Silica-supported pyrrolidine–triazole, an insoluble, recyclable organocatalyst for the enantioselective Michael addition of ketones to nitroalkenes

Recyclable organocatalysts based on hybrid silicas

“…Scheme 13 Asymmetric Michael addition of diethyl malonate to cyclohexenone Although a variety of solvents have been reported for diverse Michael additions, as seen from the examples mentioned in this section, some authors concluded, after a solvent screening, that water 195 (Scheme 10) or neat conditions 191,192 (entries 2, 3 and 4 in Table 4) were the best options taking into account activity, selectivity and environmental concerns.…”

“…The dialkyne coupling partner was obtained by treatment of 3-aminopropyl functionalized silica gel with propargyl bromide under basic conditions. Deprotection with TFA was performed on the material after the click reaction 191. The asymmetric Michael addition of ketones to nitrostyrenes was also successfully achieved by the use of silica-immobilized pyrrolidines M88 and M89 presenting an imidazolium moiety in the linker, which were obtained by a post-functionalization approach.…”

An Efficient Solvent-Free Synthesis of 2-Hydroxy-2-(trifluoromethyl)-2H-chromenes Using Silica-Immobilized L-Proline

“…Difficult recycling of homogeneous catalysts, such as piperidinium acetate, prompted us to find a suitable heterogeneous catalyst. L-Proline and its analogues have been extensively investigated as catalysts for many reactions; much effort has been dedicated to the immobilization and recycling of L-proline and its analogues with the assistance of organic and inorganic supports [33,34,35].…”

Organocatalytic Reactions Under Unusual Conditions

“…A similar polymer bound organocatalyst, as described by Pericas has been prepared also by Chuan [43] and analogous organocatalyst bonded on silica gel was described by Zhao [44]. Both of them found that p-nitrobenzoic acid must be used as co-catalyst.…”