Synthesis of photochromic 2,3-bis(5-methyl-2-phenyl-4-thiazolyl)-1,4- naphthoquinone derivatives

Article abstract of DOI:10.1016/j.jphotochem.2011.01.016

2,3-Bis(5-methyl-2-phenyl-4-thiazolyl)-1,4-naphthoquinone 1-O, its monoethylene acetal 2-O and methylated derivatives 3-O and 4-O were synthesized and their photochromic properties were investigated. While bisarylnaphthoquinone 1-O was nonphotochromic, it

Full text of DOI:10.1016/j.jphotochem.2011.01.016

Journal of Photochemistry and Photobiology A: Chemistry 219 (2011) 58–61
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Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Synthesis of photochromic 2,3-bis(5-methyl-2-phenyl-4-thiazolyl)-1,4-
naphthoquinone derivatives
Mahmut Kosea,∗, C¸ ig˘dem Yıldırım S¸ ekercia, Kazushi Suzukib, Yasushi Yokoyamab
a Department of Chemistry, Faculty of Arts and Science, Zonguldak Karaelmas University, 67100, Zonguldak, Turkey
b Department of Advanced Materials Chemistry, Graduate School of Engineering, Yokohama National University, Hodogaya, Yokohama 240-8501, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
2,3-Bis(5-methyl-2-phenyl-4-thiazolyl)-1,4-naphthoquinone 1-O, its monoethylene acetal 2-O and
methylated derivatives 3-O and 4-O were synthesized and their photochromic properties were inves-
tigated. While bisarylnaphthoquinone 1-O was nonphotochromic, its monoacetal 2-O and methylated
derivatives 3-O and 4-O displayed good photochromism. Among them, the isomer 4-O recorded a remark-
able diastereomeric excess (98.40%) with high conversion ratio (96.20%) upon UV irradiation.
© 2011 Elsevier B.V. All rights reserved.
Received 11 November 2010
Received in revised form
31 December 2010
Accepted 21 January 2011
Available online 1 February 2011
Keywords:
Photochromism
2,3-Diaryl-1,4-naphthoquinone
Diarylethene
Conversion ratio
1. Introduction
in the crystalline state [9], the later cause an absolute asymmetric
photocyclisation. Similar work also was conducted in amorphous
Thermally irreversible photochromic compounds such as
fulgides and diarylethenes have received a great attention due
to their possible application as a molecular-level switches [1].
The diarylethenes [2] are superior to the fulgides [3] in terms of
thermal stabilities and fatigue resistance. On the other hand, struc-
tural modifications of diarylethenes are limited only on the aryl
moieties. Recently a number of new photochromic systems sim-
ilar to the diarylethenes such as bisarylindenone [4], terarylene
[5], bisaryl-1,10-phenanthroline [6], and 4,5-bisarylimidazoliums
[7] have been reported. Diarylethenes and related photochromic
analogs undergo reversible ring closure/ring opening photoreac-
tions upon irradiation with different wavelengths of light. The ring
closure photoreaction of diarylethenes produces a racemic mix-
ture of two isomers. More recently a considerable attention has
been focused on the enrichment of one of the stereoisomer in the
photochromic reaction.
state [10] and in gel states by Irie et al. [11].
In this work, our achievement is to synthesis new photochromic
diarylethenes which were derived from a non-photochromic
diarylnaphthoquinone. The diarylnaphthoquinone are structurally
more modifiable than the diarylethenes. Some diarylbenzo-
quinones or naphthoquinone which possess different aryl groups
were already synthesized via slightly different methods to examine
their biological activity [12] or their photochromism [13]. However,
so far photochromic diarylquinone or naphthoquinone or their
derivatives have not yet been reported. The new photochromic iso-
mers which possess one (3-O) or two (4-O) chiral units showed
diastereoselective photochromism. In this case the chiral center
on the molecule of O-form caused fixation of the conformation via
hydrogen bonding between OH groups derived from the carbonyl
groups and nitrogen atoms on the thiazole rings. When the fixed
conformation of O-form is irradiated with UV light a diastereose-
lective ring closure is expected.
Indeed, a number of diastereoselective photochromism in the
diarylethenes was already achieved by different approaches. Main
strategy for diastereoselectivity is to place a chiral substituent on
the molecule. The chiral center helps one of the conformations of
the ring-opened forms favorable compared to others by steric and
electronic effects [8] in solution or complete conformation fixation
2. Results and discussion
2,3-Bis(5-methyl-2-phenyl-4-thiazolyl)-1,4-naphthoquinone
1-O was prepared by the Suzuki Coupling reaction [14] (Scheme 1).
The requisite thiazolyl boronic acid [10] was prepared from
4-bromo-5-methyl-2-phenylthiazole 9 and B(O-iPr)3 with n-BuLi.
Commercially available 2,3-dibromo-1,4-naphthoquinone was
used as the quinone portion. Structural modifications of 1-O on
∗
Corresponding author. Tel.: +90 3722574010; fax: +90 3722574181.
E-mail address: mahmutkose@gmail.com (M. Kose).
1010-6030/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2011.01.016

M. Kose et al. / Journal of Photochemistry and Photobiology A: Chemistry 219 (2011) 58–61
59
H
Cl
Br
O
N
N
Br2 /
N
Me-I
EtOH
+
S
O
Ph
O
Ph
Me
Ph
Me
S
S
n-BuLi /THF
S
THF
Reflux
7
8
9
Ph
NH2
1) n-BuLi /THF
2) (i-PrO)3B
Br
Br
O
O
B(OH)2
N
O
N
S
N
Ph
Me
Me
Me
Pd(PPh3)4 /K2CO3
THF
S
Ph
Ph
S
1-O
10
Scheme 1. Synthesis of 2,3-bis(phenylthiazolyl)-1,4-naphthoquinone 1-O.
the carbonyl group such as the acetal formation and mono- and
di-methylation have been carried out (Scheme 2).
Acetal formation of 1-O using 1,2-ethanediol in the presence of
p-toluenesulfonic acid (p-TSA) as a catalyst in dry benzene gives
photochromic acetal 2-O in a low yield. However, from the acetal
formation, a substantial amount of the starting 1-O was recovered
so that if the recovered compound was re-used for the synthesis the
percentage of the yield could be improved. The stability of acetal
derivatives 2-O in the silica gel column was low and slight decom-
position was observed during purification in the column. Many
Me-I
K2CO3
HO
N
OH
N
MeO
N
OMe
Acetone
N
Me
Me
Me
Me
Ph
Ph
Ph
S
S
S
Ph
S
6
5
Na2S2O4
EtOH / H2O
Me
Me
HO
Me
O
N
O
O
MeLi
HO
+
OH
THF
Ph
N
N
N
N
N
Me
Me
Me
Me
Me
Me
Ph
Ph
Ph
Ph
S
Ph
S
S
S
S
S
3-O
4-O
1-O
OH
OH
, p-TSA
Benzene
O
O
O
N
N
Me
Ph
Ph
Me
S
S
2-O
Scheme 2. Modification of 2,3-bis(phenylthiazolyl)-1,4-naphthoquinone 1-O on carbonyl groups.

60
M. Kose et al. / Journal of Photochemistry and Photobiology A: Chemistry 219 (2011) 58–61
Table 1
Absorption spectral data of 1-O and its photochromic derivatives in toluene before and after irradiation. Compound 1-O and 2-O (1 × 10−4 mol dm−3); 3-O
(7.29 × 10−5 mol dm−3) and 4-O (4.82 × 10−5 mol dm−3). Cell length: 1 cm.
Compounds
O-form
Colored isomer (pss)
ꢀmax/nm (absorbance)
Cr/% at pss
ꢀmax/nm (εmax/mol−1 dm3 cm−1
)
1
2
3
4
425 (3190)
–
305 (24,700)
315 (20,100)
485 (0.812)
490 (0.182)
529 (0.557)
505 (0.392)
–
–
96.51
96.20
attempts to protect both carbonyl groups by ethylene acetal have
failed.
This may be interpreted by the structural deformation of the con-
jugate system due to the steric compression caused by the highly
complex and congested ring system.
The conversion ratios from 3-O to 3-C and 4-O to 4-C at the pss of
313-nm light irradiation were determined by HPLC and were found
to be 96.51% and 96.20% respectively. The HPLC chromatogram of
the colored C-forms (3-C and 4-C) at the pss showed two main dias-
teromers, and the diastereomeric excesses (de) were found to be
65.16% and 98.40%, respectively.
The reason that 4-O exhibits high diastereomeric excess (de)
during the ring-closing photoreaction is not clear at this stage.
However, this could be explained by noting the electronic and steric
effect of the adjacent N atoms (and Me groups) on the thiazole ring
and the OH groups in the molecule (Scheme 3).
The carbonyl group of 1-O was also reacted with one equivalent
of MeLi. From the reaction flask three photochromic spots were
observed on TLC and the primary two of these were isolated. Since
the molecule possesses two carbonyl groups, three isomers are pos-
sible upon methylation. If both carbonyl groups are methylated, cis
and trans isomers may result. The possibility of the occurrence of
the trans isomer is more likely due to the chelation effect of the
alkoxy group generated by the first addition of MeLi to the cationic
part of the second MeLi, so that the isolated isomers were inter-
preted as 3-O (monomethylated) and 4-O (trans-dimethylated),
respectively.
One of our main interest was to synthesize bisaryl-1,4-
hydronaphthoquinone
5
and to investigate its photochromic
Compound 4-O can take two conformations before ring closure.
Presumably, in the major conformation, the N atoms of thiazole and
the OH groups are in the close proximity in the molecule, so that
they could make two sets of hydrogen bonds.
On the other hand, in the minor conformation formation of
the hydrogen bonds between the OH and N atoms is not possible.
Therefore the major conformation is thermally more stable than the
properties as well as the H-bond effect between OH groups and
N atoms of phenyl-thiazole moieties in compound 5. Therefore,
hydronaphthoquinone 5 was prepared quantitatively by treatment
of 1-O with excess sodium dithionate in equal amounts of water
and diethyl ether. During the purification on silica gel, compound
5 was partially oxidized back to the compound 1-O. To improve
the stability of compound 5, the OH groups were protected as the
methyl ethers to give compound 6. Unfortunately, even at a very
low temperature (−78 ◦C), both 5 and 6 were not photochromic.
This could be explained by the presence of stable aromatic rings.
During the photocyclisation process of the 5-O and 6-O, the three
aromatic rings should lose their aromaticity to give highly conju-
gated colored photochromes, yet this may be energetically very
difficult.
Compound 1-O was not photochromic either. A yellow-orange
solution of 1-O in toluene exhibited an absorption maximum
at 425 nm. During irradiation with 313 nm or 436 nm light, the
color intensity of the solution increased greatly. When the colored
solution (ꢀmax at 485 nm) in the photostationary state (pss) was
irradiated with visible light (570 nm), neither color nor spectral
change was observed, thus leading to the conclusion that the ring
closure is not reversible, or other unidentified photochemical side
reaction(s) may have occurred. However, when one or both car-
bonyl groups in 1-O were protected or methylated, the molecule
acquired photochromic character, so that 2-O, 3-O, and 4-O are
photochromic.
Upon irradiation with UV light, the nearly colorless solutions of
2-O, 3-O, and 4-O in toluene underwent photocyclisation to form
highly colored photochromes 2-C, 3-C, and 4-C, which reversed
back to 2-O, 3-O, and 4-O upon exposure to white light. The absorp-
tion spectral data of 1-O – 4-O and their photochromic isomers are
listed in Table 1. The ring-closing photoreaction (313 nm) from 4-O
to pss and the ring-opening photoreaction (>410 nm) from pss to
4-O in toluene are shown in Fig. 1.
The ring-closed photochrome 3-C at pss (ꢀmax = 529 nm) dis-
played bathochromic shift by 24 nm compared to the photochrome
4-C (ꢀmax = 505 nm). This can be explained by the conjugation effect
of the carbonyl group to the main chromophore on the molecular
backbone. However, photochrome 2-C displayed a shorter absorp-
tion maximum wavelength (ꢀmax = 490 nm) compared to others.
1,2
a
0.00 min
1
0,8
0,6
0,4
0,2
0
0.50 min
1.50 min
3.00 min
5.00 min
8.00 min
12.00 min
15.00 min
300
350
400
450
500
550
600
650
700
Wavelength / nm
1,2
1
b
0.00 min
1.00 min
3.00 min
7.00 min
15.50 min
30.00 min
60.00 min
0,8
0,6
0,4
0,2
0
300
350
400
450
500
550
600
650
700
Wavelength / nm
Fig. 1. Absorption spectral change of 4 in toluene (4.82 × 10−5 mol dm−3). (a) 4-O to
pss. 313 nm (0.25 mW cm−2). 0–15 min. (b) Pss to 4-O. ꢀ > 410 nm. 0–60 min.

M. Kose et al. / Journal of Photochemistry and Photobiology A: Chemistry 219 (2011) 58–61
61
Me
HO
Me
HO
Me
OH
OH
H-bond
Me
N
H-bond
N
N
N
Me
Me
Me
Me
Ph
Ph
S
Ph
S
S
Ph
S
minor conformation
major conformation
UV
UV
Me
OH
Me
Me
OH
Me
N
HO
N
HO
N
Me
Me
N
S
S
S
S
Ph
Ph
Ph
Ph
Me
Me
minor diastereomer
major diastereomer
Scheme 3. Steric/electronic stereoregulation in compound 4-O.
minor conformation so that this compound possessing trans orien-
tation of the hydroxyl groups yields extremely high diastereomeric
excess.
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Detailed synthetic procedure (SI-1); UV–Vis spectral data
for the photochromic compounds 2-O, and 3-O (SI-2–SI-3);
and HPLC charts for 3-O and 4-O during photoirradiation
(SI-4–SI-5–SI-6–SI-7–SI-8–SI-9–SI-10). Supplementary data asso-
ciated with this article can be found, in the online version, at
doi:10.1016/j.jphotochem.2011.01.016.
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