Peculiarities of N-Vinylpyrroles Protonation with Hydrogen Halides

3 Peculiarities of N-Vinylpyrroles Protonation with Hydrogen Halides 

The formation of N-(l-haloethyl)pyrrolium halides 3 from vinylpyrrolium 1 proceeds slower if high excess of hydrogen halide (HX) is present in the solution [418]. All these features are obviously due to the interrelation of kinetic and thermodynamic factors. Primary (kinetic) protonation at low temperatures leads to the formation of a-protonated forms 1 that are converted (at higher temperatures) to thermodynamically more stable uncharged adducts, N-(l-haloethyl)pyrroles 4. 

SCHEME 2.4 Protonation of the N-vinylpyrroles with hydrogen halides. 

The latter in the acid excess are again protonated at the a-position to generate cations 3. Free hydrogen halide in solution hinders deprotonation (owing to binding of the anion X to less basic and nucleophilic complex anion HX2) [423]. This explains the observed inverse dependence between HX concentration and rate of formation of N-(l-haloethyl)pyrrolium halides 3 [21,24,418]. 

---