The scale-up of microwave flow syntheses by recirculation: the chlorine-free preparation of alkyl phenyl-H-phosphinates†

In microwave (MW)-assisted flow syntheses, the size, and hence, the volume of the reactor may be a limiting factor. In this paper, we introduce a novel nonlinear accomplishment by applying recirculation within the flow system. In this way, higher conversions were attained even with a 10 mL reactor cell recirculating larger volumes (25–100 mL) of the reactants. The model reaction was the ionic liquid-catalyzed direct esterification of phenyl-H-phosphinic acid with butyl alcohol. The effect of the flow rate, the absence or presence of the catalyst, and the temperature and volume of the reaction mixture on the conversion were studied in detail. Preparative yields of 64–72% for the butyl phenyl-H-phosphinate with a 5.9–8.6 g h⁻¹ productivity were obtained. Comparative thermal experiments confirmed the special role of MW irradiation. The method was then extended to esterification with other alcohols.