Axially asymmetric metal alkyls. Part 6. Lithiation of 2,2′-dimethyl-1,1′-binaphthyl and its trimethylsilylated compounds, and of 2,2′,6,6′-tetramethyl-1,1′-biphenyl (asymmetric induction): X-ray crystal structures of monomeric [{Li(Me₂NCH₂CH₂NMe₂)} ₂{(2-CH₂C₁₀H₆)₂}].

The compound [{Li(tmen)}₂{(2-CH₂C₁₀H₆) ₂}], (6) (tmen = N,N,N′,N′-tetramethylenediamine), has been prepared via metallation, and structurally characterized using X-ray diffraction data. It is monomeric with two different lithium centres, one bridging the ipso-carbon atoms at 2.13(5) and 2.23(5) Å, the other interacting with one ipso-carbon at 2.36(5) Å and its adjacent ring carbon at 2.51(5) Å, such that one ipso carbon is bridging the lithium centres. ⁷Li N.m.r. data are consistent with this structure in toluene, below ca. -28 °C. Treatment of (6) with AsMe₂I then Mel yielded the arsonium salt [(2-Me₃AsCH₂C₁₀H₆) ₂]I₂, and with SiMe₃Cl it gave (2-Me₃SiCH₂C₁₀H₆)₂. This afforded [2-(Me₃Si)₂CHC₁₀H₆]₂ on treatment with LiBuⁿ(tmen) then SiMe₃Cl. Metallation of 2,2′,6,6′-Tetramethyl-1,1′-biphenyl using LiBuⁿ(pmdien) (pmdien = N,N,N′,N′,N″-pentamethyldiethylenetriamine) in diethyl ether yielded a monolithiated species [{Li(pmdien)}{2-CH₂-6-Me(C₆H₃) ₂-2′,6′-Me₂}] (13), also structurally characterized; the lithium centre is attached to only the ipso-carbon of the hydrocarbyl group at 2.14(2) Å. Treatment of the same biphenyl with LiBuⁿ(sp) [sp = (-)sparteine] in Et₂O yielded a dilithiated species based on the asymmetrical chiral dianion, (2-CH₂-6-MeC₆H₃)₂^2-. The degree of asymmetric induction in the metallation step was estimated to be ca. 40% using a ¹H n.m.r.-chiral lanthanide shift reagent technique on the diol (2-HOCH₂CH₂-6-MeC₆H₃)₂, formed by carboxylation then esterification and reduction of the dilithiated species.