Introduction

Solvent plays an important role in asymmetric catalytic synthesis taking part in the formation of several transition-metal complexes, stabilizing the reaction intermediates by non-covalent interactions, dissipating heat in exothermic reactions and ensuring the optimal dissolution of all the system components in a homogeneous phase. As a consequence, the solvent can markedly affect both

A. Patti, Green Approaches To Asymmetric Catalytic Synthesis, SpringerBriefs in Green Chemistry for Sustainability, DOI: 10.1007/978-94-007-1454-0_3, © Angela Patti 2011

chemical yield and selectivity for a given reaction and evidences of ''solvent effects'' have been also useful in the mechanism elucidation. Solubility and stability of the majority of organic compounds are compatible with organic solvents, that have been extensively used also in the purification of products and represent about 85% of the total mass of chemicals in manufacture of pharmaceuticals leading to the largest contribution to the E-factor magnitude. Toxicity, volatility and flammability of many organic solvents constitute a source of hazards for workers and atmospheric pollution and even polar high boiling solvents such as dimethylformamide or dimethylsulphoxide present environmental concerns since they can not be easily removed by distillation and their separation by washing otherwise affords contaminated aqueous effluent.

Under the more stringent request of ''green reactions'' many industrial processes have been reconsidered using less volatile solvents or in minimized amounts [1] as well as more biodegradable or natural-derived solvents as etha-nol, limonene and ethyl lactate but there is a growing academic and industrial research toward non-conventional and more benign alternatives. Options based on the use of non-toxic water or recyclable ionic liquids and fluorinated solvents as well as the development of solvent-free reactions have been explored in chemical and asymmetric synthesis [2]. The search for sustainable solvents has been intimately connected with the development of effective protocols, based on liquid-liquid separations, for selective recovery and reuse of catalysts [3], in many cases specifically designed in order to be compatible with non-conventional reaction media saving activity. Catalysts heterogenized on insoluble materials have been classically employed to be recovered from organic solutions by solid-liquid separations, but selective precipitation or ultrafiltration of catalysts immobilized on soluble macromolecular supports has been considered a useful alternative.

This chapter is focused on a general overview of ''green'' solvents and different techniques of catalyst recycle specifically applied to enantioselective transformations.

0 0

Post a comment