Highly reactive oligosilyltriflates - Synthesis, structure and rearrangement

Article abstract of DOI:10.1039/b408573f

The novel branched oligosilyltriflates of formula [TfO(Me ₃Si)₂Si]₂E (2a-d) [E = 0 (2a), SiMe₂ (2b), GeMe₂ (2c), SiMe₂-SiMe₂ (2d)] and [TfO(Me₃Si)₂SiSiMe

Full text of DOI:10.1039/b408573f

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P A P E R
Highly reactive oligosilyltriflates—synthesis, structure and
rearrangementw
c
b
c
b
C. Krempner,*ab U. Jager-Fiedler, C. Mamat, A. Spannenberg and K. Weichert
¨
a Department of Chemistry, The Ohio State University, 100 W. 18th Ave, Columbus, Ohio,
43210, USA. E-mail: ckrempne@chemistry.ohio-state.edu; Fax: 001- 614-2920368;
Tel: 001-6146883625
b Institut fur Chemie, Universitat Rostock, A.-Einstein-Str. 3a, 18059 Rostock, Germany
¨
¨
c Leibniz-Institut fur Organische Katalyse an der Universitat Rostock e. V., A.-Einstein-Str.
29a, 18059 Rostock, Germany
¨
¨
Received (in St. Louis, MO, USA) 7th June 2005, Accepted 17th October 2005
First published as an Advance Article on the web 3rd November 2005
The novel branched oligosilyltriflates of formula [TfO(Me3Si)2Si]2E (2a–d) [E ¼ 0 (2a), SiMe2 (2b), GeMe2
(2c), SiMe2–SiMe2 (2d)] and [TfO(Me3Si)2SiSiMe2]3SiMe (9) have been prepared by the protodesilylation of
[Ph(Me3Si)2Si]2E (1a–c), [H(Me3Si)2Si]2E (5d) and [Ph(Me3Si)2SiSiMe2]3SiMe (8) using TfOH (CF3SO3H) as
reagent in almost quantitative yields. The crystal structure of 2b is reported.
metathesis reactions by treatment of phenylbis(trimethylsilyl)-
Introduction
silylpotassium with BrCH2CH2Br, Cl2SiMe2, Cl2GeMe2 and
ClMe2SiSiMe2Cl, respectively.7 For the protodesilylation of
1a–d, the progress of the reaction was monitored by 29Si- and
19F-NMR spectroscopy.
In recent years there has been a growing interest in well-defined
polyfunctionalized oligosilanes as sources of materials with
novel electronic properties.1 The development of such com-
pounds requires new synthetic methods for the manipulation of
functional groups on the silicon backbone without Si–Si bond
cleavage. One of such useful groups is the trifluoromethane-
sulfonate group (TfO ¼ CF3SO3), which can be selectively
introduced by protodesilylation of hydrido, vinyl, allyl and
phenyl silanes with trifluoromethanesulfonic acid (TfOH)
giving the desired silyltriflates in high yields.2
Although the formation of multiple triflate functionalized
oligosilanes, often obtained as thermally unstable oils, has been
reported by protodesilylation of mainly perphenylated linear,3
cyclic4 and dendrimeric oligosilanes,5 nothing is known about
their structures in the solid state. Herein we wish to report a
rather straightforward and selective synthesis of a series of
novel isolable and crystalline permethylated oligosilanes with
two or three triflate functionalities and we report the first
crystal structure of an oligosilylbis(triflate).
When the oligosilanes 1a–c were allowed to react with two
equivalents of TfOH, immediately new fluorine signals for the
triflate group appeared at around ꢀ75 ppm in the 19F NMR
spectrum and a significant shift for the tertiary silicon atom to
lower field was observed: ꢀ70.9 (1a), ꢀ70.5 (1b) and ꢀ62.8
ppm (1c) to 40.8 (2a), 39.3 (2b) and 44.8 ppm (2c).8 The
remarkable changes in the chemical shift clearly indicate a
complete Si–Ph bond cleavage under formation of the corre-
sponding SiOTf functionality. The reaction proceeds facilely
and highly selectively yielding the oligosilylbis(triflates) 2a–c in
almost quantitative yields after removal of the solvent (eqn (1).
The triflates so obtained are surprisingly thermally stable but
highly moisture sensitive, colourless solids. They can be further
purified by crystallization from pentane at low temperatures.
The NMR spectra of the crystalline compounds are rather
straightforward and are in full agreement with the structures
proposed.
Results and discussion
Phenylsilanes, readily available and reactive derivatives to-
wards the protodesilylation reaction, are ideal starting materi-
als for the synthesis of oligosilyl triflates. Disadvantageous,
however, is that in the presence of TfOH or related electro-
philic reagents, linear oligosilanes with more electropositive
substituents such as alkyl or silyl tend to rearrange under Si–Si
bond cleavage or migration of substituents to branched struc-
tures.6 To avoid such unpredictable skeletal rearrangements,
we primarily focused on the synthesis of already branched
oligosilanes in which both phenyl groups, connected to tertiary
silicon atoms, are separated from each other by different
spacers groups.
ð1Þ
In addition, the molecular structure of 2b has been con-
firmed by X-ray crystallography. Suitable crystals were grown
from pentane solutions at RT. As seen in Fig. 1, the molecule
has a total of 7 silicon atoms and a longest chain of 5 silicon
atoms bearing two triflate groups at the 2- and 4-positions.
Most of Si–Si bond lengths lie unremarkably within the range
of 2.36–2.38 A and the Si–Si–Si bond angles are variable but
generally within the range 110–1191.
The corresponding diphenyloligosilanes 1a–d (supporting
informationw) have been synthesized in good yields via salt
w Electronic supplementary information (ESI) available: experimental
details. See DOI: 10.1039/b408573f
T h i s j o u r n a l i s & T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 5
N e w J . C h e m . , 2 0 0 5 , 2 9 , 1 5 8 1 – 1 5 8 4
1581

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The formation of 3 can be rationalized as a 1,2-oligosilyl
rearrangement in the oligosilane backbone reflecting clearly the
tendency of oligosilanes to undergo skeletal rearrangements
under formation of highly branched structures as already could
be shown by Ishikawa et al.6a Attempts to avoid such a
rearrangement by using the less polar solvent pentane led only
to mixtures of 3 and the bis(triflate) 2d, which could not be
separated.
However, the reaction of the dihydrido compound 5d with
TfOH gave the desired bis(triflate) 2d in almost quantitative
yields according to the NMR spectra (eqn (2)). To our surprise,
2d a stable crystalline solid does not tend to undergo a skeletal
rearrangement into 3, which, is in sharp contrast to the
reaction of 1d with TfOH.
Fig. 1 Molecular structure of 2b in the crystal. The thermal ellipsoids
correspond to 30% probability. Hydrogen atoms are omitted for
clarity. Selected bond lengths [A] and angles [1]: Si2–O1 1.778(2), Si3–
O4 1.780(2), S1–O1 1.520(2), S2–O4 1.494(3), Si1–Si2 2.3581(14), Si1–Si3
2.3582(14), Si3–Si6 2.3785(14), Si2–Si4 2.3821(15), S1–O1–Si2 132.2(2),
S2–O4–Si3 137.1(2), Si2–Si1–Si3 118.90(5), Si1–Si2–Si5 118.22(5), Si1–
Si2–Si4 109.90(5), Si1–Si3–Si7 116.45(5), Si1–Si3–Si6 109.80(5).
ð2Þ
The remarkable difference in the selectivity of the proto-
desilylation reaction of 1d compared to 5d, may be attributed
to an intramolecular donor–acceptor interaction of the silyltri-
flate with the phenyl group. This may result in the transient
formation of the bridged carbocationic species 7 after the first
protodesilylation step (Scheme 2). Similar 1,5-bridged cationic
systems with a thermodynamically more stable carbosilane
backbone have already been characterized by NMR spectro-
scopy and are supported by quantum-mechanical calcula-
tions,16 and 1,3-bridged cationic carbosilanes are supposed to
be intermediates in the solvolysis of silicon halides with the
[(Me2ArSi)(Me3Si)2C] group.17 A hydrido group (X ¼ H)
being only a weak donor should not interact intramolecularly
with the silyltriflate, thereby a rapid second protodesilylation
occurs without a competitive skeletal rearrangement.
The Si4–Si2–Si1–Si3–Si6 pentasilane chain approximately
adopts an all-trans conformation (T-T-T),9 which is known
to be optimal for s-conjugation.10 The geometry around the
silicon atoms Si2 and Si3 attached to the triflate groups can be
described as distorted tetrahedral. As a result of steric strain in
the molecule the angles S1–O1–Si2 [132.21] and S2–O4–Si3
[137.11] are larger then in Ph3SiOTf 11 with 129.061 but smaller
than in (But)3SiOTf 12 in which the bulky tertiary butyl groups
force a significant widening of the angle to a value of 149.31.
The length of the Si–O bonds in 2b, 1.778 A and 1.78 A, is not
only significantly longer than those normally observed for Si–O
bonds [1.64–1.66 A],13 it is also longer than in (But)3SiOTf
[1.742 A] and in Ph3SiOTf [1.743 A], implying that there is a
significant ionic component in the bonding.14
Moreover, it is the longest so far observed for tetrahedral
silyltriflates,15 reflecting the strong electronic influence of the
surrounding electropositive silyl substituents, which act as
more electron donating groups than their carbon analogues
in (But)3SiOTf. The silylium cation character in this compound
is also reflected in the sum of the three Si–Si–Si bond angles
(338.51 and 336.81) around the two silyltriflate functionalities
indicating a substantial flattening of the oligosilane backbone.
In striking contrast to the protodesilylation reactions de-
scribed above, treatment of 1d with TfOH in dichloromethane
1
yields a product which exhibits four signals in the H-NMR
spectrum, two for a SiMe3 and two for a SiMe2 group, instead
of the two signals expected for bis(triflate) 2d. However, the
structure of the liquid compound was clearly identified by
means of NMR spectroscopy as the rearranged oligsilyl(bis)-
triflate 3, which shows 6 signals in the 29Si-NMR spectrum,
two for a silyltriflate group, two for a primary silicon, one for a
quaternary and one for a secondary silicon as well (Scheme 1).
In addition, the proposed structure of 3 could be confirmed
by hydrolysis, which gave the cyclic siloxane 4 exclusively
(supporting informationw).
Scheme 1 Reaction of 1d with TfOH.
Scheme 2 Proposed mechanism for the skeletal rearrangement.
1582
N e w J . C h e m . , 2 0 0 5 , 2 9 , 1 5 8 1 – 1 5 8 4

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samples of 2a–d and 9 were obtained by recrystallization from
pentane at ꢀ40 1C.
1,1,1,4,4,4-Hexamethyl-2,3-bis(trifluoromethanesulfonato)-
2,3-bis(trimethylsilyl)tetrasilane (2a). 1a (1.355 g, 2.7 mmol),
TfOH (0.5 ml, 5.7 mmol) and pentane (10 ml). Yield 1.36 g
(78%); 1H NMR (C6D6, 250 MHz): d 0.33 (s, SiMe3, 36H); 13
C
NMR (C6D6, 63 MHz): d 0.0 (SiMe3), 119.0 (q, 1JCꢀF ¼ 318.1
Hz, CF3); 29Si NMR (C6D6, 79.5 MHz): d ꢀ8.7 (SiMe3), 40.8
(SiOTf); 19F NMR (C6D6, 235.3 MHz): d ꢀ74.4 (CF3); Anal.
Calc. for C14H36F6O6S2Si6 (647.05): C, 25.99; H, 5.61. Found:
C, 25.67; H, 5.65%.
Scheme 3 Synthesis of triflate 9.
1,1,1,3,3,5,5,5-Octamethyl-2,4-bis(trifluoromethanesulfona-
to)-2,4-bis(trimethylsilyl)pentasilane (2b). 1b (1.51 g, 2.7 mmol),
TfOH (0.5 ml, 5.7 mmol) and CH2Cl2 (ca. 10 ml). Yield 1.69 g
(89%); 1H NMR (C6D6, 250 MHz): d 0.31 (s, SiMe3, 36H),
0.44 (s, SiMe2, 6H); 13C NMR (C6D6, 63 MHz): d 0.1 (SiMe3),
To summarise, the strong p-donor interaction of the phenyl
group does not only significantly stabilize the transient bridged
species 7, but leads to an increased reaction rate of the
intramolecular 1.2-oligosilyl migration by anchimeric assis-
tance.
Finally, and with the intention of confirming the concept
successfully applied for the triflates 2a–c mentioned above, we
extended our studies to the synthesis of triflate functionalized
dendritic oligosilanes. As substrate we used compound 8, an
oligosilane dendrimer of first generation with a longest chain of
7 silicon atoms, which was prepared analogously by reaction of
three equiv. of Ph(Me3Si)Si–K with (ClMe2Si)3SiMe in good
yields (Scheme 3).
Whereas, the treatment of 8 with three equiv. of TfOH in
dichloromethane gave a mixture of several rearranged pro-
ducts of unknown composition, a microcrystalline precipitate
of the dendritic oligosilyltris(triflate) 9 was formed using
pentane as solvent. Although we were not able to obtain single
crystals of 9 suitable for X-ray diffraction studies, the structure
proposed was unambiguously evidenced by means of elemental
analysis and NMR spectroscopic data.
1
ꢀ2.7 (SiMe2), 119.1 (q, JCꢀF ¼ 318.5 Hz, CF3); 29Si NMR
(C6D6, 79.5 MHz): d ꢀ10.0 (SiMe3), ꢀ32.4 (SiMe2), 39.3
(SiOTf); 19F NMR (C6D6, 235.3 MHz): d ꢀ74.4 (CF3); Anal.
Calc. for C16H42F6O6S2Si7 (705.21): C, 27.25; H, 6.00. Found:
C, 26.34; H, 6.30%.
1,1,1,3,3,5,5,5-Octamethyl-2,4-bis(trifluoromethanesulfona-
to)-2,4-bis(trimethylsilyl)-3-germa-pentasilane (2c). 1c (1.635 g,
2.7 mmol), TfOH (0.5 ml, 5.7 mmol) and CH2Cl2 (ca. 10 ml).
Yield 1.84 g (91%); 1H NMR (C6D6, 250 MHz): d 0.30 (s,
SiMe3, 36H), 0.60 (s, GeMe2, 6H); 13C NMR (C6D6, 63 MHz):
1
d ꢀ0.2 (SiMe3), ꢀ2.1 (GeMe2), 119.1 (q, JCꢀF ¼ 317.1 Hz,
CF3); 29Si NMR (C6D6, 79.5 MHz): d ꢀ9.5 (SiMe3), 44.8
(SiOTf); 19F NMR (C6D6, 235.3 MHz): d ꢀ75.7 (CF3); Anal.
Calc. for C16H42F6O6S2Si6Ge (749.71): C, 25.63; H, 5.65; F,
15.20. Found: C, 25.03; H, 5.34; F, 14.93%.
1,1,1,3,3,4,4,6,6,6-Decamethyl-2,5-bis(trifluoromethanesulfo-
nato)-2,5-bis(trimethylsilyl)hexasilane (2d). 5d (0.40 g, 0.86
mmol), TfOH (0.16 ml, 1.89 mmol) and pentane (ca. 5 ml).
Yield 0.55 g (84%); 1H NMR (C6D6, 250 MHz): d 0.27 (s,
SiMe3, 36H), 0.50 (s, SiMe2, 12H); 13C NMR (C6D6, 75 MHz):
d 0.0 (SiMe3), ꢀ3.0 (SiMe2), 119.1 (q, 1JCꢀF ¼ 318.1 Hz, CF3);
29Si NMR (C6D6, 59.6 MHz): dꢀ32.8 (SiMe2), ꢀ10.7 (SiMe3),
42.1 (SiOTf); 19F NMR (C6D6, 235.3 MHz): d ꢀ74.7 (CF3);
Anal. Calc. for C18H48F6O6S2Si8 (763.36): C, 28.32; H, 6.34.
Found: C, 27.91; H, 6.22%.
Conclusion
The branched oligosilyltriflates 2a–d and 9 have been prepared
by protodesilylation using TfOH as reagent. Although highly
moisture sensitive, these oligosilyltriflates are thermally stable
and easy to handle crystalline compounds which can be
obtained even in larger amounts. Therefore, they might be of
interest as Lewis acid catalysts for a variety of organic trans-
formations and as reactive reagents for the protection of OH
groups in sugars or other natural products. Investigations with
regard to the latter are currently in progress.
1,1,1,3,3,5,5-Heptamethyl-2,5-bis(trifluoromethanesulfona-
to)-2,4,4-tris(trimethylsilyl)pentasilane (3). 1d (1.67 g, 2.7
mmol), TfOH (0.5 ml, 5.7 mmol) and CH2Cl2 (ca. 10 ml).
Experimental
1
Yield 1.98 g (96%); H NMR (C6D6, 250 MHz): d 0.27, 0.29
(2s, SiMe3, 2ꢁ18H), 0.47, 0.71 (2s, SiMe2, 2ꢁ6H); 13C NMR
(C6D6, 63 MHz): d 0.6, 1.4 (SiMe3), 2.8 (SiMe2), 119.1
General remarks
1
(q, JCꢀF ¼ 320.4 Hz, CF3); 29Si NMR (C6D6, 59.6 MHz):
All reactions involving organometallic reagents were carried
out under an atmosphere of argon using standard Schlenk
techniques. TfOH (CF3SO3H) was distilled prior to use,
d ꢀ10.0, ꢀ9.3 (SiMe3), ꢀ29.4 (SiMe2), 55.9 (SiMe2OTf),
43.3 (SiOTf), ꢀ122.5 (quart. Si); 19F NMR (C6D6, 235.3 MHz):
d ꢀ75.5, ꢀ74.1 (CF3); Anal. Calc. for C18H48F6O6S2Si8
(763.36): C, 28.32; H, 6.34. Found: C, 27.78; H, 6.29%.
Ph(Me3Si)2Si-K
ꢁ
3
THF,18 (Me3Si)2SiPh–SiPh(SiMe3)2,
(1a),19 and (Me3Si)2SiH–SiMe2–SiMe2–SiH(SiMe3)2, (5d),20
were prepared as previously described. The full preparation
and characterization of the oligosilanes 1b-d, 4 and 8 is given in
the supporting informationw.
1,1,1,3,3,4,5,5,7,7,7-Undecamethyl-2,6-trifluoromethanesulfo-
nato-2,6-bis(trimethylsilyl)-4-[1,1,3,3,3-pentamethyl-2-trifluoro-
methanesulfonato-2-trimethylsilyltrisilanyl]heptasilane (9).
8
General procedure for the protodesilylation of 1a–d, 5d and 8.
In a typical experiment, TfOH (1.1 equiv.) was added to a
stirred solution of 1a–d, 5d or 8 in pentane or CH2Cl2 at 0 1C.
The mixture was allowed to warm up to RT within 2 h. After
evaporation of the solvent the solid residue was dried under
high vacuum to remove the excess TfOH. Analytically pure
(1.75 g, 1.8 mmol), TfOH (0.5 ml, 5.7 mmol) and pentane
(ca. 10 ml). After a few minutes a white solid immediately
precipitates out which was isolated by filtration and drying
1
under vacuum to give pure 9 (1.3 g, 61%); H NMR (C6D6,
250 MHz): d 0.34 (s, SiMe3, 54H), 0.63 (s, SiMe2, 18H) 0.8 (s,
SiMe, 3H); 13C NMR (C6D6, 63 MHz): d 0.5 (SiMe3), 0.2
N e w J . C h e m . , 2 0 0 5 , 2 9 , 1 5 8 1 – 1 5 8 4
1583

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(SiMe2), 0.8 (SiMe), 127.0 (CF3); 19F NMR (C6D6, 235.3
MHz): d ꢀ74.1 (CF3); Anal. Calc. for C28H75F9O9S3Si13
(1188.17): C, 28.30; H, 6.36. Found: C, 28.01; H, 6.13%.
5
6
(a) A. Sekiguchi, M. Nanjo, C. Kabuto and H. Sakurai, J. Am.
Chem. Soc., 1995, 117, 4195; (b) C. Marschner and E. Hengge in
Organosilicon Chemistry II, ed. N. Auner and J. Weis, Wiley-
VCH, 1997, p. 333; (c) M. Nanjo, T. Sunaga, A. Sekiguchi and
E. Horn, Inorg. Chem. Commun., 1999, 203.
(a) M. Ishikawa, J. Iyoda, H. Ikeda, K. Kotake, T. Hashimoto
and M. Kumada, J. Am. Chem. Soc., 1981, 103, 4845; (b) J. Y.
Corey, D. M. Kraichely, J. L. Huhmann and J. Braddock-Wilk-
ing, Organometallics, 1994, 13, 3408; (c) J. Y. Corey, D. M.
Kraichely, J. L. Huhmann, J. Braddock-Wilking and A. Linde-
berg, Organometallics, 1995, 14, 2704; (d) U. Herzog andG.
Roewer, in Organosilicon Chemistry III: From Molecules to Ma-
terials, ed. N.Auner and J. Weis, VCH, Weinheim, 1998, p. 312; (e)
U. Herzog, N. Schulze, K. Trommer and G. Roewer, J. Organo-
met. Chem., 1997, 547, 133.
X-ray crystallography
The data collection for 2b was done on a STOE-IPDS dif-
fractometer using a graphite monochromated Mo Ka radia-
tion. The structure was solved by direct methods (SHELXS-86)
and refined by the full matrix least-squares method against F2
(SHELXL-93)21 All non-hydrogen atoms were refined aniso-
tropically. The hydrogen atoms were placed in theoretical
positions and refined using the riding model.
7
8
1a was first prepared by D. B. Puranik, M. P. Johnson and M. J.
Fink, Organometallics, 1989, 8, 770.
Crystal data. C16H42F6O6S2Si7, M ¼ 705.25, colourless
prism, monoclinic, space group P21/c, a ¼ 8.992(2), b ¼
16.534(3), c ¼ 23.992(5) A, b ¼ 92.85(3)1, V ¼ 3562.6(13)
A3, T ¼ 200 K, Z ¼ 4, m(Mo-Ka) ¼ 0.443 mmꢀ1, 8670
reflections measured, 4657 reflections independent of symme-
try, 3692 reflections observed [I 4 2s(I)], 334 parameters,
R1 ¼ 0.045, wR2 (all data) ¼ 0.121.z
In a previous work 2a was produced analogously using toluene as
solvent, but neither was the compound isolated nor were analytical
data given, except for the Si-NMR spectrum which is in agreement
with our data. U. Baumeister, K. Schenzel, R. Zink and K.
Hassler, J. Organomet. Chem., 1997, 543, 117.
The conformations are roughly classified as cis (C) and trans (T)
by dihedral angles of 0–301 and 150–1801, respectively.
9
10 (a) J. Michl and R. D. Miller, Chem. Rev., 1989, 89, 1359; (b) R.
West, in The Chemistry of Organic Silicon Compounds, ed. S. Patai
and Z. Rappoport, Wiley, Chichester, UK, 1989, p. 1207.
11 A. Asadi, A. G. Avent, C. Eaborn, M. S. Hill, P. B. Hitchcock, M.
M. Meehan and J. D. Smith, Organometallics, 2002, 21, 2183.
12 H.-W. Lerner, N. Wiberg, M. Bolte, H. Noeth and J. Knizek, Z.
Naturforsch. Teil B, 2002, 57, 177.
13 E. Lukevics, O. Pudova and R. Sturkovich, Molecular Structure of
Organosilicon Compounds, Horwood, Chichester, UK, 1989,
p. 180.
14 P. D. Lickiss, The Chemistry of Organic Silicon Compounds, ed. Z.
Rappoport and Y. Apeloig, Wiley, Chichester, UK, 1998, Vol. 2,
Part 1, Ch. 11.
15 The Si–OTf distances in pentacoordinate organosilicon structures
are usually longer than in tetracoordinate derivatives.
Acknowledgements
We thank Prof. Dr H. Oehme for his generous support and we
gratefully acknowledge the support of our research by the
Fonds der Chemischen Industrie.
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¨
3
4
University of Gottingen, Germany, 1993.
¨
z CCDC reference number 288229. For crystallographic data in CIF or
other electronic format see DOI: 10.1039/b408573f
1584
N e w J . C h e m . , 2 0 0 5 , 2 9 , 1 5 8 1 – 1 5 8 4