Fluoroboric acid adsorbed on silica-gel (HBF4–SiO2) as a new, highly efficient and reusable heterogeneous catalyst for thia-Michael addition to α,β-unsaturated carbonyl compounds

Fluoroboric acid adsorbed on silica-gel (HBF4–SiO2) has been found to be a new and highly efficient heterogeneous catalyst for thia-Michael addition to α,β-unsaturated carbonyl compounds under solvent-free conditions. In the case of 1,3-diaryl-2-propenones, the reactions are best carried out in MeOH. The rate of thia-Michael addition was dependent on the steric hindrance at the β-carbon of the α,β-unsaturated carbonyl substrate as well as surrounding the thiol moiety and was exploited for selective thia-Michael addition during intermolecular competition between two enones with a common thiol and between two aryl/alkyl thiols for a common enone. The methodology finds application for one-pot syntheses of 2,3-dihydro-1,5-benzothiazepines.