Synthesis of highly conjugated two-dimensional molecular scaffolds via Pd-catalyzed reactions on a tetraphenylethylene core

Article abstract of DOI:10.1055/s-2004-822929

A facile synthesis of ethylene centered highly conjugated two-dimensional molecular scaffolds is described via Pd-catalyzed coupling reactions (Sonogashira and Heck reactions) on a tetraphenylethylene core.

Full text of DOI:10.1055/s-2004-822929

LETTER
1191
Synthesis of Highly Conjugated Two-Dimensional Molecular Scaffolds via
Pd-Catalyzed Reactions on a Tetraphenylethylene Core
T
wo-dimensio
a
nal Conjug
u
ated Module
m
s
Based on Tetraphe
i
nylethy
t
lene ra Sengupta*
Department of Chemistry, Jadavpur University, Kolkata 700 032, India
Fax +91(33)24146266; E-mail: jusaumitra@yahoo.co.uk
Received 30 December 2003
Dedicated with warmth and respect to my former mentor Professor Bruce H. Lipshutz
Abstract: A facile synthesis of ethylene centered highly conjugated
two-dimensional molecular scaffolds is described via Pd-catalyzed
coupling reactions (Sonogashira and Heck reactions) on a tetraphe-
nylethylene core.
Key words: tetraphenylethylene, Sonogashira coupling, Heck
reaction, two-dimensional conjugation
Monodisperse conjugated organic molecules have attract-
ed much current attention due to their potential applica-
tions in various electronic devices.1 In particular, linear p-
conjugated oligomers of precise length and constitution
are being intensely studied as components of organic light
emitting diodes, molecular logic gates, for energy storage
and non-linear optical applications.2 Most of these studies
have traditionally used the one-dimensional (linear) p-
conjugated oligomers. We were interested in developing
conjugated oligomers of branched topologies and became
attracted to the two-dimensional conjugated module A
(Figure 1) recently proposed by Diederich et al.3,4 Three
different conjugation modes are available in A: diagonal
conjugation, lateral conjugation and cross conjugation.
Since the three p-systems could potentially interact with
each other, the overall photophysical properties of com-
Figure 1
pounds based on A were expected to be dramatically dif-
the presence of the four phenyl rings. Moreover, TPE has
a strong emission property, a feature that is absent in 1 and
provides opportunities for the synthesis of TPE centered
bichromophoric donor-acceptor dyads for energy transfer
studies.9 We hence initiated a program on the synthesis of
TPE based two-dimensional conjugated architectures
such as 2 and in this paper we describe a facile synthesis
of such motifs via fourfold Sonogashira coupling10 and
Heck reaction11 on tetra(p-iodophenyl)ethylene (5).
ferent from those of the individual one-dimensional p-
systems. Diederich et al. have prepared a variety of such
two-dimensional conjugated molecular architectures
based on a tetraethynylethylene core 1 and showed that
their properties are indeed different from their one-dimen-
sional analogs.3,5,6
We were interested in preparing tetraethynylethylene
based two-dimensional scaffolds in which all four corners
of the modules are substituted with identical aryl groups
(donors or acceptors). Such push-push or pull-pull two-di-
mensional systems promised to show interesting two-pho-
ton absorption properties.7 However, a large scale
synthesis of 1 turned out to be a difficult proposition (un-
common starting materials, a lengthy synthetic sequence
and low overall yields) and led us to consider tetraphenyl-
ethylene (TPE) as an alternative two-dimensional core.8
Compared to 1, TPE enjoys additional conjugation due to
TPE (4) was easily prepared from benzophenone (3) in
two easy steps (i: PCl5, 150 °C; ii: Cu-bronze, benzene,
reflux) on a multigram scale (Scheme 1).8 Fourfold iodi-
nation of 4 with I2-PhI(OAc)2 in CH2Cl2 or CHCl3 then
produced the key tetraiodide 5 in 80% yield.12,13 The iodi-
nation step was throughout a heterogeneous reaction and
a satisfactory conversion could be obtained only after stir-
ring at room temperature for 60 hours in the dark. Once in
hand, 5 was first subjected to fourfold Sonogashira cou-
pling reactions so as to attach alkynyl conjugation to the
TPE core. Thus, the reaction of 5 with excess TMS-acet-
ylene (8 equiv) under the standard Sonogashira-protocol
[10% PdCl2(PPh3)2, Et3N, 10% CuI, DMF, r.t.] produced
SYNLETT 2004, No. 7, pp 1191–1194
0
3.
0
6.
2
0
0
4
Advanced online publication: 10.05.2004
DOI: 10.1055/s-2004-822929; Art ID: D30603ST
© Georg Thieme Verlag Stuttgart · New York

1192
S. Sengupta
LETTER
the tetra-alkynyl product 7 in 65% yield after purification tion by silica gel chromatography (15% EtOAc in light
by silica gel chromatography (15–20% EtOAc in light pe- petroleum).15 Similarly, with excess p-dimethylamino
troleum). The TPE based tetrabromide 614 was totally in- styrene, fourfold Heck reaction of 5 gave rise to the highly
effective for this coupling reaction. The TMS groups in 7 conjugated two-dimensional oligostilbene 12 (26%). The
were removed (KOH, MeOH–THF, r.t.) to give the novel low yield in the latter case was due to losses in the isola-
TPE based tetra-acetylene 8 (80%).15
tion step caused by the sparingly soluble nature of 12.
Again, the tetrabromide 6 was ineffective for these Heck
reactions. Interestingly, 12 consists of two one-dimen-
sional quadrupolar systems having both diagonal as well
as lateral conjugation. According to Spangler et al.,16 such
donor substituted highly branched oligo(phenylene-
vinylene)s are expected to show larger two-photon ab-
sorption cross sections compared to their linear analogs.
We are currently trying to improve the solubility and yield
of products such as 12 by using long-chain p-dialkyl-
amino styrenes in these Heck reactions.
The latter may be used for further couplings with aryl ha-
lides or triflates to produce ethylene centered conjugated
tetratolan scaffolds. Fourfold Sonogashira coupling of 5
with excess phenylacetylene, however, led to an incom-
plete reaction due to precipitation of the partially coupled
intermediates from the reaction medium. Since this could
be a more convergent route to ethylene centered tetratolan
derivatives, we are currently looking for ways to circum-
vent this problem. Sonogashira coupling on 5 was also
carried out with propargyl alcohol to produce 9 (60%) af-
ter silica gel chromatography (65% EtOAc in light petro- The tetra-acrylate 11 and particularly, 12 are highly fluo-
leum).15 The four hydroxy groups in 9 were then reacted rescent compounds. While TPE showed an absorption
with 2-naphthyl carboxylic acid chloride (CH2Cl2, cat. maximum at l = 320 nm, the absorption maxima for 11
DMAP, Et3N, r.t.) to give 10 (42%),15 a prototype of TPE were found at l = 272, 310 (lmax), 330 and 375 nm (shoul-
centered bichromophoric donor-acceptor dyads suitable der). The red shift in 11 is obviously caused by the extend-
for energy transfer studies. Fourfold Heck reactions on 5 ed conjugation present in this system. Excitation of 11 at
were also studied (Scheme 2). Thus, Heck reaction of 5 either l = 330 or 375 nm produced a strong emission peak
with excess ethyl acrylate under Jeffery’s PTC conditions centered at l = 528 nm. Compared to TPE, which showed
[10% Pd(OAc)2, Bu4NBr, KOAc, DMF, 100 °C]11b led to a structured emission spectrum [lmax (emiss) = 430 nm], the
the TPE centered tetra-acrylate 11 (65%) after purifica- broad and considerably red shifted emission maximum of
Scheme 1 Reagents and conditions: (i) PCl5, 145–150 °C; (ii) Cu-bronze, benzene, reflux; (iii) I2, PhI(OAc)2, CHCl3, r.t.; (iv) 10%
PdCl2(PPh3)2, 10% CuI, Et3N, DMF, r.t.; (v) KOH, THF–MeOH, r.t.; (vi) 2-C10H7-COCl, cat. DMAP, pyridine, CH2Cl2, r.t.
Synlett 2004, No. 7, 1191–1194 © Thieme Stuttgart · New York

LETTER
Two-Dimensional Conjugated Modules Based on Tetraphenylethylene
1193
Scheme 2
(4) For two-dimensional tetraethynyl modules based on
11 indicates excimer formation. Apparently, the highly
symmetrical nature of 11 together with its high p-density
favor strong intermolecular p-stacking interactions lead-
ing to excimer formation.
cumulene and iron-cyclobutadiene cores, see: (a) van Loon,
J.-D.; Seiler, P.; Diederich, F. Angew. Chem., Int. Ed. Engl.
1993, 32, 1187. (b) Bunz, U. H. F.; Enkelmann, V. Angew.
Chem., Int. Ed. Engl. 1993, 32, 1653.
(5) (a) Hori, Y.; Noda, K.; Kobayashi, S.; Taniguchi, H.
Tetrahedron Lett. 1969, 3563. (b) Tykwinski, R. R.;
Diederich, F. Liebigs Ann. Recl. 1997, 649.
In summary, a facile synthetic route to ethylene centered
highly conjugated two-dimensional scaffolds was devel-
oped via Pd-catalyzed reactions (Sonogashira and Heck
reactions) on a tetraphenylethylene core. Photophysical
properties of such molecules are currently under investi-
gation.
(6) (a) Bosshard, C.; Spreiter, R.; Günter, P.; Tykwinski, R. R.;
Schreiber, M.; Diederich, F. Adv. Mater. 1996, 8, 231.
(b) Spreiter, R.; Bosshard, C.; Knöpfle, G. P.; Tykwinski, R.
R.; Schreiber, M.; Diederich, F. J. Phys. Chem. B 1998, 102,
29. (c) Tykwinski, R. R.; Gubler, U.; Martin, R. E.;
Diederich, F.; Bosshard, C.; Knöpfle, G. P. J. Phys. Chem. B
1998, 102, 4451. (d) Mitzel, F.; Boudon, C.; Gisselbrecht,
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Acknowledgment
Professor Nitin Chattopadhyaya is warmly thanked for the fluo-
rescence measurements. N. Pal is thanked for technical assistance.
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Synlett 2004, No. 7, 1191–1194 © Thieme Stuttgart · New York