Tris(hydridosilylethyl)boranes: Highly reactive synthons for polymeric silicon compounds

Article abstract of DOI:10.1016/S0022-328X(99)00499-4

Two methods for the synthesis of tris(hydridosilylethyl)boranes B[C₂H₄Si(CH₃)_nH_3-n] ₃ (C₂H₄ = CHCH₃, CH₂CH₂; n = 0, 1, 2; 5a-c) are reported. In the first route, tris(chlorosilylethyl)boranes B[C₂H₄Si(CH₃)_nCl_3-n] ₃ (n = 0, 1, 2; 3a-c) are reacted with LiAlH₄ in diethyl ether solution. Compounds 3a-c are prepared by hydroboration of the respective vinyl chlorosilanes (H₂C=CH)Si(CH₃)_nCl_3-n (n = 0, 1, 2; 1a-c) with borane dimethylsulfide, BH₃·S(CH₃)₂. Hydroboration of hydrido vinylsilanes (H₂C=CH)Si(CH₃)_nH_3-n (n = 0, 1, 2; 6a-c) with BH₃·S(CH₃)₂ represents an alternative route, which produces 5a-c in much higher yield. The intermediates 6a-c are obtained from 1a-c and LiAlH₄ in diethyl ether solution and, because of their difficult handling, reacted in situ. Detailed NMR studies prove boron addition to the olefinic moieties to be not regioselective, since resonance signals for products formed by α- and β-boron addition to the vinyl functions are observed. The elemental constitution of 5a-c is established by high-resolution electron ionization mass spectrometry for both the molecular ion peak and a series of fragment ions. Major fragments arise from C-C bond formation in the gas phase between two of the boron side chains in the ion dipole complex.

Full text of DOI:10.1016/S0022-328X(99)00499-4

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 592 (1999) 115–127
Tris(hydridosilylethyl)boranes: highly reactive synthons for
polymeric silicon compounds
Markus Weinmann a,*, Thomas W. Kamphowe a, Peter Fischer b, Fritz Aldinger a
a Max-Planck-Institut fu¨r Metallforschung and Institut fu¨r Nichtmetallische Anorganische Materialien, Uni6ersita¨t Stuttgart,
Pul6ermetallurgisches Laboratorium, Heisenbergstraße 5, D-70569 Stuttgart, Germany
b Institut fu¨r Organische Chemie und Isotopenforschung, Uni6ersita¨t Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany
Received 18 June 1999; accepted 29 July 1999
Abstract
Two methods for the synthesis of tris(hydridosilylethyl)boranes B[C2H4Si(CH3)nH3−n]3 (C2H4=CHCH3, CH2CH2; n=0, 1, 2;
5a–c) are reported. In the first route, tris(chlorosilylethyl)boranes B[C2H4Si(CH3)nCl3−n 3 (n=0, 1, 2; 3a–c) are reacted with
]
LiAlH4 in diethyl ether solution. Compounds 3a–c are prepared by hydroboration of the respective vinyl chlorosilanes
(H2CꢀCH)Si(CH3)nCl3−n (n=0, 1, 2; 1a–c) with borane dimethylsulfide, BH3·S(CH3)2. Hydroboration of hydrido vinylsilanes
(H2CꢀCH)Si(CH3)nH3−n (n=0, 1, 2; 6a–c) with BH3·S(CH3)2 represents an alternative route, which produces 5a–c in much
higher yield. The intermediates 6a–c are obtained from 1a–c and LiAlH4 in diethyl ether solution and, because of their difficult
handling, reacted in situ. Detailed NMR studies prove boron addition to the olefinic moieties to be not regioselective, since
resonance signals for products formed by a- and b-boron addition to the vinyl functions are observed. The elemental constitution
of 5a–c is established by high-resolution electron ionization mass spectrometry for both the molecular ion peak and a series of
fragment ions. Major fragments arise from CꢁC bond formation in the gas phase between two of the boron side chains in the ion
dipole complex. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Hydroboration; Vinylsilanes; Polysilazanes; NMR; EI-MS
silicon polymers, which additionally incorporate boron,
carbon, nitrogen and hydrogen, e.g. polyborosilazanes,
1. Introduction
borazine-modified polysilazanes, or boron-containing
polysilylcarbodiimides [6–12].
The polymeric precursors specified above are either
During the last two decades, enormous progress has
been made in the synthesis of ceramic composite mate-
rials via precursor thermolysis, by which suitable poly-
synthesized in solution or obtained by polycondensa-
meric networks are transformed into a wide variety of
tion reactions, with inevitable formation of by-prod-
amorphous ceramics [1]. Silicon-containing polymeric
ucts. Both aspects strongly limit the applicability of
compounds, e.g. polysilanes [2], polycarbosilanes [3],
such polymeric systems for specific technological appli-
polysilazanes [4], and polysiloxanes [5], have proven to
cations. We therefore searched for novel synthons that
be especially promising precursors for technologically
might be transformed into SiꢁBꢁCꢁN preceramic poly-
important ceramic components, such as fibers, coatings,
infiltrated porous media, or complex-shaped bulk parts.
mers, e.g. by polyaddition reactions, requiring neither
catalysts nor solvents, and not producing any by-prod-
Silicon-based ceramics with exceptional high-tempera-
ucts. In this context, we now report the synthesis and
ture and oxidation stability are obtained from organic
detailed structural characterization, by NMR, IR, and
mass spectrometry, of tris(hydridosilylethyl)boranes.
* Corresponding author. Tel.: +49-711-686-1231; fax: +49-711-
These highly reactive species represent a novel educt
686-1131.
type for the synthesis of SiꢁBꢁCꢁN preceramic
compounds.
E-mail address: weinmann@aldix.mpi-stuttgart.mpg.de (M. Wein-
mann)
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(99)00499-4

116
M. Weinmann et al. / Journal of Organometallic Chemistry 592 (1999) 115–127
2. Results and discussion
2.1. Synthesis of tris(hydridosilylethyl)boranes from
tris(chlorosilylethyl)boranes
The tris(hydridosilylethyl)boranes B[C2H4Si(CH3)n-
H3−n]3 (n=0, 1, 2; 5a–c) were first synthesized by
reaction of the corresponding chloro-substituted com-
pounds B[C2H4Si(CH3)nCl3−n]3 (n=0, 1, 2; 3a–c) with
LiAlH4 (4) in diethyl ether solution. The tris(chlorosi-
lylethyl)boranes 3a–c themselves were prepared by hy-
droboration of chloro vinylsilanes (H2CꢀCH)Si-
(CH3)nCl3−n (n=0, 1, 2; 1a–c) with borane dimethyl-
sulfide (2) as described previously [12a,13].
A 2 M BH3·S(CH3)2 solution in toluene was added
dropwise at 0°C to a solution of the chloro vinylsilanes
1a–c. The reaction mixture was allowed to warm to
25°C, and the volatile components were stripped off in
vacuum. The spectroscopically pure colorless oils 3a–c
thus obtained were dissolved in diethyl ether, and a
suspension of LiAlH4 (4, ꢀ10% excess relative to Cl)
in diethyl ether added dropwise at 0°C. The mixture
was refluxed for 3 h, and excess LiAlH4 removed by
treatment with (H3C)3SiCl.
The Schlenk reaction flask used for the hydrogena-
tion of the chlorosilanes was connected directly, via
distillation bridge and gas inlet tube, to a second flask
containing the borane dimethylsulfide solution in
toluene. The chloro vinylsilane was added dropwise to
the LiAlH4 suspension, and the reaction mixture heated
to 50°C. Approximately 80% of the diethyl ether–hy-
drido vinylsilane solution was distilled into the flask
with the BH3·S(CH3)2 solution. From this mixture, all
volatile components were removed at 25°C/10−1 mbar.
Compounds 5a–c were obtained by vacuum distillation
as colorless oils (86–90% yield relative to borane
dimethylsulfide).
2.3. Spectroscopic characterization
The tris(hydridosilylethyl)boranes B[C2H4Si(CH3)n-
H3−n]3 5a–c were structurally characterized by various
spectroscopic techniques, 1H-, 11B-, 13C-, and 29Si-
NMR, IR, and electron ionization mass spectrometry
(EI-MS). Their extreme sensitivity towards moisture
and their explosive reaction upon exposure to air
thwarted any attempts at elemental analysis.
Hydroboration of vinylsilanes is not regioselective as
described previously [12a,13]. Rather, the boryl group
may add, in each of the three successive hydroboration
steps, at either the a- or b-position of the vinyl
function.
After filtration and high-vacuum distillation, com-
pounds 5a–c were obtained as colorless oils. They are
extremely sensitive to moisture and explode upon expo-
sure to air (CAUTION!). While the intermediates 3a–c
are formed from 1a–c in close to quantitative yield, the
tris(hydridosilylethyl)boranes 5a–c were obtained with
at most 55% yield, due possibly to the formation of
boron hydrides. This moderate yield motivated us to
search for an alternative route to 5a–c. By starting
from hydrido vinylsilanes, formation of boron hydrides
is indeed suppressed completely.
2.2. Synthesis of tris(hydridosilylethyl)boranes from
hydrido 6inylsilanes
Due to this lack of regiospecifity, the tris(hydridosi-
lylethyl)boranes 5a–c are formed as a mixture of four
regioisomers, which can be characterized with the fol-
lowing abbreviations: aaa, aab, abb, bbb. For the
silylethyl functions, resulting from a-addition, a further
complication arises since a stereogenic center is gener-
ated at the boron-bonded 1-silylethyl carbon (indicated
by an asterisk in the above reaction scheme).
Hydrido vinylsilanes of the general structure
(H2CꢀCH)Si(CH3)nH3−n are difficult to handle in neat
form; they were therefore synthesized from the corre-
sponding chloro vinylsilanes 1a–c with LiAlH4 (4) in
diethyl ether and reacted in situ.

M. Weinmann et al. / Journal of Organometallic Chemistry 592 (1999) 115–127
117
vinylsilane. The two remaining SiH2 multiplets are con-
nected by the respective cross-peaks to signals between
1.15 and 1.23 ppm. Both the relative position and the
severely broadened lines prove these resonances to stem
from the BꢁCH(CH3)ꢁSi functions resulting from a-ad-
dition. Finally, there is a whole series of sharp
BꢁCH(CH3)ꢁSi doublets (3JH,H ꢀ7 Hz) for the various
side chains with a-addition structure at 1.06–1.13 ppm.
They are superimposed upon the BCH2 multiplets of
the b-addition side chains, which in term are marked
clearly by the prominent cross-peak to the SiCH2
multiplets.
2.3.1. NMR spectroscopy
Consequently, the H-NMR spectra of compounds
1
5a–c present a complex appearance. The individual
multiplet structures, which in part overlap, could be
assigned unequivocally, however, by 2D NMR tech-
niques. The 1H,1H COSY NMR spectrum (500.132
MHz) of B[C2H4Si(CH3)H2]3 (5b), obtained from
(H2CꢀCH)Si(CH3)H2 (6b) and BH3·S(CH3)2, is shown
as an example in Fig. 1.
On the basis of both relative chemical shift and
coupling pattern, the tightly overlapping triplets at
−0.07 to 0.05 ppm are assigned, in a straightforward
manner, to the SiCH3 groups in the various isomers.
1
All compounds 5a–c display H-NMR spectra with
comparable appearance; thus, the assignments pre-
The dominant cross-peak in the H,1H COSY NMR
1
sented above exemplarily for 5b (obtained from 6b) are
valid for the analysis of the H-NMR spectra of all
compounds 5a–c (see Table 1). The SiH protons ap-
pear as multiplets between 3.3 and 4.1 ppm. The respec-
tive SiCH3 multiplets (5b,c) are centered closely at 0
ppm and set off clearly from the SiꢁCH2 resonance
links them with the respective SiH2 multiplets in the
3.70–4.00 ppm range (3JH,H 4.1–4.2 Hz), which appear
divided into three separate complex patterns. The low-
field one is connected, via two pronounced cross-peaks,
to two multiplets at 0.50 and 0.64 ppm, which represent
SiCH2 groups, generated by b-addition of boron to the
1
Fig. 1. 1H,1H COSY-NMR spectrum (500.132 MHz) of B[C2H4Si(CH3)H2]3 (5b, 0.01 M in C6D6) obtained from (H2CꢀCH)Si(CH3)H2 (6b) and
BH3·S(CH3)2.

118
M. Weinmann et al. / Journal of Organometallic Chemistry 592 (1999) 115–127
Table 1
1H-NMR chemical shift and integral values for the aliphatic C2H4 units in tris(hydridosilylethyl)boranes 5a–c
5a from 3a
5b from 3b a
5c from 3d
5a from 6a a
5b from 6b a
5c from 6c
l SiCH2
0.24–0.53
0.8 H
0.66–0.72
7.8 H
1.21–1.40
0.8 H
0.89–1.12
2.6 H
0.51–0.72
2 H
1.08–1.26
6 H
1.08–1.26
2 H
1.32–1.52
2 H
0.44–0.67
2.8 H
1.05–1.13
4.8 H
1.14–1.22
2.8 H
1.42–1.46
1.6 H
0.33–0.53
1.4 H
0.69–1.08
7 H
0.69–1.08
1.4 H
1.20–1.32
2.2 H
0.46–0.6
2.2 H
1.04–1.23
5.6 H
1.04–1.23
2.2 H
1.26–1.49
2.0 H
0.32–0.74
3.2 H
1.05–1.12
4.2 H
1.13–1.25
3.2 H
1.28–1.46
1.4 H
Int. SiCH2
l CHCH3
Int. CHCH3
l BCH2
Int. BCH2
l CHCH3
Int. CHCH3
a:b ratio
6.5:1
2:1
1.2:1
3.3:1
1.7:1
0.9:1
a In this case, the resonance signals of CHCH3 and BCH2 overlap. The intensity of these signals was determined by subtracting the integrals
of the SiCH2 resonance signals from the sum of the integrals of the CHCH3+BCH2 signals: Int. CHCH3=Int. (CHCH3+BCH2)−Int. SiCH2.
(H2CꢀCH)Si(CH3)2H sufficiently for incipient bond for-
mation because of steric repulsion between the CHCH3
moiety and one of the SiCH3 groups of the vinylsilane.
Since for the third hydroboration step this spatial inter-
action is even more pronounced in HB[CH(CH3)Si-
(CH3)2H]2, b-addition is the more favored.
The influence of electronic effects upon the regioselec-
tivity of hydroboration of vinylsilanes is more difficult
to assess. Recent DFT (density functional theory) calcu-
lations indicate that electron-withdrawing substituents
on silicon dramatically decrease the p-electron density
around the terminal b-vinyl carbon, thus directing boron
addition to the a-position [14].
The a:b ratio determined from the bulk integrals (see
Table 1) neither gives an indication of the number of
regioisomers which it represents, nor does it reflect upon
their quantitative distribution. For this, the NMR spec-
tra have to be analyzed in much more detail. In the
following, such a spectral breakdown is represented
signals for the b-addition products (0.25–0.75 ppm).
Both resonance signals for the CHCH3 residues of the
a-addition products and the broad BꢁCH2 signals of the
b-addition products are crowded together in the 1.0–1.5
ppm range. They are sufficiently set off from the SiCH2
region, though, to allow for discrete integration for each
compound. This is of crucial importance, since the SiCH2
integral represents the share of 2-silyl-substituted ethyl
side chains in all possible regioisomers. Subtracting this
value from the bulk integral across the BCH2 and
CHCH3 signals (see Table 1) gives an integral value for
all 1-silyl-substituted side chains. The integrals for both
regioisomeric functions correspond to four protons each,
BꢁCH2CH2ꢁSi or BꢁCH(CH3)ꢁSi; thus, the a:b ratio can
be read off directly (see Table 1, last line).
The a:b ratio for B(C2H4SiH3)3 (5a), obtained by
hydroboration of vinyl trichlorosilane (1a) and subse-
quent reaction with LiAlH4 (4), is 6.5:1; 5a obtained from
vinylsilane (6a) and BH3·S(CH3)2 (2) in contrast shows
an a:b ratio of 3.3:1. For the corresponding methyl
derivatives the following a:b ratios are determined: 2:1
for the synthesis of 5b from 3b, 1.7:1 for the synthesis of
5b from 6b, 1.2:1 for synthesis of 5c from 3d, and 0.9:1
for the synthesis of 5c from 6c. This variance is caused
by (i) steric effects and (ii) electronic, i.e. inductive effects.
A schematic representation of the transition states (side-
on BꢁH//CꢀC coordination) for the second a-hydrobora-
tion step is given in Fig. 2 for the reaction of
H2BꢁCH(CH3)SiH3 with (H2CꢀCH)SiH3 (left) and of
H2BꢁCH(CH3)Si(CH3)2H with (H2CꢀCH)Si(CH3)2H
(right).
For the SiH3 derivative, i.e. in the synthesis of 5a, no
interaction (steric hindrance) is apparent between the
CHCH3 moiety in the H2BꢁCH(CH3)SiH3 intermediate
and the silicon-bonded hydrogen atoms of vinylsilane
(H2CꢀCH)SiH3. With the Si(CH3)2H derivative (synthe-
sis of 5c), in contrast, the BꢁH bond of the
H2BꢁCH(CH3)Si(CH3)2H intermediate cannot approach
the a-position of the CꢀC unit of vinyl dimethylsilane
Fig. 2. Relative topology of the transition state for the second
a-hydroboration step of H2BꢁCH(CH3)SiR2H+(H2CꢀCH)SiR2H
(R=H, left; R=CH3, right; schematic representation: view parallel
to the SiꢁCꢀC plane).

M. Weinmann et al. / Journal of Organometallic Chemistry 592 (1999) 115–127
119
Table 2
Possible stereoisomers of B[C2H4Si(CH3)H2]3 (5b; C2H4ꢀCHCH3, CH2CH2) and expected number of NMR signals for selected nuclei
a
Isomer
Chiral centers b Possible abs.
configuration
Expected no. of a/b
SiCH3
Expected no. of
CHCH3
Expected no. of
a/b Si
Expected no. of
SiCH2
aaa
3 (8)
RRR, SSS
RRS, RSR
SRR, SSR
SRS, RSS
RR, SS
RS, SR
R, S
1(a)
1(a)
1(a)
1(a)
–
–
1(a)
1(a)
1(a)
1(a)
1(a)
1(a)
aab
2 (4)
1(a)+1(b)
1(a)+1(b)
1(a)+1(b)
1(b)
1(a)
1(a)
1(a)
–
1(a)+1(b)
1(a)+1(b)
1(a)+1(b)
1(b)
1(b)
1(b)
1(b)
1(b)
4(b)
abb
bbb
S
1 (2)
0
–
14
5(a)+4(b)
5(a)
5(a)+4(b)
a Regiochemistry for each successive hydroboration step.
b The number of configurational isomers given in parentheses.
exemplarily for B[C2H4Si(CH3)H2]3 (5b), obtained from
(H2CꢀCH)Si(CH3)H2 (6b) and BH3·S(CH3)2 (2). The
formula unit C2H4 stands for both a CHCH3 and a
CH2CH2 moiety, representing the products of a- and
b-hydroboration, respectively.
of doublets (a-boron addition). Remarkably, the pri-
mary 1J (1H,29Si) triplets represent a spectral splitting of
ꢀ380 Hz, whereas the overall chemical shift range for
the individual 29Si nuclei is only 160 Hz.
The 13C{1H}-NMR spectrum of 5b validates the
findings from the 29Si-NMR. There are eight SiCH3
signals, in the range −8.5 to −7.4 ppm; the ninth line
probably coincides with one of the other signals. The
resonances for the SiCH2 groups (from b-boron addi-
tion) are observed at 3.7, 4.0, 4.2, and 5.2 ppm, the
methyl carbons of the CHCH3 units (from a-boron
addition) appear as expected at 12.0–13.2 ppm. The
boron-bonded carbon atoms are severely broadened by
the neighboring quadrupolar nucleus (BCH2 at 18.5
ppm, BCH at 20.5, 21.7, and 22.2 ppm), and thus not
resolved for the different regioisomers.
In Table 2, all possible stereoisomers that may be
produced by a- or b-boron addition in each of the three
successive hydroboration stages are listed. For each of
these potential structures, the expected number of
1
chemically non-equivalent, i.e. anisochromous H, 13C,
and 29Si positions is given.
The compilation in Table 2 shows that 14 stereoiso-
mers can be formed in all. Triple b-addition yields
a product with no stereogenic center, i.e. one isomer
only, with
a
single set each of the various
CH2ꢁCH2ꢁSiH2ꢁCH3 resonances. With double b-addi-
tion, i.e. with just one a-addition, a pair of enantiomers
is formed, though it is not differentiated in the NMR.
The product of double a-addition represents the classic
case of a meso and a DL pair of diastereoisomers for
which two signal sets are expected. Both of these sets,
just as the single set for the abb isomer, each comprise
resonance signals for a- and b-C2H4Si(CH3)H2 moi-
eties. Triple a-addition produces three stereogenic cen-
Despite the inherent complexity of the proton NMR
spectrum of 5b (cf. Fig. 1), the SiCH3 and CHCH3
resonances are sufficiently set off from the remaining
signals. Resolution enhancement (by Gaussian multipli-
cation), however, reveals once again eight triplets for
the SiCH3 protons (Fig. 4) centered at 0.038, 0.037,
0.014,. 0.001, −0.006, −0.014, −0.042, and −0.063
3
ppm with JH,H 4.1–4.2 Hz. For the CHCH3 moiety,
ters. As
a consequence, eight stereoisomers are
five doublets appear resolved at 1.070, 1.097, 1.098,
expected. Due to the C3 symmetry of the molecular
framework of compounds 5a–c, two diastereoisomers
can be distinguished by NMR: RRR and SSS on one
hand, RRS, RSR, SRR, RSS, SRS, and SSR on the
other hand.
1.112, and 1.114 ppm (3JH,H 6.7 Hz).
Other than 1H-, 13C-, and 29Si-NMR spectra, the
11B-NMR spectrum shows only one single resonance
around 80 ppm without any fine structure.
The 29Si{1H}-NMR spectrum (Fig. 3) of 5b shows
nine well-separated resonance signals between −28.8
and −32.9 ppm, indicating that all possible isomers are
indeed formed (cf. Table 2). Assignment of these reso-
nances to individual isomers failed since the 1H-coupled
29Si-NMR spectrum (Fig. 3, insert) appears highly com-
In combination, the results of the various NMR
techniques have shown that in the synthesis of
B[C2H4Si(CH3)H2]3 (5b) from (H2CꢀCH)Si(CH3)H2
(6b) and BH3·S(CH3)2, all possible regioisomers in fact
are formed. The bulk a:b ratio, summed up over the
three side chains of all isomers, is determined unequivo-
cally as 1.7:1. The relative percentage of the different
regio- and stereoisomers cannot be determined with
sufficient precision, though. The complete sets of NMR
data for all compounds 5a–c are given in Section 4.
1
plex. By JSi,H coupling (186.2–190.8 Hz; SiH2) and
2JSi,H coupling (7.0–7.2 Hz; SiCH3, SiCH2, SiCH), each
resonance is split either into a triplet of quartets of
triplets (b-boron addition) or into a triplet of quartets

120
M. Weinmann et al. / Journal of Organometallic Chemistry 592 (1999) 115–127
Fig. 3. 29Si{1H}-NMR (39.761 MHz) and 29Si-NMR spectrum (insert, 39.761 MHz) of B[C2H4Si(CH3)H2]3 (5b, 0.2 M in C6D6) obtained from
(H2CꢀCH)Si(CH3)H2 (6b) and BH3·S(CH3)2.
Fig. 4. SiCH3 resonance signals (resolution-enhanced by Gaussian multiplication) in the 1H-NMR spectrum of B[C2H4Si(CH3)H2]3 (5b) obtained
from (H2CꢀCH)Si(CH3)H2 (6b) and BH3·S(CH3)2 (500.133 MHz, 0.01 M in C6D6; digital resolution 0.04 Hz/Pt).

M. Weinmann et al. / Journal of Organometallic Chemistry 592 (1999) 115–127
121
Table 3
IR spectroscopic data of compounds 5a–c
5a
5b
5c
w(CꢁH)
w SiꢁH
2949 m, 2931 m, 2870 m
2962 m, 2908 m, 2873 m
2958 s, 2908 m, 2871 m
2146 vs
1462 m
1378 m
–
1144 m
1081 w
937 m
2133 vs
1483 m
1408 s
1253 s
1182 s
1053 m
944 s
2111 s
1467 m
1396 s
1252 s
1151 m
1093 m
n.o. a
la (Cꢁ) CH3
ls (Cꢁ) CH3
ls (Siꢁ) CH3
wa CꢁC
wa CꢁBꢁC
l CH3 rock.
a n.o., not observed.
2.3.2. IR spectroscopy
H2 to m/z=127 (5a), 155 (5b), 183 (5c), or by dehy-
drosilylation to m/z=97 (5a, elimination of SiH4), 111
(5b, elimination of H3CSiH3), and 125 (5c, elimination
of (H3C)2SiH2). For the allylic-type cations species thus
formed, various structural alternatives exist (Scheme 1).
Remarkably, there is also a series of three boron-free
fragment cations each with severely high intensity at
m/z=87, 101, 115 (5a), 115, 129, 143 (5b), 143, 157,
and 171 (5c). Their elemental constitution was estab-
lished unequivocally by dynamic high-resolution mass
spectrometry (Dm in each case B0.6 mmu). These
boron-free fragment ions cannot be formed by an elim-
The IR spectra of compounds B[C2H4Si(CH3)n-
H3−n]3, 5a–c, clearly show the characteristic group
frequencies (Table 3). Apart from the CꢁH stretching
absorptions at 2870–2962 cm−1, characteristic bands
observed are la(CꢁCH3) 1462–1483 (m); ls(CꢁCH3)
1387–1408 (m), ls(SiꢁCH3) 1252 or 1253 (m, only 5b
and 5c), wa CꢁBꢁC 1053–1093 (w–m) and the CH3
rocking vibration at 937–944 cm−1 (m–s). The most
remarkable feature in the IR spectra of compounds 5 is
the very strong SiꢁH valence at 2111–2146 cm−1; its
frequency decreases with the number of silicon-bonded
methyl groups (see Table 3), i.e. from 2146 cm−1 (5a)
to 2111 cm−1 (5c). The relative signal intensity is
attenuated in the same order. The spectra show no
indication of BꢁH vibrations, which would indicate
incomplete hydroboration.
ination process directly from M+ ; rather, they repre-
’
sent products of hydrogen or alkyl transfer in the ion
dipole complex as shown schematically in Scheme 2.
We propose that CꢁC bond formation takes place by
the transfer of either a methyl group or a hydrogen
’
atom from the C2H4SiR2H fragment to the boron
2.3.3. Mass spectrometry
center. Following BꢁC bond cleavage produces
HR2Si(H)CꢁC(CH2)SiR2H+ (m/z=101, 5a; 129, 5b;
157, 5c; route 1) or HR2Si(CH3)CꢁC(CH2)SiR2H+ (m/
z=115, 5a; 143, 5b; 171, 5c; route 2). Moreover, the
EI-MS of 5a–c delivers boron-free species with m/z=
87 (5a), 115 (5b), and 143 (5c), which must also have
been produced from the ionꢁdipole complex. In con-
trast to the mechanism proposed in Scheme 2, the boryl
cation must have been delivered with a CH2CH3 frag-
The mass spectra of compounds 5a–c each show the
molecular ion peak M+ , though with very low inten-
’
sity and with a highly analogous fragmentation pattern.
It is observed that due to the elimination of H , CH3 ,
’
’
’
’
’
H ꢁSiR2H2, CH2SiR2H and C2H4SiR2H (5a, R2=H2;
5b, R2=H(CH3); 5c, R2=(CH3)2) the corresponding
cations are formed. For 5a and 5b, the base peak at
m/z=129 and 157, respectively, arises from straightfor-
ward elimination of one hydridosilylethyl sidechain as a
’
ment of the radical species C2H4SiR2H (after isomer-
’
radical species C2H4SiR2H . In the case of 5c, the
ization of the radical fragment). Finally, also the
expected ‘light weight’ fragments SiH2R+ (31, 45) and
CH2SiH2R+ (45, 59) are detected.
respective fragment [M−C2H4Si(CH3)2H]+ (m/z=
185) has only 79% relative intensity; here, the
+
’
’
silaethylene radical cation H2C ꢀSiH(CH3)
(m/z=
58) constitutes the base peak. Fragments with higher
m/z ratio than [M−C2H4SiR2H]+ are observed in at
3. Outlook
’
most 13% relative intensity. Loss of CH3 from the
molecular ion of 5a is possible only from a CHCH3SiH3
side chain formed by a-addition. In contrast, the frag-
ment ions at m/z=155 (5a), 183 (5b), and 211 (5c) can
First orienting studies have already shown a remark-
able potential of the title compounds 5a–c for the
synthesis of novel precursors for SiꢁBꢁCꢁN ceramic
composites [15,16]. Until now, SiꢁBꢁCꢁN polymers
were usually obtained by salt elimination reactions
[10,11], reaction of borazine with polysilazanes [7,8],
or from bis(trimethylsilyl)carbodiimide and boron-
only be formed from b-hydroboration products, via the
+
’
fragmentation process M ꢁCH2SiR2H] (see Fig. 5).
The cations observed at m/z=129 (5a), 157 (5b), and
185 (5c) are further stabilized by either elimination of

122
M. Weinmann et al. / Journal of Organometallic Chemistry 592 (1999) 115–127
Fig. 5. EI mass spectra (70 eV) of B(C2H4SiH3)3 (5a, top) and B[C2H4Si(CH3)H2]3 (5b, bottom).
modified chlorosilanes [12] with elimination of chloro-
trimethylsilane. The novel precursors, derived from 5a–
c, are obtained either by base-catalyzed dehydro-
genative coupling with ammonia or primary amines, or
by hydrosilylation of vinyl- or alkinyl-substituted polysi-
lazanes.
In the first case, compounds 5a–c are dissolved in
toluene and reacted with ammonia at elevated tempera-
ture with in most cases n-butyl lithium as a catalyst [15].
The polymer is precipitated directly from the solution;
it can be separated from the solvent, soluble oligomers
and catalyst simply by filtration. The only byproduct is
molecular hydrogen.
The transformation of tris(hydridosilylethyl)-
boranes into SiꢁBꢁCꢁN precursors by reaction with
vinyl- or alkynyl-substituted polysilazanes also is a very
promising reaction. Both components which are
colorless oils are either mixed stoichiometrically or with
excess polysilazane and reacted, without solvent or
catalyst, at elevated temperature [16]. The SiꢁBꢁCꢁN
precursors thus are obtained, directly and without
any by-products, as glass-like polymers in quantitative
yield.

M. Weinmann et al. / Journal of Organometallic Chemistry 592 (1999) 115–127
123
Scheme 1. MS fragmentation of B[C2H4Si(R)2H]3 (5a–c; R=H, CH3) obtained from vinylsilanes (H2CꢀCH)Si(CH3)nH3−n and BH3·S(CH3)2.
Top values are m/z for R2=H2, bottom values are m/z for R2=(CH3)2, values in the middle are for R2=H(CH3). Relative intensities are given
in parentheses.
Scheme 2. Possible mechanism for the CꢁC bond formation in the
4. Experimental
ionꢁdipole
complex
H(R)2SiC2H4BC(CH2)Si(R)2H+·H3CCHSi-
’
(R)2H in the EI-MS (70 eV) of B[C2H4Si(R)2H]3 by either (1) alkyl
or (2) hydrogen transfer onto boron.
4.1. General comments
All reactions were carried out in a purified argon
atmosphere using standard Schlenk techniques. LiAlH4
(95%) was obtained from Merck AG and used as a
powder. Borane dimethylsulfide was obtained from
Sigma Aldrich GmbH and used as a 2 M solution in
toluene. Diethyl ether was purified by distillation from
CaH2; toluene was purified by distillation from potas-
sium. FT-IR spectra were obtained with a Bruker
1
IFS66 spectrometer as film in KBr cells. H- and 13C-
NMR spectra were recorded on a Bruker ARX500
spectrometer (nominal frequency 500.133 and 125.77
MHz, respectively). 11B- and 29Si-NMR spectra were
recorded on a Bruker AC250 spectrometer operating at
80.253 MHz (11B-NMR), 39.761 or 49.694 MHz (29Si-

124
M. Weinmann et al. / Journal of Organometallic Chemistry 592 (1999) 115–127
Table 4
Experimental details for the synthesis of tris(hydridosilylethyl)boranes (5a–c) from tris(chlorosilylethyl)boranes (3a–c)
Educt
B(C2H4SiCl3)3 (3a)
B[C2H4Si(CH3)Cl2]3 (3b)
B[C2H4Si(CH3)2Cl]3 (3c)
Weight g (mmol)
Dissolved in Et2O (ml)
LiAlH4 g (mmol)
Suspended in Et2O (ml)
(H3C)3SiCl g (mmol)
n-Pentane (ml)
29.2 (58.6)
200
5.11 (135)
200
2.2 (20)
250
40.7 (93)
400
5.48 (145)
200
2.2 (20)
300
28 (75)
200
2.27 (60)
50
1.5 (14)
100
Product
B(C2H4SiH3)3 (5a)
B[C2H4Si(CH3)H2]3 (5b)
B[C2H4Si(CH3)2H]3 (5c)
Elemental composition
Molecular mass (g mol−1
B.p.
Yield g (mmol)
Yield (%)
C6H21BSi3
188.32
C9H27BSi3
230.39
C12H33BSi3
272.64
)
44°C/10−1 mbar
6.4 (34)
55°C/5×10−2 mbar
10.5 (46)
49
80°C/10−1 mbar
10.5 (46)
49
58
Table 5
1H-, 13C{1H}-, 11B{1H}-, and 29Si{1H}-NMR chemical shifts or shift ranges (ppm) of tris(hydridosilylethyl)boranes (5a–c), obtained from
tris(chlorosilylethyl)boranes (3a–c)
a
5a
5b
5c
1H-NMR (C6D6)
l SiCH3
l SiCH2
l CHCH3
l BCH2
l CHCH3
l SiH
–
−0.03–0.09 (9 H)
−0.07–0.06 (18 H)
0.44–0.67 (2.8 H)
1.05–1.13 (4.8 H)
1.14–1.22 (2.8 H)
1.42–1.46 (1.6 H)
0.24–0.53 (0.8 H) 0.51–0.72 (2 H)
0.66–0.72 (7.8 H) 1.08–1.26 (6 H)
1.21–1.40 (0.8 H) 1.08–1.26 (2 H)
0.89–1.12 (2.6 H) 1.32–1.52 (2 H)
3.28–3.43 (6.5 H) 3.74–4.02 (6 H)
3.48–3.59 (2.5 H)
3.92–3.98 (1.5 H) 4.00–4.11 (1.5 H)
13C{1H}-NMR (C6D6)
l SiCH3
–
−8.41, −8.39, −8.37, −8.34, −8.28, −8.27,
−7.86, −7.76, −7.74
3.77, 4.24, 5.19
−5.83, −5.65, −5.15, −5.08,−4.88, −4.70,
−4.23, −3.95, −0.21
5.35, 6.20, 6.31, 7.65
l SiCH2
l BCH2
−0.02, 0.8
14.8 (br)
17.8, 18.6 (br)
14.9, 18.2 (br)
l CHCH3
l CHCH3
13.7, 13.8, 14.0
16.5 (br)
12.00, 12.23, 12.25, 12.31, 12.78, 13.22
20.3, 20.7 (br)
9.65, 9.73, 9.87, 10.04, 10.07, 10.61
21.8, 24.1, 25.0 (br)
11B{1H}-NMR (C6D6) n.d. b
80.2
n.d.
n.d.
29Si{1H}-NMR (C6D6) −57.8, −53.2,
−47.1, −46.3
−32.9, −32.4, −32.0, −31.1, −30.9, −29.6,
−29.2, −28.8
a In this case, the resonance signals of CHCH3 and BCH2 overlap. The intensity of these signals was determined by subtracting the integrals
of the SiCH2 resonance signals from the sum of the integrals of the CHCH3+BCH2 signals: Int. CHCH3=Int. (CHCH3+BCH2)−Int. SiCH2.
b n.d., not determined.
Caution: Tris(hydridosilylethyl)boranes are highly re-
active and may explode spontaneously upon exposure
to air.
NMR). Chemical shifts are reported in l units (parts
per million) downfield from TMS (l=0) with the
solvent as the reference signal): H-NMR, C6D6 l=
1
7.17; 13C-NMR, C6D6 l=127.0; 29Si-NMR: standard
extern rel. TMS (l=0), 11B-NMR: standard rel.
BF3·OEt2. EI mass spectra (70 eV) were recorded on a
MAT 8230 (Finnigan) mass spectrometer operating in
the positive ion mode at the Institut fu¨r Organische
Chemie der Universita¨t Stuttgart.
4.2. Synthesis of tris(hydridosilylethyl)boranes (5a–c)
from tris(chlorosilylethyl)boranes (3a–c)
In a typical experiment, a tris(chlorosilylethylborane)
B[C2H4Si(CH3)nCl3−n]3 (n=0, 1, 2; 3a–c) was dis-

M. Weinmann et al. / Journal of Organometallic Chemistry 592 (1999) 115–127
125
solved in diethyl ether and a suspension of LiAlH4 (used
in a slight excess) in diethyl ether added dropwise at 0°C
to. The reaction mixture was then warmed to 25°C over
night and refluxed for 3 h. After adding n-pentane and
chlorotrimethylsilane, (H3C)3SiCl, the mixture was
refluxed again for 30 min and then filtered through a plug
of Celite. All volatile components were removed in
vacuum (10−1 mbar, 25°C) and the oily colorless residue
distilled in vacuum. For experimental details (scale,
yields, b.p.) see Table 4; for NMR data see Table 5.
ether. The reaction flask was equipped with a reflux
condenser cooled at −30°C to avoid the loss of partly
hydrogenated products. At the top of the reflux con-
denser a glass tube was installed, which was connected
to a second reaction flask in which borane dimethyl-
sulfide was dissolved in toluene. After addition of the
chloro vinylsilane to the LiAlH4 suspension was com-
pleted, the reaction mixture was slowly warmed to 25°C.
Cooling of the reflux condenser was stopped and the
reaction mixture was heated further to slowly distil all
volatile components into the borane dimethylsulfide
solution which was cooled to 0°C. When the addition was
completed the solvents, by-product dimethylsulfide as
well as excess vinylsilane were carefully removed at
25°C/5×10−1 mbar. The crude products were then
purified by vacuum distillation to yield the title com-
pounds as colorless oils. For experimental details (scale,
yields, b.p.) see Table 6; for NMR data see Table 7.
4.2.1. IR spectra (KBr cells, [cm−1])
5a: w(CꢁH) 2949 m, 2931 m, 2870 m; w(SiꢁH) 2146 vs;
la(CꢁCH3) 1462 m; ls(CꢁCH3) 1378 m; w(CꢁC) 1144 m;
l(CH3 rocking) 937 m; ls(SiH3) 901 s.
5b: w(CꢁH) 2962 m, 2908 m, 2873 m; w(SiꢁH) 2133 vs;
la(CꢁCH3) 1483 m; la(CꢁCH3) 1408 s; ls(SiꢁCH3) 1253
s; w(CꢁC) 1182 s; wa(CꢁBꢁC) 1053 m; l(CH3 rocking) 944
s; k(SiH2) 889 vs, l(SiꢁC rocking) 762 m.
5c: w(CꢁH) 2958 s, 2908 m, 2871 m; w(SiꢁH) 2111 s;
la(CꢁCH3) 1467 m; ls(CꢁCH3) 1396 m; ls(SiꢁCH3) 1252
s; w(CꢁC) 1151 m; wa(CꢁBꢁC) 1093 m; k(SiꢁH) 879 vs;
l(SiꢁC rocking) 766 m.
4.3.1. IR spectra (KBr cells, [cm−1])
5a: w(CꢁH) 2949 m, 2931 m, 2870 m; w(SiꢁH) 2146 vs;
la(CꢁCH3) 1457 m; ls(CꢁCH3) 1378 m; w(CꢁC) 1144 m;
wa(CꢁBꢁC) 1081 w; l(CH3 rocking) 937 m; ls(SiH3) 901
s.
5b: w(CꢁH) 2958 m, 2927 m, 2873 m; w(SiꢁH) 2129 vs;
la(CꢁCH3) 1461 m; ls(CꢁCH3) 1386 m; ls(SiꢁCH3) 1253
m; w(CꢁC) 1180 s; wa(CꢁBꢁC) 1072 w; l(CH3 rocking)
944 m; k(SiH) 889 vs, l(SiꢁC rocking) 763 m.
5c: w(CꢁH) 2958 s, 2906 m, 2873 m; w(SiꢁH) 2113 s;
la(CꢁCH3) 1459 m; ls(CꢁCH3) 1396 m; ls(SiꢁCH3) 1251
s; wa(CꢁC) 1180 w; k(SiꢁH) 883 vs.
4.2.2. EI-MS [70 eV; m/z (rel. int.)]
5b: [M+] 230 (6); [M+−H] 229 (22); [M+−CH3] 215
(9); [M+−C2H5] 201 (10); [M+−H−Si(CH3)H3] 183
(10); [M+−CH2Si(CH3)H2] 171 (23); [M+−
C2H4Si(CH3)H2] 157 (94); [M+−C2H4Si(CH3)H2−H2]
155 (17); [Si2C6H1+5] 143 (94); [Si2C5H1+3] 129 (100);
[Si2C5H+11] 125 (19); [BSiC5H1+2] 111 (24); [SiC2H+6 ] 58
(29).
4.3.2. EI-MS [70 eV; m/z (rel. Int.)]
5a: [M+] 188 (2.5); [M+−CH3] 173 (4); [M+−
CH3−H2] 171 (2); [M+−SiH3] 157 (7); [M+−H−
SiH4] 155 (8); [M+−CH2SiH3] 143 (3); [M+−
C2H4SiH3] 129 (100); [M+−C2H4SiH3−H2] 127 (10);
[Si2C4H+11] 115 (54); [Si2C3H9+] 101 (13); [BSiC4H1+0] 97
(34); [Si2C2H7+] 87 (12); [C2H4SiH3+] 59 (8); [SiH+3 ] 31 (5).
4.3. Synthesis of tris(hydridosilylethyl)boranes (5a–c)
from hydrido 6inylsilanes (6a–c)
A chloro vinylsilane (H2CꢀCH)Si(CH3)nCl3−n (n=0,
1, 2; 1a–c) was dissolved in diethyl ether and added
dropwise at 0°C to a suspension of LiAlH4 (4) in diethyl
Table 6
Experimental details for the synthesis of tris(hydridosilylethyl)boranes (5a–c) from chloro vinylsilanes (1a–c)
Educt
(H2CꢀCH)SiCl3 (1a)
(H2CꢀCH)Si(CH3)Cl2 (1b)
(H2CꢀCH)Si(CH3)2Cl (1c)
Weight g (mmol)
Dissolved in Et2O (ml)
LiAlH4 g (mmol)
Suspended in Et2O (ml)
2 M BH3·S(CH3)2 ml (mmol)
100.0 (620)
250
22.8 (600)
200
53.5 (380)
200
9.5 (250)
150
48.3 (400)
200
4.5 (118)
100
82 (164)
60 (120)
60 (120)
Product
B(C2H4SiH3)3 (5a)
B[C2H4Si(CH3)H2]3 (5b)
B[C2H4Si(CH3)2H]3 (5c)
Elemental composition
C6H21BSi3
C9H27BSi3
230.39
C12H33BSi3
272.64
Molecular weight (g mol−1
B.p.
)
188.32
32–40°C/5×10−2 mbar
70°C/5×10−1 mbar
24.9 (108)
90
80°C/10−1 mbar
28.1 (103)
86
Yield g (mmol)
Yield (%)
27.2 (145)
88

126
M. Weinmann et al. / Journal of Organometallic Chemistry 592 (1999) 115–127
Table 7
1H-, 13C{1H}-, 11B{1H}-, and 29Si{1H}-NMR chemical shifts or shift ranges (ppm) of tris(hydridosilylethyl)boranes (5a–c), obtained from chloro
vinylsilanes (3a–c)
a
5a
5b a
5c
1H-NMR
(C6D6)
l SiCH3
l SiCH2
l CHCH3
l BCH2
l CHCH3
l SiH
–
−0.07–0.05 (9 H)
0.46–0.6 (2.2 H)
1.04–1.23 (5.6 H)
1.04–1.23 (2.2 H)
1.26–1.49 (2 H)
3.70–4.00 (6 H)
−0.12–0.05 (18 H)
0.32–0.74 (3.2 H)
1.05–1.12 (4.2 H)
1.13–1.25 (3.2 H)
1.28–1.46 (1.4 H)
0.33–0.53 (1.4 H)
0.69–1.08 (7 H)
0.69–1.08 (1.4 H)
1.20–1.32 (2.2 H)
3.42–3.63 (9 H)
3.94–4.03 (1.5 H), 4.05–4.13 (1.5 H)
13C{1H}-NMR
(C6D6)
l SiCH3
–
−8.49, −8.42, −8.40, −8.37, −8.29,
−7.88, −7.76, −7.38
3.77, 4.00, 4.22, 5.17
−1.83, −1.77, −1.70, −1.64, −1.13,
−0.94, −0.80, −0.28, 0.0
9.29, 10.27, 10.38, 11.62
18.8, 22.2
l SiCH2
l BCH2
0.00, 0.8
16.1 (br)
17.9, 18.5 (br)
l CHCH3
l CHCH3
13.3, 13.7, 13.8, 14.3, 14.7, 14.8
21.9 (br)
11.99, 12.22, 12.25, 12.77, 12.88, 13.21 13.68, 13.81, 14.03, 14.58
20.5, 21.7, 22.2 (br)
25.8, 28.0, 28.9
11B{1H}-NMR
(C6D6)
79.7
80.3
81.1
29Si{1H}-NMR −56.6, −56.3, −56.1, −55.8, −55.3,
(C6D6) −55.2, −54.6, −54.3, −53.9
−32.9, −32.4, −32.2, −32.1, −31.1,
−30.9, −29.6, −29.3, −28.9
−14.9, −14.4, −13.2, −12.4, −11.3,
−10.3, −9.8, −9.4
a In this case, the resonance signals of CHCH3 and BCH2 overlap. The intensity of these signals was determined by subtracting the integrals
of the SiCH2 resonance signals from the sum of the integrals of the CHCH3+BCH2 signals: Int. CHCH3=Int. (CHCH3+BCH2)−Int. SiCH2.
5b: [M+] 230 (1); [M+−H] 229 (0.9); [M+−CH3]
215 (1.6); [M+−C2H5] 201 (3); [M+−H−Si(CH3)H3]
183 (4); [M+−CH2Si(CH3)H2] 171 (4); [M+−
C2H4Si(CH3)H2] 157 (100); [M+−C2H4Si(CH3)H2−
H2] 155 (18); [Si2C6H+15] 143 (86); [Si2C5H1+3] 129 (89);
would like to thank Sabine Katz warmly for her sup-
port in preparative chemistry and Alf Breitschwerdt
(MPI fu¨r Festko¨rperforschung, Stuttgart) for running
the IR measurements.
[Si2C4H+11]
115
(16);
[BSiC5H+12]
111
(24);
[C2H4Si(CH3)H2+] 73 (10); [CH2Si(CH3)H+2 ] 59 (13);
[Si(CH3)H+2 ] 45 (9).
References
5c: [M+] 272 (4); [M+−H] 271 (13); [M+−CH3]
257 (5); [M+−H−Si(CH3)2H2] 211 (3); [M+−
CH2Si(CH3)2H] 199 (7); [M+−C2H4Si(CH3)2H] 185
(79); [Si2C7H1+7] 157 (52); [Si2C6H+15] 143 (31); [BSiC6H1+4]
125 (20); [C2H4Si(CH3)2H+] 87 (15); [CH2Si(CH3)2H+]
73 (23); [C2H6Si+] 58 (100).
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of Organometallics into Common and Exotic Materials: Design
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Workshop, Cap D’Agde, France, 1986. (b) M. Zeldin, K.J.
Wynne, H.R. Allcock (Eds.), Inorganic and Organometallic
Polymers, Advanced Materials and Intermediates, ACS Sympo-
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Pillot, J. Dunogue`s, Chem. Rev. 95 (1995) 1443. (e) D. Segal,
Chemical Synthesis of Advanced Ceramic Materials, Cambridge
University Press, 1987. (f) P. Wisian-Neilson, H.R. Allcock, K.J.
Wynne (Eds.), Inorganic and Organometallic Polymers II, Ad-
vanced Materials and Intermediates, ACS Symposium Series
572, American Chemical Society, Washington, DC, 1994. (g) J.
Bill, F. Aldinger, Adv. Mater. 7 (1995) 775. (h) J. Bill, F.
Aldinger, Z. Metallk. 87 (1996) 827. (i) H.-P. Baldus, M. Jansen,
Angew. Chem. 109 (1997) 338. (k) R. Richter, G. Roewer, U.
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Acknowledgements
We thank the Deutsche Forschungsgemeinschaft
(DFG), the Keramikverbund Karlsruhe Stuttgart
(KKS), and the Japan Science and Technology (JST)
for financial support. We are grateful to Dr Joachim
Bill (PML), Dr Wolfgang Weinmann (ESPE AG, Ger-
many), and Professor Dr Larry G. Sneddon (University
of Pennsylvania) for many helpful discussions. Profes-
sor Dr Gottfried Huttner and Thomas Jannack (both
ACI Universita¨t Heidelberg) are acknowledged for car-
rying out the MS investigations. Finally, the authors
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