Synthesis of 1,1′-ferrocenediyl salicylaldimine ligands and their application in titanium-initiated lactide polymerization

The synthesis of bidentate N/O 1,1'-ferrocenediyl analogues of N-aryl-salicylaldimines has been accomplished from the previously unreported 1'-hydroxyferrocenecarbaldehyde, 3. Thus, the reaction of 3 with 2,6-diisopropylaniline and with aniline leads to the isolation of N-(2,6-diisopropylphenyl)-(1'-(hydroxy)ferrocenyl)aldimine, 5, and N-phenyl-(1'-(hydroxy)ferrocenyl)aldimine, 6, respectively. However, attempts to synthesize analogues of bis(iminophenoxide) (salen) ligands by treatment of 3 with 1,2-ethylenediamine and with trans-(+/-)-1,2-diaminocyclohexane led to intractable product mixtures, while reaction with 1,2-phenylenediamine resulted in formation of a substituted benzimidazole. Titanium bis(isopropoxide) complexes of 5, 6, and, for comparative purposes, 1'-(diphenylphosphino)hydroxyferrocene have been synthesized, and in a preliminary study all three complexes were found to initiate the ring-opening polymerization of rac-lactide in a well-controlled manner.