Unsymmetrical salen-type ligands: high yield synthesis of salen-type Schiff bases containing two different benzaldehyde moieties

Salen-type Schiff base ligands incorporating two different benzylidene moie ties and a diamine backbone were synthesized in high yield (80-90%) under m ild conditions via a stepwise approach. Tn the first step, anhydrous hydroc hloric acid was used to selectively protect one amino group of the vicinal diamine backbone. The resulting ammonium salt was added to a substituted be nzaldehyde providing access to a mono-imine product. This compound reacted with an equivalent of a different benzaldehyde in the presence of triethyla mine to afford an unsymmetrical salen-type ligand. This synthetic method al lows facile access to a plethora of salen-type Schiff base ligands with eas ily tunable steric and electronic properties. (C) 2001 Elsevier Science Ltd . All rights reserved.