β-Chlorovinylsilanes as masked alkynes in oligoyne assembly: Synthesis of the first aryl-end-capped dodecayne

Article abstract of DOI:10.1039/b707681a

An aryl-end-capped dodecayne has been prepared using a four-fold fluoride-mediated dechlorosilylation of a masked dodecayne precursor containing four β-chlorovinylsilane residues that serve as masked alkynes; the unstable dodecayne product has been charac

Full text of DOI:10.1039/b707681a

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b-Chlorovinylsilanes as masked alkynes in oligoyne assembly: synthesis
of the first aryl-end-capped dodecayne{
Simon M. E. Simpkins, Michael D. Weller and Liam R. Cox*
Received (in Cambridge, UK) 22nd May 2007, Accepted 15th August 2007
First published as an Advance Article on the web 23rd August 2007
DOI: 10.1039/b707681a
(Scheme 1).12 Drawing analogy with approaches, which have
An aryl-end-capped dodecayne has been prepared using a four-
proved successful in the preparation of other non-aromatic,
p-conjugated organic oligomers,13 we initially considered enediyne
1, in which the b-chlorovinylsilane masks a third alkyne, as a
potential monomer for oligoyne assembly. However we argued
that the unsymmetrical nature of this building block would
necessarily lead to the formation of each masked oligoyne in the
series as a mixture of constitutional isomers through head-to-head
and head-to-tail couplings, something which would hinder
compound characterisation of the intermediate oligo(enediyne)
precursors, which are interesting targets in themselves.
Differentially substituting the alkyne termini in 1, and dimerising
to a centrosymmetric masked hexayne 2, would neatly circumvent
this problem and provide a larger monomer unit, which would
also allow a more rapid method for long-chain oligoyne assembly.
To this end, we recently reported a completely selective synthesis of
masked triyne 3 (Scheme 2).14 Our method allows the incorpora-
tion of a range of silyl groups into the internal (E)-b-chlorovinyl-
silane, and significantly, the differentiation of the two alkyne
termini. Masked triyne 3 provides our starting point for oligoyne
assembly. Herein we report its application in the synthesis of an
aryl-end-capped dodecayne, which represents the longest aryl-end-
capped oligoyne reported to-date.
fold fluoride-mediated dechlorosilylation of a masked dode-
cayne precursor containing four b-chlorovinylsilane residues
that serve as masked alkynes; the unstable dodecayne product
has been characterised by UV-vis absorption spectroscopy and
‘matrix-free’ MALDI-TOF mass spectrometry.
Poly(alk(2n + 1)yne), also known as carbyne (Fig. 1), represents
the fourth allotrope of carbon, after diamond, graphite and the
fullerenes.1,2 The regular alternating sequence of single bonds and
triple bonds in this polymer lends itself to an iterative synthesis
involving oxidative coupling of alkyne fragments; however, a
controlled synthesis of this form of carbon using this—or any
other—approach is beset with problems associated with its relative
instability and propensity to react further, especially in the solid
state.1,2
Oligoynes represent clipped versions of carbyne and are a more
tractable synthetic target.3–6 That said, the preparation of systems
beyond the hexayne still remains a challenging synthetic problem.
Of the various approaches to oligoyne assembly, which have been
disclosed, most rely on oxidative acetylenic coupling reactions to
construct the oligoyne framework.3,4,6,7 Whilst this strategy has
met with appreciable success, especially for systems that can be
stabilised by sterically demanding end-capping groups,4,5,8 in
particular metal-based units,3,9 this approach becomes increasingly
unsatisfactory as the length of the conjugated oligoyne increases.
For this reason, a number of alternative approaches have been
reported, which obviate the need to employ free oligoynes of any
length in oxidative acetylenic coupling.10 The work of Tykwinski
and co-workers, who have pioneered the use of the Fritsch–
Buttenberg–Wiechell reaction in oligoyne assembly, is particularly
noteworthy.5,11
Masked triyne 3 was prepared according to our established
methodology in nine steps and 52% overall yield from TMS-
acetylene.14 Dimerisation to the corresponding masked hexayne 4
using the standard Eglinton–Glaser–Hay oxidative coupling
conditions,7 however, provided the desired product in ,10%
yield. A range of Pd-mediated acetylenic coupling methods were
also investigated,15 although none offered any significant improve-
ment. Returning to the more commonly used copper-mediated
We postulated that b-chlorovinylsilanes would offer an
attractive alternative alkyne masking strategy for oligoyne
assembly, as the desired product should be accessible in a selective
manner from its protected precursor through the action of fluoride
Fig. 1 Carbyne represents a fourth allotrope of carbon.
School of Chemistry, University of Birmingham, Edgbaston,
Birmingham, UK B15 2TT. E-mail: l.r.cox@bham.ac.uk;
Fax: +44 (0)121 414 4403; Tel: +44 (0)121 414 3524
{ Electronic supplementary information (ESI) available: General experi-
mental, characterisation data, and b-chlorovinylsilanes in the synthesis of
lower order oligoynes. Estimation of the effective conjugation length for
aryl end-capped oligoynes. Scanned spectra. See DOI: 10.1039/b707681a
Scheme 1 b-Chlorovinylsilanes as masked alkynes in oligoyne assembly.
Chem. Commun., 2007, 4035–4037 | 4035
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Scheme 2 Synthesis of dodecayne 9.
coupling methodology, careful analysis of the reaction mixture by
mass spectrometry revealed dimerisation was indeed proceeding;
however the low yield of the desired hexayne 4 was a consequence
of partial premature dechlorosilylation. We hypothesised that the
relatively nucleophilic halide anion present in the reaction mixture
(CuCl and CuI are the most common sources of copper in this
type of coupling reaction) was responsible for effecting this
elimination side-reaction, and therefore a range of alternative
copper sources was investigated.{ Gratifyingly, Cu(OTf)2, which
possesses a very weakly nucleophilic triflate counteranion, effici-
ently circumvented the problem of premature dechlorosilylation
and now permitted the masked hexayne 4 to be obtained in a
greatly improved 66% isolated yield.
futile. Fortunately, as the UV-vis spectra of oligoynes are
particularly characteristic in appearance,4,11,17 and very different
from the spectrum of the masked dodecayne precursor (Fig. 2), we
were able to monitor the unmasking reaction and characterise the
oligoyne product by UV-vis spectroscopy. After careful experi-
mentation with varying quantities of TBAF, we found that
treating a solution of masked dodecayne 8 with 4 equiv. of TBAF
at room temperature, led to the formation of the dodecayne 9 over
a period of a few min. Unfortunately, even at the low
concentrations (1026 M) required for recording UV-vis spectra,
extensive decomposition of the product was evident as the UV
spectrum of dodecayne 9 decayed when the solution was assayed
after 1 h; in spite of the instability of the dodecayne we were pleased
to observe the molecular ion for the product by MALDI-TOF
With a centrosymmetric masked hexayne 4 in hand, deprotec-
tion of one of the trimethylsilyl end-caps was best achieved under
the very mild conditions afforded by AgNO3 in THF–H2O.16 The
use of K2CO3 in THF–MeOH was also effective for trimethylsilyl
deprotection; however, the rapidity of reaction meant that it
proved difficult to isolate the mono-deprotected product in
significant quantities. Monodeprotected masked hexayne 5 was
not particularly stable and hence was used directly without
purification in
a
Sonogashira coupling with 1-iodo-3,5-
bis(trifluoromethyl)benzene to provide mono-aryl-end-capped
masked hexayne 6. Usefully, the CF3 groups in this aryl end-cap
allowed the purity of the coupled product 6, and all subsequent
compounds in the sequence, to be assessed by 19F NMR.§.
Removal of the remaining TMS group in 6 with K2CO3 in THF–
MeOH, and immediate oxidative coupling of the free alkyne
intermediate 7 under our modified Eglinton–Glaser–Hay coupling
conditions, afforded masked dodecayne 8 in good yield. Four-fold
dechlorosilylation with TBAF in THF effected rapid reaction;
however all attempts to isolate the oligoyne product 9 proved
Fig. 2 UV-vis spectrum of masked dodecayne 8 (dashed line) and
dodecayne 9. Inset shows expansion of region II of spectrum for 9
highlighting the low intensity, long wavelength absorptions.
4036 | Chem. Commun., 2007, 4035–4037
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1 S. Szafert and J. A. Gladysz, Chem. Rev., 2006, 106, PR1; S. Szafert and
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analysis of the reaction mixture. Interestingly, the best results were
observed in the absence of a matrix (see ESI{).
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The UV-vis data for the masked dodecayne 8 and the four-fold
dechlorosilylation dodecayne product 9 are shown in Fig. 2." The
differences in the two spectra are immediately apparent with that
for the dodecayne product showing the typical p–p* absorptions of
oligoynes. In the region between 330 and 405 nm (region I), several
vibrational bands, exhibiting increasing intensity with increasing
wavelength are clearly observable, reaching a maximum value of
e = 295000 M21 cm21 for the most intense band at 405 nm. At
longer wavelengths (region II), a number of much weaker
(e ,6000 M21 cm21) absorption bands are present (see inset in
Fig. 2). These low-intensity bands are characteristic of aryl-end-
capped oligoynes.4,11,17 The value of 562 nm for lmax is higher than
the values reported by Hirsch,18 and Walton,19 for shorter aryl
end-capped decaynes. However, it remains to be seen as to whether
saturation has been reached for this class of oligoyne.I To probe
whether this is the case, the octadecayne trimer will be our next
target.
9 R. Dembinski, T. Bartik, B. Bartik, M. Jaeger and J. A. Gladysz, J. Am.
Chem. Soc., 2000, 122, 810.
Dodecayne 9 represents the longest oligoyne possessing aryl
termini reported to-date. Although it may be possible to improve
the stability of the dodecayne by increasing the steric bulk of the
aryl end-caps, this mode of stabilisation alone is unlikely to
provide sufficient stability for the trimer in the series, namely the
octadecayne. Instead, we are currently investigating molecular
encapsulation methods, which will provide insulation along the
entire length of the conjugated framework,20,21 rather than relying
solely on stabilising groups at the oligoyne termini.
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In summary, the synthesis of an aryl-end-capped dodecayne has
been achieved for the first time, in a controlled fashion from a
masked hexayne building block in which two of the internal
alkynes are protected as b-chlorovinylsilanes. This novel approach
to oligoyne assembly differs from the majority of previous
strategies: by using such a large monomer unit, only one oxidative
acetylenic coupling is required to access what is the longest aryl
end-capped oligoyne reported to-date. Furthermore, our approach
does not rely on oxidative coupling of long-chain terminal
oligoynes to afford the end-product; rather, the entire carbon
chain, 24 carbons in the case of dodecayne 9, is already installed
before the oligoyne is released, which prevents the formation of
shorter oligoynes resulting from loss of acetylene fragments that is
sometimes observed when traditional approaches are taken.4,5
We acknowledge The Leverhulme Trust (grant ref. F/00 094/T)
and the EPSRC (grant ref. EP/C532260/1) for post-doctoral
awards to S. M. E. S. and the University of Birmingham
(studentship to M. D. W.).
14 S. M. E. Simpkins, B. M. Kariuki, C. S. Arico´ and L. R. Cox, Org.
Lett., 2003, 5, 3971.
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J. B. Armitage, N. Entwistle, E. R. H. Jones and M. C. Whiting,
J. Chem. Soc., 1954, 147.
18 lmax for a dendronised aryl end-capped decayne is 546 nm: see
ref. 4.
19 lmax for a phenyl end-capped decayne is 549 nm; see ref. 17.
20 S. M. E. Simpkins, B. M. Kariuki and L. R. Cox, J. Organomet. Chem.,
2006, 691, 5517.
Notes and references
{ Cu(OAc)2, Cu(NO3)2, CuI, CuCl, CuCl2, CuBr, CuBr2 and Cu(OTf)2
were investigated.
21 See also: C. Klinger, O. Vostirowsky and A. Hirsch, Eur. J. Org. Chem.,
2006, 1508; J. Stahl, J. C. Bohling, E. B. Bauer, T. B. Peters, W. Mohr,
J. M. Martin-Alvarez, F. Hampel and J. A. Gladysz, Angew. Chem., Int.
Ed., 2002, 41, 1872.
§ See ESI{ for full characterisation data.
" See ESI for a full breakdown of absorption peaks and their associated
molar extinction coefficients.{
I See ESI for a more detailed analysis.{
This journal is ß The Royal Society of Chemistry 2007
Chem. Commun., 2007, 4035–4037 | 4037