Lithium silylenoid

Clark and Schleyer20 were the first to report theoretical evidence for the complexation of H2Si by halogen donors when investigating the structural isomers of the silylenoid H2SiLiF. A silylene-lithium fluoride complex 14 was found to be only 5.4 kJ moL1 less stable at the MP2/6-21G//3-21G... 

The inversion in selectivity between the insertion into an H—Si bond favored by the intermediate in the dehalogenation and the addition to a triple bond favored by the intermediate formed upon silirane pyrolysis suggests that different reactive intermediates are formed. Since free silylene is implicated in the silirane pyrolysis, the lithium-induced deiodination probably involves a silylenoid such as a complex of the silylene with Lil or with THF. For a further discussion of this possibility, see Section II.E. 

These silylenoid species have been made by lithium-tin exchange from a Si—Sn precursor96, or by reduction of chlorosilanes with lithium l-(dimethylamino)naphthalenide (LDMAN) at —78°C (equations 42 and 43)97. The resulting silyllithium species react as nucleophiles with Mc SiCI to give the corresponding disilanes (equation 44). An... 

An (alkoxysilyl)lithium, ( -BuO)Ph2SiLi, can be prepared from (t-BuO)Ph2SiSnMe3 by treatment with n-BuLi in THF (Scheme 10). This species is stable at -78°C for a few hours and behaves as an ordinary nucleophilic silyl anion 54d). (f-BuO)Ph2SiLi also behaves as an electrophile under certain conditions It undergoes self-condensation smoothly or butylation in the presence of an excess amount of n-BuLi and TMEDA (Scheme 10). (t-BuO)Ph2SiLi thus exhibits an ambiphilic nature, that is, silylenoid character. The ambiphilic reactivity of (t-BuO)Ph2SiLi, how-... 

A report by the Corriu group, however, shows that this is not necessarily the case (equation 49) . When difluorosilane was treated with lithium metal in the presence of dimethylbutadiene, the silacyclopentene was obtained in good yield but no silylene was intercepted if EtsSiH was present as the trapping agent. The product silacyclopentene could result either from reaction of the diene with a silylenoid, or from reaction of the diene with lithium and trapping of the lithium compound by the difluorosilane. In earlier studies of the alkali metal induced reactions of dihalosilanes with 1,3-dienes, it was found that the diene, rather than the dihalosilane, reacts initially with the metal. ... 

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