF. The high
Stereoselective Solid-State Synthesis
of Substituted Cyclobutanes Assisted
by Pseudorotaxane-like MOFs
selectivity for the rctt-HT (head-to-tail)
dimers
provides
a
controlled
photodimerization within a confined
environment. These results are in
striking contrast to the mixtures of
products and low yields obtained by
untemplated photodimerization in
organic solvents.
This article is protected by copyright. All rights reserved.