Table 1 shows that increasing catalyst amount initially led to high yield at short reaction time. However, 5 mol% of the catalyst was enough for the reaction. With more catalyst (10 mol%), the yield progressed very smoothly to 93% without decreasing the reaction time (entry 5). Even more catalyst (>10 mol%) not only failed to improve the yield, but also decreased the efficiency of the catalytic system (Fig. 1). This behavior was explained by the changes in polarity, pH, and components of the reaction mixture in the solvent-free system. Thus, 5 mol% of NH4H2PO4 was selected for all the reactio