Chiral 2-Aminobenzimidazoles as Recoverable Organocatalysts for the Addition of 1,3-Dicarbonyl Compounds to Nitroalkenes^†

Abstract

Chiral trans-cyclohexanediamine-benzimidazole organocatalysts promote the conjugate addition of a wide variety of 1,3-dicarbonyl compounds such as malonates, ketoesters, and 1,3-diketones to nitroolefins in the presence of TFA as cocatalyst in toluene as solvent at rt or 0 degrees C. The Michael adducts are obtained in high yield and enantioselectivity, using the chiral 2-aminobenzimidazole 7b as hydrogen-bond-mediated chiral organocatalyst. This catalyst can be recovered by acid-base extractive workup in 94% yield. The proposed bifunctional Brønsted acid-base activation role of the catalyst and the origin of the stereoselectivity of the process is in agreement with DFT calculations. According to these calculations, the protonated tertiary amine from the cyclohexanediamine backbond activates the nitroolefin, while the benzimidazole unit activates the 1,3-dicarbonyl nucleophile.