Heck reaction of bis-arenediazonium salts with vinyltriethoxy-
silane: a new synthetic protocol for poly(1,4-arylenevinylene)s†
Saumitra Sengupta* and Subir K. Sadhukhan
Department of Chemistry, Jadavpur University, Calcutta 700 032, India. Fax: 91-33-4734266
Received (in Cambridge) 2nd June 1999, Accepted 9th July 1999
a
A new synthetic protocol for poly(1,4-arylenevinylene)s via
the first-ever poly-Heck reaction of arenediazonium salts
with vinyltriethoxysilane is described.
Table 1 Physical properties of the polymers 5a,b
b
b
UV λmax
PL λmax
Mw
Mn
M /M
w
n
The discovery that poly(1,4-phenylenevinylene)s (PPVs) have
5a
5b
390 nm
338 nm
465 nm
430 nm
33118
11748
31010
10834
1.06
1.08
outstanding optoelectronic properties and can be effectively
1
used as emission layers in light-emitting diodes (LEDs) has
a
b
THF soluble fractions; polystyrene standard.
stimulated widespread activity in the synthesis of such rigid-rod
1–3
polymers.
Poly(phenylenevinylene)s are classically pre-
pared via elimination from sulfonium salt polymer precursors,
dehydrohalogenation of xylylene dihalides and Wittig or
1
McMurry condensation reactions. However, these methods
usually require harsh reaction conditions and hence are not
suitable for the synthesis of functionalized PPVs, especially for
the organic soluble variants having lateral substituents. In 1988,
Greiner and Heitz reported an alternative synthetic route to4
PPVs via Heck reaction of aromatic dibromides with ethylene.
Since then, this poly-Heck reaction strategy, by virtue of its
mild reaction conditions and high functional group tolerance
properties, has evolved as a superior methodology for the syn-
2,3
thesis of poly(phenylenevinylene)s. We have recently shown
that arenediazonium salts have several advantages over aryl
halides in Heck reactions : mild and operationally simple reac-
tion conditions (ligandless Pd-catalyst, no added base, room
temperature to 80 ЊC), short reaction times (15 min to 1 h) and
most significantly, an enhanced reactivity over aryl bromides
5
and even the iodides. In view of these and the fact that a wide
variety of anilines and bis-anilines are more readily available
than aryl halides, poly-Heck reaction of bis-arenediazonium
salts promised to be an attractive alternative for the synthesis
of poly(phenylenevinylene)s. In a model reaction, we have
described the double Heck-reaction of monoarenediazonium
salts with vinyltriethoxysilane, an inexpensive and easily
Scheme 1 Reagents and conditions: i) NaNO , dil. HCl, 0 ЊC, NaBF ;
2
4
ii) NaNO , dil. HCl, 0 ЊC, morpholine, NaHCO ; iii) for 2a,b: 10%
2
3
Pd(OAc) , EtOH, 80; iv) for 3a,b: 70% HClO (4 equiv.), 10% Pd(AOc) ,
6
2
4
2
handled ethylene-equivalent, for the synthesis of symmetrical
EtOH, 80 ЊC.
7
trans-stilbenes in high yields. Based on these results, we now
present the first ever poly-Heck reaction using bis-arene-
diazonium salts and vinyltriethoxysilane towards a facile
new synthesis of poly(arylenevinylene)s with high degrees of
regioselectivity.
appreciably soluble in CHCl and partly in THF to enable their
3
physical characterization (Table 1). The UV and PL-maxima
of these polymers, as compared to poly(phenylenevinylene)s,
expectedly underwent a hypsochromic shift. Between 5a and b,
the latter showed a lower absorption maximum as a result
of the more twisted nature of its ortho,orthoЈ-disubstituted
biphenylene unit. GPC measurements on 5a,b against a poly-
styrene standard gave average moleculer weights of ca. 33 000
and 11 000, respectively, with excellent M /M ratios (~1) which
In the event, the readily available benzidine derivatives 1a,b
were converted to the corresponding bis(diazonium tetrafluoro-
borate)s 2a,b and the latter reacted with vinyltriethoxysilane (4)
(
1 equiv.) in the presence of 5–10% Pd(OAc)2 in refluxing
EtOH–CH Cl to give the polymers 5a,b (Scheme 1). In these
2
2
w
n
reactions, the bis-triazenes 3a,b could also be used as the bis-
compares favorably with those obtained for PPVs prepared via4
8
diazonium salt surrogates. Thus, poly-Heck reaction 3a,b with
poly-Heck reactions of aromatic dibromides with ethylene.
4
(1 equiv.), under identical reaction conditions but having 4
The lower molecular weight distribution observed for 5b is per-
haps due to the electron-deficient nature of the bis-diazonium
salt 2b which favors a SET-induced dediazoniation pathway
leading to premature chain termination. The high regioselec-
equiv. of 70% HClO to liberate the bis(diazonium) salts 2a,b in
4
situ, produced the polymers 5a,b with equal efficacy. The poly-
mers were purified through filtration, Soxhlet extraction with
CHCl and reprecipitation with MeOH. Despite lacking large
1
3
tivity of this poly-Heck reaction is evident from the H and
lateral substituents, but perhaps due to the presence of the
twisted biphenylene units, these polymers were found to be
13
C NMR spectra of these polymers which showed ≤10% con-
tamination from 1,1-diarylenevinylene units present in the
chain.‡ It may be noted that poly-Heck reactions of aromatic
dibromides and ditriflates with ethylene reportedly produce up
1
3
1
†
C and H NMR spectra of 5a and 5b are available as supplementary
9
to 20% of the 1,1-disubstituted contaminants. Such structural
data available from BLDSC (SUPPL. NO. 57600, pp. 4) or the RSC
Library. See Instructions for Authors available via the RSC web page
defects result in interrupted conjugation in the polymer chain
(
http://www.rsc.org/authors).
and may profoundly influence the optical properties of the
J. Chem. Soc., Perkin Trans. 1, 1999, 2235–2236
2235
Sengupta, Saumitra
