Schiffbase ligands

The dependency of the Schiffbase ligands 11-13 on the diastereoselectivity has been reported [27,28]. 

Scheme 5. Strecker synthesis with chiral Schiffbase ligand 12 (Snapper, Hoveyda, and co-workers). TMS = trimethylsilyl.

The basic approach to prepare Co(II)-complexes of salen (N,lSr-bis(salicylidene)ethylene-diamine)-type molecules is the flexible ligand method [9]. In this process the Schiffbase ligand can diffuse by twisting into the zeolite where it becomes too large to exit by complexation with the cobalt ion. The flexible ligand method, however, was not usefiil for the preparation of Co-salophen/ zeolite catalyst, because the product was inactive in the oxidation reactions. The salophen molecule does not seem to be flexible enough and can not get into the zeolite to produce the suitable complex in the supercage. 

Novel vanadium(V)-alkyl peroxide complexes with the general formula VO(OOBu )(ROPh-salR ) (197 ROPhsalR = tridendate N-(2-oxidophenyl)salicylidenaminato Schiffbase ligand) were found to epoxidize alkenes stoichiometrically with high selectivity. " ... 

Y butenolides 22 were obtained in good yields and as essentially single stereoisomers (Table 5.2). In these studies, the tat leucine derived SchifTbase ligand 18a gave rise to the best diastereo and enantioselectivities, and even with the less expensive valine derived Schiffbase ligand, the products were formed with 97% ee. Removal of the N aryl group was readily accomplished with cerium ammonium nitrate followed by acidic hydrolysis of the intermediate aza quinone. 

Mixed Donor Atom Ligands 33.5.9.1 Schiffbase ligands... 

The only jU-superoxo complexes to have been characterized are those which contain cobalt. They are readily prepared by treatment of the corresponding /i-peroxo complexes with strong oxidants. Thompson and Wilmarth observed that MnO, HOCl, Br2, BrOf and NOf are all effective oxidants toward [(en) Co(/r-NH2,02)Co(en)2] in acidic solution whereas Fe ", HjOj, Ag" " and CtjO are not. In other systems CI2, Pb02, persulfate and Ce in nitric acid solution have also effected oxidation. A number of well-characterized //-superoxo cobalt(III) complexes are listed in Table 55. There are no reports of /i-superoxo complexes containing Schiffbase ligands and all that are known contain either terminal amine or cyano groups. 

Partial hydrolysis of a Schiffbase ligand co-ordinated to copper(ii). (E. C. Lingafelter,... 

The addition of cyanide to imine (Strecker-type reaction) can also be catalyzed by chiral Ti complexes. In a pioneering work reported by Snapper and Hoveyda, a series of titanium complexes of tripeptide-based Schiffbase ligands have been discovered by combinatorial chemistry approach for the addition of TMSCN to imine derivatives with >93% conversion of substrates and excellent enantioselectivities (85-97% ee) [227]. This methodology has provided a practical procedure for the... 

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