Synthetic protocol for diarylethenes through Suzuki-Miyaura coupling

Article abstract of DOI:10.1039/c1cc12020d

The synthesis of a variety of diarylethenes through the Suzuki-Miyaura coupling reaction of 1,2-dichlorohexafluorocyclopentene with arylboronic acids and esters has been developed. Thiophenes with various substituents such as cyano and ester functionalities can be incorporated.

Full text of DOI:10.1039/c1cc12020d

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COMMUNICATION
Synthetic protocol for diarylethenes through Suzuki–Miyaura couplingw
Satoru Hiroto, Katsuya Suzuki, Hiroki Kamiya and Hiroshi Shinokubo*
Received 9th April 2011, Accepted 9th May 2011
DOI: 10.1039/c1cc12020d
The synthesis of a variety of diarylethenes through the Suzuki–
Miyaura coupling reaction of 1,2-dichlorohexafluorocyclopentene
with arylboronic acids and esters has been developed. Thiophenes
with various substituents such as cyano and ester functionalities
can be incorporated.
Table 1 Optimization of Suzuki–Miyaura coupling with 3-thienylboronic
acida
In recent years, organic photochromic compounds have attracted
much attention due to the potential applications for photo-
switching materials.1 Among various photochromic systems,
diarylethene is one of the most prospective organic molecules
owing to its advantageous properties over other photochromic
compounds in terms of its thermal stability and fatigue
resistance as well as its intriguing photochromic behaviour.2
Among various platforms for diarylethenes, a perfluoro-
cyclopentene unit exhibits the highest reversible durability
and photo-responsibility.3 However, their synthetic procedure
has been limited so far to nucleophilic addition–elimination
reaction of perfluorocyclopentene with unstable heteroaryl-
lithium reagents at low temperatures.2a,4 Although this method
has been successfully applied to the synthesis of a variety of
diarylethenes, the reaction requires the moisture-free and cryogenic
conditions and often results in low yields. Furthermore, the
use of highly reactive organolithium species does not tolerate
the presence of a functionality such as carbonyl and cyano
groups. In addition, perfluorocyclopentene is rather volatile
(bp 27 1C) and not easy to handle. On the other hand, 1,2-dichloro-
hexafluorocyclopentene is less volatile (bp 90 1C). Here we
report the novel synthetic method for diarylethenes through
Suzuki–Miyaura cross-coupling.
Entry
Ligand
Time/h
Yield (%)
1
2
3
4
PPh3
16
16
3
0
70
67
86
S-Phos
X-Phos
X-Phos
16
a
Reaction conditions: 3-thienylboronic acid (3.0 equiv.), 1,2-dichloro-
hexafluorocyclopentene (1.0 equiv.), Pd2dba3ÁCHCl3 (1 mol% Pd),
phosphine ligand (2 mol%), K3PO4 (3.0 equiv.), 100 1C.
in the presence of X-Phos and Pd2dba3ÁCHCl3 furnishes 2a in
86% yield (Table 1, entry 4, method A). We then applied this
procedure of method A to 2-methylthienyl derivatives, because
most of the diarylethenes have 2-methyl substituents to improve
the stability of diarylethenes after photo-induced ring-closure.
Unfortunately, however, the cross-coupling with 2-methyl-
3-thienylboronic acid under the same reaction conditions with
X-Phos resulted in low yield. Eventually, the use of tricyclo-
hexylphosphine and CsF in toluene/H2O (10/1) furnished the
desired diarylethene 2b in 82% yield (Table 2, entry 6, method B).
These reaction conditions can be applied to various substrates
(Table 3). Other aromatic groups such as phenyl and 3-furyl
can be introduced to provide 2c and 2d in good yields by
method A (entries 1 and 2). Boronic esters, which can be
prepared efficiently via transition-metal catalysis, can also be a
coupling partner in this reaction. The reaction of 2,5-dimethyl-
3-thienylboronic ester 1f resulted in poor yield under the same
conditions as those with boronic acids, but the yield of 2e was
improved up to 56% by the use of 9.0 equiv. of CsF and 10 mol%
of the palladium catalyst. Under these conditions, various
arylboronic esters can be converted into the corresponding
diarylethenes. Thienyl groups with ester and cyano function-
alities can be introduced to furnish the corresponding diaryl-
ethenes 2g and 2h in moderate yields (entries 5 and 6). These
reactive substituents cannot be tolerated in the conventional
diarylethene synthesis with organolithium reagents. However,
the reaction with 2-formylthienylboronic ester 1i resulted in
low yield of 2i because of the serious side reactions (entry 7).
We anticipated that cross-coupling of 1,2-dichlorohexa-
fluorocyclopentene would be facilitated by a proper choice
of phosphine ligands. Optimization of the cross-coupling con-
ditions was conducted with 3-thienylboronic acid as a model
substrate. The use of triphenylphosphine as a ligand provided
none of the desired diarylethenes 2a. After several trials, we
found that the employment of Buchwald’s ligands, S-Phos and
X-Phos, worked nicely.5 The reaction of 3-thienylboronic acid
Department of Applied Chemistry, Graduate School of Engineering,
Nagoya University, Chikusa-ku, Nagoya 464-8603, Japan.
E-mail: hshino@apchem.nagoya-u.ac.jp; Fax: +81-52-789-5113;
Tel: +81-52-789-5113
w Electronic supplementary information (ESI) available: Experimental
procedures, spectral data for all new compounds, photophysical data,
and crystallographic data of 4a, CCDC 820786. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/
c1cc12020d
c
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Chem. Commun., 2011, 47, 7149–7151 7149

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Table
2
Optimization of Suzuki–Miyaura cross-coupling with
Table 3 Synthesis of diarylethenes with various arylboronic acids
and estersa
2-methyl-3-thienylboronic acida
Entry Substrate
Method Product Yield (%)
1
1c
1d
A
A
2c
2d
91
88
2
3
4
5
6
7
Entry
Base
Ligand
Solvent
Yield (%)
1
2
3
4
5
6
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
CsF
X-Phos
S-Phos
tBu3PÁHBF4
S-Phos
PCy3
Dioxane
Dioxane
Dioxane
Toluene/H2O
Toluene/H2O
Toluene/H2O
22
48
20
63
70
82
1e
1f
B
B
B
B
2e
2e
2g
2h
88
PCy3
56b
63b
45b
a
Reaction conditions: 2-methyl-3-thienylboronic acid (3.0 equiv.),
1,2-dichlorohexafluorocyclopentene (1.0 equiv.), Pd2dba3ÁCHCl3
(1 mol% Pd), phosphine ligand (2 mol%), base (3.0 equiv.).
1g
1h
None of the monoarylated products were observed in any cases,
even though an excess amount of dichlorocyclopentene was
employed. The reaction with thiazolylboronic ester 1j also
afforded only diarylated product 2j in 69% yield (entry 8). This
feature is notable because the conventional organolithium method
often provided monothiazolylcyclopentenes predominantly.6
The uniqueness of the present procedure was demonstrated
by the efficient introduction of porphyrin moieties into the
hexafluorocyclopentene ring, which is quite difficult by the con-
ventional organolithium method. Lithiated porphyrin derivatives
are difficult to be generated due to their electron rich charac-
teristic and reactive feature against lithium reagents.7 Although
the reaction with meso-borylporphyrins resulted in only proto-
deborylation, 2-boryl-5,10,15-tris(3,5-di-tert-butylphenyl)porphyrin
Ni(II) 3a underwent smooth coupling reaction to afford
disubstituted product 4a in 45% yield under the conditions
of method B (Scheme 1).8 However, the reaction with Zn(II)
and free-base porphyrins only afforded deborylated porphyrins
without the formation of the desired products. The structure
of product 4a was assigned by the 1H NMR spectrum, exhibiting
two singlet peaks at 9.64 and 9.31 ppm and six doublet peaks
in the region from 8.62 to 8.48 ppm. The reaction with 2-boryl-
5,10,15-trihexylporphyrin Ni(II) 3b also furnished the diarylethene
4b in 54% yield. The 1H NMR spectrum of 4b displayed
relatively downfield-shifted signals for five b-protons from
9.08 to 8.91 ppm and upfield-shifted signals for two b-protons
at 8.37 and 7.98 ppm. These upfield-shifts can be accounted for
by the aromatic ring current effect due to the adjacent porphyrin
ring, implying a strongly packed structure of 4b. The exact
structure of 4a was unambiguously elucidated by X-ray diffrac-
tion analysis (Fig. 1). The two porphyrin rings adopted a slipped
conformation with the dihedral angle of 611 between two
adjacent pyrrole planes. The bond length between C2A–C1C
is 1.45 A and the bond length between C1C–C5C is 1.35 A,
that is a typical bond length for non-conjugated C–C double
bonds. This fact suggests that each porphyrin unit does not
have significant p-conjugation to the perfluorocyclopentene
moiety. As a diarylethene, diporphyrin 4a takes the photo-
reactive antiparallel conformation. Furthermore, the intra-
molecular distance between C1A–C1B is 3.69 A, which is
1i
1j
B
B
2i
2j
29b
69b
8
a
Reaction conditions: method A: arylboronic acid (3.0 equiv.), 1,2-dichloro-
hexafluorocyclopentene (1.0 equiv.), Pd2dba3ÁCHCl3 (1 mol% Pd),
X-Phos (2 mol%), K3PO4 (3.0 equiv.), dioxane, reflux, 16 h; method B:
arylboronic acid or ester (3.0 equiv.), 1,2-dichlorohexafluorocyclopentene
(1.0 equiv.), Pd2dba3ÁCHCl3 (1 mol% Pd), PCy3 (2 mol%), CsF
b
(3.0 equiv.), toluene/H2O, reflux, 16 h. Pd2dba3ÁCHCl3 (10 mol%)
and CsF (9.0 equiv.) were used under otherwise the same conditions.
within the distance capable of photocyclization.2d Unfortunately,
however, both products 4a and 4b did not exhibit any photo-
chromic behaviour by photo-irradiation, probably due to steric
repulsion by meso-substituents on the porphyrin rings as well as
rapid quenching of the excited state by the nickel centre.
Fig. 2 shows UV/vis absorption spectra of 4a and 4b in
CH2Cl2. Both compounds showed typical spectra of porphyrins,
showing Soret bands at 409 and 410 nm and Q-bands at 532
and 574 nm and 541 and 584 nm, respectively. The broadening
of Soret bands of 4a and 4b can be explained by exciton coupling
between two porphyrin units. Compared to the corresponding
porphyrin monomer, red-shift of the Q-band was observed for
both 4a and 4b, indicating a slight electronic conjugation of
porphyrin rings and the perfluorocyclopentene moiety.9
In conclusion, we have demonstrated novel and versatile
synthesis of diarylethenes through a Suzuki–Miyaura cross-
coupling reaction. The present protocol is applicable for various
substrates bearing reactive functionalities such as cyano and
ester moieties, which cannot be compatible under the conventional
diarylethene synthesis using organolithium reagents. This procedure
c
7150 Chem. Commun., 2011, 47, 7149–7151
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Scheme 1 Suzuki–Miyaura cross-coupling of dichlorocyclopentene
with b-borylporphyrins.
Fig. 2 UV/vis absorption spectra of 4a (red solid line), 4b (blue
solid line) and 5,10,15-trihexylporphyrin Ni(II) (black dotted line) in
CH2Cl2.
(No. 471)’’ from the Ministry of Education, Culture, Sports,
Science and Technology (MEXT), Japan, and the Global COE
program in Chemistry of Nagoya University.
Notes and references
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9 For diarylethenes with covalently connected porphyrins, see:
(a) A. Osuka, D. Fujikane, H. Shinmori, S. Kobatake and
M. Irie, J. Org. Chem., 2001, 66, 3913; (b) H. D. Samacgetty
and N. R. Branda, Chem. Commun., 2005, 2840; (c) T. B. Norsten
and N. R. Branda, J. Am. Chem. Soc., 2001, 123, 1784;
(d) S. D. Straight, P. A. Liddle, Y. Terazono, T. A. Moore,
A. L. Moore and D. Gust, Adv. Funct. Mater., 2007, 17, 777;
(e) P. A. Liddel, G. Kodis, A. L. Moore, T. A. Moore and
D. Gust, J. Am. Chem. Soc., 2002, 124, 7668.
Fig. 1 X-Ray crystal structure of 4a. (a) Top view and (b) side view.
meso-Aryl substituents and hydrogen atoms are omitted for clarity.
Thermal ellipsoids are at 50% probability level.
also allows us to synthesize novel and exotic diarylethenes, as
exemplified by the efficient construction of 1,2-diporphyrinyl-
cyclopentenes.
This work was supported by a Grant-in-Aid for Scientific
Research in Priority Areas ‘‘New Frontiers in Photochromism
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 7149–7151 7151