β-Halovinylsilanes in oligoyne synthesis: a fluoride-catalysed unmasking of alkynes from β-fluorovinylsilanes

Article abstract of DOI:10.1016/j.tetlet.2008.05.081

A stereoselective halodestannylation of (E)-β-stannylvinylsilane 6 has been used to access a range of (E)-β-halovinylsilanes. Most notably, the β-fluorovinylsilane 14 was accessed in high yield using the Selectfluor reagent, and subsequently converted into a masked hexayne 22. Release of the hexayne 25 was achieved by exposing 22 to sub-stoichiometric quantities of fluoride.

Full text of DOI:10.1016/j.tetlet.2008.05.081

Tetrahedron Letters 49 (2008) 4596–4600
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Tetrahedron Letters
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b-Halovinylsilanes in oligoyne synthesis: a fluoride-catalysed unmasking
of alkynes from b-fluorovinylsilanes
*
Michael D. Weller, Benson M. Kariuki , Liam R. Cox
School of Chemistry, The University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
A stereoselective halodestannylation of (E)-b-stannylvinylsilane 6 has been used to access a range of (E)-
b-halovinylsilanes. Most notably, the b-fluorovinylsilane 14 was accessed in high yield using the Select-
fluorÒ reagent, and subsequently converted into a masked hexayne 22. Release of the hexayne 25 was
achieved by exposing 22 to sub-stoichiometric quantities of fluoride.
Received 2 May 2008
Accepted 19 May 2008
Available online 23 May 2008
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Vinylsilanes
Oligoynes
Polyynes
b-Elimination
Tin–halogen exchange
Oligoynes constitute an important class of highly p-conjugated
SnMe3
Me3SnCu•SMe2•LiBr
THF
OSiR3
organic molecule owing to their interesting physical properties and
potential applications in advanced materials.1 Whilst the Eglinton–
Glaser–Hay oxidative coupling2 of terminal alkynes provides the
most established methodology for assembling this type of conju-
gated scaffold,3 the instability of the terminal oligoynes, which
are required as intermediates in this approach, has encouraged
the development of other methodologies for oligoyne synthesis.4–8
These alternative strategies generally involve the oligoyne being
assembled first in a protected form. Unfortunately, many of these
approaches are unsuitable for long-chain oligoyne assembly be-
cause the product is not particularly stable under the unmasking
conditions.
Elimination strategies have been used widely for preparing al-
kynes,9 although only rarely for synthesising oligoynes longer than
the tetrayne.10 We postulated that b-halovinylsilanes would be
attractive alkyne precursors for oligoyne synthesis. Specifically,
we envisaged a masked oligoyne containing one or more b-halovi-
nylsilanes would undergo a highly chemoselective dehalosilylation
reaction in the presence of fluoride. Significantly, we hoped this
elimination approach would provide the corresponding oligoyne
under sufficiently mild conditions which would leave the final
product intact.11 To this end, we have developed an efficient and
flexible synthesis of a masked triyne 5 in which the internal alky-
nyl unit is protected as a (E)-b-chlorovinylsilane (Scheme 1).12 Key
R'
OSiR3
SnMe3
2
R'
R'
1
nBuLi
SiR3
SiR3
CuCl2
OH
OH
SnMe3
3
THF
Cl
R'
R'
i) MnO2
ii) CBr4,
4
PPh3
iii) LDA
R' = Me3Si, iPr3Si,
or Ar
SiR3
H
Cl
5
SiR3 = TES, TBDPS, TIPS
and SiArEt2
Scheme 1. Summary of our established route to a masked triyne containing an
internal (E)-b-chlorovinylsilane.
steps in the synthesis of 5 include (i) a completely (E)-stereo- and
regioselective Cu-mediated bis-stannylation of the unsymmetri-
cally substituted internal buta-1,3-diyne 1 to afford bis-stannane
2; (ii) a regioselective tin–lithium exchange on 2, which initiates
a 1,4-retro Brook rearrangement to install the vinylsilane in 3;
and (iii) a stereoselective chlorodestannylation of 3 to introduce
the vinyl chloride motif in 4, which can then be elaborated to
masked triyne 5 in three straightforward steps. We have used this
methodology to prepare long-chain oligoynes including the first
aryl end-capped dodecayne.13
* Corresponding author. Tel.: +44 (0)121 414 3524; fax: +44 (0)121 414 4403.
E-mail address: l.r.cox@bham.ac.uk (L. R. Cox).
Present address: School of Chemistry, Cardiff University, Main Building, Park
Place, Cardiff CF10 3AT, United Kingdom.
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.05.081

M. D. Weller et al. / Tetrahedron Letters 49 (2008) 4596–4600
4597
SitBuPh2
OH
Our synthetic strategy allows a range of end-capping groups
SitBuPh2
MnO2
and silyl substituents to be introduced into masked triyne 5. The
vinylstannane 3 potentially also allows the halogen in the internal
masked alkyne to be varied. To-date, however, we have focused
exclusively on b-chlorovinylsilanes, rather than other halogens,
since we thought that this motif would provide the right level of
reactivity to ensure that the unit is not prone to premature dechlo-
rosilylation but can still be effected when desired under mild con-
ditions upon treatment with fluoride.11 We recently postulated
that changing the chloro substituent in 5 for other halogen leaving
groups might impart useful properties on the masked oligoyne
products. We were particularly keen to investigate whether the
corresponding b-fluorovinylsilanes could be accessed, since b-
elimination might proceed under fluoride catalysis on account of
a fluoride anion being regenerated as the by-product in the elimi-
nation step. Although fluoride is only rarely used as a leaving group
in elimination reactions,14 and b-fluorovinylsilanes have not been
used in elimination routes to alkynes, we envisaged that if success-
ful, a fluoride-catalysed elimination would represent an exception-
ally mild and chemoselective unmasking strategy for oligoyne
synthesis. Herein, we report the synthesis and chemical behaviour
of the iodo-, bromo- and fluoro-analogues of vinyl chloride 4 and
show that b-fluorovinylsilanes are indeed very attractive alterna-
tives to b-chlorovinylsilanes for oligoyne synthesis.
O
CH2Cl2
0 ºC to rt
Me3Si
Br
Me3Si
Br
(E)-7
9 81%
CBr4, PPh3
CH2Cl2, 0 ºC
t
Ph2 BuSi
H
SitBuPh2
6 equiv LDA
Br
THF
-78 ºC
Me3Si
Br
11 quant
Me3Si
Br Br
10 78%
Scheme 3. Conversion of b-bromovinylsilane (E)-7 to the masked triyne 11.
A crystal structure of tribromide 10 confirmed that the bromodest-
annylation of 6 had proceeded with retention of configuration (Fig.
1).21 Treatment of 10 with an excess of LDA at À78 °C effected a
Fritsch–Buttenberg–Wiechell reaction to provide triyne 11 after
aqueous work-up. Triyne 11 was used directly in the next step
and exposed to our modified Hay coupling conditions in which
we employ a copper salt (Cu(OTf)2) which possesses a non-nucle-
ophilic counteranion to minimise premature dehalosilylation.12b,13
Unfortunately, in spite of taking this precautionary measure, triyne
11 decomposed rapidly under the reaction conditions. 1H NMR
analysis of the crude product upon work-up showed only signals
corresponding to the tert-butyldiphenylsilyl group, which was
most likely present as its corresponding silanol. The b-bromovin-
ylsilane moiety in triyne 11 is clearly much more labile than its
chloro analogue under the oxidative coupling conditions. In light
of these results, we elected not to pursue any further derivatives
of vinyl iodide 8, as we expected these would exhibit reduced sta-
bility; however, we note that vinyl iodides are excellent substrates
in metal-catalysed cross-coupling processes, and this substrate
class might have application in the preparation of heavily substi-
tuted olefins,22 since the residual vinylsilane can also serve as a
masked vinyl halide to allow further elaboration.23
Vinylstannane 3 serves as our starting material for the synthesis
of a range of b-halovinylsilanes. Ordinarily we treat 3 with CuCl2 to
provide vinyl chloride 4 in excellent yield and complete (E)-stereo-
selectivity.12 In contrast, when vinylstannane
6 was treated
with CuBr2 under similar conditions,15 the desired b-bromovinyl-
silane 7 was obtained in quantitative yield, but as a 1:1 mixture
of (E)/(Z) stereoisomers (Scheme 2).16 Much better results were
obtained using N-bromosuccinimide (NBS): addition of NBS to a
solution of 6 in CH2Cl2 at À78 °C, and warming to room tempera-
ture, provided vinyl bromide 7,17 again in excellent yield, but
now as a single stereoisomer, which was later confirmed to have
the (E)-geometry (vide infra).18 The formation of the corresponding
vinyl iodide 8 was achieved by treating 6 with molecular iodine
(Scheme 2).19,20 Although we were unable to confirm the stereo-
chemistry of the olefin in 8, we tentatively assign an (E)-configura-
tion by analogy with the examples of this reaction elsewhere in the
literature.19
We next turned our attention to the more attractive b-fluorovi-
nylsilane analogues. Although fluorine atoms have been incorpo-
rated into a diverse range of organic molecules, there are very
few examples of b-fluorovinylsilanes in the literature.24–27 Our
desired approach, in which we sought to effect fluorodestannylation
of vinylstannane 6 has literature precedent. Fluorodestannylation
b-Bromovinylsilane (E)-7 was elaborated to the masked triyne
11 using the same reaction sequence as that used to access the cor-
responding b-chlorovinylsilanes (Scheme 3). Thus, allylic oxidation
of (E)-7 with MnO2 provided aldehyde 9, and subsequent
dibromoolefination gave tribromide 10 in good yield (Scheme 3).
SitBuPh2
SitBuPh2
2.2 equiv CuBr2
OH
SnMe3
OH
THF
0 ºC to rt, 14 h
Me3Si
Me3Si
Br
7 quant.
(E):(Z) 1:1
6
SitBuPh2
1.1 equiv NBS
6
6
OH
Br
CH2Cl2
-78 ºC to rt, 1 h
Me3Si
Me3Si
(E)-7 quant.
SitBuPh2
1.1 equiv I2
OH
I
THF
0 ºC to rt, 1 h
8 96%
Scheme 2. Formation of a b-bromovinylsilane and a b-iodovinylsilane from vinyl-
stannane 6.
Figure 1. ORTEP plot of 10 confirming the (E)-stereochemistry of the internal ol-
efin. Atomic displacement parameters at 296 K.

4598
M. D. Weller et al. / Tetrahedron Letters 49 (2008) 4596–4600
1.1 equiv
Cl
R
SitBuPh2
SitBuPh2
Selectfluor®
t
Ph2 BuSi
OH
SnMe3
OH
SitBuPh2
MeCN
45 ºC, 3 h
R
R
F
R
Cl
6 R = SiMe3
13 R = Ph
12 R = SiMe3 70%
14 R = Ph 72%
23 R = SiMe3
24 R = Ph
Scheme 4. Synthesis of b-fluorovinylsilanes using SelectfluorÒ
.
Figure 2. Chloro-substituted masked hexaynes 23 and 24.13
has been accomplished using XeF2 in the presence of a silver(I)
salt.28 SelectfluorÒ (1-chloromethyl-4-fluoro-1,4-diazonia-bicyclo-
[2.2.2]octane bis(tetrafluoroborate)29) has also been used to effect
the electrophilic fluorination of vinylstannanes.30 In our hands, we
found that treatment of (E)-vinylstannane 6 with SelectfluorÒ in
acetonitrile at 45 °C led to clean consumption of the starting mate-
rial and the formation of b-fluorovinylsilane 12 in 70% yield
(Scheme 4).31 The product was obtained as a single stereoisomer,
which we have assigned as (E) in analogy with literature reports.30
A similar result was obtained when the trimethylsilyl group of 6
was replaced by a phenyl residue (13?14) (Scheme 4).
40000
23
20000
21
With two b-fluorovinylsilanes in hand, we directed our atten-
tion towards synthesising the corresponding masked hexaynes
(Scheme 5). As before, allylic oxidation of 12 to the corresponding
aldehyde 15 with MnO2, followed by dibromoolefination afforded
dibromoolefin 17 without incident. Treatment of 17 with 6 equiv
of LDA at À78 °C afforded the masked triyne 19, which was imme-
diately subjected to our modified Hay coupling conditions. This
time, the homocoupling reaction proceeded smoothly and was
complete in 4–5 h, affording the masked hexayne 21 in good yield
(Scheme 5).32 Masked hexayne 22 was obtained in a similar fash-
ion from 14 (Scheme 5).
We have previously synthesised masked hexaynes 23 and 24
(Fig. 2), which incorporate two b-chlorovinylsilanes into the conju-
gated framework.13 The UV–vis absorption spectra for fluoro-
substituted masked hexayne 21 and its chloro analogue 23 are
shown in Figure 3. The two spectra display a similar overall shape,
and the intensity of the peaks is also of the same order of magni-
tude; however, more interestingly, replacing the chloro substitu-
0
250
300
350
400
450
λ/nm
Figure 3. UV–vis spectra of masked hexaynes 21 (black line) and 23 (grey line).
hypsochromic shift in kmax. This suggests a larger HOMO–LUMO
band gap for these oligoyne precursors, which should improve
the stability of fluorinated masked oligoynes compared with their
b-chlorovinylsilane analogues. Similar observations were made
when the UV–vis spectra of 22 and 24 were compared, although
in these two cases, the spectra exhibited a bathochromic shift
owing to the conjugated
phenyl end-groups.
p-electron system extending into the
Having shown that our synthetic approach could be extended to
the synthesis of masked oligoynes containing b-fluorovinylsilanes
in place of b-chlorovinylsilanes, we were now ready to investigate
whether or not we could effect unmasking.33 We have already
shown that the elimination reaction of chloro-substituted masked
hexayne 24 requires stoichiometric quantities (i.e., 2 equiv) of TBAF
and proceeds rapidly at room temperature.13 We were therefore de-
lighted to observe that fluoro-substituted masked hexayne 22
underwent twofold defluorosilylation on exposure to a sub-stoichi-
ometric quantity of TBAF (10 mol %) in 1 h at 0 °C, to provide known
hexayne 25 in 88% isolated yield (Scheme 6). This result confirmed
our hypothesis that the unmasking of b-fluorovinylsilanes is a fluo-
ride-catalysed rather than a fluoride-mediated process.
ents in 23 with fluoro substituents leads to
a
ꢀ10 nm
SitBuPh2
SitBuPh2
MnO2
OH
O
CH2Cl2
0 ºC to rt
R
F
R
F
12 R = SiMe3
14 R = Ph
15 R = SiMe3 89%
16 R = Ph 86%
CBr4, PPh3
CH2Cl2, 0 ºC
In summary, we have further demonstrated the synthetic versa-
tility of our approach to oligoynes through the stereoselective
6 equiv
LDA
t
Ph2 BuSi
H
SitBuPh2
Br
THF
-78 ºC
R
F
R
F
Br
F
Ph
SitBuPh2
17 R = SiMe3 82%
19 R = SiMe3
20 R = Ph
t
Ph2 BuSi
18 R = Ph
91%
Ph
F
F
R
22
O2,
Cu(OTf)2•TMEDA
t
Ph2 BuSi
TBAF (10 mol%)
CH2Cl2, 0 ºC, 1 h
SitBuPh2
toluene/CH2Cl2
25 ºC
R
F
Ph
Ph
21 R = SiMe3 59%
22 R = Ph 61%
25 88%
Scheme 5. Synthesis of masked hexaynes 21 and 22.
Scheme 6. Fluoride-catalysed unmasking of 22.

M. D. Weller et al. / Tetrahedron Letters 49 (2008) 4596–4600
4599
mmax (CHCl3)/cmÀ1 3398br (O–H), 3073w, 2960m, 2897w, 2860m, 2127w
(C„C), 1713m, 1659s (C@C), 1541w, 1429m, 1391w, 1343m, 1286m, 1249m,
1216s, 1106m, 1030m, 845s, 819m, 758s, 703s, 666m, 648m; dH (300 MHz,
CDCl3) 0.25 (9H, s, Si(CH3)3), 1.01 (1H, t, J 7.0, CH2OH), 1.14 (9H, s, C(CH3)3),
3.80 (2H, d, J 7.0, CH2OH), 7.33–7.49 (6H, m, PhH), 7.68–7.81 (4H, m, PhH); dC
(75 MHz, CDCl3) À0.3 (CH3, Si(CH3)3), 19.3 (quat. C, C(CH3)3), 27.2 (CH3,
C(CH3)3), 67.3 (CH2, CH2OH), 106.5 (quat. C), 106.6 (quat. C), 124.0 (quat. C),
128.1 (CH, Ph), 129.9 (CH, Ph), 133.4 (quat. C, ipsoPh), 135.6 (CH, Ph), 148.4
(quat. C); m/z (TOF ES+) 493.1 ([M+Na]+, 100%); HRMS m/z (TOF ES+). Found
(M+Na)+ 493.0980. C24H3179BrOSi2Na requires 493.0995.
synthesis of b-fluoro-, b-bromo- and b-iodovinylsilanes from inter-
mediate vinylstannane 6. Whilst the bromo derivative proved too
labile for further application, the corresponding vinyl fluorides
were elaborated uneventfully into masked hexaynes. In contrast
to the analogous masked hexayne, which contains b-chlorovinylsi-
lane units and requires the use of stoichiometric quantities of
fluoride to effect unmasking, the new scaffold containing a b-fluo-
rovinylsilane can be unmasked using sub-stoichiometric quantities
of fluoride. This bodes well for the application of this novel class of
masked alkyne in the assembly of longer-length oligoynes, where
exceptionally mild reaction conditions will be invaluable.
18. For some recent examples of NBS-mediated bromodestannylation reactions
proceeding with retention of configuration: (a) Gracia, J.; Thomas, E. J. J. Chem.
Soc., Perkin Trans. 1 1998, 2865–2871; (b) Wipf, P.; Coish, P. D. G. J. Org. Chem.
1999, 64, 5053–5061; (c) Kittaka, A.; Asakura, T.; Kuze, T.; Tanaka, H.; Yamada,
N.; Nakamura, K. T.; Miyasaka, T. J. Org. Chem. 1999, 64, 7081–7093; (d)
Maleczka, R. E., Jr.; Gallagher, W. P. Org. Lett. 2001, 3, 4173–4176; (e) Phillips, E.
D.; Chang, H.-F.; Holmquist, C. R.; McCauley, J. P. Bioorg. Med. Chem. Lett. 2003,
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Org. Chem. 1987, 52, 4421–4423; (b) Wang, K. K.; Chu, K.-H.; Lin, Y.; Chen, J.-H.
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Acknowledgements
We thank the Engineering and Physical Sciences Research
Council and The University of Birmingham for a studentship (to
M.D.W.).
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12. (a) Simpkins, S. M. E.; Kariuki, B. M.; Aricó, C. S.; Cox, L. R. Org. Lett. 2003, 5,
3971–3974; (b) Simpkins, S. M. E.; Kariuki, B. M.; Cox, L. R. J. Organomet. Chem.
2006, 691, 5517–5523.
13. Simpkins, S. M. E.; Weller, M. D.; Cox, L. R. Chem. Commun. 2007, 4035–4037.
14. (a) Kataoka, K.; Tsuboi, S. Synthesis 1999, 452–456; (b) Hayashi, S.-I.; Nakai, T.;
Ishikawa, N. Chem. Lett. 1980, 935–938.
15. We have recently used this reagent in a completely stereoselective synthesis
(retention of configuration) of a (Z)-b-bromovinylsilane: Williams, I.; Kariuki,
B. M.; Reeves, K.; Cox, L. R. Org. Lett. 2006, 8, 4389–4392.
16. Examples of b-bromovinylsilanes in the literature are scarce: (a) Mitchell, T. N.;
Wickenkamp, R.; Amamria, A.; Dicke, R.; Schneider, U. J. Org. Chem. 1987, 52,
4868–4874; (b) Borrelly, S.; Paquette, L. A. J. Org. Chem. 1993, 58, 2714–2717;
(c) Pearson, W. H.; Postich, M. J. J. Org. Chem. 1994, 59, 5662–5671; (d)
Caporusso, A. M.; Barontini, S.; Pertici, P.; Vitulli, G.; Salvadori, P. J. Organomet.
Chem. 1998, 564, 57–59; (e) Jacobi, P. A.; Tassa, C. Org. Lett. 2003, 5, 4879–4882.
17. (E)-b-Bromovinylsilane 7: NBS (23 mg, 0.13 mmol) was added in one portion to
a solution of vinylstannane 6 (66 mg, 0.11 mmol) in CH2Cl2 (2 mL) at À78 °C.
The reaction mixture was warmed to rt and after 1 h, satd aq NH4Cl solution
(10 mL) was added. The layers were separated and the aqueous layer was
extracted with Et2O (3 Â 10 mL). The combined organic layers were washed
with H2O (3 Â 10 mL), brine (3 Â 10 mL), and dried (Na2SO4). The solvent was
removed under reduced pressure and purification of the residue by flash
column chromatography (20% Et2O in hexane) gave vinyl bromide (E)-7 as a
white solid (53 mg, quant.); Rf = 0.11 (10% Et2O in hexane); mp = 82–84 °C;
30. (a) Matthews, D. P.; Miller, S. C.; Jarvi, E. T.; Sabol, J. S.; McCarthy, J. R. Tetrahedron
Lett. 1993, 34, 3057–3060; (b) Hodson, H. F.; Madge, D. J.; Widdowson, D. A. J.
Chem. Soc., Perkin Trans. 1 1995, 2965–2968; (c) Martin, B.; Possémé, F.; Le
Barbier, C.; Carreaux, F.; Carboni, B.; Seiler, N.; Moulinoux, J.-P.; Delcros, J.-G.
Bioorg. Med. Chem. 2002, 10, 2863–2871; (d) Yu, C.-S.; Chiang, L.-W.; Wu, C.-H.;
Hsu, Z.-K.; Lee, M.-H.; Pan, S.-D.; Pei, K. Synthesis 2006, 3835–3840.
31. (E)-b-Fluorovinylsilane 12: SelectfluorÒ (246 mg, 0.69 mmol) was added in one
portion to a solution of vinylstannane 6 (350 mg, 0.63 mmol) in MeCN (10 mL)
at 45 °C. After stirring at 45 °C for 3 h, the reaction mixture was cooled to rt and
diluted with Et2O (10 mL). The suspension was filtered through a short silica
plug, eluting with Et2O (30 mL). The solvent was removed under reduced
pressure and purification of the residue by flash column chromatography (20%
Et2O in hexane) gave vinyl fluoride 12 as a colourless oil (180 mg, 70%);
Rf = 0.14 (10% Et2O in hexane); m
max (CHCl3)/cmÀ1 3583w (O–H), 3424br w (O-
H), 3073w, 3052w, 3017w, 2962m, 2898m, 2861m, 2133m (C„C), 1602m
(C@C), 1429m, 1250s, 1217m, 1174w, 1106s, 1028w, 845vs, 820w, 758vs,
703vs, 609s; dH (300 MHz, CDCl3) 0.25 (9H, s, Si(CH3)3), 1.01 (1H, t, J 6.8,
CH2OH), 1.19 (9H, s, C(CH3)3), 3.58 (2H, dd, 3JH–F 21.3, 3JH–H 6.8, CH2OH), 7.34–
7.55 (6H, m, PhH), 7.71–7.90 (4H, m, PhH); dF (282 MHz, CDCl3) À67.2 (s); dC
4
(75 MHz, CDCl3) À0.2 (CH3, Si(CH3)3), 19.1 (d, JC–F 1.7, quat. C, C(CH3)3), 27.3
(CH3, C(CH3)3), 60.3 (d, 2JC–F 29.6, CH2, CH2OH), 97.1 (d, JC–F 4.6, quat. C), 100.5
(d, JC–F 12.4, quat. C), 104.1 (d, JC–F 6.3, quat. C), 128.1 (CH, Ph), 129.9 (CH, Ph),
133.5 (d, 4JC–F 1.4, quat. C, ipsoPh), 135.7 (CH, Ph), 172.1 (d, 1JC–F 290.8, quat. C);
m/z (TOF ES+) 433.2 ([M+Na]+, 100%); HRMS m/z (TOF ES+). Found (M+Na)+
433.1808. C24H31FOSi2Na requires 433.1795.

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M. D. Weller et al. / Tetrahedron Letters 49 (2008) 4596–4600
32. Characterisation data for 21: Rf = 0.36 (10% toluene in hexane); mp 118 °C
(dec.); UV–vis (CH2Cl2): 304sh (22240), 316 (28770), 335 (32610), 362
(29080), 391 (25840); mmax (CHCl3)/cmÀ1 3071w, 2960m, 2933m, 2896m,
2859m, 2119m (C„C), 1545m, 1462w, 1428m, 1392w, 1363w, 1251m, 1216w,
1192m, 1107m, 1029w, 843s, 758s, 738m, 697s, 666m, 648m, 622m, 606s; dH
(300 MHz, benzene-d6) 0.17 (18H, s, Si(CH3)3), 1.24 (18H, s, C(CH3)3), 7.00–7.17
(12H, m, PhH), 7.57–7.75 (8H, m, PhH); dF (282 MHz, benzene-d6) À63.8 (s); dC
(75 MHz, benzene-d6) À0.4 (CH3, Si(CH3)3), 19.8 (quat. C, C(CH3)3), 28.2 (CH3,
C(CH3)3), 78.3 (dd, JC–F 42.2, 1.5, quat. C), 82.9 (app t, JC–F 5.6, quat. C), 101.6 (d,
JC–F 6.5, quat. C), 109.8 (quat. C), 114.3 (d, JC–F 6.9, quat. C), 128.2 (CH, Ph), 130.3
(CH, Ph), 132.6 (quat. C, ipsoPh), 136.5 (CH, Ph), 153.2 (dd, 1JC–F 265.7, 6JC–F 1.2,
quat. C); m/z (TOF ES+) 915.4 ([M+109Ag]+, 100%), 603.2 (26,
[(tBuPh2Si)2O + 109Ag]+); HRMS m/z (TOF ES+). Found (M+107Ag)+ 913.2548.
C
50H56F2Si4107Ag requires 913.2478.
33. Unmasking was only performed on phenyl end-capped masked hexayne 22
and not on TMS end-capped analogue 21. Under the conditions used to effect
defluorosilylation, removal of the TMS end-caps in 21 would also occur to
provide an extremely unstable free hexayne. Since the TMS protecting groups
can be removed without inducing dehalosilylation, 21 represents a convenient
monomer precursor for oligomerisation studies.