Chiral N2O2 Schiff-base complexes of titanium with a biaryl backbone: highly diastereoselective migratory insertion of alkyl at imine

Abstract

The C₂-symmetric Schiff-base proligand (H₂L) synthesized by condensation of 2,2′-diamino-6,6′-dimethylbiphenyl and 3,5-di-tert-butylsalicylaldehyde reacted in its deprotonated form with TiX₄ (X = Cl or Br) to give the isomeric mixtures α-cis- and β-cis-[MLX₂]. The molecular structure of α-cis-[TiLBr₂] was determined. The two bromide ligands occupy mutually cis co-ordination sites in a C₂-symmetric structure. Reaction of the isomeric mixture X = Cl with LiNMe₂ leads to the single isomer α-cis-[TiL(NMe₂)₂] as determined by NMR spectroscopy. Reaction of the latter complex with BCl₃ reconverts it into the chloride complex in a similar isomeric mixture as above. While attempts to alkylate the halides were unsuccessful, the reaction of H₂L with [Ti(CH₂Ph)₄] leads to α-cis-[TiL(CH₂Ph)₂] which rapidly converts into a complex [TiL^Bn(CH₂Ph)] in which one of the Ti bound benzyl groups has undergone a highly diastereoselective 1,2-migratory insertion process with one imine group. The molecular structure of this compound shows that the co-ordination sphere of the Ti atom is approximately trigonal bipyramidal. The configuration of the new chiral centre at the former imine group indicates the expected mode of cis migration in the formation of the complex. The migrated benzyl group forms an edge–face interaction with the remaining metal bound benzyl. Controlled hydrolysis of [TiL^Bn(CH₂Ph)] leads to a dimeric species [{Ti(HL^Bn)}₂-(μ-O)₂] in which the amide bond in the starting complex has been protonated to form an amine ligand.