RESEARCH
Evidence for Existence of Dimethylsilene Found by U.S. and U.S.S.R. Chemists Silicon analog of carbene seems to be a short-lived intermediate in gas-phase reactions of dimethyldichlorosilane and alkali metals; U.S. study done at Penn State, Soviet effort at Academy of Sciences in Moscow
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C&EN
APRIL
6,
1964
Evidence for the existence of dimethylsilene, (CH 3 ) 2 Si:, is being offered independently by chemists in the U.S. and the U.S.S.R. Dimethylsilene is a silicon analog of a carbene. The U.S. and Soviet work was made public within a few days of each other. In the U.S., dimethylsilene was synthesized by Dr. P. S. Skell and E. J. Goldstein of Pennsylvania State University, University Park [/. Am. Chcm. Soc, 86, 1442 ( 1 9 6 4 ) ] . The Penn State chemists believe they have generated the compound as a shortlived, gas-phase intermediate by reacting dimethyldichlorosilane with sodium-potassium vapor at 260° to 280° C. The Soviet study was made by Dr. O. M. Nefedov and M. N. Manakov of N. D. Zelinskii Institute of Organic Chemistry of the Academy of Sciences of the U.S.S.R., Moscow [Angew. Chem., 76, 270 (1964)]. They also use dimethyldichlorosilane, but treat it with lithium in tetrahydrofuran with ethylene being passed through the reaction mixture (at 0° to 10° C ) . Dimethyldichlorosilane, ethylene, metallic sodium, and benzene in an autoclave at 105° to 120° C. also give the silene intermediate, Dr. Nefedov and Mr. Manakov say. A third method is degrading polydimethylsilene at 300° C. in the presence of ethylene. Bivalent. The Penn State work is based on the requirement of a bivalent silicon intermediate for chemical reactions involving the formation of pentamethyldisilane. When the reaction of dimethyldichlorosilane with NaK vapor is carried out in a helium atmosphere, a large portion of the product is left in the reaction zone as a solid, probably polymeric. However, if a 10-fold molar excess of trimethylsilane, (CH 3 ) 3 SiH, is added to the helium stream, the dimethylsilene
is inserted into the Si—H bond to produce pentamethyldisilane, Dr. Skell and Mr. Goldstein say. The excess trimethylsilane was separated by low-temperature distillation and the pentamethyldisilane by gas chromatography. The yield of pure pentamethyldisilane is about 30%. According to Dr. Skell and Mr. Goldstein, there doesn't seem to be a satisfactory alternative hypothesis to explain the formation of pentamethyldisilane in this gas-phase system, other than the dimethylsilene intermediate reaction. A radical chain process involving ( C H 3 ) 2 ClSi- and ( C H 3 ) 3 Sican be eliminated, they add. Trimethylsilyl radical generated under comparable conditions couples in good yield to hexamethyldisilane. For example, either trimethylchlorosilane alone or a mixture of methylene bromide and trimethylsilane reacts with alkali metal vapor to produce hexamethyldisilane. Only a trace of hexamethyldisilane could be detected among the products of the reaction of dimethyldichlorosilane-trimethylsilane mixtures. These observations not only implicate a dimethylsilene intermediate, but also suggest that it's in the singlet state (where the two free electrons are paired), Dr. Skell and Mr. Goldstein point out. Spin conversion rules require that a triplet dimethylsilene react with trimethylsilane to produce two monoradicals for which geminate coupling is precluded: (CH 3 )oSi: | T + (CH 3 ) 3 SiH-> (CH 3 ) 2 HSi. t + (CH 3 ) 3 Si • j . Nongeminate coupling should lead to hexamethyldisilane, pentamethyldisilane, and S't/m-tetramethylsilane in a 1:2:1 ratio. The homo-coupling products could not be detected. All products other than pentamethyldisilane in the appropriate molecular weight
range totaled only 4 % (by weight) of the pentamethyldisilane. Therefore, the Penn State workers assign a singlet state electronic configuration to dimethylsilene. Alkylcarbenes, the carbon analogs of dimethylsilene, are not generally trapped in bimolecular processes. Instead, they rearrange to olefinic products. This pathway, ( C H 3 ) 2 C : - » C H 3 - C H = C H 2 , is not favored for dimethylsilene because of the relative instability of the silicon-to-carbon double bond. Despite the greater stability of dimethylsilene, insertion reactions involving the C—H bonds of ethane and trimethylsilane have low efficiencies. This suggests that di-
methylsilene is much less reactive than is singlet CH 2 , Dr. Skell and Mr. Goldstein say. Intermediate. The Soviet chemists' proposal for the existence of dimethylsilene also rests on the kind of products produced in the reactions they studied. Dimethyldichlorosilane treated at 0° to 10° C. with lithium in tetrahydrofuran and ethylene gives 2 to 5% yields of 1,1-dimethyl-1-silacyclopentane and 1,1,4,4-tetramethyl1,4-disilacyclohexane. Some solid and liquid high-molecular-weight compounds are also made in the reaction. Formation of silicon-containing heterocycles, telomers, and polymers can
be explained only by assuming a transient dimethylsilene intermediate, Dr. Nefedov and Mr. Manakov say. Under the conditions used by them, lithium does not add to the double bond of monoolefins, they explain. And ethylene doesn't react at 105° to 120° C. with silicon-lithium bonds such as those in R 2 SiLiCl, which is produced in the reaction between dimethyldichlorosilane and lithium. Replacing ethylene with isobutylene gives 1,1,2,2,3,3,4,4,5,5-decamethyl-1,2,3,4 - tetrasilacyclohexane, which separates out of the reaction mixture. Cyclohexene gives C 6 H 1 0 [ S i ( C H 3 ) 2 ] 4 , while 1-hexene gives the C 7 H 1 4 analog.
Penn State Work Is Based on Requirement of a Bivalent Si Intermediate for Reactions Involving Pentadimethylsilene Formation Dimethylsilene is generated by reacting NaK vapor with dimethyldichlorosilane in helium at 260° to 280°C.
Dimethylsilene
When trimethylsilane is added to the helium stream, dimethylsilene is inserted into the Si—H bond to produce pentadimethyldisilane
Dimethylsilene
Soviet Scientists Say that Formation of Si-Containing Heterocycles, Telomers, and Polymers Goes through Dimethylsilene Intermediate
Silene intermediate
APRIL
6f
196 4 C & E N
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