α-Sulfanyl and α-Selanyl Propadienyl Cations: Regioselective Generations and Cycloadditions with Thioamides and Selemides Controlled by MeNO<sub>2</sub>−H<sub>2</sub>O System
α-Sulfanyl and α-selanyl propadienyl cations were easily generated by the catalytic system, scandium triflate-nitromethane-H2O in the presence of Bu4NHSO4, to regioselectively afford the multifunctionalized thiazoles and selenazoles in high yields.