Iterative synthesis of heterotelechelic oligo(phenylene-vinylene)s by olefin cross-metathesis

Article abstract of DOI:10.1021/ol102398y

A novel iterative synthesis of heterotelechelic oligo(phenylene-vinylene)s using olefin cross-metathesis is reported. The metathesis homologation proceeds in good yields and allows for further functionalization, including the facile formation of donor-acceptor complexes and repeating sequence copolymers.

Full text of DOI:10.1021/ol102398y

ORGANIC
LETTERS
2010
Vol. 12, No. 23
5514-5517
Iterative Synthesis of Heterotelechelic
Oligo(phenylene-vinylene)s by Olefin
Cross-Metathesis
Benjamin N. Norris, Tianqi Pan, and Tara Y. Meyer*
Department of Chemistry, UniVersity of Pittsburgh, Pittsburgh,
PennsylVania 15260, United States
tmeyer@pitt.edu
Received October 4, 2010
ABSTRACT
A novel iterative synthesis of heterotelechelic oligo(phenylene-vinylene)s using olefin cross-metathesis is reported. The metathesis homologation
proceeds in good yields and allows for further functionalization, including the facile formation of donor-acceptor complexes and repeating
sequence copolymers.
In the ongoing search for more efficient organic electronics,
conjugated materials with discrete repeating sequences may
offer advantages over easier-to-prepare random copolymers.1
However, examples of such sequenced copolymers are rare.2
We are interested in developing new syntheses for and
exploring the properties of repeating sequence copolymers
(RSCs).3 Herein, we present a novel homologation strategy
for producing heterotelechelic sequenced oligo(phenylene-
vinylene)s (OPVs) and for assembling these oligomers into
RSCs.
Our synthetic approach to heterotelechelic OPVs is based
on olefin cross-metathesis (CM), which is attractive for
the following reasons: (1) simplicity, the coupling of vinyl
groups to give internal olefins avoids the need for complex
functionality in the monomer, compared to Suzuki4 or
Horner-Wadsworth-Emmons5 approaches; (2) general-
ity, the Grubbs II catalyst is highly tolerant; and (3) utility,
the resultant heterotelechelic oligomers exhibit orthogo-
nally reactive end groups that can be elaborated into more
complex materials.6 This last advantage is particularly
important since many approaches to OPVs generate
symmetric7 or low-functionality4,5,8 oligomers. The use of
orthogonally functionalized or protected monomers to pro-
duce OPVs is not a new strategy,8 but this is the first report
to utilize this approach with olefin metathesis. Prior metath-
esis approaches to OPVs have not focused on the creation
of well-defined molecules, but rather on the creation and
separation of mixtures of oligomers.9
(4) (a) Iwadate, N.; Suginome, M. Org. Lett. 2009, 11, 1899. (b)
Katayama, H.; Nagao, M.; Ozawa, F.; Ikegami, M.; Arai, T. J. Org. Chem.
2006, 71, 2699.
(5) Guerlin, A.; Dumur, F.; Dumas, E.; Miomandre, F.; Wantz, G.;
Mayer, C. R. Org. Lett. 2010, 12, 2382.
(6) For recent examples of heterotelechelic oligomers, see: (a) Berthet,
M.-A.; Zarafshani, Z.; Pfeifer, S.; Lutz, J.-F. Macromolecules 2010, 43,
44. (b) Roth, P. J.; Kim, K.-S.; Bae, S. H.; Sohn, B.-H.; Theato, P.; Zentel,
R. Macromol. Rapid Commun. 2009, 30, 1274.
(1) Beaujuge, P. M.; Amb, C. M.; Reynolds, J. R. Acc. Chem. Res. DOI:
10.1021/ar100043u. Published Online: August 20, 2010.
(2) (a) Lutz, J.-F. Polym. Chem. 2010, 1, 55. (b) Badi, N.; Lutz, J.-F.
Chem. Soc. ReV. 2009, 38, 3383.
(7) (a) Abbel, R.; Grenier, C.; Pouderoijen, M. J.; Stouwdam, J. W.;
Lecle`re, P. E. L. G.; Sijbesma, R. P.; Meijer, E. W.; Schenning, A. P. H. J.
J. Am. Chem. Soc. 2009, 131, 833. (b) Vundyala, N.; Sun, C.; Sidime, F.;
Shi, W.; L’Amoreaux, W.; Raja, K.; Peetz, R. M. Tetrahedron Lett. 2008,
49, 6386.
(3) (a) Stayshich, R. M.; Meyer, T. Y. J. Am. Chem. Soc. 2010, 132,
10920. (b) Copenhafer, J. E.; Walters, R. W.; Meyer, T. Y. Macromolecules
2008, 41, 31.
(8) (a) Jørgensen, M.; Krebs, F. C. J. Org. Chem. 2004, 69, 6688. (b)
Maddux, T. M.; Li, W.; Yu, L. J. Am. Chem. Soc. 1997, 119, 844
.
10.1021/ol102398y  2010 American Chemical Society
Published on Web 11/11/2010

Scheme 1. Homologation Strategy
Our strategy for the synthesis of heterotelechelic OPVs
involves the sequential coupling of phenyl monomers
(Scheme 1). End-to-end homologation allows for the creation
of oligomers of any desired length. The key step is a CM
reaction between olefin-terminated growing oligomers and
a vinylbenzaldehyde. The aldehyde end group of the n + 1
oligomer product can then be converted into a metathesis-
ready vinyl moiety or exploited for further functionalization.
alkene is driven toward the desired CM product, which, as
a Type IV olefin, is unreactive toward further metathesis.
Alternation between Type I and Type II vinylbenzalde-
hydes allows for stepwise growth of the oligomer. An OPV
terminated with a Type I olefin can undergo selective
homologation with a Type II monomer, 2,5-bis(hexyloxy)-
4-vinylbenzaldehyde, 1. This new OPV will be terminated
with a Type II olefin and can undergo homologation with a
Type I monomer, 4-vinylbenzaldehyde, 2, to give another
OPV terminated with a Type I olefin, and so on. Since all
of the CM products are internal olefins of Type IV,
scrambling of extant vinylene groups is suppressed.
Scheme 2. CM Selectivity with Styrenes
Type II monomer 1 was prepared from hydroquinone in
5 steps with an overall yield of 51%. Type I monomer 2
was prepared as previously reported.11 The full synthetic
scheme for monomer 1 (Scheme S1) can be found in the
Supporting Information.
The first homologation involved CM between styrene
3 (Type II) and monomer 2 (Type I) to give aldehyde
terminated OPV1a in a 75% yield (Scheme 3). This CM
requires very low catalyst loading (7.5 × 10-3 equiv) and
can be performed on multigram scales. The use of a 2-fold
excess of monomer 2 improved the yield of the cross-
coupled product. The homologation is trans-specific; the
1H NMR spectrum bears no peaks between δ 6.5 and 7.0,
the characteristic range for cis-phenylene-vinylenes.7b The
homologation, completed by a Wittig olefination with
Ph3PdCH2, gave vinyl-terminated OPV1b in a 96% yield.
Subsequent homologations followed an alternating pattern.
Each CM reaction used 3 equiv or less of the vinylbenzal-
dehyde and 0.01 equiv or less of Grubbs II catalyst,
proceeded in moderate to good yields, and was trans-specific.
The Wittig reactions all proceeded in good to excellent yields.
The excess vinylbenzaldehyde, especially monomer 1,
complicated the isolation and purification of the oligomers,
as evidenced by the lower yields of OPV2a and OPV4a
compared to those of OPV1a and OPV3a.
To promote CM over homometathesis, we exploit the
known reactivity difference between ortho-substituted and
ortho-unsubstituted styrenes.10 Styrenes lacking ortho sub-
stituents, Type I olefins, undergo rapid and reversible
homodimerization (Scheme 2). Styrenes with ortho substit-
uents, Type II olefins, are slow to homodimerize and the
coupling reaction, which produces a Type IV olefin, is nearly
irreversible. Thus, the reaction of a Type I with a Type II
The absorption and emission spectra of the OPVs were
obtained in CHCl3 (Table 1). Within each series (OPVa and
OPVb), the absorption and emission maxima, and molar
extinction coefficients, increase with increasing conjugation,
while the HOMO-LUMO gap, ∆Egap, decreases. The
absorption and emission spectra of the OPVa series are red-
(10) Chatterjee, A. K.; Choi, T.-L.; Sanders, D. P.; Grubbs, R. H. J. Am.
Chem. Soc. 2003, 125, 11360.
(9) Peetz, R.; Strachota, A.; Thorn-Csa´nyi, E. Macromol. Chem. Phys.
2003, 204, 1439.
(11) McKean, D. R.; Parrinello, G.; Renaldo, A. F.; Stille, J. K. J. Org.
Chem. 1987, 52, 422.
Org. Lett., Vol. 12, No. 23, 2010
5515

Scheme 3
.
Synthesis of OPVs
Table 1. Optical Properties of OPVsa
Scheme 4. Synthesis of Donor-Acceptor Chromophores
OPV
λmax (abs)/nm λmax (em)/nm εb/M-1 cm-1 ∆Egapc/eV
OPV1a
OPV2a
OPV3a
OPV4a
OPV1b
OPV2b
OPV3b
OPV4b
376
411
427
443
360
399
419
437
492
503
519
548
423
473
480
496
29, 100
35, 300
82, 800
77, 500
40, 500
70, 900
94, 400
118, 000
2.89
2.69
2.51
2.48
3.06
2.76
2.58
2.52
a Obtained in CHCl3 (∼10-5 M). b Calculated at absorption maximum.
c HOMO-LUMO gap estimated as the onset of absorption.
shifted relative to the OPVb series, and the HOMO-LUMO
gaps of the OPVa series are smaller than those of the OPVb
series. These effects are most pronounced in the OPV1
oligomers and become progressively smaller with increasing
conjugation. The extinction coefficients of the OPVb series
are higher than those of the OPVa series, with the difference
most pronounced in the OPV2 and OPV4 oligomers.
As an initial demonstration of the types of complex
copolymers that could be prepared from heterotelechelic
OPVs, we have converted OPV2b to OPV2e and onward
to copolymer p(OPV2e) (Scheme 5), using a Suzuki-Miyaura
coupling followed by acyclic diene metathesis (ADMET)
polymerization, chemistry amenable to producing a family
of such RSCs.3b We believe p(OPV2e) should exhibit a high
degree of head-to-tail regioregularity because OPV2e is an
AB monomer containing a Type II styrenyl olefin (head) and
a Type I aliphatic olefin (tail). Selective metathesis has
previously been used to prepare AABB alternating copoly-
mers,12 but, to the best of our knowledge, there is only one
other example of a head-to-tail polymer prepared in this
fashion.12b The incorporation of a flexible unit into the
Further functionalization of OPVs permits the physical
and electronic properties of the material to be adjusted to
meet the needs of various applications.5 We highlight the
ability of our heterotelechelic OPVs to serve as modular
platforms for orthogonal functionalization by elaborating
upon OPV2a using a Suzuki coupling to produce green-
emitting donor-acceptor chromophore OPV2c followed
by an aldol condensation to produce blue-green absorbing
but weakly emitting chromophore OPV2d (Scheme 4).
Both reactions are simple, robust, and proceed in excellent
yields.
(12) (a) Choi, T. L.; Rutenberg, I. M.; Grubbs, R. H. Angew. Chem.,
Int. Ed. 2002, 41, 3839. (b) Demel, S.; Slugovc, C.; Stelzer, F.; Fodro-
Csorba, K.; Calli, G. Macromol. Rapid Commun. 2003, 24, 636.
5516
Org. Lett., Vol. 12, No. 23, 2010

Scheme 5. Synthesis of Head-to-Tail Copolymers
Figure 1.
1H NMR spectra of the head-to-tail polymerization as a
function of time: (A) at 0 h (OPV2e); (B) after 24 h, 50%
consumption of head groups, 100% conversion to head-to-tail; (C)
after 48 h, 80% consumption of head groups, 75% conversion to
head-to-tail; (D) after 96 h, 90% consumption of head groups, 80%
conversion to head-to-tail. Head-to-head protons (δ ∼7.2) are not
assignable due to overlap. Labels (HA-H) refer to labeled protons
in Scheme 5.
backbone of a highly conjugated structure can lead to
improved solubilities, interesting liquid crystal behavior, and/
or modulated physical properties,3b and the inherent dissym-
metry of the AB copolymer could lead to enhanced opto-
electronic properties.13
OPV2e was polymerized with Grubbs II catalyst in CHCl3.
The polymerization was monitored with 1H NMR spectros-
copy by observing the disappearance of the signals for the
aliphatic alkenyl group (HA, δ ∼5.2; HB, δ ∼5.1; and HC, δ
∼5.9) and the styrenyl group (HD, δ ∼5.3; and HE, δ ∼5.8)
and the appearance of the signals for the tail-to-tail (HG, δ
∼5.5) and head-to-tail (HH, δ ∼6.7; and HI, δ ∼6.3)
connections. We estimated the percent head-to-tail based on
conversion of styrenyl groups to head-to-tail groups (Figure
1). After 24 h, no OPV2e remains, having been converted
first to the tail-to-tail intermediate followed by further
metathesis to give head-to-tail connections. At longer reaction
times there is some decrease in percent head-to-tail couplings
which can be attributed to head-to-head couplings, which
are improbable but accumulate slowly. It should be noted
that the presence of either head-to-head or tail-to-tail
couplings in the final polymer chain disrupts the sequence
of the RSC and interferes with the desired polymer direc-
tionality. Future work will focus on optimization to minimize
such mistakes and/or characterization of the effects of these
errors on properties.
In summary, we have developed a new synthetic method
for the preparation of a homologous series of heterotelechelic
OPVs. The preparation, which relies on selective CM,
produces oligomers that can be further elaborated to make
donor-acceptor complexes or repeating sequence copolymers.
A key advantage of this method is the generality; the
functional group-friendly metathesis-based couplings should
be applicable to phenylene monomers bearing substituents
other than the alkoxy groups used in this study.
Acknowledgment. We acknowledge NSF (0809289) and
the University of Pittsburgh for financial support.
Supporting Information Available: Synthetic scheme for
compounds 1 and 3, experimental procedures and charac-
terization data for all new compounds, and NMR, absorption,
and emission spectra for all OPVs. This material is available
free of charge via the Internet at http://pubs.acs.org.
(13) Cho, M. J.; Choi, D. H.; Sullivan, P. A.; Akelaitis, A. J. P.; Dalton,
L. R. Prog. Polym. Sci. 2008, 33, 1013.
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