Silylated Pyrrolidines as Catalysts for Asymmetric Michael Additions of Aldehydes to Nitroolefins

Recent Advances in Asymmetric Synthesis of Pyrrolidine-Based Organocatalysts and Their Application: A 15-Year Update

In 1971, chemists from Hoffmann-La Roche and Schering AG independently discovered a new asymmetric intramolecular aldol reaction catalyzed by the natural amino acid proline, a transformation now known as the Hajos-Parrish-Eder-Sauer-Wiechert reaction. These remarkable results remained forgotten until List and Barbas reported in 2000 that L-proline was also able to catalyze intermolecular aldol reactions with non-negligible enantioselectivities. In the same year, MacMillan reported on asymmetric Diels-Alder cycloadditions which were efficiently catalyzed by imidazolidinones deriving from natural amino acids. These two seminal reports marked the birth of modern asymmetric organocatalysis. A further important breakthrough in this field happened in 2005, when Jørgensen and Hayashi independently proposed the use of diarylprolinol silyl ethers for the asymmetric functionalization of aldehydes. During the last 20 years, asymmetric organocatalysis has emerged as a very powerful tool for the facile construction of complex molecular architectures. Along the way, a deeper knowledge of organocatalytic reaction mechanisms has been acquired, allowing for the fine-tuning of the structures of privileged catalysts or proposing completely new molecular entities that are able to efficiently catalyze these transformations. This review highlights the most recent advances in the asymmetric synthesis of organocatalysts deriving from or related to proline, starting from 2008.

Asymmetric Redox-Annulation of Cyclic Amines

Cyclic amines such as 1,2,3,4-tetrahydroisoquinoline undergo regiodivergent annulation reactions with 4-nitrobutyraldehydes. These redox-neutral transformations enable the asymmetric synthesis of highly substituted polycyclic ring systems in just two steps from commercial materials. The utility of this process is illustrated in a rapid synthesis of (−)-protoemetinol. Computational studies provide mechanistic insights and implicate the elimination of acetic acid from an ammonium nitronate intermediate as the rate-determining step.

THE REACTION MECHANISMS OF ALDEHYDES AND NITROSTYRENE CATALYZED BY A CHIRAL SILYLATED PYRROLIDINE CATALYST

The asymmetric Michael reaction of aldehydes and nitrostyrene catalyzed by a new (S)-tertbutyl-diphenyl-silyl-pyrrolidine catalyst has been investigated by using density functional theory calculations. The Re face of the enamine is effectively shielded, because of the bulky 2-substituent group on the pyrrolidine ring. For acetaldehyde, there are two different conformers of enamines. Based on the two conformers of enamines, four different reaction pathways have been considered. The calculated enantiomeric excess value is 80.27% in favor of the (R)-configuration product. For propanal, eight different reaction pathways have been considered and the eight corresponding transition states have been located. The calculated enantiomeric excess value is 98.96% in favor of the (2S, 3R)-configuration product. These calculated results are in good agreement with the experimental observations. In addition, the calculations also show that both the used solvent and the enamines play important roles in determining the stereochemical outcome of the product.