Microwave-induced conversion of stabilized phosphorus ylides to electron-poor 2H-chromenes in the presence of silica gel powder in solvent-free conditions

Protonation of the highly reactive 1:1 intermediates, produced in the reaction between triphenylphosphine and dialkyl acetylenedicarboxylates, by cyclohexan-1,3-diones leads to vinyltriphenylphosphonium salts, which undergo a Michael addition reaction with a conjugate base to produce stabilized phosphorus ylides. Silica-gel powder was found to catalyze the conversion of the stabilized phosphorus ylides to corresponding electron-poor 2H-chromenes under solvent-free conditions under microwave irradiation (0.72 KW, 9 min) or thermal conditions (95 degrees C, 45 min) in fairly good yields.