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By Gerhard Fritz"
Dedicated to Professor Rudolf Hoppe on the occasion of his 65th birthday
Carbosilanes are compounds in which the elements carbon and silicon occupy alternate
positions in the molecular framework. Formal replacement of every second carbon atom in
the diamond lattice by a silicon atom gives silicon carbide, which exists, however, in several
modifications characterized by different stacking orders. The S i c lattice is the basis of most
carbosilanes."] These are divided on the basis of structural differences into carborundanes,
silascaphanes, and molecules that no longer contain elements of the silicon carbide structure. In the carborundanes the Si-C six-membered rings are arranged in the chair form, in
the silascaphanes in the boat form. The molecular framework of the third group is derived
from barrelane and asterane. The reactivity of the carbosilanes is determined mainly by the
substituents on the skeletal atoms. The presence of SiH o r Si-halogen groups leads to a
high reactivity, while derivatives with organic groups on the silicon atoms and CH, o r C H
groups in the skeleton are much less reactive. However, even such CH2 and C H groups
permit certain characteristic reactions. Because of their structural features and the range of
substituents that can be introduced, the carbosilanes are presently of considerable interest
with respect to technological developments as precursors for ceramics.
1. Introduction
The first organosilicon compounds were obtained in the
second half of the last century by reaction of chlorosilanes
with alkylzinc o r alkylmercury compounds and by WurtzFittig syntheses (reaction of R3SiC1 with R'CI and sodium).
Their systematic study, however, only began after the discovery of Grignard reagents.[*' The "direct synthesis" of
the rnethylchlorosilanes by reaction of silicon and methyl
chloride in the presence of a copper catalyst in the 1940s
led to the industrial preparation of Me2SiC1, and thus to
the development of the chemistry of silicones.[3J Compounds whose molecular framework consists of alternating
silicon and carbon atoms have only been synthesized in
the impetus for their study did not
the last 30
come from syntheses involving organosilicon compounds
but through studies of the free radical reactions initiated
by the thermal dec0,mposition of silanes in the gas phase:
e.g., the formation of H3Si-PH2 from SiH, and PH,['] o r
the reactions of ethylene with thermally produced silyl radicals."] The thermal decomposition of methylsilanes and
methylchlorosilanes (Me,-,SiY,;
Y =C1, H) in the gas
phase leads to the preferential formation of liquid, glassy,
o r crystalline compounds with an Si-C-Si skeleton.[61
Another route to carbosilanes is provided by reactions
of elemental silicon with CH2C12or CHCI, or with C-chlorinated carbosilanes such as (CI,Si),CCI2 in the presence
of a copper catalyst.[']
Routes involving organometallic synthesis can also be
used for the construction of various carbosilanes.[xlCyclization reactions of Si-methylated, linear or monocyclic
[*] Prof. Dr. C . Fritz
lnstitut fur Anorganische Chemie der Universitlt
Postfach 63 80, D-7500 Karlsruhe (FRG)
For a definition of the term "carbosilanes," see Ref. [I]
Angew. Chem. Inr. Ed. Engl. 26 (19871 1111-I132
carbosilanes under the influence of AIBr, provide a route
to oligocyclic carb~silanes.['~
The silicon and/or carbon atoms of the carbosilanes can
be functionalized (e.g., as SiH, SiCI, or SiF, or as CCI, CF,
o r CLi). Such functionalizations open up further synthetic
possibilities. Furthermore, they permit studies of the directing influence of the substituents on the reaction course.
2. Formation of Carbosilanes
2.1. Carbosilanes by Gas-Phase Pyrolysis of SiMe, and
Me,-.SICl, (x= 1-3)
2. I . I . Experimental
The synthesis of carbosilanes on a preparative scale is
carried out using an apparatus of the type shown in Figure
1. The pump P transfers SiMe, (b.p.=26"C), which evaporates from K into the (initially evacuated) apparatus, in the
direction of the arrows through the reaction vessel
(720k5"C) at such a rate that the contact time is ca. 1 min.
Shortly after commencement of the reaction, white fumes
exit from the reaction vessel and condense out at room
temperature in the traps F,-F3. At a later stage, molten yellow products emerge from the reaction vessel and crystals
are deposited on the walls of the vessel below the oven.["'l
At the same time, H2, CH,, C2H4, and C2H6 are formed.
Since the vapor pressure of SiMe, at 20°C is ca. 720 torr,
atmospheric pressure is built up in the apparatus after 23 h (excess pressure is released via a mercury valve). The
vapor pressure of SiMe, remains constant in the system as
long as liquid SiMe, is present in the reservoir, so that
steady-state conditions are maintained in the apparatus.
SiMe4 and (in small amounts) product molecules such as
H2, CHI, C2H4, C2H6, SiH-containing alkylsilanes, and
some very low-boiling carbosilanes are continuously
0 VCH Verlagsgesel/schaft mbH, 0-6940 Weinheim, 1987
0570-0833/87/1111-1132 $ 02.50/0
I 1 11
Fig. I . Pyrolysis apparatus. R, reaction vessel (Rotosil, diameter 12 cm,
length 80 cm); K, reservoir containing SiMe, or the rnethylchlorosilane; P,
circulation pump; F,-F,, condensation traps.
forced through the reaction vessel. The pyrolysis of other
methylsilanes occurs in a n analogous manner.
2.1.2. Carbosilanes by Pyrolysis of SiMe,
The complex product mixtures obtained upon pyrolysis
of SiMe, can be separated using chromatographic techniques.["I The products containing two and three silicon
atoms are both linear and cyclic (Scheme 1).
X = SiMe,H
X = SiMe,
Scheme 2. - = Me.
Scheme 1. - = Me or H.
Schemes 2-4 show characteristic silicon-richer compounds, the molecular skeletons of which contain the
1,3,5,7-tetrasilaadamantaneunit; this unit can formally be
derived from B-Sic (zinc blende type). Scheme 2 contains
substituted 1,3,5,7-tetrasilaadamantane~,"~~
some of which
have an annelated Si-C ring, and Scheme 3 contains compounds in which such skeletons are linked by CH,
Scheme 4 contains the carborundanes, a group of compounds whose molecular skeletons contain two, three, and
four annelated 1,3,5,7-tetrasilaadamantaneunits.['31
The structural unit of a - S i c (wurtzite-type ZnS) appears
in compounds 6 and 7 . Compound 6 contains the adamantane and wurtzitane units, 7 in addition the barrelane
unit. Compound 7 represents all essential features of the
Scheme 3. - = Me
Angew. Chem. Int. Ed. Engl. 26 (1987) 1111-1132
700°C. 60 vol% of the pyrolysate obtained from Me3SiC1
consists of a fraction boiling between 160 and 200°C at
atmospheric pressure, the remainder being oils and solids
which are soluble in nonpolar solvents. MeSiCI, yields a
pyrolysate of which 85% is liquid and colorless and 15%
oily or solid (fusible).
Scheme 4. - = Me.
4H-Sic structure: the adamantane unit and the complete
wurtzitane ~keleton."~'
While the carborundanes contain structural elements of
silicon carbide, compounds 8 and 9 are members of the
silascaphanes, a group whose Si-C six-membered rings are
arranged in the boat
The main products obtained from the methylchlorosilanes include partially methylated 1,3-disilapropanes.
Their contribution to the total pyrolysate is 32% in the case
of Me3SiC1, 43% in the case of MezSiCIz, and 60% in the
case of MeSiCl,. The pyrolysate from Me,SiCI contains all
the possible chloro- and methyl-substituted compounds
with the basic structure >Si-CH,-Sif.
The main product from Me,SiCI is 10, while that from MelSiC12 and
MeSiCI, is 11. Isolation and identification of these products
was accomplished after reduction with LiAIH4, whereby
complete CI/H exchange occurs without any change in
molecular structure. This is also true for the silicon-richer
compounds. In the latter case, the 1,3,5-trisilacyclohexane
and 1,3,5,7-tetrasilaadamantanestructures are increasingly
favored when more methyl groups are present in the starting material. Thus, the 1,3,5,7-tetrasilaadamantanes12- 15
are formed from Me3SiC1 in the ratio 170 :26 :3 : 1.
2.1.3. Carbosilanes by Pyrolysis of Chloro(methyl)silanes
The carbosilanes obtained by pyrolysis of tetramethylsilane are mostly unreactive, since (apart from SiH groups
in a few cases) they are not functionalized. Carbosilanes
containing SiCl functional groups can however be obtained
by pyrolysis of the three methylchlorosilanes.~'61
The thermal decomposition of the methylchlorosilanes
in the gas phase occurs with sufficient rapidity at ca.
Angew. Chem. In!. Ed. Engl. 26 (1987) 1111-1132
13. X = Y = C l
Y =
= M
i s -x/Si--. .--
/ 'Me
15. X = Y = Me
The silicon-richer compounds formed upon pyrolysis of
Me,SiCI have not been studied in more detail. Those com1113
pounds formed upon pyrolysis of Me,SiCI, or MeSiCI,
that contain four to eight silicon atoms have been better
characterized."'] Separation of the products forming the
pyrolysate of MezSiClz yielded forty compounds containing five to seven silicon atoms; the separation was carried
o u t by chromatography following the conversion of the
SiCl groups to SiH groups, and the products were identified on the basis of N M R and MS studies"'!. Figure 2
shows the types of compound obtained, with some examples. Compound 22 is the first known carbosilane with
carbon atoms forming the bridgeheads of an adamantane
skeleton; i.e., a 2,4,6,8.9,10-hexasilaadamantane derivative.
The polycyclic products formed in the pyrolysis of
MeSiC1, are mostly carbosilanes with the asterane
structure[''. 19] (Scheme 5).
\I /
The Si : C ratio of the methylsilanes has a considerable
influence on the structure of the pyrolysis products. As the
number of methyl groups in the starting material is reduced in the series SiMe,, MeSiCI,, MeCI,Si-CH,-SiCI,,
the formation of carbosilanes with an Si :C ratio of 1 : 1 is
increasingly favored. Carbosilanes with an adamantane
structure are formed preferentially from SiMe,, but never
from MeSiC1,; in support of this finding, no carbosilanes
with the asterane structure are formed from SiMe,. The
Si : C ratios of some molecular skeletons are given in
Scheme 6.
Scheme 5
1 : 1.5
I-ip. 2 I j p e \ 01 cumpound with l'ibe tu eighl ~ t l i ~ oatom5
lormcd upon pyrolysis of MelSiC12. 16, example of a substituted 1.3,5-trisilacyclohexane;
17, 18, examples of substituted !,3,5,7-tetrasilabicyclo[3.3.l]nonanes; 19, 20,
examples of substituted 1,3,5,8-tetrasilabicyclo[2.2.2]octanes;21, 22, molecules with the adamantane skeleton; 23, 24, molecules containing the 1,3disiiacyclopentane unit. - = H.
1 : 1.43
1 : 1.25
1 : l
1 : l
Scheme 6 .
Angew. Chem. I n [ . Ed. Engl. 26 (1987) 1111-1132
2.1.4. Mechanism of the Formation of Carbosilanes in the
Gas-Phase Pyrolysis of Methylsilanes
Prior to the discussion of the mechanism we should note
the following observations. The compounds formed from
SiMe, contain methyl groups and, to a lesser extent, hydrogen atoms as the substituents on the silicon atoms: only
in a few cases are structures with Si-Si bonds observed.
The compounds derived from methylchlorosilanes bear
methyl and chlorine substituents on the silicon atoms.
Neither C-chlorinated carbosilanes nor carbon polymers
are observed. The formation of the pyrolysate occurs via
complex reaction sequences which have until now not
been sufficiently clarified in detail. Clearly, the preparative
method used permits low-molecular-weight reaction products to participate in the overall reaction, owing to the permanent circulation of volatile molecules through the apparatus. The formation of silicon-rich products of higher molecular weight is favored by a thermal aftertreatment. Taking into account the experimental facts, the discussion of
the mechanism has to include the following considerations:
the primary reaction in the thermal decomposition of
the methylsilanes in the gas phase;
the importance of insertion of the CH, group into the
Si-Si bond;
the possible participation of initially produced, simple
carbosilanes in the formation of higher-molecularweight ones.
a) The first step in the thermolysis of SiMe, is described
by reaction (1). In order to account for the formation of
+ Me
formed, respectively. The existence of radicals has not yet
been proved, and species with Si-C double bonds formed
at 700°C in the gas phase should be regarded in a more
general sense as reactive intermediates. However, it appears correct to define the primary reaction as one in
which a n Si-C bond undergoes homolysis to give a silyl
and a methyl radical.
b) It is noteworthy that compounds with SiH groups are
found among the pyrolysis products of SiMe,. Since silyl
radicals are formed in the pyrolysis of the methylsilanes,
their recombination to give compounds with Si-Si bonds is
to be expected. The formation of the SiH groups is due to
the insertion of the C H 2 group into the Si-Si bond according to reaction (4). Examples are provided by hexamethyl>Si-SicCH3
disilane [reaction (5)][”] and trisilacyclopentane 2123.243
[reaction (611.
c) i n order to obtain information pertinent to the participation of simple carbosilanes in the total pyrolysis reaction,
the thermal behavior of the linear compounds (Me3Si)2CH2,
(Me3Si-CHz),SiMez, and (Me3Si-CH2-SiMe2)2CH2 and
the cyclic compounds 25, 26, and 27 was studied. Ther-
compounds with the 3 S i - C H 2 - S i t skeleton, reaction
(2) also seems likely. Of particular importance is the isolaMe$-Me
+ Me
+ CH,
tion of the cyclic compounds 25 and 26. These are also
formed when the methylsilanes are decomposed in a cold
plasma ;Izo1 under these conditions, thermally induced sec-
ondary reactions cannot occur. These compounds are presumably formed via reaction (3), followed by dimerization
and trimerization.
+ Me
Helm and
and later Davidson et al.12’b1have
suggested a primary reaction in which radicals or reactive
intermediates with a silicon-carbon double bond are
Angew. Chem. Inr. Ed. Engl. 26 (1987) 1111-1132
mally induced rearrangements are most likely to occur
when the molecular framework of the compound is
strained. The studies on the disilacyclobutane derivative
25 show the influence of temperature;[251while at 300320°C polymeric Si-C-Si chains are formed, at 480-500°C
molecular structures are formed which correspond to those
observed in the pyrolysis of SiMe,. The six-membered ring
compound 26, in contrast, is much more stable. At 500°C
the disilacyclopentene 27 yields 1,3,5,7-tetrasilaadamantanes and the bicyclooctene 28 as well as further products
of the SiMe, pyrolysis. Thus it appears that some of the
compounds formed in the pyrolysis of SiMe4 react further,
under the reaction conditions, as intermediates in the construction of the silicon-richer compounds. The studies carried out until now have not permitted a complete characterization of the higher-molecular-weight carbosilanes in
the methylsilane pyrolysates; they occur as solid, glasslike,
fusible products.
2.2. Carbosilanes by Reaction of Silicon with CHzCIz,
CHC13, and CCI,
2.2.1. Reaction of CH2C12with Silicon
As early as 1945, following the direct synthesis of the
methylchlorosilanes, a US patent reported the reaction of
silicon with CH2Clzin the presence of copper;Iz6]the products were stated to include linear compounds,
X3Si(CH2),SiX2Y ( X = C l , Y = H or CI, n = 1-4) and cyclic
compounds (SiC12CH2), ( x = 3 and larger). We were later
able to isolate the crystalline six-membered ring com-
pound (C12Si-CH2)3 29 and the eight-membered ring
compound (C12Si-CH2), 30 from such reaction^.^'^'^'^
Apart from these and the linear compounds 11 and 31,
only viscous and nondistillable products are formed. The
reaction of silicon with CH2C12in a fluidized bed at 350°C
favors the formation of such substances. Since Sic1 groups
hinder chromatographic separation, they were first converted into SiH groups by reduction with LiAIH,. The
compounds obtained (see Fig. 3) were then separated by
HPLCf7."] (for further details see Refs. [4, 71).
a) Si,C,-,H,,
b) Si,C,H,,+,
Figure 4a. In this case, too, the reaction in a fluidized bed
favors the formation of higher-molecular-weight products;
these could be separated by chromatography and identified after transforming the Sic1 groups into SiH groups
(see Refs. [7, 41 and Fig. 4b-d).
a) (C13Si)2CH211, HCIzSi-CH2-SiC13, CI3Si-CHCI-SiCI3,
HCI2Si-CH(SiCI3),, (C13Si)3CH,(C13Si-CH2)zSiC1231,
b) Unbranched carbosilanes with terminal SiH, groups,
e.g., H3Si(CH2-SiH2)4CH2-SiH3
Carbosilanes with one or two C branches, e.g.,
d) 1,3,5-Trisilacyclohexaneswith one, two, or three Si substituents
at the C atoms, e.g.,
(two terminal SiH3 groups)
(one terminal SiH, and one terminal
CH3 group)
Fig. 4. Compounds formed in the reaction of CHCI, with silicon (Cu catalyst).
Fig. 3 . Types
(Cu catalyst).
aubstances formed in the reaction of silicon with CH2C12
The products of the reaction of silicon with CH2C12contain the corresponding Si-chlorinated compounds. Those
of higher molecular weight are honeylike, tarry, or solid
glassy substances. The proportion of such substances is ca.
20%. The viscosity of the SiH-containing compounds is
considerably lower.
R . Miiller and c ~ - w o r k e r s [also
~ ~ ,studied
~ ~ ~ the reaction
of silicon with CCI,. In agreement with Miiller's results,
the reaction in a fluidized bedf7]leads mainly to the following substituted methane, ethylene, and acetylene derivatives:
(C13Si),CH, (CI,Si),CCI, (CI,Si),C,
2.2.2. Reaction of CHCl, and CC14 with Silicon
R . Miiller et aLtZx1
studied the reaction of silicon with
CHC13 and were able to identify the compounds shown in
Angew. Chem. In;. Ed. Engi. 26 (1987) 1111-1132
2.2.3. Mechanism of the Reaction of CH2C12,CHCI,, and
CCI., with Silicon
chlorinated carbosilanes. Thus, (CI3Si),CCl2 33 reacts at
300-350°C with silicon (Cu catalyst) to give the carbosilanes shown in Figure 5 (white crystals); the four-membered rings are joined in a spirocyclic manner via carbon
The reactions of silicon (Cu catalyst) with CHzC12 and
CHCI, in a fluidized bed afford almost solely carbosilanes.
With CH2CI2, 70-80% of the products consist of nonThe replacement of the chlorines on silicon by methyl
branched carbosilanes, while with CHCI, similar amounts
groups has a considerable effect on this reaction, as can be
of C-branched carbosilanes and C-substituted trisilacycloseen by the reaction of 34
hexanes are formed; no bi- and polycyclic carbosilanes are
observed. An increase in the reaction temperature always
(Me3Si)2CC12 34
leads to a decrease in the amounts of Si-richer carbosilanes. These results can be understood o n the basis of the
and of the partially methylated derivatives. A
mechanism proposed by Klebanskq and F i k h t e n g ~ l ’ t s [ ~ ’ , ~ ~>Si-CHCI-Sic
unit is formed which prohibits the forfor the reaction of silicon (Cu catalyst) with MeCI. The admation of cyclic compounds. Thus, Me,Si-CCI,-SiMe,CI
sorption of the CH2C12molecules o n the catalyst surface is
gives (among other compounds) isomers of composition
a crucial step, the complete reaction occurring at this surSi3C6HI6Cl4,e.g., compound 35.[331The formation of the
face. The possibility that further reaction steps occur in the
spiro arrangement from perchlorinated carbosilanes apgas phase, leading finally to the observed products, can be
pears to be favored; thus the products of the reaction of
excluded. This hypothesis is supported by the following
the octachloro derivative 36 with elemental silicon at
observations : no further reaction occurs when isolated carbosilanes, such as (C13Si-CHz-SiCIz)2CHz, are passed
over activated silicon (Cu catalyst) at 350°C; changes occur only at 500°C after three days. Linear carbosilanes
with four or more Si atoms undergo degradation to give
the six-membered ring compound 29, which is not formed
from 31.l3’l
2.2.4. C-Spirocyciic Carbosilanesfrom Perchlorinated
Carbosilanes and Silicon (Cu Catalyst)
The reactions of chlorinated methanes with silicon described in Sections 2.2.1 and 2.2.2 can be applied to C(C~,Si),cc~,
330°C include the spirocyclic compound 37 as well as further compounds such as those having the empirical formulas Si7C6H9CI3and Si, ,C9HlzC120,whose structures are not
completely clear.‘351 These reactions, the mechanism of
which is not known, lead to a new group of carbosilanes.
3. The Organometallic Synthesis of Carbosilanes
Functional substituents on silicon are of great importance for the synthesis and chemical behavior of the carbosilanes. Most substitution reactions with organometallic
compounds occur via the Si-halogens, particularly via the
SiCl group.[361The cleavage of phenyl groups from silicon
with HBr or HI has proved to be extremely useful for the
synthesis of carbosilanes from organosilicon compounds
and for the preparation of halosilanes with SiH groups
(such as SiH3Br)[371[reaction (7)J The cleavage of phenyl
+ HBr
+ C6H6
groups is slowed down, or even prevented, by the introduction of electronegative substituents on the Si atom.
3.1. The Synthesis of Oligocyclic Carbosilanes
Fig. 5. Carbosilanes formed by reaction of 33 with silicon (Cu catalyst).
Angew. Chem. in,. Ed. Engl. 26 (1987) 1111-1132
The organometallic synthesis of cyclic and polycyclic
carbosilanes has been reviewed.[” It is made possible by
the introduction of Si-protecting groups (e.g., phenyl),
which are transformed into functional groups in the step
preceding the final one of ring formation. An example is
the synthesis of trans- and cis-nonamethyl- 1,3,5,7,9-pentasiladecalin 40 : construction of the linear, protected carbo( Ph M ezSi-C H2-Si Me2- CH,),Si Ph Me
silane 38 ; cleavage of the Si-phenyl bonds with the formation of the bromo compound 39; reaction of 39 with
CHBr3 and lithium to give the trans and cis compounds
A further example is the synthesis of the trans-trans
and cis-trans perhydrophenalene derivatives 45 and 46,
synthesis of the chain 41 ; Br2 cleavage of
one phenyl group on each Si atom to give 42; reduction
with LiAIH, to give 43; ring formation with lithium to
yield 44 ; once again, Si-Ph cleavage; bromination of the
SiH groups and reaction with CHBr3 and lithium to give a
product mixture from which the isomers 45 and 46 are
41, X = P h ; 42, X = B r ; 43, X = H
3.2. Synthesis of C-Bridged Carbosilanes
So far, the syntheses of oligocyclic carbosilanes always
make use of functional substituents at silicon and thus lead
to bridging via silicon atoms. The pyrolysis of methylsilanes (Section 2.1) and the rearrangement reactions of carbosilanes induced by AICI3 (Section 4) also lead in general
to similar Si-bridged carbosilanes. The situation is illustrated in Scheme 7.
Scheme 7
In the isolated carborundanes the Si atoms occupy
bridgehead positions (a). The formation of compounds
with carbon bridgeheads and the Si6C4 skeleton (b) is an
exception’’’] (cf. 22). An approach to compounds of this
type requires further progress in the syntheses of C-functional carbosilanes. In Sections 3.2.1 and 3.2.2 suitable
reactions at the carbon atoms of the carbosilane skeleton
will be described.
/---7i7 I .
I s i q s i
Lithiation at Skeletal Carbon Atoms of Si-Methyfated
In contrast to the possibilities for reactions involving the
substituents on the Si atom, the C H 2 group in the carbosilanes appears unreactive. In order to expand the scope of
carbosilane synthesis it is necessary to incorporate the latter group into the synthetic scheme. Reactions of perchlorinated carbosilanes with alkyllithium or alkylmagnesium
compounds are not suitable, since the reactivity of the
CC12 group is enhanced by its position between the two
SiC12groups: for example, reactions with MeMgCl lead to
skeletal changes (see Section 5.1.3). However, one possibility lies in the selective acid-base reactionf4’]according to
reaction (8).
+ LiR
The synthesis of the heptasila[4.4.4]propellane 4Sr401
achieved in an analogous manner from 47 (after replacing
the phenyl groups by bromine), CCI4, and lithium.
+ RH
Reaction (8) requires a reagent that selectively lithiates
s S i - C H 2 - S i t groups but does not react with either
(3Si),CH, >SiMe, >SiMe2, or -%Me3 groups. These criteria are satisfied by the complex between nBuLi and
N , N . N’, N’-tetramethylethylenediamine (TM E DA).‘421The
model substance used for our studies was 1,1,3,3,5,5-hexamethyl-1,3,5-trisilacyclohexane 26, which can undergo
several successive lithiations and ~ilylations.[~”
The reaction of 26 with nBuLi/TMEDA in hexane at 30°C gives
the monolithiated compound 49 quantitatively in 2-3 h
(Scheme 8).
Angew. Chem. In(. Ed Engl. 26 (1987) 1111-1332
ment of 51 to 55 or of 53 to 56 occurs in TMEDAIhexane
at higher temperatures.
Compound 50 contains primary, secondary, and tertiary
C atoms and is thus a model substance for studying the
thermodynamic acidity of this class of compound:
(>Si),CH > +Si-CH,-Sit
> >SiMe. Since MeLi in
THF is a weaker base than BuLi in TMEDA, but sterically
less demanding, the reaction with MeLi should be thermodynamically controlled. Indeed, the reaction of 50 with
MeLi in THF/ether (7 : 1) is complete in 24 h at 20°C. Subsequent reaction with Me3SiC1 leads, as expected, to 57.
Further examples are provided by the lithiation of nonamethyl- 1,3,5,7,9-pentasiIadecaIin 40 and of the silabarrelane 62.[501
Scheme 8. Stepwise lithiation and silylation of compound 26 to give 54
3.2.2. Metalation of the CBr2 Group
The reaction is thermodynamically controlled, the carbanion formed being stabilized by two SiMe2-CH2
groups. N o further metalation occurs with an excess of
BuLi at 20°C. Compound 49 is stable for several weeks; it
undergoes quantitative silylation with Me,SiC1 to afford
50. The treatment of 50 with BuLi can in principle lead to
two products, the formation of either the thermodynamically more stable arrangement (3Si)JLi (stabilization of
the carbanion) or the kinetically preferred arrangement
>Si-CHLi- S i c .
The use of nBuLi/TMEDA leads to compound 51, owing to the large size of the reagent and the high shielding of
the (>Si),CH group. The reaction of 51 with Me3SiCI
gives compound 52: the cis and trans isomers are formed
in a ratio of 1 :2.[431
The further reaction of 52 (80% trans
and 20% cis isomer) with nBuLi/TMEDA at 20°C confirms the selectivity of the lithiation; compound 53 is
formed. Silylation of 53 gives compound 54 (94% cis-trans
and 6% cis-is isomer[431).The lithium compounds 51 and
53 are the kinetically controlled reaction products and are
formed quantitatively. The thermodynamicatly favored
lithium compounds are 55 and 56, in which the anion is
stabilized by three methylated silyl groups. The rearrange-
The photobromination of the trisilacyclohexane 26 permits the conversion of a CH2 group into a CBr2 group.1441
The product 58 can be metalated at - 100°C in a mixture
of THF, ether, and pentane (4 : 1 : 1) according to Scheme
Scheme 9.
This reaction principle leads to various synthetic possibilities, e.g., that shown in Scheme
The spirocyclic
compound 60 is formed extremely selectively (cf. Section
As N. Wiberg and G. Wagner have
at a position next to a Si-halogen group and subsequent
elimination of LiX leads to the formation of a silaalkene
when the necessary sterically demanding groups are pres-
Angenz. Chem. Inr. Ed. Engl. 26 (1987) 1111-1132
Scheme 10.
under similar conditions with AIBr, to give 26 and
thence 3.19]Si-methylated carbosilane chains as substituents
at the tertiary C atom of a carbosilane form an additional
six-membered ring with AlBr,, as shown in Scheme 11.
Both cis and trans isomers of pentasiladecalin 40 were
In contrast, if the side chain is shortened so that ring
closure to give a six-membered ring is no longer possible,
carbosilanes with the barrelane (62), decalin (40, cis and
trans), o r diadamantane ( 5 ) skeleton are formed[’’ (Scheme
‘ S i T
+ 40
+ SiMe,
6OoC. 3 d
ent. They prepared compound 61, which contains a S i c
double bond.
+ CH4
‘ 62
Me2Si=C(SiMe,)SiMe(rBu)Z 61
4. Polycyclic Carbosilanes via Rearrangement of
Si-Methylated Linear and Monocyclic Carbosilanes
with AlBr, or AlC13
The organometallic syntheses of carbosilanes have not
yet opened a convenient route to the 1,3,5,7-tetrasilaadamantanes: thus cyclization reactions that proceed without pyrolysis are of interest. Such a reaction is that of
26 with AIBr, to give 1,3,5,7-tetrasilaadamantane 3 and SiMe4.[471Linear carbosilanes react
+ SiMe, + CH,
Scheme 12. - = Me.
According to this principle, compound 63 can be converted via a double ring closure into the all-trans compound 45 [reaction (9)J
Scheme 1 I . - = Me
Compound 45 is a convex molecule (the cap of a
sphere) in which all axial methyl groups point upwards,
the axial CH2 protons and the C H group downwards.i481
Steric factors play an important part in these ring closure reactions. While the transformation of the C-substituted compound 57 into the carbosilane 64 occurs with
elimination of CH4 (Scheme 13),l9] a corresponding reaction of compounds 65 and 66 (with further SiMe, substituents on the skeletal carbon atoms) no longer occurs on
steric grounds.1491The sole reaction is a partial bromination
with’substitution of a methyl group.
This steric hindrance is not present in compound 67, so
that it can eliminate methane to give the silascaphane
6Si5’l (Scheme 14).
Angew. Chem. Int. Ed. Engl. 26 (1987) 1111-1132
This provides a relatively simple route to Si-methylated
1,3,5,7-tetrasilaadamantane 3.1491
In many cases the compounds initially formed are further transformed when they are subjected to the influence
of AIBr3 for a longer time. Thus, compound 40, which is
formed in a yield of ca. 90% according to Scheme 11,
reacts further in the reaction mixture to give derivatives of
1,3,5,7-tetrasilaadamantane,along with 26 and SiMe,.[”]
The barrelane derivative 62 rearranges under similar conditions to give derivatives of 3.ls2]The 1,3,5,7-tetrasilaadamantane structure element is stable towards AIBr3.
The reaction without solvent leads after 8 d at 80°C to the
compounds shown in Scheme 16.
Scheme 13.
\ /
4 , X = Me
X = Br
Scheme 14. - = Me
Ring formation accompanied by methane elimination
also occurs when carbosilane chains of varying lengths are
attached to a Si atom of a carbosilane ring (Scheme 15).
X = Y = Me;
X = Me, Y = Br;
X = Br, Y = Me
Scheme 16. - = Me.
Scheme IS. - = Me.
Angew. Chem. inr. Ed. Engl. 26 (1987) 1111-1132
An increase in the reaction time leads to the formation
of insoluble polycarbosilanes of this type. The reaction
with AIC13 is slower, so that the isolation of the isomers 69
and 70 is possible.[521
The ring formation of Si-methylated carbosilanes with
concurrent elimination of SiMe, is induced by the polarization of the Si-C bond by AlBr3 (Scheme 17), as has been
shown for partially deuterated c a r b ~ s i l a n e s . [ ~ ' ~
Me3 Si:-L:
Br3AI- -
i e2
+ SiMe, + ALBr,
5.1.1. Reactions of (C13Si),CC12 with MeMgCl and MeLi
The reactions of (Cl,Si)2CC12 33, the simplest member
of this series, with MeMgCl or MeLi (excess with respect
to the total number of C1 atoms) leads to the compounds
shown in Scheme 18.[571
(Me3Si)2C=CH276 (85%) (3%), (Me,Si),CHMe 77 (2%) (23%1,
(Me,Si),CMe, (2%) (25%),(Me,Si),CHCI (2%) (28%)
Scheme 18. Yields of the reactions with MeMgCl in parentheses; yields of
the reactions with MeLi in curly brackets.
Scheme 17.
The ring closure reaction with SiMe, elimination is
sometimes accompanied by a ring closure with methane
elimination; in some cases the latter is the sole reaction. It
is due to an initial cleavage of a Si-Me bond by AlBr, to
give >Si-Br and MeAIBrz 72. Thus, the pure compounds
71 and 72, synthesized independently, react in a molar ratio of 1 : I with the formation of almost no side
to give the tetrasilaadamantane 3 and CH,.
When 33 is treated with MeMgC1, a total of 55 different
1,3-disilapropanes can be detected depending on the molar
ratio of the reactants. These products have varying chlorine content and can be divided into six groups according
to changes in the carbon bridge (CC12, CHMe, C=CHMe,
CHCl, CH2, C=CH,)."*]
The reaction with MeLi occurs in a more straightforward manner: it is initiated by the methylation of the CClZ
(C13Si)zCC12 MeLi
- -- Me
+ 2 CH,
+ AlBr,
Very pure AlBr3 and AlC13 d o not induce the ring closure reaction; this only occurs when small amounts of BrZ
or HBr are added or with commercially available AlBr3,
(C13Si)zCMeCI LiCl
Reaction of 33 with 1-1.5 molar equivalents of MeLi
gives 78 in 93% yield, while reaction with 2 molar equivalents gives (CI3Si),CMeZ in 90% yield. Only then does the
methylation of the SiC13 groups occur, so that the reaction
of 33 with 4 molar equivalents of MeLi gives
(C1MeZSi)ZCMe2in 86% yield.[s81
The study of the course of reaction was first concentrated on the formation of the methylidene group in the
main product 76 of the reaction with MeMgCl. The formation of methylidene groups is always observed when an
isolated Si-CC1,-Si group is present in a carbosilane, as
is shown for example by 2,2-dichlorohexamethyl-l,3,5-trisilacyclohexane and the carbosilane 79.167.681
The only pos-
5. Reactions of Carbosilanes
The question arises as to whether the known reactions of
functionalized silanes also occur for carbosilanes ; in particular, what influence is exerted by groups such as CH,,
CC12, SiCl, SiF, and SiH. The following model compounds
were chosen to give an answer to this question:
(C13Si),CH2 11, (C13Si-CH2),SiC12 31, (C12Si-CH2)3 29,
5.1. Reactions of Si- and C-Chlorinated Carbosilanes with
MeMgCl and RLi
The conversion of CH, groups into CCl, o r CHCl
groups in carbosilane skeletons can be carried out via photochlorination without cleavage of the Si-C-Si skeleton.
The photochlorination can be controlled in such a way
that intermediates with one and two CCI2 groups can be
preparatively isolated. Examples of such products are:
sible intermediates for their formation from 33 are derivatives with CClZor CMeCl groups, as all others can be excluded because of their low reactivity. The study of the
reaction mechanism was thus carried out using 2,2-dichloro-l,3-disilapropanes chlorinated and methylated at
silicon to different degrees as well as 2-chloro-2-methyl1,3-disilapropanes, which were available as pure comp o u n d ~ . ' ~The
~ , ~compounds
80 are indeed precursors for
the formation of (T\Si),C=CH2.
- -- Me,
C-metalation of 81 does not occur with MeMgCl or
MeLi, but can be carried out with magnesium or lithium.
Angew. Chem. Ini. Ed. Engl. 26 (1987) 1111-1132
Complete reaction of 81 with lithium in cyclohexane or
benzene gives 76 and 77 in the ratio of 5 5 :45(see Scheme
+ MeMgCL
+ MeMgCL
& 82
- MeCL
Scheme 2 I
+ LiH
Either Si-methylation or C-metalation can occur upon
reaction of the 2,2-dichloro- 1,3-disilapropanes with
MeMgCI. C-metalation dominates when the degree of
chlorination at silicon is low, while Si-methylation is more
important when it is high. Thus, for example, all products
obtained from Me3Si-CCIZ-SiMe,C1 are formed by attack at the bridging C atom, while ca. 40% of those from
MeCI2Si-CClz-SiC1, are due to Si-methylation. The metalation, and thus the formation of the methylidene and
CHCl groups, however, is much faster if, besides methyl
groups, chlorine substituents are still present on silicon.
Scheme 19
The reaction with magnesium in ether occurs in an analogous manner, but the compounds 76 and 77 are formed
in the ratio of 43 :57. This change in product ratio is explained by Scheme 20.
P H 3
+ HMgCl
Scheme 20
The intermediate 82 formed initially reacts further with
elimination of HMgCI, which in turn reduces unreacted 81
t o 77 ; these reactions are accompanied by reaction of
ether with the intermediate and subsequent ether cleavage, which also leads to 77. This mechanism is confirmed
by the reaction of (Me,Si),CCI(CD,) with magnesium
in ether: according to Scheme 20, this compound should
split off DMgCl to give (Me3Si),C=CD,. The reaction of (Me3Si),CCI(CD3) with DMgCl should
give (Me3Si)ZCD(CD3), while ether cleavage by the
intermediate gives
(Me3Si)*CH(CD3). The
compounds (Me3Si),C=CDZ, (Me3Si)$ZD(CD3), and
(Me3Si)2CH(CD,) are indeed formed in the ratio of
3 :3 :1.I601
Si-chlorinated deri.vatives of 81 form the methylidene
group even with MeMgC1, since the intermediate is formed
more readily when the partial negative charge on carbon is
stabilized more strongly by electronegative substituents on
(Me,Si),CCI, 34 reacts with MeMgCl about as fast as 81
with Mg to give the final product 76. However, 34 reacts
faster to give the intermediate 82 than does 81. The reaction of 34 can thus not begin with a methylation at the
CCI, group followed by a metalation, but must occur in
the reverse manner (Scheme 21); this can also be understood in terms of the better stabilization of the carbanionic
Angew. Chem. Int. Ed. Engi. 26 (1987) 1111-1132
5.1.2. Formation and Reaction of MeJi- CCl(Li)-SiMe,Cl
Great importance was attached to the proof of the Clithiation of perchlorinated carbosilanes, which has been
verified for (C13Si)2CC1233 and (CIzSi-CC12)3 75 by subsequent trapping reactions at - 100°C with Me1 or Me3SiI.
Thus, starting from 33/n-BuLi, it is possible in this way to
prepare (CI,Si),C(Cl)Me 78 and (C13Si)2C(C1)SiMe3 preparatively (yield 80%). Compound 75 behaves in a corresponding manner.I6’] Warming of (CI,Si),CCI(Li) leads to
the formation of LiCI, the chlorine atom of which is derived from one of the SiCI, groups. In this way, the disilabicyclo[l.l.O]butane 85 (white crystals, m.p. = 28°C) is obtained as the end product of the C-lithiation.[62J
The precursors of 85 are two 1,3-disilacyclobutanes
formed initially from the carbenoid 84. They are lithiated
by 8 4 in T H F according to Scheme 22.1631
2 Me3Si-CCL2-SiMe2CI
2 nBufi
2 Me3Si-CCl(Li)-SiMe2CI
Scheme 22.
5. I . 3. Reactions of (CI,Si- CC12),SiC12with MeMgCI
and MeLi
The reaction of 73 with MeMgCl can be explained by
the mechanism in Scheme 25.
The reaction of the trisilapentane 73 with an excess of
MeMgCl (molar ratio 1 :18) in ether leads to the compounds shown in Scheme 23; the main products are 86
and SiMe4.Lh41
Me3Si H
Scheme 23
Reactions of 73 with lesser amounts of MeMgCl give
less of the compounds shown in Scheme 23, and at a molar
ratio of 1 : 1 the product is 90.
+ SiMe,
Scheme 25
The number of CClz groups in the 1,3,5-trisilapentane
determines the course of the reaction. Thus the reaction of
74 with MeMgCl leads to 91 (70%) and that of 90 to 87 as
The first step is a metalation at a CCI2 group to give 94.
It is followed by an intermolecular elimination of MgClz
to give 95, which undergoes @-eliminationand Si-methylation to give 86 and SiMe,. The initial reaction of 73 with
MeLi is analogous. Compound 86 reacts with an excess of
MeLi according to reaction (lo), as is shown by the forma-
the major product. The reaction of 73 with a n excess of
MeLi'651occurs in a similar manner to give SiMe,, 87, 88,
92, and 93 as the main products, which are shown along
with the byproducts in Scheme 24.
+ 2MeLi
+ LiC-CLi
tion of acetylene in the hydrolysis of the reaction product
97. The vinylic CI atom in the intermediate 96, which is
not isolated, can be methylated by MeLi to give 92
(Scheme 24), so that no further reaction occurs. The formation of 87 is due to the reaction of 90 (CC1,-lithiation
and subsequent ether cleavage). An important difference
between the behavior of MeLi and MeMgCl with respect
to 73 lies in the formation of 93; it is formed via the Clithiated intermediate 98 (Scheme 26).
Scheme 24.
Scheme 26. - = Me or C1
Angew. Chem. Inr. Ed. Engi. 26j1987) 1111-1132
The carbanion of the Li compound 98 is much more
reactive than the corresponding Mg derivative 94 and attacks not only at the CCI, group but also at the terminal Si
atom with the formation of 93. Compound 92 rearranges
catalytically, undergoing 1,3-isomerization in the presence
of traces of moisture o r acids, to give 88 (Scheme 27).
Me@ 4
CIMao Mee
c 0
Scheme 29.
Scheme 2 1
The product ratio is strongly influenced by the MeLi
concentration during the reaction.
The initial step is the cleavage of the vinylic chlorine
atom under the influence of MeMgCl: the cation thus
formed rearranges to the energetically more favored C = C
linkage and then adds the nucleophilic anion Mee with
concomitant methylation at silicon to give 79.
The experiment gives the following information regarding the ring contraction of 75 to 103: the reaction begins
5.1.4. Reactions of (Ci2Si- CCr,), with MeMgCI
The perchlorinated compound 75 reacts with MeMgCl
to give the products of ring contraction and ring cleavage'66.671shown in Scheme 28.
with a magnesium-halogen exchange at a skeletal carbon
atom. The primary reaction products 105-108 have been
Et20 B
1 04
Scheme 28
The linear derivatives 99 and 100 are formed preferentially when MeMgCI is used in excess (for the transformation of 99 to 100, compare Scheme 20). The formation of
the 1,3-disilacyclopentene 103 is the decisive step in the
reaction sequence: 103 is formed as white crystals in ca.
98% yield when 75 is treated with MeMgCl (molar ratio
1 :4) and reacts with further MeMgCl to give mainly the
acetylene derivatives 79, 104, and 99,[66J
which are also
formed directly when 75 is treated with an excess of
MeMgC1. This reaction proceeds via 103 (Scheme 29; for
the reaction 75- 103, see Scheme 30).
Angew. Chem. lnt. Ed. Engl. 26(1987) 1111-1132
Scheme 30.
Scheme 30 describes the course of the reaction. The first
step is the magnesium-halogen exchange of 75 to give
109, which can react further by two routes. Route A results
in a rearrangement under the influence of MeMgCl of the
bicyclic intermediate formed to give 112 ; Si-methylation
by further MeMgCl gives 103. Route B leads from 109 via
ether cleavage to 105. The acidic C H proton of 105 reacts
with MeMgCl to give CH, and 109, provided that no metalation of a CClz group in 105 by MeMgCl occurs.
Whether route A o r route B is preferentially followed depends on the concentration of MeMgCl; if it is low, 105
containing the CHCl group is formed, while if it is high
the rearrangement via route A is favored. The complete
course of the subsequent reactions is determined by the
key compound 103[681
[reaction (1 I)].
Scheme 32.
reaction (1 1) does not occur, apparently because of the favored methylation of the vinylic CI atom.
5.2. Reactions of C-Chlorinated SiH-Containing
The Sic1 groups in CH2-containing carbosilanes can be
converted by LiAIH, into SiH groups without any change
in the molecular structure.'691 Examples are provided by
the transformations of 31 and 29.
(C13Si-CH2)2SiC12--t (H3Si-CH2)2SiH2
The cleavage reaction giving 79 (Scheme 29) starts from
the vinylic chlorine substituent: if this position is methylated (here a side reaction), no cleavage takes place, and
the disilacyclopentenes 114, 101, and 102 are formed. All
This reaction also permits the formation of SiH- and
CC1-containing derivatives from perchlorinated compounds, as shown below:
the linear reaction products can be explained by the reactions of the functional groups in 79.
5.1.5. Reactions of SiCI-Containing and Partially
C-Chlorinated Carbosilanes with MeMgCl
The reaction of (C12Si-CC12)3 75 with MeMgCl to give
the 1,3-disilacyclopentene skeleton (such as 103) according to Scheme 30 requires the presence of at least two CClz
groups for the ring contraction: it should not occur when
only one such group is present. Thus, the reaction of compound 36 with an excess of MeMgCl proceeds without
any change in ring size to give 115 (Scheme 31). Compound 36 thus reacts like an acyclic compound containing
an -SiC12-CC12-siC12gro~p.[*~.~''
Scheme 3 I
However, compound 116 (with two CClz groups) undergoes ring contraction with an excess of MeMgCl to give
117 (Scheme 32) in ca. 80% yield. This reaction proceeds
analogously to that shown in Scheme 30.f671Rearrangement with formation of linear compounds according to
However, side reactions such as cleavage of the Si-C
bond and formation of C H groups are always observed. It
must be noted that violent explosions will occur when pure
SiH- and CCI-containing compounds (such as 118) are
heated above room temperature: however, their solutions
in ether or hydrocarbons have shown themselves to be
safe. Their reactivity is due to hydrogen-chlorine exchange
[reaction (12)]. This reaction can be used for the formation
of partially chlorinated SiH-containing compounds, as the
example in Scheme 33 shows."o1
(C12Si-CC12)3 75
3 (H3Si-CH2)2SiH2
3 H2ClSi-CH2-SiHCI-CH2-SiH,
Scheme 33
The influence of the CClz group o n the SiH bond is
shown by the following comparison: while in (H2Si-CH2),
the SiH group can only be methylated with extreme difficulty by MeMgCl in Et20, such a methylation does occur
Angew. Chem. Inr. Ed. Engi. 26 (1987) 1111-1132
under analogous conditions for derivatives containing the
CCl, group (e.g., formation of 120 from 119).
Derivatives containing SiHMe groups are also formed,
but there is no formation of the C=CH, group (cf. Scheme
3 1). The reaction of the 1,3,5-trisilacyclohexane 121 (containing two CCl, groups) with MeMgCl gives 122 ; thus the
reactions shown in Schemes 31 or 32 do not occur.
(F3Si)2CC12+ MeLi
(Me3Si)2CC12+ LiF
Additional products are 125 and 126 as well as further
Similarly, 127 reacts to
partially methylated
give 128.
The studies thus present the following picture:
(CI,Si-CH2), reacts with MeMgCl to give (Me,Si-CH,),
23, while (H2Si-CHJ3 does not react under these conditions. Methylation of the neighboring SiH2 groups is only
possible when a CCI, group is introduced; the latter does
not undergo substitution. However, the route from
(H2Si-CC12)3to (Me$-CCI,),
with MeMgCl is not feasible, since the introduction of three CCI, groups favors
cleavage of the Si-C-Si skeleton during the reaction with
5.3. Reactions of C-Chlorinated SiF-Containing
The fluorination of Sic1 groups in carbosilanes can be
carried out using ZnF,; almost no Si-C bond cleavage is
observed. The preparation of (F3Si-CH2),SiF2 and
(F,Si-CC12)2SiF2 is most readily carried out by reaction of
the corresponding chlorinated compounds with ZnF, in
In a similar manner, (F2Si-CH2)3
is formed from (C12Si-CH2), 29 and the adamantane 123
from 12.i”J
a) F3Si-CC12H, MeF2Si-CC12-SiMeF2, Me2FSi-CCI2-SiMeF2
b) Me,Si-CC12-SiMe3 34, Me3Si-CC12-SiMe2-CH2-SiMe3,
Me3Si-CC12-SiMeF-CH2-SiMe3, (Me3Si-CC12)2SiMe2
c) Me3%-C-C-SiMe, 86, 34,
99, SiMe,
Scheme 34. Reaction of 129 in EtZO with a) MeMgCl in a molar ratio of i : I.
b) excess MeMgCI, and c) MeLi in excess ( I : 10).
In this case the cleavage of the molecular skeleton is the
predominant reaction; it is folIowed by Si-methylation.
The reaction with MeLi leads to, among other compounds,
86 and shows that this stronger metalating agent is also
capable of C-metalation and thus is able to induce the accompanying skeletal change^."'^
The initial reaction of Si-fluorinated 1,3,5-trisilacyclohexanes (e.g., 130) is also Si-methylati~n.[’~]
As with the
linear derivatives, ring cleavage is favored by polar solvents (e.g., THF) and MeLi.
However, it is not possible to use perchlorinated carbosilanes and ZnF, to form cyclic SiF- and CCI-containing
carbosilanes, since decomposition reactions occur which
may even lead to the formation of SiF,. Such carbosilanes
can be formed via photochlorination of SiF-containing
derivatives; e.g., reaction of (F2Si-CH&
The SiF groups in Si-fluorinated, C-chlorinated carbosilanes determine the course of their reactions. Thus, 124
reacts with MeLi according to reaction (13).
Angew. Chem Inr. Ed. Engl. 26 (1987) 1111-1132
In contrast, 129, which, unlike 127, contains no CH2
groups, behaves in a quite different manner; the products
of three different methylation reactions are shown in
Scheme 34.
(F,Si-CCI2), can be methylated by MeMgC1, affording
(MeFSi-CCIJ3 in 86% yield. Further reaction with
MeMgCl or MeLi leads to changes in the ring structure.
The reaction of 130 with an excess of MeMgCl (Scheme
35) makes this clear. The initial reaction is the Si-methylation to give 131. At this degree of methylation the metalation of the CCl, group occurs; it leads via ring contraction
to 133, which rearranges to give 117 and 134. Thus, above
a certain degree of Si-methylation, the reaction occurs according to Scheme 30 and reaction (1 I).
groups; e.g., (HMe2Si)2CF, is formed. Reaction with
LiPMe2 leads to substitution at the silicon atom: reaction
(MezP- Si Me,)& F2.[741
‘ L
Scheme 35.
5.4. Reactions of CF2-ContainingCarbosilanes
Carbosilanes containing the >Si-CF2-Sic
group can
be prepared via the insertion of difluorocarbene into the
Si-Si bond.[?,]
+ CF2
carbene CF, is formed by thermolysis of
Further examples are provided by the reactions of 137 and 138 and of the trisilane 139.[741
insertion into the Si-Si bond is only observed when the
terminal Si atoms bear one or more - I substituents
5.5. The Cleavage of the Si-Me Bond in Carbosilanes
The clean cleavage of the Si-C bond and the influence
of substituents on this reaction are of considerable interest.
The alkaline cleavage is favored by the chlorination of the
a-carbon a t ~ m ~ The
. ~ Si-Ph
~ ~ . bond
~ ~ is
~ cleaved in
aqueous HCI solution.[761These reactions have the disadvantage that the SiOH groups thereby formed condense to
form >Si-O-Sit
units and therefore the functionality at
silicon is lost. The cleavage of the Si-Ph bond by pure HBr
is a basic requirement for the organometallic synthesis of
carbosilanes (see Section 3).[371Under these conditions the
Si-Me bond is stable. Its cleavage by Me,SiCI or acetyl
chloride with AIC13 as a catalyst has been described by Kumadu and c o - w ~ r k e r s . ~A’ cleavage
reaction using HSiCI3
and catalyzed by H,PtCI, was described by Benkeser et
al.17’l and extended by Hengge et al.[791to the Si-chlorination of methylated silanes with Si-Si bonds. Calas et al.[’’l
reported the formation of Me,SiCI from SiMe, and ICI
(Scheme 36); Eaborn et al.[x71have reported similar reactions.
Me3Si(CH2),SiMe3 + 2 Me,SiCI 3
C1Me2Si(CH2),SiMe2CI + 2SiMer (n = 1-4)
+ HSiCI, aR,SiCi + MeHSiCI,
SiMed + 2 IC1
+ MeCl + I2
Scheme 36.
+ (FMezSi-CFz)2SiMe2
(F, C1, Br, OMe). These substituents weaken the Si-Si bonds
and favor the CF, insertion. In the absence of such
substituents (e.g., for %,Me6) the CF, insertion does not
occur, and more highly fluorinated disilanes (such as
F2MeSi-SiMeF, and Si2F6) decompose at temperatures
below that required to generate CF, from MeSnCF3.
In contrast to comparable C-chlorinated carbosilanes,
136 reacts with MeMgCl or MeLi even at lower temperatures not with C-metalation but with Si-alkylation, the
products being 140 and 141. Similarly, reaction with PhLi
and PhMgBr gives 142 and 143. Compound 136 and its
derivatives react with LiAIH, with formation of SiH
1 I28
(X = F, CI, Br, OMe)
These reactions convert the unreactive SiMe group into
a functional SiCl group. The reaction with ICI can be extended to numerous Si-methylated carbosilanes,‘*” leading
in each case to the replacement of at the most one methyl
group at each silicon atom.[*’’Thus, the end product of the
reaction of (Me2Si-CH2)3 26 is (CIMeSi-CH&, while in
1,3,5,7-tetramethyl-1,3,5,7-tetrasilaadamantane3 all SiMe
groups are converted into SiCl groups. Further examples
for the reaction with ICl are:
Angew. Chem. In[. Ed. Engl. 26 (1987) 1111-1132
A higher degree of chlorination is possible with ICl and
catalytic amounts of AlBr,:[*l1
shown by the reaction of 58 to give 149: ICI causes the
selective cleavage of an Si-Me bond at the silicon atom in
the position “puru” to the CBr, group.[*’]
The CH2-SiMe3 group in compound 144 is not degraded by ICl; instead, one C1 atom is introduced at each
Si atom. The initial reaction is the formation of 145, which
is then converted into the end product 146. Compound
147 behaves similarly, though in a side reaction (8%) there
is ring opening at the Si atom bearing the side chain. Csilylated derivatives behave similarly: for example, 54
6. The Influence of Substituents on Carbosilanes
The substituents on the Si and C atoms of carbosilanes
have an effect on both the reactivity of the remaining
groups and the stability of the molecular skeleton, as
shown by the following examples:
a) The SiH group in carbosilanes such as (H2Si-CH2)3
reacts only slowly with MeMgCl in Et20 to give Simethylated products. However, if the SiH group is in a
position next to a CCl, group, it can be relatively
easily substituted (with Si-methylation); the CCl, group
is not attacked.
b) The Sic1 group is normally much more readily alkylated by RLi or RMgCl than is the CCl, group. However, in the case of perchlorinated carbosilanes such as 73
or 75 the reaction with MeMgCl commences at the
CCl, group; the primary reaction is followed by further
characteristic reactions. At - 100°C nBuLi lithiates the
C atom of 33 without attacking the Sic1 groups. The
behavior of C-chlorinated carbosilanes containing
either SiF or SiH groups towards MeMgCl is quite different: in these cases the initial reaction occurs at the
SiF or SiH groups, and the CCI, group is at first not
involved in the reaction (e.g., conversion of (F3Si),CC12
into (Me,Si),CCI, 34).
(C13Si-CC12)2SiC12 (CI2Si-CCI2),
Me, Si-Si
1 48
reacts with ICI to give 148, and (Me,Si-CHMe), forms
Not only C1 substituents at silicon atoms but also the
CBr, group stabilizes the neighboring Si-Me bonds, as is
Angew. Chem. I n [ . Ed. Engl. 26(1987) 1111-1132
c) In carbosilanes with CC12 groups the cleavage of an
Si-C bond in the molecular skeleton occurs relatively
readily, e.g., when one attempts to convert 33 into
(H,Si),CCl, or 75 into (H2Si-CC12), using an excess of
LiAlH,. Such cleavages of Si-C bonds are also observed in reactions of SiH- and CC1-containing carbosilanes with organometallic compounds, such as in the
reaction of (HZSi-CC12), with MeMgCl.
d) Si-methylated linear carbosilanes or Si-methylated cyclic carbosilanes with Si-rnethylated side chains rearrange under the influence of AlBr3 (formation of sixmembered rings with elimination of SiMe,). This cyclization reaction does not occur with the corresponding
compounds that are partially chlorinated at silicon.
e) The reaction of linear and cyclic Si-methylated carbosilanes with ICl leads only to the substitution of
one SiMe group at the Si atom concerned: thus
(C1MeSi-CH,)3 is formed from (Me2Si-CH2)3 26,
while the reaction with ICl/AIBr3 gives (C12Si-CH2)3
1 I29
29. Here the second Si-Me bond is also cleaved, but
the Si-C-Si skeleton of the molecule is not attacked.
The behavior of the carbosilanes is mainly determined
by the polarity of the Si-C bond, which is influenced by
the substituents. Because of the electronegativities of the
elements concerned, the bond polarizations in a carbosilane such as (H3Si-CH2)2SiH2 are as follows:
( 3 0
C-chlorination leads via the electron withdrawal by chlorine to a decrease in the shielding at the C atom and thus
to a decrease in the shielding at the Si atom. This in turn
causes an increase in the polarity of the Si-H bond. Thus
the increased reactivity of the SiH group caused by C-chlorination is explained, as well as the fact that the Si-C bond
in such compounds is more readily cleaved. Similarly, methylation at silicon must (since Me acts as an electron-donating substituent) decrease the reactivity of the H atoms
on the silicon concerned and thus reduce the tendency for
Si-C cleavage. The Si- and C-chlorinated derivatives differ
from those that are SiH-containing and C-chlorinated in
that the reactivity of the CClz groups is increased. The reason for this is that with increased C-chlorination more
electrons of the chlorines in SiCl groups are available for
an increased shielding of the Si atom, so that the Sic1
group becomes less polar and less reactive. The behavior
of the SiF- and CCI-containing carbosilanes, such as 150,
more nearly approaches that of SiH- and CCI-containing
carbosilanes (e.g., 118) than that of SiCl- and CCI-containing derivatives (e.g., 73). The reaction of 127 with
MeMgCl commences with Si-methylation and not with the
C-metalation observed in the corresponding Si- and Cchlorinated compounds.
Polarity variations of the Si-C bond also determine the
chemical behavior observed in the reactions of Si-methylated carbosilanes with ICI and ICVAIBr,. In the reaction
with ICI, only one methyl group in the SiMe, group is substituted by CI. The introduction of such an Sic1 group decreases the polarity of the remaining Si-Me bonds, so that
the possibility of a nucleophilic cleavage is decreased. This
is also the reason that the ring closure reaction of methylated carbosilanes with elimination of SiMe, n o longer OCcurs when a n SiCl group is present. The generally lower
bond polarity of the Si-CH2-Si bonds in comparison with
that of the Si-Me bond explains the observed stability of
the carbosilane skeleton with respect to cleavage. Aluminum halides polarize not only the Si-Me bond but also the
ICI molecule and thus make possible further Si-Me cleavage and higher degrees of chlorination. These experimental results and their qualitative interpretation are in agree1130
ment with quantum-mechanical calculations for simple silicon-carbon compounds.[821
7. Polycarbosilanes
The favorable properties of silicon carbide (chemical resistance, density 3.2 g ~ m - hardness
~ ,
9.5) for use in modern ceramics have also led to an increasing interest in polymeric carbosilanes. These substances are obtained either
via pyrolysis of m e t h y l ~ i l a n e s [or
~ , via
~ ~ prolonged thermal
treatment of p~lysilanes.[~'~
S . Yajirn~'*~'
has reported the
formation of S i c fibers by thermal treatment of methylpolysilanes. Polysilanes of the type (Me,Si), (n = 5-35) are
formed from Me2SiCI2 and Na/K in T H F ; various cyclic
and polycyclic polycarbosilanes are obtained by the cocondensation of Me2SiCI2and MeSiCI, with Na/K.[831On
heating to ca. 450°C these polysilanes rearrange: C H 2 derived from SiCH, groups inserts into Si-Si bonds with concomitant formation of SiH groups according to reactions
( 5 ) and (6). The polycarbosilanes so formed["] are converted into @-Sicat 1300°C. Because of the formation of
volatile products, this stepwise procedure involves considerable material loss. The mechanism of the formation of
these polycarbosilanes as well as their further structural
variations are not yet well understood.
In the pyrolysis of SiMe, (Fig. 1, Section 2) the Si-poor
carbosilanes initially formed are involved in the subsequent reactions to give the Si-richer products : an example
is provided by (Me2Si-CH2), 25 (Section 2.1.4). The
studies showed that in a closed system 25 is converted
at temperatures between 200 and 280°C without gas
evolution into linear polycarbosilanes of the type
Me3Si(CH2-SiMe2),-CH2-SiMe3 ( M , = 250000, temperature-dependent ; colorless gel-like substances, still soluble
in toluene).[251When these are subjected to higher temperatures (65OoC, 4 h), a partial degradation to give low-molecular-weight carbosilanes is accompanied by the formation
of insoluble products.
The polycarbosilanes obtained by the pyrolysis of methylsilanes are not linear (as revealed by their composition); however, their structures have not been unambiguously determined. Since the lower-molecular-weight carbosilanes formed from the various methylsilanes (varying
Si :C ratios) have different structures, differences in the
structures of the polymers are also to be expected. This becomes clear in the thermal treatment of the compounds in
Figure 2 (Section 2.1.3) obtained from Me2SiC12. Between
420 and 620°C there is a marked increase in viscosity, accompanied by elimination of SiCI, and MeSiCI,; low-molecular-weight carbosilanes are formed. Since most of the
end groups of the compounds in Figure 2 are initially SiCIj
groups, the formation of SiCI, indicates a condensation
with formation of additional Si-C bonds which link these
molecular skeletons: they thus become structural elements
in the polymers.['81
8. Summary and Outlook
An extremely large number of structurally different carbosilanes have now been synthesized in different ways. BeAngew. Chem. Inr. Ed. Engl. 26(1987) 1111-1132
cause of their chemical behavior, the carbosilane frameworks can b e described neither as “quasi-C-skeletons” nor
as silicone-like systems (despite the fact that the Si-CH2Si grouping is isoelectronic with Si-0-Si). It is noteworthy
that the chemical stability of the Si-C-Si molecular structures is higher than that of SiMe end groups and that their
stability increases still further when electronegative substituents are introduced at the Si atoms. Numerous questions
however remain to be answered. Thus the formation of Siricher and high-molecular-weight carbosilanes in the pyrolysis of methylsilanes is still not sufficiently well understood. Further studies on the reaction of silicon with
CH,CI, to give unbranched linear carbosilanes should
serve to increase considerably our knowledge of catalytic
processes at Si surfaces. The same is true for the catalyzed
reactions of perchlorinated carbosilanes with silicon,
which will provide routes to new carbosilane structures.
Organometallic synthesis from organosilicon compounds
alone appears to be of only limited use for the construction
of polycyclic carbosilanes, though the possibilities provided by its combination with cyclization reactions induced by AIBr3 are extremely interesting. In many ways a
more quantitative physicochemical foundation for carbosilane chemistry is required; for example, calorimetric studies could provide valuable information.
Studies on the construction and structure of polymeric
carbosilanes are at present mainly of only a qualitative nature. The use of carbosilanes in high-performance ceramics[*‘l requires not only the formation of polymers of different types and information on the structures of such polymers but also a complete understanding of the reactions
that they undergo on further thermal treatment. This is a
challenge for both the chemist and the materials scientist.
I thank my co-workers (whose names appear in the list of
references) for their dedication. The work discussed here
could not have been carried out without the support of the
Fonds der Chemischen Industrie and the Deutsche Forschungsgemeinschaft. I feel myserf particularly indebted to
these institutions.
Received: May 22, 1987 [A 643 IE]
German version: Angew. Chem. 99 (1987) 1150
Translated by Prof. T. N . Mitchell, Dortmund
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The 100th anniversary issue of ANGEWANDTE CHEMIE (January 1988) will contain the following
A. Eschenmoser
Vitamin Biz: Experiments Concerning the Origin of Its Molecular Structure
J. Falbe et al.
Natural Fats and Oils-Renewable
H. Gerischer
Light as a Probe for the Investigation of Electrochemical Surface Reactions
R. Huber
Flexibility and Rigidity of Proteins and Protein-Pigment Complexes
J.-M. Lehn
Supramolecular Chemistry- Molecules, Supermolecules, and Molecular Devices
H. Ringsdorf
Molecular Architecture and Function of Polymeric Oriented Systems
A. Simon
Clusters of Valence-Electron-Deficient Metals-Structure,
G. Wilke
100 Years of Organonickel Chemistry
Raw Materials for Chemical Industry
Bonding, and Properties
Angew. Cbem. Inf. Ed. Engl. 26 (19871 1111-1132
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