Silaallenes synthesis

Barton and co-workers" performed flash vacuum pyrolysis (FVP) on trimethyl-silylvinylmethylchlorosilane (30), resulting in the production of trimethylchlorosi-lane (30%), trimethylvinylsilane (11.5%), and most interestingly, ethynylmethyl-silane (34, 11.9%). A proposed mechanism for the synthesis of 34 (Scheme 10) begins with the lo.ss of trimethylchlorosilane to form silylene 31, which can rearrange either to silaallene 32 or to silirene 33, both of which can lead to the isolated ethynylsilane. 

West et al. have recently described the synthesis and reactions of a 1-germaallene. Germaallene 76 (Eq. (7)) is analogous to silaallene 59a and is synthesized by intermolecular addition of f-butyllithium to precursor 75, followed by salt elimination at —78 C. This germaallene is not stable above 0 C in solution, but remains intact until heated above 90°C in the solid state. In either case, the... 

The first synthesis of stable 1-silaallenes follows closely an established route developed in carbon chemistry292. An overall nucleophilic 1,3 substitution at propargylic halides gives allenes in good yields (equations 206 and 207). In organosilicon chemistry, special attention must be paid to avoid direct substitution at the silicon which is the preferred site for nucleophilic attack. 

Of the 2-silaallenes no sufficiently stable species have yet been isolated, so that no spectroscopic verification is available up to date. The synthesis of possible 2-silaallenes is therefore treated along with the description of their reactivity in Section n.C below. 

The successful isolation of 1-silaallenes and 1-germaallenes raises the expectation of synthesis of a novel class of allenic compounds having an sp-hybridized silicon unit, that is, 2-silaallene derivatives. Although the generation of 2-silaallene (f-Bu)-CH2-CH=Si=CH-CH2-(t-Bu) was postulated in the reaction of (H2C=CH)2SiCl2 with 2-equivalent of t-BuLi, no distinct evidence on the generation of such a compound has been obtained so far. 

Summary Reactions of dichlorodivinylsilane and LitBu in a molar ratio 1 1 and 1 2 lead to highly reactive intermediates, which can be trapped by suitable trapping agents. From that mono and double addition products are formed, which provide experimental hints for the intermediate formation of the neopentylsilene H2C=CH(Cl)Si=CHCH2/Bu 3 and the 2-silaallene tBuCH2CH=Si=CHCH2tBu 4. In particular, the formation of double cycloadducts from 2 with Lifflu is a preparatively facile route for the synthesis of silaspirocycles such as 12, 14, and 15, which could be characterized by single crystal X-ray structure analysis. 

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