Selective Activation of Fluoroalkenes with N-Heterocyclic Carbenes: Synthesis of N-Heterocyclic Fluoroalkenes and Polyfluoroalkenyl Imidazolium Salts

Abstract

Selective reactions between nucleophilic N,N′-diaryl-heterocyclic carbenes (NHCs) and electrophilic fluorinated alkenes afford NHC fluoroalkenes in high yields. These stable compounds undergo efficient and selective fluoride abstraction with Lewis acids to give polyfluoroalkenyl imidazolium salts. These salts react at Cβ with pyrrolidine to give ammonium fluoride-substituted salts, which give rise to conjugated imidazolium-enamine salts through loss of HF. Alternatively, reaction with 4-(dimethylamino)-pyridine provides a Cα-pyridinium-substituted NHC fluoroalkene. These compounds were studied using multinuclear NMR spectroscopy, mass spectrometry, and X-ray crystallography. Insight into their electronic structure and reactivity was gained through the use of DFT calculations. The unusual suspects: N,N′-Diaryl-heterocyclic carbenes react with fluorinated alkenes to afford N-heterocyclic fluoroalkenes in high yields. Efficient and selective fluoride abstraction with Lewis acids affords the first examples of polyfluoralkenyl imidazolium salts. These salts react with pyrrolidine and DMAP through different pathways to provide unique compounds.