PS-BEMP-catalyzed solvent-free Knoevenagel condensation under microwave activation: Extension to weakly acidic β-sulfuramides

The Knoevenagel condensation is a fundamental transformation for carbon–carbon bond formation. Herein, we report a polystyrene-supported 2-tert-butylimino-2-diethylamino-1,3-dimethylperhydro-1,3,2-diazaphosphorine (PS-BEMP)-catalyzed, solvent-free Knoevenagel protocol conducted at room temperature for highly activated substrates and under microwave-assisted conditions for less reactive active methylene compounds. Aromatic and heteroaromatic aldehydes reacted with a range of active methylene partners to afford the corresponding α,β-unsaturated products in isolated yields of up to 98% within 4–15 min. Most products were isolated after catalyst filtration and solvent evaporation without column chromatography. The protocol was further extended to weakly acidic β-sulfuramides (pKₐ ≈ 23), providing sulfur-containing vinyl derivatives in isolated yields of 21–90%, including several previously unreported compounds. PS-BEMP was recovered and reused over five cycles with only a moderate decrease in catalytic performance. However, no conversion was observed with selected highly stabilized 1,3-dicarbonyl substrates, including diethyl malonate, ethyl and tert-butyl acetoacetates, and 2,4-pentanedione, thereby defining a limitation of the methodology.