Fluorine-Substitution Effect in Intermolecular Stetter Reactions

摘要

The N-heterocyclic carbene (NHC)catalyzed intermolecular Stetter reaction of vari-ous heteroaromatic aldehydes with differentnitroalkenes is reported. The involved precatalyst,triazolium salt i , exhibits its properties due to acis-backbone fluorination. Compared to non-fluorinated NHC catalysts, the former outperformsthese in terms of reactivity and selectivity. X-raystudies of the discussed precatalysts showed,that 1 forms an exo-conformed pucker on thebackbone, thus enabling a superior fixation of thesubstrates in the transition state. This fairly unex-pected behavior suggests that the stereoelectron-ic effects of fluorine overrule its rather small stericimpact, thus dictating the favored conformation.