Molecules 2009, 14, 4779-4789; doi:10.3390/molecules14114779 molecules ISSN 1420-3049 www.mdpi.com/journal/molecules Article Silica Sulfuric Acid Promotes Aza-Michael Addition Reactions under Solvent-Free Condition as a Heterogeneous and Reusable Catalyst Yan Wang, Yan-Qin Yuan and Sheng-Rong Guo * Department of Chemistry and Biology, Lishui University, Lishui, Zhejiang 323000, China * Author to whom correspondence should be addressed; E-Mail: guosr9608@163.com. Received: 3 November 2009; in revised form: 19 November 2009 / Accepted: 23 November 2009 / Published: 23 November 2009 Abstract: A highly efficient, inexpensive, recyclable, convenient, and green protocol for chemoselective aza-Michael addition reactions of amines/thiols to α,β-unsaturated compounds using silica sulfuric acid (SSA or SiO 2 -SO 3 H) was developed. This method is simple, convenient and the title compounds are produced in good to excellent yields. Keywords: amines; thiols; silica sulfuric acid (SSA); Michael reaction; α,β-unsaturated olefins Introduction The Michael reaction has been studied for over a century. The conjugate addition of amines to carbon–carbon double bonds is a useful protocol in synthetic organic chemistry [1–4]. It is used extensively in the synthesis of pharmaceutical intermediates, peptide analogues, antibiotics, and other biologically active molecules and drugs [5–10]. In the past few years, a number of alternative procedures have been developed for the conjugate addition of amines to α,β-unsaturated carbonyl compounds. In particular, various Lewis acid catalyzed reactions have been reported. This reaction has been investigated using catalysts such as lanthanum trichloride (LaCl 3 ) [11], bromodimethylsulfonium bromide [12], silica supported perchloric acid [13], cerium(IV) ammonium nitrate (CAN) [14,15], β-cyclodextrin [16], zirconium(IV) chloride [17], samarium(III) triflate [18], ZrOCl 2 ·8H 2 O on montmorillonite K 10 [19], N-methylimidazole [20], Cu(acac) 2 /ionic liquid [21], 1,8-diazabicyclo[5.4.0] OPEN ACCESS