Environmentally friendlier organic transformations on mineral supports under non-traditional conditions

Synthetic organic reactions performed under non-traditional conditions are gaining popularity, primarily to circumvent growing environmental concerns. A solvent-free approach that involves microwave ( MW) exposure of neat reactants ( undiluted) catalyzed by the surfaces of less expensive and recyclable mineral supports, such as alumina, silica, clay, or doped surfaces, is described which is applicable to a wide range of deprotection, condensation, cyclization, rearrangement, oxidation, and reduction reactions, including rapid one-pot assembly of heterocyclic compounds from in situ-generated reactive intermediates. The strategy is adaptable to multi-component reactions for rapid assembly of a library of compounds. The application of microwaves and ultrasound as successful alternative energy sources is described for the selective epoxidation of alkenes and alpha,beta-unsaturated ketones over hydrotalcites as catalysts. The ability of green solvents such as supercritical (sc) CO2 to dissolve many reactive gases like H-2 and O-2, and also a variety of organic compounds, can be exploited to facilitate many important industrial transformations in this medium, wherein the improved reactant solubility and minimized interphase mass-transfer limitations lead to enhanced reaction rates and unusual product selectivity. Consequently, the use of sc-CO2 as an attractive medium for the selective hydrogenation of maleic anhydride and alpha,beta-unsaturated aldehydes, such as cinnamaldehyde, over supported metallic catalysts (Pd/Al2O3) is illustrated. Furthermore, recent developments in the areas of microwave or ultrasound-expedited reactions, and the use of supercritical CO2 in organic transformations are also reviewed. The salient eco-friendly features of these processes, namely the selectivity, the ease of experimental manipulation, and the enhanced reaction rates, are highlighted. Use of the above non-traditional methods promises to overcome many of the difficulties associated with conventional reactions and offers both process and environmental advantages in many cases.