EncapsulationofachiralMn III (salen)complexinordered mesoporoussilicas:anapproachtowardsheterogenizing asymmetricepoxidationcatalystsfornon-functionalizedalkenes Rukhsana I. Kureshy, * Irshad Ahmad, Noor-ul H. Khan, Sayed H. R. Abdi, Kavita Pathak and Raksh V. Jasra Silicates and Catalysis Discipline, Central Salt and Marine Chemicals Research Institute (CSMCRI), Bhavnagar 364 002, Gujarat, India Received 29 July 2005; accepted 12 September 2005 Available online 14 November 2005 Abstract—Two immobilized chiral Mn III (salen) complexes covalently anchored on modified MCM-41 (50 A ˚ ) and SBA-15 (75 A ˚ ) were prepared using 3-aminopropyltriethoxysilane as a reactive surface modifier to afford comparable or even higher enantioselec- tivity than homogeneous catalysts for the enantioselective epoxidation of a series of smaller to bulkier alkenes. The catalyst immo- bilized in silica with larger pore diameters was found to be more active. Compared to homogeneous catalysts, the heterogenized catalysts are more stable and can be recycled four times with retention of enantioselectivity. Ó 2005 Elsevier Ltd. All rights reserved. 1.Introduction Homogeneous catalysts usually display higher activities and enantioselectivities than heterogeneous catalysts for enantioselective catalytic reactions. However, the prod- uct separation and catalyst recovery is difficult for homogeneous systems. 1 An immobilized enantioselec- tive catalytic system is one of the most promising strat- egies to produce single optically active enantiomers due to easy separation and recycling of the catalyst. 2 One general way to transform a homogeneous catalytic reac- tion into heterogeneous process involves anchoring of active catalytic sites on a solid support by a linker group. Such immobilization of Mn III (salen) comp- lexes also effectively prevents the formation of inactive dimeric or polymeric l-oxo intermediate species through isolation of the active metal sites. 3 Several papers have recently appeared on the immobilization of chiral Mn III (salen) complexes onto solid supports such as organic polymers 4–6 and inorganic solids of various porosity 2b,7–12 with moderate to excellent results for epoxidation of non-functionalized alkenes. The solid chiral catalysts accommodated in the pores or cavities of the porous materials show differences in reactivity and enantioselectivity of the epoxidation reaction. 2b,8,10 It is, therefore, intuitive to graft the active metal com- plex onto solid supports of varied pore sizes to examine the influence of surface and pores on catalytic activ- ity and enantioselectivity (confinement effect) of the Mn III (salen) complex without loosing its merits. Meso- porous materials have received extensive attention due to their ordered pore structure, good thermal stability, large surface area and ease of modification by utiliz- ing the available surface silanol groups for the anchor- ing of a homogeneous catalyst. 13 Herein, we have synthesized the siliceous MCM-41 and SBA-15 to immobilize the chiral Mn III (salen) complex 1 using 3-aminopropyltriethoxysilane (APTES) as a reactive surface modifier. The immobilized catalysts 1a (complex 1 supported on MCM-41) and 1b (complex 1 supported on SBA-15) lead to markedly higher ee for 4-chlorosty- rene (69–71%) than the homogeneous complex 1 and other reported immobilized Mn III (salen) complexes 8,14 anchored on MCM-41. Furthermore, catalysts 1a and 1b epoxidized bulkier alkenes with high chiral induction (up to 96%) and the catalysts are reusable for up to four cycles. 0957-4166/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetasy.2005.09.022 * Corresponding author. Fax: +91 278 2566970; e-mail: rkureshy@ csmcri.org Tetrahedron: Asymmetry 16 (2005) 3562–3569 Tetrahedron: Asymmetry