Dicationic chiral Mn(III) salen complex exchanged in the interlayers of montmorillonite clay: a heterogeneous enantioselective catalyst for epoxidation of nonfunctionalized alkenes

Abstract

Dicationic chiral MnIII salen complexes 1–4 were prepared from the reaction of 1S,2S-(+)-1,2-diaminocyclohexane/1S,2S-(−)-diphenyldiamine with 2-hydroxy-3-t-Bu-5-(triisooctylaminomethyl) benzaldehyde chloride/2-hydroxy-3-t-Bu-5-(triethylaminomethyl) benzaldehyde chloride and were exchanged in montmorillonite clay to give supported complexes 1′–4′. Enantioselective epoxidation of styrene, indene, and 2,2-dimethyl-6-nitro chromene mediated by 1–4 complexes (0.4–2 mol%) as catalysts in the presence of pyridine N-O as axial base with NaOCl as an oxidant gave quantitative epoxide yield (>99%) under biphasic homogeneous conditions. On employment of catalysts 1′–4′ under similar epoxidation condition, the epoxide selectivity in the case of styrene was found to be in the range 68–74%. However, there was a significant enhancement in ee (69–70%) compared to its homogeneous catalysts 1–4 (ee; 41–52%). Importantly, for the bulkier substrates like indene and chromene the selectivity were comparable both in homogeneous and heterogeneous system, probably due to the flexible T–O–T layers of clay allowing greater accessibility to the MnIII salen residing between the layers of clay. Further, the catalyst 1′–4′ could be recovered easily and recycled efficiently several times by a simple separation method.