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

The C-2-symmetric Schiff-base proligand (H2L) synthesized by condensation o f 2,2'-diamino-6,6'-dimethylbiphenyl and 3,5-di-tert-butylsalicylaldehyde r eacted in its deprotonated form with TiX4 (X = Cl or Br) to give the isomer ic mixtures alpha-cis- and beta-cis-[MLX2]. The molecular structure of alph a-cis-[TiLBr2] was determined. The two bromide ligands occupy mutually cis co-ordination sites in a C2-symmetric structure. Reaction of the isomeric m ixture X = Cl with LiNMe2 leads to the single isomer alpha-cis-[TiL(NMe2)(2 )] as determined by NMR spectroscopy. Reaction of the latter complex with B Cl3 reconverts it into the chloride complex in a similar isomeric mixture a s above. While attempts to alkylate the halides were unsuccessful, the reac tion of H2L with [Ti(CH2Ph)(4)] leads to alpha-cis-[TiL(CH2Ph)(2)] which ra pidly converts into a complex [TiLBn(CH2Ph)] in which one of the Ti bound b enzyl groups has undergone a highly diastereoselective 1,2-migratory insert ion process with one imine group. The molecular structure of this compound shows that the co-ordination sphere of the Ti atom is approximately trigona l bipyramidal. The configuration of the new chiral centre at the former imi ne group indicates the expected mode of cis migration in the formation of t he complex. The migrated benzyl group forms an edge-face interaction with t he remaining metal bound benzyl. Controlled hydrolysis of [TiLBn(CH2Ph)] le ads to a dimeric species [{Ti(HLBn)}(2)-(mu-O)(2)] in which the amide bond in the starting complex has been protonated to form an amine ligand.