Synthesis and photochromic behaviour of new dipyrrolylperfluorocyclopentenes

Article abstract of DOI:10.1039/b302623j

The synthesis of new photoswitchable dipyrrolylethenes having different π-conjugated chain lengths, employing Wittig-Horner reaction, Knoevenagel condensation, Suzuki palladium catalyzed cross-coupling or Sonogashira coupling, is reported. Their photochemical behaviour, and in particular, their thermal stability has been investigated upon continuous irradiation with light of appropriate wavelengths.

Full text of DOI:10.1039/b302623j

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Synthesis and photochromic behaviour of new
dipyrrolylperfluorocyclopentenes
´
Arnault Heynderickx, Ali Mohamed Kaou, Corinne Moustrou, Andre Samat* and
Robert Guglielmetti
´
Groupe de Chimie Organique et Materiaux Moleculaires, UMR 6114 CNRS, Universite de la
Mediterranee, Faculte des Sciences de Luminy, Case 901, 13288, Marseille Cedex 9, France.
´
´
´
´
´
E-mail: samat@luminy.univ-mrs.fr; Fax: 33 4 91 82 93 01
Received (in Montpellier, France) 7th March 2003, Accepted 21st May 2003
First published as an Advance Article on the web 26th August 2003
The synthesis of new photoswitchable dipyrrolylethenes having different p-conjugated chain lengths,
employing Wittig–Horner reaction, Knoevenagel condensation, Suzuki palladium catalyzed cross-coupling
or Sonogashira coupling, is reported. Their photochemical behaviour, and in particular, their thermal
stability has been investigated upon continuous irradiation with light of appropriate wavelengths.
Introduction
In recent years, organic photochromic compounds that rever-
sibly change their physical and chemical properties upon irra-
diation have received increasing attention due to their
potential applications for optoelectronic devices.1 So far, var-
ious photochromic families, such as spiropyranes,2 spiro-
oxazines,3 azobenzenes,4 fulgides,5 and diarylethenes6 have
been developed to satisfy the specific needs of each new device.
Scheme 1 Photochromism of dipyrrolylethenes.
In the field of the design of materials undergoing variable
optical transmission such as ophthalmic plastic lenses, photo-
chromic compounds must cope with several requirements: a
thermally reversible colour change, a relatively fast thermal
fading at room temperature (few seconds), a low fatigue or
chemical instability upon repeated cycling or continuous irra-
diation, a highly efficient photoresponse in the near-UV, and
a minimal quantum yield for bleaching with visible light in
order to preserve the colour under sunlight exposure.
Spiropyranes, spironaphthoxazines and naphthopyrans have
been studied to these aims, but a major problem arised from
their fatigue resistance even if spironaphthoxazines brought
important improvement. Further development of new systems
is warranted.
Diarylethenes having perfluorocyclopentene moieties may
be promising candidates for these practical applications owing
to their excellent characteristics regarding degradation even
in the presence of air during photocyclisation.6,7 Various
diarylethenes can undergo more than 104 cycles of colora-
tion–bleaching before significant evidence of fatigue. It is
worthwhile to note that the thermal stability of the colored
closed-ring isomers is dependent on the aryl group type. When
the aryl groups are furan, thiophene, selenophene or thiazole
rings, the closed form are stable and do not return to the
open-ring isomer even at 80 ꢀC. On the other hand, photogen-
erated closed-ring isomers of diarylethenes with pyrrole7a,b,8
(Scheme 1), indole or phenyl rings undergo thermally reversi-
ble reaction.6 Other parameters such as bulky substituents or
strong electron-withdrawing substituents can assist the thermal
reversibility.9
Results and discussion
Syntheses
The synthetic approach to these compounds began with Irie’s
1,2-bis(2-cyano-1,5-dimethyl-4-pyrrolyl)hexafluorocyclopen-
tene 19. Despite its highly attractive synthetic interest as
precursor, the expensive and rather volatile material octafluoro-
cyclopentene and the low yield (11%) occurring during the
double substitution reaction of octafluorocyclopentene with
4-lithio-1,5-dimethyl-2-pyrrolcarbonitrile is a major disadvan-
tage for applications.
In order to improve the yield for the preparation of 1,
the bromine–lithium exchange, performed from reaction
between 4-bromo-1,5-dimethyl-2-pyrrolcarbonitrile with
slight excess of n-butyllithium atꢁ100 ꢀC in THF, was exam-
ined (Scheme 2). The lithio derivative was trapped by a mix-
ture of HCl, ethanol and THF, at various reactions times.
When the reaction time is less then 15 minutes, a small amount
of starting material is still recovered and for longer reaction
times, the yield is decreasing.
a
The most efficient conditions were obtained for a reaction
time of 15 minutes, affording 1,5-dimethyl-2-pyrrolcarbonitrile
in excellent yield. Then the lithio intermediate was reacted with
an equimolar amount of octaperfluorocyclopentene, at low
temperature, for longer time than the protocol previously
described. With this optimized reaction condition, the diaryl-
ethene 1 can be conveniently prepared on a multigram scale,
in 74% yield.
In that context, the synthesis of new dipyrrolylperfluoro-
cyclopentenes and their photochromic properties having differ-
ent p-conjugated chain lengths have been investigated.
Selective reduction of the cyano groups10 performed with
diisobutylaluminium hydride (Dibal-H) in dichloromethane,
followed by hydrolysis, gave the key dialdehyde 2 in 79% yield.
DOI: 10.1039/b302623j
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Scheme 2 Synthesis of products 1–9 by Wittig–Horner reaction and Knoevenagel condensation. Reagents and conditions: (i) 1.05 equiv. n-Buli,
THF, ꢁ100 ꢀC, t min then hydrolysis by a mixture (HCl–EtOH–THF; 1 2 2), ꢁ78 ꢀC. (ii) 1.05 equiv. n-Buli, THF, ꢁ100 ꢀC, 15 min; (iii) 0.45 equiv.
octaperfluorocyclopentene, ꢁ100 ꢀC, 30 min then ꢁ78 ꢀC, 4 h and ꢁ30 ꢀC, 2 h. (iv) Dibal-H–CH2Cl2 , ꢁ50 ꢀC to 0 ꢀC; (v) 2-dimethoxyphosphinyl-
1,3-benzodithiol (1.05 or 2.2 equiv.), n-Buli, THF, ꢁ78 ꢀC ! 20 ꢀC; (vi) 2-dimethoxyphosphinyl-4,5,6,7-tetrahydro-1,3-benzodithiol (1.05 or 2.2
equiv.), n-Buli, THF, ꢁ78 ꢀC ! 20 ꢀC; (vii) malononitrile (1.05 or 2.2 equiv.), benzene, piperidine, 20 ꢀC.
Further Wittig–Horner reaction of 2 with phosphonate carb-
anion (1.05 or 2.2 equiv.), generated in vivo from deprotonation
of 2-dimethoxyphosphinyl-1,3-benzodithiol11 or 2-dimethoxy-
phosphinyl-4,5,6,7-tetrahydro-1,3-benzodithiol12 with n-butyl-
lithium, was achieved to produce compounds 3–6. Then, the
aldehydes 2, 3 and 5 were converted by Knoevenagel condensa-
tion13 employing malononitrile (1.05 or 2.2 equiv.) to give
symmetric and unsymmetric compounds 7–9 as illustrated in
Scheme 2.
et al.,19 using dimethylformamide dimethylacetal in alcohol.
The compound 11 was isolated in 96% yield starting from
10, in refluxing methanol, providing that an excess of sodium
methylate was added, after one night of reaction, so as to
accelerate the reaction of the intermediate N-acylformamidine.
The methoxycarbonyl group can easily be hydrolysed,20 at
reflux temperature, to afford the corresponding carboxylic acid
12 in excellent yield.
Dipyrrolylethenes 7 and 8 may be expected to display pro-
nouncedly different and reversible nonlinear optical features
between the two forms (open and closed), switching being trig-
gered by UV and visible irradiation. They constitute donor-
p-acceptor (D-p-A) systems comprising 1,3-dithiol moieties
as terminal electron-donating group and dicyanomethylene as
electron-accepting group.14
The next target was the 1,2-bis(2-iodo-1,5-dimethylpyrrol-
4-yl)perfluorocyclopentene 13 which could be regarded as an
excellent precursor for the design of future extended p-conju-
gated systems through palladium-catalyzed coupling reactions.
The synthetic approach to the diiodo compound15 13 can be
performed from the dicyano compound 1 as illustrated in
Scheme 3.
Attempts to hydrolyse the cyano group in one step to diacid 12
using the standard method16 (concentrated potassium hydroxide
in water or ethylene glycol at higher temperature), were unsuc-
cessful even prolonging the reaction time, giving diamide 10
as major product (76%) and negligible yield of diacid 12. Alter-
native routes, involving oxidation of dialdehyde 2 with alkaline
silver oxide17 for the preparation of 12 or TMSCl in alcohol at
reflux18 for the preparation of diester 11 were unsuccessful.
Finally, the conversion of 10 into the carboxylic ester 11,
was performed according to the method reported by Anelli
Scheme 3 Preparation of 1,2-bis(2-iodo-1,5-dimethylpyrrol-4-yl)per-
fluorocyclopentene, precursor of extended p-conjugated systems. Re-
agents and conditions: (i) aq. 2M NaOH, EtOH, reflux, 15 h, 76%;
(ii) (CH3)2NCH(OCH3)2 , CH3OH, reflux, 10 h then 2 eq CH3ONa,
CH3OH, 2 h, 85%; (iii) aq. 2M NaOH, H2O, CH3OH, reflux, 1 h 30
then HCl, 96%; (iv) I2 , KI, NaHCO3 , CH3OH, H2O, 60 ꢀC, 2 h, 95%.
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The iodinative decarboxylation15c–e of diacid 12 (Scheme 3)
was performed, by heating at 60 ꢀC for 2 h, a mixture of iodine
and potassium iodide diluted in water–methanol solution, to
lead to 1,2-bis(2-iodo-1,5-dimethylpyrrol-4-yl)perfluorocyclo-
pentene 13 in near quantitative yield.
The Suzuki palladium catalyzed cross-coupling between aryl
boronic acids and haloarenes or aryl triflates, in the presence
of base, is an extremely powerful tool in organic synthesis to
access the Csp2–Csp2 bond. These coupling reactions work in
the presence of many functional groups, give easily removable
nontoxic by-products, and use nontoxic and readily available
organoboranes. Consequently, this methodology has been
already used in the synthesis of biologically active compounds
containing the pyrrole ring.21
Then, a palladium-catalyzed Suzuki reaction of diiodo com-
pound 13 with phenylboronic acid in DME–water solution in
the presence of sodium carbonate and a catalytic amount of
Pd(PPh3)4 gave the corresponding coupled-product 14 in mod-
erated yield (52%). The thiophene analogue 15 has been pre-
pared analogously in 86% yield (Scheme 4).
CuI as effective catalyst, afforded 16 in nearly quantitative
yield. Using an excess of ethynyltrimethylsilane, at 70 ꢀC
for 17 h, the coupled alkynylsilane 18 could be prepared in
quantitative yield. Deprotection of the trimethylsilyl group
was realised under mild conditions by treatment of 18 with
KOH in MeOH–CH2Cl2 to give 19 in 90% yield.
A modified version of Sonogashira coupling, carried out
under phase-transfer condition,29 allowed the synthesis of
compound 17. Then compound 16 was reacted with the
2-iodothiophene, at reflux for 20 h, in the presence of benzyl-
triethylammonium chloride as phase-transfer agent, aqueous
5.5 N NaOH as base, benzene as solvent, and a mixture of
[PdCl2(PPh3)2] and CuI as catalysts, to lead to 17 in 92% yield.
It is noteworthy that the mechanism of the coupling reaction in
these conditions is most likely the ‘‘retro’’ version of the
‘‘Favorski–Babayau carboxy-ethynylation’’, in which the
diethynyl derivative 19 is primarily formed, reacting then with
the 2-iodothiophene to afford the desired product 17.
Photochromic behavior
Among the various methods for introducing alkynyl groups
on aromatic nuclei, the most powerful and useful is the Pd-
catalyzed cross-coupling reaction of halogenoarenes with
terminal alkynes, which was reported for the first time by
Sonogashira–Hagihara22 and Cassar23 in 1975. Usually, this
cross-coupling reaction proceeds in the presence of catalytic
amounts of Pd-complexes such as [Pd(PPh3)4] or [PdCl2-
(PPh3)2] in an amine as solvent. A variation of this synthetic
procedure involves the use of protecting groups.24 Coupling
of halogenoarenes with the commercially available ethynyltri-
methylsilane or with 2-methylbut-3-yn-2-ol provides (2-aryl-
ethynyl)trimethylsilane25 or 3-arylalkynol26 respectively. The
compounds generate a terminal arylacetylene moiety by
removal of the protecting group, which can be involved in
another Sonogashira coupling to afford diarylalkynes.27 This
approach28 was extended to the preparation of dipyrrolyl-
ethenes 16–19 as illustrated in Scheme 5.
The photocyclisation of most of these new dipyrrolylethenes
was achieved, by irradiation of ethylacetate or toluene solution
(3.10ꢁ5 M) in the presence of air, at wavelengths correspond-
ing to the absorption band of the open-ring isomer (Table 1).
Attempts to perform the photocyclisation reaction of the push-
pull substances 7, 8, their mono aldehyde precursors 3 and 5,
the dialdehyde 2 and the dicyanomethylene 9 were unsuccess-
ful in various solvents (ethyl acetate, toluene and acetonitrile),
at different temperatures (283 K to 303 K) between 304 nm to
406 nm. An example of photochromic behavior is given for
compound 4 in Fig. 1.
In each case a photostationary state was observed. Indeed,
the closed-ring isomers are thermally unstable and the decay
of the coloration is greatly dependent on the nature of the sub-
stituent. The range of the half-time varies about 103 times from
6 s to 6000 s. The most stable cyclised forms are obtained with
phenyl or ethynyl substituents.
Sonogashira–Hagihara reaction of 1,2-bis(2-iodo-1,5-
dimethylpyrrol-4-yl)perfluorocyclopentene 13 with the com-
mercially available 2-methylbut-3-yn-2-ol in Et2NH (room
temperature, 15 h), in the presence of [PdCl2(PPh3)2] and
Colored forms are also sensitive to visible light, leading to
the initial open form (Fig. 1f).
Characteristic absorption bands of the closed-ring isomers
appeared from 618 nm to 747 nm. Compounds 4 and 6 exhibit
red-shifted lmax values compared to the parent dicyanodipyr-
rolylethene 1, with lmax located into the near infrared region.
Conclusion
In this work, the improvement of the synthesis of 1,2-bis(2-
cyano-1,5-dimethyl-4-pyrrolyl)hexafluorocyclopenteneisdemon-
strated. Using this compound as starting material, we
succeeded in the preparation of eighteen new dipyrrolyl-
ethenes. Most of these compounds are photochromic, the
substitution pattern allowing the half-life time scale for the
colored form to be extended.
Further studies aiming at substituting the 2-position
of dipyrrolylethenes are currently in progress and will be the
subject of a future report.
Experimental
General
Melting points were determined on a Buchi 510 apparatus
¨
in capillary tubes and are uncorrected. The 1H NMR and
13C NMR spectra were recorded in deuterated solvent on a
Brucker AM 250 spectrometer (250 and 62.5 MHz, respec-
¨
Scheme 4 Suzuki coupling reaction. Reagents and conditions: (i)
phenylboronic acid, DME, EtOH, Pd(PPh3)4 , aq. 2 M Na2CO3 ,
70 ꢀC, 18 h, 52%; (ii) 2-thienylboronic acid, DME, EtOH, Pd(PPh3)4 ,
Na2CO3 , 80 ꢀC, 32 h, 86%.
tively) using tetramethylsilane as the internal standard. Col-
umn chromatography was carried out using silica gel Merck
60 (230–400 mesh from Merk Co.). Silica TLC was conducted
New J. Chem., 2003, 27, 1425–1432
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ꢀ
=
Scheme 5 Sonogashira–Hagihara coupling reaction. Reagents and conditions: (i) [PdCl (PPh ) ], CuI, Et NH, HC C(CH ) OH, 20 C, 15 h,
=
94%; (ii) [PdCl (PPh ) ], CuI, Et N, HC CSi(CH ) , 70 C, 17 h, 98%; (iii) KOH, MeOH, CH Cl , 20 C, 14 h, 90%; (iv). [Pd(PPh ) ], CuI,
2
3 2
2
3 2
ꢀ
ꢀ
=
=
2
3 2
3
3 3
2
2
3 4
benzene, aq. 5.5 N NaOH, PhCH2NEt3+Clꢁ, reflux, 20 h, 92%.
on precoated aluminium sheets (60 F254) with a 0.2 mm thick-
ness (Aldrich Chemical Co.). The electron impact mass spectra
(Ms) were obtained on a spectrometer HP 5973 Masse Selec-
tive Detector. Elemental analysis was performed using a
Perking-Elmer EA 240 instrument.
Tetrahydrofuran was distilled prior to use, under argon,
from sodium benzophenone ketyl and used immediately.
Metalation was performed under an argon atmosphere and
reagents were handled with syringes through septa.
Photoirradiation was carried out by using a Xenon lamp
(150 W) equipped with a monochromator.
Solvents (SDS Company, France) were used without further
purification and were dried over sieves if necessary.
Syntheses
Table 1 Photochromic behavior of new dipyrrolylperfluorocyclo-
pentenes
1,2-bis(2-cyano-1,5-dimethylpyrrol-4-yl)perfluorocyclopen-
tene (1)8. To a stirred and cooled (ꢁ100 ꢀC) solution of
lmax (nm) of the
open-ring isomer
lmax (nm) of
the closed-ring Half-life time
Compound (e ꢂ 10ꢁ3 Mꢁ1 cmꢁ1
)
isomer
(s) (293 K)
1a
2b
311 (13)
312 (27)
630
—
59 (lit 37)
—
4b
336 (17)
304 (50)
735
—
174
—
5b
6b
352 (33)
333 (8), 394 (10)
399 (30)
747
—
63
7b
—
—
8b
—
—
9b
388 (38), 406 (36)
322 (8.9)
312 (14)
—
10a
11a
12a
14a
15a
17a
18a
19a
631
660
644
618
656
694
657
636
597
6
312 (12)
337 (10)
9
5400c
660c
4350
134c
5991
302 (16), 342 (9)
328 (33)
337 (9)
Fig. 1 UV-Vis absorption spectra change of 4 (3.10ꢁ5 M in EtOAc)
measured at room temperature at different stages of the photochromic
reaction. Irradiation (l ¼ 336 nm) intervals are every 2 minutes: (a)
starting from open-ring isomer; (b) after irradiation for 2 minutes;
(c) 4 minutes; (d) 6 minutes; (e) photostationary state under irradia-
tion; (f) 30 minutes after irradiation was stopped or after irradiation
with 735 nm light for 5 minutes, starting from the photostationary
state.
333 (6.8)
a
b
Ethyl acetate solution (3.10–5 M). Toluene solution (3.10ꢁ5 M).
The symbol — means that the studied compound is not photosensible.
The thermal bleaching constant of the colored closed-form, measured
c
at lmax, obeys a first-order kinetic. 333 K.
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4-bromo-1,5-dimethyl-2-pyrrolcarbonitrile8 (2 g, 10.05 mmol)
in anhydrous THF (25 mL) was added a 2.5 M solution of
n-butyllithium in hexane (4.22 mL, 10.55 mmol). The bro-
mine–lithium exchange was completed after 15 minutes. Then,
octafluorocyclopentene (0.61 mL, 4.52 mmol) was introduced
via a syringe. The resulting solution was maintained at
ꢁ100 ꢀC for 30 min, then allowed to warm to ꢁ78 ꢀC for
another 3 h and finally to warm gently to ꢁ30 ꢀC. After
quenching with a cold (ꢁ30 ꢀC) mixture of HCl–EtOH–THF
(1 : 2 : 2 in volume, 1 mL), water (10 mL) was added at room
temperature. The layers were separated and the aqueous layer
extracted CH2Cl2 (2 ꢂ 15 mL). The combined organic layers
were dried over MgSO4 , filtered, and concentrated in vacuo.
The crude product was purified by chromatography on silica
gel (eluent, CH2Cl2–toluene 5 : 2) to afford the dipyrrolylethene
1 (1.38 g, 3.34 mmol, 74%). Spectroscopic and analytical
measurements were entirely consistent with those reported
previously.8
chromatography (Neutral Alumina, CH2Cl2–cyclohexane
1
1 : 1) as a brownish solid; mp: 155 ꢀC. H NMR (CDCl3): d
1.72 (s, 3H, CH3), 1.85 (s, 3H, CH3), 3.33 (s, 3H, CH3), 3.80
(s, 3H, CH3), 6.15 (s, 1H), 6.22 (s, 1H), 7.03–7.18 (m, 7H),
9.44 (s, 1H, CHO). Analysis calculated for C26H20F6N2OS2
(554.57): C, 56.31; H, 3.64; N, 5.05%. Found: C, 56.18; H,
3.37; N, 4.80%.
1,2-bis[2-(1,3-benzodithiol-2-ylidenemethyl)-1,5-dimethylpyr-
rol-4-yl]perfluorocyclopentene (4). The general procedure with
2 (33 mg, 0.08 mmol) gave 46 mg (0.066 mmol, 83%) of the title
compound after column chromatography (Silica gel, CH2Cl2–
cyclohexane 1 : 1). Recrystallization from methanol–toluene
provided analytically pure product 4 as brownish needles;
1
mp: 249 ꢀC. H NMR (CDCl3): d 1.74 (s, 3H, CH3), 3.33 (s,
3H, CH3), 6.28 (s, 1H), 6.35 (s, 1H), 7.00–7.19 (m, 8H). Ana-
lysis calculated for C33H24F6N2S4 (690.81): C, 57.37; H, 3.50;
N, 4.06%. Found: C, 57.45; H, 3.62; N, 4.22%.
1-[2-(4,5,6,7-tetrahydro-1,3-benzodithiol-2-ylidenemethyl)-1,5-
dimethylpyrrol-4-yl]-2-(2-formyl-1,5-dimethylpyrrol-4-yl)per-
fluorocyclopentene (5). The general procedure with 2 (100 mg,
0.24 mmol) gave 84 mg (0.15 mmol, 63%) of the title com-
pound after column chromatography (Silica gel, CH2Cl2–
1,2-bis(2-formyl-1,5-dimethylpyrrol-4-yl)perfluorocyclopen-
tene (2). To a stirred and cooled (ꢁ50 ꢀC) solution of 1 (1.39 g;
3.37 mmol), in anhydrous CH2Cl2 (60 mL) was added drop-
wise a 1 M solution of diisobutylaluminium hydride in hexane
(16.86 mL, 16.85 mmol). After completion of the addition, the
solution was allowed to warm to 0 ꢀC and the stirring was con-
tinued for 5 h at the same temperature. Excess of aqueous sul-
furic acid (5%) was added and the resulting solution was
adjusted to pH 8 with potassium carbonate. After decanting,
the aqueous layer was extracted with dichloromethane (2 ꢂ
20 mL). The combined organic layers were washed with brine
(10 mL), dried over MgSO4 and filtered.
Concentration in vacuo followed by purification of the crude
solid with silica gel chromatography (eluent, CH2Cl2–cyclo-
hexandiethyl ether 4 : 4: 2) provided dialdehyde 2 (1.03 g,
2.46 mmol, 73%) as a clear brown solid; mp: 209 ꢀC. 1H
NMR (CDCl3): d 1.69 (s, 3H, CH3), 3.77 (s, 3H, CH3N),
6.96 (s, 1H, H3), 9.42 (s, 1H, CHO). MS: m/z ¼ 418 (molecu-
lar ion). Analysis calculated for C19H16F6N2O2 (418.33): C,
54.55; H, 3.86; N, 6.70%. Found: C, 54.28; H, 3.72; N, 6.67%.
1
cyclohexane 1 : 1) as a solid; mp: 192 ꢀC. H NMR (CDCl3):
d 1.35 (s, 3H, CH3), 1.70 (m, 4H, CH2), 1.83 (s, 3H, CH3),
2.20 (m, 4H, CH2), 3.29 (s, 3H, CH3), 3.77 (s, 3H, CH3),
6.03 (s, 1H), 6.15 (s, 1H), 7.00 (s, 1H). MS: m/z ¼ 558 (mole-
cular ion). Analysis calculated for C26H24F6N2OS2 (558.60): C,
55.90; H, 4.33; N, 5.01%. Found: C, 55.95; H, 4.36; N, 4.90%.
1,2-bis[2-(4,5,6,7-tetrahydro-1,3-benzodithiol-2-ylidenemethyl)-1,5-
dimethylpyrrol-4-yl]perfluorocyclopentene (6). The general
procedure with 2 (33 mg, 0.08 mmol) gave 37 mg (0.05 mmol,
67%) of the title compound after column chromatography
(Neutral Alumina, CH2Cl2–cyclohexane 1 : 3) as a dark-brown
solid; mp: 239 ꢀC. 1H NMR (CDCl3): d 1.68 (m, 4H, CH2),
2.09 (s, 3H, CH3), 2.19 (m, 4H, CH2), 3.27 (s, 3H, CH3),
6.17 (s, 1H), 6.20 (s, 1H). Analysis calculated for
C33H32F6N2S4 (698.88): C, 56.71; H, 4.01; N, 5.01%. Found:
C, 56.65; H, 3.88; N, 4.96%.
¨
Knoevenagel reaction of aldehydes (2), (3) and (5) with
Wittig–Horner reaction of 2-dimethoxyphosphinyl-1,3-benzo-
dithiol or 2-dimethoxyphosphinyl-4,5,6,7-tetrahydro-1,3-benzo-
dithiol with aldehyde (2). General procedure for preparation
of compounds (3)–(6). Freshly distilled trimethylphosphite
(0.03 mL, 0.24 mmol) and sodium iodine (36 mg, 0.24 mmol)
were added successively to a stirred solution of 1,3-benzo-
dithiolium tetrafluoroborate (57 mg, 0.24 mmol) or 4,5,6,7-
tetrahydro-1,3-benzodithiolium hexafluorophosphate (72 mg,
0.24 mmol) in dry acetonitrile (1 mL), under a argon atmo-
sphere at 20 ꢀC. The reaction took place rapidly and was
slightly exothermic. Stirring was continued for 2 h, whereupon
the solvent was evaporated under reduced pressure. The resi-
due was dissolved in dry THF (10 mL) under argon and a
2.5 M solution of n-butyllithium in hexane (96 ml, 0.24 mmol)
was syringed dropwise at ꢁ78 ꢀC. After 30 min, a solution of
aldehyde 2 (0.33 equiv. or 1 equiv.) in dry THF (10 mL) was
added to the solution of phosphonate carbanion. The resulting
mixture was stirred for 10 min at ꢁ78 ꢀC, and then allowed to
warm to 20 ꢀC for 2 h. THF was evaporated under reduced
pressure, water (10 mL) added and the residue was extracted
with CH2Cl2(3 ꢂ 10 mL). The combined extracts were dried
over MgSO4 , filtered, and concentrated under reduced pres-
sure. Purification of products was achieved by column chroma-
tography (Silica gel or neutral Alumina) with eluents as
indicated below. Further purification, if necessary, could be
achieved by recrystallization.
malononitrile. General procedure for preparation of compounds
(7)–(9). To a stirred solution of aldehyde 2, 3, or 5 (0.45 mmol)
and malononitrile (66 mg, 1 mmol) in dry benzene (5 mL) at 20
ꢀC was added a catalytic amount of piperidine. The resulting
mixture was stirred at 20 ꢀC for 48 h. After cooling, the reac-
tion mixture was poured into water (5 mL). The organic layer
was separated and the aqueous layer was extracted with
CH2Cl2 (3 ꢂ 10 mL). The combined organic layers were dried
over MgSO4 and the solvent were removed under reduced
pressure. Neutral alumina or silica gel column chromatogra-
phy of the residue afforded the desired products.
1-[2-(1,3-benzodithiol-2-ylidenemethyl)-1,5-dimethylpyrrol-4-
yl]-2-[2-(2,2-dicyanoethen-1-yl)-1,5-dimethylpyrrol-4-yl]per-
fluorocyclopentene (7). The general procedure with 3 (252 mg,
0.45 mmol) gave 233 mg (0.38 mmol, 84%) of the title
compound after two successive recrystallizations, from metha-
nol–cyclohexane and from CH2Cl2–cyclohexane as a red-
brown solid; mp: 235 ꢀC. 1H NMR (CDCl3): d 1.92 (s, 6H,
CH3), 3.42 (s, 3H, CH3), 3.63 (s, 3H, CH3), 6.26 (s, 1H),
6.32 (s, 1H), 7.17–7.30 (m, 2H), 7.48 (s, 1H), 7.84 (s, 1H). Ana-
lysis calculated for C29H20F6N4S2 (602.62): C, 57.80; H, 3.35;
N, 9.30%. Found: C, 57.85; H, 3.38; N, 9.22%.
1-[2-(4,5,6,7-tetrahydro-1,3-benzodithiol-2-ylidenemethyl)-
1,5-dimethylpyrrol-4-yl]-2-[2-(2,2-dicyanoethen-1-yl)-1,5-
dimethylpyrrol-4-yl]perfluorocyclopentene (8). The general
procedure with 5 (254 mg, 0.45 mmol) gave 145 mg (0.23
mmol, 52%) of the title compound after column chromatogra-
phy (Neutral Alumina, CH2Cl2–cyclohexane 1 : 1) as a orange
solid; mp: 256 ꢀC. 1H NMR (CDCl3): d 1.72 (m, 4H, CH2),
1.81 (s, 3H, CH3), 1.87 (s, 3H, CH3), 2.18 (m, 4H, CH2),
1-[2-(1,3-benzodithiol-2-ylidenemethyl)-1,5-dimethylpyrrol-4-
yl]-2-(2-formyl-1,5-dimethylpyrrol-4-yl)perfluorocyclopentene
(3). The general procedure with 2 (100 mg, 0.24 mmol) gave
74 mg (0.13 mmol, 56%) of the title compound after column
New J. Chem., 2003, 27, 1425–1432
1429

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3.29 (s, 3H, CH3), 3.49 (s, 3H, CH3), 5.98 (s, 1H), 6.14 (s, 1H),
7.37 (s, 1H), 7.74 (s, 1H). Analysis calculated for
C29H24F6N4S2 (606.65): C, 57.42; H, 3.99; N, 9.24%. Found:
C, 57.32; H, 3.90; N, 9.09%.
After cooling, the resulting solution was concentrated to near
dryness in vacuo and water was added (10 mL). The residue
was extracted with AcOEt (3 ꢂ 20 mL). The organic phase
was vigorously stirred with a saturated aqueous solution of
Na2S2O3ꢃ5H2O (10 mL), washed with water (10 mL), dried
over MgSO4 then filtered and evaporated. The crude product
was purified by column chromatography (silica gel, eluted by
CH2Cl2–cyclohexane 1 : 1) to give 13 (122 mg, 0.2 mmol) in
95% yield as an oil which crystallised slowly upon storage to
1,2-bis[2-(2,2-dicyanoethen-1-yl)-1,5-dimethylpyrrol-4-yl]per-
fluorocyclopentene (9). The general procedure with 2 (190 mg,
0.45 mmol) gave 178 mg (0.34 mmol, 77%) of the title com-
pound after column chromatography (Silica gel CH2Cl2–
diethyl ether 5 : 1) and recrystallisation from benzene–cyclo-
1
1
hexane as a solid; mp: 256 ꢀC. H NMR (acetone d6): d 2.02
a yellow solid; mp: 210 ꢀC. H NMR (CDCl3): d 1.67 (s, 3H,
(s, 3H, CH3), 3.79 (s, 3H, CH3), 7.72 (s, 1H, H3), 8.14 (s,
1H). Analysis calculated for C25H16F6N6 (514.43): C, 58.37;
H, 3.13; N, 16.34 Found: C, 58.32; H, 3.11; N, 16.26.
CH3), 3.37 (s, 3H, CH3N), 6.41 (s, 1H, H3). MS: m/z ¼ 614
(molecular ion). Analysis calculated for C17H14F6I2N2 (614.11):
C, 33.25; H, 2.30; N, 4.56%; Found: C, 33.28; H, 2.36; N,
4.62%.
1,2-bis(2-carboxamide-1,5-dimethylpyrrol-4-yl) perflurocyclo-
pentene (10). To a solution of 1 (2 g, 4.85 mmol) in ethanol (30
mL) was poured 20 mL of aqueous NaOH (2M). The resulting
mixture was refluxed and stirred for 15 h. After cooling, the
solvents were evaporated under vaccum. Water (20 mL) was
added and the mixture extracted with AcOEt (4 ꢂ 20 mL).
The collected organic layer were dried over MgSO4 , filtered,
and concentrated in vacuo. The crude product was purified
by chromatography on silica gel (AcOEt–EtOH 18 : 1 as elu-
ent) to afford the diamide 10 (1.653 g, 3.7 mmol, 76%) as a
dark-yellow solid; mp > 250 ꢀC. 1H NMR (DMSO): d 1.43
(s, 3H, CH3), 3.60 (s, 3H, CH3N), 6.88 (m, 2H, H3 , NH),
7.51 (m, 1H, NH). MS: m/z ¼ 448 (molecular ion). Analysis
calculated for C19H18F6N4O2 (448.36): C, 50.90; H, 4.05; N,
12.50%. Found: C, 51.38; H, 4.34; N, 11.84%.
1,2-bis(1,5-dimethyl-2-phenylpyrrol-4-yl)perfluorocyclopentene
(14). To a mixture of 1,2-bis(2-iodo-1,5-dimethylpyrrol-4-yl)-
perfluorocyclopentene 13 (200 mg, 0.32 mmol), DME (4 mL)
and a 2 M aqueous solution of Na2CO3 (1 mL, 2 mmol) was
added phenylboronic acid (119 mg, 9.98 mmol) dissolved in
a minimum of ethanol (0.5 mL). The resulting solution was
degassed and flushed with an argon flow for 30 minutes. Then,
the catalyst Pd(PPh3)4 (37 mg; 0.032 mmol) was quickly added
and the resulting suspension was degassed again for 15 min.
The mixture was heated and stirred under positive nitrogen
pressure, at 70 ꢀC, for 18 h. After cooling, the solvents were
evaporated to near dryness and water (5 mL) was added.
The aqueous layer was extracted with ether (2 ꢂ 20 mL). The
organic layers were combined and dried over MgSO4 and
filtered on Celite. After evaporation of the solvent in vacuo,
the crude product was purified by chromatography on silica
gel (CH2Cl2–cyclohexane 3 : 7) to afford 14 (87 mg; 0.17 mmol,
52%) as a white solid; mp: 186 ꢀC. 1H NMR (CDCl3): d 1.71 (s,
3H, CH3), 3.39 (s, 3H, CH3N), 6.29 (s, 1H, H3), 7.24 (m, 5H,
phenyl). MS: m/z ¼ 514 (molecular ion). Analysis calculated
for C29H24F6N2 (514.50): C, 67.70; H, 4.70; N, 5.44%. Found:
C, 67.72; H, 4.81; N, 5.46%.
1,2-bis(2-methoxycarbonyl-1,5-dimethylpyrrol-4-yl)perflurocy-
clopentene (11). To a solution of diamide 10 (1.26 g, 2.81
mmol) in methanol (40 mL) was added dimethylformamide
dimethylacetal (2.26 mL, 16.8 mmol). The reaction was
refluxed overnight and a solution of MeONa, prepared from
Na (650 mg, 28 mmol) in 10 mL of MeOH, was introduced.
The mixture was heated for another 3 h. After cooling, 10 mL
of a saturated aqueous NH4Cl solution was added and the
solvents were removed under reduced pressure. The aqueous
layer was extracted with CH2Cl2 (3 ꢂ 20 mL). The organic
phase was dried over MgSO4 , filtered and concentrated to give
a crude residue, which was purified by chromatography on
silica gel (dichloromethane–diethyl ether; 8 : 2 as eluent) lead-
ing to 11 in 85% yield (1.14 g, 2.39 mmol) as a white solid;
1,2-bis(1,5-dimethyl(2-(2-thienyl)pyrrol-4-yl)perfluorocyclopen-
tene (15). To a mixture of 1,2-bis(2-iodo-1,5-dimethylpyrrol-
4-yl)perfluorocyclopentene 13 (150 mg; 0.24 mmol), DME
(10 mL) and a 2 M aqueous solution of Na2CO3 (7.5 mL, 15
mmol) was added 2-thienylboronic acid (94 mg; 0.732 mmol)
dissolved in a minimum of ethanol (1 mL). The resulting solu-
tion was degassed and flushed with an argon flow for 30 min.
Then, the catalyst Pd(PPh3)4 (56 mg; 0.049 mmol) was quickly
added and the resulting suspension was degassed again for
15 min. The mixture was heated and stirred under positive
nitrogen pressure, at 80 ꢀC for 32 h. After cooling, the solvents
were evaporated to near dryness and water (5 mL) was added.
The aqueous layer was extracted with ether (2 ꢂ 20 mL). The
combined organic layers were dried over MgSO4 and filtered
on Celite. After evaporation of the solvent in vacuo, the crude
product was purified by chromatography on silica gel (diethyl
ether–cyclohexane 3 : 7) to afford 15 (110 mg; 0.21 mmol) in
1
mp: 151 ꢀC. H NMR (CDCl3): d 1.72 (s, 3H, CH3), 3.83 (s,
3H, CH3), 3.86 (s, 3H, CH3), 7.13 (s, H, H3). MS: m/
z ¼ 478
(molecular
ion).
Analysis
calculated
for
C21H20F6N2O4 (478.38): C, 52.72; H, 4.21; N, 5.86%. Found:
C, 52.66; H, 4.11; N, 5.78%.
1,2-bis(2-carboxy-1,5-dimethylpyrrol-4-yl)perflurocyclopentene
(12). A stirred mixture of compound 11 (900 mg, 1.9 mmol), a
2 M aqueous NaOH solution (5.6 mL) and methanol (15 mL)
was refluxed for 1 h 30 min. The resulting solution was concen-
trated to approx. 2/3 of its volume, cooled with an ice bath
and cautiously acidified with concentrated HCl. The resulting
precipitate was collected, washed with cold water and dried to
yield pure diacid 12 (0.821 g, 1.82 mmol, 96%) as a white solid;
mp > 250 ꢀC. 1H NMR (DMSO): d 1.54 (s, 3H, CH3), 3.67 (s,
3H, CH3N), 6.85 (s, 1H, H3), 12.60 (m, 1H, CO2H). Analysis
calculated for C19H16F6N2O4 (450.33): C, 50.67; H, 3.58; N,
6.22%. Found: C, 50.52; H, 3.49; N, 6.11%.
1
86% yield as a brown solid; mp: 164 ꢀC. H NMR (CDCl3):
d 2.21 (s, 3H, CH3); 3.56 (s, 3H, CH3); 6.48 (s, 1H, H3), 7.05
(dd, 1H, J ¼ 1.2, 3.6 Hz); 7.11 (dd, 1H, J ¼ 3.4, 5.1 Hz);
7.34 (dd, 1H, J ¼ 1.2, 5.1 Hz, 1H). MS: m/z ¼ 526 (molecular
ion). Analysis calculated for C25H20F6N2S2 (526.56): C, 57.02;
H, 3.83; N, 5.32%. Found: C, 56.96; H, 3.79; N, 5.30%.
1,2-bis(2-(3-hydroxy-3-methylbut-1-ynyl)-1,5-dimethylpyrrol-
4-yl)perfluorocyclopentene (16). A round-bottomed flask, equip-
ped with a magnetic stirrer was charged with [PdCl2(PPh3)2]
(365 mg; 0.52 mmol) and CuI (49.6 mg; 0.26 mmol), 13
(800 mg; 1.3 mmol), and 2-methylbut-3-yn-2-ol (656 mg;
7.8 mmol), in Et2NH (8 mL), purged with argon and stirred
at 20 ꢀC for 15 h. Et2NH was removed in vacuo, the residue
diluted with benzene (10 mL). The mixture was filtered
1,2-bis(2-iodo-1,5-dimethylpyrrol-4-yl)perfluorocyclopentene
(13). To a solution of diacid 12 (100 mg; 0.21 mmol) in metha-
nol (5 mL) and water (2 mL) containing NaHCO3(93 mg, 1.11
mmol), at 60 ꢀC, was treated dropwise with a solution of I2 (118
mg, 0.47 mmol), KI (206 mg, 1.24 mmol) in methanol (3 mL)
and water (1 mL). The mixture was stirred at 60 ꢀC for 5 h. The
progress of the reaction was monitored by either TLC or GC.
1430
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cules and Systems, Elsevier, Amsterdam, 1990; (c) C. B. Mc Ardle,
Applied Photochromic Polymer Systems, Blackie, Glasgow, 1992;
(d ) J. C. Crano and R. J. Guglielmetti, Organic Photochromic
and Thermochromic Compounds, Kluwer Academic/Plenum
Publishers, New York, 1999, vol. 1 and 2.
(a) F.-M. Raymo and S. Giordani, J. Am. Chem. Soc., 2002,
124(9), 2004; (b) E. A. Gonzalez de Los Santos, M. J.
Lozano-Gonzalez and A. F. Johnson, J. Appl. Polym. Sci.,
1999, 71, 259; (c) G. E. Collins, L. S. Choi, K. J. Ewing, V.
Michelet, C. M. Bowen and J. D. Winkler, Chem. Commun.,
1999, 321.
through a Celite pad thoroughly rinsed with benzene (2 ꢂ 7
mL). The combined filtrate were evaporated, and the residue
was purified by chromatography on silica gel (cyclohexane–
acetone 100 : 0 to 30 : 70) to give 16 (645 mg, 1.22 mmol)
in 94% yield as a brown solid; mp: 165 ꢀC. 1H NMR
(CDCl3): d 1.44 (s, 6H), 1.49 (s, 3H), 3.29 (s, 3H), 6.33 (s,
1H). Analysis calculated for C27H28F6N2O2 (526.51): C, 61.59;
H, 5.36; N, 5.32%. Found: C, 61.52; H, 5.31; N, 5.25%.
2
1,2-bis(2-(2-thienylethynyl)-1,5-dimethylpyrrol-4-yl)perfluorocy-
clopentene (17). To a deaerated solution of 16 (160 mg; 0.3
mmol) and 2-iodothiophene (153 mg; 0.73 mmol) in benzene
(8 mL), CuI (5 mg; 0.272 mmol), Pd(PPh3)4 (31 mg; 0.72
mmol), and PhCH2NEt3+Clꢁ (4 mg; 0.193 mmol) were rapidly
added. Deaerated aq. 5.5 N NaOH (8 mL) was then added,
and the mixture was stirred for 20 h at reflux. The biphasic
mixture was allowed to cool and filtered through Celite and
the latter washed carefully with benzene (2 ꢂ 10 mL) and the
aqueous layer of the filtrate extracted with benzene (2 ꢂ 10
mL). The combined organic layers were washed with water
(2 ꢂ 10 mL) and brine (2 ꢂ 10 mL), dried over MgSO4 , filtered
and evaporated. The residue was purified by chromatography
on silica gel (gradient elution, cyclohexane–diethyl ether
100 : 0 ! 60 : 40) to afford 17 (158 mg, 0.28 mmol) in 92% yield
as a yellow solid; mp: 213 ꢀC. 1H NMR (CDCl3): d 1.66 (s, 3H,
CH3), 3.48 (s, 3H, CH3), 6.57 (s, 1H), 6.94 (dd, J ¼ 3.7, 5.1 Hz,
1H), 7.15–7.25 (m, 2H). MS: m/z ¼ 574 (molecular ion). Ana-
lysis calculated for C29H20F6N2S2 (574.60): C, 60.62; H, 3.51;
N, 4.88%. Found: C, 60.58; H, 3.41; N, 4.76%.
3
4
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6
7
(a) K. Sayo, M. Iwamoto, S. Hayashi, K. Kuroda, K. Uchida and
M. Irie, Jpn. Kokai Tokkyo Koho 2000 JP 2000344693; (b) M.
Irie, K. Sayo, R. Sumiya and Y. Horikawa, Jpn. Kokai Tokkyo
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Soc. Jpn., 1998, 71, 985; (d ) J. C. Owrutsky, H. H. Nelson, A. P.
Baronavski, O. K. Kim, G. M. Tsivgoulis, S. L. Gilat and J. M.
Lehn, Chem. Phys. Lett., 1998, 293, 555; (e) K. Higashiguchi,
K. Matsuda, S. Kobatake, T. Yamada, T. Kawai and M. Irie,
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8
9
V. Z. Shirinyan, M. M. Krayushkin, L. I. Belen’kii, L. G.
Vorontsova, Z. A. Starikova, A. Yu. Martynkin, V. L. Ivanov
and B. M. Uzhinov, Chem. Heterocycl. Compd. (Engl. Transl.),
2001, 37, 1, 85.
1,2-bis(2-trimethylsilylethynyl-1,5-dimethylpyrrol-4-yl)perfluoro-
cyclopentene (18). A mixture of 16 (179 mg; 0.34 mmol),
ethynyltrimethylsilane (133 mg; 1.35 mmol), [PdCl2(PPh3)2]
(100 mg; 0.014 mmol), CuI (13 mg; 0.07 mmol) and anhydrous
Et3N (10 mL) was stirred at 70 ꢀC for 17 h. After cooling, the
solvent was evaporated and the residue was diluted with ben-
zene (10 mL). The resulting solution was washed with water
(3 ꢂ 5 mL), the organic layer was dried over MgSO4 , filtered
through Celite and the filtrate was evaporated. The residue
was purified by chromatography on silica gel (gradient elution,
cyclohexane–diethyl ether 100 : 0 ! 80 : 20), to afford 18 (189
mg, 0.34 mmol) in 98% yield as a brown solid; mp: 165–
166 ꢀC. 1H NMR (CDCl3): d 0.287 (s, 9H, SiMe3); 2.20
(s, 3H, CH3); 3.52 (s, 3H, NCH3); 6.63 (s, 1H, H3). MS:
m/z ¼ 554 (molecular ion). Analysis calculated for
C27H32F6N2Si2 (554.72): C, 58.46; H, 5.81; N, 5.05%. Found:
C, 58.42; H, 5.83; N, 4.98%.
10 K. v. d. Hoef, M. A. Hempenius, J. H. Hoek and J. Lugtenburg,
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1803; (d ) A. S. Benahmed-Gasmi, P. Frere, A. Belyasmine,
K. M. A. Malik, M. B. Hursthouse, A. J. Moore, M. R. Bryce,
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Stratford, J. Med. Chem., 1999, 42, 4071.
1,2-bis(2-ethynyl-1,5-dimethylpyrrol-4-yl)perfluorocyclopentene
(19). To a solution of 18 (150 mg; 0.27 mmol) in CH2Cl2
(8 mL) was added dropwise at 0 ꢀC a solution of KOH
(45 mg, 0.8 mmol) in MeOH (5 mL). The mixture was stirred
for 14 h at 20 ꢀC. The progress of the reaction was monitored
by either TLC or GC (cyclohexane–acetone 1 : 1). After eva-
poration, the residue was diluted with diethyl ether (15 mL)
and the resulting solution was washed with water (3 ꢂ 15
mL), dried over MgSO4 , filtered and evaporated. After purifi-
cation by column chromatography on silica gel (cyclohexane–
diethyl ether 8 : 2), the compound 19 was isolated in 90% yield
1
(100 mg; 0.24 mmol) as a brown solid; mp: 128 ꢀC. H NMR
(CDCl3): d 1.618 (s, 3H, CH3); 3.33 (s, 1H); 3.44 (s, 3H,
NCH3); 6.52 (s, 1H, H3). MS: m/z ¼ 410 (molecular ion)
Analysis calculated for C21H16F6N2 (410.36): C, 61.46; H, 3.93;
N, 6.83%. Found: C, 61.42; H, 3.87; N, 6.79%.
18 F. T. Luo and A. Jeevanandam, Tetrahedron Lett., 1998, 9455.
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