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}]
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)22–. 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.