Synthesis and characterization of multinanometer-sized expanded dendralenes with an iso-poly(triacetylene) backbone

Article abstract of DOI:10.1002/1522-2675(200207)85:7<2169::AID-HLCA2169>3.0.CO;2-Z

A series of [n]dendralenes (n = 3, 4, 8, 3b-d (Fig. 1)) expanded with buta-1,3-diynediyl moieties between the C=C bonds were prepared by repetitive acetylenic scaffolding of 3-(cyclohexylidene)penta-1,4-diyne building blocks (Scheme). These remarkably uns

Full text of DOI:10.1002/1522-2675(200207)85:7<2169::AID-HLCA2169>3.0.CO;2-Z

Helvetica Chimica Acta Vol. 84 (2001)
2169
Synthesis and Characterization of Multinanometer-Sized Expanded
Dendralenes with an iso-Poly(triacetylene) Backbone
by Estelle Burri and FrancÀois Diederich*
Laboratorium f¸r Organische Chemie, ETH-Hˆnggerberg, HCI, CH-8093Z¸rich
and
Mogens Br˘ndsted Nielsen*
Department of Chemistry, University of Southern Denmark, Odense University, Campusvej 55,
DK-5230 Odense M
A series of [n]dendralenes (n ˆ 3 , 4, 8,3b d (Fig. 1)) expanded with buta-1,3-diynediyl moieties between
the CˆC bonds were prepared by repetitive acetylenic scaffolding of 3-(cyclohexylidene)penta-1,4-diyne
building blocks (Scheme). These remarkably unstable iso-poly(triacetylene) (iso-PTA) oligomers were
characterized by 1H- and 13CÀNMR (Fig. 3 and Table 1), IR, and UV/VIS (Figs. 4 and 6 and Table 2)
spectroscopy, as well as mass spectrometry (Fig. 2). The expanded [8]dendralene contains 40 C(sp)- and C(sp2)-
atoms in the backbone and represents the longest iso-PTA oligomer prepared to date. HOMO-LUMO Gap
energies were determined as a function of oligomeric length (Fig. 5 and Table 3), providing insight into the
degree of p-electron delocalization in these cross-conjugated chromophores. A continous drop in the HOMO-
LUMO gap with increasing number of monomeric repeating units provides evidence that cross-conjugation
along the oligomeric backbone is effective to some extent. The limiting HOMO-LUMO gap energy for an
infinitely long, buta-1,3-diynediyl-expanded dendralene was extrapolated to about 3.3 3.5 eV. The conforma-
tional preferences of the expanded dendralenes were analyzed in semi-empirical calculations, revealing
energetic preferences for planar or slightly twisted s-cis and −U-shaped× geometries.
1. Introduction. Dendralenes are polyene hydrocarbons in which CˆC bonds are
aligned in a cross-conjugated arrangement [1a]. This requires the presence of at least
three CˆC bonds, and the simplest dendralene, therefore, is 3-(methylidene)penta-1,4-
diene (a [3]dendralene). Novel synthetic approaches to dendralenes are currently
being developed [1d,e], and these chromophores are increasingly investigated for their
structural, electronic, and advanced materials properties [1]. Upon insertion of one or
more acetylene units between the CˆC bonds, series of expanded dendralenes are
obtained (Fig. 1). Diederich and co-workers [2] reported in 1995 the synthesis of the
iso-poly(triacetylene)s 1a d by acetylenic scaffolding starting from appropriate
tetraethynylethene (TEE, 3,4-(diethynyl)hex-3-ene-1,5-diyne) precursors. Compounds
1c and 1d in this series are the first examples of expanded dendralenes, with buta-1,3-
diynediyl bridges inserted between the CˆC bonds. More recently, Tykwinski and co-
workers [3] introduced a new class of expanded dendralenes 2c i with the iso-
poly(diacetylene) backbone, featuring ethynediyl spacers between the CˆC bonds (the
names iso-poly(diacetylene) (iso-PDA) and iso-poly(triacetylene) (iso-PTA) were
introduced by Tykwinski and co-workers [3b]). In addition to compounds 2a i, with
peripheral isopropylidene moieties, derivatives with peripheral cyclohexylidene and
adamantylidene fragments were also reported [3d].

2170
Helvetica Chimica Acta Vol. 84 (2001)
Fig. 1. The parent class of dendralenes [1], iso-poly(triacetylene)s 1a d [2], 3a d (this work), and 4a d [5]
featuring buta-1,3-diynediyl-expanded dendralenes, and iso-poly(diacetylene)s 2a i [3d] containing ethynediyl-
expanded dendralenes
We became interested in a direct comparison between the properties of buta-1,3-
diynediyl- and ethynediyl-expanded dendralenes [4]. However, the peripheral
perethynylation in the earlier iso-PTA series 1a d prevented direct comparison to
the alkyl-substituted iso-PDAs such as 2a i, and the real consequences of expanding
the bridges between the CˆC bonds from a C2- to a C4-fragment could not be extracted
from these two series. Therefore, we decided to prepare the new series of iso-PTAs 3a
d, featuring the expanded dendralenes 3b d, with peripheral cyclohexylidene groups
rather than alkyne substituents (as in 1a d). Here, we report the synthesis and
characterization of 3b d as well as UV/VIS-spectral investigations on the extent of
cross-conjugation in these novel expanded dendralene chromophores. During the
preparation of this paper [4], Tykwinski and co-workers [5] reported the synthesis of
the related iso-PTAs 4a d but with peripheral Me groups and elongated acetylenic end
groups. In addition to the structural differences, the two studies contrast in the synthetic
strategy, with the formation of 3a d being based on sequential oxidative acetylenic
coupling [6] and the preparation of 4a d taking advantage of Pd-catalyzed
C(sp)ÀC(sp2) cross-coupling [7] for the construction of the oligomeric backbone.
2. Results and Discussion. 2.1. Synthesis of the Expanded Dendralenes 3b d. The
differentially protected penta-1,4-diyne 5 was prepared by Pd-catalyzed cross-coupling
between (i-Pr)3SiCꢀCH and vinyl triflate 6 [3a] (Scheme). Next, 5 was selectively
deprotected (K2CO3 in MeOH/THF) to give 7, which was oxidatively homo-coupled
under Hay conditions [8], to afford iso-PTA dimer 3a. Treatment of 3a with 2 equiv. of
Bu4NF gave the terminally bis-deprotected dimer 8 as a surprisingly stable white solid.
The deprotection had to be performed at ca. À308, since the reaction at room
temperature led to complete decomposition. Treating 3a with only 0.1 0.2 equivalents

Helvetica Chimica Acta Vol. 84 (2001)
2171
Scheme. Synthesis of Expanded Dendralenes 3b
d
a) (i-Pr)3SiCꢀCH (excess), [Pd(PPh3)4], CuI, Et2NH, THF, r.t. b) K2CO3, MeOH, THF, r.t. c) CuCl, TMEDA,
CH2Cl2, air, r.t. d) Bu4NF (2 equiv.), THF, À308; e) Bu4NF (0.1 0.2 equiv.), THF, À308; f) Bu4NF (0.36
equiv.), THF, À308. Tf ˆ Triflate. TMEDA ˆ N,N,N',N'-tetramethylethylenediamine.
of Bu4NF at À308 yielded a mixture of unreacted (3a), mono-deprotected (9), and bis-
deprotected (8) dimer. Progress of the reaction upon dropwise addition of Bu4NF was
conveniently monitored by TLC (SiO2; hexanes/CH2Cl2 6 :1), since the three
compounds had significantly different Rf values (3a: 0.54, 9: 0.40, 8: 0.27). When
TLC indicated a product ratio of ca. 1:1:1, no more Bu4NF was added. The
requirement of less than 0.5 equiv. suggests that the first deprotection step (3a ! 9) is
significantly faster than the second one (9 ! 8). After separation by column
chromatography, mono-deprotected 9 was hetero-coupled under Hay conditions with
mono-deprotected 7, to yield expanded [3]dendralene 3b, together with 3a and

2172
Helvetica Chimica Acta Vol. 84 (2001)
expanded [4]dendralene 3c as by-products. To enforce formation of the [3]dendralene
as the major product, an excess of monomeric 7 was used. The desired product 3b was
isolated in pure form by repeated column chromatography.
Mono-deprotected iso-PTA dimer 9 was also readily homo-coupled, which
provided the expanded [4]dendralene 3c. Gratifyingly, mono-deprotection of 3c with
Bu4NF could again be monitored by TLC, since unreacted 3c and the corresponding
terminally mono- and bis-deprotected derivatives featured significantly different Rf
values. Pure mono-deprotected [4]dendralene 10 was isolated by column chromatog-
raphy and subsequently homo-coupled under Hay conditions, affording expanded
[8]dendralene 3d, the longest iso-PTA oligomer prepared so far.
This new series of expanded dendralenes was found to be highly unstable,
experiencing both light and temperature sensitivity. In this respect, they are very
different from the related series of TEE oligomers 1a d. Thus, the expanded [3]- and
[4]dendralenes 1c and 1d are air- and light-stable yellow solids melting above 1008 [2].
Interestingly, the two extra geminal acetylene moieties in each monomeric repeat unit
in 1a d add a remarkable stability to the oligomeric iso-PTA scaffolds, which is not
well-understood at present. The instability possessed by the cyclohexylidene-substi-
tuted iso-PTAs 3a d was moreover reflected in the fact that we were not able to obtain
the related macrocycles, the expanded radialenes. Thus, all attempts of macrocycliza-
tion, starting from 3a c after desilylation, failed, employing the same dilute Hay-
coupling conditions that had successfully been used for constructing TEE-based
expanded radialenes [9].
2.2. Characterization of the New Expanded Dendralenes. The iso-PTA dimer 3a and
the [4]- (3c) and [8]- (3d) dendralenes are solids, whereas [3]dendralene 3b is an oil.
The [8]dendralene is less soluble than the shorter analogs, but it is still possible to
dissolve fair amounts in CH2Cl2 or CHCl3 for spectral characterization.
The Fourier-transform matrix-assisted laser-desorption-ionization (FT-MALDI)
mass spectrum of 3d (Fig. 2) confirmed the purity of this long oligomer, which contains
a total of 40 C-atoms in the backbone. The base peak in the spectrum corresponds to
‡
‡
the [M ‡ Na] ion (m/z 1473.8990, C106H122NaSi2 ; calc. 1473.8983).
1
In the H-NMR spectra of 3a d, the cyclohexylidene protons were found to
resonate at about the same chemical-shift values, that is as two multiplets at d 1.6 and
2.5 ppm, respectively. The terminal (i-Pr)3Si protons appeared as a singlet at 1.1 ppm in
the whole series. The ratio between the integrated intensities of cyclohexylidene and (i-
Pr)3Si resonances changed correctly according to the number of monomeric repeating
units and, hence, confirmed the structures.
The 13C-NMR spectra are quite informative, especially in the 80 170 ppm region.
It is possible to observe the different C(sp) and C(sp2) resonances from the iso-PTA
backbones as well as the cyclohexylidene C(sp2) signals. Fig. 3 and Table 1 show the
spectra of 3a d and, for comparison, of the bis-(Me3Si)-protected monomer 11 [10].
The cyclohexylidene C(sp2) resonances appear at the highest chemical shifts (d
163 170 ppm), while the signals in the region of 75 to 102 ppm correspond to the
backbone C-atoms. In the spectra of 3a c and 11 (as well as of 5), all the chemically
different C(sp2) resonances can be differentiated, whereas in the spectrum of
[8]dendralene 3d, some C(sp2) resonances seem to be overlapping. However, the
number of C-atoms per signal can be evaluated from the relative signal intensities and

Helvetica Chimica Acta Vol. 84 (2001)
2173
Fig. 2. FT-MALDI Mass spectrum of 3d (matrix: 2,5-dihydroxybenzoic acid)
Fig. 3. Partial 13C-NMR spectra of 11 (a) and 3a (b) in CDCl3, and of 3b (c), 3c (d), and 3d (e) in CD2Cl2

2174
Helvetica Chimica Acta Vol. 84 (2001)
Table 1. 13C-NMR Chemical Shifts [ppm] for iso-PTAs 3a d and Comparison Compound 11. Numbers in
brackets refer to the number of C-atoms corresponding to a given resonance.
Compound
Cyclohexylidene C(sp2)
Backbone C(sp) and C(sp2)
11
3a
3b
163.4 [1]
165.5 [2]
167.0 [2], 169.2 [1]
96.1 [2], 98.3 [2], 101.1 [1]
75.8 [2], 78.5 [2], 93.3 [2], 97.9 [2], 101.9 [2]
75.2 [2], 76.3[2], 77.3[2], 79.1 [2], 93.6 [2], 96.6 [1], 97.4 [2],
101.7 [2]
3c
167.0 [2], 169.6 [2]
75.1 [2], 76.0 [2], 76.4 [2], 77.1 [2], 77.9 [2], 79.1 [2], 93.6 [2],
96.5 [2], 97.4 [2], 101.7 [2]
3d
167.0 [2], 169.6 [2], 169.9 [4]
75.2 [2], 76.0 [2], 76.1 [8], 76.4 [2], 77.2 [2], 77.8 [8], 77.9 [2],
79.1 [2], 93.6 [2], 96.4 [4], 96.5 [2], 97.4 [2], 101.7 [2]
is in accordance to the structure. The different cyclohexylidene C(sp2) resonances are
readily differentiated as well (Table 1). In the IR-spectrum of bis-deprotected dimer 8,
a strong band appears at 3312 cmÀ1, corresponding to the CÀH vibration of the
terminal acetylenes. Since a similar band does not appear in any of the spectra of the
silyl-protected iso-PTA oligomers 3a d, it establishes the absence of unprotected
products in these compounds.
2.3. UV/VIS Spectroscopy. The UV/VIS spectra in CHCl3 of the iso-PTA oligomers
5 and 3a d (with expanded dendralenes 3b d) are shown in Fig. 4. All compounds in
the series, except from monomer 5, show the same vibrational fine-structure pattern.
Moreover, it is evident that the molar extinction coefficient increases almost
proportionally as the number of monomeric repeat units (n) increases. It ranges from
e ˆ 18600 mÀ1 cmÀ1 at lmax ˆ 308 nm for 3a to e ˆ 99000 mÀ1 cmÀ1 at lmax ˆ 316 nm for 3d
(Table 2).
Table 2. Molar Extinction Coefficients of the Band at ca. lmax 310 mm Compared to the Increasing Number of
Monomeric Repeat Units (n) in the iso-PTA Oligomers
Compound
lmax [nm]
e [MÀ1 cmÀ1
]
e/n [MÀ1 cmÀ1
]
3a
3b
3c
3d
308
312
313
316
18600
32200
42200
99000
9300
10700
10600
12400
Another interesting observation is the red-shift of the end-absorption as the
number of repeat units in the dendralenes increases. Three different approaches were
used to estimate the HOMO-LUMO gap, either from i) the longest-wavelength
absorption maximum lmax (with emax), ii) the wavelength at emax/2, or iii) the intercept
value (ltg) of the tangent to the curve close to the end-absorption [3d]. The obtained

Helvetica Chimica Acta Vol. 84 (2001)
2175
Fig. 4. UV/VIS Spectra in CHCl3 of 5 (a), 3a (b), 3b (c), 3c (d), and 3d (e)
Table 3. HOMO-LUMO Gap Energies (Eg) Calculated by Three Different Methods
Compound
Eg (lmax) [eV]
Eg (l(emax/2)) [eV]
Eg (ltg) [eV]
5
3a
8
3b
3c
3d
4.35
3.65
3.68
3.61
3.59
3.56
4.28
3.58
3.61
3.52
3.50
3.48
4.16
3.49
3.50
3.42
3.39
3.35
energies are tabulated in Table 3. All three methods show the same trend: the HOMO-
LUMO gap energy decreases as n increases.
The HOMO-LUMO gaps of 5 and 3a d (series a, Fig. 5) were compared to those
of iso-PTAs 1a d [2] composed of monomeric TEE units (series b) and of iso-PDAs
2b i [3d] (series c). This comparison allows analysis of the influence of peripheral
substituents and the number of acetylenes between the CˆC bonds on the electronic
properties of the expanded dendralenes.
The two extra acetylenes in each TEE repeating unit expectedly lower the HOMO-
LUMO gap in the iso-PTA series b by ca. 0.8 eV compared to iso-PTA series a with
peripheral cyclohexylidene residues. This lowering is readily explained by more-
extended linear p-electron conjugation.
The influence of the number of acetylene moieties between the CˆC bonds is
evident when comparing series a and c: Upon moving from iso-PDA to iso-PTA
oligomers, the HOMO-LUMO gap energy is lowered by 0.4 eV. The difference
between the peripheral groups (cyclohexylidene vs. isopropylidene) is negligible as

2176
Helvetica Chimica Acta Vol. 84 (2001)
Fig. 5. Correlations of the HOMO-LUMO gaps in iso-PTAs 5 and 3a-d (series a), iso-PTAs 1a d (series b) [2],
~
&
^
and iso-PDAs 2b i (series c) [3 d]. Corresponds to lmax
,
to l at emax/2, and to ltg.
shown by comparison of the UV/VIS spectra of iso-PTA dimers containing the two
types of peripheral groups [11].
UV/VIS and structural data [1] strongly indicate the absence of cross-conjugation in
the parent, nonexpanded dendralenes. In contrast, all three series of expanded
dendralenes show some extent of cross-conjugation as indicated by the shift of the end-
absorption with increasing number of monomeric subunits (and increased possible
longest-conjugation path). However, it is not straightforward to extrapolate when
saturation occurs, i.e., at which oligomeric length the Eg value of an infinite polymer is
reached [12], in particular since the differently-sized oligomers in one series may have
different conformational preferences (vide infra). A tentative extrapolation for series a
(3a d) indicates a minimal Eg value of 3.3 to 3.5 eV for infinite buta-1,3-diynediyl-
expanded dendralenes.
It is also interesting to compare the new iso-PTA series 3a d with the linearly-
conjugated PTAs composed of (E)-1,2-diethynylethene (DEE, (E)-hex-3-ene-1,5-
diyne) repeating units [13]. The effective conjugation length in these oligomers is
reached at ca. 10 monomeric DEE units, corresponding to a limiting HOMO-LUMO
gap of 2.5 2.8 eV, that is a reduction of ca. 0.8 eV relative to cross-conjugated iso-
PTAs.
The spectrum of bis-deprotected iso-PTA dimer 8 was recorded to obtain more
information on the influence of the (i-Pr)3Si end groups on the UV/VIS absorptions
(Fig. 6). Compared with bis-protected dimer 3a, the spectrum of unprotected 8 reveals
a small blue-shift of the longest-wavelength absorptions (ca. 3nm) and some changes in
the spectral shape are also apparent. But overall, the (i-Pr)3Si end groups seem to have

Helvetica Chimica Acta Vol. 84 (2001)
2177
Fig. 6. UV/VIS Spectra of bis[(i-Pr)3Si]-protected dimer 3a (a) and bis-deprotected dimer 8 (b)
only a small influence on the electronic absorptions as is also evident from the
comparision of the HOMO-LUMO gap energies in Table 3.
A comparison of lowest-energy absorptions was also made between 8 and a related
compound devoid of cross-conjugation. Thus, (Z,Z)-deca-2,8-diene-4,6-diyne (12) [14]
possesses a linear conjugation pathway of exactly the same length as the longest
possible one in 8. The two lowest-energy absorptions of 8 occur at lmax 325 and 337 nm,
whereas the lowest-energy absorption for 12 is at lmax 312 nm, with a very small
shoulder at 326 nm (in hexane). The bathochromic shifts observed for 8 relative to 12
indicate a significant degree of cross-conjugation in 8.
The analysis of the UV/VIS spectral data, including the control experiments,
indicates that the slight but non-negligible red-shifts of the longest wavelength maxima
and the optical end-adsorption with increasing length of the expanded dendralene do
reflect some further extension of p-electron delocalization via cross-conjugative
pathways as the number of monomeric repeat units increases.
2.4 PM3 Calculations. To gain insight into possible conformational preferences of
the cyclohexylidene-substituted iso-PTAs 3a, c and d, semi-empirical geometry
optimizations were performed at the PM3(perturbation method )3 level with

2178
Helvetica Chimica Acta Vol. 84 (2001)
Hyperchem [15], with the terminal (i-Pr)3Si groups substituted, for computational ease,
by H-atoms. Various starting geometries were used, and some of the obtained local
conformational minima are depicted in Fig. 7.
Fig. 7. PM3-Optimized structures of iso-PTA oligomers 3a,c,d. The (i-Pr)3Si terminal groups are replaced in the
calculations by H-atoms. Energies are given relative to the near-planar all-s-trans conformation of the oligomer.
For the dimer, the planar s-trans conformation, which was observed in several X-ray
crystal structures of PTA oligomers [2][16], was not found to be the most favorable
geometry. Thus, the s-cis and a slightly twisted s-trans conformer were À4.5 and
À4.6 kcal molÀ1, respectively, lower in energy than the planar s-trans conformer. This
relative sequence of conformational energies was confirmed by single-point calcu-
lations on the PM3-optimized structures, employing the Gaussian 98 program package
[17]. At the HF/6-31 ‡ G(d) level of theory; the calculated relative energies were 0
(planar s-trans), À8.9 (s-cis), and À9.2 (twisted s-trans) kcal molÀ1.
The planar all-s-trans conformation of the expanded [4]dendralene was also
calculated as the highest-energy geometry. A slightly twisted s-trans conformation was
found to be by À2.6 kcal molÀ1 lower in energy whereas a geometry with a dihedral
angle, defined by the four C(sp2)-atoms in the iso-PTA backbone, of ca. 908 was lower
in energy by À3.0 kcal molÀ1. In the most-stable calculated conformation
(À4.7 kcal molÀ1), this dihedral angle approachs 08, leading to a partly planar U-
shaped conformation.

Helvetica Chimica Acta Vol. 84 (2001)
2179
A significant difference between the parent and buta-1,3-diynediyl-expanded
dendralenes becomes apparent: the [4]dendralene prefers a dihedral angle of ca. 728
[1c,e], whereas the expanded [4]dendralene seems to be more stable in the near-planar
U-shape with a dihedral angle approaching 08, which should be favorable for p-electron
conjugation. Apparently, the spacing induced by the four acetylenic C-atoms between
the CˆC bonds is enough to reduce the steric interactions between the peripheral
cyclohexylidene moieties, thereby allowing a more-planar p-chromophore. Further-
more, it has been previously shown [18] that effective p-electron conjugation between
directly connected CꢀC and CˆC bonds does not require full planarity of the C4
fragment in view of the cylindrical p-electron conjugation in the acetylenic bond. Both
factors should enhance cross-conjugation in the expanded dendralenes as compared to
the parent compounds.
The calculations performed on the expanded [8]dendralene showed once more that
the planar all-s-trans conformation was not the most-stable one. The lowest energy
(À11.1 kcal molÀ1) was calculated for a U-shaped geometry with five monomeric
repeating units in one plane and the remaining three in another one.
It should be pointed out that the lowest-energy conformations obtained in this study
are not necessarily the global minima. However, the present data do indeed reveal one
general trend for iso-PTA oligomers: planar or slightly twisted s-cis and U-shaped
conformations are energetically more favorable than the all-s-trans conformations,
which is in contrast to the findings in PTAs with linearly conjugated backbones.
3. Conclusions. A series of expanded [n]dendralenes 3b d (n ˆ 3, 4, 8) with an
iso-poly(triacetylene) (iso-PTA) backbone and peripheral cyclohexylidene substitu-
ents were synthesized from mono- and dimeric precursors and fully characterized.
These compounds are remarkably less stable than the TEE-based expanded
dendralenes and are sensitive to both light and temperature. The UV/VIS spectra of
3b d reveal interesting electronic properties: their HOMO-LUMO gap energies (Eg)
are located between those of the corresponding TEE-based expanded dendralenes (1c
and d) and expanded dendralenes with the iso-poly(diacetylene) (iso-PDA) backbone
(2c i). Their spectra show a slight red-shift as the number of monomeric repeat units
increases, which indicates a small but non-negligible degree of p-electron cross-
conjugation along the oligomeric backbone. The HOMO-LUMO gap energy for an
infinitely long buta-1,3-diynediyl-expanded dendralene is estimated to 3.3 3.5 eV.
Calculations performed at the PM3level suggest that planar or slightly twisted s -cis and
U-shaped conformations of iso-PTA oligomers are of lower energy than all-s-trans
conformations, which is in contrast to the experimentally demonstrated conformational
preferences of linearly p-conjugated PTA oligomers. The study once again underlines
the great value of the oligomeric approach in exploring the electronic properties of
polymers with novel conjugated backbones [12][19].
Experimental Part
General. Chemicals were purchased from Aldrich and Fluka, and used as received. All reactions, except
Hay couplings, were carried out under a positive pressure of Ar. For the Hay couplings, the following mixture
was used as Hay catalyst: CuCl (0.13g) and N,N,N,N-tetramethylethylenediamine (TMEDA, 0.16 g) in CH2Cl2
(4.5 ml). Column chromatographic (CC) purification refers to flash chromatography with the solvent mixture in

2180
Helvetica Chimica Acta Vol. 84 (2001)
the given ratio on SiO2-60 (230 400 mesh). M.p.: B¸chi 510 melting-point apparatus; uncorrected. UV/VIS
(lmax [nm] (e [MÀ1 cmÀ1])): CARY 500 UV/VIS-NIR spectrophotometer. IR [cmÀ1]: Nicolet 600 FT-IR
spectrometer. 1H- and 13C-NMR: Varian Gemini 300 MHz spectrometer; chemical-shift values are reported in
ppm relative to residual solvent peaks. EI-MS VG-Tribid instrument; high-resolution- (HR) MALDI spectra:
IonSpec Fourier Transform instrument by a two-layer technique (tl), with 2,5-dihydroxybenzoic acid (DHB) as
matrix and the compound typically dissolved in CH2Cl2. Elemental analyses were performed by the Mikrolabor
at the Laboratorium f¸r Organische Chemie, ETH Z¸rich.
3-Cyclohexylidene-1-(triisopropylsilyl)-5-(trimethylsilyl)penta-1,4-diyne (5). Compound
6 (1.38 g,
4.06 mmol) [3a] was dissolved in Ar-degassed THF (35 ml) and Et2NH (5 ml). (Triisopropylsilyl)acetylene
(2.70 ml, 12.2 mmol), [Pd(PPh3)4] (116 mg, 0.100 mmol), and CuI (62 mg, 0.326 mmol) were added, and the
mixture was stirred at r.t. under Ar for 19 h. Et2O (200 ml) was added, and the org. phase was washed with H2O
(200 ml) and sat. aq. NH4Cl soln. (2 Â 100 ml), dried (MgSO4), and concentrated in vacuo. CC (SiO2; hexanes,
then hexanes/CH2Cl2 10 :1) afforded 5 (1.22 g, 81%). Colorless oil. UV/VIS (CHCl3): 268 (15600), 285 (sh,
9100). IR (CCl4): 2958s, 2942s, 2892m, 2865s, 2150m, 2120w, 1593w, 1462m, 1448w, 1408w, 1383w, 1366w, 1348w,
1337w, 1283w, 1250m, 1231w, 1187w, 1132w, 1113w, 1072w, 1011m, 996w, 940w, 919w, 897w, 883m, 858s, 853s,
845s. 1H-NMR (300 MHz, CDCl3): 0.19 (s, 9 H); 1.09 (s, 21 H); 1.55 1.66 (m, 6 H); 2.47 2.54 (m, 4 H).
13C-NMR (75 MHz, CDCl3): 0.12; 11.5; 18.8; 26.2; 27.6 (2Â); 32.8; 32.9; 92.7; 96.0; 98.7; 101.5; 103.1; 162.3. EI-
‡
‡
MS: 372 (M ), 329 ([M À Pr] ), 287, 259, 129, 73, 59. Anal. calc. for C23H40Si2 (372.74): C 74.11, H 10.82; found:
C 74.22, H 10.82.
3,8-Bis(cyclohexylidene)-1,10-bis(triisopropylsilyl)deca-1,4,6,9-tetrayne (3a). A mixture of 5 (300 mg,
0.805 mmol) and K2CO3 (111 mg, 0.805 mmol) in wet MeOH (10 ml) and THF (10 ml) was stirred at r.t. under
Ar for 1.5 h. Et2O (200 ml) and H2O (200 ml) were added, and the org. phase was dried (MgSO4) and
concentrated in vacuo to a few ml. The residue was dissolved in CH2Cl2 (15 ml), whereupon Hay catalyst
(1.5 ml) was added, and the mixture was stirred under air for 19.5 h. CC (SiO2; hexanes/CH2Cl2 6 :1) afforded 3a
(207 mg, 86%) as an oily product that solidified upon standing. White solid. M.p. 738. UV/VIS (CHCl3): 265
(24800), 273 (27000), 290 (20300), 308 (18600), 317 (sh, 14400), 328 (12200), 340 (11500). IR (CCl4): 2941s,
2891m, 2865s, 2146m, 1578w, 1462m, 1448m, 1383w, 1366w, 1348w, 1336w, 1317w, 1281w, 1255w, 1229w, 1177w,
1130w, 1117w, 1071w, 1052w, 1018w, 1006w, 996w, 921w, 883m, 854w. 1H-NMR (300 MHz, CDCl3): 1.09 (s, 42 H);
1.55 1.67 (m, 12 H); 2.50 2.58 (m, 8 H). 13C-NMR (75 MHz, CDCl3): 11.4; 18.8; 26.2; 27.7; 27.9; 33.2 (2Â);
‡
‡
75.8; 78.5; 93.3; 97.9; 101.9; 165.5. EI-MS: 599 (M ), 556 ([M À Pr] ), 513, 471, 256, 193, 157, 115, 87, 59. Anal.
calc. for C40H62Si2 (599.10): C 80.19, H 10.43; found: C 80.36, H 10.24.
3,8,13-Tris(cyclohexylidene)-1,15-bis(triisopropylsilyl)pentadeca-1,4,6,9,11,14-hexayne (3b). A soln. of 3a
(118 mg, 0.197 mmol) in THF (25 ml) was degassed under Ar and cooled to À308. Bu4NF (1m in THF, 0.04 ml,
0.04 mmol) was added dropwise over 30 min, and the reaction was carefully followed by TLC (SiO2; hexanes/
CH2Cl2 6 :1). To the mixture containing 3a, 8, and 9, Et2O (200 ml) was added, and the org. phase was washed
with H2O (2 Â 200 ml), dried (MgSO4), and concentrated to a few ml. CC (SiO2; hexanes/CH2Cl2 6 :1) gave
pure 9 (Rf 0.40) as the second fraction.
A mixture of 5 (100 mg, 0.268 mmol) and K2CO3 (37 mg, 0.27 mmol) in wet MeOH (5 ml) and THF (5 ml)
was stirred at r.t. under Ar for 1 h. Et2O (200 ml) and H2O (200 ml) were added, and the org. phase, containing 7,
was dried (MgSO4) and concentrated to a few ml.
Mono-deprotected 7 and 9 were dissolved in CH2Cl2 (15 ml), and Hay catalyst (3ml) was added. After
stirring under air for 18 h, CH2Cl2 (200 ml) was added, and the org. phase was washed with H2O (2 Â 200 ml),
dried (MgSO4), and concentrated to a few ml. CC (SiO2; hexanes/CH2Cl2 8 :1) afforded 3b (25 mg, 17%). White
oily solid. UV/VIS (CHCl3): 263 (39600), 275 (37600), 292 (31200), 312 (32200), 332 (23200), 343 (sh, 20400).
IR (CCl4): 2941s, 2891m, 2865s, 2147m, 1578w, 1462m, 1448m, 1439w, 1383w, 1367w, 1348w, 1336w, 1316w, 1281w,
1271w, 1255w, 1229w, 1173w, 1124w, 1110w, 1072w, 1017m, 1004w, 996m, 920w, 883m, 854w. 1H-NMR (300 MHz,
CD2Cl2): 1.07 (s, 42 H); 1.54 1.68 (m, 18 H); 2.48 2.60 (m, 12 H). 13C-NMR (75 MHz, CD2Cl2): 11.3; 18.6;
26.0; 26.1; 27.8; 28.0; 33.3; 33.5; two coincident peaks not observed; 75.2; 76.3; 77.3; 79.1; 93.6; 96.6; 97.4; 101.7;
‡
‡
‡
167.0; 169.2. HR-MALDI-MS (DHB-tl): 740.5137 (M , C51H72Si2 ; calc. 740.5173), 763.5070 ([M ‡ Na] ,
‡
C51H72NaSi2 ; calc. 763.5065).
3,8,13,18-Tetrakis(cyclohexylidene)-1,20-bis(triisopropylsilyl)icosa-1,4,6,9,11,14,16,19-octayne (3c). A soln.
of 3a (821 mg, 1.37 mmol) in THF (150 ml) was degassed under Ar and cooled to À308. Bu4NF (1m in THF,
0.15 ml, 0.15 mmol) was added dropwise over 70 min, and the reaction was followed carefully by TLC (SiO2;
hexanes/CH2Cl2 6 :1). To the mixture containing 3a, 8, and 9, Et2O (200 ml) was added, and the org. phase
washed with H2O (3 Â 200 ml), dried (MgSO4), and concentrated to a few ml. CC (SiO2; hexanes/CH2Cl2 6 :1)
gave pure 9 (Rf 0.40) as the second fraction. The residue was dissolved in CH2Cl2 (60 ml), then Hay catalyst

Helvetica Chimica Acta Vol. 84 (2001)
2181
(5.5 ml) was added, and the mixture was stirred under air for 17 h. Et2O (200 ml) was added, and the org. phase
was washed with H2O (3 Â 200 ml), dried (MgSO4), and concentrated in vacuo. CC (SiO2; hexanes/CH2Cl2 4 :1)
afforded 3c (171 mg, 28%). White solid. M.p. 708. UV/VIS (CHCl3): 263 (51100), 275 (45500), 293 (39900), 313
(42200), 334 (31600), 345 (sh, 26400). IR (CCl4): 2940s, 2892m, 2864s, 2145w, 1576w, 1462m, 1448m, 1440w,
1383w, 1367w, 1348w, 1337w, 1281w, 1261w, 1255w, 1229w, 1172w, 1144w, 1127w, 1109w, 1071w, 1041w, 1018m,
1010m, 1003m, 997m, 919w, 883m, 854w. 1H-NMR (300 MHz, CDCl3): 1.09 (s, 42 H); 1.55 1.69 (m, 24 H);
2.48 2.58 (m, 16 H). 13C-NMR (75 MHz, CD2Cl2): 11.5; 18.6; 26.0; 26.1; 27.8; 28.0; 33.3 (2Â), 33.3; 33.5; two
coincident peaks not observed; 75.1; 76.0; 76.4; 77.1; 77.9; 79.1; 93.6; 96.5; 97.4; 101.7; 167.0; 169.6. HR-MALDI-
‡
‡
MS (DHB-tl): 905.5867 ([M ‡ Na] , C62H82NaSi2 ; calc. 905.5858). Anal. calc. for C62H82Si2 (883.50): C 84.29, H
9.35; found: C 84.27, H 9.39.
3,8,13,18,23,28,33,38-Octakis(cyclohexylidene)-1,40-bis(triisopropylsilyl)tetraconta-1,4,6,9,11,14,16,19,21,
24,26,29,31,34,36,39-hexadecayne (3d). A soln. of 3c (171 mg, 0.194 mmol) in THF (50 ml) was degassed under
Ar and cooled to À308. Bu4NF (1m in THF, 0.07 ml, 0.07 mmol) was added dropwise over 60 min, and the
reaction was carefully followed by TLC (SiO2; hexanes/CH2Cl2 5 :1). To the mixture containing unreacted,
mono-, and bis-deprotected 3c, Et2O (200 ml) was added, and the org. phase was washed with H2O (2 Â 200 ml),
dried (MgSO4), and concentrated to a few ml. CC (SiO2; hexanes/CH2Cl2 5 :1) afforded pure 10 as the second
fraction. This compound was dissolved in CH2Cl2 (20 ml), then Hay catalyst (1.5 ml) was added, and the mixture
was stirred under air for 17 h. CC (SiO2; hexanes/CH2Cl2 5 :1) afforded 3d (26 mg, 18%) containing a slight
amount of impurities. Pure 3d was obtained by precipitation from CH2Cl2/hexanes in the refrigerator. White
solid. M.p. dec. > 1208. UV/Vis (CHCl3): 263 (sh, 120800), 277 (94200), 296 (87600), 316 (99000), 335 (76900),
348 (sh, 62000). IR (CCl4): 2960m, 2933s, 2890w, 2862m, 2145w, 1700w, 1653w, 1559m, 1460w, 1448m, 1439w,
1
1347w, 1336w, 1281w, 1261s, 1228w, 1171w, 1098s, 1015s, 885w, 856w. H-NMR (300 MHz, CD2Cl2): 1.07 (s, 42
H); 1.51 1.68 (m, 48 H); 2.46 2.58 (m, 3 2 H).13C-NMR (75 MHz, CD2Cl2): 11.5; 18.6; 26.0; 26.1; 27.8; 28.0;
33.3; 33.5; fourteen coincident peaks not observed; 75.2; 76.0; 76.1 (4Â); 76.4; 77.2; 77.8 (4Â); 77.9; 79.1; 93.6;
‡
96.4 (2Â); 96.5; 97.4; 101.7; 167.0; 169.6; 169.9 (2Â). HR-MALDI-MS (DHB-tl): 1473.8990 ([M ‡ Na] ,
‡
C
106H122NaSi2 ; calc. 1473.8983).
3,8-Bis(cyclohexylidene)deca-1,4,6,9-tetrayne (8). A soln. of 3a (103mg, 0.172 mmol) in THF (20 ml) was
degassed under Ar and cooled to À308, then Bu4NF (1m in THF, 0.34 ml, 0.34 mmol) was added dropwise. Et2O
(200 ml) was added, and the org. phase was washed with H2O (2 Â 200 ml), dried (MgSO4), and concentrated to
a few ml. CC (SiO2; hexanes/CH2Cl2 6 :1) provided 8 (15 mg, 30%). White solid. M.p. 1138. UV/VIS (CHCl3):
259 (24500), 271 (20900), 287 (15800), 293 (sh, 14300), 306 (18100), 313 (17100), 325 (14300), 337 (14400). IR
(CCl4): 3312s, 2962m, 2936s, 2894w, 2859m, 2837w, 1584w, 1448m, 1440w, 1348w, 1281w, 1261s, 1230w, 1161w,
1099s, 1015s, 855w. 1H-NMR (300 MHz, CDCl3): 1.55 1.68 (m, 12 H); 2.51-2.57 (m, 8 H); 3 .09 s(, 2 H).
13C-NMR (75 MHz, CDCl3): 26.0; 27.7; 27.8; 33.1; 33.2; 75.9; 78.4; 79.1; 79.7; 96.4; 166.6. MALDI-MS (DHB-tl):
‡
286 (M ). Anal. calc. for C22H22 (286.42): C 92.26, H 7.74; found: C 92.18, H 7.66.
This work was supported by a grant from the ETH Research Council and by the Fonds der Chemischen
Industrie.
REFERENCES
[1] a) H. Hopf, Angew. Chem. 1984, 96, 947; Angew. Chem., Int. Ed. 1984, 23, 948; b) M. H. Palmer, J. A. Blair-
Fish, P. Sherwood, J. Mol. Struct. 1997, 412, 1; c) P. T. Brain, B. A. Smart, H. E. Robertson, M. J. Davis,
D. W. H. Rankin, W. J. Henry, I. Gosney, J. Org. Chem. 1997, 62, 2767; d) S. Fielder, D. D. Rowan, M. S.
Sherburn, Angew. Chem., 2000, 112, 4501; Angew. Chem., Int. Ed. 2000, 39, 4331; e) H. Hopf, Angew.
Chem. 2001, 113, 727; Angew. Chem., Int. Ed. 2001, 40, 705.
[2] A. M. Boldi, J. Anthony, V. Gramlich, C. B. Knobler, C. Boudon, J.-P. Gisselbrecht, M. Gross, F. Diederich,
Helv. Chim. Acta 1995, 78, 779.
[3] a) S. Eisler, R. R. Tykwinski, Angew. Chem. 1999, 111, 2138; Angew. Chem., Int. Ed. 1999, 38, 1940; b) Y.
Zhao, R. R. Tykwinski, J. Am. Chem. Soc. 1999, 121, 458; c) S. Eisler, R. McDonald, G. R. Loppnow, R. R.
Tykwinski, J. Am. Chem. Soc. 2000, 122, 6917; d) Y. Zhao, K. Campbell, R. R. Tykwinski, J. Org. Chem.
2002, 67, 336.
[4] E. Burri, Diploma Thesis, Wintersemester 2001/02, ETH Z¸rich.
[5] Y. Zhao, R. McDonald, R. R. Tykwinski, J. Org. Chem. 2002, 67, 2805.

2182
Helvetica Chimica Acta Vol. 84 (2001)
[6] P. Siemsen, R. C. Livingston, F. Diederich, Angew. Chem. 2000, 112, 2740; Angew. Chem., Int. Ed. 2000, 39,
2632.
[7] −Metal-catalyzed Cross-coupling Reactions×, Eds. F. Diederich, P. J. Stang, Wiley-VCH, Weinheim, 1997.
[8] A. S. Hay, J. Org. Chem. 1962, 27, 3320.
[9] A. Boldi, F. Diederich, Angew. Chem. 1994, 106, 482; Angew. Chem., Int. Ed. 1994, 33, 468; J. Anthony, A.
Boldi, C. Boudon, J.-P. Gisselbrecht, M. Gross, P. Seiler, C. B. Knobler, F. Diederich, Helv. Chim. Acta 1995,
78, 797; M. Schreiber, R. R. Tykwinski, F. Diederich, R. Spreiter, U. Gubler, C. Bosshard, I. Poberaj, P.
G¸nter, C. Boudon, J.-P. Gisselbrecht, M. Gross, U. Jonas, H. Ringsdorf, Adv. Mater. 1997, 9, 339; M. B.
Nielsen, M. Schreiber, Y. G. Baek, P. Seiler, S. Lecomte, C. Boudon, R. R. Tykwinski, J.-P. Gisselbrecht, V.
Gramlich, P. J. Skinner, C. Bosshard, P. G¸nter, M. Gross, F. Diederich, Chem. Eur. J. 2001, 7, 3263.
[10] D. A. Shultz, K. P. Gwaltney, H. Lee, J. Org. Chem. 1998, 63, 4034.
[11] Y. Zhao, S. C. Ciulei, R. R. Tykwinski, Tetrahedron Lett. 2001, 42, 7721.
[12] R. E. Martin, F. Diederich, Angew. Chem. 1999, 111, 1440; Angew. Chem., Int. Ed. 1999, 38, 1350.
[13] R. E. Martin, U. Gubler, J. Cornil, M. Balakina, C. Boudon, C. Bosshard, J.-P. Gisselbrecht, F. Diederich, P.
¬
G¸nter, M. Gross, J.-L. Bredas, Chem. Eur. J. 2000, 6, 3622.
[14] T. Bruun, P. K. Christensen, C. M. Haug, J. Stene, N. A. Sˆrensen, Acta Chem. Scand. 1951, 5, 1244; H. G.
Viehe, Chem. Ber. 1959, 92, 3064.
[15] Hyperchem Pro 5.1, Hypercube, Inc., Gainesville, Florida, 1997.
[16] M. J. Edelmann, M. A. Estermann, V. Gramlich, F. Diederich, Helv. Chim. Acta 2001, 84, 473.
[17] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, V. G. Zakrzewski,
J. A. Montgomery Jr., R. E. Stratmann, J. C. Burant, S. Dapprich, J. M. Millam, A. D. Daniels, K. N. Kudin,
M. C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S.
Clifford, J. Ochterski, G. A. Petersson, P. Y. Ayala, Q. Cui, K. Morokuma, D. K. Malick, A. D. Rabuck, K.
Raghavachari, J. B. Foresman, J. Cioslowski, J. V. Ortiz, A. G. Baboul, B. B. Stefanov, G. Liu, A. Liashenko,
P. Piskorz, I. Komaromi, R. Gomperts, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A.
Nanayakkara, C. Gonzalez, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, J. L.
Andres, C. Gonzalez, M. Head-Gordon, E. S. Replogle, J. A. Pople, GAUSSIAN 98, Rev. A.7, Gaussian,
Inc., Pittsburgh, PA, 1998.
[18] A. P. H. J. Schenning, J.-D. Arndt, M. Ito, A. Stoddart, M. Schreiber, P. Siemsen, R. E. Martin, C. Boudon,
J.-P. Gisselbrecht, M. Gross, V. Gramlich, F. Diederich, Helv. Chim. Acta 2001, 84, 296.
[19] −Electronic Materials: The Oligomer Approach×, Eds. K. M¸llen, G. Wegner, Wiley-VCH, Weinheim, 1997.
Received April 12, 2002