Addition of Hydrogen Halides to the Pyrrole Ring

Obviously, here there is a formation of covalent bonding between halogen anion and positively charged carbon atom C4 with simultaneous addition of the HX molecule across the vinyl group. The intermediate pyrroline is protonated by the third HX molecule generating dihydropyrrolium cation 5. 

It is intriguing that dihydropyrrolium cafions are not detected in the case of the corresponding NH-pyrroles. It can be explained by the inaease of electrophilicity of the pyrrole ring after the introduction of acceptor substituent CHXMe. Also, the addition of HX to the pyrrole ring is not obsCTved for 3-alkyl-N-vinylpyrroles, likely due to electron-donating effect of the alkyl substituent lowering the electrophilicity of the pyrrole ring. 

As far as regioselectivity and structure of the dihydropyrrolium cations 5 are concerned, the reactions of 2-substituted N-vinylpyrroles with HX (X=C1, Br) give the same ontcomes. This is confirmed by nearly complete concordance of their H NMR parameters including spin-spin coupling constants. The main distinction is that the addition of HBr is almost exhaustive, whereas for HCl, owing to weaker nucleophi-licity of chloride ion, the equilibrium is shifted toward pyrrolium cation 3. 

contrary to the common opinion about the inability of the pyrrole cycle to the addition reactions, the results discussed previously testify that in some special cases, such reactions are possible. 

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