Effects of internal and external carboxylic acids on the reaction pathway of organocatalytic 1,4-addition reactions between aldehydes and nitroolefins

Kinetic and NMR spectroscopic studies revealed that the reaction pathway of conjugate addition reactions between aldehydes and nitroolefins depends on the presence or absence of a suitably positioned carboxylic acid group within the catalyst. The intermediate nitronate is trapped intramolecularly either by protonation (in the presence of a well positioned intramolecular carboxylic acid) or by C-C bond formation to a cyclobutane intermediate (in the absence of an intramolecular proton donor). These differences in the reaction pathway are reflected in different rate limiting steps of the reaction. The studies demonstrated that the preferred reaction pathway and thereby the rate limiting steps of the reaction can be influenced by additives of different acidity or the position of an intramolecular carboxylic acid group within the catalyst.