Synthesis of α-lithioarylmethanes of m-xylene and its α-trimethylsilyl derivatives; crystal structure of [{Li(Me2NCH2CH2NMe2)}2{C6H4(CHSiMe3)2-m}]

Abstract

Crystalline organolithium complexes [{Li(pmdien)}{C₆H₄(CH₂R)(CHR′)-m}](R = R′= H, R = R′= SiMe₂, pmdien =NNN′N″N″-pentamethyldiethylenetriamine), [{Li(tmen)}{C₆H₄Me(CHSiMe₃)m}](tmen =NNN′N′-tetramethylethylenediamine), and [{Li(tmen)}₂{C₆H₄(CHSiMe₃)₂-m}] have been prepared by the metallation of m-xylene or the appropriate α-silylated m-xylene using LiBuⁿ–tertiary amine in hexane at ca. 20 °C. Lithiation of C₆H₄Me[CH(SiMe₃)₂]-m and C₆H₄(CH₂SiMe₃)[CH(SiMe₃)₂]-m using LiBuⁿ(tmen) yielded respectively –CH(SiMe₃)₂ and –CH₂SiMe₃ metallated species (not isolated) whereas C₆H₄[CH(SiMe₃)₂]₂-m is unaffected under the same conditions. The crystal structure of the compound [{Li(tmen)}₂{C₆H₄(CHSiMe₃)₂}]-m has been determinedand shows two non-equivalent lithium centres. One lithium is η² bonded to the hydrocarbyl moiety interacting through a carbon bearing a–SiMe₃ group [2.14(1)Å] and its nearest carbon [2.30(1)Å], the other Li is η³ bonded, on the opposite side of the C₈ plane, with Li–C distances of 2.14(1)Å to the other carbon bearing a SiMe₃ group, 2.26(1)Å to its adjacent carbon, and 2.44(1)Å to the next.