Ultimate diastereoselectivity in the ring closure of photochromic diarylethene possessing facial chirality

Article abstract of DOI:10.1039/c0cc00240b

A bisthienylethene with hitherto unprecedented facial chirality imposed by a triethyleneglycol bridge on a thiophene ring was synthesized and its photochromic ring closure was shown to occur with 100% diastereoselectivity upon UV-light irradiation.

Full text of DOI:10.1039/c0cc00240b

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Ultimate diastereoselectivity in the ring closure of photochromic
diarylethene possessing facial chiralityw
Tatsuya Shiozawa, Mohammed K. Hossain, Takashi Ubukata and Yasushi Yokoyama*
Received 5th March 2010, Accepted 7th May 2010
First published as an Advance Article on the web 20th May 2010
DOI: 10.1039/c0cc00240b
A bisthienylethene with hitherto unprecedented facial chirality
imposed by a triethyleneglycol bridge on a thiophene ring was
synthesized and its photochromic ring closure was shown to
occur with 100% diastereoselectivity upon UV-light irradiation.
here on the use of this third chirality, i.e., facial chirality, on a
diarylethene to induce 100% diastereoselectivity of photo-
chromic ring closure in various solvents, and the concept is
shown in Scheme 1. If one of the two surfaces of an aromatic
ring (ring A) is occupied by a bulky attachment, ring closure is
possible when the other aromatic ring (ring B) approaches
from the back of ring A. Perfect control of the diastereo-
selectivity during ring closure could, thus, be realized.
Our molecular design and the expected photochromic reac-
tion pathways are shown in Scheme 2. The target open form
1O has a bridge across the surface of thiophene ring A which
prevents access of the other thiophene B to the side of the
bridge. Therefore, this molecule can take only one antiparallel
(cyclizable) conformation. We chose the C2 and C4 carbon
atoms to span the bridge because C3 is used to connect with
hexafluorocyclopentene while C5 is useful in attaching to a
functional substituent. We decided to introduce a phenyl
group on C5 as a typical example. As the bridge, we chose
triethylene glycol (TEG) which is long enough to connect
the methylene carbon atoms on C2 and C4, yet short enough
to prohibit isomerization of the facial chirality with the rope-
jumping action of the TEG bridge over a substituent, such as
the phenyl group, on C5.
Among the photochromic compounds, diarylethenes1 are one
of the most investigated families due to their high durability,
simple synthetic pathways to specifically designed molecules,
and an attractive feature of a modifiable conjugation system
spread all over the molecule which can be switched by photo-
chemical ring-closure and ring-opening reactions. Recent research
into this third property has led to the development of switches
for: (1) the reactivity of the central ethene moiety,2 (2) the
conjugation/deconjugation of the p-conjugated systems attached
to the end of the hexatriene,3 (3) the conjugation path around
the central ethene moiety,4 (4) communication between the
functional groups on both ends of the aryl groups,5,6 and (5)
the hybridization mode of the carbon atoms involved in the
ring closure between sp2 and sp3.7 The hybridization switch
of the carbon atoms induces the generation of a pair of
stereogenic centers upon photocyclization which results in
the generation of a pair of isomeric molecules—enantiomers,
if there are no other chiral units.7
Our group8 and others9–12 have been interested in the
research of how to control the absolute stereochemistry of these
newly generated sp3 carbon atoms not only for its scientific
significance but also for its potential application to biological
materials,13 elaboration of functional materials such as liquid
crystals14–16 and supramolecular systems,10 and other applica-
tions requiring stereoselectivity.17 Most of the previous work
adopted a diastereoselective protocol in which one or two
inherent chiral units were introduced into a molecule in order
to induce an asymmetric ring-closing reaction.
Loosely classified, there are three categories of asymmetry:18
asymmetric carbon atoms, axial chirality, and facial chirality.
We have so far proposed the use of asymmetric carbon atoms8
and axial chirality19 to induce a high diastereoselectivity for
the photochromic ring closure of diarylethenes and fulgides
which both exhibit thermally irreversible photochromic
6p-electrocyclizations. Similar to the case of axial chirality,
facially chiral compounds do not have asymmetric carbon
atoms. In fact, facial chirality has been successfully introduced
to thermally reversible photochromic azobenzenes.20 We report
Thus, a bisthienylethene 1O possessing a facial chirality
which generates only one diastereomer of the closed form
(1C-real) upon photoirradiation was designed. First, we carried
out DFT calculations21 which proved that one of the two
possible antiparallel conformers (1O-major) in which the other
thiophene (B) is located at the back side of the bridge is more
stable by 32.8 kJ molꢀ1 than the other (1O-minor). If the
population at 25 1C is calculated from the difference in heat-
of-formation (DHf) by disregarding the entropy term, the ratio
is 564 800/1. In addition, DHf of the two closed-ring C-forms
(1C-real and 1C-imaginary) is 194.1 kJ molꢀ1, which is too
large to be imaginable.
Encouraged by the calculation results, we undertook the
synthesis of 1O. The synthetic route is shown in Scheme 3. Starting
from 2,5-dibromothiophene, 2,4-dibromo-3,5-bis(chloromethyl)-
thiophene (2)22 was obtained in 49% yield. Interestingly, regio-
selective partial Suzuki coupling on C5 of 2 to introduce a
Department of Advanced Materials Chemistry, Graduate School of
Engineering, Yokohama National University, Hodogaya, Yokohama,
240-8501, Japan. E-mail: yyokoyam@ynu.ac.jp
w Electronic supplementary information (ESI) available: Experimental
details and Fig. S1–S16. See DOI: 10.1039/c0cc00240b
Scheme 1 Molecular modelling concept of facially chiral diarylethenes.
Chem. Commun., 2010, 46, 4785–4787 | 4785
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Scheme 2 Molecular design and photochromic reactions of 1.
phenyl group generated 3-bromo-2,4-bis(hydroxymethyl)-5-phenyl-
thiophene (3) with a 57% yield. This may be the result of
solvolysis of the benzylic chlorides under aqueous basic con-
ditions. The position where the phenyl was introduced was
envisaged by the difference in reactivity and steric hindrance,
the NOE experiments on 3 showing only one of the methylene
protons exhibiting NOE signals,23 and ultimately determined
by the absorption maximum wavelength of the final bisthienyl-
ethene 1C (559 nm in hexane) which is similar to the value of
1,2-bis(2,4-dimethyl-5-phenyl-3-thienyl)hexafluorocyclopentene
4C (562 nm),24 whereas the corresponding 2-thienyl compounds
are known to have absorption bands at shorter wavelengths
than the 3-thienyl compounds.25 Bridging of the C2 and C4
hydroxymethyl groups by a TEG chain by reacting 3 with
triethylene glycol bis(4-toluenesulfonate) in THF in the presence
of NaH under high-dilution conditions afforded the bridged
thiophene 5, though the yield was merely 3%. Introduction of
5 to 1-(2,4-dimethyl-5-phenyl-3-thienyl)heptafluorocyclopentene
626 gave the desired 1O with a 44% yield after purification with
silica gel column chromatography.23
Fig. 1 Absorption spectral changes of 1 in hexane. (a) 1O to pss. 313 nm
(0.71 mW cmꢀ2). 0 to 15 min. (b) Pss to 1O. 512 nm (1.24 mW cmꢀ2).
0 to 150 min.
achieved and the ring-opening reaction upon 512 nm light
irradiation to the pss solution are shown in Fig. 1. A clear
isosbestic point (287 nm) was observed.
The components existing at the pss were examined thoroughly
by HPLC with three different columns and three different
solvent systems. A typical HPLC chromatogram of 1 at pss in
hexane detected at a wavelength of 559 nm is shown in Fig. 2.
1H NMR analysis of the compounds was also carried out,
however, not even any slight evidence of the formation of the
minor C-form was observed. From these results obtained
along with the calculation results, it was concluded that 1O
showed ultimate diastereoselectivity within the detection limit
of the minor diastereomer upon photochromic ring closure
under any of the reaction conditions carried out. The spectro-
scopic data as well as quantum yields of the photoreactions in
the three solvents used are described in Table 1.
Photochromic reactions of 1O were carried out in hexane,
toluene, and ethyl acetate. A change in the absorption spectra
during the ring-closing reaction of 1O in (hexane) with 313 nm
light irradiation until the photostationary state (pss) was
Subsequently, 1O was resolved into enantiomers by an
HPLC equipped with a chiral column (Daicel Co. Chiralpak
IA with CHCl3–hexane = 4.5: 95.5 as the eluent) to investigate
Fig. 2 Detection of diastereomers at the pss of 313 nm irradiation of
1O in hexane. Column: Wakosil 5SIL, Eluant: 33% v/v ethyl acetate–
hexane. Flow rate: 0.5 mL minꢀ1. Detection wavelength: 559 nm.
Scheme 3 Synthetic pathway to 1O.
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Table 1 Absorption spectroscopic data and quantum yields of the
photoreactions of 1
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Solvent
max1O/nm (ea)
Hexane
Toluene
Ethyl acetate
b
l
267 (24 400)
559 (9630)
0.30
0.017
0.0030
86
—
567 (9840)
0.31
268 (2500)
563 (9610)
0.33
0.019
0.0036
86
lmax 1O/nm (ea)
F
F
F
OC(313)
CO(313)
CO(512)
0.015
0.0033
88
CRc(%)
a
b
Molar absorption coefficient/cmꢀ1 molꢀ1 dm3. No absorption
c
maximum was observed due to solvent absorption. Conversion ratio
to the coloured form at pss.
its chiroptical properties and racemization possibility upon
thermal treatment.
The two enantiomers obtained (1Of: moving faster on HPLC
and 1Os: moving slower, 50: 50 ratio) showed a pair of mirror-
image CD spectra.23 At pss, a new CD band appeared in the
visible region due to the absorption of the C-form.
One of the enantiomers (1Of) was refluxed in toluene
(bp 110 1C) for 5 h and its racemization behavior was monitored
by HPLC with a chiral column. As anticipated, no evidence of
racemization of 1Of was observed, showing the phenyl group
at C5 of the thiophene (ring A) is sufficiently large enough to
prohibit the jump-rope-like motion of the TEG bridge which
causes the racemization.
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In conclusion, we have succeeded in synthesizing a bisthienyl-
ethene with hitherto unprecedented facial chirality imposed
by a TEG bridge on a thiophene ring and it showed photo-
chromic ring closure with 100% diastereoselectivity upon UV
irradiation.
This work was supported by a Grant-in-Aid for Scientific
Research in Priority Areas ‘‘New Frontiers in Photochromism
(No. 471)’’ from the Ministry of Education, Culture, Sports,
Science and Technology (MEXT), Japan.
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