Reactions of trimethylsiloxychlorosilanes with lithium metal – On the mechanism of the formation of trimethylsiloxysilyllithium compounds LiSiRR’(OSiMe3)

The reaction pathway for the formation of the trimethylsiloxysilyllithium compounds (Me3SiO)RR′SiLi (2a: R = Et, 2b: R = iPr, 2c: R = 2,4,6-Me3C6H2 (Mes); 2a–c: R′ = Ph; 2d: R = R′ = Mes) starting from the conversion of the corresponding trimethylsiloxychlorosilanes (Me3SiO)RR′SiCl (1a–d) in the presence of excess lithium in a mixture of THF/diethyl ether/n-pentane at −110 °C was investigated.The trimethylsiloxychlorosilanes (Me3SiO)RPhSiCl (1a: R = Et, 1b: R = iPr, 1c: R = Mes) react with lithium to give initially the trimethylsiloxysilyllithium compounds (Me3SiO)RPhSiLi (2a–c). These siloxysilyllithiums 2 couple partially with more trimethylsiloxychlorosilanes 1 to produce the siloxydisilanes (Me3SiO)RPhSi–SiPhR(OSiMe3) (Ia–c), and they undergo bimolecular self-condensation affording the trimethylsiloxydisilanyllithium compounds (Me3SiO)RPhSi–RPhSiLi (3a–c). The siloxydisilanes I are cleaved by excess of lithium to give the trimethylsiloxysilyllithiums (Me3SiO)RPhSiLi (2). In the case of the two trimethylsiloxydisilanyllithiums (Me3SiO)RPhSi–RPhSiLi (3a: R = Et, 3b: R = iPr) a reaction with more trimethylsiloxychlorosilanes (Me3SiO)RPhSiCl (1a, 1b) takes place under formation of siloxytrisilanes (Me3SiO)RPhSi–RPhSi–SiPhR(OSiMe3) (IIa: R = Et, IIb: R = iPr) which are cleaved by lithium to yield the trimethylsiloxysilyllithiums (Me3SiO)RPhSiLi (2a, 2b) and the trimethylsiloxydisilanyllithiums (Me3SiO)RPhSi–RPhSiLi (3a, 3b). The dimesityl-trimethylsiloxy-silyllithium (Me3SiO)Mes2SiLi (2d) was obtained directly by reaction of the trimethylsiloxychlorosilane (Me3SiO)Mes2SiCl (1d) and lithium without formation of the siloxydisilane intermediate. Both silyllithium compounds 2 and 3 were trapped with HMe2SiCl giving the products (Me3SiO)RR′Si–SiMe2H and (Me3SiO)RPhSi–RPhSi–SiMe2H.

Graphical abstractDepending on substituents the reaction pathways of the formation of (Me3SiO)RR′SiLi differ. The formation of (Me3SiO)RPhSiLi (R = Et, iPr) takes place via the intermediates (Me3SiO)RPhSi–(RPhSi)n–SiPhR(OSiMe3) (n = 0, 1) which are cleaved by excess lithium. (Me3SiO)MesPhSiLi is formed via Me3SiO(MesPhSi)2OSiMe3 and its following cleavage by Li. (Me3SiO)Mes2SiLi is directly obtained by reductive lithiation without formation of intermediates.Figure optionsDownload full-size imageDownload as PowerPoint slide