The power of trichlorosilylation: isolable trisilylated allyl anions, allyl radicals, and allenyl anions

Abstract

Treatment of hexachloropropene (Cl₂CC(Cl)–CCl₃) with Si₂Cl₆ and [nBu₄N]Cl (1 : 4 : 1) in CH₂Cl₂ results in a quantitative conversion to the trisilylated, dichlorinated allyl anion salt [nBu₄N][Cl₂CC(SiCl₃)–C(SiCl₃)₂] ([nBu₄N][1]). Tetrachloroallene Cl₂CCCCl₂ was identified as the first intermediate of the reaction cascade. In the solid state, [1]⁻ adopts approximate C_s symmetry with a dihedral angle between the planes running through the olefinic and carbanionic fragments of [1]⁻ of CC–Si//Si–C–Si = 78.3(1)°. One-electron oxidation of [nBu₄N][1] with SbCl₅ furnishes the distillable blue radical 1˙. The neutral propene Cl₂CC(SiCl₃)–C(SiCl₃)₂H (2) was obtained by (i) protonation of [1]⁻ with HOSO₂CF₃ (HOTf) or (ii) H-atom transfer to 1˙ from 1,4-cyclohexadiene. Quantitative transformation of all three SiCl₃ substituents in 2 to Si(OMe)₃ (2^OMe) or SiMe₃ (2^Me) substituents was achieved by using MeOH/NMe₂Et or MeMgBr in CH₂Cl₂ or THF, respectively. Upon addition of 2 equiv. of tBuLi, 2^Me underwent deprotonation with subsequent LiCl elimination, 1,2-SiMe₃ migration and Cl/Li exchange to afford the allenyl lithium compound Me₃Si(Li)CCC(SiMe₃)₂ (Li[4]), which is an efficient building block for the introduction of Me, SiMe₃, or SnMe₃ (5) groups. The trisilylated, monochlorinated allene Cl₃Si(Cl)CCC(SiCl₃)₂ (6), was obtained from [nBu₄N][1] through Cl⁻-ion abstraction with AlCl₃ and rearrangement in CH₂Cl₂ (1˙ forms as a minor side product, likely because the system AlCl₃/CH₂Cl₂ can also act as a one-electron oxidant).