Acetylenic scaffolding on solid support: Poly(triacetylene)-derived oligomers by Sonogashira and Cadiot-Chodkiewicz-type cross-coupling reactions

Article abstract of DOI:10.1039/b210184j

We report the first synthesis of poly(triacetylene)-derived oligomers by Pd(0)-catalysed Sonogashira and Cadiot-Chodkiewicz-type cross-coupling reactions on solid support. Oligo(phenylene triacetylene)s, members of a new class of linearly π-conjugated oligomers with all-carbon backbones, feature very high fluorescence intensities.

Full text of DOI:10.1039/b210184j

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Acetylenic scaffolding on solid support: poly(triacetylene)-derived
oligomers by Sonogashira and Cadiot–Chodkiewicz-type
cross-coupling reactions†
Nils F. Utesch and François Diederich*
Laboratorium für Organische Chemie, ETH-Hönggerberg, HCI, CH-8093 Zürich, Switzerland
Received 16th October 2002, Accepted 11th November 2002
First published as an Advance Article on the web 9th December 2002
We report the first synthesis of poly(triacetylene)-derived
oligomers by Pd(0)-catalysed Sonogashira and Cadiot–
Chodkiewicz-type cross-coupling reactions on solid
support. Oligo(phenylene triacetylene)s, members of a new
class of linearly ꢀ-conjugated oligomers with all-carbon
backbones, feature very high fluorescence intensities.
Investigations of monodisperse π-conjugated oligomers of
precise length and constitution provide specific information
about the structural, electronic and optical properties of their
polydisperse polymeric counterparts.1 Fluorescent conjugated
oligomers are explored for incorporation into organic electro-
luminescent devices.2 Recently, the testing of rigid conjugated
oligomers as molecular wire components in nanoelectronics
devices has attracted particular interest.3 With the extensive
development of solid-phase organic synthesis in the 1990s,4
monodisperse molecular wires, in particular oligothiophenes5
and oligo(phenylene ethynylene)s2,6 are increasingly being
prepared on polymeric support, taking advantage of versatile
oligomeric length control and facilitated workup and purifi-
cation protocols.
Here we describe the first polymer-supported synthesis of
poly(triacetylene)7-derived monodisperse oligomers using
Pd(0)-catalysed Sonogashira8 and Cadiot–Chodkiewicz-type9
cross-couplings as key steps in the construction of the
acetylenic scaffolds. Whereas the former has found abundant
application in the solid-phase synthesis of oligo(phenylene
ethynylene)s,2,6 only one report has appeared on Cadiot–
Chodkiewicz couplings on solid support.10 With the oligo-
(phenylene triacetylene)s 1–4, we report the first members of
a new family of stable, highly fluorescent molecular wires.
Attachment of mono-deprotected diethynylethene (DEE,
(E)-hex-3-ene-1,5-diyne) 814a to the resin-bound linker by
Sonogashira coupling proceeded smoothly and provided the
polymer-supported alkyne 9 (Scheme 1). Subsequent cleavage
from the support with MeI2,6a–c gave the (phenylene tri-
acetylene) monomer 1 in excellent yield (83% starting from 8).‡
All cleavage reactions in this study required 24 h at 120 ЊC
in agreement with previous reports15 that electron-withdrawing
groups in the para-position (such as the DEE moiety in 9)16
slow down the conversion to the aryl iodide.
After protodesilylation of 9, the resulting polymer-
supported, terminally deprotected alkyne was subjected to
Pd(0)-catalysed Cadiot–Chodkiewicz cross-coupling with 10,14b
which was followed by cleavage of the products from the solid
support with MeI. Under all experimental conditions, which
were optimised in a systematic way while carefully excluding O2,
a mixture of three products was obtained, namely the desired
cross-coupled DEE dimer 11, homo-coupled DEE dimer 12
and uncoupled monomer 13.† The highest cross-coupling yield
(11: 37%, 12: 10% and 13: 2%) was obtained using 5.0 equiv. of
10 and 0.2 equiv. of [Pd2(dba)3] in the absence of CuI at 45 ЊC.
Starting from 1, on the other hand, repetitive cycles of
protodesilylation, followed by Pd(0)-catalysed Sonogashira
cross-coupling, allowed the efficient modular construction of
the oligo(phenylene triacetylene)s 2–4 on solid phase (Scheme
2). Purification of the products, cleaved from the solid support
with MeI, was readily accomplished by column chromato-
graphy (SiO2, hexane–CH2Cl2 mixtures as eluent), and the pure
oligomers were obtained in quantities of 100–200 mg. The
reduced yields of the higher oligomers 3 and 4 were not due to
less efficient cross-coupling on the polymeric support but rather
to their increasing insolubility which affected the workup.
Such problems should be readily avoided in future work by
introducing alkyl side chains on the phenyl moieties.
For our investigations, we chose the Merrifield resin11 (chloro-
methylated polystyrene, 0.4 mmol Cl gϪ1 resin) as solid support
in combination with a triazene linker. 1-Aryltriazene linkers
are well known for their compatibility with Pd(0)-catalysed
coupling reactions2,6 and for their facile cleavage from the
resin.12,13 The evaluation of possible linkers in preliminary
liquid-phase studies revealed, in our hands, substantial
advantages of the benzyloxyethyl derivative 56c over the benzyl
derivative 6,2,6a,b in terms of cleavage efficiency and yield. There-
fore, we used the resin-linker combination 7, for which elemental
analysis revealed an iodide loading of ≈ 0.2 mmol gϪ1 resin.
† Electronic supplementary information (ESI) available: typical
procedures for the reactions on solid phase and physical as well as
spectral data of compound 2. See http://www.rsc.org/suppdata/ob/b2/
b210184j/
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 2 3 7 – 2 3 9
237

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Scheme 1 Pd(0)-catalysed Cadiot–Chodkiewicz cross-coupling on solid phase. Reagents and conditions: (i) [Pd2(dba)3] (dba = dibenzylidene
acetone), CuI, PPh3, Et3N, 70 ЊC, 20 h; (ii) MeI, 120 ЊC, 24 h, 83% (2 steps); (iii) NaOH, THF–MeOH, 70 ЊC, 3 h; (iv) [Pd2(dba)3], CuI, LiI,
C6H6, 1,2,2,5,5-pentamethylpiperidine (PMP), 45 ЊC, 24 h; (v) MeI, 120 ЊC, 24 h, 37% (11), 10% (12), 2% (13).
Scheme 2 Synthesis of oligo(phenylene triacetylene)s by Pd(0)-catalysed Sonogashira cross-coupling on solid phase. Reagents and conditions: (i)
NaOH, THF–MeOH, 70 ЊC, 3 h; (ii) 1, [Pd2(dba)3], CuI, PPh3, Et3N, 70 ЊC, 24 h; (iii) MeI, 120 ЊC, 24 h.
not yet observed, i.e. the effective conjugation length1b of
Table 1 Electronic absorption and emission data for the oligo-
(phenylene triacetylene)s 1–4 in CHCl3 at r.t.
oligo(phenylene triacetylene)s involves more than 40 conju-
gated carbon atoms. Interestingly, dimer 2 and, in particular,
Absorption
Emission
trimer 3 and tetramer 4 display strong fluorescence emission
with the highest quantum yield,17 ΦF = 0.69, being measured for
trimer 3. This strong luminescence is in sharp contrast to the
poor emitting behaviour of the corresponding oligo(tri-
acetylene) oligomers, with the dimer and trimer being only
weakly fluorescent (ΦF = 0.01)16 and the tetramer not emitting
at all. The high fluorescence quantum yields of 2–4 are quite
remarkable in view of their terminal iodine substituents which
should promote intersystem crossing to the corresponding
triplet excited state. Investigations of the electroluminescence
properties of 3 and 4 are underway.
λmax/nma
b
d
Compound
(ε/MϪ1cmϪ1
)
λmax/nmc
ΦF
1
2
3
4
337 (41100)
360 (54200)
381 (97800)
364
414
436
444
0.01
0.24
0.69
0.60
384 (122000)
a Longest-wavelength absorption maximum. b Molar extinction co-
efficient. c Fluorescence emission maximum. d Fluorescence quantum
yield, determined by using anthracene (ΦF = 0.33) as reference com-
pound.17
In conclusion, we accomplished the first synthesis of oligo(tri-
acetylene)-related oligomers by Pd(0)-catalysed acetylenic
scaffolding on solid phase. Whereas optimal conditions for
polymer-supported Cadiot–Chodkiewicz-type coupling reac-
tions still remain to be found, Sonogashira cross-coupling
proceeded smoothly and led rapidly to the new oligo(phenylene
triacetylene) oligomers 1–4 featuring intense longest-wave-
length absorption bands and, most interestingly, a dramatically
The UV/VIS spectral data of the novel oligo(phenylene
triacetylene)s 1–4 in CHCl3 are depicted in Fig. 1. The longest
wavelength absorption maxima are shifted bathochromically
with increasing oligomeric length, from λmax = 337 (1) to 384 (4)
nm, and the corresponding molar extinction coefficients also
increase in the same direction, reaching ε = 122000 MϪ1 cmϪ1 for
tetramer 4 (Table 1). Saturation of the optical properties is
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 2 3 7 – 2 3 9
238

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enhanced fluorescence emission as compared to the corre-
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Notes and references
‡ All new compounds were fully characterised by IR, UV/VIS, 1H and
13C NMR, mass spectrometry and microanalysis. Overall yields for the
products obtained after cleavage from the resin refer to the iodide load-
ing of the polymer-bound aryltriazene 7. In the optimisation of the
Pd(0)-catalysed Cadiot–Chodkiewicz coupling, only side product 13
was isolated in pure form, whereas the yields of 11 and 12, which could
hardly be separated, were determined by calibrated HPLC. For full
characterisation, 11 and 12 were independently prepared by liquid-
phase synthesis. Electronic supplementary information (ESI) includes
typical procedures for the reactions on solid phase and physical as well
as spectral data of compound 2.
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and G. Wegner, Wiley-VCH, Weinheim, 1997; (b) R. E. Martin
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O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 2 3 7 – 2 3 9
239