Regiodivergent Synthesis of Bis(4-oxycoumarin)-based Dioxabicycles: Exploration of [4 + 4] (Heterocyclo)reversion/addition and 1,5-Hydrogen Shift Photochromism

Article abstract of DOI:10.1021/acs.orglett.0c00904

Two isomeric dioxabicyclic molecular skeletons were constructed by employing the concepts of divergent synthesis. A base-mediated and an acid-catalyzed pseudo-three-component reaction of two equivalents of 4-hydroxycoumarin and (Z)-3-chloro-3-phenylacryla

Full text of DOI:10.1021/acs.orglett.0c00904

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Letter
Regiodivergent Synthesis of Bis(4-oxycoumarin)-based
Dioxabicycles: Exploration of [4 + 4] (Heterocyclo)reversion/addition
and 1,5-Hydrogen Shift Photochromism
Sameer Vyasamudri and Ding-Yah Yang*
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ABSTRACT: Two isomeric dioxabicyclic molecular skeletons
were constructed by employing the concepts of divergent synthesis.
A base-mediated and an acid-catalyzed pseudo-three-component
reaction of two equivalents of 4-hydroxycoumarin and (Z)-3-
chloro-3-phenylacrylaldehyde yielded the corresponding bis(4-
oxycoumarin)-based 2,6- and 2,8-dioxabicycles, respectively. The
prepared colorless 2,6-dioxabicycles turned red upon UV
irradiation and underwent the reverse reaction when exposed to visible light. The photochromism was proposed to proceed via
a sequential [4 + 4] (heterocyclo)addition/reversion and 1,5-hydrogen shift on the basis of photogenerated product-trapping
experiments.
1
0
11
hotochromism is defined as the reversible transformation
of a chemical species between two forms by the
receptors, and the modulation of magnetic properties.
Nevertheless, because of its low conversion efficiencies and its
P
12
absorption of electromagnetic radiation, in which the two
forms have different absorption spectra. Compounds with
photochromic properties may have wide applications in the
tendency to undergo side reactions with oxygen, anthracene
photochromism is not as widely used as the unimolecular
photoreactions of azobenzenes or diarylethenes. Whereas the
efficiency of anthracene dimerization may be substantially
enhanced by connecting two anthracenes together via a
1
2
3
areas of electronic display, information storage, and so on.
Various organic compounds exhibiting photochromism and
4
8e
their unique properties have been reported in the literature, to
linker, the resulting bisanthracene still suffers from the
5
name a few, the trans/cis isomerization of azobenzene, the
undesired competition of unsymmetrical dimerization as well
as other side reactions such as [4 + 2], [2 + 2], or [6 + 6]
cycloadditions. Therefore, the development of an oxygen-
insensitive, regiospecific [4 + 4] cycloaddition-based photo-
chromic dye with sharp color variation as an output property
remains to be a challenging task for organic chemists. In our
continuing efforts to explore photochromic colorants with
novel molecular scaffolds as well as new photochromic
mechanisms, we proposed to design 2,6-dioxabicycle-based
photochromic dye 1 in which two oxygen atoms were
introduced onto the molecule, aiming to eliminate the possible
oxygenation reaction and other undesired cycloadditions
6
electrocyclic ring-opening/closing reaction of diarylethenes,
7
napthopyranes, and the dimerization photoreaction of
8
anthracene (Scheme 1a).
The photochromic property of anthracene not only results
in a change of absorption but also leads to a large change in the
dipole between two states, which can be used for many
9
different applications, including smart materials, photoactive
Scheme 1. (a) Photodimerization and Oxidation of
Anthracene and (b) Design Rationale for the Potential 2,6-
Dioxabicycle-based Photochromic Dye 1
(
Scheme 1b).
To enhance the ring-opening/closing efficiency upon
irradiation, the proposed bicycle 1 was assembled by
connecting two molecules of diene from head to tail (i.e.,
oxygen to carbon), followed by bridging via a one-carbon
Received: March 13, 2020
©
XXXX American Chemical Society
https://dx.doi.org/10.1021/acs.orglett.0c00904
A
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Letter
linker between the two tail carbons. We hypothesized that
when compound 1 is irradiated, compound 2 will be generated
if the reaction proceeds with a [4 + 2] cycloreversion
mechanism, whereas compound 3 will be formed if the
reaction proceeds with a [4 + 4] cycloreversion mechanism.
Here we report the efficient synthesis of bis(4-oxycoumarin)-
based dioxabicycles via a multicomponent reaction. The
photochromic properties and the plausible mechanism of the
prepared compounds were investigated by the trapping and
subsequent characterization of the photogenerated product.
In contrast with anthracene, in which benzene is used to
stabilize the cyclic adduct (dianthracene), we intended to
employ a coumarin moiety to serve this purpose. After a
literature search, we realized that the coumarin-based 2,6-
Scheme 3. Pseudo-Three-Component Synthesis of
Dioxabicycles 7a−d and 8a−d
13
dioxabicycle 4a has been previously prepared by the pseudo-
three-component reaction of 2 equiv of 4-hydroxycoumarin
with pentane-2,4-dione, as shown in Scheme 2. To investigate
Scheme 2. Pseudo-Three-Component Synthesis of 2,6-
Dioxabicycles 4a and 4b
The mechanism for the formation of dioxabicycle 7a under
basic conditions via this pseudo-three-component reaction is
proposed to proceed with a sequence of 1,4-addition,
cyclization, 1,2-addition, and a cyclization process. Intriguingly,
the proposed sequence of 1,4- and 1,2-additions is reversed
when the reaction is performed under acidic conditions,
generating dioxabicycle 8a as the major product. (See the SI
for details.)
their potential photochemical properties, we synthesized the
2
,6-dioxabicycles 4a and 4b accordingly, but they were found
be light-insensitive. This crucial information prompted us to
relook into our proposed design strategy, and we speculated
that the replacement of the bridgehead methyl group of 4b
with a hydrogen atom might lead to a light-sensitive
dioxabicycle.
With dioxabicycles 7a−d and 8a−d in hand, we then
evaluated their photochromic properties. Upon ultraviolet
irradiation (352 nm) in acetonitrile, the 2,8-dioxabicycles 8a−
d were found to be light-insensitive and remain intact even
under prolonged irradiation. Conversely, the 2,6-dioxabicycles
7a−c turned from colorless to red soon after irradiation
(except for 7d, in which the color variation was not so
obvious). For instance, with the increase in the exposure time
of 7a to UV light, a new absorption band with a peak
wavelength around 554 nm gradually increased on the UV−vis
spectra, as shown in Figure 1. When irradiated with visible light
(580 nm), the photogenerated product in acetonitrile decayed
away with the disappearance (i.e., turning colorless) of the 554
nm band (Figure 1, inset). The photochromic process was
reversible and could be repeated 10 times without a significant
loss of absorption at the 554 nm band. Furthermore, the
photogenerated product of 7a was found to be stable in the
dark at room temperature for at least 72 h and at 80 °C for 10
h. These observations suggest that the conversion between 7a
and its photogenerated product can be categorized as a P-type
photochromism. It should be noted that whereas the coumarin
moiety was employed to harness the fluorescence emission as a
potential output property in our original design, unfortunately,
in the current system, we did not observe the fluorescence
emission for either 7a or its photogenerated product.
Our previous efforts on the photochromism of oxazabi-
1
4
cycles and divergent synthesis of lamellarins and their
1
5
derivatives propelled us to judiciously choose 4-hydroxycou-
marin and (Z)-3-chloro-3-phenylacrylaldehyde to construct
the designed dioxabicyclic core. To begin with, the reaction of
4
-hydroxycoumarin (5) with (Z)-3-chloro-3-phenylacrylalde-
hyde (6, R = H) in the presence of triethylamine (1 equiv) as
1
a base in toluene under refluxed conditions for 4 h furnished
the 2,6-dioxabicycle 7a in 32% yield. Interestingly, a minor
product (<5%) of 2,8-dioxabicyclic isomer 8a was also
detected. This observation led us to further explore the
possible divergent synthesis of 7a and 8a by screening of
reaction conditions with different acids and bases. As a result,
we found that when 4-hydroxycoumarin (5) was reacted with
6
(R = H) using N-methylmorpholine (NMM, 1.5 equiv) as a
1
base in dioxane at 90 °C for 6 h, the 2,6-dioxabicycles 7a−d
were obtained as the exclusive products (Scheme 3). In
contrast, when the reaction was carried out in the presence of a
catalytic amount of B(OH) in toluene at 120 °C for 8 h, the
3
2
,8-dioxabicycles 8a−d were generated in good yield,
exclusively. The structures of dioxabicyclic isomers 7a−d and
8
Although the photogenerated product of 7a was thermally
stable, all attempts to isolate it failed, mainly due to the low
photoconversion yield. Moreover, some photogenerated
product reverted back to the original form in daylight during
the purification process. To gain insights into the structure of
the photogenerated product, the compounds generated by the
irradiation of 7b and 7c were trapped in situ by an excess of
diazomethane in acetonitrile and hydrogen gas in the presence
a−d can be easily differentiated by their proton NMR spectra.
The characteristic triplet absorption peaks that appeared at a
chemical shift of 5.50 to 6.05 ppm were assigned to the
bridgehead hydrogen for 2,6-dioxabicycles 7a−d, whereas
those at 4.40 to 4.82 ppm were assigned to 2,8-dioxabicycles
8
a−d. The molecular structures of dioxabicycles 7a and 8a
were further verified by the X-ray crystallography. (See the SI.)
B
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Scheme 5. Proposed Mechanism for Trapping of the
Photogenerated Product 11
give the zwitterion 12. The subsequent 1,2-vinyl migration of
1
2 to insert the methylene group into the molecular backbone
along with expelling of nitrogen gas affords the compound 13.
The facile 1,5-hydrogen shift of 13 from the central methylene
hydrogen to the coumarin carbonyl oxygen atom yields the 4-
hydroxycoumarin 14. The final methylation of 14 with the
third equivalent of diazomethane affords the isolated
compound 9.
Figure 1. Absorption spectra of 7a (3.5 × 10−5 M in CH CN)
3
Similar to that of 11, we speculate that the photogenerated
product 15 is formed upon the exposure of 7c to UV
irradiation, as outlined in Scheme 6. The central double bond
obtained with different exposure times (352 nm), 0−21 min, in
increments of 1.5 to 2 min. (inset) Absorption spectra of the
photogenerated product (3.5 × 10− M in CH CN) obtained with
5
3
different exposure times (580 nm), 0−12 h, in increments of 0.5 to 6
h.
Scheme 6. Proposed Mechanism for Trapping of the
Photogenerated Product 15
of palladium on carbon in methanol followed by diazomethane
to afford compounds 9 and 10, respectively, as shown in
Scheme 4. The molecular structures of both 9 and 10 were
Scheme 4. Trapping of the Photogenerated Product
of 15 is then hydrogenated by the exposure to the first
equivalent of hydrogen gas to give the compound 16. The
subsequent 1,5-hydrogen shift of the methylene hydrogen to
the coumarin carbonyl oxygen atom of 16 yields the olefin 17.
Further hydrogenation of the central double bond of 17 with
the second equivalent of hydrogen gas affords 18, which is
unstable and hence is methylated with 2 equiv of diazomethane
to receive the isolated product 10.
fully characterized by spectroscopic analysis. (See the SI for
details.) The isolation of 9 and 10 indicates that the
photogenerated product of 7b reacted with 3 equiv of
diazomethane, and the photogenerated product of 7c reacted
with 2 equiv of hydrogen and diazomethane gases each during
the trapping process. On the basis of the in situ diazomethane
and hydrogen trapping results depicted in Scheme 4, the
structures of the photogenerated products of 7b and 7c were
then elucidated.
Scheme 5 outlines the proposed mechanism for trapping of
the photogenerated product 11. Upon irradiation, the
dioxabicycle 7b is converted to the ring-opened product 11.
In the presence of an excess of diazomethane, the photo-
generated product 11 is then swiftly methylated, followed by
the 1,4-addition of the second equivalent of diazomethane to
Scheme 7 depicts the plausible mechanism for the
photochromic switch between 7a and the photogenerated 20
on the basis of the aforementioned two trapping experiments.
Upon irradiation, the 2,6-dioxabicycle 7a presumably proceeds
with [4 + 4] heterocycloreversion to give the bis(chromane-
2,4-dione) 19, which can easily undergo a 1,5-hydrogen shift
from the central methylene hydrogen to the carbonyl oxygen
atom to yield the red photogenerated product 20. The color of
20 can presumably be attributed to the intramolecular charge
transfer between the electron-rich 4-hydroxycoumarin moiety
and the electron-poor 3-methylenechromane-2,4-dione moi-
16
ety. This hypothesis is partially supported by the UV−vis
C
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Letter
Scheme 7. Proposed Photochromic Mechanism between 7a
and 20
Experimental details and spectral data of 4a,b, 7a−d,
8
a−d, 9, and 10 (PDF)
Accession Codes
CCDC 1952779−1952780 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
Ding-Yah Yang − Department of Chemistry, Tunghai
University, 40704 Taiwan, Republic of China; orcid.org/
0
000-0002-3611-2042; Email: yang@thu.edu.tw
Author
Sameer Vyasamudri − Department of Chemistry, Tunghai
University, 40704 Taiwan, Republic of China
absorption spectra of the photogenerated compound in
solvents with different polarities. (See the SI.) When exposed
to visible light, the colored 20 can be reverted back to the
colorless 2,6-dioxabicycle 7a via the reverse process, that is, a
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c00904
Notes
1
,5-hydrogen shift and a [4 + 4] heterocycloaddition. To the
best of our knowledge, the interconversion between ring-
closed 7a and ring-opened 20 represents the first photo-
chromic system that involves the combination of a [4 + 4]
heterocycloreversion/heterocycloaddition with a 1,5-hydrogen
shift as the switching mechanism.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Ministry of Science and Technology of the
Republic of China, Taiwan, for financially supporting this
research under contract no. MOST 108-2113-M-029-001.
Although, unlike their isomeric 2,6-dioxabicycles 7a−d, the
2
,8-dioxabicycles 8a−d were found to be photochemically
inactive, the structural resemblance of the 2,8-dioxabicyclic
core to the bridged [3.3.1]dioxabicyclic natural products such
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