α-Lithium alkyls of α-trimethylsilyl-p-xylenes and the synthesis of p-C₆H₄(CH₂SnMe₃)₂ from a new di-Grignard reagent: Crystal structure of [{Li(Me₂NCH₂CH₂NMe₂)} ₂{p-C₆H₄(CHSiMe₃)₂}]

Metallation of the p-xylene compounds p-C₆H₄(CHRR′) (CHR″R‴) (R,R′,R″,R‴ = H or SiMe₃) with [LiBuⁿ(tmen)] (tmen = NNN′N′-tetramethylethylenediamine) or [LiBuⁿ(pmdien)] (pmdien = NNN′N″N″-pentamethyldiethylenetriamine) occurs selectively at the benzylic carbon atom(s) (C_α) in a manner dependent on the degree of substitution and the tertiary amine. Four of the organolithium complexes generated have been isolated as crystalline solids and the molecular structure of one, [{Li(tmen)}₂-{p-C₆H₄(CHSiMe₃) ₂}], has been determined from single-crystal X-ray diffraction data. The p-xylenediyl moiety is planar with the trimethylsilyl groups trans to each other, being related by an inversion centre. There is evidence of a dominant p-quinodimethanide bonding contribution, with each lithium, on opposite sides of the C₈H₆Si₂ plane, interacting unsymmetrically with both C_α and its adjacent ring-carbon atom [Li-C_α 2.10(1) and Li-C_β 2.38(1) Å]. A high-yield synthesis of a di-Grignard reagent derived from 1,4-bis(chloromethyl)benzene in thf (tetrahydrofuran) which yields a compound of composition [Mg_1.5Cl(thf)₂(C₈H₈)]_n after ca. 1 h is described. The utility of this compounds is illustrated by the synthesis of p-C₆H₄(CH₂SnMe₃)₂ from SnMe₃Cl.