Synthesis of hybrid masked triyne-phenylene axial rods containing (E)-β-chlorovinylsilanes in the π-conjugated framework

Article abstract of DOI:10.1021/jo901766p

(Chemical Equation Presented) A two-directional synthesis of a masked hexayne 7, in which two β-chlorovinylsilanes protect two of the internal alkynes, is reported. The key step involves the Pd-catalyzed oxidative dimerization of alkyne 10 to provide diyn

Full text of DOI:10.1021/jo901766p

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Synthesis of Hybrid Masked Triyne-Phenylene Axial Rods Containing
(E)-β-Chlorovinylsilanes in the π-Conjugated Framework
Michael D. Weller, Benson M. Kariuki,‡,† and Liam R. Cox*
School of Chemistry, The University of Birmingham, Edgbaston, Birmingham, B15 2TT, United Kingdom.
‡To whom details regarding the X-ray structures should be addressed. E-mail: kariukib@cardiff.ac.uk.
†Present address: School of Chemistry, Cardiff University, Park Place, Cardiff, CF10 3AT, U.K
l.r.cox@bham.ac.uk
Received August 15, 2009
A two-directional synthesis of a masked hexayne 7, in which two β-chlorovinylsilanes protect two of
the internal alkynes, is reported. The key step involves the Pd-catalyzed oxidative dimerization of
alkyne 10 to provide diyne 12, which is elaborated into centrosymmetric masked hexayne 7 in four
steps. Masked hexayne 7 is a constitutional isomer of masked hexayne 2, which has been used as a
monomer unit for oligoyne assembly. Although masked hexayne 7 was not as convenient a building
block as 2 for application in oligoyne assembly, one of its precursors, namely alkyne 10, could be used
successfully in Sonogashira couplings, which allowed the incorporation of aromatic spacers and the
formation of hybrid masked triyne-phenylenes 20 and 28. Compounds 20 and 28 both contain
removable end-groups, which will permit their application as building blocks for the assembly of
classes of long-chain, π-conjugated rod-like molecules. Rod-like molecule 34, which possesses a
similar conjugated scaffold as 28, was also prepared by using a similar strategy. Treatment of 34 with
TBAF effected a 2-fold dechlorosilylation to provide a rigid rod molecule 35 in which two phenylene
units interrupt an octayne scaffold.
Introduction
materials have focused primarily on the synthesis of
oligoynes.3-7 These molecules represent clipped versions of
carbyne,8 an elusive allotrope of carbon, which has aroused
considerable interest over the years, and, in many ways,
represents the ultimate molecular wire.9 The controlled
preparation of oligoynes of any appreciable length
(goctayne), however, let alone of carbyne itself, presents a
state-of-the-art synthetic challenge that has yet to find an
efficient solution.10 Working toward this goal, we have
Linearly π-conjugated organic materials display optoelec-
tronic properties that make them important candidates for
molecular wires1 in the emerging field of molecular electro-
nics.2 Our own research efforts in the area of π-conjugated
*To whom correspondence should be addressed. Phone: þ44 (0)121 414
3524. Fax: þ44 (0)121 414 4403.
(1) (a) Wang, C.; Bryce, M. R.; Gigon, J.; Ashwell, G. J.; Grace, I.;
Lambert, C. J. J. Org. Chem. 2008, 73, 4810–4818. (b) For a review see:
Robertson, N.; McGowan, C. A. Chem. Soc. Rev. 2003, 32, 96–103.
(2) Vuillaume, D. C. R. Phys. 2008, 9, 78–94.
(8) Cataldo, F. Polyynes: Synthesis, Properties and Applications; Taylor &
Francis: New York, 2005.
ꢀ
(3) Simpkins, S. M. E.; Kariuki, B. M.; Arico, C. S.; Cox, L. R. Org. Lett.
2003, 5, 3971–3974.
(4) Simpkins, S. M. E.; Kariuki, B. M.; Cox, L. R. J. Organomet. Chem.
2006, 691, 5517–5523.
(5) Simpkins, S. M. E.; Weller, M. D.; Cox, L. R. Chem. Commun. 2007,
4035–4037.
(6) Weller, M. D.; Cox, L. R. C. R. Chim. 2009, 12, 366–377.
(7) Weller, M. D.; Kariuki, B. M.; Cox, L. R. Tetrahedron Lett. 2008, 49,
4596–4600.
(9) Sun, A. W.; Lauher, J. W.; Goroff, N. S. Science 2006, 312, 1030–1034.
(10) For leading references see: (a) Zheng, Q.; Bohling, J. C.; Peters, T. B.;
Frisch, A. C.; Hampel, F.; Gladysz, J. A. Chem. Eur. J. 2006, 12, 6486–6505.
(b) Eisler, S.; Slepkov, A. D.; Elliott, E.; Luu, T.; McDonald, R.; Hegmann,
F. A.; Tykwinski, R. R. J. Am. Chem. Soc. 2005, 127, 2666–2676. (c) Gibtner,
T.; Hampel, F.; Gisselbrecht, J.-P.; Hirsch, A. Chem. Eur. J. 2002, 8, 408–
432. (d) Eastmond, R.; Walton, D. R. M.; Johnson, T. R. Tetrahedron 1972,
28, 4601–4616.
7898 J. Org. Chem. 2009, 74, 7898–7907
Published on Web 09/23/2009
DOI: 10.1021/jo901766p
r
2009 American Chemical Society

Weller et al.
JOCArticle
SCHEME 1. Action of Fluoride on a β-Halovinylsilane
Generates an Alkyne
SCHEME 2. Assembly of Dodecayne 4 from Masked Triyne 1
pioneered the use of β-halovinylsilanes11 as masking
groups12 for alkynes in oligoyne assembly (Scheme 1).3-7
This elimination approach13 is particularly attractive since
fluoride-induced release of the alkyne proceeds under mild
conditions that do not affect the oligoyne product. More-
over, the oligoyne precursors themselves represent a new
class of oligo(enediyne)s14 containing heteroatom appen-
dages (i.e., a halogen and a silyl substituent), which provide
a useful means to modulate the optoelectronic properties of
the π-conjugated backbone.
Our synthetic strategy generally begins with masked triyne
1 in which the central (E)-β-chlorovinylsilane serves to
protect the internal alkyne. Dimerization of 1 provides a
centrosymmetric masked hexayne 2, which we have elabo-
rated into aryl end-capped masked dodecayne 3 (Scheme 2).5
Treatment of masked dodecayne 3 with TBAF effects a 4-fold
dechlorosilylation to provide the corresponding dodecayne 4,
which is the longest aryl end-capped oligoyne reported to date.15
Phenyl end-capped masked hexaynes based on β-chloro- (5)
and β-fluorovinylsilanes (6) have also been prepared in a similar
fashion to masked hexayne 2 (Figure 1), and unmasked to
afford 1,12-diphenyldodeca-1,3,5,7,9,11-hexayne.5,7 In the case
of masked hexayne 6, the unmasking can be performed by using
a substoichiometric quantity of TBAF since a fluoride anion is
regenerated in each elimination cycle.7
While slightly longer than our route to its isomer 2 in terms of
number of steps, the synthesis of 7 displays increased flex-
ibility, which enables the introduction of aromatic spacer
groups into the carbon-rich framework to generate hybrid
masked phenylene-butadiynylene and masked phenylene-
hexatriynylene π-conjugated building blocks.16,17
Having established an efficient route to a masked hexayne
2 in which the chloro substituents occupy “inside” positions,
and the silyl groups “outside” positions,3,5,6 we were keen to
access its constitutional isomer 7 in which the chloro and silyl
groups have been interchanged (Figure 2). We wanted to
investigate how this structural change affects the efficiency of
the synthesis of this alternative oligoyne building block, as
well as the optoelectronic properties of the resulting masked
hexayne. We now report the synthesis of masked hexayne 7.
FIGURE 1. Masked hexaynes in which two alkynes are protected
as β-chloro- and β-fluorovinylsilanes.
(11) (a) Cunico, R. F.; Dexheimer, E. M. J. Am. Chem. Soc. 1972, 94,
2868–2869. (b) Danheiser, R. L.; Carini, D. J. J. Org. Chem. 1980, 45, 3925–
3927. (c) Danheiser, R. L.; Carini, D. J.; Kwasigroch, C. A. J. Org. Chem.
1986, 51, 3870–3878. (d) Marshall, J. A.; Maxson, K. J. Org. Chem. 2000, 65,
630–633. (e) Yamaguchi, R.; Kawasaki, H.; Yoshitome, T.; Kawanisi, M.
Chem. Lett. 1982, 1485–1486. (f) Schmitz, C.; Rouanet-Dreyfuss, A.-C.;
Tueni, M.; Biellmann, J.-F. J. Org. Chem. 1996, 61, 1817–1821. (g) Oda, H.;
Sato, M.; Morizawa, Y.; Oshima, K.; Nozaki, H. Tetrahedron 1985, 41,
3257–3268.
FIGURE 2. Masked hexayne 7 is a constitutional isomer of masked
hexayne 2.
(12) For other alkyne masking strategies in oligoyne synthesis see: 1,1-
dibromoolefins: (a) Reference 10b. (b) Luu, T.; Elliott, E.; Slepkov, A. D.;
Eisler, S.; McDonald, R.; Hegmann, F. A.; Tykwinski, R. R. Org. Lett. 2005, 7,
51–54. (c) Eisler, S.; Chahal, N.; McDonald, R.; Tykwinski, R. R. Chem. Eur. J.
2003, 9, 2542–2550. (d) Eisler, S.; Tykwinski, R. R. J. Am. Chem. Soc. 2000,
122, 10736–10737. Cyclobutene-1,2-diones: (e) Rubin, Y.; Lin, S. S.; Knobler,
C. B.; Anthony, J.; Boldi, A. M.; Diederich, F. J. Am. Chem. Soc. 1991, 113,
6943–6949. [4.3.2]Propellatrienes: (f) Tobe, Y.; Fujii, T.; Naemura, K. J. Org.
Chem. 1994, 59, 1236-1237. 1-Amino-1,2,3-triazoles: (g) Adamson, G. A.;
Rees, C. W. J. Chem. Soc., Perkin Trans. 1 1996, 1535–1543. Dicobaltocta-
carbonyl complexes: (h) Diederich, F.; Rubin, Y.; Chapman, O. L.; Goroff, N. S.
Helv. Chim. Acta 1994, 77, 1441–1457.
Results and Discussion
Our starting point in the synthesis of masked hexayne 7
was (E)-β-chlorovinylsilane 8, which is readily prepared in
(16) For selected examples of phenylene-butadiynylenes see: (a) West,
K.; Wang, C.; Batsanov, A. S.; Bryce, M. R. Org. Biomol. Chem. 2008, 6,
1934–1937. (b) West, K.; Hayward, L. N.; Batsanov, A. S.; Bryce, M. R. Eur.
J. Org. Chem. 2008, 5093–5098. (c) Alam, M. A.; Kim, Y.-S.; Ogawa, S.;
Tsuda, A.; Ishii, N.; Aida, T. Angew. Chem., Int. Ed. 2008, 47, 2070–2073.
(13) Orita, A.; Otera, J. Chem. Rev. 2006, 106, 5387–5412.
€
(d) McNeil, A. J.; Muller, P.; Whitten, J. E.; Swager, T. M. J. Am. Chem. Soc.
2006, 128, 12426–12427.
(17) For a review on axial rod-like molecules see: Schwab, P. F. H.; Levin,
M. D.; Michl, J. Chem. Rev. 1999, 99, 1863–1933.
(14) (a) Nielsen, M. B.; Diederich, F. Chem. Rev. 2005, 105, 1837–1867.
(b) Martin, R. E.; Diederich, F. Angew. Chem., Int. Ed. 1999, 38, 1350–1377.
(15) For a recent review concerning the synthesis of oligoynes see:
Chalifoux, W. A.; Tykwinski, R. R. C. R. Chim. 2009, 12, 341–358.
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Weller et al.
SCHEME 3. Synthesis of Alkyne 10 and Bromo-alkyne 11
JOCArticle
five steps from 1,4-bis(trimethylsilyl)buta-1,3-diyne and is a
key intermediate in our synthesis of masked hexayne 2.3 For
the synthesis of masked hexayne 7, we first needed to process
the alkyne in 8 and then elaborate the alcohol into a second
alkyne, which would form the terminal units of our target
molecule. Anticipating base-promoted 1,4-Brook rearrange-
ment and subsequent elimination to the alkyne would be a
problem when manipulating alcohol 8, we chose to protect
the alcohol functionality in 8 first as its THP ether; thus
treatment of 8 with 3,4-dihydro-2H-pyran (DHP) in the
presence of p-toluenesulfonic acid (p-TSA)18 afforded the
corresponding THP ether 9 in 93% yield. Removal of the
TMS group in 9 could now be achieved under mildly basic
conditions19 to provide terminal alkyne 10 in 99% yield
(Scheme 3). Since halo-alkynes are useful building blocks
in alkyne coupling chemistry,20,21 we also showed that TMS-
protected alkyne 9 could be converted into the correspond-
ing bromo-alkyne 11 in similarly excellent yield by reaction
with 1 equiv each of silver(I) nitrate and NBS in acetone
(Scheme 3).22 A subsequent Cadiot-Chodkiewicz coupling21
of 11 with phenylacetylene as the alkyne coupling partner was
accomplished in 60% yield (see the Supporting Information).
Terminal alkyne 10 proved to be a rather poor substrate
for homocoupling with use of the Eglinton-Glaser-Hay
approach,23 even under the reaction conditions that we have
developed to avoid premature dechlorosilylation.4,5 The
desired dimerization product could be obtained in this way
but only in poor yields (e24%) and with poor recovery of
starting material. We therefore turned our attention to palla-
dium-catalyzed variants of this homocoupling reaction,23-25
and were particularly attracted to the method of Zhang and
co-workers, who reported excellent yields for a palladium-
catalyzed oxidative homocoupling of alkynes, using ethyl
bromoacetate as the promoter.25 Treatment of a solution of
terminal alkyne 10 in THF with DABCO (1.2 equiv), bis(tri-
phenylphosphine)palladium(II) dichloride (5 mol %), copper-
(I) iodide (5 mol %), and ethyl bromoacetate (60 mol %) in the
presence of air resulted in the formation of dimer 12 in excellent
yield (Scheme 4). When ethyl bromoacetate was excluded from
the reaction mixture, no dimerization product was observed.
Interestingly, and in contrast to Zhang’s observations, we also
found that exposure of the reaction mixture to air was essential
for the coupling to proceed.
13 in hand, we expected its conversion into our target masked
hexayne 7 would be straightforward using our previously
reported sequence of oxidation, dibromoolefination,27,28
followed by a Fritsch-Buttenberg-Wiechell-like reaction;27
however, this turned out not to be the case. Double allylic
oxidation of diol 13 with manganese(IV) oxide proved to be
surprisingly problematic: as well as forming the desired bis-
aldehyde product 14, a significant quantity (up to 25% by
1
integration in the H NMR spectrum of the crude reaction
mixture) of an unknown byproduct was also obtained.29
After screening a range of other oxidizing agents, it was
found that treatment of diol 13 with PCC in dichloro-
˚
methane in the presence of Celite and activated 5 A molec-
ular sieves30 was effective in minimizing side reactions and
provided the desired bis-aldehyde 14 in 87% yield. Bis-
aldehyde 14 was then converted uneventfully into the corre-
sponding bis-dibromoolefin 15 under standard conditions
(Scheme 4).28 The final step in the synthesis of masked
hexayne 7 required the installation of the terminal alkyne
groups. Using reaction conditions that we routinely employ
to introduce the terminal alkyne in masked triyne 1, tetra-
bromide 15 was treated with an excess (12 equiv) of LDA
in THF at -78 °C; however, under these conditions, we
observed rapid decomposition. Since the reaction of dibro-
moolefins with LDA generates an intermediate lithium
acetylide, and the corresponding alkyne on aqueous workup,
we attempted to trap this reactive intermediate by addition of
chlorotrimethylsilane after 15 min; however, in contrast to
carrying out a similar reaction, which we have used to
prepare the bis-TMS end-capped masked triyne analogue
of 1 in excellent yield,3 this again failed to provide the desired
compound. Fortunately a slight modification of the reaction
conditions led to improved results: since chlorotrimethylsi-
lane has been shown to be compatible with LDA at -78 °C,31
we included this electrophile from the start of the reaction in
order that it could act as an in situ quench for the generated
The THP protecting groups in the dimer 12 were removed
by using PPTS in THF/ethanol (1:1) at 55 °C (Scheme 4),26 to
provide the diol 13, which was poorly soluble in most organic
solvents with the exception of dimethyl sulfoxide. With diol
(18) Earl, R. A.; Townsend, L. B. Org. Synth. 1981, 60, 81–87.
(19) Manini, P.; Amrein, W.; Gramlich, V.; Diederich, F. Angew. Chem.,
Int. Ed. 2002, 41, 4339–4343.
(20) Cadiot, P.; Chodkiewicz, W. Couplings of Acetylenes; Viehe, H. G.,
Ed.; Marcel Dekker: New York, 1969.
(21) (a) Chodkiewicz, W.; Cadiot, P. C. R. Hebd. Seances Acad. Sci. 1955,
241, 1055–1057. (b) Chodkiewicz, W. Ann. Chim. (Paris) 1957, 819–869.
(22) Nishikawa, T.; Shibuya, S.; Hosokawa, S.; Isobe, M. Synlett 1994,
485–486.
(23) Siemsen, P.; Livingston, R. C.; Diederich, F. Angew. Chem., Int. Ed.
2000, 39, 2633–2657.
(24) (a) Batsanov, A. S.; Collings, J. C.; Fairlamb, I. J. S.; Holland, J. P.;
Howard, J. A. K.; Lin, Z.; Marder, T. B.; Parsons, A. C.; Ward, R. M.; Zhu,
J. J. Org. Chem. 2005, 70, 703–706. (b) Li, J.-H.; Liang, Y.; Xie, Y.-X. J. Org.
Chem. 2005, 70, 4393–4396.
(25) Lei, A. W.; Srivastava, M.; Zhang, X. M. J. Org. Chem. 2002, 67,
1969–1971.
(26) Miyashita, M.; Yoshikoshi, A.; Grieco, P. A. J. Org. Chem. 1977, 42,
3772–3774.
(27) (a) Corey, E. J.; Fuchs, P. L. Tetrahedron Lett. 1972, 3769–3772.
(b) Knorr, R. Chem. Rev. 2004, 104, 3795–3849.
(28) Ramirez, F.; McKelvie, N.; Desai, N. B. J. Am. Chem. Soc. 1962, 84,
1745–1747.
(29) This side product was represented by the presence of an additional
singlet at ∼10.3 ppm in the 1H NMR spectrum and was inseparable from the
desired bis-aldehyde 14.
(30) Herscovici, J.; Antonakis, K. J. Chem. Soc., Chem. Commun. 1980,
561–562.
(31) Corey, E. J.; Gross, A. W. Tetrahedron Lett. 1984, 25, 495–498.
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SCHEME 4. Synthesis of Masked Hexayne 7
lithium acetylide intermediates. In this way, masked hexayne
7 could be obtained in 37% yield after purification by
column chromatography (Scheme 4).32
π-conjugated framework do not seem to have a signi-
ficant impact on these features of the UV-vis spectra. It is
interesting to note, however, the difference in the relative
intensities of the peaks in the two spectra: isomer 7, with the
chlorine atoms occupying “outside” positions and the silyl
groups “inside” positions, exhibits molar absorptivities that
are almost 2-fold higher than those for its constitutional
isomer 2.
In summary, masked hexayne 7 was prepared from alco-
hol 8 in seven steps and 17% overall yield. By contrast, its
constitutional isomer 2 can be accessed from 8 in only four
steps with an overall yield of 52%. For these reasons, masked
hexayne 7 is not as attractive a building block as 2 for
oligoyne assembly. Contrasting the physical properties of
these two isomers is interesting: masked hexayne 2 is gen-
erally isolated as pale yellow needles (mp 170 °C dec) whereas
its constitutional isomer 7 is an amorphous yellow solid that
exhibits a considerably lower melting point (mp 60-62 °C).
The UV-vis absorption spectra for masked hexayne 7 and
its constitutional isomer 2 are shown in Figure 3. In the case
of this physical parameter, masked hexayne 7 shows a small
bathochromic shift of 4 nm in λmax compared with 2;
however, the spacing between the characteristic absorp-
tion bands in the spectra of these two isomers is the same;
the positions of the silyl and chloro substituents along the
Although masked hexayne 7 is a less convenient building
block than 2 for the synthesis of oligoynes, the observation
that one of its precursors, namely 10, proved to be a good
substrate in a Pd-catalyzed oxidative dimerization reaction
suggested that this intermediate could be a useful coupling
partner in a Sonogashira reaction.33 The introduction of
aromatic spacer groups between two masked triynes of the
type contained in 1 should significantly alter the optoelec-
tronic properties of the material, and potentially open the
way to new and interesting classes of oligomers.34 For
example, aromatic spacer groups should reduce the effi-
ciency of π-electron delocalization along the linear conju-
gated framework through competition with localized
aromatic π-electron conjugation. This reduced delocaliza-
tion is accompanied by a shift in electronic end absorptions
to higher energy.34 To investigate these proposals, we chose
to introduce a phenylene spacer group between two of our
masked triyne units. This was achieved by substituting the
Pd-catalyzed homodimerization used to prepare diyne 12
with a double-Sonogashira cross-coupling reaction. Thus
Sonogashira coupling of terminal alkyne 10 with 1,4-diio-
dobenzene (0.5 equiv) afforded the desired phenylene diethy-
nylene 16 in 81% yield (Scheme 5). The THP protecting
groups in 16 were removed by using PPTS to afford diol 17 in
(32) Masked hexayne 7 was obtained in crude (∼90% purity) form in
89% yield after column chromatography (1% f 2% Et2O in hexane). It was
poorly stable to long durations on silica, which accounts for the low isolated
yield of 7.
(33) (a) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett.
1975, 4467–4470. (b) Doucet, H.; Hierso, J.-C. Angew. Chem., Int. Ed. 2007,
46, 834–871.
(34) Martin, R. E.; Wytko, J. A.; Diederich, F.; Boudon, C.; Gisselbrecht,
J.-P.; Gross, M. Helv. Chim. Acta 1999, 82, 1470–1485.
FIGURE 3. UV-vis spectra of masked hexaynes 7 (solid line) and
2 (dashed line).
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FIGURE 4. ORTEP plots of diol 17 (left) and tetrabromide 19 (right). Atomic displacement parameters at 293 K.
SCHEME 5. Synthesis of Hybrid Masked Triyne-phenylene 20
89% yield, allowing the same sequence that had been used to
access masked hexayne 7 to be carried out on this new
substrate. Reaction of diol 17 with PCC, in the presence of
the π-orbitals of the neighboring enyne units. In the case of
tetrabromide 19, the phenylene ring and the two 1,1-dibro-
moolefins are now not situated in the plane of the β-chloro-
vinylsilane moieties. This solid-state conformation suggests
quite poor overlap of the π-orbitals along the chain and
therefore a lower level of π-conjugation for this compound
(at least in the solid state). A torsion angle of 61° is observed
between each 1,1-dibromoolefin and its neighboring internal
olefin; this presumably minimizes the 1,3-steric repulsions
between the chlorine atom of the β-chlorovinylsilane unit
and the cis-bromine atom of the proximal dibromoolefin.
Treatment of tetrabromide 19 with 12 equiv of LDA in
THF at -78 °C in the presence of 10 equiv of chlorotri-
methylsilane afforded hybrid masked triyne-phenylene 20 in
excellent yield (Scheme 5). It would appear that incorporat-
ing the phenylene spacer group had resulted in an improved
stability of the lithium intermediates generated in this reac-
tion as well as the product itself (toward column chromato-
graphy) when compared with masked hexayne 7 whose syn-
thesis and isolation had proven to be much more problematic.
˚
Celite and activated 5 A molecular sieves, afforded bis-
aldehyde 18 in 80% yield. Again, these reagents and reaction
conditions were important to avoid the generation of the
same type of side product described above in the synthesis of
bis-aldehyde 14. Bis-aldehyde 18 was next converted into its
corresponding tetrabromide 19 by using carbon tetrabro-
mide and triphenylphosphine (Scheme 5). The crystal struc-
tures of diol 17 and tetrabromide 19 were determined and
these are shown as ORTEP plots in Figure 4.
In the case of diol 17, four ellipsoids corresponding to the
two oxygen atoms in the molecule are present in the solved
X-ray structure showing that the two hydroxyl groups each
occupy two positions, each with 50% probability. The
relative orientation of the two β-chlorovinylsilane moieties
is anti to each other across the π-conjugated framework. The
phenylene spacer group is located in the plane of the
π-conjugated framework to provide optimal overlap with
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SCHEME 6. Synthesis of Axial Rod 28
To investigate the scope of this type of reaction sequence
further, we reacted alkyne 10 with 1-iodo-4-(trimethyl-
silylethynyl)benzene 2135 under Sonogashira coupling con-
ditions to provide phenylene ethynylene 22 in good yield
(Scheme 6). The trimethylsilyl group in 22 was removed in
the usual fashion using anhydrous potassium carbonate in
THF/methanol (Scheme 6). The resulting terminal alkyne in
23 was noticeably less stable than alkyne 10 and the product
was therefore used immediately in the next synthetic step.
Thus, terminal alkyne 23 was subjected to our modified
Eglinton-Glaser-Hay coupling conditions with copper(II)
triflate as the source of copper.4-7 The triflate counteranion
in this salt is weakly nucleophilic and therefore less suscep-
tible to promoting a premature dechlorosilylation compared
to other copper salts (e.g., copper chloride or acetate)
commonly used in this type of homodimerization.23 This
coupling reaction afforded phenylene ethynylene 24, in
which the β-chlorovinylsilane moieties are now separated
by a diphenylenebuta-1,3-diynyl spacer module. Using our
now established methodology, the THP protecting groups of
24 were removed to afford diol 25 in 73% yield. Although
diol 25 containing this much longer π-conjugated framework
compared to our previous substrates exhibited poor solubi-
lity in most organic solvents, we were still able to perform a
PCC oxidation to provide the corresponding bis-aldehyde 26
in good yield, although it was now necessary to perform this
reaction at 0 °C to minimize the formation of side products
(see above) with this more reactive substrate. In light of its
increased lability, bis-aldehyde 26 was converted immedi-
ately into the corresponding tetrabromide 27 in good yield
with carbon tetrabromide and triphenylphosphine. Finally
reaction of bis-dibromoolefin 27 with LDA, once again with
chlorotrimethylsilane as in situ quench, gave axial rod 28 in
55% yield (Scheme 6).
An alternative route to the above class of rigid rod
molecule was developed starting from (E)-β-chlorovinylsi-
lane 29,3 which is an intermediate in our synthesis of masked
hexayne 2. Treatment of dibromoolefin 29 with 6 equiv of
LDA in THF at -78 °C, followed by addition of acetone,
gave tertiary alcohol 30 in 89% yield (Scheme 7). The TMS
group of 30 was cleaved selectively with anhydrous potas-
sium carbonate in THF/methanol. The resulting terminal
alkyne 31 was then cross-coupled with aryl iodide 21 under
Sonogashira conditions to afford phenylene ethynylene 32 in
52% yield over two steps. A second TMS deprotection again
proceeded uneventfully to afford terminal alkyne 33, which
was immediately subjected to our modified Eglinton-Glaser-
Hay coupling conditions to provide the target axial rod
34 in 41% yield over two steps (Scheme 7). Rigid rod
molecules 28 and 34 are approximately 2.6 nm in length
(35) Anderson, S. Chem. Eur. J. 2001, 7, 4706–4714.
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SCHEME 7. Synthesis of Axial Rod 34 and Two-Fold Dechlorosilylation to 35
TABLE 1. Absorption Peaks in the UV-Vis Spectra of 2, 5, 7, 20, 28,
and 34
compd
λ λ
max (nm) ε (M-1cm-1) compd max (nm) ε (M-1cm-1)
2
402
372
345
327
313 sh
430 sh
402
38 800
37 300
38 100
34 800
27 000
37 100
45 700
48 900
38 900
66 600
60 600
35 900
40 600
47 300
41 200
20
399
383 sh
374
352 sh
320
403 sh
381
355 sh
398 sh
377
53 200
53 200
57 900
36 500
21 900
72 100
102 000
63 900
76 500
107 000
65 700
5
7
28
34
383
364 sh
406
375
356 sh
348
324
350 sh
314 sh
FIGURE 5. UV-vis spectra of phenylene-containing compounds
20, 28, and 34, and masked hexayne 5.
provide compound 35 in which the two phenylene rings
interrupt an octayne framework (Scheme 7).
The UV-vis spectra for 20, which possesses a phenylene
spacer group, and for molecular rods 28 and 34 are shown in
Figure 5, along with the spectrum for phenyl end-capped
masked hexayne 5. The relevant signals in the UV-vis
absorption spectra of target compounds 7, 20, 28, and 34
are also summarized in Table 1 along with data for masked
hexaynes 2 and 5. Comparing the UV-vis spectrum of 5 with
masked hexaynes 2 and 7, which are of similar basic structure
(see Figure 3), reveals the ability for the π-conjugated system
to extend into the phenyl end-groups in 5 resulting in a
bathochromic shift of λmax (to 430 nm). Moving the phenyl
units from the termini of the masked hexayne, as in 5, to the
middle, as in 20, clearly interrupts the conjugation pathway
in the masked hexayne, which has the effect of removing the
characteristic profile for absorption associated with our
(measured from the terminal carbons in the conjugated
scaffold) and, together with masked dodecayne 3, represent
the longest π-conjugated frameworks containing β-chloro-
vinylsilanes that have been prepared to date.
Since the dimethylmethanol end-capping groups present
in 34 are much more robust to basic conditions than the TMS
protecting groups used to end-cap the conjugated frame-
work in compounds 7, 20, and 28, rigid rod 34 provided
us with an opportunity to effect elimination of the two
β-chlorovinylsilanes without having to worry about generat-
ing compounds that possess unstable terminal hexatriynes
(which would be produced were we to effect dechlorosilyla-
tion on compounds 20 and 28). To this end, compound 34
was treated with TBAF under our standard conditions.5 The
2-fold dechlorosilylation proceeded smoothly at 0 °C to
7904 J. Org. Chem. Vol. 74, No. 20, 2009

Weller et al.
JOCArticle
masked hexaynes. As expected, the extinction coefficient, ε,
of strong absorptions is enhanced by increasing the length of
the π-conjugated framework (compare 20 with 28 and 34),
and changing the end-cap from trimethylsilyl to dimethyl-
methanol has little effect on the UV-vis absorption spectra.
Experimental Section
(E)-3-tert-Butyldiphenylsilyl-4-chloro-5-tetrahydropyranyloxy-
1-trimethylsilylpent-3-en-1-yne (9). p-TSA (11 mg, 0.060 mmol)
was added to a solution of alcohol 8 (2.50 g, 5.85 mmol) in
CH2Cl2 (60 mL) at 0 °C. After 5 min, 3,4-dihydro-2H-pyran
(1.06 mL, 11.7 mmol) was added in one portion at 0 °C. After
being stirred for 3 h, the reaction mixture was diluted with
CH2Cl2 (40 mL) before being washed with NaHCO3 solution
(3 ꢀ 30 mL). The organic phase was separated, washed with
H2O (3 ꢀ 50 mL) and brine (3 ꢀ 50 mL), and dried (MgSO4).
The solvent was removed under reduced pressure. Purification
of the residue by flash column chromatography (10% Et2O in
hexane) gave THP ether 9 as a white solid (2.78 g, 93%, single
anomer); Rf 0.44 (10% Et2O in hexane); mp 119-121.5 °C; IR
(Nujol) 2131m (CtC) cm-1; 1H NMR (300 MHz, CDCl3) δ 0.22
(s, 9H), 1.12 (s, 9H), 1.22-1.48 (stack, 5H), 1.52-1.71 (m, 1H),
3.04-3.27 (stack, 2H), 3.55 (d, J=12.1 Hz, 1H), 3.73 (br s, 1H),
3.82 (d, J = 12.1 Hz, 1H), 7.29-7.46 (stack, 6H), 7.66-7.79
(stack, 4H); 13C NMR (75 MHz, CDCl3) δ -0.3 (CH3), 18.2
(CH2), 19.1 (C), 25.1 (CH2), 27.3 (CH3), 29.8 (CH2), 60.5 (CH2),
69.1 (CH2), 97.5 (CH), 104.9 (C), 106.5 (C), 120.0 (C), 127.6
(CH), 127.8 (CH), 129.4 (CH), 129.5 (CH), 133.0 (C), 133.5 (C),
135.5 (CH), 135.7 (CH), 152.1 (C); MS (TOF ESþ) m/z 533.0
([MþNa]þ, 100%); HRMS (TOF ESþ) calcd for C29H3935ClO2-
Si2Na [M þ Na]þ 533.2075, found 533.2084. Anal. Calcd for
C29H39ClO2Si2: C, 68.13; H, 7.69. Found: C, 68.23; H, 7.78.
General Procedure for the K2CO3/MeOH-Mediated Depro-
tection of TMS-Alkynes: (E)-3-tert-Butyldiphenylsilyl-4-chloro-
5-tetrahydropyranyloxy-pent-3-en-1-yne (10). Anhydrous K2CO3
(0.78 g, 5.7 mmol) was added to a solution of TMS-alkyne 9(1.32g,
2.58 mmol) in THF/MeOH (1:1, 25 mL) at 0 °C. After 3 h, NH4Cl
solution (20 mL) was added. The phases were separated and the
aqueous phase was extracted with Et2O(3ꢀ 15 mL). The combined
organic layers were washed with brine (3 ꢀ 15 mL) and dried
(MgSO4). The solvent was removed under reduced pressure.
Purification of the residue by flash column chromatography (5%
Et2O in hexane) gave alkyne 10 as a colorless oil (1.12 g, 99%, single
THP anomer); Rf 0.47 (33% Et2O in hexane); IR (film) 3286 s (H;
CtC), 2083 w cm-1; 1H NMR (300 MHz, CDCl3) δ 1.15 (s, 9H),
1.23-1.50 (stack, 5H), 1.54-1.75 (m, 1H), 3.06-3.32 (stack, 2H),
3.60 (d, J=12.1 Hz, 1H), 3.70 (s, 1H), 3.76 (br s, 1H), 3.88 (d, J=
12.1 Hz, 1H), 7.29-7.50 (stack, 6H), 7.67-7.82 (stack, 4H); 13C
NMR (75 MHz, CDCl3) δ 18.2 (CH2), 19.1 (C), 25.1 (CH2), 27.4
(CH3), 29.8 (CH2), 60.7 (CH2), 69.0 (CH2), 83.8 (C), 88.9 (CH),
97.6 (CH), 119.3 (C), 127.7 (CH), 127.9 (CH), 129.5 (CH), 129.6
(CH), 132.9 (C), 133.4 (C), 135.5 (CH), 135.7 (CH), 153.1 (C); MS
(TOF ESþ) m/z 461.2 ([M þ Na]þ, 100%); HRMS (TOF ESþ)
calcd for C26H3135ClO2SiNa [MþNa]þ 461.1680, found 461.1683.
(2E,8E)-3,8-Di(tert-butyldiphenylsilyl)-2,9-dichloro-1,10-bis-
(tetrahydropyranyloxy)-deca-2,8-dien-4,6-diyne (12). DABCO
(138 mg, 1.23 mmol), [(PPh3)2PdCl2] (36 mg, 0.051 mmol),
and ethyl bromoacetate (70 μL, 0.63 mmol) were added to a
stirred solution of alkyne 10 (450 mg, 1.03 mmol) in THF (12 mL)
under N2. CuI (10 mg, 0.053 mmol) was added. The reaction
mixture was stirred under N2 for 15 min before exposing the vessel
to air. After 20 h, the reaction mixture was diluted with CH2Cl2
(20 mL) and filtered through a short silica plug, eluting with 20%
Et2O in hexane (60 mL). The solvent was removed under reduced
pressure and purification of the residue by flash column chroma-
tography (20% Et2O in hexane) afforded dimer 1236 as a white
foam (367 mg, 93%); Rf 0.32 (33% Et2O in hexane); IR (Nujol)
2923 s, 1460 s, 1122 m cm-1; 1H NMR (300 MHz, CDCl3) δ 1.12
(s, 18H), 1.23-1.48 (stack, 10H), 1.52-1.73 (m, 2H), 3.02-3.31
Conclusions
We have shown that a modification in our synthesis of
masked hexayne 2 allows access to its constitutional isomer 7
through a two-directional synthesis strategy. While masked
hexayne 7 displays similar UV-vis properties to its isomer 2,
its synthesis is three steps longer and practically less straight-
forward. Bis-TMS end-capped hexayne 7 also proved to
be more unstable than our “standard” masked hexayne 2,
which is likely to be mirrored in the relative stability of its bis-
TMS deprotected building block that we would need for
oligomerization studies. For these reasons, we believe that
masked hexayne 2, in which the chloro groups occupy
“inside” positions and silyl groups “outside” positions, is the
better building block for long-chain oligoyne assembly.6,15
Where the route to masked hexayne 7 is attractive lies in the
functionalization of the intermediates, specifically THP
ether 10, which has allowed us to incorporate other π-conju-
gated units (i.e., phenylene units) into the middle of the
masked hexayne framework. Interrupting the linear conju-
gation of the masked hexayne has a significant effect on the
optical properties of these hybrid products, as evidenced by
their UV-vis spectra, and also increases the stability of some
of these building blocks, which may be useful in future
oligomerization studies. The solubility problems associated
with some of the particularly long conjugated frameworks
can be addressed in the future by judicious substitution on
the aromatic units.
From a synthetic viewpoint, this study has shown that
β-chlorovinylsilanes are stable to the conditions for the
Sonogashira cross-coupling reaction and Cadiot-Chodkie-
wicz coupling. Moreover, the palladium-mediated oxidative
coupling of alkyne 10 to provide dimer 12 proved to be an
extremely efficient method for dimerizing this alkyne and
may be a useful alternative for similar couplings where our
modified Eglinton-Glaser-Hay coupling conditions fail.
By introducing phenylene units into the π-conjugated frame-
work, we were able to access a series of phenylene ethyny-
lenes containing β-chlorovinylsilanes. Although this study
has focused on assembling axial rod-like π-conjugated sys-
tems,17 which contain masked alkynes in the form of the
β-chlorovinylsilane moiety, we have shown previously that
this alkyne masking group undergoes dechlorosilylation
upon exposure to TBAF.5 This oligoyne unmasking strategy
was further verified by the synthesis of 35 from 34, which
demonstrates that these systems provide an entry into
alkyne-rich π-conjugated frameworks that are not readily
accessible by other means.16 While compound 34 contains
relatively robust dimethylmethanol end-capping groups,
compounds 20 and 28 both contain more labile TMS end-
capping groups. Since we have shown previously that this
silyl protecting group can be removed without effecting
dechlorosilylation of the internal β-chlorovinylsilane motifs,
these compounds represent useful monomer precursors
for oligomerization studies, which will be a focus for
future work.
(36) Bis-THP derivatives 12, 16, and 24 are presumably 1:1 mixtures of
diastereoisomers even though there is no evidence for this in the NMR
spectra for these compounds.
J. Org. Chem. Vol. 74, No. 20, 2009 7905

Weller et al.
JOCArticle
(stack, 4H), 3.59 (d, J=12.5 Hz, 2H), 3.76 (br s, 2H), 3.86 (d, J=
byproduct. The suspension was filtered through a silica plug and
the plug was washed with 20% Et2O in hexane (2 ꢀ 25 mL). The
solvent was removed under reduced pressure to give bis-dibro-
mide 15 as a yellow solid, which was used without further puri-
fication (0.44 g, 77%); Rf 0.27 (5% Et2O in hexane); mp 126-
128 °C; IR (Nujol) 2922 s, 1463 s, 1108 s, 700 s cm-1; 1H NMR
(300 MHz, CDCl3) δ 1.15 (s, 18H), 6.51 (s, 2H), 7.36-7.50
(stack, 12H), 7.66-7.80 (stack, 8H); 13C NMR (75 MHz,
CDCl3) δ 19.4 (C), 27.8 (CH3), 86.6 (C), 88.3 (C), 98.8 (C),
122.5 (C), 128.2 (CH), 130.0 (CH), 131.9 (C), 135.1 (CH), 135.8
(CH), 146.2 (C); MS (TOF ESþ) m/z 1037.4 ([MþNa]þ, 100%),
577.5 (23); HRMS (TOF ESþ) calcd for C42H4079Br435Cl2Si2Na
[MþNa]þ 1032.8677, found 1032.8721. Anal. Calcd for C44H40-
Br4Cl2Si2: C, 52.04; H, 3.97. Found: C, 51.93; H, 4.16.
12.5 Hz, 2H), 7.30-7.48 (stack, 12H), 7.67-7.82 (stack, 8H); 13
C
NMR (75 MHz, CDCl3) δ 18.3 (CH2), 19.2 (C), 25.1 (CH2), 27.4
(CH3), 29.8 (CH2), 60.8 (CH2), 69.1 (CH2), 85.0 (C), 85.2 (C), 97.7
(CH), 119.4 (C), 127.8 (CH), 128.0 (CH), 129.6 (CH), 129.7 (CH),
132.7 (C), 133.2 (C), 135.6 (CH), 135.7 (CH), 154.3 (C); MS (TOF
ESþ) m/z 897.5 ([MþNa]þ, 100%); HRMS (TOF ESþ) calcd for
C52H6035Cl2O4Si2Na [Mþ Na]þ 897.3305, found 897.3318.
General Procedure for the Deprotection of THP Ethers:
(2E,8E)-3,8-Di(tert-butyldiphenylsilyl)-2,9-dichloro-deca-2,8-dien-
4,6-diyn-1,10-diol (13). Bis-THP ether 12 (1.82 g, 2.07 mmol)
was dissolved in THF/EtOH (1:1, 40 mL) and the resulting
solution was heated with stirring at 55 °C. PPTS (0.10 g, 0.41 mmol)
was added. After being stirred at 55 °C for 26 h, the reaction
mixture was cooled to rt and H2O (100 mL) was added. The
resulting off-white precipitate was collected by suction filtration
and washed with toluene (40 mL) to afford diol 13 as an off-
white solid (0.97 g, 66%). More product was obtained from the
filtrate: the organic layer of the combined filtrates was separated
from the aqueous layer and dried (MgSO4). The solvent was
removed under reduced pressure followed by dissolution of the
residue in CH2Cl2 (10 mL). The crude material was preadsorbed
onto silica and purified by flash column chromatography (33%
Et2O in hexane) to give an off-white solid (0.22 g, 15%). The two
samples of the diol 13 were combined (1.19 g, 81%); Rf 0.20
(50% Et2O in hexane); mp 223-228 °C; IR (CHCl3) 3563 m
(O;H), 3534 m (O;H), 3442 br w (O;H), 2114 w (CtC)
cm-1; 1H NMR (300 MHz, CDCl3) δ 1.13 (s, 18H), 3.78 (s, 4H),
7.34-7.51 (stack, 12H), 7.67-7.82 (stack, 8H), OH resonances
not observed; 13C NMR (75 MHz, DMSO-d6) δ 18.7 (C), 27.3
(CH3), 63.8 (CH2), 83.1 (C), 84.7 (C), 115.6 (C), 128.1 (CH),
130.0 (CH), 132.2 (C), 135.2 (CH), 158.9 (C); MS (TOF ESþ)
m/z 729.2 ([M þ Na]þ, 100%); HRMS (TOF ESþ) calcd for
C42H4435Cl2O2Si2Na [Mþ Na]þ 729.2155, found 729.2143.
General Procedure for the Synthesis of TMS-Alkynes from the
Corresponding 1,1-Dibromoolefins: (3E,9E)-4,9-Di(tert-butyldi-
phenysilyl)-3,10-dichloro-1,12-bis(trimethylsilyl)dodeca-3,9-dien-
1,5,7,11-tetrayne (7). n-BuLi (1.96 mL of a 2.4 M solution in
hexane, 4.6 mmol) was added over 5 min to a prechilled (0 °C)
solution of i-Pr2NH (0.64 mL, 4.9 mmol) in THF (5 mL). After
30 min, the resulting LDA solution was cooled to -78 °C and
transferred via cannula to a cooled (-78 °C) solution of bis-
dibromide 15 (398 mg, 0.394 mmol) and Me3SiCl (0.50 mL,
3.9 mmol) in THF (5 mL). After 45 min, the resulting purple
solution was quenched at -78 °C with NH4Cl solution (20 mL)
and allowed to warm to rt. The phases were separated and
the aqueous layer was extracted with EtOAc (3 ꢀ 20 mL). The
combined organic extracts were washed with brine (3 ꢀ 20 mL)
and dried (MgSO4). The solvent was removed under reduced
pressure to afford a viscous, orange oil. Purification by flash
column chromatography (13% toluene in hexane) gave bis-
TMS-alkyne 7 as a bright yellow solid (121 mg, 37%); Rf 0.32
(3% Et2O in hexane); mp 60-62 °C; UV (CH2Cl2) λmax, nm
(ε) 290 sh (32700), 301sh (37400), 314 sh (41200), 324 (47300),
348 (40600), 356 sh (35900), 375 (60600), 406 (66600); IR
˚
General Procedure for the PCC/Celite/5 A Molecular Sieves-
1
Mediated Oxidation of Diols to Enals: (2E,8E)-3,8-Di(tert-butyl-
diphenylsilyl)-2,9-dichloro-deca-2,8-dien-4,6-diyn-1,10-dial (14).
Diol 13 (510 mg, 0.722 mmol) was added to a stirred suspension
(CHCl3) 2964 m, 1489 m, 1109 s cm-1; H NMR (300 MHz,
CDCl3) δ -0.27 (s, 18H), 1.15 (s, 18H), 7.33-7.46 (stack, 12H),
7.69-7.81 (stack, 8H); 13C NMR (100 MHz, CDCl3) δ -1.2
(CH3), 19.6 (C), 27.5 (CH3), 88.2 (C), 91.1 (C), 101.7 (C), 111.2
(C), 125.5 (C), 127.6 (CH), 129.4 (CH), 132.3 (C), 132.5 (C),
136.0 (CH); MS (TOF ESþ) m/z 946.5 ([M þ 109Ag]þ, 100%),
944.6 (43, [Mþ107Ag]þ);HRMS(TOFESþ) calcdforC50H5635Cl2-
Si4107Ag [M þ Ag]þ 945.1887, found 945.1862.
˚
of Celite (250 mg) and freshly activated 5 A molecular sieves
(250 mg) in CH2Cl2 (3 mL). PCC (620 mg, 2.88 mmol) was then
added in one portion at rt. After 10 h, the reaction mixture was
diluted with CH2Cl2 (15 mL) and filtered through a short silica
plug. The product was eluted with 33% Et2O in hexane (100 mL),
taking care not to bring through the dark band of Cr residues.
Removal of the solvent under reduced pressure resulted in
precipitation of a yellow solid of bis-aldehyde 14, which was
collected by suction filtration (430 mg, 87%); Rf 0.48 (40% Et2O
in hexane); UV (CH2Cl2) λmax, nm (ε) 315 sh (10740), 331
(13150), 349 sh (10710), 375 (10370), 406 (9100); IR (CHCl3)
General Procedure for the Sonogashira Coupling. Aryl iodide
(1.0 equiv) and terminal alkyne (1.0 equiv) were dissolved in
Et3N (0.1 M concentration) and the mixture was degassed with
two freeze-thaw saturate with N2 cycles. To this solution was
added either [Pd(OAc)2] (5 mol %) and PPh3 (10 mol %) or
[(PPh3)2PdCl2] (5 mol %) and the mixture was again degassed
with a freeze-thaw saturate with N2 cycle. The reaction mixture
was warmed to rt before adding CuI (2.5 mol %). The reaction
mixture was carefully degassed by boiling briefly under reduced
pressure and then flushing with N2. After stirring at rt overnight,
hexane was added, and the reaction mixture was filtered through
Celite. The solvent was removed under reduced pressure and the
residue was purified by flash column chromatography to afford
the product.
1683 s (CdO) cm-1 1H NMR (300 MHz, CDCl3) δ 1.20
;
(s, 18H), 7.34-7.52 (stack, 12H), 7.64-7.78 (stack, 8H), 9.21
(s, 2H); 13C NMR (75 MHz, CDCl3) δ 19.6 (C), 27.5 (CH3), 91.3
(C), 97.8 (C), 128.5 (CH), 130.5 (CH), 132.5 (C), 135.5 (CH),
137.9 (C), 150.0 (C), 183.5 (CH); MS (TOF ESþ) m/z 757.5
([M þ Na þ MeOH]þ, 50%) 725.4 (100, [M þ Na]þ); HRMS
(TOF ESþ) calcd for C42H4035Cl2O2Si2Na [MþNa]þ 725.1842,
found 725.1840.
General Procedure for the Dibromoolefination of Aldehydes:
(3E,9E)-4,9-Di(tert-butyldiphenylsilyl)-3,10-dichloro-1,1,12,12-
tetrabromododeca-1,3,9,11-tetraen-5,7-diyne (15). A solution of
PPh3 (1.20 g, 4.56 mmol) in CH2Cl2 (5 mL) was added via a
dropping funnel over 5 min to a cooled (0 °C) solution of CBr4
(0.77 g, 2.3 mmol) in CH2Cl2 (5 mL), resulting in the formation
of a golden yellow solution. After 1 h at 0 °C, a solution of
aldehyde 14 (0.40 g, 0.57 mmol) in CH2Cl2 (5 mL) was added
dropwise over 5 min. The dropping funnel was rinsed with
additional CH2Cl2 (2 mL). After 1 h, hexane (50 mL) was added
to the solution with vigorous stirring to precipitate the Ph3PdO
(E)-1,4-Di[30-tert-butyldiphenylsilyl-40-chloro-50-tetrahydro-
pyranyloxypent-30-en-10-ynyl]benzene (16). Phenylene ethyny-
lene 16 was synthesized according to the general procedure for
the Sonogashira reaction. Alkyne 10 (2.310 g, 5.272 mmol) was
coupled with 1,4-diiodobenzene (0.870 g, 2.64 mmol) in the pre-
sence of Pd(OAc)2 (59 mg, 0.26 mmol), PPh3 (138 mg, 0.520 mmol),
and CuI (25 mg, 0.17 mmol) in Et3N (50 mL). After 20 h, the
reaction mixture was diluted with hexane (120 mL) and filtered
through Celite. The solvent was removed under reduced pres-
sure and purification of the residue by flash column chroma-
tography (10% Et2O in hexane) gave phenylene ethynylene 1636
7906 J. Org. Chem. Vol. 74, No. 20, 2009

Weller et al.
JOCArticle
as a white foam (2.060 g, 81%); Rf 0.74 (50% Et2O in hexane);
IR (CHCl3) 2192 w (CtC) cm-1; 1H NMR (300 MHz, CDCl3) δ
1.24 (s, 18H), 1.28-1.53 (stack, 10H), 1.58-1.78 (m, 2H),
3.06-3.37 (stack, 4H), 3.70 (d, J = 12.1 Hz, 2H), 3.85 (br s,
2H), 3.98 (d, J = 12.1 Hz, 2H), 7.33-7.54 (stack, 16H), 7.75-
7.89 (stack, 8H); 13C NMR (75 MHz, CDCl3) δ 18.2 (CH2), 19.2
(C), 25.1 (CH2), 27.4 (CH3), 29.8 (CH2), 60.7 (CH2), 69.3 (CH2),
92.4 (C), 97.6 (CH), 99.7 (C), 119.7 (C), 123.5 (C), 127.8 (CH),
127.9 (CH), 129.5 (CH), 129.6 (CH), 131.1 (CH), 133.0 (C),
133.4 (C), 135.6 (CH), 135.7 (CH), 151.4 (C); MS (TOF ESþ)
m/z 1059.6 ([M þ 109Ag]þ, 100%), 1057.6 (24, [M þ 107Ag]þ),
428.9 (36); HRMS (TOF ESþ) calcd for C58H6435Cl2O4-
Si2107Ag [M þ Ag]þ 1057.2771, found 1057.2747; Anal.
Calcd for C58H64Cl2O4Si2: C, 73.16; H, 6.77. Found: C, 73.19;
H, 6.83.
7.37 (s, 4H), 7.69-7.82 (stack, 8H); 13C NMR (75 MHz,
benzene-d6) δ 19.6 (C), 28.3 (CH3), 93.4 (C), 99.2 (C), 103.7
(C), 123.4 (C), 124.2 (C), 128.6 (CH), 130.3 (CH), 131.8 (CH),
132.7 (C), 135.9 (CH), 136.3 (CH), 144.2 (C). Satisfactory MS
data could not be obtained for this compound. Crystals sui-
table for X-ray analysis were obtained from hexane and con-
firm the structure of this compound (see the Supporting
Information).
(E)-1,4-Di[30-tert-butyldiphenysilyl-40-chloro-60-trimethylsilyl-
hex-30-en-10,50-diynyl]benzene (20). Bis-TMS-alkyne 20 was
synthesized according to the synthesis of TMS-alkynes from
the corresponding 1,1-dibromoolefins (see the synthesis of 7).
Bis-dibromide 19 (229 mg, 0.210 mmol) was reacted under the
standard conditions and after 1 h, the crude product was
isolated with use of the standard workup conditions. Purifica-
tion of the residue by flash column chromatography (2% Et2O
in hexane) gave bis-TMS-alkyne 20 as a pale yellow foam (190 mg,
99%); Rf 0.30 (3% Et2O in hexane); UV (CH2Cl2) λmax, nm (ε) 320
(21900), 352 sh (36500), 374 (57900), 383 sh (53200), 399 (53200);
IR (CHCl3) 2133 m (CtC) cm-1; 1H NMR (300 MHz, CDCl3)
δ -0.23 (s, 18H), 1.25 (s, 18H), 7.34-7.49 (stack, 16H), 7.76-7.86
(stack, 8H); 13C NMR (75 MHz, CDCl3) δ -1.1 (CH3), 19.8 (C),
27.7 (CH3), 93.2 (C), 101.9 (C), 106.1 (C), 109.1 (C), 123.8 (C),
126.1 (C), 127.5 (CH), 129.3 (CH), 129.6 (C), 131.3 (CH), 132.8
(C), 136.0 (CH); MS (TOF ESþ) m/z 1023.5 ([M þ 109Ag]þ,
100%), 1021.5 (29, [M þ 107Ag]þ), 749.3 (44, [M-tBuPh2SiCl þ
Ag]þ); HRMS (TOF ESþ) calcd for C56H6035Cl2Si4107Ag
[M þ Ag]þ 1021.2200, found 1021.2235; HRMS (TOF ESþ)
calcd for C56H6035Cl2Si4109Ag [M þ Ag]þ 1023.2197, found
1023.2195.
1,4-Di[40-(700-hydroxy-700-methylocta-100,300,500-triynyl)phenyl]-
buta-1,3-diyne (35). TBAF (75 μL of a 1 M solution in THF,
0.08 mmol) was added to a solution of phenylene ethynylene 34
(19 mg, 0.019 mmol, see the Supporting Information) in THF
(4 mL) at 0 °C. After 1 h, NH4Cl solution (8 mL) was added at
0 °C and the reaction mixture was warmed to rt. The aqueous
layer was extracted with Et2O (3 ꢀ 8 mL) and the combined
organic extracts were washed with H2O (3 ꢀ 8 mL) and brine
(3 ꢀ 8 mL) and dried (Na2SO4). Removal of the solvent under
reduced pressure gave phenylene-oligoyne 35 as an off-white
solid, which was collected by suction filtration (7 mg, 76%);
Rf 0.31 (70% Et2O in hexane); UV (CH2Cl2) λmax, nm
(ε, normalized) 390 (0.93), 360.5 (1.00), 335 (0.58), 312 (0.31),
284 (0.40), 263 (0.50); 1H NMR (300 MHz, acetone-d6) δ 1.50
(s, 12H), 7.66 (s, 8H); MS (EI) m/z 462 ([M]þ, 100%), 447 (29,
[M - CH3]þ), 429 (11), 419 (12), 404 (25), 389 (7), 368 (9), 361
(11), 346 (15), 313 (11); HRMS (EI) calcd for C34H22O2 Mþ
462.1620, found 462.1609.
(E)-1,4-Di[30-tert-butyldiphenylsilyl-40-chloro-50-hydroxypent-
30-en-10-ynyl]benzene (17). Diol 17 was synthesized according to
the general procedure for the THP deprotection of THP ethers
(see the synthesis of 13). Bis-THP ether 16 (2.06 g, 2.17 mmol)
was reacted under the standard conditions and after 24 h, the
crude product was isolated by using the standard workup
procedure. Purification of the crude product by flash column
chromatography (25% Et2O in hexane) gave diol 17 as colorless
crystals (1.51 g, 89%); Rf 0.35 (50% Et2O in hexane); mp >140 °C
1
dec; IR (Nujol) 3554 m (O-H), 3484 br m (O-H) cm-1; H
NMR (300 MHz, CDCl3) δ 1.21 (s, 18H), 3.82 (s, 4H),
7.38-7.51 (stack, 16H), 7.74-7.84 (stack, 8H), OH resonances
not observed; 13C NMR (75 MHz, CDCl3) δ 19.2 (C), 27.3
(CH3), 65.6 (CH2), 92.1 (C), 99.9 (C), 119.2 (C), 123.5 (C), 128.1
(CH), 129.9 (CH), 131.3 (CH), 133.3 (C), 135.6 (CH), 153.2 (C);
MS (TOF ESþ) m/z 805.7 ([M þ Na]þ, 17%), 424.4 (100);
HRMS (TOF ESþ) calcd for C48H4835Cl2O2Si2Na [M þ Na]þ
805.2468, found 805.2454. Anal. Calcd for C48H48Cl2O2Si2: C,
73.54; H, 6.17. Found: C, 73.48; H, 6.07. Crystals suitable for
X-ray analysis were obtained from the column eluent and
confirm the structure of this compound (see the Supporting
Information).
(E)-1,4-Di[30-tert-butyldiphenylsilyl-40-chloro-50-oxo-pent-30-en-
10-ynyl]benzene (18). Bis-aldehyde 18 was synthesized according
˚
to the general procedure for the PCC/Celite/5 A molecular
sieves-mediated oxidation of diols to enals (see the synthesis of
14). Diol 17 (500 mg, 0.639 mmol) was reacted under the
standard conditions, and after 14 h, the crude product was
isolated by using the standard workup procedure to give bis-
aldehyde 18 as a green foam, which was used without further
purification (399 mg, 80%); Rf 0.59 (40% Et2O in hexane); IR
(CHCl3) 2181s (CtC), 1682s (CdO) cm-1; 1H NMR (300 MHz,
CDCl3) δ 1.27 (s, 18H), 7.33-7.57 (stack, 16H), 7.67-7.82
(stack, 8H), 9.27 (s, 2H); 13C NMR (75 MHz, CDCl3) δ 19.5
(C), 27.6 (CH3), 93.6 (C), 114.3 (C), 123.9 (C), 128.3 (CH), 130.3
(CH), 131.8 (CH), 132.9 (C), 135.5 (CH), 139.2 (C), 147.5 (C),
183.8 (CH); MS (TOF ESþ) m/z 801.3 ([M þ Na]þ, 100%);
HRMS (TOF ESþ) calcd for C48H4435Cl2O2Si2Na [M þ Na]þ
801.2155, found 801.2169. Anal. Calcd for C48H44Cl2O2Si2: C,
73.92; H, 5.69. Found: C, 74.04; H, 5.78.
(E)-1,4-Di[30-tert-butyldiphenylsilyl-40-chloro-60,60-dibromo-
hexa-30,50-dien-10-ynyl]benzene (19). Bis-dibromide 19 was syn-
thesized according to the general procedure for the dibromoo-
lefination of aldehydes (see the synthesis of 15). Bis-aldehyde 18
(357 mg, 0.459 mmol) was reacted with CBr4 (607 mg, 1.83 mmol)
and PPh3 (960 mg, 3.66 mmol) under the standard conditions
and the product was isolated by using the standard workup
procedure; the product was eluted from the short silica plug with
10% Et2O in hexane (60 mL). The solvent was removed under
reduced pressure to give bis-dibromide 19 as a bright green foam
(268 mg, 54%); Rf 0.33 (20% Et2O in hexane); mp 85 °C dec; IR
(film) 2858 s, 1591 m, 1428 s, 1107 s cm-1; 1H NMR (300 MHz,
benzene-d6) δ 1.27 (s, 18H), 6.66 (s, 2H), 7.10-7.25 (stack, 12H),
Acknowledgment. We thank the University of Birmingham
and the Engineering and Physical Sciences Research Council
for a studentship (to M.D.W.). The NMR spectrometers
used in this research were funded in part through Birming-
ham Science City: Innovative Uses for Advanced Materials
in the Modern World (West Midlands Centre for Advanced
Materials Project 2), with support from Advantage West
Midlands (AWM) and partially funded by the European
Regional Development Fund (ERDF).
Supporting Information Available: General experimental
details, experimental procedures, and compound characteriza-
tion data for 11 and its Cadiot Chodkiewicz coupling product
with phenyl acetylene, 22, 24-28, 32, and 34, X-ray data for 17
and 19, scanned 1H NMR spectra, and 13C NMR spectra for all
new compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
J. Org. Chem. Vol. 74, No. 20, 2009 7907