Efficient Tertiary Amine/Weak Acid Bifunctional Mesoporous Silica Catalysts for Michael Addition Reactions

The Michael reaction, which involves 1,4-addition of a nucleophile (preferably a C-nucleophile) to an a,b-unsaturated compound, offers a simple yet effective way of making C C bonds, which makes it one of the most widely used reactions in both industry and academia. The reaction, as first reported by Arthur Michael in 1887, involves primarily the attack of a Cnucleophile, generated from compounds with active methylene groups, on the b position of unsaturated carbonyl compounds, giving the corresponding enol/enolate, followed by subsequent protonation and tautomerization of the enol to its keto form, yielding the so-called 1,4-addition product. Thus, the customary steps in the reaction include the deprotonation of the parent active methylene compounds with basic reagents and the protonation of enolate with either the conjugate acid of that same base or the solvent. The ease of the reaction and the involvement of a base and the corresponding conjugate acid to catalyze the reaction have made the development of efficient acid or base catalysts, which can catalyze the reactions reversibly and effectively, the subject of intense research in catalysis. The concept has been applied to this reaction (in both stereoselective and non-stereoselective manner) by using metal-based Lewis acid catalysts and organocatalysts. Compared with organometallic catalytic systems, organocatalysts offer advantages because most of them are easy to handle, are stable under many different synthetic conditions, and have highly tunable synthetic skeletons. In this particular context of Michael addition reaction, the most common organocatalytic systems are derived from organoamines because they have been found to be basic enough to take up the protons from the active methylene groups and generate carbanions, and their conjugate acids (ammonium ions) are proven to be capable of delivering the protons back to the enolate at the end of the catalytic cycle. Alternating modes of activation, that is, either activation of carbonyl using acid catalysts or use of highly reactive organometallic reagents to form the nucleophile, have also been used to make some of the most important synthetic skeletons. However, until reWe describe the development and application of efficient bifunctional acid–base mesoporous silica catalysts, denoted hereafter as Ext-SBA-15-NMe2, comprising tertiary amine and silanol (weak acid) groups for the Michael addition reaction. The catalysts were synthesized by grafting tertiary amine containing organosilane into the mesoporous channel pores of SBA15 mesoporous silica in polar protic solvent (isopropyl alcohol) or nonpolar solvent (toluene). The resulting materials, Ext-SBA15-NMe2-IPA or Ext-SBA-15-NMe2-Tol, respectively, were used as catalysts for the Michael addition reactions between trans-b-nitrostyrene(s) and active methylene compounds, such as malononitrile, acetylacetone, and diethylmalonate, at different temperatures. A variable-temperature NMR-array technique was used to monitor the reactant conversion and the reaction progress. Among the active methylene compounds tested with trans-b-nitrostyrene, malononitrile gave the highest conversion of 90% in 0.3 h at 0 8C if catalyzed by the catalyst ExtSBA-15-NMe2-IPA, the silanol groups of which were not passivated by trimethysilyl ( SiMe3) groups. Compared with ExtSBA-15-NMe2-Tol, the Ext-SBA-15-NMe2-IPA material (whose the amine groups were grafted in polar-protic solvents), in particular, demonstrated an effective cooperative bifunctional catalysis because of its optimized proportions of tertiary amine groups and a significant number of surface silanol groups that were judiciously left behind on the material with this optimized synthetic strategy. The catalytic activity of this material was found to be significantly higher than that of the corresponding material that was grafted in toluene and that contained less optimized proportions of these two catalytic groups (i.e. , amine and silanol groups). The bifunctional catalysts also showed good recyclability upon washing with acetone, which offered an effective way of regenerating the catalyst free from any undesired product/substrate bound on the surface of the catalysts.