Chiral (Salen) Cobalt Complexes Encapsulated in Mesoporous MFI as an Enantioselective Catalyst for Asymmetric Ring Opening of Terminal Epoxides

Abstract

MFI structural zeolite (ZSM-5 and Sililcalite) was treated with hot NaOH solution to make mesoporous channels in the microporous crystals. Inner mesopore size could be controlled by changing the concentration of NaOH solution. The pore structure of alkaline-treated MFI zeolite was studied by instrumental analysis. To obtain the cage type mesopores, Ti-coating on the outside MFI crystals before alkaline treatment was investigated to be the most effective. Co(III) salen complexes monomers were successfully encapsulated in mesoporous MFI-type zeolite by ''ship-in-a-bottle'' method. The heterogeneous catalyst could be applied in asymmetric ring opening of terminal epoxides by phenol derivatives. It showed very high enantioselectivity with yield in the catalysis.

Ring-Opening of Oxiranes using Taeniolite-Supported Tris(β-Diketonato)Zirconium

“…The synthetic route for such compounds commonly proceeds via ring-opening of oxiranes (epoxides) using traditional Lewis acids such as Cu(BF 4 ) 2 [2,21], Sn(OTf) 2 [11], BF 3 [22], Sc(OTf) 3 [24], and traditional solid acids such as amberlyst-15 [10]. Modified Lewis acid complexes based on salen [5,12] and porphyrin [23] ligands have also been reported. Some heterogeneous catalysts such as zeolites [1,18], monmorillonite K-10 [4], silica gel [6], mesoporous aluminosilicate [9,12,14,15,20], CuO/SiO 2 [19], and sulphated yttria-zirconia [25] have been reported as catalysts in the ring-opening of oxiranes.…”

“…Modified Lewis acid complexes based on salen [5,12] and porphyrin [23] ligands have also been reported. Some heterogeneous catalysts such as zeolites [1,18], monmorillonite K-10 [4], silica gel [6], mesoporous aluminosilicate [9,12,14,15,20], CuO/SiO 2 [19], and sulphated yttria-zirconia [25] have been reported as catalysts in the ring-opening of oxiranes. An alternative reaction method using a microwave technique has also been examined [13], even though such a method still experiences difficulties in scale-up processes.…”

Cobalt-Catalysed Bond Formation Reactions; Part 2

“…Among these approaches were (macrocyclic) oligomeric cobalt(III)-Salen complexes, [142][143][144][145] cobalt-Salen complexes bonded to calix [4]arenes 146 or polymer-supported cobalt catalysts, [147][148][149][150] as well as the application of a cationic-modified cobalt-Salen complex utilised in ionic liquids. 151 Heterogeneous immobilisation was realised via encapsulated zeolites, 152,153 by electrostatic interactions on SBA-16 silica, 154,155 on carbon monolith 156 and on gold colloids. 157 All these investigations focused either on the increase of reactivity and selectivity or on the recycling of the cobalt catalysts for applications in green chemistry.…”

Recoverable chiral salen complexes for asymmetric catalysis: recent progress

“…The same team also reported a procedure in which they trapped a complex inside the cavity of a zeolite. 76 This synthesis first implied the introduction of both the aldehyde and the chiral diamine as ligand precursors inside the zeolite. The cobalt salt of 4-nitrobenzenesulfonic acid was then added to form the desired complex.…”