Strategies for copper bis(oxazoline) immobilization onto porous silica based materials

A copper(II) chiral bis(oxazoline) homogeneous catalyst (CuBox) was anchored onto mesoporous materials using three procedures: method 1, first anchoring of the bis(oxazoline) functionalized with 3-iodopropyltrimethoxysilane onto hexagonal mesoporous silica, SBA-15 and silica gel and then copper(II) coordination; method 2, anchoring of the CuBox functionalized with 3-iodopropyltrimethoxysilane onto hexagonal mesoporous silica, SBA-15 and silica gel; and method 3, first anchoring onto silica gel of 3-iodopropyltrimethoxysilane and then anchoring of the CuBox. In methods 1 and 3, the remaining free silanol groups of the silica materials were also capped using trimethylsilane groups. The bis(oxazoline) ligand was also encapsulated onto copper(II) exchanged zeolites NaY and NaUSY as a simpler strategy for the homogeneous catalyst immobilization, method 4. The materials were characterized by ICP–AES, elemental analyzes, 13C and 29Si MAS NMR, FTIR, thermogravimetry and nitrogen adsorption at −196 °C. The materials were tested as heterogeneous catalysts in the benchmark reaction of cyclopropanation of styrene to check the effect of the immobilization strategy on the catalytic parameters as well as on their reutilization in several catalytic cycles.

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► Cu(II) chiral Box anchored onto mesoporous silica materials of varying pore size.
► Box ligand also encapsulated onto copper(II) exchanged zeolites.
► Material with smaller mesopores evidences reduction in the pore sizes.
► Heterogeneous catalysts in the cyclopropanation of styrene at room temperature.
► The enantioselectivities and styrene conversions increase with decreasing pore size.