Synthesis of sulphonated mesoporous phenolic resins and their application in esterification and asymmetric aldol reactions

Mesoporous phenolic resins were functionalized with sulphonic acid groups by four different types of sulphonation procedures: (i) direct sulphonation on the aromatic ring, (ii) alkyl sulphonation of the aromatic ring, and functionalizations of the phenolic hydroxyl surface by using an aryl silane, 2-(4-chlorosulphonylphenyl)ethyl trichlorosilane (iii) or a propyl silane, 3-mercaptopropyltrimethoxysilane (iv). The highest acidity loadings were obtained through direct sulphonation with fuming sulphuric acid (1.90 mmol H⁺ g⁻¹) or chlorosulphonic acid (1.31 mmol H⁺ g⁻¹) and these materials showed the highest conversion (97+ %) in Fischer esterification of acetic acid with propanol. However, the alkyl sulphonic groups, obtained through sulphonation procedure (ii) showed the highest stability in terms of maintenance of their acidity after use in consecutive catalytic runs or leaching treatments. This was demonstrated both through evaluation of the regenerated catalysts in a consecutive esterification run and during a leaching resistance test in aqueous medium. Moreover, the developed sulphonated mesoporous phenolic resins are presented as novel support for the non-covalent immobilization of an l-phenylalanine derived chiral diamine organocatalyst for asymmetric aldol reactions. The immobilization is established by an acid–base interaction between the sulphonic acid group and the amine function. The acidity and in particular the electronic withdrawing environment of the sulphonic acid groups influence enormously the catalytic performance of the non-covalent immobilized chiral diamine catalyst (aromatic > aliphatic).