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Divalent Silicon Intermediates.

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JULY 1 9 6 9
PAGES 469-534
Divalent Silicon Intermediates
By W. H. AtwellL*I and D. R. Weyenberg
Like carbenes, silylenes are assumed 10 occur as intermediates in many reactions. A detailed account of the reactivity of the silylenes is given and the reductive and thermolytic
methods used for their preparation are discussed. Insertion and interception reactions
emphasize the formal relationship between these species and the carbenes. Silacyclopropenes and silacyclopropanes have not, as yet, been isolated.
1. Introduction and Nomenclature
The considerable attention devoted to divalent carbon
compounds (carbenes) has had a great impact on organic chemistry. Although the systematic and sustained investigation of the chemistry of divalent silicon
compounds (silylenes) is of more recent origin and
much less complete, it is a l r e a d y apparent that it will
have an equivalent impact on the field of organosilicon
Because these divalent species overlap the traditional domain
of both organic and inorganic chemistry, several different
nomenclatures are currently being used to name the specific
species as well as the generic class of intermediates.
In this review compounds are referred to as “silylenes” if the
silicon radical is either unsubstituted or partially substituted
(e.g. :SiH2 = silylene, :SiHF = fluorosilylene) or if a parent
group directly attached to silicon is capable of being substituted (e.g. :SiMez = dimethylsilylene). When fully substituted by groups that cannot be further substituted, derivatives of divalent silicon are frequently given inorganic
binary-type names (e.g. :SiF2 = silicon difluoride. :Si(CN)Z =
silicon dicyanide) 111.
Although the name “silenes” has been previously applied to
the general class o f divalent silicon intermediates, it has been
recently suggested [1,2J that this generic name is inappropriate because of possible confusion with unsaturated silicon
compounds, should they exist. The alternative name, “silicenes” has also been proved as inappropriater31. In this
[*I Dr. W. H. Atwell and Dr. D. R. Weyenberg
Organometallic Research
Dow Corning Corporation
Midland, Mich. 48640 (USA)
[l] We wish to thank Dr. K . L . Loening (Chemical Abstracts
Service) for this information.
[2] P. P . Gaspar, 8. D.Pale, and W. Eckelman, J. Amer. chem.
Soc. 88, 3878 (1966); see also [6].
131 For appropriate references and discussion see, V . Baiant,
V . Chvalovsky, and J. Rathousky : Organosilicon Compounds.
Academic Press, New York, 1965, Vol. 2.
Angew. Chem. internat. Edit.
Vol. 8 (1969) 1 No. 7
review, we will use the more appropriate term “silylene”
when referring to the generic class of divalent silicon intermediates. Also, since the division in nomenclature based on
the substituent (and its ability to be further substituted)
appears to be somewhat arbitrary, we will refer to all specific
intermediates as derivatives of “silylene.”
While the preparation and the spectral observation of
the inorganic silylenes (dichlorosilylene in particular)
were reported over thirty years ago [41, o n l y recently
have detailed studies of these intermediates appeared (51.
Silylenes have been suggested as intermediates in
the “direct” synthesis of chlorosilanes [6-81, and have
been demonstrated as intermediates in the preparation
of silicon by the hydrogen reduction of chlorosilanes [9-141.
141 R. Schwartz and G . Pietsch, Z . anorg. allg. Chem. 232, 249
(1937); R. K . Asundi, M . S . Karim, and R. Samuel, Proc. physic.
Soc. 50, 581 (1938).
[5] For a recent general review on the inorganic, organometallic,
and organic analogs of carbenes, see 0.M . Nefedov and M . N .
Manakov, Angew. Chem. 78, 1039 (1966); Angew. Chem. internat. Edit. 5 , 1021 (1966).
[6] S. A . Golubtsov, K. A. Andrianov, R. A . Turetskaya, Z . V.
Belikova, I. V . Trofimova, and N . G . Morozov, Proc. Acad. Sci.
USSR, Chem. Sect. (English translation) 151, 656 (1963).
[7] V. I . Zubkov, M. V. Tikhomirov, K . A. Andrianov, and S . A.
Golubtsov, Proc. Acad. Sci. USSR, Chem. Sect. (English translation) 159, 599 (1964).
181 J . Joklik and V . Baiant, Collect. czechoslov. chem. Commun. 29, 603, 834 (1964).
[ 9 ] H . Schafer and J . Nickl, Z . anorg. allg. Chem. 274, 250
E. G . Bylander, J. electrochem. Soc. 109, 1171 (1962).
K. Reuschel, Z . anorg. allg. Chem. 332, 113
1111 E. Sirfl and
I121 W. Steinmaier, Philips Res. Rep. 18, 75 (1963).
[13] R . R. Monchamp, W. J . McAleer, and P. I . Pollak, J. electrochem. SOC.111, 879 (1964).
[14] 0. AIstrup and C. 0. Thomas, J. electrochem. SOC.112, 319
The objective of this review will be to discuss the reported silylene syntheses and to place in perspective
those reports which contribute most to our present
knowledge of the preparation, structure, and properties
of these intermediates [151.
reduction of other metal halides by silicon has been
+ 2 Cu
2 CuCl+ Si
CaF2+ Si
+ :Sic12
+ Ca+ :SiFz
Related vapor phase reductions have also been employed for the preparation of dimethylsilylene.
2. Methods of Preparation
Essentially all reactions used for the preparation of
silylenes can be divided into two general methods:
(A) The reduction of halosilanes, and (B) the thermal
decomposition of appropriate substrates. Variations
of each of these have been employed for the preparation of inorgano- as well as organosilylenes.
2.1. Reduction Methods
Of the reactions included under this method, the most
intensely studied have been those involving the reaction of silicon with silicon tetrahalides. Reactions
260-280 "C
+ 2 K ( N ~ )__
+ 2 KCl+
Many of the products obtained from the reaction o f
dichlorodiorganosilanes and metals, in various aprotic
solvents, have been explained in terms of silylene intermediates [51. However, we do not feel that the formation of silylenes has been well established in many of
the reactions and a more complete discussion of this
area is included in Section 5 of this review.
Several reports show that the high-temperature reduction of silicon tetrachloride to silicon with hydrogen
involves the intermediate formation of dichlorosilylene [10-131. However, utilization of the dichlorosilylene produced via this route for other syntheses
have not been reported.
of this type have been reported for a variety of
silicon halides in which (X = fluorine[W171, chlorine [9,14,18-221, bromine [231, and iodine 124-261). These
reactions take place only at temperatures above 800 "C
and most of the above studies were carried out by
passing the silicon tetrahalide over silicon at 1000 to
1500 O C ; excellent yields of difluorosilylene (90 %) [271
and dichlorosilylene (95 %) [221 have been obtained at
1450 and 135O"C, respectively. Various forms of
silicon, such as silicon carbide, silicon alloys, and binary silicides of polyvalent metals have been used in the
synthesis of difluorosilylene 1161. In addition, the
1181 K. Wieland and M . Heise, Angew. Chem. 63, 438 (1951).
[19] P. F. Antipin and V. V. Sergeev, 2. prikl. Chim. 27, 737
[20] T . Zshino and A . Matsumoto, Technol. Rep. Osaka Univ. 13,
487 (1963); Chem. Abstr. 61, 6449 (1964).
[21] R . Teichmann and E. Wol/, Z. anorg. allg. Chem. 347, 145
[22] P. Timms, Chem. Engng. News 45, Nr. 39, p. 57 (1967);
Inorg. Chem. (Washington) 7, 387 (1968).
1231 a) E. Wolfand C. Herbst, Z. anorg. allg. Chem. 347, 113
(1966); b) Z. Chem. 7, 34 (1967).
i24] H . Schafer and B. Morcher, Z. anorg. allg. Chem. 290, 279
[25] R. C. Newman and J . Wakefield, J. electrochem. SOC. 110,
1068 (1963).
[26] D. M . Schmeisser and K . Friederick, Angew. Chem. 76, 782
(1964); Angew. Chem. internat. Edit. 3, 699 (1964).
[27] J . C. Thompson and J . L . Margrave, Science (Washington)
155, 669 (1967).
+ H2
+ 2 HCI + :Sic12
2.2. Thermal Methods
The second general method for generating silylenes
involves the thermal degradation of an appropriate
substrate. Two of these methods, the thermolysis of
polysilanes and of 7-silanorbornadienes, have provided
a very convenient source of silylenes at moderate
temperatures (170-300 "C).
The thermal degradation of hexahalodisilanes, first
noted by Friedel and Ladenburg 1301, gives silanes and
higher polysilane compounds [5,31-331. However, it is
[I51 A vast amount of work concerned with the suboxides and
subsulfides of silicon is considered beyond the scope of this
[16] D . C. Pease, US-Pat. 2840588 (June 24, 1958); Chem.
Abstr. 52, 19245 (1958).
[I71 P. L. Timms, R . A. Kent, T. C. Ehlert, and J. L. Margrave,
J. Amer. chem. SOC.87, 2824 (1965).
S i L + (Silz),
only during recent years that the intermediacy of silylenes in these thermolysis reactions has been established.
+ :SiF2
The much more facile thermolysis of alkoxydisilanes[341
has rendered organosilylenes available at considerably
lower temperatures. These thermolyses occur at 170
1281 A. S . Kana'an and J. L. Margrave, Inorg. Chem. 3, 103
(1 964).
[29] P. S. Skelland E. J. Goldstein, J. Amer. chem. SOC. 86, 1442
(1964); Chem. Engng. News 42, 40 (1964).
[30] C. Friedel and A . Ladenburg, Liebigs Ann. Chem. 203, 241
[31] M . Schmeisser and K . P. Ehlers, Angew. Chem. 76, 281
(1 962).
[32] N . W. Kohlschiitter and H. Mattner, Z. anorg. allg. Chem.
282, 169 (1955).
1331 A . Pflugmacher and Z. Rohrmann, Z. anorg. allg. Chem. 290,
101 (1957).
[34] a) W. H. Atwell and D . R. Weyenberg, J. organometallic
Chem. 5, 594 (1966); b) Chem. Engng. News 45, No. 38, p. 30
(1967); c) J. Amer. chem. SOC. 90, 3438 (1968).
Angew. Chem. internat. Edit. f Vol. 8 (1969) f No. 7
to 300 "C (with half-lives of 1-3 h) in sealed tubes and
lead to a silane and a series of polysilanes via a formal
redistribution Of silicon-silicon and SiliCOn-OXygen
of hetero-substituted polysilanes, where the heteroatom possesses an unshared electron pair [34,351.
It should be noted that the above considerations are
applicable to only the thermal redistributions of poly22s "C
silanes. A variety of base-catalyzed redistributions of
n(MeO)MezSiSiMez(OMe) -+
(n-1)MezSi(OMe)2 + M e O ( ~ i ~ e z ) , + l O ~ e silicon-silicon bonds with silicon-halogen and siliconoxygen bonds are known 15,361, but there is no evidence
that these reactions involve silylenes [34c7361. In fact,
bonds. Recent successful attempts to intercept reacno
silylenes have been intercepted by acetylenes in the
tive intermediates and kinetic studies with syrn-dimethbase-catalyzed redistribution of alkoxydisilanes[34cJ.
oxytetramethyldisilane provide convincing evidence
The thermal decomposition of polydimethylsilylene
that these reactions involve a ratedetermining and
has been reported [5,37-391 to give dimethylsilylene.
unimolecular thermolysis of the disilane to give a
silane and a silylene.
m 300'C
The thermolysis of sym-dimethoxytetramethyldisilane
is first-order in disilane and is not accelerated by the
presence of trapping agents (such as acetylenes, dienes,
alcohols, etc.) which efficiently intercept the silylene
intermediate. Although the alkoxypolysilanes offer a
new and convenient source of silylenes, it should be
noted that the reactivity of the starting disilane toward
the silylene can be a complicating factor in synthesis,
i.e., the trapping agent must compete with the starting
disilane. The capture of the silylene by the disilane
(via insertion into the siIicon-oxygen bond) and other
trapping agents will be discussed in Section 4 of this
This reaction appears to be quite general for alkoxypolysilanes with the more highly alkoxylated compounds undergoing thermolysis at lower temperatures.
The reactions
+ :Si(OMe)z
200 "C
(MeO)MezSiSiMez(OMe) ----+
+ :SiMeZ
-+ MezSi(0Me)z + 2 :SiMez
225 "C
275 "C
also proceed quite nicely in the vapor phase at400 "C
with reaction times of only a few seconds. They most
likely involve an a-elimination mechanism, e.g.,
n :SiMez
(n m 55)
The fact that not all polydimethylsilylenes decompose
with such easel401 and that the yields of silylene
"trapped" products are generally low (1-6 %) [37J,
renders this route somewhat unattractive for the
production of such intermediates [411.
A second thermal method which has been useful in
elucidating the chemistry of silylenes is the thermolysis
of 7-silanorbornadienes. This reaction has been utilized [5,40,421 for the preparation of dimethyl- and dip henylsilylene.
= Ph,
Me o r Ph.
It has been suggestedL431 that silylenes are formed in
the photolytic and thermal decomposition of a variety
of metal silyls.
+ AIMe3 + :SiMez
+ Hg+ n :SiMez
While the synthetic utility of such methods has not
been demonstrated, their potential as photolytic or
low-temperature sources of silylenes seems worthy of
further investigation.
1351 W . H. Atwell and D . R. Weyenberg, unpublished.
[36] W . H. Atwell and D . R . Weyenberg, J. organometallic
Chem. 7, 71 (1967).
[37] M. E. Vol'pin, Yu. D . Koreshkov, V. G . Dulova, and D . N.
Kursanov, Tetrahedron 18, 107 (1962).
I381 0. M . Nefedov, G. Garzo, T. Szekei, and V. I. Shiryaev,
Proc. Acad. Sci. USSR, Chern. Sect. (English translation) I64,
945 (1965).
[39] a) 0. M. Nefedov and M . N . Manakov, Angew. Chem. 76,
270 (1964); Angew. Chem. internat. Edit. 3, 226 (1964); b) Chem.
Engng. News 42, p. 40 (1964).
[401 H. Gilman, S . G. Cottis, and W. H . Atwell, J. Amer. chem.
SOC. 86, 1596 (1964).
I e.
I411 The possibility that the formation of dimethylsilylene from
MeOSi, ,OMe
--+ MezSi(OMe), + :SiMe2
these polymers may be (i) due to the presence of siloxane impurSi
ities, or, (ii) related to the homolytic type of decomposition
reported for hexamethyldisilane, has to our knowledge not been
adequately considered.
1421 H. Giiman, S. G. Coftis,and W. H . Atwell, J . Amer. chem.
SOC.86, 5584 (1964).
and, thus, constitute another variation of the type of a1431 E. Wiberg, 0. Stecker, H . J . Andrascheck, Z . Kreuzbichler,
elimination reaction which leads to the formation of
and E. Staude, Angew. Chem. 75, 516 (1963); Angew. Chem.
carbenes. This appears to be a fairly general reaction
internat. Edit. 2, 507 (1963).
Angew. Chem. internat. Edit. J Vol. 8 (1969) 1 No. 7
47 1
Silylene formation has also been suggested in the
thermal decomposition of a variety of monosilanes,
> 1000°C
+ :Sic12
> 900°C
+ Xz+ :Six2
X = C1 [18,441,I [23b,261
m 400"C
Hz+ :SiH2
- and in a solid matrix at low temperature by infrared
spectroscopy 1561. The structures of several other
silylenes have been determined from spectral studies
(see Table 1).I n all cases the angle (0)formed by the
two substituents on the silicon is considerably less
than the tetrahedral angle. This same phenomenon is
noted in carbenes where the bond angle in difluoromethylene has been recently shown to be 104.9 [5*1.
Table 1.
Structural parameters of silylenes.
as well as in related decompositions using other
energy sources such as photolysis 146-481, glow discharge [4,49,501,and neutron irradiation [21.
-+ H2+
C1 and Br
uv H2+
SiF4 __ + F2+ :SiF2
92 [bl
R = F
101 [cl
[a] Values refer to the singlet ground state of the
3. Structure and Physical Properties
[b] Predicted value was 95 (see ref. 1571).
[c]Previous values were O
1.49 A (see ref. [Sol).
The recent and extensive studies on difluorosilylene by
Margrave and co-workers [271 have provided our most
definitive picture of the spectral and physical properties of the silylenes. Difluorosilylene with a half-life of
approximately 150 seconds at ambient temperature
and 0.1 torr pressure is the most stable of the silylenes
and is considerably more stable than its carbon analog,
difluoromethylene. This exceptional stability allows
the transport of difluorosilylene away from the reaction
zone in the gas phase for study of its physical properties and of its chemistry. Although quantitative data
are absent, none of the related silylenes are sufficiently
stable to allow this kind of treatment.
Difluorosilylene has been studied in the gas phase by
microwave [27,511,electron spin resonance 1521, ultraby infrared spectroscopy 1551
violet absorption C27-53-54],
I441 H. Schafer, 2. anorg. allg. Chem. 274, 2651 (1963).
[45] J. H . Purnell and R . Walsh, Proc. Roy. Sac. (London),
Ser. A 293, 543 (1966).
[46] G. Herzberg and R . D . Verma, Canad. J. Physics 42, 395
1471 I. Dubois, G. Herzberg, and R . D . Verma, J. chem. Physics
47, 2462 (1967).
[48] a) 0.P . Strausz, K . Obi, and W . K . Duholke, J. Amer. chem.
SOC.90, 1359 (1968); b) J. Amer. chem. SOC.91, 1622 (1969).
[49] J. W . C . Johns, A . W. Chantry, and R . F. Barrow, Trans.
Faraday SOC.54, 1580 (1958).
[50] D . R . Rao and P . Venkateswarlu, J. molecular Spectroscopy
7, 287 (1967).
[51] V. M. Rao, R . F. Curljr., P . L. Timms, and J. L . Margrave,
J. chem. Physics 43, 2557 (1965).
1521 H. P . Hopkins, J . C . Thompson, and J. L. Margrave, J. Amer.
chem. SOC.90,901 (1968).
[53] V . M . Rao and R . F. Curl j r . , J. chem. Physics 45, 2032
[54] V . M . Kkanna, G . Besenbruch, and J. L. Margrave, J. chem.
Physics 46, 2310 (1967).
1551 V . M . Khanna, R . Hauge, R . F. Curl j r . , and J. L . Margrave,
J. chem. Physics 47, 5031 (1967).
124' and ro SiF =
The heats of formation of the dihalosilylenes are given
in Table 2. These data with the reported[23,591 heats
of formation of the corresponding silicon tetrahalides
give the enthalpies for the reaction,
Si+ Six4
2 :Six2
which are shown in Table 2. These data suggest that
:SiF2, which has the greatest life-time of the halosilylenes, is actually the least stable of the halosilylenes
relative to the zero- and tetravalent silicon species.
This instability may be related to the previously
observed thermodynamic stability associated with the
accumulation of fluorine about a silicon atom 1601.
Table 2. Heats of formation of dihalosilylenes
and calculated heats of reaction for
Si Six4 + 2 :Six*.
-139 [281
-39.5 121, 23allal
-10.0 [23al
+18.7 123bI
[56] J. M . Bassler, P. L . Timms, and J. L. Margrave, Inorg.
Chem. 5, 729 (1966).
[57] P . C . Jordan, J. chem. Physics 44, 3400 (1966).
[58] F. X. Powell and D . R . Lide jr., J. chem. Physics 45, 1067
[59] J. C. McDonald, C. H. Williams, J. C. Thompson, and J. L .
Margrave, Proceedings of Symposium "Applications of Mass
Spectrometry in Inorganic Chemistry", Advances in Chemistry
Series, Washington 1966.
[60] D . R. Weyenberg, A . E. Bey, H . F. Stewart, and W . H.
Atwell, J. organometallic Chem. 6,583 (1966).
Angew. Chem. internat. Edit.
Vol. 8 (1969)/ NO. 7
on co-condensation at low temperatures with a variety
4. Reactions
It is only in recent years that the chemistry of the silylenes has been examined and that reagents have been
discovered that will intercept (trap) them. As with any
highly reactive intermediate, the chemistry of a silylene
is basically a consideration of the competitive reactions
available to it under the conditions in which it is generated. These are summarized in Scheme1 below, where,
after generation by reduction or thermolysis (see Section 2), the silylene may either polymerize, react with
its precursor, or react with a “trapping reagent”
which is added to intercept the species. The fate of the
silylene and the products of the reaction will depend
on the relative rates of these three processes. In this
T Polymerization
-__ +
iTrapping Agent
Scheme 1
study [28J of SiFz/SiF4 mixtures has confirmed the
presence of paramagnetic species under these conditions which may represent diradical species, [.(SiF2),.].
4.2. Insertion into Single Bonds
A rather wide variety of reactions has been reported
which involve the insertion of silylene into a single
bond. In all cases either X or Y has been a hydrogen or
one of the more electronegative elements such as a
halogen or an alkoxy group.
of substrates. Under these conditions the majority of
products contain silicon-silicon bonds and it appears
that partial polymerization (dimerization, trimerization, etc.) occurs prior to reaction with the added
“trapping” agents. A recent electron spin resonance
R & + XY
section we will attempt to classify the known chemistry of silylenes into a limited number of rather
general reactions. The reactions we will consider are
polymerization, insertion into single bonds, and the
addition to multiple bonds - three reactions which
also characterize most of the known chemistry of
+ BC13
:Sic12 + PCl3
SilyIenes, when generated in the absence of other reagents, usually undergo polymerization to give polysilanes. The many examples of such silylene polymern :SIX2
+ (SiXz)n
ization include cases where X = F[16,17,311,
C] 122,23,44,611, Br C33,621, and 1[24,26,30,631.
This reaction may be analogous to the dimerization of
carbenes to form olefins[641, with the difference in
The insertion of silylenes into a variety of MX bonds
(where M represents boron, carbon, or phosphorus
and X represents halogen or alkoxy) may be a rather
general reaction. Both dichlorosilylene1221 and difluorosilylene 127,651 react with boron and phosphorus
4.1. Polymerization
+ R2Si
+ C13SiPC12
halides. In the case of difluorosilylene and phosphorus trifluoride, unstable silicon-phosphorus compounds were formed L7-71. The difluorosilylene-boron
trichloride and dibromosilylene-boron trifluoride systems give similar products but are complicated by
halogen exchange reactions.
:SiBrz+ BF3 + FBrzSiBF2
Related reactions with alkyl halides have also been
reported. Also, the reactions of difluorosilylene and
products being due to the known [31 instability of
multiple bonds between silicon atoms.
In all cases, with the possible exception of difluorosilylene, a variety of trapping agents can compete with
the polymerization process. Difluorosilylene is quite
unreactive in the gas phase 1171 but undergoes reaction
1611 ff. Schafer and B. Morcher, Colloquium SOC.inorg. Chem.
internat. Union pure appl. Chem. 1954, p. 24.
[62] D . M. Schmeisser and M . Schwarzmann, Z . Naturforsch.
I l b , 278 (1956).
[63] E. Wolf and M . Schonkerr, Z . Chem. 2, 154 (1962).
[64] J . H i m : Divalent Carbon. The Roland Press Company,
New York 1964; W. Kirmse: Carbene Chemistry. Academic
Press, New York 1964.
Angew. Chem. internat. Edit.
/ Vol. 8 (1969)/ No.
trifluoroethylene or fluorobenzene (see Section 4.3.2)
are formally an insertion into the carbon-fluorine
bond. The anomalous behavior of difluorosilylene is
1651 P. L. Timms, T. C . Ehlert, and J. L. Margrave, J. Amer.
chem. SOC.87, 3819 (1965).
[66] P . L. Timms, unpublished.
[67] J . L. Margrave, unpublished studies presented a t the “Polysilane Symposium”, University of Pennsylvania, Philadelphia,
Pa., April 15, 1967.
again illustrated in the above examples, however, 1:1
adducts being obtained with other MX compounds.
+ Br2SiF2
:SiF2+ Brz
The last two reactions are additional examples of 1:l
adducts from difluorosilylene. It has been suggested
that the “direct” synthesis of trichlorosilane from
silicon and hydrogen chloride involves this type of
insertion reaction.
There are now several examples of silylenes reacting
with alkoxy- or halo-substituted polysilanes. While
+ :SiMez
+ MeO(SiMez),+lOMe
+ :Sic12
+ CI(SiC12),+1 CI
( n 3 2)
the above products could arise via reaction with the
silicon-silicon bond, the following example shows that
this reaction proceeds via insertion in the Si-X bond.
+ HSiC13
The similarity of these reactions of silylenes to the reactions of carbenes with hydrogen halides 1701, alcohols [64,70,711, and silicon and germanium hydrides [TO]
is very striking. However, insertion into carbonhydrogen bonds, which is a common reaction with
carbenes, has not been observed with silylenes. The
low efficiency of dimethylsilylene insertions into the
(MeO)MezSiSiMez (OMe)
+ :SiMe(OMe)
( M e0)M ezSiSiSiMez (OM e)
This is consistent with the proposed mode of decomposition of these alkoxy-substituted polysilanes which
involves an a-elimination of the terminal silicon, Le.,
the reverse of the above silicon-oxygen insertion reaction [34~1.
While the above data suggest that the reaction of
silylenes with the silicon-oxygen or silicon-halogen
bonds of polysilanes is rather general, several attempts
to carry out related reactions with monosilanes have
been unsuccessful. Thus, silicon tetrafluoride did not
react with difiuorosilylene, and the reaction of dimethylsilylene with dimethyldimethoxysilane was shown to
be kinetically unimportant in the thermolysis of the
corresponding disilane [351. With additional data on
the relative reactivities of various SIX bonds, this
‘Me2Si: +
MeZSi(0Me)z +
other products
(ref. [35])
carbon-hydrogen bonds of ethane and trimethylsilane
has been cited in support of the suggested decreased
reactivity of this silylene relative to methylene [291.
4.3. Carbon-Carbon Multiple Bonds
Certainly the most popular and probably the most
general method for intercepting carbenes involves
trapping with unsaturated hydrocarbons. Most of our
information regarding carbene chemistry has come
from these reactions which yield cyclopropanes or
cyclopropenes. Silylenes also appear to react with a
variety of unsaturated organic compounds; however,
to date, no stable, isolable silacyclopropanes or silacyclopropenes are known 1721.
4.3.1. A l k y n e s
reaction could become a valuable synthetic route to
Silylenes react with a variety of X-H bonds to give
1:l adducts:
+ Me3SiH +
:SiH(SiMe3) + Me3SiH
:SiMe(OMe) + MeOH
:Si(OMe)z+ MeOH
+ HBr
:SiF2+ HzS
+ (Me3Si)zSiH~
+ MeHSi(0Me)Z
+ HF2SiSiFzBr+ HFzSiSH
+ 2GeH4
[29, 351
HF2SiSiF3 [691
+ H3GeSiF2SiFzH
[68] P. S . SkeNand P. W. Owen, J. Amer. chem. SOC.89, 3933
[69] A. G. MacDiarmid and Y. L. Baay, unpublished.
Since the initial report 1371 that diphenylacetylene
could be employed as a “trapping” agent for dimethylsilylene and the original incorrect assignment of the
structure of the products was corrected [5,34~1, a
variety of acetylenes and silylenes have been employed
in this reaction. In all cases the product is the corresponding 1,4-disilacyclohexadiene. It is generally
assumed that the disilacyclohexadiene arises via a
silacyclopropene and it has been further suggested 1401
that a dimerization through the x-system was involved.
[70] D. Seyferth in: Proc. R. A. Welch Foundation Conference
Chem. Res. IX Organometallic Compounds, Robert A. Welch
Foundation, Houston, Texas, 1966, p. 84ff.
[71] A. M . Trozzolo, W. A. Yager, G . W. Griffin, K. Kristinsson,
and I. Sarkar, J. Amer. chem. SOC.89, 3357 (1967).
[72] K . A. Andrianov and L. M . Khanashvili, Organometallic
Chem. Rev. 2, 141 (1967).
Angew. Chem. internat. Edit.
1 Vol. 8 (1969) 1 No. 7
R = R1
[5, 34,37,40]
Me, R2 = Ph
Me, R2 = Et
= Ph, R2 = Ph
Me, R1 = OMe, R2
Me, R1 = OMe, Rz
Me, R1 = OMe, R2
However, reaction of dimethylsilylene with a mixture
of dimethyl and diphenylacetylene gave only three disilacyclohexadienes. The structure of the “mixed” disilacyclohexadiene eliminated from further consideration the x-dimerization mechanism for disilacyclohexadiene formation. An alternative route involving
gives, in addition to vinyldimethylsilane, cyclic and
polymeric products which are also viewed as derivatives of the silacyclopropane [51. It should be noted
that although the above examples demonstrate that
dimethylsilylene reacts with simple olefins, the nonconjugated olefins cannot successfully compete with
simple alkoxydisilanes for the silylenes, and, therefore,
this convenient source of silylenes is not satisfactory
for this class of reactions. The relative reactivities of
various unsaturated organic compounds are compared
in Section 4.4.
The reactions of difluorosilylene with ethylene [161,
acrylonitrile 1161, and tetrafluoroethylene 116,731 had
been reported to give solid polymeric products. In a
recent study, however, two monomeric products were
identified from the ethylene reaction [731.
H2C=CH2 +
Reaction of difluorosilylene and trifluoroethylene
gave several vinylsilane derivatives 1731. While these
products may be accounted for via insertion into the
a rather specific dimerization across the carbon-
FzC = C H F
silicon ring bonds of the silacyclopropene intermediates
has been suggested [34cJ.
4.3.2. Alkenes
The reaction of dimethylsilylene with ethylene in the
vapor phase has been reported 1291. While the product
isolated is vinyldimethylsilane,it has been suggested 1291
C-F bonds (see Section 4.2), their formation may also
be rationalized by isomerization of a silacyclopropane
intermediate. The reaction of difluorosilylene and
cyclohexene has been thoroughly studiedl741, and,
again, the products are most easily explained via dimerization of a silacyclopropane.
that this reaction involves formation and isomerization
of 1,I-dimethyl-1-silacyclopropanerather than direct
insertion of the silylene into the carbon-hydrogen
bond. The condensed phase reaction of dimethyl
silylene (from 7-silanorbornadienes) and ethylene
[(MezSi), -(CH2CH2),],
Conjugated olefins such as butadienes have also been
utilized as trapping agents. Difluorosilylene and 2,3dimethylbutadiene gave only solid polymeric products 1161. Silylenes generated via pyrolysis of alkoxydi-
Me Me
(ref. [351)
M e d ‘OMe
[73] J. C. Thompson, P. L. Timms, and J. L. Margrave, Chem.
Commun. 1966, 566.
[741 A . G . MacDiarmid and F. M . Rabel, unpublished.
Angew. Chem. internat. Edit.
1 Vol. 8 (1969) 1 No. 7
silanes and of 7-silanorbornadienes, however, gave
silacyclopentenes. The dienes, like the acetylenes,
compete successfully with the alkoxydisilanes for the
silylenes. While the silacyclopentene is formally a
product of 1,Caddition of the silylene, a reaction path
involving formation and thermal isomerization of a
vinyl-substituted silacyclopropane is favored 1341.
tions are closely related to known methods of generating
silylenes and have been interpreted in terms of silylene intermediates, present evidence suggests alternative mechanisms.
In some cases, even the intermediacy of “silylenoid” “61
species is unlikely.
5.1. Catalyzed Disproportionation of Polysilanes
A number of base-catalyzed disproportionation reactions of
substituted polysilanes have been reported 1791. Silylene
intermediates have been suggested 151 for some of these reactions which are formally similar to the thermal disproportionations (Section 2.2) - reactions known to involve silylenes.
Dimethylsilylene from the pyrolytic methods is not
intercepted by simple aromatic compounds [35,40,421.
However, a variety of derivatives have been isolated
from the co-condensation of difluorosilylene with
amine or
n SizC16 ___ - --+
n Sic14
ammonium halide
n EtC12SiSiCI3
+ (SiClz),
+ (SiC12),
+ Me3Si(SiMez),CN
n EtSiCI,
n Me3SiSiMezCN 3 (n-1) Me3SiCN
n = 2-5
2 MeO(SiMe&OMe
4.4. Relative Reactivities
The trapping of silylenes from the pyrolysis of methoxydisilanes provides some insight into the relative
reactivity of the trap, for it must always compete with
the precursor disilane for the silylene. From these and
related studies with the 7-silanorbornadienes, we see
that saturated hydrocarbons and benzene are quite
inert and do not compete with the polymerization of
the silylene. Ethylene reacts with the silylene but
cannot compete with the disilane. Acetylenes and
conjugated dienes are more reactive than the methoxydisilane with the dimethylsilylene. Thus, we have
the following series of relative reactivities toward dimethylsilylene.
< ethylene < dimethoxytetramethyldisilane <
dienes and alkynes
However, to date, there is no direct evidence to support this
suggestion and the one attempt to examine this point suggests
a quite different mechanism. Thus, while the thermolysis of
1,2-dimethoxytetramethyldisilanein the presence of diphenylacetylene gives a disilacyclohexadiene derivative, none of the
iatter is obtained in the base-catalyzed reaction of thisidisilane [34c1.
benzene or fluorobenzenes 1751. Reaction with benzene
or toluene, however, follows a different path to give
structures of the type shown below.
+ MeO(SiMe&OMe
MeO(SiMe,), OMe
(n = 1-4)
These base-catalyzed reactions are best explained in terms of
a silicon-silicon (SiSi) and silicon-ligand (SiL) redistribution
of the general type[36*79.*51:
+ SiSiL
+ Si-L+ -SiSiSi-
[76] The term “silylenoid” analogous to “carbenoid” 177, 781 is
suggested for the general class of intermediates which exhibit
reactions qualitatively similar to those of silylenes without
necessarily being free divalent siiicon species.
[77] G. L. Closs and R . A . Moss, J. Amer. chem. SOC.86, 4042
[78] G . Kobrich, Angew. Chem. 79, 15 (1967); Angew. Chem.
internat. Edit. 6, 41 (1967).
1791 M. Kumada and K . Tamao, Advances organometallic Chem.
6, 19 (1968).
[SO] C. J. Wikins, J. chem. SOC.(London) 1953, 3409.
5. Concerning the Proposed Intermediacy
Silylenes in Other Organosilicon Reactions
A number of organosilicon reactions are known (in addition
to those previously discussed) which involve the formal
transfer of a n RZSi= moiety. Although some of these reac[75] P. L. Timms, D . D . Stum-p, R . A. Kent, and J. L. Margrave,
J. Amer. chern. SOC.88,940 (1966).
1811 G. D . Cooper and A. R . Gilbert, J. Amer. chem. SOC.82,5042
182) G . Urry, J. inorg. nuclear Chern. 26, 409 (1964).
[83] E. Wiberg and A. Neumaier, Angew. Chem. 74, 514 (1962);
Angew. Chem. internat. Edit. I , 517 (1962).
[84] J. V. Urenovitch and A. G . MacDiarmid, J. h e r . chem.
SOC.85, 3372 (1963).
1851 An exception to this may be the thermally induced reactions
of the cyano-substituted polysilanes 1841.
Angew. Chem. internat. Edit.
Vol. 8 (1969) 1 No. 7
2 Me3SiC1+ 2 PhCH=CH2
5.2. Reaction of R2SiX2 with Metals
+ 2 Li
Although some of the most convincing evidence for dimethylsilylene has come from the reaction of dichlorodimethylsilane
and potassium in the gas phase1291 (Section 2.1), the nature
of the reaction of alkali metals and dihalodiorganosilanes in
the condensed phase is much more obscure. Dihalodiorganosilanes react rapidly with alkali metals in a variety of solvents
to yield polysilanes c79.861, and a similar reaction occurs more
slowly with halotriorganosilanes 179,861.
+ 2 Na +
(Me3Si)PhC=CPh(SiMe3) L 2 NaCl
2 Me3SiC1 1 PhCECPh
2 Me3SiC1
CH2=CH-CH=CHz + 2 Na
RzSiC12 + 2 M
+ 2 MCI
R3SiSiR3 + 2 NaCl
Of specific interest is the class of reactions where the combination of an alkali metal, a dichlorodiorganosilane, and an
unsaturated organic compound yield organic products containing the R2Si= unit. The reactions have been citedfsl as
examples of syntheses involving silylenes.
MezSiC12 + 2 PhCH=CH,
2 Li
MezSiClz + CH,=CH-C=CH,
2 Na
2 Li
2 Me3SiC1
2 (CF3)&0
2 Li
Related reactions with olefins which react less readily with
alkali metals may indeed involve initial reaction between the
chlorosilane and metal. Particularly, the reaction of dimethyldichlorosilane, lithium, and ethylene is more consistent with
Me3Si SiMe,
Me, .,Me
Likewise, the related reaction with hexafluoroacetone was
initially thought 1921 to involve a silylene, but most likely
involves initial reaction of the alkali metal and the
ketone 193,941.
+ 2 LiCl
+ 2 ArCrCAr + 4 Na +
2 NaCl
An attempt to intercept silylene intermediates in the reaction
of dichlorodiphenylsilane and lithium using cyclohexene was
unsuccessful [87L
+ (RZSi),
2 R$iCI+ 2 Na
2 MeZSiClz
2 NaCl
Me2SiC12 + H2C=CH2
it is highly unlikely that any of the above reactions involve
silylenes as nearly identical reactions are obtained in each
case with a chlorotriorganosilane. These reactions involve
the trapping, via reaction with a chlorosilane, of short-lived
organometallic reagents from the alkali metal made olefin,
i.e., a “disilylation” reaction “38,901. This latter scheme is
more consistent with the effects of stoichiometry, solvent, and
metal on the structure and stereochemistry of the products
than is one involving silylene intermediates.
[86] H. Gilman, W . H . Atwell, and F. K . Cartledge, Advances organometallic Chem. 4, 1 (1966); H. Gilman and G. L. Schwebke,
ibid. I , 89 (1964).
[87] H. Gilman and D . J. Peterson, J . h e r . chem. Soc. 87,2389
I881 D. R . Weyenberg, L. H. Toporcer, and A. E. Bey, J. org.
Chemistry 30, 943 (1965).
I891 R . West and R . E. Bailey, J. Amer. chem. SOC. 85, 2871
[901 D . R . Weyenberg,L. H. Toporcer, and L. E. Nelson, J. org.
Chemistry 33, 1975 (1968).
Angew. Chem. internat. Edit.
Vol. 8 (1969) No. 7
a silylene or “silylenoid” intermediate (Section 4.3) 139aI.
Because of these complicating factors, the reaction of alkali
metals and halosilanes in aprotic solvents has been of limited
value in the study of silylenes.
The authors are grateful to Drs. A . G. MacDiarmid,
J. L. Margrave, and P. L. Timms for allowing use of
their data prior to publication.
Received: July 8, 1968
[A 702 IEI
German version: Angew. Chem. 81,485 (1969)
[91] D . R . Weyenberg and A . E. Bey, unpublished.
[92] R . A . Braun, J. Amer. chem. SOC.87, 5516 (1965).
[93] C. L. Frye, R . M . Salinger, and T. J. Patin, J. Amer. chem.
Soc. 88, 2343 (1966).
[94] A. F. Janzen, P . E. Rodesifer, and C.J. Willis, Chem. Commun. 1966, 672.
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