Investigation of oxygen-free Sonogashira step growth synthesis of mono-terminated di-tert-butyl-substituted oligo(phenylene ethynylene)s (OPEs)

Article abstract of DOI:10.1016/j.tet.2012.11.063

A series of oligomers, the hetero-coupled mono-terminated di-tert-butyl-substituted phenylene ethynylene dimer, trimer, tetramer, and pentamer have been successfully synthesized in a stepwise process and characterized. Their electronic properties have bee

Full text of DOI:10.1016/j.tet.2012.11.063

Tetrahedron 69 (2013) 1105e1111
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Investigation of oxygen-free Sonogashira step growth synthesis
of mono-terminated di-tert-butyl-substituted oligo(phenylene
ethynylene)s (OPEs)
,
a,
Xinyan Bai a, Xueyi Chen a, Charles Barnes b, Jerry R. Dias a , T.C. Sandreczki
*
*
a Department of Chemistry, University of Missouri-Kansas City, 5100 Rockhill Road, Kansas City, MO 64110, United States
b Department of Chemistry, University of Missouri-Columbia, 601 South College Avenue, Columbia, MO 65211, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 October 2012
Received in revised form 16 November 2012
Accepted 19 November 2012
Available online 28 November 2012
A series of oligomers, the hetero-coupled mono-terminated di-tert-butyl-substituted phenylene ethy-
nylene dimer, trimer, tetramer, and pentamer have been successfully synthesized in a stepwise process
and characterized. Their electronic properties have been studied by UVevis and fluorescence spectros-
copy. Both UV absorption and FL emission maxima have red shift trends with increasing conjugation
length of the oligomers. In addition, homo-coupling products inevitably complicate the hetero-coupling
reaction. With increasing substituent group size on the terminal alkyne, the amount of homo-coupling
product is increased.
Keywords:
Oligo(phenylene ethynylene)s
spesp2 Conjugated systems
Oxygen-free Sonogashira reactions
Carbonecarbon bond formation
Fluorescence emmission
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
alternating phenyleneeacetylene system and greatly decreases
a polymer’s solubility, quantum yield, and purity.8 In addition, the
The use of phenylene ethynylene polymers and oligomers for
fabrication of electronic and optoelectronic devices such as organic
photovoltaic cells,1 organic field-effect transistors2 (OFETs), and
organic light-emitting diodes3 (OLEDs) is of current interest. In
recent years, great progress has been made in the synthesis of
various rigid rod-like (poly) p-aryl acetylene polymers and triple-
bond-containing oligomers as potential organic semi-
conductors;1c,4 however, due to the intrinsic limitations of many
synthetic strategies, it has been impossible to make perfect highly
conjugated arylene ethynylene polymers and large oligomers. De-
fects in the conjugated system lead to inferior electronic properties
and limit their application in molecular electronics and photonics.5
The Sonogashira coupling reaction is currently the only efficient
synthesis of arylene ethynylene oligomers by Sonogashira coupling
is also complicated by the presence of competing homo-coupling
induced by the CuI catalyst and the oxidizing agent (O2). As part of
our effort in this field, we have investigated the unsymmetric
alkynylation of electron-enriched aryl dihalides and their applica-
tion to the stepwise-expansion of spesp2 conjugated systems to
form various mono-terminated di-tert-butyl-substituted arylene
ethynylene oligomers by using oxygen-free Sonogashira reaction
conditions.8 In this study, we present a method to obtain soluble
iodo-substituted oligomers. These oligomers are the key com-
pounds for production of even numbered oligomers. They are also
important building blocks for synthesizing more extended spesp2
conjugated systems.
and useful method for the formation of p-conjugated triple-bond-
containing systems.6 The hetero-coupling of iodo-substituted aro-
matic compounds with terminal alkynes is complicated by the
homo-coupling of the two terminal alkynes to themselves.7 In
particular, the polymerization between di-iodo aromatic com-
pounds and dialkynyl aromatic compounds forms polymers con-
taining a number of diyne units forming ABBA type polymers in
competition with the ABAB type polymers. This breaks the
2. Results and discussion
The syntheses of mono-terminated di-tert-butyl-substituted
oligo(phenylene ethynylene) series are summarized in Schemes
1e4. The iodo-substituted monomer 1,3-di-tert-butyl-5-iodo-2-
methoxybenzene (2) was synthesized by methylation of the com-
mercially available 2, 6-di-tert-butylphenol,9 followed by iodin-
ation using iodine monochloride (Scheme 1). It was then coupled
with trimethylsilylacetylene under Sonogashira conditions to give
* Corresponding authors. Tel.: þ1 816 235 2284; e-mail address: diasj@umkc.edu
(J.R. Dias).
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.11.063

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X. Bai et al. / Tetrahedron 69 (2013) 1105e1111
Scheme 1. Synthesis of dimer 4a.
Scheme 4. Synthesis of pentamer 7a.
satisfactory, 97%, 80%, 60%, and 40%, respectively, for oligomers
4ae7a. The competing homo-coupling side reactions become more
dominant as the terminal alkyne gets larger, even though the re-
action is conducted under oxygen-free conditions. When a small-
substituent terminal alkyne, such as compound 3, was used in
this reaction, it yielded the homo-coupling side product, homo2, in
only 3%. But when the larger 4b, 5b, and 6b were used as the ter-
minal alkynes, the corresponding homo-products homo4, homo6,
and homo8 were obtained in 20%, 35e40%, and 60%, yields, re-
spectively. It is not clear why a larger substituent group on the
terminal alkyne leads to more homo-coupling products. But it
might be due to decreasing solubility leading to the monomers
precipitating onto the catalyst.
Scheme 2. Synthesis of trimer 5a.
1H NMR spectra of oligomers unambiguously identify each
oligomer. For example, as shown in Fig. 1, the formation of 5a, 6a,
and 7a was confirmed by integration of three signals above
7.00 ppm, i.e., 10, 14, and 18 protons, respectively. The two
aromatic protons on di-tert-butyl methoxyphenyl give a singlet
at 7.43 ppm. The signals for the four protons on the phenyl-
ethynyltrimethylsilane unit appear at 7.46 ppm, and all other pro-
tons on the middle phenylene groups can be found at around
7.50 ppm. The triple-bond carbons exhibit 13C NMR shift from 87 to
105 ppm as shown in Fig. 2.
Scheme 3. Synthesis of tetramer 6a.
the TMS protected monomer, followed by deprotection of TMS
using TBAF to give monomer 3. This was then reacted with excess
1,4-diiodobenzene to afford iodo-substituted dimer 4. A small
amount of homo-coupling side product homo2 was also isolated.
Repeating this cycle of mono-alkynylation of diiodophenylene,
coupling with TMSA and then deprotecting afforded trimer 5a
(Scheme 2), tetramer 6a (Scheme 3), and pentamer 7a (Scheme 4)
with corresponding homo4, homo6, and homo8 as side products,
respectively. The two tert-butyl groups on the phenyl ring greatly
increased the solubility of the oligomers. Pentamer 7a remains
soluble in most organic solvents, while mono-iodo-substituted
pentamer 7 has very limited solubility even in chloroform. The
hetero-coupling reaction yields for this series of oligomers are
Fig. 1. Partial 1H NMR spectra of oligomers 5a, 6a, and 7a in CDCl3.
The carbon directly connected to the trimethylsilyl group is
shifted to 96.5 ppm due to the shielding effect resulting from
electron donation by the trimethylsilyl group. The next succeeding
carbon is shifted to 104.5 ppm due to the influence of the end ad-
jacent aromatic group. The two carbons (assigned to C16 and C17)
next to di-tert-butyl methoxyphenyl ring resonate at 92.0 and

X. Bai et al. / Tetrahedron 69 (2013) 1105e1111
1107
Fig. 4. The crystal structure of 5b. The displacement ellipsoids were drawn at the 50%
probability level.
Fig. 2. Partial 13C NMR spectra of oligomers 5a, 6a, and 7a in CDCl3.
Fig. 5. The crystal structure of homo4. The displacement ellipsoids were drawn at the
50% probability level.
87.5 ppm, respectively. From oligomer 5a to 7a the additional tri-
ple-bond carbon shifts are all in narrow range between 90 and
91 ppm, which indicate that chemical environment of these car-
bons is very similar.
coplanar for the same reason. The terminal and internal rings are
rotated with respect to each other. The distortions from linearity in
4 are also less dramatic than the other two molecules, again partly
due to the inversion center.
Very few structural studies of these series of spesp2 conjugated
systems have been conducted, due to the lack of oligomers with
high purity. In this study, upon slow solvent evaporation, we were
able to obtain single crystals of oligomers suitable for X-ray anal-
ysis. Our structural analysis of these spesp2 conjugated oligomers
reveals their deviations from planarity in the solid form and thus on
the effectiveness of their conjugation. This will assist our un-
derstanding of the relationship of oligomer’s UVevis absorption
and fluorescence emission spectra. The introduction of solubility
enhancement groups causes a twisting of rings, thus disturbing
conjugation. The conjugation systems in this paper contain di-tert-
butyl groups and the crystal structures 5a, 5b, and homo4 shown
here (Figs. 3e5) reveal how these groups twist the conjugation
plane, which affects their photo emission and presumably their
tendency for self-assembly.
For oligomer 5a the motif contains two non-identical molecules,
A and B in Fig. 3. Although these molecules are parallel to each
other and are oriented head to tail, consistent with P-1, The B tert-
butyl groups exhibit slight rotational disorder. In Fig. 3, only the
principal position is shown. Both the di-tert-butyl methox-
yphenylene and the trimethylsilyl phenylene rings are twisted out
of plane. The dihedral angle between the di-tert-butyl methox-
yphenylene ring and the central ring is 19.30ꢀ. The C6eC1eC2 bond
angle of 5a is 122.4ꢀ instead of 120ꢀ, probably due to the inward
squeezing of the tert-butyl on C(1) of 5a, a similar phenomenon to
what we found previously for the iodinated cyclopentadienone
derivative having two tert-butyl groups.10 The bond angles for
C3eC4eC5, C19eC18eC23, C20eC21eC22, C27eC26eC31, and
C28eC29eC30, in which the central carbons are adjacent to the
triple bond, are 119ꢀ. All the other bond angles on the phenylene
rings are 120.5ꢀ except for the bond angles of C3eC2eC1 and
C5eC6eC1 being 116.5ꢀ and 117.3ꢀ, respectively.
The structure of oligomer 5b is shown in Fig. 4. The three phenyl
rings are not in the same plane, especially the di-tert-butyl
methoxyphenyl ring, which is significantly twisted out of plane for
a torsion angle of 25.5ꢀ. Without the silyl group on the remote end
of the triple bond, 5b is more twisted and bent away from the pep
conjugation plane than the corresponding structure of 5a. In ad-
dition, the triple-bond linkage is bent. The bond angles between the
carbons on the phenyl rings and the acetylenes are 173.98ꢀ
(C4eC7eC8),175.04ꢀ (C7eC8eC9),177.01ꢀ (C12eC15eC16),177.54ꢀ
(C15eC16eC17), and 179.03ꢀ (C20eC23eC24), respectively. The
two bulky tert-butyl groups on the phenyl ring and the un-
symmetrical structure of this molecule could be responsible for the
bending of bond angles and the twisting of the phenyl rings in the
conjugation system, as well as crystal packing forces.
As shown in Fig. 5, oligomer homo4 is also not planar although it
features a symmetrical structure. The two terminal di-tert-butyl
methoxyphenyl rings are in the same plane; however, the two
coplanar central rings are twisted with respect to the plane con-
taining the terminal rings. Also, no bending of the triple bonds is
Fig. 3. The crystal structure of 5a. The displacement ellipsoids were drawn at the 50%
probability level.
The structures of oligomer 5a, 5b, and homo4 have been con-
firmed by single crystal X-ray diffraction. All three compounds
crystallize in the triclinic space group P-1. Due to the steric hin-
drance of the two tert-butyl groups, the packing of molecules is
constrained. For example, the phenylene rings are not in the same
plane and the ethynylene bonds are bent for all three oligomers. In
4, the central rings are exactly coplanar due to symmetry, and the
molecule lies on an inversion center. The terminal rings are

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X. Bai et al. / Tetrahedron 69 (2013) 1105e1111
Table 1
evident. The above solid state data for oligomers 5a, 5b, and homo4
suggest that the twist of the phenyl rings and the bond bending
induced by tert-butyl groups are most likely due to packing forces.
This is not present in gas phase or solution.
Summary of the absorption and fluorescence emission spectra (CH2Cl2) of oligomers
4ae7a
Oligomer lmax (abs) (nm) lmax (emission) (nm) Stokes shift (nm) Ff
4a
5a
6a
7a
312
350
346
356
366
387
388
395
54
37
42
39
0.343
0.536
0.986
0.992
UVevis and fluorescence spectra for this series of oligomers are
shown in Figs. 6 and 7, respectively. The spectroscopic data are
summarized in Table 1. The absorption maxima are red-shifted
with increasing conjugation length. There is a 38 nm red shift
when comparing 4a to 5a (n from 2 to 3), but the red shift from 6a
to 7a is only 10 nm, so that with each repeat unit increase, the red
shift difference becomes smaller. For emission spectra, there is
a 21 nm red shift as n varies from 2 to 3, with the fluorescence
maximum changing from 366 to 387 nm. Compounds 5a and 6a
have almost the same emission maximum (387 and 388 nm, re-
spectively) even though 6a is more conjugated than 5a. Finally, 7a
has its maximum emission at 395 nm and is thus red-shifted by
29 nm compared to 4a. The overall trend is that fluorescence
maxima gradually red-shift as the number of phenylene ethylene
units increases. The oligomers with conjugation lengths of more
than three units are highly fluorescent. As is shown in Table 1, the
quantum yield is 0.34 for dimer 4a, 0.54 for trimer 5a, 0.98 for
tetramer 6a, and 0.99 for pentamer 7a, as measured by Jobin Yvon
method.11
Our investigation is limited to studying only oligomers with
conjugation lengths varying from 2 to 5, and so it is unclear
whether the red shift for an extended series of oligomers would
reach a limiting value. Unfortunately, achieving pure longer oligo-
mers (n>5) is difficult due to the increased presence of homo-
coupling side product and low solubilities, which make purifica-
tion an issue. Homo-coupling is non-negligible and competes
strongly with hetero-coupling, when the terminal alkynes have
large or bulky substituent groups, even under oxygen-free
conditions.
3. Conclusion
With di-tert-butyl substituents on the end phenyl ring,
a twisting of the inner phenyl rings and a bending of acetylene
bonds are observed in the corresponding crystal structures,
probably due to packing forces. UVevis and fluorescence studies
show that there is a red shift for this series of oligomers 4ae7a
with each additional phenylene ethynylene unit in the conjugated
system. The quantum yield is convergent to 1.0 as the number of
phenylene ethynylene unit increases. As the size of the coupling
partners increases, the efficiency of the Sonogashira coupling
decreases, even if there is no steric impediment at the site of
reaction.
4. Experimental
4.1. General methods
1H NMR and 13C NMR spectra were recorded at 400 and
100 MHz. Single crystal X-ray analyses were performed on a Bruker
SMART 1K CCD area detector system at the University of Missouri-
Columbia. Reagent grade tetrahydrofuran (THF) was distilled from
sodium potassium alloy. Triethylamine (Et3N) was distilled from
CaH2 under N2 atmosphere. Column chromatography was per-
formed on silica gel (MERCK C60). All Sonogashira coupling re-
actions were performed using our oxygen-free two-chamber
reaction system as described in the literature.8
Fig. 6. Normalized absorption spectra of oligomers 4ae7a in CH2Cl2 solution.
4.2. 1,3-Di-tert-butyl-2-methoxy benzene (1)
2,6-Di-tert-butylphenol (10.0 g, 48 mmol), NaH (2.30 g,
58 mmol), and iodomethane (8.22 g, 58 mmol) were added to DMF
(50 mL) at 0 ꢀC for 2 h. The reaction mixture was poured into cold
water, extracted with diethyl ether, and dried with Na2SO4. The
crude product was purified by flash column and dried in a vacuum
oven to afford 1. 1H NMR (400 MHz, CDCl3)
d
7.35 (d, 2H, J¼6.4 Hz),
6.98 (m, 1H), 3.67 (s, 3H), 1.39 (s, 18H). 13C NMR (100 MHz, CDCl3)
159.7 (C1), 144.2 (C2, C6), 126.5 (C3, C5), 123.2 (C4), 64.4 (C7), 35.8
(C8, C12), 31.9 (C9, C10, C11, C13, C14, C15).
d
Fig. 7. Fluorescence emission spectra of oligomers 4ae7a in CH2Cl2 solution.

X. Bai et al. / Tetrahedron 69 (2013) 1105e1111
1109
4.3. 1,3-Di-tert-butyl-5-iodo-2-methoxy benzene (2)
C12), 31.9 (C9, C10, C11, C13, C14, C15). HRMS (EI) calcd for C23H27IO
[Mþ] 446.1107, found at 446.1121, 3.1 ppm.
4.6. ((4-((3,5-Di-tert-butyl-4-methoxyphenyl)ethynyl)phenyl)
ethynyl)trimethylsilane (4a)
Compound 1 (5.0 g, 23 mmol) was dissolved in 30 mL CH2Cl2
and ICl (4.5 g, 27.6 mmol) was added into slowly into solution at
0 ꢀC for 1 h. The reaction mixture was poured into H2SO3 solution,
washed with Na2CO3 for twice, extracted by CH2Cl2, and dried with
Na2SO4. It afforded compound 2 in 95% yield. 1H NMR (400 MHz,
Compound 3 (2.0 g, 8.20 mmol), ((4-iodophenyl)ethynyl)tri-
methylsilane (2.9 g, 9.84 mmol), Pd(PPh3)2Cl2 (0.4 mol %), CuI
(0.4 mol %), Et3N (0.5 mL), and THF (10 mL) were placed in a spe-
cially designed round bottom flask and reacted under oxygen-free
Sonogashira reaction conditions (similar conditions to compound
3). The resulting products were purified by column chromatogra-
phy and dried in a vacuum oven to afford 4a. 1H NMR (400 MHz,
CDCl3)
CDCl3)
d
7.50 (s, 2H), 3.67 (s, 3H), 1.39 (s, 18H). 13C NMR (100 MHz,
159.6 (C1), 146.4 (C2, C6), 135.7 (C3, C5), 88.1 (C4), 64.4
d
(C7), 35.8 (C8, C12), 31.9 (C9, C10, C11, C13, C14, C15). HRMS (EI)
calcd for C15H23IO [Mþ] 346.0794, found at 346.0784, 2.8 ppm.
4.4. 1,3-Di-tert-butyl-5-ethynyl-2-methoxybenzene (3)
CDCl3)
d
7.44e7.46 (m, 4H), 7.42 (s, 2H), 3.70 (s, 3H), 1.44 (s, 18H),
160.4 (C1), 144.2 (C2, C6),
0.26 (s, 9H). 13C NMR (100 MHz, CDCl3)
d
131.9 (C3, C5), 131.4 (C19, C23), 130.2 (C20, C22), 123.8 (C21), 122.6
(C18), 117.2 (C4), 104.8 (C24), 96.1 (C25), 92.2 (C16), 87.6(C17), 64.5
(C7), 35.9 (C8, C12), 32.1, 32.0 (C9, C10, C11, C13, C14, C15), 0.1
(SieCH3). HRMS (EI) calcd for C28H36OSi 416.2535, found at
416.2533, 0.4 ppm.
4.7. 1,3-Di-tert-butyl-5-((4-ethynylphenyl)ethynyl)-2-metho-
xybenzene (4b)
Compound 2 (2.0 g, 5.7 mmol), ethynyltrimethylsilane (0.67 g,
6.84 mmol), Pd (PPh3)2Cl2 (0.4 mol %), CuI (0.4 mol %), Et3N (1.0 mL),
and THF (10 mL) were placed in a specially designed long-neck
round bottom flask. Gas was removed from the reaction mixture
by three liquid nitrogen freezeethaw cycles under high vacuum,
and then the flask was flame-sealed. After 48 h, the flask was
opened and solvents were evaporated. The crude product was then
purified by column chromatography and dried in the oven. The
obtained product was desilylated by TBAF (1.60 g, 0.005 mol) in
THF; the reaction mixture was extracted with diethyl ether and
dried with Na2SO4. The crude product was purified by flash column
and dried in a vacuum oven to afford 3. 1H NMR (400 MHz, CDCl3)
Compound 4a (2.0 g, 4.8 mmol) and TBAF (1.50 g, 5.76 mmol)
were reacted in THF for 2 h. The reaction mixture was poured into
cold water, washed with diethyl ether, and dried with Na2SO4. The
solvent was evaporated to give the crude product. Using a flash
column, pure product 4b was obtained. 1H NMR (400 MHz, CDCl3)
d
7.39 (s, 2H), 3.68 (s, 3H), 2.99 (s, 1H), 1.41 (s, 18H). 13C NMR
(100 MHz, CDCl3) 160.5 (C1), 144.0 (C2, C6), 130.6 (C3, C5), 116.2
d
d
7.43e7.48 (m, 4H), 7.42 (s, 2H), 3.69 (s, 3H), 3.17 (s, 1H), 1.41 (s,
(C4), 84.5 (C16), 75.5 (C17), 64.4 (C7), 35.7 (C8, C12), 31.9 (C9, C10,
C11, C13, C14, C15). HRMS (ESI) calcd for C17H25O [MþH] 245.1895,
found at 245.1905, 4.1 ppm.
18H). 13C NMR (100 MHz, CDCl3)
d
160.3 (C1), 144.1 (C2, C6), 132.0
(C19, C23), 131.4 (C3, C5), 130.1 (C20, C22), 124.1 (C21), 121.5 (C18),
117.1 (C4), 92.2 (C16), 87.4 (C17), 83.4 (C24), 78.7 (C25), 64.4 (C7),
35.8 (C8, C12), 31.9 (C9, C10, C11, C13, C14, C15). HRMS (ESI) calcd
for C25H29O [MþH] 345.2218, found at 345.2206, 3.5 ppm.
4.5. 1,3-Di-tert-butyl-5-((4-iodophenyl)ethynyl)-2-methoxybe-
nzene (4)
4.8. 1,3-Di-tert-butyl-5-((4-((4-iodophenyl)ethynyl)phenyl)
ethynyl)-2-methoxybenzene (5)
Compound 3 (0.5 g, 2.05 mmol), 1,4-iodobenzene (1.35 g,
4.10 mmol), Pd(PPh3)2Cl2 (0.4 mol %), CuI (0.4 mol %), Et3N (0.3 mL),
and THF (5 mL) were dissolved in THF and reacted under our pre-
viously described oxygen-free Sonogashira reaction conditions.8
After 3 days, the flask was opened and the solvents were evapo-
rated, the crude product was then purified by column chromatog-
raphy, and dried in an oven to afford 8. 1H NMR (400 MHz, CDCl3)
1H NMR (400 MHz, CDCl3)
d
7.67 (d, 2H, J¼8.8 Hz), 7.48e7.53 (m,
4H), 7.43 (s, 2H), 7.25 (d, 2H, J¼8.4 Hz), 3.67 (s, 3H), 1.41 (s, 18H). 13
C
NMR (100 MHz, CDCl3) d 160.5 (C1), 144.3 (C2, C6), 137.8 (C28, C30),
133.3 (C27, C31), 131.7 (C19, C23, C20, C22), 130.3(C3, C5), 124.0
(C26), 122.8 (C18), 122.5 (C21), 117.3 (C4), 94.5 (C16), 92.5 (C17),
90.8 (C24), 90.3 (C25), 87.8 (C29), 64.6 (C7), 36.0 (C8, C12), 32.2 (C9,
C10, C11, C13, C14, C15). HRMS (ESI) calcd for C31H32IO [MþH]
547.1498, found at 547.1511, 2.4 ppm.
d
7.68 (d, 2H, J¼8.8 Hz), 7.41 (s, 2H), 7.25 (d, 2H, J¼8.4 Hz), 3.69 (s,
3H), 1.43 (s, 18H). 13C NMR (100 MHz, CDCl3)
d
160.3 (C1), 144.1 (C2,
C6), 137.4 (C20, C22), 133.0 (C19, C23), 130.1 (C3, C5), 123.1 (C18),
117.0 (C4), 93.7 (C16), 91.6 (C17), 86.9 (C21), 64.4 (C7), 35.8 (C8,

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X. Bai et al. / Tetrahedron 69 (2013) 1105e1111
4.9. ((4-((4-((3,5-Di-tert-butyl-4-methoxyphenyl)ethynyl)phe-
nyl)ethynyl)phenyl)ethynyl)trimethylsilane (5a)
130.1 (C35, C39, C3, C5, C19, C23, C20, C22, C27, C31, C28, C30, C36,
C38), 123.7 (C37), 123.1, 123.0, 122.9, 122.4 (C34, C29, C26, C21, C18),
117.1 (C4), 104.5 (C40), 96.4 (C41), 92.3 (C16), 91.2 (C17), 91.0 (C24),
90.9 (C25), 90.7 (C32), 87.6 (C33), 64.4 (C7), 35.8 (C8, C12), 31.9 (C9,
C10, C11, C13, C14, C15), 0.1 (SieCH3). HRMS (FAB, NBA) calcd for
C
44H44OSi [Mþ] 616.3161, found at 616.3189, 4.5 ppm.
4.13. 1,3-Di-tert-butyl-5-((4-((4-((4-ethynylphenyl)ethynyl)
phenyl)ethynyl)phenyl)ethynyl)-2-methoxybenzene (6b)
1H NMR (400 MHz, CDCl3)
(s, 2H), 3.7 (s, 3H), 1.46 (s, 18H), 0.26 (s, 9H). 13C NMR (100 MHz,
CDCl3) 160.3 (C1), 144.1 (C2, C6), 131.5, 131.5 (C19, C23, C20, C22,
d 7.47e7.53 (m, 4H), 7.45 (s, 4H), 7.43
d
C27, C31), 131.4 (C3, C5), 130.1 (C28, C30), 123.7 (C29), 123.1, 123.0,
122.4 (C26, C21, C18), 117.1 (C4), 104.6 (C32), 96.4 (C33), 92.3 (C16),
91.1 (C17), 90.7 (C24), 87.6 (C25), 64.4 (C7), 35.8 (C8, C12), 31.9 (C9,
C10, C11, C13, C14, C15), 0.1 (SieCH3). HRMS (EI) calcd for C36H40OSi
[Mþ] 516.2848, found at 516.2823, 4.8 ppm.
1H NMR (400 MHz, CDCl3)
(s, 2H), 3.71 (s, 3H), 3.20 (s, 1H), 1.46 (s, 18H). 13C NMR (100 MHz,
CDCl3) 160.3 (C1),144.1 (C2, C6),132.1,131.6,131.6,131.5,131.5 (C3,
d 7.50e7.55 (m, 8H), 7.49 (s, 4H), 7.46
d
4.10. 1,3-Di-tert-butyl-5-((4-((4-ethynylphenyl)ethynyl)phe-
C5, C19, C23, C20, C22, C27, C31, C28, C30, C35, C39), 130.1 (C36,
C38), 123.76 (C37), 123.5, 123.2, 122.9, 122.4, 122.1 (C34, C29, C26,
C21, C18), 117.2 (C4), 92.36(C16), 91.3 (C17), 91.1 (C24), 90.8 (C25),
90.7 (C32) 87.6(C33), 83.2 (C40), 79.1(C41), 64.4 (C7), 35.8 (C8, C12),
32.0 (C9, C10, C11, C13, C14, C15).
nyl)ethynyl)-2-methoxybenzene (5b)
4.14. 1,3-Di-tert-butyl-5-((4-((4-((4-((4-iodophenyl)ethynyl)
phenyl)ethynyl)phenyl)ethynyl)phenyl)ethynyl)-2-
methoxybenzene (7)
1H NMR (400 MHz, CDCl3)
(s, 2H), 3.70 (s, 3H), 3.18 (s, 1H), 1.44 (s, 9H). 13C NMR (100 MHz,
CDCl3) 160.3 (C1), 144.1 (C2, C6), 132.1, 131.5, 131.5, 131.4 (C3, C5,
d 7.50e7.53 (m, 4H), 7.48 (s, 4H), 7.43
d
C19, C23, C20, C22, C27, C31), 130.1 (C28, C30), 123.8 (C29), 123.5,
122.3, 122.0 (C26, C21, C18), 117.1(C4), 92.3 (C16), 91.2 (C17), 90.5
(C24), 87.6 (C25), 83.2 (C32), 79.0 (C33), 64.4 (C7), 35.8 (C8, C12),
31.9 (C9, C10, C11, C13, C14, C15). HRMS (ESI) calcd for C33H33
[MþH] 445.2531, found at 445.2508, 5.2 ppm.
O
1H NMR (400 MHz, CDCl3)
7.41 (s, 2H), 7.25 (d, 2H, J¼8.4 Hz), 3.70 (s, 3H), 1.44 (s, 18H).
d
7.70 (d, 2H, J¼8.8 Hz), 7.51 (m, 12H),
4.11. 1,3-Di-tert-butyl-5-((4-((4-((4-iodophenyl)ethynyl)phe-
nyl)ethynyl)phenyl)ethynyl)-2-methoxybenzene (6)
4.15. ((4-((4-((4-((4-((3,5-Di-tert-butyl-4-methoxyphenyl)
ethynyl)phenyl)ethynyl)phenyl)ethynyl)phenyl)ethynyl)phe-
nyl)ethynyl)trimethylsilane (7a)
1H NMR (400 MHz, CDCl3)
d
7.70 (d, 2H, J¼8.8 Hz), 7.51 (m, 8H),
7.43 (s, 2H), 7.25 (d, 2H, J¼8.4 Hz), 3.70 (s, 3H), 1.44 (s, 18H). 13C
1H NMR (400 MHz, CDCl3)
7.43 (s, 2H), 3.7 (s, 3H), 1.44 (s, 18H), 0.26 (s, 9H). 13C NMR
(100 MHz, CDCl3) 160.3 (C1), 144.1 (C2, C6), 131.9, 131.6, 131.5,
131.5, 131.4, 130.1 (C43, C47, C35, C39, C3, C5, C19, C23, C20, C22,
C27, C31, C28, C30, C36, C38, C44, C46), 123.8 (C45), 123.2, 123.1,
123.0, 123.0, 122.4 (C42, C37, C34, C29, C26, C21, C18), 117.2 (C4),
104.6 (C48), 96.5 (C49), 92.3 (C16), 91.2 (C17), 91.1 (C24), 91.0
(C25), 91.0 (C32), 90.9 (C33), 90.7 (C40), 87.6 (C41), 64.4 (C7), 35.8
(C8, C12), 32.0(C9, C10, C11, C13, C14, C15), 0.1 (SieCH3). HRMS
(FAB, NBA) calcd for C52H48OSi [Mþ] 716.3474, found at 716.3478,
0.6 ppm.
d 7.51e7.52 (m, 12H), 7.46 (s, 4H),
NMR (100 MHz, CDCl3) d 160.3 (C1), 144.1 (C2, C6), 137.6 (C36, C38),
133.1 (C35, C39), 131.5, 131.5, 131.5, 131.5, 130.1 (C3, C5, C19, C23,
C20, C22, C27, C31, C28, C30), 123.7 (C34), 123.1 (C18), 122.8 (C29),
122.5 (C26), 122.3 (C21), 117.1 (C4), 94.4 (C16), 92.29 (C17), 91.2
(C24), 90.7 (C25), 90.4 (C32), 90.4 (C33), 87.58 (C37), 64.4 (C7), 35.8
(C8, C12), 31.9 (C9, C10, C11, C13, C14, C15). HRMS [FAB, NBA] calcd
for C39H35IO [Mþ] 646.1733, found at 646.1753, 3.1 ppm.
d
4.12. ((4-((4-((4-((3,5-Di-tert-butyl-4-methoxyphenyl)ethy-
nyl)phenyl)ethynyl)phenyl)ethynyl)phenyl)ethynyl)trime-
thylsilane (6a)
4.16. Homo4
1H NMR (400 MHz, CDCl3)
(s, 2H), 3.7 (s, 3H), 1.44 (s, 18H), 0.25 (s, 9H). 13C NMR (100 MHz,
CDCl3) 160.3 (C1), 144.1 (C2, C6), 131.9, 131.5, 131.5, 131.5, 131.4,
d 7.49e7.54 (m, 8H), 7.46 (s, 4H), 7.43
The homo-coupling product was separated from the mono-
iodo-substituted oligomer 5 by column chromatography. A ratio of
d

X. Bai et al. / Tetrahedron 69 (2013) 1105e1111
1111
hexanes to CH2Cl2 (5:1) was used to elute the column. H NMR
(400 MHz, CDCl3) 7.50 (s, 8H), 7.43 (s, 4H), 3.70 (s, 6H), 1.44 (s,
36H). 13C NMR (100 MHz, CDCl3)
160.6 (C1), 144.4(C2, C6), 132.6,
(5a). Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.tet.2012.11.063.
d
d
131.7, 130.4, 124.8, 121.2, 117.2, 110.0, 93.3 (C16), 87.7 (C17), 82.4
(C24), 77.2 (C25), 64.6 (C7), 36.0 (C8, C12), 32.2 (C9, C10, C11, C13,
C14, C15). HRMS (FAB, NBA) calcd for C50H54O2 [Mþ] 686.4124,
found at 686.4113, 1.6 ppm.
References and notes
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€
€
4.17. Homo6
ꢀ
ꢀ
HRMS (FAB, NBA) calcd for C66H62O2 [Mþ] 886.4750, found
886.4779, 3.3 ppm.
4.18. Homo8
3. (a) Kulkarni, A. P.; Tonzola, C. J.; Babel, A.; Jenekhe, S. A. Chem. Mater. 2004, 16,
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HRMS (FAB, NBA) calcd for C82H70O2 [Mþ] 1086.5376, found
1086.6.
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Acknowledgements
This work was supported in part by grants from the Office of the
Vice Provost for Research (UMKC Research Incentive Fund Grant
K0710) and by the UM Board of Curators (K0906077) (to J.R.D.) and
by the Army Research Office (W911NF-10-1-0476) (to T.C.S.). We
thank (Dr. Todd D. Williams, Mr. Robert Drake, Mr. Larry Seib) of the
KU Mass Spectrometry Laboratory for acquiring the ESI spectra. LCT
premier was purchased with support from NIH SIG S10 RR019398
(T.D.W.).
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Supplementary data
1H and 13C NMR spectra and X-ray crystallographic data (CIF).
The crystal structures of 5a, 5b, and Homo4 have been deposited at
the Cambridge Crystallographic Data Centre. The deposition num-
bers are CCDC 903809 (Homo4), CCDC 903810 (5b), and 903811
10. Chen, X.; Barnes, C.; Bai, X.; Sandreczki, T. C.; Peng, Z.; Kadnikova, E. N.; Dias, J.
R. Chem. Commun. 2010, 8171.
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