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

Abstract

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 α,β-unsaturated ketones over hydrotalcites as catalysts. The ability of “green” solvents such as supercritical (sc) CO2 to dissolve many reactive gases like H2 and O2, 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 α,β-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.