Novel synthesis of bridged phenylthienylethenes and dithienylethenes via Pd-catalyzed double-cyclization reactions of diarylhexadienynes

Article abstract of DOI:10.1021/ol0600281

Bridged phenylthienylethenes and dithienylethenes were synthesized via Pd-catalyzed double-cyclization reactions of (Z,Z)-1,6-diaryl-1,5-hexadien-3- ynes. Pd-catalyzed as well as photoinduced Z/E isomerization of the products were also investigated.

Full text of DOI:10.1021/ol0600281

ORGANIC
LETTERS
2006
Vol. 8, No. 6
1197-1200
Novel Synthesis of Bridged
Phenylthienylethenes and
Dithienylethenes via Pd-Catalyzed
Double-Cyclization Reactions of
Diarylhexadienynes
S. M. Abdur Rahman,† Motohiro Sonoda, Miyako Ono, Koji Miki, and
Yoshito Tobe*
DiVision of Frontier Materials Science, Graduate School of Engineering Sciences,
Osaka UniVersity, and CREST, Japan Science and Technology Agency (JST), 1-3
Machikaneyama, Toyonaka, Osaka 560-8531, Japan
tobe@chem.es.osaka-u.ac.jp
Received January 5, 2006
ABSTRACT
Bridged phenylthienylethenes and dithienylethenes were synthesized via Pd-catalyzed double-cyclization reactions of (Z,Z)-1,6-diaryl-1,5-hexadien-
3-ynes. Pd-catalyzed as well as photoinduced Z/E isomerization of the products were also investigated.
Thiophene-based diarylethenes are considered as the key
components of a number of advanced technologies including
nonlinear optical, electronic, and photochromic devices.1
Styrylthiophenes (phenylthienylethenes) having a donor
substituent on the phenyl ring and an acceptor substituent
on the thiophene ring are known to behave as good NLO-
phores.2 It has been shown that bridging between the double
bond and the aryl ring of diarylethenes results in the
enhancement of NLO properties and electric conductivity
because of the enhanced capability of π-electron delocal-
ization owing to enforced planarization and rigidification.3
Dithienylethenes are among the most promising photochro-
mic compounds.4 Moreover, unsymmetrical bridged diaryl-
ethenes are attractive in view of their potential as novel
molecular rotors and switches.5
We previously reported a versatile and convenient route
to construct (E)-1,1′-biindenylidene (1) and its unsymmetrical
derivatives possessing a bridged diarylethene structure from
a series of diphenyldienyne derivatives via 5-exo tandem
cyclization.6a,7 In all cases, the (E)-isomers were obtained
exclusively presumably via the isomerization of the vinylpal-
(3) (a) Hudson, R. D. A.; Manning, A. R.; Nolan, D. F.; Asselberghs, I.;
Boxel, R. V.; Persoons, A.; Gallagher, J. F. J. Organomet. Chem. 2001,
619, 141. (b) Raimundo, J.-M.; Blanchard, P.; Gallengo-Planes, N.; Mercier,
N.; Ledoux-Rak, I.; Hierle, R.; Roncali, J. J. Org. Chem. 2002, 67, 205
and references therein.
(4) Special issue on photochromism: Irie, M. Chem. ReV. 2000, 100,
1685.
(5) (a) Koumura, N.; Zijlstra, R. W. J.; van Delden, R. A.; Harada, N.;
Feringa, B. L. Nature 1999, 401, 152. (b) Feringa, B. L. Nature 2000, 408,
151. (c) Feringa, B. L. Acc. Chem. Res. 2001, 34, 504. (d) Kottas, G. S.;
Clarke, L. I.; Horinek, D.; Michl, J. Chem. ReV. 2005, 105, 1281.
† On leave from the University of Dhaka, Bangladesh.
(1) (a) Munn, R. W.; Ironside, C. N. Principles and Application of
Nonlinear Optical Materials; Chapman & Hall: London 1993. (b) Prasad,
P. N.; Williams, D. J. Introduction to Nonlinear Optical Effects in Molecules
and Polymers; Wiley: New York, 1991. (c) Balzani, V.; Venturi, M.; Credi,
A. Molecular DeVices and Machines; Wiley-VCH: Weinheim, 2003. (d)
Feringa, B. L. Molecular Switches; Wiley-VCH: Weinheim, 2001.
(2) (a) Zhang, J. X.; Dubois, P.; Je´ro˜me, R. J. Chem. Soc., Perkin Trans.
2 1997, 1209. (b) Wu, X.; Wu, J.; Liu, Y.; Jen, A. K-Y. J. Am. Chem.
Soc. 1999, 121, 472.
10.1021/ol0600281 CCC: $33.50
© 2006 American Chemical Society
Published on Web 02/24/2006

Scheme 1. Pd-Catalyzed Double Cyclization of Diaryldienynes
Scheme 2
ladium intermediate because of steric hindrance in the (Z)-
isomers.6a,8 As an extension of our work, we planned to apply
the reaction to (di)thienyldienynes because of the following
possibilities in the cyclization products: (i) further elabora-
tion of bridged diarylethenes containing (a) thiophene ring-
(s) to potentially useful materials as described above and
(ii) formation of the (Z)-isomers owing to the absence of
the inner peri hydrogen atoms.9 The latter issue is indispen-
sable for application of the products to photochromic
systems. In this context, we report herein the efficient
synthesis of phenylthienylethene and dithienylethene deriva-
tives by Pd-catalyzed double cyclization reactions of readily
available diarylhexadienynes and their Z/E isomerization
catalyzed by a Pd catalyst or induced by photoexcitation
(Scheme 1).
The preparations of the starting materials 2a-e for the
cyclization are shown in Scheme 2. For the preparation of
phenylthienyldienynes 2a and 2b, bromothienylethene 4a10
or 4b was prepared from the corresponding thiophenecar-
boxaldehyde in excellent yields through the Corey-Fuchs
protocol11 followed by the stereoselective hydrogenolysis
reported by Uenishi.12 The Sonogashira coupling of 4a or
4b with 36a afforded (Z,Z)-2a and 2b, respectively, together
with their (E,Z)-isomers as minor components.13 Pure (Z,Z)-
isomers were isolated by preparative GPC. Dithienyldienynes
2c and 2d were also synthesized by the coupling of
bromothienylbutenyne 5 with 4a or 4b.14 Compound 5 was
prepared by the Sonogashira coupling of 1-bromo-2-(3-
bromothienyl)ethene15 with ethynyltrimethylsilane and sub-
sequent deprotection of the trimethylsilyl group.
Dithienyldienyne 2e was prepared from 1-bromo-2-(2-
bromothienyl)ethene (7), derived from bromothiophenecar-
boxaldehyde (6)16 by the same method as that used for the
synthesis of 4a and 4b, and thienylbutenyne 8 derived from
4b by the same method as that used for the synthesis of 5.
It should be noted that, by changing the position of the enyne
bond formation, only (E,E)-isomer 2e was obtained.
The tandem cyclization reactions of 2a-e were conducted
under the conditions optimized in the synthesis of 1.6a When
2a was treated with Pd(OAc)2 (0.2 equiv), PPh3 (0.4 equiv),
Bu4NBr (1 equiv), and K2CO3 (3 equiv) in DMF at 80 °C,
the 5-exo cyclization proceeded readily to form the doubly
cyclized products, bridged phenylthienylethene 9a, as a
mixture of (E)- and (Z)-isomers in 85% yield (Z/E ) 75/25)
(Figure 1). In contrast to the case of 1, the (Z)-isomer was
formed as a major product. Pure (Z)-9a and (E)-9a were
(6) (a) Rahman, S. M. A.; Sonoda, M.; Itahashi, K.; Tobe, Y. Org. Lett.
2003, 5, 3411. We also reported 6-endo tandem cyclization of diaryldienynes
by flash vacuum pyrolysis (FVP): (b) Sonoda, M.; Itahashi, K.; Tobe, Y.
Tetrahedron Lett. 2002, 43, 5269.
(7) A similar type of domino-Heck double cyclization reaction was
reported: (a) Tietze, L. F.; Kahle, K.; Raschke, T. Chem. Eur. J. 2002, 8,
401. (b) Bruye`re, D.; Bouyssi, D.; Balme, G. Tetrahedron 2004, 60, 4007.
(c) Ohno, H.; Yamamoto, M.; Iuchi, M.; Tanaka, T. Angew. Chem., Int.
Ed. 2005, 44, 5103.
(8) (Z)-Biindenylidenes should be deformed to a nonplanar conformation
because of the steric hindrance between the two inner peri hydrogen atoms.
According to the calculations by the DFT method at the B3LYP/6-31G**
level, twist angles of the central double bond of (E)- and (Z)-biindenylidenes
were estimated to be 0° and 14.9°, respectively.
(9) The DFT calculations at the B3LYP/6-31G** level predict that the
twist angles of the central double bond of (Z)-isomers of 9a, 9b, and 9c are
11.4°, 6.1°, and 8.4°, respectively, whereas those of both 9d and 9e are 0°.
The corresponding angles of the (E)-isomers, (E)-9a-e, are essentially 0°.
(10) Recently, the synthesis of (Z)-vinylthiophenes was reported: Hay-
ford, A.; Kaloko, J., Jr.; El-Kazaz, S.; Bass, G.; Harrison, C.; Corprew, T.
Org. Lett. 2005, 7, 2671.
(14) The 1H NMR spectrum of 2c showed two sets of vinyl proton signals
with J ∼ 12 Hz (δ 7.07, 6.97, 6.00, and 5.85 ppm) for the (Z,Z)-isomer.
1
The presence of the other isomers of 2c were confirmed clearly by the H
NMR spectrum of the residual fractions after isolation of pure (Z,Z)-isomer
using preparative GPC. Thus, two doublet of doublet signals, due to the
vinyl protons adjacent to the acetylenic carbons, with J ∼ 12 Hz (δ 5.84
and 5.71 ppm) and four doublet of doublet signals with J ∼ 16 Hz (δ 6.32,
6.25, 6.19, and 6.14 ppm) were assigned to the two (E,Z)- and (E,E)-isomers.
The vinyl proton signals adjacent to aromatic rings, which appeared at lower
field, were concealed by the aromatic proton signals. Simiarly, a doublet
of doublet signal with J ∼ 16 Hz at δ 6.27 ppm and that with J ∼ 12 Hz
at δ 5.85 ppm in the spectrum of 2d were ascribed to (E,Z)-2d.
(15) Tietze, L. F.; Lu¨cke, L. P.; Major, F.; Mu¨ller, P. Aust. J. Chem.
2004, 57, 635.
(11) Corey, E. J.; Fuchs, P. L. Tetrahedron Lett. 1972, 3769.
(12) Uenishi, J.; Kawahama, R.; Yonemitsu, O.; Tsuji, J. J. Org. Chem.
1998, 63, 8965.
(13) The presence of (E,Z)-isomers was indicated by the 1H NMR spectra,
which showed sets of vinyl proton signals with J ∼ 16 Hz (δ 7.45 and
6.43 ppm for (E,Z)-2a and δ 7.36 and 6.38 ppm for (E,Z)-2b) and those
with J ∼ 12 Hz (δ 6.96 and 5.73 ppm for (E,Z)-2a and δ 6.73 and 5.78
ppm for (E,Z)-2b).
(16) Christophersen, C.; Begtrup, M.; Ebdrup, S.; Petersen, H.; Vedso,
P. J. Org. Chem. 2003, 68, 9513.
1198
Org. Lett., Vol. 8, No. 6, 2006

Figure 1. Phenylthienylethenes 9a,b and dithienylethenes 9c-e.
isolated by preparative GPC.17 In the 1H NMR spectrum of
(Z)-9a, a diagnostic signal was observed at δ 8.30 ppm which
was assigned to the inner peri hydrogen of the benzene ring.
The prominent downfield shift relative to the corresponding
proton of (E)-9a (δ 7.87 ppm) is ascribed to the anisotropic
deshielding effect of the closely located thiophene ring. The
stereochemical assignment of (E)-9a was secured by the
observation of a nuclear Overhauser enhancement between
the peri hydrogen and one of the vinyl hydrogens of the
cyclopentadiene ring fused to the thiophene ring. The
formation of (Z)-9a may be interpreted by the relatively
diminished steric repulsion between the â-hydrogen of the
thiophene ring and the inner peri hydrogen of the benzene
ring in the diarylethene structure.9 Interestingly, (E)-9a was
obtained as the major product when the reaction was
performed for a relatively long period (54% yield, Z/E )
35/65). In this case, the Z/E ratio of the products was found
to be dependent on the reaction time, indicating that the
isomerization is catalyzed by the palladium catalyst employed
for the cyclization as described later. Hexadienyne 2b was
also converted efficiently to 9b (89%, Z/E ) 89/11). Though
complete separation of (E)-9b and (Z)-9b was not achieved,
samples enriched with either isomer were obtained.
Figure 2. ORTEP drawings of (Z)-9e: (crystal A) (a) side view,
(b) top view; (crystal B) (c) side view, (d) top view.
place even with a smaller amount of the catalyst (0.05 equiv)
and the phosphine ligand (0.1 equiv) with similar efficiency.
The geometry of the double bond of (Z)-9e was confirmed
by the X-ray crystallographic analysis of crystals obtained
by recrystallization from chloroform/hexane (crystal A) or
ether (crystal B) as shown in Figure 2. In both crystal
structures, two molecules of (Z)-9e stacked to form a dimer.
The closest interatomic distances between non-hydrogen
atoms of the two molecules of crystals A and B are 3.56
(C(5)-C(12′)) and 3.45 Å (C(4)-C(3′)), respectively. The
most striking difference between the two crystal structures
is the orientation of the two molecules of (Z)-9e; it is
antiparallel in crystal A while parallel in crystal B. In crystal
B, the intermolecular S-S distance (S(1)-S(1′)) (3.80 Å)
is slightly longer than the sum of the van der Waals distance
(3.7 Å).19 In both structures, the molecules adopt planar
conformations with dihedral angles (C(4)-C(7)-C(8)-
C(11)) of 1.1° and 0.2°, respectively. The intramolecular
atomic distances between the two sulfur atoms (S(1)-S(2))
(3.26 Å in crystal A and 3.22 Å in crystal B) are smaller
than the sum of the van der Waals radii, indicating the
presence of S-S interaction,20 which may be responsible for
its exclusive formation.
Reactions of 2c-d were less efficient, giving bridged
dithienylethenes 9c and 9d in 38% (Z/E ) 77/23) and 49%
yields (Z/E ) 83/17), respectively. As was the case with
9b, the (E)- and (Z)-isomers of 9c and 9d could not be
separated entirely. In the 1H NMR spectra of the mixture of
(E)- and (Z)-isomers of 9c and 9d, the signals of the
â-hydrogen of the thiophene ring of the (Z)-isomers, which
are located inside the U-shaped molecular framework, appear
at relatively low field compared with the corresponding
proton of the (E)-isomers. In both cases, the reactions
proceeded slowly and a portion of the starting material was
not consumed.18 In remarkable contrast, in the case of 2e,
the tandem cyclization occurred efficiently and (Z)-9e was
obtained exclusively in 87% yield. The reaction of 2e took
The result of the reaction of 2a, forming initially (Z)-9a
and then (E)-9a with increasing reaction time, suggests that
the former is kinetically favored and the latter is thermody-
namically favored. Accordingly, we examined the isomer-
ization reaction between the (Z)- and (E)-isomers for all
products under the catalytic conditions. We also investigated
photochemical isomerization between the isomers. The
results are summarized in Table 1. When (Z)-9a (Z/E ) 100/
(17) Dark unknown materials, which were insoluble in CHCl3, were
formed gradually on standing a solution of (Z)-9a in CHCl3 (or CDCl3). It
was not possible, therefore, to measure the absorption spectrum of (Z)-9a
accurately.
(18) The low yields of 9c and 9d were partly ascribed to the decomposi-
tion of the products under the reaction conditions.
(19) (a) Pauling, L. The Nature of the Chemical Bond, 3rd ed.; Cornell
University Press: Ithaca, 1960. (b) Ladd, M. F. C. Structures and Bonding
in Solid State Chemistry; Ellis Horwood: West Sussex, 1979.
(20) (a) Aso, Y.; Yui, K.; Ishida, H.; Otsubo, T.; Ogura, F.; Kawamoto,
A.; Tanaka, J. Chem. Lett. 1988, 1069. (b) Mazaki, Y.; Kobayashi, K. J.
Chem. Soc., Perkin Trans. 2 1992, 761.
Org. Lett., Vol. 8, No. 6, 2006
1199

Table 1. Isomerization between (Z)- and (E)-Isomers of 9a-ea
Scheme 3
substrate
(Z/E)
time
(h)
productb
(Z/E)
yieldc
(%)
entry
conditions
1
2
3
4
5
6
7
8
9
Ad
A
A
A
A
B
B
B
B
B
B
9a (100/0)
9a (100/0)
9a (0/100)
9b (89/11)
9e (100/0)
9a (60/40)
9b (89/11)
9c (82/18)
9d (95/5)
9e (100/0)
9e (9/91)
1
3.5
2
48
12
6
1
2
12
3
2
9a (100/0)
9a (30/70)
9a (0/100)
9b (59/41)
9e (100/0)
9a (0/100)
9b (16/84)
9c (10/90)
9d (39/61)
9e (41/59)
9e (37/63)
NDe
85f
NDe
55f
NDe
56
73
72
86
76
) 0/100) (entry 6). Photoisomerization of Z-enriched 9b (Z/E
) 89/11) also proceeded smoothly to afford an E-enriched
9b (Z/E ) 16/84) (entry 7). Fast isomerization was observed
in the case of 9c (Z/E ) from 82/18 to 10/90) (entry 8).
Z-Enriched 9d (Z/E ) 95/5) isomerized slowly to give
slightly E-enriched 9c (Z/E ) 39/61) (entry 9). Interestingly,
isomerization of (Z)-9e (Z/E ) 100/0), which was not
effected under the catalytic conditions, proceeded smoothly
to furnished a Z/E mixture of 9e (Z/E ) 41/59) (entry 10).
This isomerization was proven to have reached at photosta-
tionary state from the result of the isomerization of E-
enriched 9e (Z/E ) 9/91) (entry 11).
10
11
83
a Conditions A: Pd(OAc)2 (0.2 equiv), PPh3 (0.4 equiv), DMF 80 °C.
Conditions B: photoirradiation in THF with a high-pressure mercury lamp
in a Pyrex tube. b Product ratio was determined by HPLC or 1H NMR.
c Yields were determined by 1H NMR using an internal standard (1,4-
dioxane). d Without the palldium catalyst. e Not determined. f Isolated yield.
In the UV-visible spectra, 9a-e17 show two absorption
maxima in the area of 360-430 nm and weak absorptions
extending to ca. 600 nm were observed, which are respon-
sible for their deep red-brown color (Figure S1 in the
Supporting Information). The absorption maxima of the
samples enriched with the E-isomers showed bathochromic
shifts (ca. 5-10 nm) relative to those of the corresponding
Z-enriched samples. It is attributed to the better conjugation
pathway and the higher planarity of the (E)-isomers than the
(Z)-isomers.9
In conclusion, we have accomplished the first synthesis
of bridged phenylthienylethenes and dithienylethenes. In
contrast to the synthesis of (E)-biindenylidene (1), the (Z)-
isomers of the products were obtained selectively, which
could be transformed into the corresponding (E)-isomers by
Pd-catalyzed or photoisomerizaion method. Further elabora-
tion of this method to the synthesis of more extended
heteroaromatic π-electronic systems with potential photonic
and electronic applications is the subjects of our future
investigation.
0) was treated with Pd(OAc)2 and PPh3 in DMF at 80 °C,
an E-enriched mixture of 9a (Z/E ) 30/70) was obtained in
85% yield (entry 2). In the absence of the palladium catalyst,
no isomerization was observed (entry 1). The reverse mode
of the isomerization, i.e., E to Z, did not proceed (entry 3).
When 9b (Z/E ) 89/11) was subjected to the same catalytic
reaction conditions, the isomerization occurred slowly (Z/E
) 59/41) (entry 4). In the case of 9c (Z/E ) 77/23) and 9d
(Z/E ) 83/17), decomposition of the substrates and the
products precluded us from studying their isomerization.18
In contrast, (Z)-9e (Z/E ) 100/0) was inactive under the
catalytic conditions (entry 5). A plausible mechanism for the
isomerization process is shown in Scheme 3 for 9b. Thus,
the reaction is initiated by re-coordination of the sulfur atom
of the (Z)-isomer to the palladium center, followed by
isomerization of the centeral double bond and elimination
of the palladium complex to form the (E)-isomer. The slow
isomerization rate of (Z)-9b may be attributed to the steric
hindrance between the inner peri hydrogen of the benzene
ring and the palladium complex because the sulfur atom is
located in the bay region of the U-shaped structure of (Z)-
9b. In the case of (Z)-9e, chelation to the palladium center
by the two sulfur atoms might stabilize the Z-form, prevent-
ing it from the isomerization.
Supporting Information Available: Experimental pro-
cedures, characterization data, and NMR spectra of new
compounds, UV-vis spectra of 9a-e, and X-ray crystal-
lographic data (CIF) of (Z)-9e. This material is available free
of charge via the Internet at http://pubs.acs.org.
In contrast to the catalytic method, photoisomerization of
Z-enriched 9a (Z/E ) 60/40) proceeded smoothly by irradia-
tion with a high-pressure mercury lamp to afford (E)-9a (Z/E
OL0600281
1200
Org. Lett., Vol. 8, No. 6, 2006