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

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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.

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