Synthesis of polyynes to model the sp-carbon allotrope carbyne

Article abstract of DOI:10.1038/nchem.828

Carbyne is an allotrope of carbon composed of sp-hybridized carbon atoms. Although its formation in the laboratory is suggested, no well-defined sample is described. Interest in carbyne and its potential properties remains intense because of, at least in

Full text of DOI:10.1038/nchem.828

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PUBLISHED ONLINE: 19 SEPTEMBER 2010 | DOI: 10.1038/NCHEM.828
Synthesis of polyynes to model the sp-carbon
allotrope carbyne
Wesley A. Chalifoux and Rik R. Tykwinski†
*
Carbyne is an allotrope of carbon composed of sp-hybridized carbon atoms. Although its formation in the laboratory is
suggested, no well-defined sample is described. Interest in carbyne and its potential properties remains intense because
of, at least in part, technological breakthroughs offered by other carbon allotropes, such as fullerenes, carbon nanotubes
and graphene. Here, we describe the synthesis of a series of conjugated polyynes as models for carbyne. The longest of
the series consists of 44 contiguous acetylenic carbons, and it maintains a framework clearly composed of alternating
single and triple bonds. Spectroscopic analyses for these polyynes reveal a distinct trend towards a finite gap between the
highest occupied molecular orbital and the lowest unoccupied molecular orbital for carbyne, which is estimated to be
∼485 nm (∼2.56 eV). Even the longest members of this series of polyynes are not particularly sensitive to light, moisture
or oxygen, and they can be handled and characterized under normal laboratory conditions.
y mass, carbon is the fourth most-abundant element in the samples for n ¼ 6 and 8 (ref. 10). The attempted synthesis of
universe, and it is essential to all known forms of life1. In longer derivatives, however, gave TES[10], TES[12] and TES[16],
B
terms of naturally occurring carbon, typically two forms (allo- which could be manipulated only in solution, but the longest mol-
tropes) are encountered, namely diamond (sp3-hybridized carbon) ecule, TES[16], could not be purified10. Polyynes Pt[n] were isolated
and graphite (sp2-hybridized carbon). In each case, it is the bulk and characterized up to n ¼ 14; Pt[14] is the longest isolable
properties of these allotropes that are of interest, properties that polyyne reported to date11.
differ quite dramatically for each allotropic form. For example,
Recognizing that large, sterically demanding end-capping groups
diamond is the hardest known naturally occurring material and should provide synthetic access to much longer polyynes, we used
also a good electrical insulator. Graphite, however, is much softer the tris(3,5-di-t-butylphenyl)methyl moiety (Tr*, Fig. 1)12 and
and shows high conductivity2. Carbon allotropes discovered more report here the synthesis of polyynes with unprecedented length:
recently, such as fullerenes, nanotubes and graphene (all composed the series 1a–1j is composed of polyynes of up to n ¼ 22 acetylenic
of sp2-hybridized carbon) each also show unique properties, often bonds (that is, 44 contiguous sp-carbons). Structure–property ana-
different from those of either diamond or graphite, and they offer lyses for polyynes 1a–1j provide predictions as to the highest occu-
the promise of technological breakthroughs3.
pied molecular orbital (HOMO)–lowest unoccupied molecular
Next in the series of carbon-atom hybridization from sp3- to sp2- orbital (LUMO) gap and structure of carbyne.
is sp-hybridization and the least well-known of the carbon allo-
tropes, carbyne (Fig. 1). Whereas diamond and graphite feature
R
R
R
R
n
Et3Si
SiEt3
an atomic carbon skeleton with covalent bonding in three and
two dimensions, respectively, carbyne is expected to feature a
linear or approximately linear one-dimensional framework.
Carbyne is proposed to exist in interstellar dust4, meteorites5 and
as a by-product of shock-fused graphite6. There are reports of the
laboratory formation of carbyne by a number of routes, including
gas-phase deposition techniques, electrochemical synthesis, dehy-
drohalogenation of polymers and others7,8. In all these cases, the
resulting product or material is ill-defined, and the specific proper-
ties of carbyne thus remains unrealized.
In the absence of a well-defined or indisputable sample of
carbyne, synthetic chemists addressed the question of its potential
properties through the formation of homologous series of polyyne
molecules (Fig. 1)9. More specifically, this involved the synthesis
of oligomers of defined length and composed only of acetylenic seg-
ments that are end-capped with a sterically bulky group designed to
stabilize the polyyne chain. For a particular set of oligomers, conver-
gence of spectroscopic data versus length to a constant value (satur-
ation) can be evaluated to shed light on the expected properties of
n
Carbyne
PAr3
Polyyne
TES[n]
n = 6, 8, 10, 12, 16
PAr3
Ar Pt
PAr3
Pt
Ar
Me
Ar =
n
PAr3
Pt[n]
n = 3–6, 8, 10, 12, 14
t-Bu
t-Bu
t-Bu
Tr*
Tr*
n
1a n = 4 1f n = 14
Tr* =
1b n = 6
1c n = 8
1d n = 10
1e n = 12
1g n = 16
1h n = 18
1i n = 20
1j n = 22
t-Bu
t-Bu
t-Bu
the polymer carbyne. A notable example is that of the triethylsilyl Figure 1 | Schematic structures of carbyne, polyyne and several polyyne
end-capped series TES[n] (Fig. 1), which provided isolable species.
Department of Chemistry, University of Alberta, Edmonton, Alberta, T6G 2G2 Canada; †Present address: Institut fu¨r Organische Chemie, Friedrich-
Alexander-Universita¨t, Erlangen-Nu¨rnberg, Henkestrasse 42, 91054 Erlangen, Germany. e-mail: rik.tykwinski@chemie.uni-erlangen.de
*
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.828
Table 2 | Synthesis of polyynes 1a–1j (n 5 4–22).
ARTICLES
Table 1 | Synthesis of polyynes 4e–4k (c 5 5–11).
Cu(OAc)2•H2O
K2CO3
Dimerization
conditions
Desilylation
conditions
Tr*
Sii-Pr3
Tr*
H
Tr*
Tr*
n
c
a
Tr*
H + Me3Si
Sii-Pr3
Tr*
Sii-Pr3
a
b
4
2
1
c
2,6-lutidine/
MeOH/CH2Cl2
(1:1:1)
2
3
4
4
2, yield, %
1, yield, %
–
2b (a ¼ 2)†
1a (n ¼ 4), 92}
1b (n ¼ 6), 66}
1c (n ¼ 8), 97}
Entry
Substrate 2
Substrate 3
4, % yield
4c (c ¼ 3)*
4d (c ¼ 4)*
4e (c ¼ 5)
4f (c ¼ 6)
4g (c ¼ 7)
4h (c ¼ 8)
4i (c ¼ 9)
4j (c ¼ 10)
4k (c ¼ 11)
2c (a ¼ 3), 91‡
2d (a ¼ 4), 100§
2e (a ¼ 5), 99§
2f (a ¼ 6), 93§
2g (a ¼ 7)§ꢁ
1
2b (a ¼ 2)
2c (a ¼ 3)
2c (a ¼ 3)
2d (a ¼ 4)
2e (a ¼ 5)
2f (a ¼ 6)
2g (a ¼ 7)
3c (b ¼ 3)
3c (b ¼ 3)
3d (b ¼ 4)
3d (b ¼ 4)
3d (b ¼ 4)
3d (b ¼ 4)
3d (b ¼ 4)
4e (c ¼ 5), 84
4f (c ¼ 6), 63
4g (c ¼ 7), 73
4h (c ¼ 8), 77
4i (c ¼ 9), 42
4j (c ¼ 10), 52
4k (c ¼ 11), 35
1d (n ¼ 10), 26#, 74**
1e (n ¼ 12), 41#, 86**
1f (n ¼ 14), 90**
1g (n ¼ 16), 76**
1h (n ¼ 18), 81**
1i (n ¼ 20), 57**
1j (n ¼ 22), 18**
2
3
4
5
6
7
2h (a ¼ 8)§ꢁ
2i (a ¼ 9)§ꢁ
2j (a ¼ 10)§ꢁ
2k (a ¼ 11)§ꢁ
*The synthesis of 4c and 4d is described in the Supplementary Information (pp S15–S17). †Diyne 2b
was produced by a different method, see Supplementary Information (pp S12–S13). ‡Desilylation
using tetrabutylammonium fluoride, THF, room temperature. §Desilylation using CsF in
tetrahydrofuran/H2O (5:1), room temperature. ꢁProduct carried on directly to next step.
}Oxidative dimerization using CuCl, tetramethylethylenediamine, CH2Cl2, O2. #Oxidative
Results and discussion
The formation of the synthetic precursors to polyynes 1a–1j isdesigned
around a chain-extension sequence using the terminal polyynes 2b–2g
in a reaction with differentially end-capped polyynes 3c and 3d, where
a and b represent the number of acetylene units in precursors 2 and 3,
respectively (Table 1). The building blocks 2b–2d and 3c and 3d were
.
dimerization using Cu(OAc)2 H2O (excess), pyridine, CH2Cl2. **Oxidative dimerization using
.
Cu(OAc)2 H2O (excess), 2,6-lutidine, CH2Cl2.
assembled readily based on reported methods (see Supplementary coupling conditions18 (CuCl, tetramethylethylenediamine, CH2Cl2,
Information (pp S12–S18) and Jahnke and Tykwinski13), and the syn- O2) readily converted 2b–2d into tetrayne 1a, hexayne 1b and
thesis of 2e–2g is described below. This chain extension was accom- octayne 1c, respectively. These conditions were not, however, particu-
plished efficiently using a modified Eglinton–Galbraith protocol to larly successful when applied to the conversion of 2e and 2f into
generate unsymmetrical Tr*–[C;C]c–Sii-Pr3 polyynes (4e–4k)14,15
.
decayne 1d and dodecayne 1e, respectively. In these cases, the formation
Specifically, allowing a mixture of the appropriate terminal polyyne of side products rendered purification and isolation of the desired poly-
2b–2g (a ¼ 2–7) and either 3c (b ¼ 3) or 3d (b ¼ 4) to react in the ynesdifficult. SwitchingtothetypicalEglinton–Galbraithcouplingcon-
.
.
presence of excess Cu(OAc)2 H2O, K2CO3 and 2,6-lutidine resulted ditions (Cu(OAc)2 H2O, pyridine, THF) gave decayne 1d and
in the formation of polyynes 4e–4k in good to moderate yields dodecayne 1e without the troublesome by-products encountered
(Table 1, entries 1–4 and entries 5–7, respectively). Three notable with the Hay protocol, but only in modest isolated yields of 26% and
aspects of this process are: (1) removal of the trimethylsilyl groups 41%, respectively. By using CH2Cl2 as the reaction solvent and the
of 3c and 3d was effected in situ under the Eglinton–Galbraith reaction less nucleophilic base 2,6-lutidine, however, the yields of 1d and 1e
conditions, which eliminated the need for an independent desilylation were improved significantly. Gratifyingly, thisset of conditionswas gen-
step16, (2) use of 2,6-lutidine in place of the typical base, pyridine, erally applicable, and terminal polyynes 2g–2k thus provided the
was essential to the success of the reaction and (c) 4i, 4j and 4k rep- remaining members of the series, 1f–1j. The isolated polyynes span a
resent the longest unsymmetrical polyynes isolated and characterized range of colours, starting with tetrayne 1a, which is colourless. Hexa-
to date.
and octaynes 1b and 1c are both yellow, and decayne 1d marksthe tran-
Removal of the terminal i-Pr3Si groups from polyynes 4c–4k to sition from yellow to orange. Polyynes 1e–1h are all isolated as orange
give terminal polyynes 2c–2k (Table 2) was first explored using solids and the longest derivative, 1j, is orange–red in colour.
tetrabutylammonium fluoride as a desilylation agent at low temp-
Ultraviolet–visible spectroscopy is a primary tool for polyyne
erature. Under these conditions 4c was desilylated successfully, characterization because these molecules typically display well-
but significant decomposition was observed for longer derivatives. resolved spectra with distinctive vibrational fine structures19.
Ultimately, desilylation was achieved using CsF in a 5:1 mixture Furthermore, the lowest energy wavelength of significant absorp-
of tetrahydrofuran (THF)/H2O, which provided polyynes 2d–2k tion, lmax, shows a steady shift to lower energy as a function of
with little apparent decomposition based on ultraviolet–visible spec- polyyne length, and this trend documents the lowering of the
troscopic analyses (see Supplementary Fig. S1).
HOMO ꢀ LUMO bandgap energy (Eg) as the conjugation length
The stabilizing influence of the Tr* end-capping group was already of the molecule increases. Numerous studies used lmax values for
quite evident at this point, in that terminal polyynes up to seven triple shorter polyynes with n ¼ 2–14 as a predictive tool for carbyne
bondsinlength(2g) could be isolated neat and characterized by 1H and based on the premise that the HOMO ꢀ LUMO bandgap should
13C NMR, infrared and ultraviolet–visible spectroscopy, as well as by eventually reach an asymptotic limit, which would be a reasonable
mass spectrometry (although within minutes the terminal polyyne estimate of lmax (and thus Eg) for carbyne. Typically, a plot of Eg
2g showed signs of decomposition as a solid). To our knowledge, the versus 1/n was examined (where n is the number of acetylene
longest terminal polyyne previously isolable as a stable, neat solid units) and extrapolation to the y-intercept then provided an estimate
was a tetrayne17. Given the instability observed for 2g in the solid of lsat for an infinite polyyne chain (that is, lsat ¼ lmax at the satur-
state, attempts were not made to isolate the longer terminal polyynes ation length of the series). For example, this analysis was reported
2h–2k. Thus, after formation by desilylation, the products 2g–2k, for the series i-Pr3Si–[C;C]n–Sii-Pr3 (n ¼ 2–10) and predicted a
after work up, were taken on directly to the formation of 1f–1j. limiting value of lsat ¼ 570 nm (ref. 20), nearly identical to the
Polyynes 2h–2k could, however, be obtained pure in solution and value predicted for aryl end-capped polyynes of the same lengths
characterized by high-resolution matrix-assisted laser desorption/ (lsat ¼ 570 nm)21. Similarly, a plot of Eg versus 1/n reported for
ionization mass spectrometry, as well as by ultraviolet–visible and the organometallic series Pt[n] up to n ¼ 14 predicted a limiting
infrared spectroscopy.
value of lsat ¼ 573 nm (ref. 11), which is somewhat higher than
towards the synthesis of polyynes 1a–1j (Table 2). Standard Hay to n ¼ 12 (lsat ≈ 492–527 nm)22.
Oxidative dimerization of precursors 2b–2k was the final step that determined for an earlier series of Pt-terminated polyynes up
968
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a
b
500
475
450
425
400
375
350
325
300
275
250
225
1,695cm–1
λsat =485nm @ n ~ 48
~
λmax
458nm
1,534cm–1
n =22
λ(n) = λ∞– (λ∞–
λ
1)e–a(n–1)
λmax
451nm
λ
∞ =486nm
λ1 =175nm
a =0.116
n =20
0
4
8
12 16 20 24 28 32 36 40 44 48
λmax
443nm
n
c
n =18
λmax
432nm
n =16
λmax
419nm
n =14
λmax
400nm
69 68 67 66 65 64 63 62 61 60 59 58 57
ppm
n =12
d
λmax
376nm
n =10
C7
C5
C4
C3
C2
C9
C11
C1
C6
C8
λmax
347nm
n =8
4f
λmax 310nm
n =6
λmax 268nm
n =4
C7'
C7
C6
C4
C2
C5
C3
230 260 290 320 350 380 410 440 470
λ (nm)
1b
Figure 2 | Spectroscopic and crystallographic characterization of polyynes 1a–1j. a, Ultraviolet–visible spectra of 1a–1j polyynes as measured in hexanes;
wavelength of lmax for each derivative is shown in blue and vibrational separations of lowest energy absorptions for 1j are shown in magenta. a.u. ¼ arbitrary
units. b, Convergence of the absorption maxima in the series of 1a–1j polyynes according to equation (1), where l1 ¼ 486 nm, l1 ¼ 175 nm and a ¼ 0.116.
c, 13C NMR spectrum (in CDCl3) of 1j in the range 69–57 ppm, which shows convergence of the sp-carbon resonances to a median value of ꢂ63.7 ppm
(outlying signals at 68.66 and 57.38 ppm arise from the endmost acetylenic carbons). d, X-ray crystallographic structures for 4f and 1b shown as space-
filling models (left) and ORTEP representations (right, 20% probability level, hydrogen atoms not shown); selected bond lengths (Å) for 4f: C(1)–C(2)
1.213(3), C(2)–C(3) 1.357(4), C(3)–C(4) 1.209(3), C(4)–C(5) 1.360(4), C(5)–C(6) 1.211(3), C(6)–C(7) 1.358(3), C(7)–C(8) 1.202(3), C(8)–C(9) 1.360(4),
C(9)–C(10) 1.210(3), C(10)–C(11) 1.364(3), C(11)–C(12) 1.196(3); selected bond lengths (Å) for 1b: C(2)–C(3) 1.192(3), C(3)–C(4) 1.363(3), C(4)–C(5)
1.205(3), C(5)–C(6) 1.353(3), C(6)–C(7) 1.201(3), C(7)–C(7′) 1.362(5).
The ultraviolet–visible spectra for 1a–1j are plotted in Fig. 2a, Following the usual procedure (see above), a plot of the solution
and show the expected vibrational fine structure (for example, optical bandgap Eg versus 1/n gave a good fit to the data for
spacing of 1,534 and 1,695 cm21 for the three lowest energy absorp- series 1a–1j and predicted a limiting value of lsat ≈ 564 nm
tions of 1j). The bathochromic shift in lmax as a function of length (2.20 eV; see Supplementary Fig. S2). There is, however, a potential
spans from lmax ¼ 268 nm for 1a to lmax ¼ 458 nm for 1j. The problem with this estimation, as previously documented by Meier
extinction coefficients for lmax increase almost uniformly as a func- et al. for other rigid, conjugated oligo- and polymers23. A plot
tion of length with values that approach one million for 1i (1 ¼ of Eg versus 1/n assumes that Eg continues to decrease uniformly
866,000 at lmax ¼ 451 nm and 1 ¼ 936,000 at l ¼ 420 nm). as a function of length through to infinity. It is well-established,
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.828
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however, that an effective conjugation length (also referred to as a
confinement length, convergence length or delocalization length)
should exist for p-conjugated organic oligomers. This leads to a
convergent limit for properties such as lmax or Eg at lengths
much shorter than infinity24. Indeed, the plot of Eg versus 1/n for
series 1 revealed a conspicuous deviation from linearity for the
longest members of the series (n ≥ 10), which suggested a better
fit was possible (see Supplementary Fig. S2). Following the Meier
protocol23, the data for series 1a–1j were analysed using an expo-
nential function (equation (1)).
Table 3 | 13C NMR alkyne carbon resonances for polyynes
1a–1j.
Compound Alkyne carbon chemical shifts in CDCl3 (ppm)
1a
1b
1c
1d
69.06, 63.00, 62.36, 57.19
68.78, 62.96, 62.90, 62.75, 62.42, 57.29
68.65, 63.55, 63.49, 62.97, 62.60, 62.50, 62.19, 57.33
68.60, 63.96, 63.78, 63.49, 63.11, 62.69, 62.54, 62.28,
62.06, 57.36
1e
1f
68.57, 64.17, 63.92, 63.77, 63.49, 63.18, 62.86, 62.52, 62.50,
62.17, 61.99, 57.36
68.57, 64.28, 64.00, 63.93, 63.69, 63.45, 63.24, 63.01,
62.74, 62.48, 62.44, 62.12, 61.96, 57.37
68.56, 64.34, 64.05, 64.01, 63.81, 63.61, 63.45, 63.29, 63.12,
62.93, 62.69, 62.48, 62.41, 62.10, 61.95, 57.37
68.55, 64.36, 64.06, 64.04, 63.85, 63.68, 63.55, 63.42,
63.31, 63.19, 63.05, 62.87, 62.65, 62.47, 62.39, 62.08, 61.94,
57.37
l(n) = l1 − (l1 − l1)e−a(n−1)
(1)
1g
1h
where n is again the number of acetylene units, l(n) is lmax for
a polyyne of length n (l1 is thus lmax of the monomer with n ¼ 1,
calculated according to equation (1)) and l1 is the limiting value
as n ꢀ 1. The parameter a indicates how fast saturation (conver-
gence) is approached, and it can also be used to compare convergence
characteristics between different oligomer series. Finally, a practical
definition is required that relates the approach to l1 and lsat, and
the suggestion of Meier is adopted here23, taking into account the
accuracy of the ultraviolet–visible spectrometer (+1 nm): lsat is
defined by fulfilment of the relationship l1 – l(n) ≤ 1 nm.
1i
1j
68.63, 64.50, 64.19, 64.19, 64.03, 63.88, 63.77, 63.67,
63.58, 63.51, 63.43, 63.33, 63.20, 63.03, 62.81, 62.54, 62.54,
62.20, 62.03, 57.39
68.66, 64.60, 64.27, 64.27, 64.14, 64.00, 63.91, 63.83,
63.77, 63.72, 63.66, 63.61, 63.54, 63.44, 63.33, 63.15, 62.93,
62.64, 62.58, 62.29, 62.10, 57.38
Applying the lmax data for 1a–1j into equation (1) generated the in size between the Tr* (diameter, d ≈ 14 Å) and i-Pr3Si (d ≈ 7 Å)
parameters l1 ¼ 486+5 nm, l1 ¼ 175+13 nm and a ¼ 0.116+ groups. Thus, whereas the i-Pr3Si groups do little to shield the
0.008 (for details and error analysis, see Supplementary Table S2). polyyne chain, the Tr* groups extend well out over the sp-carbon
Figure 2b shows the convergence of lmax values for 1a–1j based framework, and offer significantly more steric shielding against
on equation (1) and predicts a lsat for carbyne of 485 nm the prospect of intermolecular interactions that reportedly lead to
(2.56 eV) by about n ¼ 48. This lsat value is substantially higher decomposition (schematically shown in red, Fig. 2d)9,27,28. Finally,
in energy than that predicted by the linear extrapolation method analysis of bond lengths reveals that C–C and C;C bonds for
of Eg versus 1/n (lsat ≈ 564 nm, 2.20 eV). Interestingly, the low both structures remain within the expected ranges26–28
value of a ¼ 0.116 shows that saturation for polyynes was reached
.
quite slowly in comparison to that for other conjugated oligomers, Conclusions
which probably reflects that effective conjugation for polyynes is The experimental data reported here are thus consistent with the
not dependent on molecular conformation23. Given the extensive prediction of carbyne as a polyyne-like material with a structure
set of lmax data for polyyne series 1a–1j, we believe this to be the composed of alternating single and triple bonds with a finite
most accurate estimation to date for the finite HOMO–LUMO bandgap of lmax ¼ 485 nm (2.56 eV). The successful synthesis of
gap of carbyne. This analysis also provides excellent evidence that the longer derivates 1i and 1j as persistent molecules is significant,
a polyyne structure of alternating single and triple bonds will be and suggests that longer polyynes, and even carbyne, are viable
maintained for carbyne, that is, Peierls distortion will be upheld25,26
.
targets for stepwise synthesis.
All synthesized polyynes 1a–1j were sufficiently stable and
soluble for 13C NMR spectroscopic analysis, which provided
additional insight into the prospective properties of carbyne
(Table 3). 13C NMR spectroscopy confirmed that the longest
member of the series, 1j, was still a polyyne rather than carbyne;
quite amazingly, 21 of the 22 resonances for unique sp-carbons
were observed (two overlapping signals were found at 64.27 parts
per million (ppm)). The two endmost alkyne carbons experienced
the greatest influence from the end-capping group and appeared
at ꢂ69 and ꢂ57 ppm. The remaining carbon signals resonated
within a narrow window of 62.10–64.60 ppm and converged
towards a median value of 63.7 ppm (Fig. 2c and Supplementary
Fig. S3). Thus, these data suggest that the 13C NMR spectrum of
carbyne probably consists of a broadened signal centred at a chemi-
cal shift of ꢂ63.7 ppm, a value quite consistent with that expected
from a polyynic framework composed of alternating single and
triple bonds. This proposed chemical shift is consistent with other
predictions11,20–22, albeit more precise.
Methods
General procedure: modified Eglinton–Galbraith coupling. To a solution of
2 (0.297 mmol) and 3 (1.49 mmol) in CH2Cl2 (10 ml), MeOH (10 ml) and
2,6-lutidine (10 ml) was added K2CO3 (1.49 mmol) and the mixture was stirred
.
for 10 minutes at room temperature. To the mixture was added Cu(OAc)2 H2O
(2.97 mmol) and the reaction was stirred at room temperature until deemed
complete by thin-layer chromatography (TLC) (ꢂ17 hours). The reaction was
quenched by the addition of saturated aqueous NH4Cl (25 ml) and the resulting
mixture extracted with hexanes (100 ml). The organic phase was washed with
saturated aqueous NH4Cl (25 ml), H2O (2 × 25 ml) and brine (25 ml), dried over
MgSO4 and filtered. The resulting solution was filtered through a plug of silica, the
solvent removed in vacuo and the crude product purified by column
chromatography (silica, hexanes) to yield 4.
General procedure: desilylation using CsF. To a solution of 4 (1.288 mmol) in
THF (100 ml) and H2O (20 ml) was added CsF (1.54 mmol). The reaction was
stirred at room temperature until deemed complete by TLC analysis (ꢂ1 hour).
The reaction was quenched by the addition of saturated aqueous NH4Cl (50 ml)
and the resulting mixture extracted with hexanes (100 ml). The organic layer was
washed with H2O (50 ml) and brine (50 ml), dried over MgSO4 and filtered. The
solvent was removed in vacuo and the crude product purified by passing through
a plug of silica gel with CH2Cl2/hexanes (1:5) to yield 2 (a ¼ 3–6).
General procedure: formation of 1f–1j (n ≥ 14). To a solution of 4 (0.10 mmol) in
THF (25 ml) and H2O (5 ml) was added CsF (0.12 mmol) and the reaction
monitored by TLC for completion (ꢂ20 minutes). The reaction was quenched by the
addition of saturated aqueous NH4Cl (15 ml) and the resulting mixture extracted
with hexanes (50 ml). The organic layer was washed with H2O (15 ml) and brine
X-ray crystallographic analysis of 4f and 1b offered an opportu-
nity to examine structural aspects and a means to visualize the steric
shielding that results from the presence of the Tr* groups (Fig. 2d).
Molecule 4f adopts a gentle bend in the solid state, and the bond
angles for centrosymmetric 1b show an S-shaped conformation;
in both cases the C–C;C bond angles range from about 175 to
1798. Clearly evident in the structures is the dramatic difference (15 ml), dried over MgSO4 and filtered. The resulting solution was concentrated to
970
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.828
ARTICLES
ꢂ5 ml, and the crude intermediate 2 was passed through a plug of silica gel with
CH2Cl2/hexanes (1:5). The filtrate was concentrated to ꢂ25 ml and diluted with
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.
CH2Cl2 (25 ml). Cu(OAc)2 H2O (0.510 mmol) and 2,6-lutidine (1.0 ml) were
added and the reaction mixture was stirred at room temperature for 24 hours.
The reaction mixture was concentrated to ꢂ5 ml, the crude product purified by
passing through a plug of silica gel with CH2Cl2/hexanes (1:10) and the solvent
removed in vacuo to yield 1.
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16. Haley, M. M. et al. One-pot desilylation/dimerization of ethynyl- and
butadiynyltrimethylsilanes. Synthesis of tetrayne-linked
Solid-state analyses. Single crystals of 4f suitable for X-ray crystallographic analysis
were grown by slow evaporation of a CDCl3/hexanes (1:1) solution at room
ꢀ
temperature. C70H98Si, Mr ¼ 967.57, triclinic crystal system, space group P1 (No. 2),
a ¼ 10.2575(7) Å, b ¼ 14.6634(11) Å, c ¼ 21.9595(16) Å, a ¼ 76.520(1)8,
dehydrobenzoannulenes. Tetrahedron Lett. 38, 7483–7486 (1997).
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structures of isolable terminal aryl hexatriyne and octatetrayne derivatives:
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b ¼ 89.555(1)8, g ¼ 86.272(1)8, V ¼ 3,205.0(4) Å3, Z ¼ 2, rcalcd ¼ 1.003 g cm23
,
m ¼ 0.073 mm21, l ¼ 0.71073 Å, T ¼ 2100 8C, 2umax ¼ 50.508, total data
collected ¼ 22,875, R1 ¼ 0.0635 for 6,600 observed reflections with [Fo2 ≥ 2s(Fo2)],
wR2 ¼ 0.1884 for 586 variables and 11,578 unique reflections with [Fo2 ≥ 23s(Fo2)],
residual electron density ¼ 0.407 and 20.258 e Å23. CCDC no. 772622.
Single crystals of 1b suitable for X-ray crystallographic analysis were
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grown by slow evaporation of a xylene solution at room temperature. C98H126
,
ꢀ
Mr ¼ 1,303.99, triclinic crystal system, space group P1 (No. 2), a ¼ 10.5799(10) Å,
b ¼ 10.7732(10) Å, c ¼ 19.6616(18) Å, a ¼ 84.7319(14)8, b ¼ 75.3405(14)8,
g ¼ 87.4321(14)8, V ¼ 2,158.4(3) Å3, Z ¼ 1, rcalcd ¼ 1.003 g cm23
,
m ¼ 0.056 mm21, l ¼ 0.71073 Å, T ¼ 2100 8C, 2u
¼ 50.768, total data
max
collected ¼ 15,603, R1 ¼ 0.0538 for 4,479 observed reflections with [Fo2 ≥ 2s(Fo2)],
wR2 ¼ 0.1535 for 486 variables and 7,893 unique reflections with [Fo2 ≥ 23s(Fo2)],
residual electron density ¼ 0.408 and 20.195 e Å23. CCDC no. 772621.
The diameters of the endgroups for 4f and 1b were estimated from the radius
as measured from the central carbon or silyl atom of the end-capping group to
the outermost proton, based on the X-ray crystallographic structures, using
Mercury CSD 2.2.
Received 14 April 2010; accepted 27 July 2010;
published online 19 September 2010
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Acknowledgements
This work was supported by the University of Alberta and the Natural Sciences and
Engineering Research Council of Canada (NSERC) through the Discovery Grant program.
W.A.C. thanks the NSERC (Postgraduate Scholarship-D) and the Alberta Ingenuity Fund
for scholarship support. We also thank F. Marsiglio for discussions and M. Ferguson and
R. McDonald for solving the X-ray structures for 4f and 1b, respectively.
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Author contributions
W.A.C. designed the experiments, and performed the syntheses, characterization and
property studies. W.A.C. and R.R.T co-wrote the paper. R.R.T conceived the project.
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Additional information
The authors declare no competing financial interests. Supplementary information and
chemical compound information accompany this paper at www.nature.com/
naturechemistry. Reprints and permission information is available online at http://npg.nature.
com/reprintsandpermissions/. Correspondence and requests for materials should be
addressed to R.R.T.
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