The competition between Si-Si and Si-C cleavage in functionalised oligosilanes: Their reactivity with elemental lithium

Article abstract of DOI:10.1039/b920846a

The reaction of aryl-substituted disilanes with elemental lithium is a common method for the preparation of lithiosilanes and the subsequent synthesis of functionalised oligosilanes, especially of enantiomerically pure compounds. A series of alkyl- and arylsubstituted di- and trisilanes has been investigated with respect to their reactivity against elemental lithium. Thereby, depending on the substituents, silicon-silicon bond cleavage of the central Si-Si unit and/or silicon-carbon bond cleavage to arenes are observed. Quantum chemical studies provide a deeper insight into the ongoing processes. The reactive centre can be estimated by both, bond elongation after electron transfer and frontier orbital interactions (π-bonding and σ-antibonding part). Aromatic substituents at the silicon atoms proved to be necessary for the processing of any cleavage reaction in the studied systems by stabilising the radical anion after electron transfer at the corresponding di- or trisilane. Yet, selective cleavage reactions do not depend on the number of arenes.

Full text of DOI:10.1039/b920846a

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The competition between Si–Si and Si–C cleavage in functionalised
oligosilanes: their reactivity with elemental lithium†
Christian Da¨schlein and Carsten Strohmann*
Received 7th October 2009, Accepted 23rd November 2009
First published as an Advance Article on the web 14th January 2010
DOI: 10.1039/b920846a
The reaction of aryl-substituted disilanes with elemental lithium is a common method for the
preparation of lithiosilanes and the subsequent synthesis of functionalised oligosilanes, especially of
enantiomerically pure compounds. A series of alkyl- and arylsubstituted di- and trisilanes has been
investigated with respect to their reactivity against elemental lithium. Thereby, depending on the
substituents, silicon–silicon bond cleavage of the central Si–Si unit and/or silicon–carbon bond
cleavage to arenes are observed. Quantum chemical studies provide a deeper insight into the ongoing
processes. The reactive centre can be estimated by both, bond elongation after electron transfer and
frontier orbital interactions (p-bonding and s-antibonding part). Aromatic substituents at the silicon
atoms proved to be necessary for the processing of any cleavage reaction in the studied systems by
stabilising the radical anion after electron transfer at the corresponding di- or trisilane. Yet, selective
cleavage reactions do not depend on the number of arenes.
Introduction
Results and discussion
Functionalised lithiosilanes1,2 are versatile reagents in organic and
organometallic chemistry, e.g. for the nucleophilic introduction of
protecting groups, synthesis of silyl substituted transition metal
complexes or silicon based polymers.3,4 Besides the synthesis
of lithiosilanes via the reaction of chlorosilanes with elemen-
tal lithium,5 their preparation via selective silicon–silicon bond
cleavage with lithium has become the method of choice as
no disrupting lithium chloride is formed.2c,3b,3c,6 According to
a general formulation of the reactivity of simple disilanes by
Tamao and Kawachi,3c systems possessing at least one aromatic
substituent at silicon can be cleaved selectively at their central
Si–Si unit using lithium in polar solvents such as thf. Contrary to
solely alkyl substituted disilanes, they referred the electron transfer
(ET) from the metal to the disilane (the first step of the cleavage
reaction) and therefore the following cleavage of the Si–Si bond
in aryl substituted systems to the decrease of the energy of the
lowest unoccupied molecular orbital (LUMO). Nevertheless, this
has mainly been studied for simple and often symmetric disilanes
whereas investigations concerning the reactivity of more complex
systems are rare.
Experimental studies
In 2002 and 2007 we reported the selective synthesis of
the enantiomerically pure lithiosilanes 2 and 5 via selective
cleavage of the Si–Si unit (for the synthesis of 2)4c and selective
cleavage of the Si–C unit (for the synthesis of 5),4e respectively, in
the functionalised disilanes (R)-1 and (R)-3 (see Scheme 1).8,9
As part of our studies on aminomethyl and amino substituted
silanes,4b-4f,7 we herein report the synthesis of a series of func-
tionalised di- and trisilanes and their reactivity against elemental
lithium. A combination of experimental and theoretical studies
has been used to elucidate the preferred centre of reactivity and
the reasons for the observed competing Si–Si and Si–C cleavage
reactions.
Scheme 1 Synthesis of the enantiomerically pure lithiosilanes 2 and 5 via
selective Si–Si (upper row) and Si–C (lower row) cleavage in the disilanes
(R)-1 and (R)-3.4c,4e
Most recently, we realised that the related trisilane (R)-4 yields
a product mixture consisting of the trapping products of a
Si–Si cleavage and of a Si–C cleavage to the aromatic substituent
(see Scheme 2). Thus, the reactivity of (R)-4 against lithium
represents a transition from the selective Si–Si in 1 to the Si–C
Anorganische Chemie, Technische Universita¨t Dortmund, Otto-Hahn-
Straße 6, 44227 Dortmund, Germany. E-mail: mail@carsten-strohmann.de;
Fax: (+) 49-231-755-7062; Tel: (+) 49-231-755-3807
† Electronic supplementary information (ESI) available: Computational
details. See DOI: 10.1039/b920846a
2062 | Dalton Trans., 2010, 39, 2062–2069
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compound 9, we were now able to investigate the impact of the
aromatic substituents on the cleavage reactions. It has to be noted
that silane 10 has been synthesised in its racemic form, as the
stereo information is not of relevance for these reactivity studies.
To examine the reactivity of the shown compounds against
elemental lithium, each silane was added to lithium (granals)
◦
in thf.12 After a reaction time of 6 h at 0 C,13 each solution
was trapped with a twofold excess of Me3SiCl. In each case, we
expected a silicon–silicon and/or a silicon–carbon bond cleavage
to the aromatic substituent of the central silicon (see Scheme 4).
Scheme 2 Reaction of the enantiomerically pure trisilane (R)-4 with
elemental lithium.
cleavage in 3. Thereby, the thermodynamically stronger Si–C bond
is preferential cleaved at -78 ◦C.9,10
Based on these results, we wondered about the cause for
the different reactivity of the three reactants (R)-1, (R)-3 and
(R)-4, and if we can estimate the observed reactive centre in
functionalised oligosilanes in general.
For a systematic study of the reactivity of di- and trisilanes
against elemental lithium, we first synthesised – in addition to the
already known compounds (R)-1,4c (R)-34e and (R)-44b – the three
disilanes 8, 9 and 10 (see Scheme 3).
Scheme 4 Expected reactions (Si–Si vs. Si–C cleavage) in functionalised
di- and trisilanes; R = Me or Ph; R¢ = Ph or SiR3; T = temperature.
Afterwards, the product mixtures were analysed via GC/MS
analysis and NMR spectroscopy. Based on the obtained results the
investigated silanes can be separated into four different categories:
(i) systems reacting under selective Si–Si cleavage, (ii) systems
reacting under selective Si–C cleavage, (iii) systems resulting in
a mixture of both possible reactions and (iv) systems without any
reaction. Fig. 1 and Table 1 summarise the described reactions
Table 1 Experimentally investigated silanes, reaction temperatures and
observed reactions
Scheme 3 Synthesis of the disilanes 8, 9 and 10.
8 and 9 have been synthesised according to the previously
reported method for 14c starting with the (chloromethyl) sub-
stituted disilanes 11 (for 8) and 12 (for 9) via an amination
reaction with a twofold excess of piperidine in toluene. 10
was synthesised via selective cleavage of the Si–C bond with
elemental lithium in diphenylmethyl(piperidinomethyl)silane (13)
and subsequent trapping reaction of the formed lithiosilane with
dimethylphenylchlorosilane.11 The newly synthesised disilanes
have been chosen as additional systems for our investigations, as
this series of functionalised silanes feature a gradual decrease of
the number of aromatic substituents at silicon atoms. Starting with
the triphenyl substituted disilane 1 to the solely alkyl substituted
Ratio Si–Si :
Si–C cleavage
System Reaction temperature Observed reaction
1
8
4
-78 ◦C/0 ◦Ca
Selective Si–Si cleavage 100 : 0
Selective Si–Si cleavage 100 : 0
0 ◦
C
-78 ◦Cb
Mixture of Si–Si and
Si–C cleavage
Mixture of Si–Si and
Si–C cleavage
15 : 85
10
0 ◦Cb
58 : 42
3
9
-50 ◦C/0 ◦Ca
Selective Si–C cleavage 0 : 100
No reaction
0 ◦
C
—
a Both reaction temperatures are known in literature. b Detailed investiga-
tions follow below.
Fig. 1 Experimentally investigated silanes and their experimentally determined reactivity against elemental lithium [the reactive bond(s) is(are) labelled
in dark red].
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Table 3 Lengths of the Si–Si and Si–C bonds of all investigated silanes
(for Fig. 1: reactive centres are indicated in dark red; blue, green
and red refers to the number of aromatic substituents in each
compound).
Observed
reaction
Bond length
Si–Si/A
Bond length
Si–Ci/A
˚
˚
Optimised system
The obtained results allow the following conclusions: (i) there
is no distinct trend between the number of aromatic substituents
at the silicon atoms and the observed reactive centre; (ii) yet, the
aromatic substituents are obviously necessary for the procedure
of any reaction (no transformation takes place in case of the
solely alkyl substituted disilane 9). Most remarkably, silane 10
undergoes no selective cleavage of its Si–Si bond despite its
phenyl substituent. Instead, a product mixture resulting from
both possible reactions could be determined. Contrary, the mono
phenyl substituted disilane 8 shows selective cleavage of the
silicon–silicon bond, although it is strongly related to the disilane
3, reacting under selective Si–C cleavage. Hence, albeit very useful
for simple disilanes, the above mentioned postulate of Tamao and
Kawachi3c obviously has to be modified for more complex systems.
1
Si–Si cleavage
Si–Si cleavage
Si–Si cleavage
Si–Si cleavage
Mixture
Mixture
Mixture
2.388
2.442
2.378
2.380
2.380
2.413
2.381
2.415
2.377
2.389
2.377
2.376
1.896
1.874
—
∑
1 -
8
∑
8 -
—
4
1.898
1.865
1.897
1.869
1.898
1.872
—
∑
4 -
10
∑
10 -
Mixture
3
Si–C cleavage
Si–C cleavage
None
∑
3 -
9
∑
9 -
None
—
“∑-” indicates the corresponding radical anion.
been reported for other functionalised silanes.16 Astonishingly, the
lowest energy of the LUMO is not found for disilane 1 (selective
Si–Si cleavage) – as expected – but for disilane 10, which showed
both possible reactions in the experiment. The highest value is
observed for the solely alkyl substituted disilane 9. Altogether, no
clear tendency can be found for the energy of the frontier orbitals.
How can the reactivity of these silanes against elemental lithium
be understood? One common feature of all reactions is the fact,
that the bond cleavage always takes place between the central,
(piperidinomethyl) substituted silicon and one of its substituents
(SiR3 or Ph). In almost all cases, the electron transfer results in
an elongation of the Si–Si bond. The highest change is found
Theoretical studies
To get a deeper insight into the experimentally observed reac-
tivities of the silanes against elemental lithium, we performed
quantum chemical calculations for all investigated molecules. The
geometries of the stationary points were initially optimised by
using density functional theory with the hybrid B3LYP functional
and the 6-31+G(d) basis set.14 Harmonic vibrational frequency
analyses – to establish the nature of the global minima – were
performed on the same level showing no imaginary frequencies.
The starting structural parameters for the systems were taken
from the X-ray data of (S)-1·HCl7b and (R)-3·HCl.4e All other
compounds were constructed based thereon with Chem3D.
The cleavage of element–element bonds with lithium (alkali
metals, respectively) is supposed to proceed via double electron
transfer from the metal to the investigated system, creating a
radical anion after the first ET before the second ET induces the
bond cleavage.3c,4b,15 Thus, each molecule was optimised both, as
a neutral compound and as a radical anion. Possible interactions
between the radical anion and the metal have not been considered
in these calculations. According to the postulate of Tamao and
Kawachi,3c additional aromatic substituents should result in a
decrease of the energy of the LUMO. Thus, we first examined the
energy of the frontier orbitals of the neutral systems as well as their
HOMO–LUMO gap (LUMO = lowest unoccupied molecular
orbital; HOMO = highest occupied molecular orbital; SOMO =
single occupied molecular orbital). The results can be found in
Table 2.
˚
between disilane 1 and its radical anion (Dd = 0.054 A) which
undergoes selective cleavage of this bond. In fact, there is an almost
continuous decrease of the bond elongation over the disilane
10 and the trisilane 4 (showing Si–Si and Si–C cleavage in the
˚
˚
experiment) with values of 0.034 A and 0.033 A until only an
elongation of 0.012 A in the disilane 3 (which shows no Si–Si
cleavage in the experiment anymore). Finally, silane 9 – without
any reaction – actually shows a slight shortening of this bond of
˚
˚
˚
0.001 A. The only exception of this trend is disilane 8 (0.002 A, Si–
Si cleavage). Contrary, the Si–C distances show no clear tendency.
Although the electron transfer does result in a shortened bond in
all cases, the smallest bond shortening was observed in disilane
1 and not – as expected – in the trimethyl substituted system 3
showing the cleavage of this bond in the experiment (Si–C bond
shortenings: 1: 0.022 A; 4: 0.033 A; 10: 0.028 A; 3: 0.026 A). Thus,
although the change of the bond lengths is already indicating a
difference between the investigated silanes, it can not be the only
reason for the diverse reactivities. Table 3 summarises the described
bond elongations and shortenings.
˚
˚
˚
˚
Both, HOMO and LUMO of all investigated silanes as well as
the HOMO–LUMO gap have comparable energy. Similar values
for HOMO and LUMO and the HOMO–LUMO gap have already
Recognising the frontier orbitals (HOMO, LUMO and SOMO)
of all investigated silanes one similarity can be seen (see Fig. 2):
the LUMO of all compounds are located with almost the same
shape at the aromatic substituents, except of disilane 9 without any
reaction. This observation confirms the statement of Tamao and
Kawachi3c concerning the necessity of the arenes for a successful
cleavage reaction. Obviously, an orbital of suitable energy and
shape is required in the first step of the bond cleavage (the electron
transfer), to stabilise the transferred electron and initiate the
subsequent cleavage reaction. Although the frontier orbitals are
in general comparable, some significant differences with respect
Table 2 HOMO and LUMO energies as well as the HOMO–LUMO gap
of all optimised silanes
Optimised system HOMO/a.u. LUMO/a.u. Gap/a.u. Gap/eV
1
-0.21490
-0.20960
-0.21380
-0.21331
-0.21396
-0.21774
-0.02754
-0.02343
-0.02384
-0.02962
-0.02262
-0.01371
-0.18736
-0.18617
-0.18996
-0.18369
-0.19134
-0.20403
-5.10
-5.07
-5.17
-4.99
-5.21
-5.55
8
4
10
3
9
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Fig. 2 HOMO (second from the left) and LUMO (second from the right) of the neutral systems as well as the SOMO (right) of the radical anions;
Molekel plots;17 threshold values: cutoff 0.04. In the schematic drawing (left), the reactive bond(s) are indicated in red.
to p-bonding and s-antibonding parts of the relevant bonds can
be found in all cases. The SOMO is almost exclusively located
at the aromatic substituent. In disilane 1 with selective Si–Si
bond cleavage as well as in 4 and 10 with partly Si–Si cleavage,
a significant p-bonding part can be found between the silicon
and the phenyl group (also present in 8, but to the arene at
the other silicon). This p-bonding part leads to the stabilising
of the corresponding bond. Thereby, this p-bonding part is mostly
pronounced in 1 where no cleavage of this bond was observed
in the experiment. Together with the present s-antibonding part
(destabilising; most pronounced in disilane 1) between the two
silicon atoms, this is consistent with the experimental reactivity.
This is confirmed by the trimethylsilyl substituted disilane 3
(selective Si–C cleavage): on the one hand, 3 exhibits no s-
antibonding part between the two silicon atoms and on the other
hand, it shows the less distinct p-bonding part to the arene. None
of these effects is visible in 9 (no reaction). Fig. 3 shows (simplified)
the described orbital interactions.
Based on these observations, the reactivity of all compounds
investigated can be understood:
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Table 4 Product distribution between the tetrasilane 6 (Si–C cleavage)
and the disilane 3 (Si–Si cleavage), depending on the reaction temperature
Entry T/◦C
Si–C cleavage (tetrasilane 6) Si–Si cleavage (disilane 3)
1
2
3
4
5
+ 45
60
40
32
20
15
13
RT (~20) 68
-50
76
85
87
-78
~ -90
Fig. 3 (Strongly) Simplified ChemDraw picture showing destabilising
s-antibonding part between the two silicon atoms (left) and stabilising
p-bonding part to the arene (right).
1 (selective cleavage of the Si–Si bond): the electron transfer
results in the weakening of the silicon–silicon bond due to the
bond elongation and the s-antibonding part between both silicon
atoms. The cleavage of the Si–C bond is not observed due to the
significant p-bonding part.
3 (selective cleavage of the Si–C bond): although the electron
transfer does also result in an elongated Si–Si bond, the SOMO
doesn’t possess any s-antibonding part. Together with the weakest
p-bonding part to the arene, the ET finally results in the cleavage
of the most weakened bond, the Si–C bond.
4 and 10 (Si–Si as well as Si–C cleavage): the ET results in an
elongation of the central Si–Si bond in both compounds weak-
ening this bond, which is further amplified by a s-antibonding
part (although not as distinct as in 1). Moreover, the p-bonding
part to the arene (stabilising the bond to the phenyl group) is less
pronounced compared to 1. This finally results in the cleavage
of both possible bonds. It should be noted, that the SOMO of 4
also possesses a s-antibonding part between Si2 and Si3. Anyhow,
as it is less pronounced than between Si1 and Si2, no competing
cleavage between both Si–Si bonds does occur.
8 (selective cleavage of the Si–Si bond): the ET only results
in a slight elongation of the Si–Si bond in disilane 8, yet, no
s-antibonding part is effected. Contrary, the strong p-bonding
part to the aromatic substituent obviously amplifies this bond
so much, that no Si–C cleavage can occur. As a result, the
thermodynamically weakest Si–Si bond is cleaved after the electron
transfer.
Scheme 5 Reaction of trisilane 4 with elemental lithium to tetrasilane
6 (Si–C cleavage) and disilane 3 (Si–Si cleavage); T = temperature.
◦
◦
Increasing the temperature from -90 C to +45 C resulted in
an increase of the Si–Si bond cleavage, whereas low temperatures
led to the preferred cleavage of the thermodynamically stronger
Si–C bond (see Table 4). The ratio between 6 and 3 varies between
60 : 40 at +45 ◦C and a considerable preference of the tetrasilane 6
at -90 ◦C (87 : 13). A further decrease of the temperature is limited
due to the melting point of thf and the difficulty to maintain the
low temperature.
Afterwards, disilane 10 was reacted likewise with lithium for
6 h at three different temperatures (~15 ◦C, 0 ◦C and -78 ◦C)
and subsequently trapped with two equivalents of Me3SiCl (see
Scheme 6). Finally, the crude products were investigated via
GC/MS and NMR spectroscopy.
Temperature-dependence of the reactive centre in trisilane 4 and
disilane 10 in the experiment
As described above, reactions of trisilane 4 as well as disilane 10
with elemental lithium lead to product mixtures of both possible
cleavage reactions (Si–Si and Si–C) at the chosen temperature
(-78 ◦C for 4, 0 ◦C for 10). Often, the strict control of temperature
is the method of choice to distinguish between two possible
reaction pathways. To investigate a possible influence of the chosen
reaction temperature on the product composition of the cleavage
reactions (see Scheme 4) of 4 and 10, respectively, with elemental
lithium, we investigated both reactions at different temperatures.
At first, 4 was treated as described with elemental lithium
and subsequently trapped with Me3SiCl (see Scheme 5; see also
Scheme 2) at five◦different temperatures, starting with +45 ◦C
(thf)18 until ~ -90 C (n-pentane/N2).19 The crude products were
again investigated via GC/MS and NMR spectroscopy.
Scheme 6 Reaction of racemic disilane rac-10 with elemental lithium to
silanes rac-15 (Si–C cleavage) and rac-3 (Si–Si cleavage); T = temperature.
As anticipated, the product composition between Si–C cleavage
(trisilane 15) and Si–Si cleavage (disilane 3) is also influenced by the
reaction temperature (see Table 5). Yet, contrary to trisilane 4, the
weaker Si–Si bond is cleaved preferentially in disilane 10 at lower
temperatures, what is in agreement with the chemical expectations.
Nevertheless, this temperature dependency is less pronounced than
in the trisilane 4. Table 5 summarises the experimentally obtained
ratios between both products.
Thus, simple variation of the temperature in the reaction of
trisilane 4 and disilane 10 with elemental lithium, respectively,
leads to a preference of one of both possible reactions. In both
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Table 5 Product distribution between Si–Si and Si–C bond cleavage in
disilane 10 depending on the reaction temperature
Synthesis
(R)-1,4c (R)-34e and (R)-44b have been synthesised according to
literature procedures.
Entry
T/◦C
Si–C cleavage
Si–Si cleavage
1
2
3
RT (~15)
44
42
33
56
58
67
Synthesis of 1,1,2,2-tetramethyl-1-phenyl-1-(piperidinomethyl)-
disilane (8). The synthesis of 8 was performed analogue to the
synthesis of 1.4c Therefore, a solution of 10.0 g (41.2 mmol)
1-(chloromethyl)-1,2,2-trimethyl-1,2-diphenyldisilane and 8.06 g
(94.7 mmol) piperidine, dissolved in 50 ml toluene, was heated
under reflux for 18 h. After removal of all volatile compounds in
vacuo, the crude product was dissolved again in 80 ml n-pentane,
separated from all salts and finally cleaned through bulb-to-bulb
0
-78
cases, the amount of the preferred cleavage product at room
temperature is increased at lower temperatures (Si–C cleavage for
3; Si–Si cleavage for 10). This preference of the main product at low
temperatures is the expected course for two competing, kinetically
controlled reaction pathways (with different reaction barriers).
Further effects influencing or regulating these different reaction
barriers are not clear at the moment.
◦
distillation (oven temperature: 132 C; pressure 3.2 10-4 mbar).
1
Yield: 9.72 g (33.3 mmol, 81%); H-NMR (400.1 MHz, CDCl3):
d = 0.06 [s, 6H; Si(CH3)2], 0.34 [s, 6H; Si(CH3)2], 1.25–1.35 (m,
2H; NCCCH2), 1.45–1.50 (m, 4H; NCCH2C), 1.91 (s 2H; SiCH2),
2.15–2.25 (m, 4H; NCH2CC), 7.25–7.35 (m, 3H; aromat. H), 7.45–
Conclusions
1
13
7.50 (m, 2H; aromat. H); { H} C-NMR (100.6 MHz, CDCl3):
d = -3.7, -3.4 (2C each) [Si(CH3)2], 23.9 (1C) (NCCCH2), 26.3
(2C) (NCCH2), 50.4 (1C) (SiCH2N), 58.5 (2C) (NCH2C), 127.6
(2C) (C-m), 128.2 (1C) (C-p), 133.9 (2C) (C-o), 139.6 (1C) (C-i);
We herein presented the synthesis of a series of alkyl and aryl
substituted di- and trisilanes and their reactivity against elemental
lithium. Depending on the substituents at silicon selective Si–Si
and Si–C cleavages as well as mixtures of both possible reactions
have been observed. Aromatic substituents at the silicon atoms
proved to be necessary for the viability of any cleavage reaction
as they are stabilising the radical anion after electron transfer to
the corresponding di- or trisilane. Yet, selective cleavage reactions
do not depend on the number of arenes. The observed reactivity
in the respective silane can be understood via bond elongation, s-
antibonding and p-bonding of involved bonds. Si–Si bond elonga-
tion in combination with s-antibonding between the silicon atoms
and p-bonding between silicon and the aromatic substituents
results in the selective cleavage of the thermodynamically weakest
Si–Si bond (e.g. disilane 1). Weakened p-bonding and missing s-
antibonding topple the reactivity towards a selective Si–C bond
cleavage (disilane 3).
Furthermore, if both possible cleavage reactions – Si–Si and Si–
C – occur in the experiment (silanes 4 and 10), the observed ratio
between Si–Si and Si–C cleavage feature a reaction temperature
dependency in favour of the kinetic product. Thereby, decreasing
the temperature resulted in cleavage of the thermodynamically
stronger Si–C bond in trisilane 4, whereas the weaker Si–Si bond
was cleaved in the disilane 10.
1
29
{ H} Si-NMR (59.6 MHz, CDCl3): d = -21.0 (1Si) (NCSi), -19.8
(1Si) (NCSiSi).
Synthesis of 1,1,2,2,2-pentamethyl-1-(piperidinomethyl)disilane
(9). The synthesis of 9 was performed analogue to the synthesis
of 8. Yield: 8.73 g (38.1 mmol, 86%); 1H-NMR (400.1 MHz,
CDCl3): d = 0.07 [s, 9H; Si(CH3)3], 0.42 [s, 6H; Si(CH3)2],
1.25–1.35 (m, 2H; NCCCH2), 1.45–1.55 (m, 4H; NCCH2C),
1
13
1.89 (s 2H; SiCH2), 2.15–2.25 (m, 4H; NCH2CC); { H} C-
NMR (100.6 MHz, CDCl3): d = -3.9 (2C) [Si(CH3)2], -2.1
(3C) [Si(CH3)3], 23.7 (1C) (NCCCH2), 26.1 (2C) (NCCH2), 49.6
1
29
(1C) (SiCH2N), 58.3 (2C) (NCH2C); { H} Si-NMR (59.6 MHz,
CDCl3): d = -21.5 (1Si) (NCSi), -20.2 (1Si) (NCSiSi).
Synthesis of 1,2,2-trimethyl-1,2-diphenyl-1-(piperidinomethyl)-
disilane (10). 235 mg (33.9 mmol) lithium in 10 ml
thf were combined with 5.00
g (16.9 mmol) diphenyl-
methyl(piperidinomethyl)silane (13) at 0 ◦C and stirred at this tem-
perature for 5 h. Afterwards, the brown solution of the lithiosilane
was added at -78 ◦C to 6.36 g (37.2 mmol) dimethylphenylchlorosi-
lane, resulting in the immediate decolouration of the mixture,
before it was allowed to warm to room temperature. After removal
of all volatiles in vacuo the residue was suspended in 15 ml of
2M NaOH and extracted 5 times with 15 ml diethyl ether. The
combined organic layers were extracted 5 times with 10 ml of
2M HCl and afterwards set to pH 12 with NaOH. Finally the
aqueous layer was extracted 5 times with 15 ml of diethyl ether
and the combined organic layers were dried over Na2SO4. After
removal of all volatile compounds in vacuo, the crude product
was ◦cleaned through bulb-to-bulb distillation (oven temperature:
220 C; 1.0 10-3 mbar). Yield: 3.97 g (11.2 mmol, 71%); 1H-NMR
(400.1 MHz, CDCl3): d = 0.34, 0.36, 0.37 (s, 3H each; SiCH3),
At the moment we are trying to elucidate the reactivity of further
functionalised silanes against elemental lithium and the effects
of electron-withdrawing substituents at silicon to the preferred
reactive centre.
Experimental
General details
All experiments were carried out under a dry, oxygen-free argon
atmosphere using standard Schlenk techniques. Involved solvents
were dried over sodium and distilled prior to use. NMR spectra
were recorded on an AMX-400 Bruker spectrometer at 22 ◦C. As-
signing the signals was supported by additional DEPT-135, C,H-
and H,H-COSY experiments. GC/MS analysis were performed
on a ThermoQuest TRIO-1000 (EI = 70 eV); Column; Zebron,
Capillary GC Column, ZB-1.
1.25–1.35 (m, 2H; NCCCH2), 1.40–1.50 (m, 4H; NCCH2C),
2
2.15–2.30 (m, 4H; NCH2CC), 2.16, 2.27 (AB-system, JAB
=
1
13
14.3 Hz, 2H; SiCH2), 7.20–7.55 (m, 10H; aromat. H); { H} C-
NMR (100.6 MHz, CDCl3): d = -5.4 (1C) (NCSiCH3), -3.5, -3.4
(1C each) [Si(CH3)2], 23.8 (1C) (NCCCH2), 26.3 (2C) (NCCH2),
48.9 (1C) (SiCH2N), 58.5 (2C) (NCH2C), 127.56, 127.61 (2C each)
(all C-m), 128.34, 128.37 (1C each) (all C-p), 134.07, 134.16 (2C
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Table 6 Total (SCF) and zero-point energies (ZPE) of all calculated
systems
chemicals. C. D. thanks the Studienstiftung des deutschen Volkes
for a doctoral scholarship.
Compound
SCF/Hartree
ZPE/Hartree
Notes and references
1
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-1260.569402
-1260.557420
-1069.246902
-1069.227136
-1643.940472
-1643.938856
-1260.569191
-1260.557901
-1630.339755
-1630.324531
-1452.255706
-1452.251796
-1260.987286
-1260.971240
-1068.883119
-1068.865497
∑
1 -
1 For some selected pioneer works on functionalised lithiosilanes, see:
(a) L. H. Sommer and R. Mason, J. Am. Chem. Soc., 1965, 87, 1619;
(b) L. H. Sommer, J. E. Lyons and H. Fujimoto, J. Am. Chem. Soc.,
1969, 91, 7051; (c) E. Colomer and R. J. P. Corriu, J. Chem. Soc., Chem.
Commun., 1976, 176; (d) E. Colomer and R. J. P. Corriu, J. Organomet.
Chem., 1977, 133, 159.
2 For studies on silyl-anions, see: (a) J. B. Lambert, M. Urdaneta-Perez
and H.-N. Sun, J. Chem. Soc., Chem. Commun., 1976, 806; (b) J. B.
Lambert and M. Urdaneta-Perez, J. Am. Chem. Soc., 1978, 100, 157;
(c) M. Flock and C. Marschner, Chem.–Eur. J., 2002, 8, 1024; (d) M.
Flock and C. Marschner, Chem.–Eur. J., 2005, 11, 4635.
8
∑
8 -
4
∑
4 -
10
∑
10 -
3
∑
3 -
9
∑
9 -
3 (a) I. Fleming, R. S. Roberts and S. C. Smith, J. Chem. Soc., Perkin
Trans. 1, 1998, 1215; (b) U. Schubert and A. Schenkel, Transition Met.
Chem., 1985, 10, 210; (c) K. Tamao and A. Kawachi, Adv. Organomet.
Chem., 1995, 38, 1; (d) H.-S. Oh, L.-S. Park and Y. Kawakami, Chirality,
2003, 15, 646; (e) W. Uhlig, J. Organomet. Chem., 2003, 685, 70;
(f) I. Fleming in Organocopper Reagents, ed. R. J. K. Taylor, Oxford
University Press, Oxford, U.K., 1994, pp 257.
1
29
each) (all C-o), 138.2, 139.0 (1C each) (all C-i); { H} Si-NMR
(59.6 MHz, CDCl3): d = -23.3 (1Si) (NCSi), -21.2 (1Si) (NCSiSi).
General specification for the cleavage of the Si–Si and Si–C
bonds with elemental lithium in silanes 4 and 10. For the reactivity
studies, 100 mg (10: 283 mmol; 4: 286 mmol) of the corresponding
silane were dissolved in 1 ml of thf and reacted with two equivalents
of elemental lithium (granals) for 6 h at the temperature given in
Table 4 (for 4) and Table 5 (for 10). Afterwards, 2.2 equivalents
of trimethylchlorosilane was added at -78 ◦C resulting in the
decolouration of the corresponding mixture, before it was allowed
to warm to room temperature. After removal of all volatiles in
vacuo the residue was suspended in a minimum amount of n-
pentane and separated from all salts. Finally, the obtained product
mixtures were investigated via GC/MS and NMR spectroscopy
without further purification. The obtained product ratios are given
in Table 4 (for 4) and Table 5 (for 10), respectively.
4 For other selected works on lithiosilanes, see: (a) M. Omote, T.
Tokita, Y. Shimizu, I. Imae, E. Shirakawa and Y. Kawakami,
J. Organomet. Chem., 2000, 611, 20; (b) C. Strohmann and C. Da¨schlein,
Organometallics, 2008, 27, 2499; (c) C. Strohmann, J. Ho¨rnig and D.
Auer, Chem. Commun., 2002, 766; (d) C. Strohmann, M. Bindl, V. C.
Vraaß and J. Ho¨rnig, Angew. Chem., 2004, 116, 1029; C. Strohmann,
M. Bindl, V. C. Vraaß and J. Ho¨rnig, Angew. Chem., Int. Ed., 2004, 43,
1011; (e) C. Strohmann, C. Da¨schlein, M. Kellert and D. Auer, Angew.
Chem., 2007, 119, 4864; C. Strohmann, C. Da¨schlein, M. Kellert and
D. Auer, Angew. Chem., Int. Ed., 2007, 46, 4780; (f) D. Auer, M. Kaupp
and C. Strohmann, Organometallics, 2004, 23, 3647; (g) M. Nanjo, M.
Masayuki, Y. Ushida, Y. Awamura and K. Mochida, Tetrahedron Lett.,
2005, 46, 8945; (h) D. Bravo-Zhivotovskii, I. Ruderfer, S. Melamed, M.
Botoshansky, B. Tumanskii and Y. Apeloig, Angew. Chem., 2005, 117,
749; D. Bravo-Zhivotovskii, I. Ruderfer, S. Melamed, M. Botoshansky,
B. Tumanskii and Y. Apeloig, Angew. Chem., Int. Ed., 2005, 44, 739;
(i) T. I. Ku¨ckmann, F. Dornhaus, M. Bolte, H.-W. Lerner, M. C.
Holthausen and M. Wagner, Eur. J. Inorg. Chem., 2007, 1989; (j) D.
Scheschkewitz, Angew. Chem., 2004, 116, 3025; D. Scheschkewitz,
Angew. Chem., Int. Ed., 2004, 43, 2965; (k) H.-W. Lerner, Coord. Chem.
Rev., 2005, 249, 781.
Determination of the product distribution
The product distributions between Si–Si and Si–C cleavage have
been determined via a combination of GC/MS and NMR
spectroscopic analysis. Thereby, the results obtained from the
GC/MS measurements have been used to determine the obtained
products, whereas their ratios have been determined via integration
of relevant methyl groups in the 1H NMR spectra.
5 For some examples, see: (a) M. Oestreich, G. Auer and M. Keller,
Eur. J. Org. Chem., 2005, 184; (b) C. Strohmann, O. Ulbrich and D.
Auer, Eur. J. Inorg. Chem., 2001, 1013; (c) H.-W. Lerner, S. Scholz, M.
Bolte and M. Wagner, Z. Anorg. Allg. Chem., 2004, 630, 443.
6 For some examples, see: (a) H. Gilman, D. J. Peterson and D.
Wittenberg, Chem. Ind. (London), 1958, 1479; (b) A. G. Brook and
H. G. Gilman, J. Am. Chem. Soc., 1954, 76, 278; (c) H. Gilman and
T. C. Wu, J. Am. Chem. Soc., 1951, 73, 4031.
7 For more examples concerning our studies, see: (a) C. Da¨schlein
and C. Strohmann, Eur. J. Inorg. Chem., 2009, 43; (b) C. Da¨schlein,
V. H. Gessner and C. Strohmann, Acta Crystallogr., Sect. E: Struct.
Rep. Online, 2008, 64, o1950; (c) C. Strohmann and C. Da¨schlein,
Chem. Commun., 2008, 2791; (d) H. Ott, C. Da¨schlein, D. Leusser,
D. Schildbach, T. Seibel, D. Stalke and C. Strohmann, J. Am. Chem.
Soc., 2008, 130, 11901; (e) A. Hameau, F. Guyon, M. Knorr, C.
Da¨schlein, C. Strohmann and N. Avarvari, Dalton Trans., 2008, 4866;
(f) C. Strohmann, C. Da¨schlein and D. Auer, J. Am. Chem. Soc., 2006,
128, 704; (g) V. H. Gessner, C. Da¨schlein and C. Strohmann, Chem.–
Eur. J., 2009, 15, 3320; (h) C. Da¨schlein, J. O. Bauer and C. Strohmann,
Angew. Chem., 2009, 121, 8218; C. Da¨schlein, J. O. Bauer and C.
Strohmann, Angew. Chem., Int. Ed., 2009, 48, 8074; (i) C. Da¨schlein
and C. Strohmann, Z. Naturforsch., 2009, 64b, 1558.
8 The selective cleavage of the Si–C bond in functionalised silanes is a
desired reaction. Yet, this reaction type was hithereto mainly observed
as an unwanted side reaction, e.g. see: (a) M. Porchia, N. Brianese,
U. Casellato, F. Ossola, G. Rossetto, P. Zanella and R. Graziani,
J. Chem. Soc., Dalton Trans., 1989, 677; (b) A. L. Allred, R.T. Smart
and D. A. Van Beek Jr., Organometallics, 1992, 11, 4225; (c) H. Tsuji,
A. Toshimitsu and K. Tamao, Chem. Heterocycl. Compd., 2001, 37,
1369.
Computational details
All calculations were done without symmetry restrictions. Starting
coordinates were obtained with Chem3DUltra 10.0. Optimisation
and additional harmonic vibrational frequency analyses (to es-
tablish the nature of stationary points on the potential energy
surface) were performed with the software package Gaussian 03
(Revision D.01 and Revision E.01) on the B3LYP/6-31+G(d)
level.14 Thereby, no imaginary frequencies were obtained. The
starting structural parameters for the systems were taken from the
X-ray data of (S)-1·HCl7b and (R)-3·HCl.4e All other compounds
were constructed based thereon with Chem3DUltra 10.0. The total
(SCF) and zero-point energies (ZPE) of all systems can be found
in Table 6. All coordinates are available in the ESI.†
We are grateful to the Deutsche Forschungsgemeinschaft for
financial support. Furthermore we acknowledge the Wacker
Chemie AG and Chemetall GmbH for providing us with special
2068 | Dalton Trans., 2010, 39, 2062–2069
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9 The low temperature (-78 ◦C/-50 ◦C) in the synthesis of the shown
lithiosilanes is necessary to prevent the racemisation process occurring
at temperatures above -30 ◦C; for a detailed description, see: D.
Auer, J. Ho¨rnig and C. Strohmann in Organosilicon Chemistry V:
From Molecules to Materials, ed. N. Auner and J. Weis, Wiley-VCH,
Weinheim, 2003, pp. 167.
10 Although not isolated in pure form, the intermediate lithiosilane 7 is
only the seventh highly enantiomerically enriched lithiosilane known
in literature so far.
B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A. Petersson, H.
Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X.
Li, J. E. Knox, H. P. Hratchian, J. B. Cross, V. Bakken, C. Adamo, J.
Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R.
Cammi, C. Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A.
Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich,
A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck,
K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S.
Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz,
I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y.
Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson,
W. Chen, M. W. Wong, C. Gonzalez and J. A. Pople, Gaussian, Inc.,
Wallingford CT, 2004.
11 C. Strohmann, D. Schildbach and D. Auer, J. Am. Chem. Soc., 2005,
127, 7968.
12 The beginning of a reaction is indicated by a change of the colour: the
formation of any lithiosilane results in the change of the previously
colourless solution to a dark red, dark green or dark brown solution.
13 0 ◦C have been chosen as reaction temperature to minimized by-
products caused by ether-cleavage.
15 For some selected work dealing with element-element bond cleavage
reactions, see: (a) J.-M. Save´ant, Advances in Electron Transfer Chem-
istry, 1994, 4, 53; (b) L. Eberson, Acta Chem. Scand., 1999, 53, 751;
(c) F. Maran, D. D. M. Wayner and M. S. Workentin, Adv. Phys. Org.
Chem., 2001, 36, 85; (d) R. O. Al-Kaysi and J. L. Goodman, J. Am.
Chem. Soc., 2005, 127, 1620; (e) Z. Grobelny, Eur. J. Org. Chem., 2004,
2973; (f) E. D. Lorance, W. H. Kramer and I. R. Gould, J. Am. Chem.
Soc., 2004, 126, 14071; (g) S. Jakobsen, H. Jensen, S. U. Pedersen and
K. Daasbjerg, J. Phys. Chem. A, 1999, 103, 4141; (h) S. Antonello, K.
Daasbjerg, H. Jensen, F. Taddei and F. Maran, J. Am. Chem. Soc.,
2003, 125, 14905; (i) A. B. Meneses, S. Antonello, M. C. Arevalo and
F. Maran, Electrochim. Acta, 2005, 50, 1207; (j) R. A. Marcus and N.
Sutin, Biochim. Biophys. Acta, 1985, 811, 265.
14 (a) Gaussian 03, Revision D.01, M. J. Frisch, G. W. Trucks, H. B.
Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A.
Montgomery Jr., T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam,
S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani,
N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota,
R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao,
H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian, J. B. Cross,
V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann,
O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, P. Y.
Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg, V. G.
Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain, O. Farkas, D. K.
Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz,
Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov, G.
Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T.
Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe,
P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, C. Gonzalez and
J. A. Pople, Gaussian, Inc., Wallingford CT, 2004; (b) Gaussian 03,
Revision E.01, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, J. A. Montgomery Jr., T. Vreven, K. N.
Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone,
16 For one example, see: D.-R. Bai and S. Wang, Organometallics, 2004,
23, 5958.
17 P. Flu¨kinger, H. P. Lu¨thi, S. Portmann and J. Weber, Molekel 4.3, Swiss
Center for Scientific Computing, Manno, Switzerland, 2000-2002.
18 Due to the relatively high reaction temperature, several byproducts
caused by ether cleavage could be obtained in the reaction mixture.
Yet, we were able to focus on the desired tetra- and disilane.
19 The average temperature in this entry was about -90 ◦C; yet, intermit-
tent, slightly higher or lower temperatures were existent.
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 2062–2069 | 2069
©