Study On The Aldol Reaction Catalyzed Cooperatively By Heterogeneous Acid-base Catalysts

Acid-base cooperativity is commonly invoked in enzymatic and catalytic antibody catalysis.This paradigm of biological catalysis has inspired the synthesis of bifunctional catalysts with superior activity and selectivity,including enantioselectivity.Small molecular bifunctional organocatalyst imitated enzyme,especially with L-proline and its derivatives has attracted extensive research activities and experienced a huge development.The main advantages of these catalysts are that the reaction can be performed with excellent stereoselectivity under mild conditions.From a practical point of view,it would be desirable to have the catalyst immobilized so that its recovery,reuse and product purification can be facilitated in order to reduce costs,especially when the catalyst is obtained after several synthetic steps.The direct catalytic asymmetric aldol reaction is a highly versatile and powerful C-C bond forming reaction and has been widely used in constructing natural and nonnatural products.In this dissertation,we demonstrate the synthesis of bifunctional mesoporous catalysts containing an acidic site in the framework and a basic site on the porous surface by postgrafting of 1,5,7-triazabicyclo[4.4.0]dec-5-ene(TBD,a sterically hindered organic base with comparable strength to KOH) onto Al-MCM-41 molecular sieve.Al-MCM-41 usually has relatively weaker Brφnsted acid sites and stronger Lewis acid sites.Because of the space hindrance between the acidic sites in the framework and the immobilized TBD base sites, they can cohabit independently without mutual destruction.Therefore,a cooperative catalysis originated from the acidic and basic sites of the bifunctional heterogeneous catalyst shows an acid-base cooperativity in catalyzing the aldol condensation of acetone and various aldehydes to selectively giveβ-hydroxycarbonyl compounds.The binary catalyst system,consisting of the double hydrogen-bonding of urea as acidic center and primary amine of phenylene diamine as basic center within same a molecule,can effectively catalyze the aldol reaction of actone with p-nitrobenzaldehyde.We have demonstrated that a general acid group,urea,can activate substrates in cooperation with a general base group,primary amine,to catalyze aldol reaction that involve carbonyl activation. By fine-tuning the relative position and proper spatial arrangement of different cooperative functional groups,the functional catalysts could effectively catalyze in the aldol reaction of actone with p-nitrobenzaldehyde catalyzed by bifunctional solid catalyst.The high TOF value indicates a genuine and superior catalytic performance in comparison with those of other bifunctional catalysts between different molecules.We demonstrate the synthesis of bifunctional mesoporous catalysts containing a simple chiral primary-tertiary diamine/Brφnsted acid conjugates in combination with on the porous surface by postgrafting of chiral trans-N,N-dialkylated diaminocyclohexanes onto mesoporous materials.The bifunctional catalysts can catalyze effectively the asymmetric aldol reaction of acetone with various aldehydes.A confinement effect originating from mesoporous was observed in this reaction.The organic-inorganic hybrid catalysts were synthesized by covalent anchoring of chiral diamine in the nanopores of MCM-41 and SBA-15.Modified SBA-15 catalyst obtained higher ee value than that obtained for the same catalysts anchored on the silica gel and MCM-41.Because of the confinement effect of the nanopores,the enantioselectivity increase with increasing of nanopore size in the asymmetric aldol reaction.