The efficient synthesis of unsymmetrical oligo(phenylenevinylenes)

Article abstract of DOI:10.1016/j.tetlet.2007.11.073

Selective DDQ oxidation of 2,5-bis(methylmethoxy)-1,4-di(octyloxy)benzene generates an unsymmetrical subunit suitable for the preparation of substituted tetrameric oligo(phenylenevinylenes) in good to excellent yields.

Full text of DOI:10.1016/j.tetlet.2007.11.073

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Tetrahedron Letters 49 (2008) 526–528
The efficient synthesis of unsymmetrical oligo(phenylenevinylenes)
Timothy Smith and David A. Modarelli*
Department of Chemistry and The Center for Laser and Optical Spectroscopy, The University of Akron,
Akron, OH 44325-3601, United States
Received 26 June 2007; revised 12 November 2007; accepted 14 November 2007
Available online 19 November 2007
Abstract—Selective DDQ oxidation of 2,5-bis(methylmethoxy)-1,4-di(octyloxy)benzene generates an unsymmetrical subunit suitable
for the preparation of substituted tetrameric oligo(phenylenevinylenes) in good to excellent yields.
Ó 2007 Elsevier Ltd. All rights reserved.
1
. Introduction
from which a growing oligomeric chain can then be con-
structed. Unfortunately, monoformylation reactions are
often plagued with unwanted byproducts and low to
moderate yields. As part of an ongoing program to pre-
pare novel unsymmetrical OPVs, we sought a more
selective and synthetically simpler method of producing
these oligomers (Chart 1). Such a facile and versatile
synthesis is described in this Letter.
Probing the characteristics of well-defined p conjugated
systems has been an active area of research over the past
few years. The synthesis of unsymmetrically substituted
oligo(phenylenevinylenes) (OPVs) may have uses in
1
2
3
LED devices, organic photovoltaics, and NLO appli-
4
cations.
To efficiently prepare unsymmetrical
oligo(phenylenevinylenes), the facile synthesis of an
unsymmetrical central subunit that can be produced in
high yields is highly desirable. The most common
method for the production of unsymmetrical subunits
involves di-iodination of 1,4-di(octyloxy)benzene
2. Results and discussion
The selective oxidation of substituted methyl benzyl
ethers with DDQ to their corresponding mono-alde-
5
followed by monoformylation with n-BuLi and DMF,
6
hydes has previously been described. The rate at which
the oxidation occurs was found to depend on both the
reaction concentration and the substitution pattern on
the aromatic ring. In particular, the presence of electron
donating substituents on the ring was found to acceler-
ate the direct conversion of dimethyl benzyl ethers to
mono-aldehydes in excellent yields (ꢀ99% isolated).
This method appeared ideal for the preparation of an
unsymmetrical subunit useful for OPV synthesis
(Scheme 1).
R2
OR
1
2
: R = H
R = H
2
: R = OCH3 R = H
R = H
R = H
: R = OCH3 R = NO
1
1
2
OR
R = -octyl
3: R = NO2
1
2
R1
4
5
6
7
: R = NH2
1
2
n
1
2
2
2
: R = Br
: R = Br
R = NO
R = NH
1
2
1
2
2
OR
OR
Chart 1. Unsymmetrical OPVs 1–7.
CHO
OCH3
(a)
H CO
H CO
3
3
Keywords: Oligomers; Phenylene vinylene; Asymmetric synthesis;
Photovoltaic; Wittig; Horner–Wadsworth–Emmons; Mono-formylated;
Coupling.
R = n-octyl
OR
8
OR
9
*
Corresponding author. Tel.: +1 330 972 7366; fax: +1 330 972
370; e-mail: dmodarelli@uakron.edu
7
2 2 2
Scheme 1. Reagents and conditions: (a) DDQ, CH Cl /H O, rt, 99%.
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.11.073

T. Smith, D. A. Modarelli / Tetrahedron Letters 49 (2008) 526–528
527
OR
Br
O
P
OR
OR
CHO
OEt
OEt
(a)
H CO
3
Br
H CO
3
OR
9
R =
n
-octyl
10
11
Br
O
P
OR
(
b)
OEt
OEt
(c)
R1
OHC
OR
13 R1 = H
12
1
1
4 R1 = OCH3
5 R1 = NO2
Br
OR
OR
(
d)
1
2
3
: R1 = H
: R1 = OCH3
: R1 = NO2
OR
R1
OR
4: R1 = NH2
1
1
1
6 R1 = H
7 R1 = OCH3
8 R1 = NO2
R1
Scheme 2. Reagents and conditions: (a) tBuOK, THF, rt, 12 h, 92%; (b) DDQ, CH
Styrene, Pd(OAc) , K PO , DMA, 140 °C, 19 h, 50–75%.
2 2 2
Cl /H O, rt, 96%; (c) tBuOK, THF, rt, 12 h, 74–84%; (d)
2
3
4
The synthesis of unsymmetrical OPVs using mono-alde-
hyde 9 is shown in Scheme 2. Dimethyl ether 8 was pre-
pared from the corresponding dibromide by reaction
with sodium methoxide in dry methanol in 96% yield.
The selective oxidation of 8 to 9 was found to proceed
best using a 0.02 M solution of 8 and DDQ in a 10:1
mixture of methylene chloride and water at room tem-
perature. The conversion to mono-aldehyde 9 was found
by TLC to occur quantitatively (99%) in 15 min. Purifi-
cation of 9 was accomplished by washing the organic
thetic approach presented here, we used two different
reaction pathways to prepare tetramers 1–4 (Scheme 2)
and 5–7 (Scheme 3) from this precursor.
For the synthesis of tetramers 1–3 (Scheme 2), the inter-
mediate bromide 12 was reacted with the para-substi-
tuted aryl phosphonates 13–15 under HWE coupling
conditions to produce trimers 16–18. The subsequent
Heck coupling reactions with styrene produced tetra-
mers 1–3. Nitro-substituted tetramer 3 was converted
to its amine-substituted analog 4 by reduction with
layer with satd NaHCO and removing the organic sol-
3
vent under reduced pressure. The intermediate dimeric
bromide 11 was prepared using a Horner–Wadsworth–
Emmons (HWE) coupling reaction of 9 with aryl phos-
phonate ester 10, and then converted to 12 in 96% yield
with DDQ. To demonstrate the versatility of the syn-
SnCl in ethanol (80 °C, 18 h) in 60% yield.
2
The procedure used to synthesize tetramers 5–6 is out-
lined in Scheme 3. In this synthesis, 12 is reacted under
Heck coupling conditions with para-nitrostyrene to give
NO2
Br
OR
OR
(
a)
O N
2
OHC
OHC
OR
1
2
19
OR
O
P OEt
OEt
(
b)
R1
1
1
3 R = Br
1
NO2
4 R = OCH3
1
OR
5
: R = OCH3
1
OR
6: R = Br
1
R1
Scheme 3. Reagents and conditions: (a) Pd(OAc) , NaOAc, tetramethylguanidine, DMA, 140 °C, 16 h, 50%; (b) tBuOK, THF, rt, 12 h, 67–69%.
2

528
T. Smith, D. A. Modarelli / Tetrahedron Letters 49 (2008) 526–528
the trimeric aldehyde 19 in ꢀ50% crude yield. HWE
coupling with either 13 or 14 provided the correspond-
ing methoxy-nitro (5) and bromo-nitro-substituted (6)
tetramers, which were subsequently purified by column
chromatography. The bromo-nitro-substituted tetramer
References and notes
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was then converted to the amine-substituted analog 7
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2
version of 3 to 4.
In summary, the selective oxidation of a methyl benzyl
ether produces an intermediate we have used to synthe-
size unsymmetrical mono- and disubstituted OPVs. We
believe this synthetic procedure is more versatile and
simpler to work with than previous synthetic methods.
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Supplementary data associated with this article can be
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