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

Dicationic chiral Mn-III 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 14 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 Will 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. (C) 2003 Elsevier Inc. All rights reserved.