New Chiral Calixsalen Chromium Complexes: Recyclable Asymmetric Catalysts

• Published in: Chemistry : a European journal Vol. 16; no. 36; pp. 11108 - 11114
• Main Authors: Zulauf, Anaïs, Mellah, Mohamed, Schulz, Emmanuelle
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 24.09.2010; WILEY‐VCH Verlag; Wiley; Wiley Subscription Services, Inc
• Subjects: asymmetric catalysis; Asymmetry; Bimetals; Catalysis; catalyst recycling; Catalysts; Chemistry; Chemistry, Multidisciplinary; Chromium; Derivatives; heterogeneous catalysis; N,O ligands; O ligands; Physical Sciences; Recycled; ring opening; Science & Technology; Transformations; POLYMERS; TERMINAL EPOXIDES; heterogeneous catalysis; DIELS-ALDER REACTIONS; MECHANISM; MACROCYCLES; asymmetric catalysis; ENANTIOSELECTIVE ADDITION; EPOXIDATION; N,O ligands; catalyst recycling; ring opening; HYDROLYTIC KINETIC RESOLUTION; SALEN COMPLEXES; RACEMIC SECONDARY ALCOHOLS
• DOI: 10.1002/chem.201001012
• ISSN: 0947-6539

A chiral N,N′‐bis(salicylidene)ethylenediamine (salen) polymer has been prepared by a condensation reaction between a thiophenedisalicyladehyde derivative and (S,S)‐cyclohexane‐1,2‐diamine. This polymeric compound was demonstrated to possess a cyclic structure with two to five repetitive units. The addition of chromium(II) salts led to the generation of a chiral catalyst that could be recovered as an insoluble powder. The performance of this new calixsalen‐type catalyst was examined in various transformations, particularly in its ability to promote nucleophilic epoxide ring opening under heterogeneous conditions. The target products were obtained in high yields and with improved selectivity compared with those obtained by using analogous linear polymers. The arrangement of the catalytic sites in the cyclic structure is probably more suitable for the necessary cooperative bimetallic pathway of this demanding reaction. The catalyst could be successfully recycled. This approach represents the first use of calixsalen complexes under heterogeneous catalytic conditions. Just reuse it! Chiral calixsalen (salen=N,N′‐bis(salicylidene)ethylenediamine) chromium complexes (see scheme) were prepared and used as heterogeneous asymmetric catalysts. They proved to be particularly competent for epoxide ring opening compared with their linear analogues in terms of selectivity. These cyclic polymers could be easily recovered by simple filtration and efficiently reused.