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}] and [Li{(Me2NCH2CH2)2NMe}{2-CH2-6-Me(C6H3)2-2′,6′-Me2}]

Abstract

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.