Benzoin solid support

The synthesis of 2,4,5-triarylimidazoline derivatives from aldehydes and alumina-supported ammonium acetate under solvent-free conditions using MW irradiation has been explored by Kaboudin and Saadati [184]. A similar solid-supported route from benzoin, aldehydes, and ammonium acetate, possibly involving air as an oxidant, had been reported earlier (Scheme 8.73) [185]. 

Leznoff et al. described the use of polymer supports for the mono-protection of symmetrical dialdehydes, describing oxime formation, H img-reaction, crossed aldol-condensation, benzoin-condensation and Grignard- reaction on solid support. 

Clayfen (0.125 g) was thoroughly mixed with neat benzoin lc (0.106 g, 0.5 mmol) in the solid state using a vortex mixer and the material was placed in an alumina bath inside the microwave oven and irradiated. Upon completion of the reaction, monitored on TLC (hexane-AcOEt, 10 1, v/v), the product 2c was extracted into methylene chloride. That the effect is not purely thermal is supported by the fact that this reaction could be completed in 18 h in an oil bath at a comparable temperature of 65 °C. 

Copper(II) sulfate supported on alumina has been sucessfully used to catalyse the oxidation of benzoins under microwave.58 Microwave radiation can drastically reduce the reaction time and enable milder conditions to be used in a wide range of reactions catalysed by solids. Full industrial exploitation is, however, yet to be realised. 

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