An iterative method for the synthesis of symmetric polyynes

Article abstract of DOI:10.1002/ejoc.201200442

An iterative synthetic route for obtaining symmetric polyynes was developed, consisting of a series of iodination and Stille coupling reactions. The starting materials employed in this pathway are simple and can be prepared easily. Polyynes containing up to seven Ca=C bonds were synthesized using this method. This route is particularly effective for accessing polyynes with an odd number of Ca=C bonds and has allowed for the synthesis of a new iodine-capped polyyne, diiododecapentayne. Copyright

Full text of DOI:10.1002/ejoc.201200442

Job/Unit: O20442
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FULL PAPER
DOI: 10.1002/ejoc.201200442
An Iterative Method for the Synthesis of Symmetric Polyynes
Racquel C. DeCicco,[a] Allison Black,[a] Lei Li,[a] and Nancy S. Goroff*[a]
Keywords: Alkynes / Cross-coupling / Synthetic methods / Palladium
An iterative synthetic route for obtaining symmetric polyynes
was developed, consisting of a series of iodination and Stille
coupling reactions. The starting materials employed in this
pathway are simple and can be prepared easily. Polyynes
containing up to seven CϵC bonds were synthesized using
this method. This route is particularly effective for accessing
polyynes with an odd number of CϵC bonds and has allowed
for the synthesis of a new iodine-capped polyyne, diiodode-
capentayne.
length polyynes based on symmetric couplings to diiodopo-
lyyne intermediates under mild reaction conditions.
Introduction
The synthesis of polyynes has been the subject of much
research in recent years, based on potential applications in
optics and electronics.[1] Third-order nonlinear optical
properties are of particular interest, with theoretical and
experimental studies demonstrating that sp-hybridized car-
bon chains have large multiphoton absorption cross-sec-
tions.[1,2] Several groups have examined the mechanism of
charge transport in polyynes and have suggested that they
can behave as molecular wires.[3] Polyyne chains may also
be used as models to predict the properties of carbyne, the
putative linear allotrope of carbon, and more generally to
understand the fundamentals of conjugation.[1] Polyynes
have also been pursued as possible precursors to polydiacet-
ylenes, nanotubes, fullerenes, graphitic materials, and other
carbon-rich compounds.[4] In addition to applications in
materials science, polyynes occur in a variety of natural
products with biological activity.[5]
Walton and co-workers pioneered the synthesis of ex-
tended polyynes with up to 16 CϵC bonds using Hay cou-
pling conditions.[6] Diederich and co-workers used solution-
spray flash vacuum pyrolysis (SS-FVP) to form shorter po-
lyynes from substituted 3,4-dialkynyl-3-cyclobutene-1,2-di-
ones.[7] Tykwinski and co-workers recently synthesized
chains of as many as 22 CϵC bonds by combining tradi-
tional oxidative couplings with the Fritsch–Buttenburg–
Wiechell (FBW) rearrangement of dibromoolefinic precur-
sors.[8] While these methods have proven effective in many
instances, they require the use of unstable terminal alkynes,
complex starting materials, or harsh reactions conditions.
Therefore, we envisioned an iterative pathway to moderate-
Few examples of cross-coupling reactions using diiodo-
polyynes have been reported. Hirsch and co-workers used
Cadiot–Chodkiewicz conditions to couple bridged terminal
alkynes and diiodoacetylene, which resulted in low yields
and product mixtures.[9] Similarly, after the methods de-
scribed here were under development, the synthesis of di-
arylpolyynes from diiodoacetylene was reported by Cataldo
and co-workers. These reactions yielded mixtures of po-
lyynes of different lengths, and the individual components
were not isolated.[10] In addition, Bruce and co-workers re-
ported the palladium-catalyzed cross-coupling of diiodoalk-
ynes and gold(I) alkynyl complexes.[3a,11] These specific ex-
amples highlight the potential of diiodopolyynes to provide
a general synthetic route towards symmetric polyynes.
The method presented here provides for the synthesis
and isolation of polyynes containing four to seven CϵC
bonds, via a series of iodination and Stille coupling reac-
tions (Scheme 1). Palladium-catalyzed coupling reactions
bring together short symmetric iodine-capped polyyne rods
and silyl-protected tin acetylides. Although there are rela-
tively few examples of Stille-type alkyne–alkyne cou-
plings,[12] we have found efficient conditions for this reac-
tion. The resulting silyl-capped polyyne can subsequently
be iodinated to make a new diiodopolyyne for successive
coupling reactions. Each iodination/coupling cycle symmet-
rically increases the length of the resulting polyyne by four
carbon atoms, without requiring the preparation or isola-
tion of a terminal alkyne intermediate. The diiodopolyynes
that are central to this synthesis are relatively stable com-
pared to analogous terminal or bromoalkynes, thus adding
to the appeal of this route. Phenyl alkynyl tin reagents can
also be used to prepare the corresponding phenyl-capped
polyynes. This method is especially beneficial for providing
a straightforward route of accessing symmetric polyynes
with an odd number of CϵC bonds.
[a] Department of Chemistry, State University of New York,
Stony Brook, New York 11794-3400, USA
Fax: +1-631-632-7960
E-mail: nancy.goroff@sunysb.edu
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201200442.
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R. C. DeCicco, A. Black, L. Li, N. S. Goroff
FULL PAPER
heptayne 7b is due to difficulties in isolating the product,
but also appears to result from the instability of intermedi-
ate diiodopentayne 5a, as described below. Attempts to pre-
pare a heptayne with bulkier end caps using the tin acet-
ylide of (tert-butyl)dimethylsilyl (TBDMS) ethyne were not
successful. Thus, heptaynes appear to strain the limits for
this method, although there are very few alternatives for
heptayne synthesis.
Scheme 1. General scheme for the synthesis of symmetric polyynes.
Results and Discussion
Polyynes containing an even number of CϵC bonds are
derived here from diiodobutadiyne (2a), which can be ob-
tained from the iodination of commercially available bis(tri-
methylsilyl)butadiyne (2c). Polyynes containing an odd
number of CϵC bonds are accessed from diiodohexatriyne
(3a). Trimethylsilyl-capped precursor 3c is not commercially
available; however, it can be synthesized from propargyl
alcohol in three straightforward steps, as shown in
Scheme 2.[15] Our initial route towards 3c encompassed the
synthesis of 2,4-hexadiyn-1,6-bis(p-toluenesulfonate), as re-
ported by Rubin and co-workers,[7] but we found that going
through dibromohexadiyne 10 greatly minimized the risk of
undesired polymerization.
One major advantage of this route is that the required
starting materials can be obtained by straightforward syn-
thetic procedures.[13] The diiodoalkyne precursors in this
method are prepared via direct iodination of trimethylsilyl-
capped alkynes using AgNO3 and N-iodosuccinimide
(NIS).[13a] Incorporating different tin acetylides (i.e., 1b–d)
allows for end cap variety among the polyynes that have
been obtained using this method. Trimethylsilyl, triisoprop-
ylsilyl, and phenyl tin acetylides, prepared according to lit-
erature procedures,[13b] are coupled to C4I2 (2a), C6I2 (3a),
C8I2 (4a), and C10I2 (5a) to produce the respective tetraynes
(i.e., 4b, 4c), pentaynes (i.e., 5b–d), hexaynes (i.e., 6b, 6d),
and heptaynes (i.e., 7b), as shown in Table 1. Yields for the
products are largely dependent on the techniques required
To avoid the potential toxicity of the tin reagents and
for isolation. Byproduct bis(trimethylsilyl)butadiyne (2c) is byproducts in the Stille reaction, the synthesis of polyynes
removed from the trimethylsilyl-capped polyynes using vac- using Negishi coupling conditions was also explored.[16]
uum sublimation rather than chromatography, thus avoid- TMS, TIPS, and Ph-capped tetraynes were targeted from
ing the difficulty of separating compounds with similar po- reaction of diiodobutadiyne (2a) with the corresponding
larity on silica. Compounds 4b and 5b are isolated via zinc acetylide. Unfortunately, preparing the zinc acetylides
recrystallization techniques. However, similar methods were proved unreliable in these experiments, and the product
not effective for purifying longer analogs. The low yield of yields were typically significantly lower than those obtained
Table 1. Polyyne synthesis via Stille coupling.[a]
[a] Compounds are numbered according to the number of triple bonds they contain, with letters designating the end groups: a = iodine,
b = TIPS, c = TMS, d = Ph. Conditions: Mole ratio of organostannane/diiodoalkyne = 2:1, Pd(PPh3)2Cl2 (12 mol-%), CuI (23 mol-%),[14]
THF, 10 h. [b] All yields are for the isolated product, based on diiodopolyyne starting material. [c] An excess amount of the organostann-
ane was used, CuI (13 mol-%).
2
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An Iterative Method for the Synthesis of Symmetric Polyynes
bonds. To compensate for this effect and to enhance the
rate of reaction for longer polyynes, the concentration of
AgNO3 and NIS was increased. By reducing the time re-
quired for complete iodination, we minimized decomposi-
tion of the target diiodopolyynes.
As part of this iterative route, we have prepared the novel
compound 1,10-diiodo-1,3,5,7,9-decapentayne (5a). This
diiodopolyyne is less stable than its shorter analogs, decom-
posing rapidly at room temperature and exploding at 55–
56 °C. When working with this compound, it is crucial to
keep it in solution and cooled below 0 °C, as black insoluble
Scheme 2. Synthesis of bis(trimethylsilyl)hexatriyne (3c).
under Stille coupling conditions. Similarly, copper acet- material forms in solution relatively quickly above 0 °C.
ylides were prepared and employed under Cadiot–Chodkie- Keeping the diiodopentayne in a dry ice/acetone bath al-
wicz conditions.[17] Unfortunately, these reactions did not lowed successful reaction with triisopropyl tin acetylide 1b
favor cross-coupling, as product mixtures consistently con- to make bis(triisopropylsilyl)heptayne (7b), which has also
tained significant amounts of homocoupled starting materi- not been reported until now.
als.
The diiodopolyynes prepared here are interesting com-
Stille couplings between tin acetylides and haloalkynes pounds in their own right. Based on their Lewis acidity,
are relatively rare, with only a few reported examples.[12] We they can be aligned in the solid state for topochemical poly-
therefore carried out model experiments based on the Stille merization, as a means of accessing carbon-rich materi-
coupling of iodophenylacetylene and trimethylsilyl tin acet- als.[19] We have prepared the ordered polymer of diiodo-
ylide 1c. These experiments indicated that the concentration butadiyne (2a) and are currently pursuing methods for con-
of copper iodide is particularly important to promote cross- trolled polymerization of the longer analogs.
coupling in this reaction. Previous studies of the coupling
of tin acetylides with aryl halides have suggested that a cop-
per cocatalyst transmetalates with the organostannane to
form a copper acetylide, which in turn may transmetalate
Conclusions
with palladium at a faster rate than the stannane alone.[18]
The presence of copper iodide thus minimizes homocou-
pling of the iodoalkyne. Optimizing the reaction conditions
gave catalyst ratios for the symmetric double coupling of
the diiodopolyynes of 12 mol-% palladium (6 mol-% per
iodine) and 23 mol-% copper (11.5 mol-% per iodine).
In addition to catalyst load, we examined the order and
rate of reagent addition. Initial investigations revealed the
importance of both these parameters. Experimental trials
where compound 2a and tin acetylide 1c were combined in
solution and then slowly added together to the catalyst mix-
ture provided better results than when all reagents were
combined in one pot. This method increases the effective
concentration of the catalyst, relative to the alkyne reac-
tants. Adding the diiodoalkyne and the tin acetylide to the
catalyst mixture over a period of 10–11 h proved optimal,
whereas shorter addition times resulted in increased homo-
coupling of the tin acetylide, as well as the unwanted forma-
tion of longer polyynes, as determined by 13C NMR spec-
troscopy. Rapid addition evidently promotes decomposition
The iterative iodination/Stille coupling method described
here has proven to be an effective route towards tetraynes,
pentaynes, and hexaynes. Polyynes with an even number of
CϵC bonds are all derived from bis(trimethylsilyl)buta-
diyne (2c), whereas the polyynes with an odd number of
CϵC bonds are prepared from bis(trimethylsilyl)hexatriyne
(3c). The choice of end group is not made until the final
step, thus adding flexibility to this route. The decreased sta-
bility of the longer diiodoalkyne precursors is addressed by
simple modifications in temperature, reaction rate, and or-
der of reagent addition. Thus, starting from simple building
blocks, we can prepare polyynes with a range of lengths
and end groups.
Experimental Section
General Methods: Reagents were purchased reagent grade from Al-
drich, Fischer Scientific/Acros Organics, VWR, Strem, or GFS
Chemicals and were used without further purification, except where
stated. Tetrahydrofuran was distilled under an atmosphere of argon
of diiodobutadiyne (2a), via successive homocoupling reac- from sodium/benzophenone. Copper iodide was purified by
recrystallization. High-purity hexanes should be used when prepar-
ing silyl-capped polyynes to avoid possible grease contamination
caused by the low polarity of these compounds. All reactions were
performed under an inert argon atmosphere, unless stated other-
wise. Iodination and coupling reactions of polyynes were carried
out in aluminum-foil-wrapped flasks in an unlighted hood. All di-
iodopolyynes were washed with aqueous Na2S2O3 immediately
prior to use. Column chromatography: alumina (50–200 mesh)
from Acros Organics, silica gel-60 (230–400 mesh) from Sorbent
Technologies. Thin-Layer Chromatography (TLC): plastic sheets
tions. However, diyne 2c, formed from homocoupling of or-
ganostannane 1c, is easily separated from the product using
vacuum sublimation. The ability to recover the diyne and
use it in future iodination reactions adds to the overall effi-
ciency of this method.
The procedure used for iodinating the silyl-capped po-
lyynes required modification to prepare longer diiodoalk-
yne intermediates. Iodination of longer chains was observed
to proceed more slowly than shorter analogs, which may be
due to complexation of the silver to the internal CϵC covered with silica gel purchased from Acros. Melting points were
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R. C. DeCicco, A. Black, L. Li, N. S. Goroff
FULL PAPER
measured with a Thomas Hoover Capillary melting point appara-
tus. 1H NMR and 13C NMR spectra were obtained by using Varian
Gemini 300 MHz, Inova 400 MHz, or Inova 500 MHz instruments
and were taken in deuterated chloroform unless noted otherwise.
Mass spectra (EI) and high-resolution mass spectra (EI) were ob-
after the first HCl additions, the cloudy precipitate disappeared,
but the addition was continued until the white cloudy suspension
remained steady. This warm solution was allowed to cool down to
room temperature and placed in the freezer. Compound 4b was
isolated as a light yellow solid (0.16 g, 77% yield). 1H NMR
tained from the University of Illinois SCS Mass Spectrometry Lab- (400 MHz, CDCl3, 25 °C): δ = 1.09 ppm. 13C NMR (100 MHz,
oratory.
CDCl3, 25 °C): δ = 89.6, 85.6, 62.2, 61.4, 18.5, 11.3 ppm.[1]
Safety Statement: The diiodopolyynes, while more stable than analo-
gous terminal polyynes or dibromopolyynes, still must be handled
with caution. These compounds are shock explosives and have been
observed to become less stable over time. They should therefore be
stored in solution below 0 °C and should be used within a week of
preparation. Caution should also be taken when preparing and hand-
ling the tin acetylides, as trimethyltin chloride and analogous or-
ganotin compounds have been declared toxic by all means of expo-
sure.[20]
1,8-Bis(trimethylsilyl)-1,3,5,7-octatetrayne (4c): Diiodobutadiyne
(2a; 0.239 g, 0.792 mmol) and trimethylsilyl(trimethylstannyl)eth-
yne (1c; 0.414 g, 1.58 mmol) were dissolved in dry THF (20 mL) in
a heart-shaped flask wrapped in aluminum foil. The flask was
placed in an ice bath (0 °C). CuI (0.036 g, 0.19 mmol, 24 mol-%)
and Pd(PPh3)2Cl2 (0.065 g, 0.093 mmol, 12 mol-%) were combined
in a round-bottom flask wrapped with foil, and the system was
degassed and backfilled with Ar. THF (5 mL) was added to the
flask, and the solution of 2a and 1c was added to the catalyst mix-
ture via cannula over a period of 10 h. The mixture was stirred for
2 h after the addition, solvent was removed in vacuo, and the excess
amount of the catalyst was removed using a short plug (SiO2/hex-
anes). Vacuum sublimation was used to isolate 4c as a brown oil
that contained solid needles (0.11 g, 59% yield). 1H NMR
(500 MHz, CDCl3, 25 °C): δ = 0.21 ppm. 13C NMR (125 MHz,
CDCl3, 25 °C): δ = 88.01, 87.83, 62.16, 62.14, –0.57 ppm.[21]
Diiodobutadiyne (2a), diiodohexatriyne (3a), and diiodooctatet-
rayne (4a) were each prepared from the corresponding trimethyl-
silyl-capped polyynes (that is, 2c, 3c, and 4c, respectively) according
to previously described methods.[19b,21] 2,4-Hexadiyne-1,6-diol (9)
and 1,6-dibromo-2,4-hexadiyne (10) were also prepared using pre-
viously reported methods.[15b,15c]
General Procedure for the Preparation of Tin Acetylides[13b]
1,12-Bis(triisopropylsilyl)-1,3,5,7,9,11-dodecahexayne (6b): In
a
Triisopropylsilyl(trimethylstannyl)ethyne (1b): Triisopropylsilyl
acetylene (4.50 g, 25.0 mmol) was dissolved in THF (50 mL), and
the solution was cooled to –30 °C using a dry ice/acetone bath.
nBuLi (1.6 m in hexanes, 15.0 mL, 25.0 mmol) was added dropwise
over 15 min. After stirring for 15 min at –30 °C, SnMe3Cl (1 m in
THF, 25.0 mL, 25.0 mmol) was added, and the mixture was stirred
for an additional 30 min before warming to room temperature and
stirring for 1 h. Hexanes (100 mL) was added to the reaction mix-
ture, resulting in the formation of a white precipitate. The mixture
was extracted with H2O (25 mL), and the organic layer was dried
with MgSO4. The solution was filtered, and the solvent was re-
moved in vacuo, leaving a cloudy yellow liquid. Vacuum distillation
(≈90 °C) was used to isolate 1b as a colorless liquid (5.50 g, 64%
yield). 1H NMR (400 MHz, CDCl3, 25 °C): δ = 1.07 (br., 21 H),
0.28 (s, 9 H) ppm. 13C NMR (100 MHz, CDCl3, 25 °C): δ = 115.1,
113.5, 18.6, 11.2, –7.6 ppm.[22]
heart-shaped flask wrapped in aluminum foil, diiodotetrayne (4a;
0.150 g, 0.430 mmol) and triisopropylsilyl(trimethylstannyl)ethyne
(1b; 0.57 g, 1.6 mmol) were dissolved in dry THF (20 mL). CuI
(0.010 g, 0.053 mmol, 13 mol-%), and Pd(PPh3)2Cl2 (0.035 g,
0.05 mmol, 12 mol-%) were combined in a round-bottom flask
wrapped with foil, and the system was degassed and backfilled with
Ar. THF (5 mL) was added to the flask, and both flasks were kept
in ice baths (0 °C). The solution of 4a and 1b was added to the
catalyst mixture via cannula, over a period of 10 h. After addition,
the mixture was stirred for 2 h at room temperature, and the solvent
was removed in vacuo. Column chromatography (SiO2/hexanes)
was used to isolate 6b as a light yellow solid (0.092 g, 47% yield).
1H NMR (500 MHz, CDCl3, 25 °C): δ = 1.08 ppm. 13C NMR
(125 MHz, CDCl3, 25 °C): δ = 89.40, 87.03, 62.71, 62.66, 62.36,
61.26, 18.56, 11.29 ppm.[1]
1,12-Bis(diphenyl)-1,3,5,7,9,11-dodecahexayne (6d): In
a heart-
Trimethylsilyl(trimethylstannyl)ethyne (1c): Colorless liquid (79%
shaped flask wrapped in aluminum foil, diiodotetrayne (4a; 0.132 g,
0.377 mmol) and phenyl(trimetylstannyl)ethyne (1d; 0.194 g,
0.732 mmol) were dissolved in dry THF (20 mL). CuI (0.018 g,
0.095 mmol, 25 mol-%) and Pd(PPh3)2Cl2 (0.032 g, 0.046 mmol,
12 mol-%) were combined in a round-bottom flask wrapped with
foil, and the system was degassed and backfilled with Ar. THF
(5 mL) was added to the flask, and both flasks were kept in ice
baths (0 °C). The solution of 4a and 1d was added to the catalyst
mixture via cannula, over a period of 11 h. After addition, the mix-
ture was stirred for 2 h at room temperature, and solvent was re-
moved in vacuo. The excess amount of the catalyst was removed
using a short plug (SiO2/hexanes). Column chromatography (SiO2/
hexanes) was used to isolate 6d as a light orange solid (0.018 g,
16% yield). 1H NMR (500 MHz, CDCl3, 25 °C): δ = 7.56–7.54
(m, 4 H), 7.42–7.41 (m, 2 H), 7.36–7.33 (m, 4 H) ppm. 13C NMR
(125 MHz, CDCl3, 25 °C): δ = 133.5, 130.4, 128.5, 120.1, 77.5, 74.3,
67.2, 64.6, 63.6, 62.5 ppm.[25]
1
yield). H NMR (400 MHz, CDCl3, 25 °C): δ = 0.27 (s, 9 H), 0.15
(s, 9 H) ppm. 13C NMR (100 MHz, CDCl3, 25 °C): δ = 117.4,
113.1, 0.2, –7.7 ppm.[23]
1
Phenyl(trimethylstannyl)ethyne (1d): Yellow liquid (55% yield). H
NMR (400 MHz, CDCl3, 25 °C): δ = 7.45–7.44 (m, 2 H), 7.26–7.25
(m, 3 H), 0.35 (s, 9 H) ppm. 13C NMR (100 MHz, CDCl3, 25 °C):
δ = 131.8, 128.1, 128.0, 123.5, 108.9, 93.2, –7.7 ppm.[24]
1,8-Bis(triisopropylsilyl)-1,3,5,7-octatetrayne (4b): Diiodobutadiyne
(2a; 0.150 g, 0.500 mmol) and triisopropylsilyl(trimethylstannyl)-
ethyne (1b; 0.370 g, 1.07 mmol) were dissolved in dry THF (15 mL)
in a heart-shaped flask wrapped in aluminum foil. CuI (0.019 g,
0.10 mmol, 20 mol-%) and Pd(PPh3)2Cl2 (0.035 g, 0.050 mmol,
10 mol-%) were combined in a round-bottom flask wrapped with
foil, and the system was degassed and backfilled with Ar. THF
(8 mL) was added to the flask. The solution of 2a and 1b was added
to the catalyst mixture via cannula over 10 h. After addition, the
mixture was stirred for 2 h, and the solvent was removed. The resi-
due was purified by column chromatography (Al2O3/hexanes). To
recrystallize 4b, the mixture of 4b and 1,4-bis(triisopropylsilyl)-1,3-
butadiyne (2b) was dissolved in a minimum amount of warm
CHCl3 ≈45 °C, causing a white precipitate to appear. Upon stirring
1,6-Bis(trimethylsilyl)-1,3,5-hexatriyne (3c): Potassium tert-but-
oxide (0.551 g, 4.91 mmol) was dissolved in THF (20 mL), and this
solution was cooled to –78 °C using a dry ice/acetone bath. A solu-
tion of 1,6-dibromo-2,4-hexadiyne (2g; 0.290 g, 1.23 mmol) in THF
(20 mL) was added to the reaction mixture via cannula. After stir-
4
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An Iterative Method for the Synthesis of Symmetric Polyynes
ring for 2 h, the mixture reached –60 °C, and trimethylsilyl chloride
(0.63 mL, 4.9 mmol) was added. The reaction was allowed to stir
for 3.5 h, without further cooling. Saturated aq. NH4Cl (30 mL)
was added, and the mixture was extracted with hexanes (3ϫ
30 mL). Removal of the solvent resulted in 3c as a mixture of
brown oil and short needle-like crystals (0.17 g, 63% yield). 1H
1,10-Diiodo-1,3,5,7,9-decapentayne (5a): 1,10-Bis(trimethylsilyl)-
1,3,5,7,9-decapentayne (5c; 0.096 g, 0.36 mmol) was dissolved in
acetone (200 mL) in a round-bottom flask wrapped with aluminum
foil. AgNO3 (0.186 g, 1.09 mmol) and N-iodosuccinimide (0.488 g,
2.17 mmol) were added, and the reaction mixture was allowed to
stir at room temperature for 4.5 h. Ice water (200 mL) and hexanes
NMR (400 MHz, CDCl3, 25 °C): δ = 0.20 ppm. 13C NMR (200 mL) were added, and the aqueous layer was extracted with
(100 MHz, CDCl3, 25 °C): δ = 87.9, 87.4, 61.9, –0.6 ppm.[7]
hexanes (3ϫ 100 mL). The combined organic layers were washed
with 15% aq. Na2S2O3. The resulting yellow solution was dried
with MgSO4. Filtration and removal of solvent resulted in 5a as a
yellow solid that decomposed at 55–56 °C (0.11 g, 82% yield). 13C
NMR (100 MHz, CDCl3, 25 °C): δ = 79.0, 62.6, 61.4, 59.3,
2.3 ppm. MS (EI): m/z (%): 373 (100) [M]+, 246 (18) [M – I]+, 128
(60) [I]+, 119 (42) [M – 2I]+. HRMS (EI): calcd. for C10I2
373.80900; found 373.80923.
1,10-Bis(triisopropylsilyl)-1,3,5,7,9-decapentayne (5b): In a heart-
shaped flask wrapped in aluminum foil, diiodohexatriyne (3a;
0.123 g, 0.377 mmol) and triisopropylsilyl(trimethylstannyl)ethyne
(1b; 0.253 g, 0.734 mmol) were dissolved in dry THF (20 mL). CuI
(0.016 g, 0.084 mmol, 22 mol-%) and Pd(PPh3)2Cl2 (0.035 g,
0.050 mmol, 13 mol-%) were combined in a round-bottom flask
wrapped with foil, and the system was degassed and backfilled with
Ar. THF (5 mL) was added to this flask, and both flasks were kept
in ice baths (0 °C). The solution of 3a and 1b was added to the
catalyst mixture via cannula over a period of 9 h. After stirring for
an additional 2 h at room temperature, the solvent was removed in
vacuo, and the excess amount of the catalyst was removed using a
short plug (SiO2/hexanes). Warm MeOH/CHCl3 (10:1) was used
for recrystallization. The mixture was placed in a 45 °C water bath,
and 3 n HCl was added dropwise until a visible yellow suspension
formed. The liquid was cooled to room temperature and placed in
the freezer. Vacuum filtration was used to isolate 5b as a yellow
1,14-Bis(triisopropylsilyl)-1,3,5,7,9,11,13-tetradecaheptayne (7b): In
a heart-shaped flask wrapped in aluminum foil, diiodopentayne
(5a; 0.056 g, 0.15 mmol) and triisopropylsilyl(trimethylstannyl)eth-
yne (1b; 0.10 g, 0.29 mmol) were dissolved in dry THF (20 mL).
CuI (0.007 g, 0.037 mmol, 24 mol-%) and Pd(PPh3)2Cl2 (0.013 g,
0.018 mmol, 12 mol-%) were combined in a round-bottom flask
wrapped with foil, and the system was degassed and backfilled with
Ar. THF (5 mL) was added to the flask, and both flasks were kept
in ice baths (the diiodopentayne and tin solution was kept between
–15 and –30 °C, and the catalyst mixture was kept at 0 °C). The
solution of 5a and 1b was added to the catalyst mixture via cannula
over a period of 10 h. After stirring for an additional 2 h at room
temperature, the solvent was removed in vacuo and the excess
amount of the catalyst was removed using a short plug (SiO2/hex-
anes). Column chromatography (SiO2/hexanes) was used to isolate
7b as dark yellow oil (0.007 g, 10% yield). 1H NMR (500 MHz,
CDCl3, 25 °C): δ = 1.09 ppm. 13C NMR (125 MHz, CDCl3, 25 °C):
1
solid (0.065 g, 40% yield). H NMR (400 MHz, C6D6, 25 °C): δ =
0.99 ppm. 13C NMR (100 MHz, C6D6, 25 °C): δ = 90.65, 87.18,
63.42, 63.41, 62.49, 18.95, 11.87 ppm.[7]
1,10-Bis(trimethylsilyl)-1,3,5,7,9-decapentayne (5c): In
a heart-
shaped flask wrapped in aluminum foil, diiodohexatriyne (3a;
0.113 g, 0.347 mmol) and trimethylsilyl[(trimethylstannyl)ethynyl]-
ethyne (1c; 0.179 g, 0.686 mmol) were dissolved in dry THF
(20 mL). CuI (0.015 g, 0.079 mmol, 23 mol-%) and Pd(PPh3)2Cl2
(0.028 g, 0.041 mmol, 12 mol-%) were combined in a round-bottom
flask wrapped with foil, and the system was degassed and back-
filled with Ar. THF (5 mL) was added to the flask, and both flasks
were kept in ice baths (0 °C). The solution of 3a and 1c was added
to the catalyst mixture via cannula over a period of 10 h. After
stirring for an additional 2 h at room temperature, the solvent was
removed in vacuo and the excess amount of the catalyst was re-
moved using a short plug (SiO2/hexanes). Vacuum sublimation was
δ
= 89.34, 87.39, 62.97, 62.93, 62.62, 62.26, 61.20, 18.49,
11.27 ppm. HRMS (EI): calcd. for C32H42Si2 482.28251; found
482.28314.
Supporting Information (see footnote on the first page of this arti-
1
cle): Copies of the H NMR and 13C NMR for the prepared com-
pounds and HRMS for novel compounds.
Acknowledgments
1
used to isolate 5c as a brown solid (0.060 g, 61% yield). H NMR
This work was supported by the National Science Foundation
(CHE-0446749 and CHE-0911540). We thank Liang Luo and Da-
vid Connors for helpful discussions.
(400 MHz, CDCl3, 25 °C): δ = 0.21 ppm. 13C NMR (100 MHz,
CDCl3, 25 °C): δ = 88.63, 87.71, 62.62, 62.23, 62.17, –0.65 ppm.[26]
1,10-Bis(phenyl)-1,3,5,7,9-decapentayne (5d): In
a heart-shaped
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flask wrapped in aluminum foil, diiodohexatriyne (3a; 0.198 g,
0.608 mmol) and phenyl(trimethylstannyl)ethyne (1d; 0.321 g,
1.21 mmol) were dissolved in dry THF (20 mL). CuI (0.026 g,
0.14 mmol, 22 mol-%) and Pd(PPh3)2Cl2 (0.051 g, 0.073 mmol,
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ditional 2 h at room temperature, the solvent was removed in vacuo
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NMR (400 MHz, CDCl3, 25 °C): δ = 7.56–7.54 (m, 4 H), 7.44–7.40
(m, 2 H), 7.36–7.32, (m, 4 H) ppm. 13C NMR (100 MHz, CDCl3,
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62.8 ppm.[7]
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/KAP1
Date: 09-07-12 15:51:51
Pages: 7
R. C. DeCicco, A. Black, L. Li, N. S. Goroff
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Received: April 5, 2012
Published Online: ᭿
6
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 0000, 0–0

Job/Unit: O20442
/KAP1
Date: 09-07-12 15:51:51
Pages: 7
An Iterative Method for the Synthesis of Symmetric Polyynes
Symmetric Polyynes
R. C. DeCicco, A. Black, L. Li,
N. S. Goroff* .................................... 1–7
Symmetric polyynes containing four to
seven CϵC bonds were prepared by using
a series of iodination and Stille coupling re-
actions. The simple starting materials that
are required in this iterative route are easily
prepared and relatively stable. This route is
especially effective when targeting polyynes
with an odd number of CϵC bonds.
An Iterative Method for the Synthesis of
Symmetric Polyynes
Keywords: Alkynes / Cross-coupling / Syn-
thetic methods / Palladium
Eur. J. Org. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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7