Introduction

The major costs of asymmetric catalytic processes are usually related with the preparation of chiral catalysts and much efforts have been devoted to their structural modifications in order to make them more efficient, more recyclable and compatible with environmentally benign solvents. However, the same strategies developed for the optimization of catalyst performances in terms of sustainability can be also applied to other reaction components with the aim to reduce the amount of toxic or hazardous reagents and introduce simplification in the purification procedures, that in many case are a non-negligible source of solvent waste. As an example, in the asymmetric hydroxylation of alkenes, promoted by

A. Patti, Green Approaches To Asymmetric Catalytic Synthesis, SpringerBriefs in Green Chemistry for Sustainability, DOI: 10.1007/978-94-007-1454-0_4, © Angela Patti 2011

Cinchona alkaloid-osmium complexes, toxicity and volatility of the metal oxide and its possible contamination in the products represent serious obstacles for industrial applications, more than the cost and availability of the chiral ligands. Since the use of supported ligands in some cases has not resulted in satisfactory suppression of osmium leaching, due to the reversibility of metal-ligand coordination, the direct immobilization of osmium tetroxide by microencapsulation into polystyrenes or by electrostatic interactions with amino-functionalized silica or resins has been explored as an alternative strategy [1]. Labelling of substrates or reagents with removable fluorous or ionic liquid tags offers great potentiality in multistep as well as in combinatorial synthesis, allowing selective separation of final products from reaction mixtures by liquid-liquid or fluorous solid phase extraction [2, 3].

Other contributes to the improvement of synthetic efficiency could come from the use of microwave irradiation and the development of continuous-flow reactors, as technological tools for the enhancement of reaction rate and productivity. Although these technologies have been mainly focused on organic synthesis, the interest in their application for asymmetric transformations is growing in parallel with the increased availability of more stable and selective catalysts.

Besides the modifications of the different factors influencing the outcome of known asymmetric processes in a ''green direction'', the discovery of novel reactivity and new reaction design could lead to a substantial and innovative advance in this field of organic chemistry. In this context, the development of multicomponent and ''cascade'' reactions could open new scenarios in the synthesis of chiral compounds bearing multiple stereogenic centers, with a direct advantage in decreasing the tedious and expensive steps of purification and pro-tection/deprotection of the intermediates.

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