Kinetic studies on enamine catalysis provided insight into the rate determining step(s) of peptide catalyzed conjugate addition reactions between aldehydes and nitroolefins. They demonstrate that not enamine formation but both the reaction of the enamine with the electrophile and hydrolysis of the resulting imine are rate limiting. These results allowed for reducing the catalyst loading by a factor of 10 to as little as 0.1 mol %. This is the lowest catalyst loading that has been achieved so far in enamine catalysis with low molecular weight catalysts for a broad range of substrates.

Analysis of the structural and functional requirements within the asymmetric peptidic catalyst H-D-Pro-Pro-Asp-NH(2) led to the development of the closely related peptide H-D-Pro-Pro-Glu-NH(2) as an even more efficient catalyst for asymmetric conjugate addition reactions of aldehydes to nitroolefins. In the presence of as little as 1 mol % of H-D-Pro-Pro-Glu-NH(2), a broad range of aldehydes and nitroolefins react readily with each other. The resulting gamma-nitroaldehydes were obtained in excellent yields and stereoselectivities at room temperature. Within the structure of the peptidic catalysts, the D-Pro-Pro motif is the major contributor to the high stereoselectivities. The C-terminal amide and the spacer to the carboxylic acid in the side-chain of the C-terminal amino acid are responsible for the fine-tuning of the stereoselectivity. The peptidic catalysts not only allow for highly effective asymmetric catalysis under mild conditions, but also function in the absence of additives.

The peptide H-D-Pro-Pro-Glu-NH2 is a highly effective catalyst for conjugate addition reactions between aldehydes and nitroethylene. Only 1 mol % of H-d-Pro-Pro-Glu-NH2 and a 1.5-fold excess of aldehyde with respect to nitroethylene suffice to obtain gamma-nitroaldehydes and, after reduction, monosubstituted gamma-nitroalcohols in excellent yields and optical purities. The products can be readily converted into gamma2-amino acids, thereby opening an effective direct entry into this important class of compounds.

Bridging the gap: Conformational analysis of a peptidic catalyst for aldol reactions led to the development of H‐D‐Pro‐Pro‐Asp‐NH2 as a highly efficient catalyst for conjugate addition reactions between aldehydes and nitroolefins (see scheme). Only 1 mol % of catalyst suffices to obtain γ‐nitroaldehydes in excellent yields and stereoselectivities.

The rate determining steps of organocatalytic conjugate addition reactions between aldehydes and nitroolefins depend on the presence or absence of a suitably positioned carboxylic acid moiety within the catalyst.