Synthesis of dendritic stilbenoid compounds: Heck reactions for the periphery and the core

Article abstract of DOI:10.1016/S0040-4039(01)02339-5

A convergent synthetic strategy for stilbenoid dendrimers is described in which multifold Heck reactions have been used to construct both the peripheral and the core stilbene units. Solution optical properties of one of the dendrimers indicated some energy transfer having taken place from the periphery to the core.

Full text of DOI:10.1016/S0040-4039(01)02339-5

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 1117–1121
Synthesis of dendritic stilbenoid compounds: Heck reactions for
the periphery and the core
Saumitra Sengupta,* Subir Kumar Sadhukhan, Rajkumar Sunil Singh and Nilasish Pal
Department of Chemistry, Jadavpur University, Kolkata 700 032, India
Received 25 October 2001; revised 26 November 2001; accepted 6 December 2001
Abstract—A convergent synthetic strategy for stilbenoid dendrimers is described in which multifold Heck reactions have been used
to construct both the peripheral and the core stilbene units. Solution optical properties of one of the dendrimers indicated some
energy transfer having taken place from the periphery to the core. © 2002 Elsevier Science Ltd. All rights reserved.
Since 1990, when Holmes et al. first reported that
poly(p-phenylenevinylenes) (PPVs) can be effectively
used as the emitting layer in light emitting diodes
(LEDs),1 interest in the synthesis and optical properties
of stilbenoid compounds has increased substantially.2,3
While most studies have dealt with linear
oligophenylenevinylenes,3 it has been recognized
recently that dendritic stilbenoid compounds are
promising candidates for applications in organic
LEDs.4 Stilbenoid dendrimers offer a number of advan-
tages over the linear PPVs. Due to their highly
branched structures, such dendrimers are expected to be
less aggregated and hence, should not suffer from
fluorescence-quenching due to intermolecular interac-
tions.5 Moreover, their optical properties, especially the
emitted color and quantum yields, can be fine tuned via
appropriate choice of the fluorescent core. However,
despite such promise and given the fact that the synthe-
sis of functional dendrimers has rapidly burgeoned in
recent years,6 there are only a handful of reports on the
synthesis of dendritic stilbenoid compounds.4,7 In con-
tinuation of our interest in the synthesis of novel conju-
gated aromatic architectures for optoelectronic
applications, we now present a convergent synthesis of
stilbenoid dendrimers via the strategic use of multifold
Heck reactions.
thons are usually required, both for dendron
preparation as well as for the core-coupling reactions.
Syntheses of stilbenoid dendrimers via a sequential
Heck reaction (for dendron propagation) and Horner–
Emmons condensation (for core-coupling) have also
been reported.4,7a However, stilbene synthesis via Wittig
or Horner–Emmons reactions usually give rise to a
Until now, Wittig or Horner–Emmons reactions have
been mostly used for the synthesis of stilbenoid den-
drimers.7 For such strategies, special bifunctional syn-
Scheme 1. (i) NaNO2, conc. H2SO4, 0°C to rt then morpho-
line, aq. Na2CO3; (ii) styrene, Pd(OAc)2, Bu4NBr, KOAc,
DMF, 90°C; (iii) CH2ꢀCHSiMe3 (10 equiv.), Pd(dba)2,
CH3CN, 0°C to rt; (iv) TFA, CH2Cl2, rt.
Keywords: stilbenoid dendrimers; Heck reaction; fluorescence; energy
transfer.
* Corresponding author. Fax: 91 033 4734266; e-mail: jusaumitra@
yahoo.co.uk
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)02339-5

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S. Sengupta et al. / Tetrahedron Letters 43 (2002) 1117–1121
of the styrylsilane 4 and the desired distyryl styrene 5 in
good yield. Protodesilylation of 4 (to 5) could be easily
carried out by treatment of the mixture with TFA,
resulting in a 60% overall yield of 5 from 3.11 However,
despite its brevity and synthetic novelty, the above
sequence became prohibitively expensive on a large
scale due to the high cost of vinyl trimethylsilane which
was required in large excess.
cis/trans mixture of products which is indeed the case
for all the reported synthesis of stilbenoid den-
drimers,4,7 necessitating an extra isomerization step to
produce the desired trans-stilbene linkages. It must be
mentioned that lack of stereochemical homogeneity in
the dendrimer structure can severely affect its optoelec-
tronic properties. In view of these and given the fact
that Heck arylation of styrenes provides a highly trans-
selective synthetic route to stilbenoid compounds,8 we
decided to use the Heck arylation reaction to construct
all the stilbenoid linkages (both at the core and in the
periphery) of our dendrimers.
In an alternative approach, we started with methyl
anthranilate 6 which via dibromination (to give 7, 85%)
followed by diazotization (NaNO2, conc. HCl then aq.
NaBF4) and protodediazoniation reactions (FeSO4,
DMF) gave methyl 3,5-dibromobenzoate 8 (71%) in a
good overall yield on a multigram scale (Scheme 2).
Twofold Heck reaction on 8 with 4-t-butylstyrene
under Jeffery’s conditions then produced 9 in 75%
yield. The ester group in 9 was reduced (LiAlH4, THF,
rt) to give the benzyl alcohol 10 (78%) and the latter
oxidized (MnO2, CH2Cl2, rt) to produce the 3,5-bis-sty-
ryl benzaldehyde 11 in 85% yield.12 A more convergent
route to 11 via twofold Heck reaction on 3,5-dibro-
mobenzaldehyde was also examined but gave inferior
yields (ca. 15%) under the above conditions, pre-
sumably due to the sensitive nature of the aldehyde
function in the starting material and/or the product.4
Wittig reaction of 11 (Ph3P+MeI−, n-BuLi, THF) then
For a convergent approach, our immediate goal was to
find an efficient synthetic route to a 3,5-distyryl styrene
dendron. Towards this goal, 3,5-diiodoaniline 1 was
converted to the corresponding triazene 2 (76%) and
the latter subjected to twofold Heck reaction with
styrene under Jeffery’s conditions (Pd(OAc)2, Bu4NBr,
KOAc, DMF, 90°C)8c to produce trans,trans-3,5-di-
styryl triazene 3 in 87% yield (Scheme 1).9 Protection of
the amino group in 1 as a triazene9 was necessary to
drive the Heck reaction to completion. The triazene
moiety in 3 was then converted to the diazonium
tetrafluoroborate (HBF4, ether) and then subjected to a
Heck reaction with excess vinyl trimethylsilane
(Pd(dba)2, CH3CN, 0°C to rt),10 which led to a mixture
Scheme 2. (i) Br2, HOAc; (ii) NaNO2, conc. HCl, 0°C then satd aq. NaBF4; (iii) FeSO4, DMF, rt; (iv) 4-t-butyl styrene, Pd(OAc)2,
Bu4NBr, KOAc, DMF, 90°C; (v) LiAlH4, THF, rt; (vi) MnO2, CH2Cl2, rt; (vii) Ph3P+MeI−, n-BuLi, THF; (viii) Pd(OAc)2,
Bu4NBr, KOAc, DMF, 90°C.

S. Sengupta et al. / Tetrahedron Letters 43 (2002) 1117–1121
1119
produced the desired 3,5-distyryl styrene dendron 12 in
78% yield after chromatographic purification on silica
gel.
The dendron 12 was used to prepare a stilbenoid den-
drimer having a low band-gap anthracene core. The
anthracene core was chosen for its high fluorescence
yields as well as to create an energy sink in our den-
drimer.13 Thus, twofold Heck reaction of 12 with 9,10-
dibromoanthracene 13, again under Jeffery’s phase
transfer conditions, gave rise to the first generation
dendrimer 14 in 40% yield (Scheme 2). The dendrimer
1
was characterized by its H and 13C NMR spectra.14
1
The H NMR spectrum of 14 showed uniformly large
coupling constants for the vinylic protons (J 15–16 Hz)
indicating an all-trans configuration, thereby attesting
to the superior stereoselectivity of the Heck reaction
vis-a`-vis Wittig methodologies in trans-stilbene
synthesis.
Figure 2. PL spectrum of 14: (a) uexc=318 nm; (b) uexc=414
nm.
The photoluminescence (PL) spectrum of 14 in CH2Cl2
(excitation at 412 nm) showed an emission band at 470
nm due to the anthracene core (Fig. 2b). Excitation at
318 nm i.e. at the absorption maximum of the stilbene
units, resulted in an emission band at 418 nm, typical of
stilbenoid compounds, together with a shoulder at 470
nm (Fig. 2a). The appearance of this shoulder at 470
nm indicated that some energy transfer had taken place
from the peripheral stilbene units to the anthracene
core.4,16 Both these emission spectra, however, showed
considerable tail emission extending to ca. 650 nm,
probably arising from the eximer states of the anthra-
cene core. The photoluminescence excitation (PLE)
spectrum of 14 obtained by monitoring the emission at
470 nm produced a large excitation band at 316 nm
with a shoulder at 416 nm (Fig. 3). This also suggests
that there is some optical coupling between the surface
and the core chromophores of the dendrimer. Since the
peripheral stilbene units in 14 are mutually meta-ori-
ented and hence cross-conjugated, it is unlikely that
energy transfer would take place via a through-bond
mechanism (Dexter’s mechanism), although in view of
the short distance between the stilbene units and the
The absorption spectrum of 14 (1.08×10−5 M in
CH2Cl2) is shown in Fig. 1. The peaks at 228 (log m 4.7)
and 318 nm (log m 5.0) were assigned to the peripheral
stilbene units [umax for 11: 230 and 316 nm] whereas
those at 268 (log m 4.9) and 412 nm (log m 4.2) were due
to the anthracene core [umax for the model 9,10-bis-(p-t-
butylstyryl)anthracene:15 228, 266, 310 (shoulder) and
414 nm]. The spectrum is thus a superimposition of
peaks arising from the peripheral and central chro-
mophores. This was as expected in view of the meta-
orientation of the peripheral and central chromophores
in 9. At higher concentrations of 9, the shape and
chromicity of the spectral bands changed, perhaps due
to aggregation effects. Similar concentration dependent
spectral changes have also been reported for a number
of stilbenoid and anthracene derivatives and ascribed to
aggregate formation.5,7,13
Figure 1. Absorption spectrum of 14.
Figure 3. PLE spectrum of 14 monitored at 470 nm.

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S. Sengupta et al. / Tetrahedron Letters 43 (2002) 1117–1121
Scheme 3.
anthracene core, some contribution from orbital over-
lap cannot be ruled out. A Fo¨rster’s type through-space
(singlet“singlet) mechanism is more likely to operate in
this system, especially in view of spectral overlap
between the stilbene fluorescence and the core absorp-
tion bands. Although, at this moment, we do not have
a satisfactory explanation for the inefficient energy
transfer in 14, the presence of only four peripheral
energy collection sites and also that these are in the
form of para-substituted stilbene units which usually
have very short fluorescence lifetimes,17 may be the
reason for the low transduction of energy.18
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As an additional example of our Heck reaction strat-
egy, a twofold Heck reaction of 12 was also carried out
with trans-4,4%-dibromostilbene (15)19 which led to the
highly fluorescent dumb-bell shaped dendrimer 1620
having an all-trans distyrylstilbene core (Scheme 3).
However, the isolated yield of 16 was poor (16%),
mainly due to limited solubility of the mono- and
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In conclusion, a new synthetic strategy for dendritic
stilbenoid compounds has been described which uses
multifold Heck reactions to construct both the periph-
eral and core stilbene units. Since multifold Heck reac-
tions on poly-haloarenes is currently an established
synthetic repertoire,8 synthesis of stilbenoid dendrimers
having high core-branching can be readily achieved via
the present strategy. We are presently exploring these
possibilities.
Acknowledgements
Professor Samaresh Bhattacharya and Pradipta
Purkayastha are warmly thanked for some spectral
data. This work was supported by a grant-in-aid
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8.06 (d, J 1.5 Hz, 2H). 11: lH (CDCl3, 300 MHz) 1.34 (s,
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