Fluorine-Substitution Effect in Intermolecular Stetter Reactions

Significance

The N-heterocyclic carbene (NHC) catalyzed intermolecular Stetter reaction of various heteroaromatic aldehydes with different nitroalkenes is reported. The involved precatalyst, triazolium salt 1, exhibits its properties due to a cis-backbone fluorination. Compared to non-­fluorinated NHC catalysts, the former outperforms these in terms of reactivity and selectivity. X-ray studies of the discussed precatalysts showed, that 1 forms an exo-conformed pucker on the backbone, thus enabling a superior fixation of the substrates in the transition state. This fairly unexpected behavior suggests that the stereoelectronic effects of fluorine overrule its rather small steric impact, thus dictating the favored conformation.