A straightforward access to photochromic diarylethene derivatives via palladium-catalysed direct heteroarylation of 1,2-dichloroperfluorocyclopentene

Article abstract of DOI:10.1039/c2cc37046h

A novel and efficient palladium-catalysed direct di-heteroarylation of 1,2-dichloroperfluorocyclopentene with a variety of heteroarenes is reported, giving rise to 1,2-di(heteroaryl)ylperfluorocyclopentene photochromic compounds. The reaction proceeds wit

Full text of DOI:10.1039/c2cc37046h

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COMMUNICATION
A straightforward access to photochromic diarylethene derivatives
via palladium-catalysed direct heteroarylation of
1,2-dichloroperfluorocyclopentenew
´
Kassem Beydoun, Julien Roger, Julien Boixel, Hubert Le Bozec, Veronique Guerchais* and
Henri Doucet*
Received 28th September 2012, Accepted 16th October 2012
DOI: 10.1039/c2cc37046h
A novel and efficient palladium-catalysed direct di-heteroarylation
of 1,2-dichloroperfluorocyclopentene with a variety of heteroarenes
is reported, giving rise to 1,2-di(heteroaryl)ylperfluorocyclopentene
photochromic compounds. The reaction proceeds with thiazoles,
thiophenes or furan derivatives and tolerates various substituents.
The palladium-catalysed direct arylation or vinylation of hetero-
aromatics has recently emerged as a very powerful method for
the preparation of substituted heteroaromatics.1–5 However,
there are still limitations for these reactions in terms of substrate
scope. If the coupling of various aryl halides5 with heteroarenes
has been largely described, on the other hand, the coupling of
vinyl halides and especially vinyl chlorides has attracted much
less attention.6
Scheme 1 Photochromism of (a) 3-thienyl/4-thiazolyl and (b) 2-thienyl/
Research in design and preparation of photoswitchable
molecules such as diarylethene containing heterocyclic rings
(DAE) has been driven by their potential applications in
optical data storages and molecular switches.7 The chemistry
of photochromic DAE derivatives, in which the two heteroaryl
rings are connected to the ethene bridge at the C3-thienyl or
C4-thiazolyl position has been widely developed (Scheme 1a).7
By contrast, the design of DAE derivatives incorporating
bis(2-thienyl)ethene and bis(5-thiazolyl)ethene fragments, whose
photochromic and optical properties are different from the
former species, has been much less explored (Scheme 1b).8
DAE derivatives are generally prepared in two steps via
lithiation of thiophenes or thiazoles and requires the use of the
toxic and volatile octafluorocyclopentene C5F8.7,8 Therefore,
to overcome these limitations, a straightforward and selective
method for the synthesis of 1,2-di(heteroaryl)perfluorocyclo-
pentenes is highly desirable.
5-thiazolyl containing DAE derivatives.
Suzuki–Miyaura cross-coupling (Scheme 2, top).9 Based on
this result, and also on our results on palladium-catalysed
direct arylation or vinylation of heteroaromatics,2,6a we
decided to investigate the synthesis of 1,2-di(heteroaryl)-
perfluorocyclopentenes via the direct coupling of 1,2-dichloro-
perfluorocyclopentene with various heteroarenes (Scheme 2,
bottom).
The palladium-catalysed direct heteroarylation for access to
1,2-di(heteroaryl)perfluorocyclopentenes would present con-
siderable advantages. It would allow us to reduce the number
of steps to prepare these compounds. In addition, such couplings
Very recently, it has been reported that dithienylper-
fluorocyclopentene can be easily obtained in high yield by
Institut Sciences Chimiques de Rennes, UMR 6226 CNRS-Universite´
´
´
de Rennes 1 ‘‘Organometalliques, Materiaux et Catalyse’’, 35042
Rennes Cedex, France. E-mail: henri.doucet@univ-rennes1.fr,
veronique.guerchais@univ-rennes1.fr; Tel: +33 (0)2 23 23 63 84
w Electronic supplementary information (ESI) available: Procedures;
NMR data; additional absorption data and UV-visible spectra for
compounds 2, 3a, 4, 6 and 12. See DOI: 10.1039/c2cc37046h
Scheme 2
c
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Chem. Commun., 2012, 48, 11951–11953 11951

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are expected to present a better functional group tolerance,
which would allow a straightforward modification of the
nature of the substituents and hence of the photochromic
properties of the resulting 1,2-di(heteroaryl)perfluorocyclo-
pentene derivatives.
PdCl(C3H5)(dppb) as the catalyst also produces 1a in high yield,
whereas the use of 5 mol% Pd(OAc)2 in the absence of phosphine
ligand was completely ineffective (Table 1, entries 7 and 8).
The scope of this reaction was examined using various hetero-
arenes (Scheme 3). A good yield of 68% in 2 was obtained by
coupling of 1,2-dichloroperfluorocyclopentene with 2-ethyl-
4-methylthiazole. By using a 1 : 1 ratio of these reactants, the
formation of a mixture of the mono-heteroarylated cyclopentene
and 2 in a 4 : 1 ratio was observed by GC/MS. However, under
these conditions, 48% of the 1,2-dichloroperfluorocyclopentene
was recovered unreacted. A lower yield in 3a was obtained from
2-phenyl-4-methylthiazole due to the formation of a mixture of the
mono- and di-heteroarylated alkenes 3a and 3b in a 38 : 43 ratio
after 24 h. Then, the reactivity of several thiophene derivatives was
examined. From 2-acetyl-4-methylthiophene, 4 was obtained in
69% yield using 5 mol% PdCl(C3H5)(dppb) as the catalyst. No
significant formation of the mono-heteroarylated product was
formed. Next, in order to obtain building blocks allowing a simple
access to a variety of di(heteroarylated)perfluorocyclopentenes, we
employed 2-acetyl-4-chlorothiophene as the reactant. The desired
We now report (i) conditions for the palladium-catalysed
direct coupling of heteroaromatics with 1,2-dichloroperfluoro-
cyclopentene (ii) the synthesis of a variety of new DAE
derivatives and preliminary studies on their photochromic
properties (iii) that this coupling method tolerates several
useful functional groups.
Keeping in mind the reaction conditions employed for the direct
arylation of heteroaromatics in our previous works,2,6a we initially
examined the influence of the nature of the solvent for the coupling
of 1,2-dichloroperfluorocyclopentene with 2-i-butylthiazole
(Table 1). We observed that, the use of 5 mol% Pd(OAc)2 as the
palladium source, 5 mol% PCy3 as the ligand, DMAc as the
solvent using KOAc as a base and Bu4NBr as an additive at 150 1C
did not result in the formation of the target product 1a (Table 1,
entry 1). Similar results were obtained at 130 1C using either PCy3
or dppb as the ligands and cyclopentyl methyl ether (CPME) as the
solvent (Table 1, entries 2 and 3). On the other hand, the use of
CPME as the solvent for this reaction, using 10 mol% Pd(OAc)2 as
the palladium source and 10 mol% PCy3 as the ligand at 130 or
120 1C, without addition of Bu4NBr, afforded cleanly the desired
product 1a in 74% yield (Table 1, entry 4). Surprisingly, no trace of
the mono-heteroarylated product 1b was detected under these
conditions. We had previously observed that CPME promotes
the palladium-catalysed direct arylations in high yields with
some heteroaromatic derivatives.10 This solvent presents several
advantageous features such as limited miscibility in water and low
formation of peroxides.11 A slightly lower reaction temperature of
120 1C allowed the increase of the yield in 1a to 91% (Table 1,
entry 5). It should be noted that the use of
5 mol%
Table 1 Influence of the reaction conditions for palladium-catalysed
direct coupling of 1,2-dichloroperfluorocyclopentene with 2-i-
butylthiazole
Temp./
1C
Ratio Yield
Solvent Bu4NBr 1a : 1b (%)
Entry Catalyst (mol%)
1
2
3
4
5
6
7
8
Pd(OAc)2/PCy3 (10) 150
Pd(OAc)2/PCy3 (10) 130
Pd(OAc)2/dppb (10) 130
Pd(OAc)2/PCy3 (10) 130
Pd(OAc)2/PCy3 (10) 120
DMAc 1 equiv. nd
CPME 1 equiv. nd
CPME 1 equiv. nd
0
0
0
CPME —
CPME —
CPME —
CPME —
CPME —
100 : 0 74
100 : 0 91 (82)
100 : 0 59
Nd
97 : 3 90 (80)
Pd(OAc)2/PCy3 (5)
Pd(OAc)2 (5)
120
120
0
PdCl(C3H5)(dppb) (5) 120
Conditions: catalyst: [Pd], 2-i-butylthiazole (1.5 mmol), 1,2-dichloro-
perfluorocyclopentene (0.5 mmol), KOAc (1.5 mmol), under argon,
16 h, GC and NMR yields. Yields in parentheses are isolated. CPME:
cyclopentyl methyl ether.
Scheme 3 Palladium-catalysed direct coupling of heteroaromatics
with 1,2-dichloroperfluorocyclopentene: substrate scope.
c
11952 Chem. Commun., 2012, 48, 11951–11953
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product 5 was obtained in 67% yield. Interestingly, in the course of
this reaction no cleavage of the C–Cl bond on thiophene was
observed. Then, we employed a 2-formyl-4-bromothiophene
protected as an acetal. The target compound 6 was isolated in
80% yield, again without cleavage of the C–Br bond of thiophene.
Finally, the reactivity of three thienoquinolines and two fluoro-
quinolines was examined. The desired products 7–11 were obtained
in 46–79% yields.
offers a fast and direct access to symmetric and unsymmetric 1,
2-di-(heteroaryl)perfluorocyclopentenes. This protocol is applicable
to a range of functions, including reactive ones, on the heteroarene.
Such functional group tolerance allows the easy modification of the
electronic structure of such derivatives, a strategy enabling the
tuning of their optical and photochromic properties. Moreover,
due to its higher boiling point, the handling of 1,2-dichloroperfluor-
ocyclopentene is more convenient than octafluorocyclopentene.
For these reasons, this process should give a more economically
viable and environmentally attractive access to several 1,
2-di(heteroaryl)ylperfluorocyclopentenes.
As the synthesis of the mono-heteroarylated alkene 3b was
possible, we attempt to synthesize a 1,2-di(heteroaryl)perfluoro-
cyclopentene derivative bearing two different heteroaryl units.
From 3b and protected 2-formyl-4-bromothiophene, 12 was
obtained in 75% yield (Scheme 4).
K. B. is grateful to the CNRS and ‘‘Region Bretagne’’ for a
´
PhD grant. This work was supported by ANR COMET.
The photochromic behaviour of compounds 2, 3a, 4, 6 and 12
has been monitored by UV-visible absorption (Fig. 1, Fig. S1 and
Table S2 (see ESIw)). The thienyl- and the thiazolyl-based
compounds 6 and 2 were used as representative examples. Fig. 1
shows their absorption spectral changes in dichloromethane upon
light irradiation. The initial solution of 6 was colorless with an
intense absorption band in the UV region (lmax = 346 nm)
(Fig. 1a). Upon UV irradiation (l = 350 nm), the solution turned
yellow with a concomitant decrease of the bands in the UV and the
appearance of a new band in the visible (lmax = 430 nm). The
1H NMR spectrum displayed two new singlets at 5.93 and 5.77,
upfield shifted by ca. 0.3 ppm, which are assigned to the thiophene
protons. These features are consistent with the formation of the
closed form,8a with a photocyclization conversion of 83%. The
thiazole derivative 2 shows a similar photochromic behaviour under
the same conditions (Fig. 1b). For both compounds 6 and 2, the
yellow color of the solution was easily bleached with visible light
irradiation (l = 450 nm) with the initial spectrum being fully
recovered (Fig. 1), indicating the fully reversible closed-to-open
photoisomerization.
Notes and references
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2 For selected recent examples of palladium-catalysed direct arylation
from our laboratory: (a) L. Chen, J. Roger, C. bruneau,
P. H. Dixneuf and H. Doucet, Chem. Commun., 2011, 47, 1872;
(b) M. Baloch, R. J. Roy, D. Roy, K. Beydoun and H. Doucet, RSC
Adv., 2011, 1, 1527; (c) K. Beydoun, M. Zaarour, J. A. G. Williams,
H. Doucet and V. Guerchais, Chem. Commun., 2012, 48, 1260.
3 A. Ohta, Y. Akita, T. Ohkuwa, M. Chiba, R. Fukunaga, A. Miyafuji,
T. Nakata, N. Tani and Y. Aoyagi, Heterocycles, 1990, 31, 1951.
4 For selected examples of palladium-catalysed direct arylation of
thiophenes: (a) S. Pivsa-Art, T. Satoh, Y. Kawamura, M. Miura and
M. Nomura, Bull. Chem. Soc. Jpn., 1998, 71, 467; (b) L. Lavenot,
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5 (a) H. Kamiya, S. Yanagisawa, S. Hiroto, K. Itami and
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6 For examples of palladium-catalysed direct couplings of vinyl halides
with heteroarenes: (a) A. L. Gottumukkala, F. Derridj, S. Djebbar
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Scheme 4 Palladium-catalysed synthesis of unsymmetric DAE 12.
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Fig. 1 UV-visible absorption spectra of the open form (black line),
photostationary state after UV irradiation at 350 nm (red line) and
after irradiation at 450 nm (dashed grey line) of (a) 6 and (b) 2.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 11951–11953 11953