One central challenge for XXI century chemists is the development of sustainable processes that do not represent a risk either to humanity or to the environment. In this regard, the search for more efficient and clean alternatives to achieve the chemical activation of molecules involved in chemical transformations has played a prominent role in recent years. The use of microwave or UV-Vis light irradiation, and mechanochemical activation is already widespread in many laboratories. Nevertheless, an additional condition to achieve “green” processes comes from the point of view of so-called atom economy. The removal of solvents from chemical reactions generally leads to cleaner, more efficient and more economical processes. This review presents several illustrative applications of the use of sustainable protocols in the synthesis of organic compounds under solvent-free reaction conditions.
A convenient synthetic procedure for the preparation of chiral thiohydantoins derived from natural α‐amino acids is reported. Salient features of this method include ready availability of the starting materials, mild reaction conditions, and high yields of the desired products. The prepared thiohydantoins were tested as organocatalysts in asymmetric Michael addition reactions, under solvent‐free reaction conditions. Thiohydantoin 10 d turned out to be the most efficient catalyst, and its catalytic activity can be significantly enhanced by the use of an equimolar amount of salicylic acid as additive. Furthermore, highest diastereo‐ and enantioselectivity is achieved when the reaction is carried out at −5.5 °C, without detriment of the reaction rate. Regarding the reaction’s scope, the asymmetric Michael reaction catalyzed by 10 d was found to maintain its efficiency with various Michael acceptors.
The application of six novel α,β-dipeptides as chiral organocatalysts in the asymmetric Michael addition reaction between enolizable aldehydes and N-arylmaleimides or nitroolefins is described. With N-arylmaleimides as substrates, the best results were achieved with dipeptide 2 as a catalyst in the presence of aq. NaOH. Whereas dipeptides 4 and 6 in conjunction with 4-dimethylaminopyridine (DMAP) and thiourea as a hydrogen bond donor proved to be highly efficient organocatalytic systems in the enantioselective reaction between isobutyraldehyde and various nitroolefins.
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