Tetra(alkynyl)silanes, a 3,6-disila-triyne, a 3,6,9-trisila-tetrayne, a 1,3,4,6-tetrasiladiyne, and bis(trimethylstannyl)ethyne. molecular structures and solid-state NMR studies

Article abstract of DOI:10.1515/znb-2010-0204

The molecular structures of three alkynylsilanes, tetrakis(ethynyl-p-tolyl) silane, 3,3,6,6,-tetramethyl-3,6-disila-triyne, 3,3,6,6,9,9,-hexamethyl-3,6,9- trisila-tetrayne, and of bis(trimethylstannyl)-ethyne have been determined by X-ray diffraction. The same alkynylsilanes, and in addition 1,2- bis(trimethylsilylethynyl)-1,1,2,2-tetramethyldisliane, were studied by solid-state ¹³C and ²⁹Si MAS NMR spectroscopy. The results of these measurements were compared with crystallographic evidence and also with relevant solution-state NMR data.

Full text of DOI:10.1515/znb-2010-0204

Tetra(alkynyl)silanes, a 3,6-Disila-triyne, a 3,6,9-Trisila-tetrayne,
a 1,3,4,6-Tetrasiladiyne, and Bis(trimethylstannyl)ethyne.
Molecular Structures and Solid-state NMR Studies
Bernd Wrackmeyera, Ezzat Khana,b, Amin Badshaha,c, Elias Mollaa,d, Peter Thomaa,
Oleg L. Toka, Wolfgang Miliuse, Rhett Kempea, and Ju¨rgen Senkere
a Anorganische Chemie II, Universita¨t Bayreuth, 95440 Bayreuth, Germany
b Department of Chemistry University of Malakand, Chakdara, Dir(Lower), N.W.F.P., Pakistan
c Department of Chemistry, Quaid I Azam University, Islamabad, Pakistan
d Department of Chemistry, Jahangirnagar University, Savar, Dhaka-1342, Bangladesh
e Anorganische Chemie I, Universita¨t Bayreuth, 95440 Bayreuth, Germany
Reprint requests to Prof. Dr. B. Wrackmeyer. E-mail: b.wrack@uni-bayreuth.de
Z. Naturforsch. 2010, 65b, 119 – 127; received November 27, 2009
The molecular structures of three alkynylsilanes, tetrakis(ethynyl-p-tolyl)silane, 3,3,6,6,-tetra-
methyl-3,6-disila-triyne, 3,3,6,6,9,9,-hexamethyl-3,6,9-trisila-tetrayne, and of bis(trimethylstannyl)-
ethyne have been determined by X-ray diffraction. The same alkynylsilanes, and in addition 1,2-
bis(trimethylsilylethynyl)-1,1,2,2-tetramethyldisliane, were studied by solid-state 13C and 29Si MAS
NMR spectroscopy. The results of these measurements were compared with crystallographic evi-
dence and also with relevant solution-state NMR data.
Key words: Alkynes, Silanes, Organotin Compounds, NMR, X-Ray
Introduction
stannanes both by X-ray diffraction [12 – 14] and
by solid-state 13C and 119Sn NMR [9, 10, 12]. In
the present work, we report on three further exam-
ples of molecular structures of alkynylsilanes (1b,
2, 3) and combine this information with that from
solid-state 13C and 29Si NMR spectroscopy for 1 – 4
(Scheme 1). In addition, the molecular structure of
bis(trimethystannyl)ethyne (5) has been determined
which complements the thorough 13C and 119Sn NMR
spectroscopic study of the same molecule [9, 10].
Alkynylsilanes are useful starting materials in or-
ganic and organometallic synthesis [1 – 3]. Some
alkynylsilanes are commercially available, and many
examples can be readily prepared by conventional
methods, starting from the respective chlorosilanes.
Similarly, alkynylstannanes also deserve attention, al-
though they are somewhat more difficult to prepare and
to handle owing to the greater reactivity of the Sn–C
bonds, when compared with Si–C bonds. The molec-
ular structures of several alkynylsilanes have already
been reported [4 – 8], and in some cases solid-state
13C and 29Si NMR measurements have been carried
out [9 – 11]. There are also studies on solid alkynyl-
Results and Discussion
The tetraalkynylsilane 1b was prepared in the usual
way [15] by the reaction of SiCl4 with four equivalents
Scheme 1. Alkynylsilanes and bis(tri-
methylstannyl)ethyne, studied here.
c
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120
B. Wrackmeyer et al. · Solid-state NMR Studies
of the alkynyllithium reagent. The triyne 2 [16] and the
tetrayne 3 [17] were obtained via stepwise procedures
as reported. The disilane derivative 4 was obtained
from the reaction of 1,2-dichlorotetramethyldisilane
with two equivalents of trimethylsilylethynyllithium.
Bis(trimethylstannyl)ethyne (5) was prepared from the
reaction of Li2C2 with two equivalents of Me3SnCl as
described [18]. Recently, we have reported that a mod-
ification of the work-up procedure also affords the use-
ful trimethylstannylethyne as a side product in moder-
ate yield [19].
Fig. 2. Molecular structure of the triyne 2 in the solid
state (ORTEP, 40 % probability ellipsoids; hydrogen atoms
omitted for clarity). Selected bond lengths (pm) and an-
gles (deg): Si1–C1 182.90(13), Si1–C3 183.20(12), C1–C2
118.2(2), C3–C3 120.4(2); C1–Si1–C3 106.66(6), C1–Si1–
C5 109.95(8), C3–Si1–C5 108.70(6), C1–Si1–C4 109.29(7),
C3–Si1–C4 109.19(6), C5–Si1–C4 112.86(9), C2–C1–Si1
178.73(15), C3–C3–Si1 178.82(13).
X-Ray structural studies of the alkynylsilanes 1b, 2, 3,
and of bis(trimethylstannyl)ethyne (5)
The molecular structures of the alkynylsilanes 1b, 2,
3, and of bis(trimethylstannyl)ethyne (5) are shown in
the Figs. 1, 2, 3, and 4, respectively. Apparently inter-
molecular interactions appear to be weak or negligible
in all cases. All distances and angles are in the expected
range [4 – 8].
In all cases, the surroundings of the silicon and
tin atoms correspond to slightly distorted tetrahedra.
Expectedly, the distortion is small in 1b, mainly due
to the slight difference in the orientation of the aryl
groups (see, however, the solid-state 13C NMR spec-
tra in Fig. 6). There is a center of inversion in the
triyne 2 which is absent in the tetrayne 4, and this has
consequences for the solid-state 13C and 29Si NMR
spectra of the latter (vide infra). It should be noted
that the optimized geometries in the gas phase of 2
Fig. 3. Molecular structure of the tetrayne 3 in the solid
state (ORTEP, 40 % probability ellipsoids; hydrogen atoms
omitted for clarity). Selected bond lengths (pm) and an-
gles (deg): Si1–C1 183.93(15), Si1–C3 184.06(15), Si2–
C14 183.96(16), Si2–C4 183.97(15), Si3–C13 183.80(17),
Si3–C2 184.39(16), C1–C2 120.8(2), C3–C4 120.5(2), C7–
C14 118.3(2), C12–C13 118.0(2); C1–Si1–C3 106.73(6),
C14–Si2–C4 105.92(8), C13–Si3–C2 106.16(8), C2–C1–Si1
178.97(12), C1–C2–Si3 178.01(13), C4–C3–Si1 178.16(13),
C3–C4–Si2 177.97(13), C12–C13–Si3 178.59(15), C7–
C14–Si2 179.14(14); C13–Si3–Si1–C3 60.2, C14–Si2–Si1–
C1 57.2.
and 3, calculated [20] at the B3LYP/6-311+G(d,p)
level of theory (Fig. 5), show different conforma-
tions as far as the mutual orientation of the alkynyl
groups is concerned. Therefore, it can be assumed that
the packing in the crystal lattice is affected signifi-
cantly by the type of alkynyl moieties. In the case
of 2, the solid-state 13C NMR spectrum (vide infra)
indicates the presence of a second modification. Fi-
nally, the X-ray diffraction study of 5 shows that there
is a center of inversion which answers one question
left open in previous solid-state NMR studies of this
compound [9, 10].
Fig. 1. Molecular structure of Si(C≡C–C6H4-4-Me)4, 1b,
in the solid state (ORTEP, 40 % probability ellipsoids; hy-
drogen atoms omitted for clarity). Selected bond lengths
(pm) and angles (deg): Si–C1 181.5(2), C1–C2 120.3(3), C2–
C3 143.2(3), C3–C4 138.9(3), C3–C8 138.6(3); C1–Si–C10
110.8(9), C2–C1–Si 172.4(19), C1–C2–C3 177.7(2), C4–
C3–C8 117.8(2).
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B. Wrackmeyer et al. · Solid-state NMR Studies
121
Table 1. 13C and 29Si NMR data of alkynylsilanes 1 – 4 in solution and in the solid state.
δ
13C (≡C)
δ
78.6
13C (Si–C≡)
δ
13C (R)
δ
29Si
1a
solid:
104,7
4.5 (Me)
−95.1
108.0, 106.2
105.6, 105.2
78.5, 77.7
77.1, 76.6
5.9, 5.5, 4.6, 4.4 (Me)
−97.6
1ba
107.5
86.5
120.2(1), 133.3(2), 130.1(3), 141.0(4),
21.8 (Me)
−93.8
118.6(1), 132 1 (broad,2,3), 140.2(4),
22,7 (Me)
solid:
109.7
85.7
−92.7
−40.7
2
94.6(1)
86.1(2),
110.6(4)
84.5(2)
−0.2 (SiMe2)
solid:
98.4(1)
94.8(1)
1.2, 0.4 (SiMe2)
−40.0
110.9(4)
3
86.4(2)
110.8(4), 111.1(5)
88.6(2,10)
113.5, 112.3
111.1, 110.4
(4,5,7,8)
0.0 (Si(3)Me2), −0.1 (Si(6)Me2)
1.3, 1.0, 0.5, 0.0, −0.5 (Si(3,6)Me2)
−39.9(3)
−41.0(6)
solid:
98.5
97.0 (1,11)
−38.9, −39.3(3)
−41.4(6)
4b
111.9 (C–SiMe2)
117.1 (C–SiMe3)
114.5
0.0 (SiMe3), −2.9 (SiMe2)
−18.9 (SiMe3)
−37.8 (SiMe2)
solid:
3.3 (SiMe3),
0.6, 0.2, −0.1, −0.4 (SiMe2)
−18.9, −19.1 (SiMe3)
−38.0, −38.1 (SiMe2)
119.3, 119.4
a Measured in [D8]THF; 1J(29Si,13C) = 128.9 Hz, 2J(29Si,13C) = 26.5 Hz; b 13C NMR data given in ref. [1d] without assignment; measured
here in C6D6, and the assignment is based on the coupling constants, as follows: 1J(29Si(Me2),13C≡) = 69.6 Hz, 2J(29Si(Me3),13C≡) =
12.5 Hz, 1J(29Si(Me3),13C≡) = 77.8 Hz, 2J(29Si(Me2),13C≡) = 10.5 Hz, 1J(29Si(Me2),13C(Me)) = 49.8 Hz, 1J(29Si(Me2),13C(Me)) =
56.3 Hz.
Fig. 4. Molecular structure of bis(trimethylstannyl)ethyne,
5, in the solid state (ORTEP, 40 % probability ellipsoids;
Fig. 5. Calculated [B3LYP/6-311+G(d,p)] and optimized
hydrogen atoms omitted for clarity). Selected bond lengths
gas-phase geometries of the triyne and the tetrayne, to be
(pm) and angles (deg): Sn1–C1 212.8(3), Sn1–C2 210.6(3),
compared with the solid-state structures in Figs. 2 and 3.
Sn1–C3 213.8(3), Sn1–C4 213.8(3), C2–C2A 120.2(6); C2–
Sn1–C1 107.87(12), C2–Sn1–C3 106.31(12), C1–Sn–C3)
and 29Si nuclear shielding between liquid and solid
112.79(14), C1–Sn–C4 112.00(12), C3–Sn1–C4 111.03(13),
C2–C2A–Sn 179.6(4).
state are most pronounced (a few ppm) for 13C nu-
clei in peripheral positions, such as ≡C–H or SiMe
groups, which is in support of different conformations
preferred on average in solution when compared with
Solid-state 13C and 29Si NMR spectroscopic studies
The 13C and 29Si chemical shifts of the com- the solid state.
pounds 1 – 4 measured in solution differ very little
The solid-state 13C MAS NMR spectrum of 1b
from those measured in the solid state (Table 1), in- (Fig. 6) is only consistent with the result of the X-ray
dicating weak or negligible intermolecular interac- diffraction by considering a fast exchange regime on
tions. This fact is now clearly established by the re- the NMR time scale, in which reorientation of the aryl
sults of the X-ray diffraction studies also for the groups takes place, leading on average to four identi-
bis(trimethylstannyl)ethyne(5). The differences in 13
C
cal surroundings for the alkynyl carbon atoms as well
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122
B. Wrackmeyer et al. · Solid-state NMR Studies
1
Fig. 6. Solid-state 75.1 MHz 13C{ H} MAS
NMR spectra of 1b (recycle delay 20 s; upper
trace 1536 transients; lower trace 512 transients).
The exchange-broadened 13C2,3 NMR signals
could only be observed at high spinning rates
(lower trace). Rotational side bands (lower trace)
are indicated by asterisks. At lower spinning rate
(upper trace), only the fairly sharp 13C NMR sig-
nals for alkynyl carbons, aryl carbons C1, C4
and CH3 were readily observed together with the
rotational side bands. The solid-state 59.6 MHz
1
29Si{ H} NMR spectrum of 1b (insert; single
pulse, recycle delay 20 s) is a fairly sharp line,
even without MAS.
as the C1, C4 and CH3 carbon atoms. The proposed ro- center of inversion in solid 3 is also illustrated by two
tation of the aryl groups is evident by inspection of the different 29Si NMR signals for Si3,9 in addition to the
exchange-broadened 13C2,3 NMR signals. In the case signal for Si6.
of 1a which was studied previously, motional averag-
ing was slow on the NMR time scale, and the four dif-
ferent propynyl groups, found by X-ray diffraction [8],
were clearly distinguished in the solid-state 13C NMR
spectrum [11] (see also Table 1).
According to the molecular structure of 2 in the
crystal, the solid-state 29Si and 13C MAS NMR spectra
(Fig. 7) should be simple, showing one and five signals,
respectively. However, in addition to the expected sig-
nals, there are three 13C NMR signals of minor inten-
sity in the alkynyl range as marked. Therefore, in the
bulk material of 2, a small amount of a second mod-
ification is most likely present, of which the 29Si and
13C(SiMe2) NMR signals are not resolved.
In the case of the disilane derivative 4, attempts
failed to obtain suitable single crystals for X-ray
diffraction. However, the solid-state 13C (Fig. 9) and
29Si (Fig. 10) MAS NMR spectra of the micro-
crystalline material could be measured. The differ-
ent 13C NMR signals for the alkynyl carbons and for
all methyl groups at the disilane unit clearly indicate
the absence of a mirror plane or a center of inver-
sion in the solid-state molecular structure of 4. Dif-
ferent 13C(SiMe3) signals were not resolved. In ad-
dition to expectedly small differences in the chem-
ical shifts of the SiMe3 groups (see the 29Si NMR
spectrum in Fig. 10), these 13C NMR signals may
be slightly exchange-broadened by 2π/3 propeller
Solid-state 13C and 29Si MAS NMR spectra of the jumps [9, 10]. The assignment of the alkynyl 13C NMR
tetrayne 3 (Fig. 8) are in perfect agreement with the signals was based on the solution-state 13C NMR spec-
results from X-ray diffraction. Except of the methyl tra. The solid-state 29Si MAS NMR spectrum mea-
groups linked to the central silicon atom, and the sured at high spinning rate shows two resolved pairs
alkynyl carbon atoms C2,10, all other 13C NMR signals of signals for the SiMe3 and SiMe2 groups, consistent
are clearly resolved. The absence of a mirror plane or a with the 13C NMR spectra. At low spinning rate, the
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B. Wrackmeyer et al. · Solid-state NMR Studies
123
1
1
Fig. 7. Solid-state 100.6 MHz 13C{ H} and 79.5 MHz 29Si{ H} MAS NMR spectra of the triyne 2 (recycle delay 8 s; 64
transients for 13C and 26 transients for 29Si).
1
1
Fig. 8. Solid-state 100.6 MHz 13C{ H} and 79.5 MHz 29Si{ H} NMR MAS spectra of the tetrayne 3 (recycle delay 8 s; 32
transients for 13C and 29Si). Rotational side bands (rotation 6 kHz) are indicated.
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124
B. Wrackmeyer et al. · Solid-state NMR Studies
1
Fig. 9. Solid-state 100.5 MHz 13C{ H} NMR spectrum of the disilane derivative 4 (recycle delay 10 s; 492 transients; rotation
9 kHz).
1
Fig. 10. Solid-state 79.5 MHz 29Si{ H} NMR spectrum of the disilane derivative 4 measured at different spinning rates
(recycle delay 8 s; 32 transients). At a spinning rate of 970 Hz the rotational side bands become visible, and their pattern is
consistent with non-axially symmetric (SiMe2) and close to axially symmetric (SiMe3) 29Si shielding tensors.
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B. Wrackmeyer et al. · Solid-state NMR Studies
125
Table 2. Crystallographic data of the alkynylsilanes 1b, 2, 3 and of bis(trimethylstannyl)ethyne (5).
1b
C36H28Si
needle
2
3
5
Formula
C8H14Si2
prism
C14H20Si3
prism
C6H18Sn2
prism
Crystal
Dimensions, mm3
0.93×0.14×0.14
0.60×0.50×0.50
0.66×028×0.09
0.36×0.23×0.11
T, K
133(2)
193(2)
193(2)
133(2)
Crystal system
Space group
tetragonal
l41/a
trigonal
R3
monoclinic
P21/n
monoclinic
P21/c
¯
Lattice parameters
a, pm
b, pm
c, pm
1966.5(2)
1966.5(2)
729.00(11)
90
90
90
4
1612.1(2)
1612.1(2)
1253.3(2)
90
90
120
9
1635.2(5)
553.9(5)
1958.9(5)
90
103.4(5)
90
913.3(2)
559.3(1)
1221.7(3)
90
97.49(2)
90
2
α, deg
β, deg
γ, deg
Z
4
0.3
µ, mm−1 (MoKα )
0.1
0.2
4
Diffractometer
— Stoe IPDS II; MoKα ,λ = 71.073; graphite monochromator —
Absorption correction
ϑ range, deg
nonea
2.1 – 35.7
17538
1000
85
0.112 / 0.051
0.19 / −0.27
nonea
numerical
1.47 – 26
22889
2705
234
0.099 / 0.030
0.14 / −0.31
numerical
1.18 – 26
2192
1166
49
0.041 / 0.017
0.55 / −0.39
2.18 – 26.8
10922
1158
Reflections collected
Independ. refl. [I ≥ 2σ(I)]
Refined parameters
wR2/R1[I ≥ 2σ(I)]
Max. / min. resid. electron density, e pm−3 ×10−6
a
83
0.076 / 0.075
0.23 / −0.15
Absorption corrections did not improve the parameter set.
rotational side bands expectedly indicate 29Si shield- oven-dried glassware, and dry solvents were used through-
out. nBuLi in hexane (1.6 M), 1-ethynyl-4-methylbenzene,
ing tensor patterns typical of a non-axial symmetry of
ethynyltrimethylsilane, SiCl4, and Me4Si2Cl2 were commer-
cial products. The 3,3,6,6,-tetramethyl-3,6-disila-triyne (2)
[16], 3,3,6,6,9,9-hexamethyl-3,6,9-trisila-tetrayne (3) [17],
and bis(trimethylstannyl)ethyne (5) [18] were prepared
adopting the literature procedures. Single crystals of 2 and
the SiMe2 groups and close to axial symmetry for the
SiMe3 groups.
Conclusions
Crystallographic evidence and solid-state 13C and 3 were collected after slow sublimation at ambient tem-
29Si MAS NMR results are in good agreement for
perature. Single crystals of 5 were grown from concen-
trated hexane solutions at ambient temperature. Solution-
state NMR measurements were carried out in [D8]THF (1b)
and C2DCl2 (4) in 5 mm o. d. tubes at 296 1 K: Var-
ian Inova 300 or 400 MHz and Bruker ARX 250 spec-
trometer for 1H, 13C, and 29Si NMR; chemical shifts are
given with respect to Me4Si [δ1H [D7]THF/C6D5H = 1.73,
7.15; δ13C ([D8]THF/C6D6) = 25.4/128.0; δ29Si = 0 for
Ξ(29Si) = 19.867184 MHz]. Chemical shifts δ1H are given
alkynylsilanes. Dynamic processes such as reorien-
tation of aryl groups are mirrored by exchange-
broadened 13C NMR signals. Furthermore, the solid-
state NMR spectra indicate the presence of mod-
ifications so far not readily apparent from sin-
gle crystal X-ray diffraction studies, as shown in
the case of the triyne 2. The crystal structure
study of bis(trimethylstannyl)ethyne (5) has answered
the question left open after temperature-dependent
solid-state 13C and 119Sn MAS NMR investigations
[9, 10], the results of which are in perfect agree-
ment with the particular inversion symmetry of this
molecule.
to 0.04 ppm, δ13C and δ29Si to 0.1 ppm. Solid-state 13
C
and 29Si NMR spectra were measured at ambient temper-
ature with Bruker Avance II 300 and Bruker Avance 400
spectrometers, using 4 mm zirconia rotors in double-bearing
probe heads. All MAS spectra were measured with variable
amplitude cross polarization (VACP) [21], and the SPINAL
pulse sequence [22] was used to achieve efficient 1H de-
coupling. EI-MS spectra: Finnigan MAT 8500 spectrometer
(ionisation energy 70 eV) with direct inlet. The m/z data re-
fer to the isotopes 1H, 12C, and 28Si. The melting points (un-
corrected) were determined using a Bu¨chi 510 melting point
apparatus.
Experimental Section
Starting materials and measurements
The preparations and all handling of samples were car-
ried out under an inert atmosphere (Ar), and carefully
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126
B. Wrackmeyer et al. · Solid-state NMR Studies
Synthesis of tetraalkyn-1-ylsilane 1b
(70 eV): m/z = 310 (10) [M]+, 295 (28) [M–15]+, 73 (100)
[SiMe3]+.
Tetrachlorosilane (3 mL; 10 mmol) was added slowly
◦
at low temperature (−78 C) to a suspension of Li–C≡C–
X-Ray structure analyses of compounds 1b, 2, 3, and 5
C6H4-4-Me (39.4 mmol) in 75 mL of hexane. The reaction
mixture was allowed to warm to r. t., heated to reflux for 2 h
and stirred for further 10 h at r. t. The solid materials were
separated and washed with an excess of toluene. The toluene
fractions were collected, and all volatiles were removed in
a vacuum. The solid residue left was mainly the product 1b
(3.8 g, 77 %). It was washed with hexane and dissolved in
THF. Needle-like crystals (m. p. > 263 ◦C, decomp.) suitable
for X-ray structural analysis were grown from concentrated
THF solutions at r. t. – 1H NMR (400 MHz) data: δ = 2.4,
7.3, 7.6 (s, m, m, 28H, 4-Me-C6H4).
The X-ray crystal structural analyses of 1b, 2, 3, and 5
were carried out at 133(2) K (1b), 193(2) K (2, 3) on sin-
gle crystals selected in perfluorinated oil [23] at r. t., using
a Stoe IPDS II (MoKα radiation, λ = 71.069 pm) system
equipped with an Oxford Cryostream low-temperature unit.
Structure solutions and refinement were accomplished using
SIR97 [24], SHELXL-97 [25], and WINGX [26]. Pertinent
data are given in Table 2. Since the solid-state NMR spectra
of 1b were measured at r. t. and showed dynamic processes,
the X-ray diffraction of 1b was repeated at r. t. (Stoe IPDS I)
with a single crystal sealed in a Lindemann capillary. The re-
sults were essentially identical with those obtained at 133 K.
CCDC 753787 (1b), 753788 (2), 753789 (3), and 753790
(5) contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data request/cif.
Synthesis of 1,3,4,6-tetrasiladiyne 4
A solution of 1,2-dichloro-tetramethyldisilane (4.14 g,
35.3 mmol) in hexane (20 mL) was added at room tem-
perature within 15 min to a suspension of freshly prepared
trimethylsilylethynyllithium (71 mmol) in hexane (300 mL).
The mixture was heated at reflux for 12 h, insoluble materials
were filtered off, and the solvent was removed in a vacuum to
leave a colorless powder. After recrystallization from pentane
at −20 ◦C, the pure disilane derivative 4 was obtained (m. p.
Acknowledgements
Support of this work by the Deutsche Forschungsgemein-
schaft is gratefully acknowledged. E. K. and A. B. thank the
◦
36 – 38 C; 6.8 g, 62 %). – 1H NMR (250 MHz in C6D6): DAAD and HEC (Pakistan), and E. M. the Alexander-von-
δ = 0.24 (s, 12H, SiMe2), 0.02 (s, 18H, SiMe3). – EI-MS Humboldt Stiftung for fellowships.
[1] a) T. Hiyama, A. Mori, Science of Synthesis 2002,
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