Peralkynylated buta-1,2,3-trienes: Exceptionally low-rotational barriers of camulenic C=C bonds in the range of those of peptide C-N bonds

Article abstract of DOI:10.1002/chem.200400218

A variety of 1,1,4,4-tetraal kynylbutatrienes and 1,4-dialkynylbutatrienes was synthezized by dimerization of the corresponding gem-dibromoolefins. Both ¹H and ¹³C NMR spectroscopy indicated that the di- and tetraalkynylated butatrie

Full text of DOI:10.1002/chem.200400218

FULL PAPER
Peralkynylated Buta-1,2,3-Trienes: Exceptionally Low Rotational Barriers of
ꢀ
Cumulenic C=C Bonds in the Range of Those of Peptide C N Bonds
Audrey Auffrant,[a] Bernhard Jaun,*[a] Peter D. Jarowski,[b] Kendall N. Houk,*[b] and
FranÁois Diederich*[a]
Dedicated to Prof. Julius Rebek, Jr. on the occasion of his 60th birthday
Abstract: A variety of 1,1,4,4-tetraal
kynylbutatrienes and 1,4-dialkynylbuta-
trienes was synthezized by dimerization
of the corresponding gem-dibromoole-
1,1,4,4-tetraalkynylbutatrienes, the acti-
vation barrier DG° was measured by
magnetization transfer to be around
20 kcalmolꢀ1, in the range of the barri-
er for internal rotation about a peptide
bond. Unlike the tetraalkynylated
[3]cumulenes, 1,4-dialkynylbutatrienes
are more difficult to isomerize and
could, in one case, be obtained isomeri-
cally pure. Based on experimental data,
the rotational barrier DG° for 1,4-di-
alkynylbutatrienes is estimated to be
around 25 kcalmolꢀ1. The hypothesis of
a stabilizing effect of the four alkynyl
substituents on the proposed but-2-
yne-1,4-diyl singlet diradical transition
state of this cis trans isomerization is
further supported by a computational
study.
1
fins. Both H and 13C NMR spectrosco-
py indicated that the di- and tetraal-
kynylated butatrienes are formed as a
mixture of cis and trans isomers. Varia-
ble temperature NMR studies evi-
denced a facile cis trans isomerization,
thus preventing the separation of these
isomers by gravity or high-performance
liquid chromatography (HPLC). For
Keywords: alkynes ¥ cumulenes ¥
density functional calculations ¥ iso-
merization ¥ magnetization transfer
Introduction
rationalized in terms of an increasing conjugative stabiliza-
tion of the proposed 908-twisted, singlet diradical-like transi-
tion state. Extrapolation to an infinite number of conjugated
double bonds gave an activation enthalpy of 18 kcalmolꢀ1.[3c]
Concerning cumulenes,[4] the rotational barriers (DG°
[kcalmolꢀ1]) for 1,3-dimethylallene (46.2),[5] 1,4-dimethylbu-
tatriene (31.8),[6] and 1,6-di(tert-butyl)-1,6-diphenylhexapen-
taene (20.0)[7a] have been measured, revealing strongly facili-
tated bond rotation with increasing cumulenic length.[7b] Fo-
cusing on butatrienes, the large bond length alteration evi-
denced by X-ray analysis, represents another remarkable
structural parameter. With a length of 1.22 1.25 ä, the cen-
tral double bond, is much shorter than the terminal ones,
measuring around 1.35 ä.[8,9] An interesting property result-
ing from this structural peculiarity is the facile, thermally in-
duced, 1,4-free-radical polymerization of butatrienes.[10]
In our efforts to develop new versatile building blocks for
acetylenic scaffolding in one, two, or three dimensions,[11] we
became interested in polyalkynylated [n]cumulenes. While
several 1,3-alkynylallenes have recently been prepared,[12]
symmetrically substituted 1,1,4,4-tetrakis[(trialkylsilyl)ethyn-
yl]butatrienes are the only known polyalkynylated [3]cumu-
lenes.[8b] Here, we report the synthesis of a series of various-
ly functionalized 1,1,4,4-tetraethynylbutatrienes and give a
preliminary account of their interesting optoelectronic prop-
The activation energy for the thermal cis trans isomeriza-
tion of 1,2-dideuteroethene has been determined as
65 kcalmolꢀ1.[1] Doering et al. showed that the rotational
barrier is considerably lowered in conjugated polyenes.
Thus, the activation enthalpy (DH° [kcalmolꢀ1]) for thermal
cis trans isomerization across the central double bond in a
series of semirigid conjugated all-trans polyenes decreases
from hexa-1,3,5-triene (38.9), to deca-1,3,5,7,9-pentaene
(31.9), to tetradeca-1,3,5,7,9,11,13-heptaene (27.8), and to
octadeca-1,3,5,7,9,11,13,15,17-nonaene (24.5).[2,3] This was
[a] Dr. A. Auffrant, Prof. Dr. B. Jaun, Prof. Dr. F. Diederich
Laboratorium f¸r Organische Chemie
ETH-Hˆnggerberg, HCI, 8093 Z¸rich (Switzerland)
Fax : (+41)1-632-1109
E-mail: diederich@org.chem.ethz.ch
[b] P. D. Jarowski, Prof. Dr. K. N. Houk
Department of Chemistry and Biochemistry
University of California, Los Angeles
California 90095-1569 (USA)
Fax : (+1)310-206-1843
E-mail: houk@chem.ucla.edu
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
2906
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/chem.200400218
Chem. Eur. J. 2004, 10, 2906 2911

2906 2911
erties. In particular, we demonstrate their surprisingly facile
thermal cis trans isomerization, featuring rotational barriers
DG° in the range of those observed for the internal rotation
lent yields.[18] The intensely colored aryl- and ferrocenyl-sub-
stituted tetraethynylbutatrienes 1a d are remarkably stable
and can be stored for months in the solid state at ꢀ308C.
On the other hand, 1,4-dialkynylbutatrienes, in particular
(iPr)3Si-protected 2d, exhibit only limited stability; this may
actually have prevented the isolation of product from the di-
merization of 4a and 4c.
about C N bonds in amides[13] and peptides.[14] Our theoreti-
ꢀ
cal study suggests that the exceptionally low rotational bar-
rier results in part from the large stabilization imparted to a
but-2-yne-1,4-diyl singlet diradical transition state by the
four alkynyl substituents. Stabilization of the ground state
by the alkynyl groups also plays a role in the observed ge-
ometry and the magnitude of the observed barrier.
1
Both H and 13C NMR spectra clearly showed that the di-
and tetraalkynylated butatrienes were formed as mixtures of
cis and trans isomers (in a ratio close to 1:1.5, without con-
figurational assignment).
Results and Discussion
Electronic absorption spectroscopy: First investigations of
the optical properties of these mixtures yielded promising
results. The UV-visible spectrum (CH2Cl2) of the purple
donor-substituted 1b (Figure 1) is dominated by a strong,
broad, longest wavelength absorption band at lmax =573 nm
(e=40500 Lmolꢀ1 cmꢀ1) and features an end-absorption
around 650 nm (1.84 eV). Upon acidification with p-toluene-
sulfonic acid, the purple solution turns yellow, while the in-
tense absorption at 573 nm disappears, the most intense ab-
Synthesis: The preparation[15] of the 1,1,4,4-tetraalkynylbuta-
trienes 1a d and the 1,4-dialkynylbutatrienes 2a d was ac-
complished by dimerization of the corresponding gem-dibro-
moolefins 3a d and 4a d, respectively (Scheme 1).[8b] The
gem-dibromoolefins were obtained in 20 58% yield by ad-
dition of the appropriate lithium acetylide to either (iPr)3Si-
protected propynal (!3a d) or paraformaldehyde (!4a
d), oxidation of the resulting
propargyl alcohols with MnO2,
and dibromoolefination.[16] The
dimerization to the butatrienes
was first attempted by metalla-
tion with Zn, followed by addi-
tion of a catalytic amount of
CuBr. This method, which
proved successful for the prepa-
ration of fluorinated buta-
trienes,[17] gave no product in
the present case. Alternatively,
treatment of the dibromoolefins
with one equivalent of nBuLi in
Et2O at ꢀ1108C followed by
one equivalent of [CuI¥PBu3] at
ꢀ858C afforded the alkynylated
Figure 1. Electronic absorption spectra of 1b in pure CH2Cl2 (c), after addition of p-toluene-4-sulfonic acid
(b), and after neutralization with Et3N (a).
butatrienes in modest to excel-
sorption band being shifted to 468 nm (2.56 eV). Neutraliza-
tion with triethylamine regenerates the original spectrum.
Such a behavior allows us to assign the strong longest wave-
length absorption band at lmax =573 nm as a charge-transfer
band, resulting from efficient intramolecular charge-transfer
interactions between the electron-donating anilino groups
and the electron-accepting all-carbon core. A comparison
with the similarly substituted tetraethynylethene (lmax
=
459 nm; Dlmax =114 nm, DE=0.53 eV) demonstrates that
the two additional C(sp) atoms strongly increase the elec-
tron-acceptor strength of the [3]cumulene core.
The cis trans isomerization: Based on the available data for
butatrienes,[6,15] we expected barriers to rotation high
enough to allow isolation and characterization of the pure
isomers at ambient temperature. However, exhaustive at-
tempts to separate the cis and trans isomers of 1,1,4,4-tet-
raalkynylbutatrienes 1a d by gravity or high-performance
Scheme 1. Synthesis of tetra- and dialkynylated butatrienes 1a d and 2a
ꢀ
d. a) n-BuLi, THF, ꢀ208C, then (iPr)3SiCꢁC CHO or paraformalde-
hyde; b) MnO2, Et2O, 20 8C; c) CBr4, PPh3, Zn (except for 3a and 3b),
CH2Cl2 or benzene (for 3a and 3b), 208C; yields over three steps;
d) nBuLi, Et2O, ꢀ1108C, then [CuI¥PBu3], ꢀ85!208C.
Chem. Eur. J. 2004, 10, 2906 2911
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¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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F. Diederich, K. Houk, B. Jaun et al.
FULL PAPER
Table 1. Activation parameters for the cis trans isomerization of tetraal-
kynylbutatrienes 1a and 1d.
liquid chromatography (HPLC) all failed. Therefore, we
eventually had to consider the possibility that cis trans iso-
merization would occur rapidly under ambient conditions,
thereby preventing the isolation of isomerically pure prod-
1
ucts. Variable-temperature (VT) H NMR studies (between
258C and 1008C) were undertaken as shown in Figure 2 for
the bisferrocenyl derivative 1d.
1a[a]
1d[b]
T [K] k1 [sꢀ1
]
T [K] k1 [sꢀ1
]
323.3
333.5
350.9
361.9
0.0798ꢃ0.0035 323.7
0.104ꢃ0.021
0.282ꢃ0.016
0.525ꢃ0.090
0.924ꢃ0.13
0.203ꢃ0.005
0.823ꢃ0.01
1.624ꢃ0.1
333.3
343.8
351.4
K
0.87ꢃ0.05
17.5ꢃ0.5
ꢀ9.4ꢃ2
0.68ꢃ0.0044
16.6ꢃ0.8
ꢀ11.7ꢃ3
20.1ꢃ1.2
DH°[c]
DS°[d]
DG°(298 K)[e] 20.3ꢃ0.8
[a] R=3,5-(tBu)2C6H3. [b] R=(C5H5)Fe(C5H4). [c] In kcalmolꢀ1. [d] In
calKꢀ1 molꢀ1. [d] In kcalmolꢀ1
.
for 1b and 1c could not be performed, since it was impossi-
ble to selectively excite a ™doublet∫ (J=9 Hz) for the aro-
matic protons of one isomer without affecting the other one.
Unlike the tetraethynylated [3]cumulenes, 1,4-dialkynyl-
1
butatrienes are more difficult to isomerize. Indeed, VT H
NMR (CDCl2)2 did not show any broadening for compound
2b up to 1008C (Supporting Information). Moreover for
compound 2d isomer separation by chromatography (SiO2,
hexane) was successful and trans-2d was isolated in pure
form in 13% yield.[20] However, prolonged heating of pure
trans-2d in toluene restored the mixture of cis and trans iso-
mers (Supporting Information). We conclude that the rota-
tional barrier DG° for 1,4-dialkynylbutatrienes such as 2d is
around 25 kcalmolꢀ1. A complete kinetic study to accurately
determine the activation parameters was not possible, be-
cause of the chemical instability of the two isomers.
Figure 2. VT 1H NMR spectra of 1d in (CDCl2)2.
1
The H NMR spectrum in (CDCl2)2 at 258C features two
triplets at 4.46 and 4.43 ppm, one for each isomer and as-
signable to two protons of the h5-C5H4 rings, one broad
signal at 4.25 ppm corresponding to the other protons of the
h5-C5H4 rings of both isomers, and two singlets at 4.18 and
4.17 ppm, each for the five h5-C5H5 protons of one isomer.
Upon heating, all resonances move downfield, while, more
interestingly, the characteristic signals of the two isomers
broadened and partially merged at 1008C. Similar VT NMR
spectral behavior was observed for the other tetraalkynyl
derivatives 1a c. Since coalescence would require heating to
temperatures well above 1208C at which the compounds
start to decompose, the determination of the activation pa-
rameters for the thermal cis trans isomerization by line
shape analysis was not further pursued. Rather, these data
were obtained by magnetization transfer[19] after selective in-
version of the signal of one isomer (for a depiction of the
spectral evolution, see the Supporting Information). The
temperature range investigated is the maximum window as
determined by the need to have significant transfer within
the delay time and on the other hand not too much longitu-
dinal relaxation of the signal. The results are gathered in
Table 1 and evidence a surprisingly low rotational barrier
DG° of 20.3 and 20.1 kcalmolꢀ1 at 298 K for the tetraalkynyl
derivatives 1a and 1d, respectively (for the Eyring plots, see
Supporting Information). These values for rotation around a
C=C double bond are in the range of those for rotation
about peptide bonds! Magnetization transfer experiments
Computational study: The comparison between the rotation-
al barriers DG° for 1,4-dimethylbutatriene (31.8 kcalmolꢀ1),[6]
1,4-dialkynylbutatriene 2d (ꢂ25 kcalmolꢀ1), and 1a/1d
(ꢂ20.2 kcalmolꢀ1) clearly demonstrates the dramatic stabi-
lizing effect of the alkynyl substituents on the transition
state of the isomerization process. Computational results at
the unrestricted B3LYP6-31G(d) level of theory calculated
with Gaussian 98,[21] show that the rotation of ethynyl-,
methyl-, and ethenyl-substituted butatriene, as well as the
parent butatriene,[10c] proceeds via perpendicular singlet dir-
adical transition states (Scheme 2).
The barrier to rotation and the ground state geometry are
both influenced by introduction of radical stabilizing groups.
The effect of substitution can be clearly illustrated by geo-
metric analysis of the ground and transition states
(Figure 3).
The ground-state a bond lengths (Scheme 2) of the deriv-
atives are shortened relative to butatriene, while the b bond
length is elongated. This effect is a function of the radical
stabilizing ability of the substituent on the ground-state dira-
dicaloid resonance form. The ground-state g bond length
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Chem. Eur. J. 2004, 10, 2906 2911

Peralkynylated Buta-1,2,3-Trienes
2906 2911
Figure 3. Ground- (left) and transition-state (right) geometries for butatriene, trans-1,4-dimethylbutatriene, tetramethylbutatriene, trans-1,4- diethenylbu-
tatriene, tetraethenylbutatriene, diethynylbutatriene, and tetraethynylbutatriene. Unrestricted B3LYP6-31G(d) computed bond lengths a, b, and g are
given along with experimental values in parentheses. a) See reference [22] for details. b) From the crystal structure analysis of the (Me3Si)4 derivative in
reference [8b]. c) Averaged bond lengths, see reference [8b].
sponding tetrasubstituted derivative. The g bonds for the
tetra-substituted derivatives tend to be longer than in the
corresponding disubstituted butatriene. In the transition
state a formal, doubly propargylic, diradical exists; these
geometric effects are expected to be enhanced. For example,
butatriene experiences a decrease in the a bond length of
0.01 ä and an increase in the b bond length of 0.043 ä,
while tetraethynylbutatriene has double the affect in the a
bond (0.02 ä) and an increase in b of 0.053 ä, a full 0.01 ä
more than the increase in butatriene. In the transition state,
Scheme 2. Bond assignments, a, b, and g, and singlet diradical transition-
state torsional geometries for unsubstituted and methyl-, ethenyl-, and
ethynyl-substituted butatriene derivatives.
the
a bond in tetraethenylbutatriene is shortened by
0.027 ä, and the b bond length is elongated by 0.062 ä. This
represents the largest geometric change in going from the
ground to transition state as expected from the larger radi-
cal stabilizing ability of this substituent.
Barriers to rotation for butatriene, trans-1,4-dimethylbuta-
triene, tetramethylbutatriene, trans-1,4- diethenylbutatriene,
tetraethenylbutatriene, trans-1,4-diethynylbutatriene, and
tetraethynylbutatriene were computed. Transition-state free
energies and enthalpies were calculated by adding the zero-
point energy and thermal correction to the calculated elec-
tronic energies (Table 2).
Comparison of the calculated barrier for tetraethynylbuta-
triene to the experimental for 1a shows good agreement for
the enthalpy of activation (16.0 vs 17.3 kcalmolꢀ1). The cal-
culated enthalpy of activation for dimethylbutatriene is also
in good agreement with experiment (29.0 vs 31.2 kcal
molꢀ1).[6] The free energy of activation, in both cases, devi-
ates to a greater degree. The 25 kcalmolꢀ1 estimate for the
tends to decrease in length as this ability increases. The pre-
dicted order of radical stabilizing ability is ethenyl>ethyn-
yl>methyl. The ethenyl and ethynyl groups are comparably
stabilizing, although both are considerably more stabilizing
than methyl.[23] With tetramethylbutatriene, methyl substitu-
tion has a minimal geometric effect on the a bond (1.271 to
1.269 ä), but a more pronounced effect on the b bond
(1.318 to 1.325 ä). Both ethynyl and ethenylation result in
large geometrical changes. The a bond in tetraethynylbuta-
triene is shorter than this bond length in butatriene by
0.02 ä, and the b bond length is longer by 0.034 ä. The tet-
raethenyl derivative experiences a nearly identical effect on
the geometry of the butatriene moiety as that of tetraethyn-
yl derivative. In all cases the 1,4-disubstituted butatriene ex-
periences smaller changes upon substitution than the corre-
Chem. Eur. J. 2004, 10, 2906 2911
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F. Diederich, K. Houk, B. Jaun et al.
FULL PAPER
Table 2. Unrestricted B3LYP6-31G(D) calculated rotational-energy bar-
riers [kcalmolꢀ1] for butatriene derivatives. The enthalpies and free ener-
gies of activation are given at 298 K. Singlet triplet vertical transition
energy gaps [DHSTVERT] are given for both the singlet ground and transi-
tion state geometries.
Conclusion
In summary, we have observed an unexpectedly facile cis
trans isomerization of novel di- and tetraalkynylbutatrienes,
proceeding most probably through singlet diradical transi-
tion states that benefit from a dramatic stabilization by the
alkynyl substituents. With four alkynyl substituents, the bar-
rier for cis trans isomerization in [3]cumulenes is lowered to
DG° around 20 kcalmolꢀ1, thereby resembling the rotational
barrier for the peptide bond.
VERT
Compound
Calculated DH°(DG°)
DH
ST
butatriene
butatriene TS
butatriene TS
0.0
64.2
2.2
28.5(28.1)
29.3(28.9)[a]
trans-1,4-dimethylbutatriene
trans-1,4-dimethylbutatriene TS
0.0
29.0(28.3)
62.8
2.1
tetramethylbutatriene
tetramethylbutatriene TS
0.0
29.4(28.7)
61.2
1.9
Experimental Section[16]
trans-1,4-diethynylbutatriene
trans-1,4-diethynylbutatriene TS
0.0
21.2(21.2)
42.7
1.4
General: All reactions were carried out under an inert atmosphere (of
argon or nitrogen) by applying a positive pressure. Chemicals were pur-
chased from commercial suppliers and used as received.
trans-1,4-diethenylbutatriene
trans-1,4-diethenylbutatriene TS
0.0
20.1(20.8)
41.0
1.3
Thin-layer chromatography (TLC) was conducted on alumina sheets pre-
coated with 0.25 mm Macherey-Nagel silica gel, with fluorescent indica-
tor. Column chromatography was carried out with SiO2 60 (particule size
0.04 0.063 mm, 230 400 mesh) from Fluka and distilled technical sol-
vents.
tetraethenylbutatriene
tetraethenylbutatriene TS
0.0
9.4(11.4)
28.9
0.5
tetraethynylbutatriene
tetraethynylbutatriene TS
0.0
16.0(16.8)
31.0
0.8
Melting points (m.p) were measured in open capillaries with a B¸chi 540
apparatus and are uncorrected.
[a] B3LYP6-311+G(2df,p).
Infrared-red (IR) spectra were recorded on a Perkin Elmer FT16000
spectrometer. Selected absorption bands are reported in wavenumbers
(cmꢀ1
)
and their relative intensity described as vs (very strong),
s
(strong), m (medium), w (weak) or vw (very weak).
free energy of activation of 1,4-diethynylbutatriene is sup-
ported by the computed value of 21.2 kcalmolꢀ1. Interesting-
ly, tetramethylation increases the barrier to rotation by
about 1 kcalmolꢀ1, although methyl groups should stabilize
the diradical transition state, thereby lowering the barrier to
rotation. This effect can be explained by the corresponding
ground-state stabilization of the cumulenic center imparted
by these same methyl groups. Ethynyl substituents also sta-
bilize the ground state, but the stabilization of the diradical
transition state is much larger resulting in a net decrease of
the transition state energy. The same argument can be made
for the ethenyl derivatives, which were found to have signifi-
cantly stabilized transition-state energies relative to their
ethynyl 1,4-disubstituted and tetrasubstituted counterparts.
The tetraethenyl derivative, with a low barrier of only
DH° =9.4 kcalmolꢀ1, and the 1,4-disubstituted compound
(DH° =20.1 kcalmolꢀ1) are predicted to undergo rapid iso-
merization and/or rotation at ambient temperatures. Inter-
estingly, the conformation of the ground state for the tetrae-
thenyl derivative is in the C2v point group, whereas the tran-
sition state geometry (D2d point group) corresponds to the
rotation of a C2h symmetric conformer.
Vertical triplet singlet energy gaps were also calculated
(Table 2). A pure diradical, in which the spatial overlap of
the SOMOs is zero, orthogonal in the present case, will
have a triplet singlet gap of zero. The energy of the triplet
in the ground-state geometry is dramatically lowered by
both ethenyl and ethynyl substituents; however, in all cases
the energy gap is nonetheless large. In the transition-state
geometry the triplet singlet gap is small, with the singlet
state also of lower energy, and approaches zero as the radi-
cal stabilization increases.
300 MHz 1H and 75 MHz 13C NMR spectra were recorded on Varian
Gemini 300 spectrometer. Chemical shifts are indicated in ppm downfield
from tetramethylsilane using the solvent×s peak as internal reference
(dH =7.25 ppm, dC =77.2 ppm).
Elemental analyses were performed by the Mikroelementaranalytisches
Laboratorium at ETH Hˆnggerberg.
General procedure for butatriene synthesis
Synthesis of 1,8-bis(3,5-di-tert-butylphenyl)-3,6-bis[(triisopropylsilyl)ethy-
nyl]octa-3,4,5-ene-1,7-diyne (1a):
A solution of nBuLi in hexane
(0.8 mmol, 0.53 mL) was added to a solution of 3a (0.461 g, 0.8 mmol) in
Et2O (7 mL) at ꢀ1108C. After stirring for 1 h at ꢀ1008C, a solution of
[CuI¥PBu3] (0.314 g, 0.8 mol) in Et2O (7 mL) was added, and the resulting
red solution was stirred for 1 h at ꢀ858C. After warming to 208C within
5 h, stirring was continued for 14 h. The mixture was filtered through
SiO2, and the solvent was removed under reduced pressure affording an
orange solid, which was purified by chromatography (SiO2; hexane/
CH2Cl2 9:1) to provide 1a (235 mg, 69%) as a mixture of cis and trans
conformers in a 39:61 ratio (1H NMR, without configurational assign-
ment). Rf =0.16 (hexane); m.p. 1228C (decomp); 1H NMR (300 MHz,
CDCl3) for the major isomer: d=1.15 (m, 42H; SiCH(CH3)3), 1.33 (s,
18H; CH3), 7.38 (d, J=2 Hz, 4H; CHAr), 7.43 ppm (t, J=2 Hz, 2H;
CHAr); for the minor isomer: d=1.16 (m, 42H; SiCH(CH3)3), 1.33 (s,
18H; CH3), 7.34 (d, J=2 Hz, 4H; CHAr), 7.43 ppm (t, J=2 Hz, 2H;
CHAr); 13C NMR (75 MHz, CDCl3): d=151.2 and 151.0 (CAr), 149.1
and 149.0 (C=C=C), 126.5 and 126.4 (CHAr), 124.0 (CHAr), 121.6 and
121.5 (CAr), 104.8 and 104.7 (Cꢁ), 100.4 and 100.0 (Cꢁ), 98.4 and 98.2
(Cꢁ), 88.0 (ꢁC), 87.6 (=C), 35.0 (C(CH3)3), 31.5 (CH3), 18.8 and 18.7
(CH3), 11.7 and 11.6 ppm (CH); IR (CCl4): n˜ =2964 (vs), 2866 (s), 2187
(m), 1866 (s), 1589 (m), 1539 (w), 1463 (m), 1427 (w), 1394 (w), 1364
(m), 1248 (w), 1184 (w), 1158 (w), 1072 (vw), 904 (w), 878 (m), 833 (w),
812 cmꢀ1 (vs); UV/Vis (CH2Cl2): l (e)=481 (41500), 456 (35500), 443 nm
(30600 Lmolꢀ1 cmꢀ1); MALDI-MS (matrix DCTB): m/z: 837 [M+], 794
[M+ꢀiPr]; HR-MALDI: m/z calcd for C58H84Si2 [M+]: 836.6112; found:
836.6117 (100) [M+].
Magnetization transfer measurements: The exchange rates were deter-
mined by the selective inversion variant of the Forsÿn Hoffman techni-
que[19a] by using the pulse sequence of PFGSE-NOE[19b c]and iSNOB[19d]
selective inversion pulses.
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Peralkynylated Buta-1,2,3-Trienes
2906 2911
Temperatures (ꢃ0.1 K) were measured with an internal Pt-100 resistance
thermometer inserted into the probehead instead of the NMR tube
before and after each measurement.
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spectroscopy, FT-IR spectroscopy, EI- or MALDI-MS, and elemen-
tal analysis or high-resolution MS. Detailed experimental proce-
dures and analytical characterizations will be reported elsewhere.
13C NMR resonances of the two central C(sp) atoms in the buta-
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Acknowledgement
This research was supported by the ETH Research Council, the German
Fonds der Chemischen Industrie, and the United States National Science
Foundation. We thank Philipp Zumbrunnen for the magnetization trans-
fer measurements.
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Received: January 21, 2004
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Published online: April 26, 2004
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¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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