Global Electrophilicity Study of the Reaction of Pyrroles with N-Halo Compounds and the Rate-Determining Step

Pyrroles (nucleophiles) can react with N-halo compounds (electrophiles) to give either addition-elimination or halogenation products. Global electrophilicity (w) has been used to study electrophile-nucleophile combinations. The w values of the N-halo compounds (n = 22) used were calculated using DFT/B3LYP. No correlation was observed between w and the pathway of reaction/non-reaction. A single parameter (w) could not explain the results of the competing reactions that are taking place in this system–reactions that appear to depend on the N-halogen (Cl, Br, I) nucleophilicity/basicity and possibly hardness of the leaving group. These calculations confirmed that the rate-determining step of the addition-elimination process was not the formation of an sigma-complex, but its subsequent reaction. Calculations suggested that deprotonation of the sigma-complex was the rate-determining step in the halogenation reactions. It was also possible to examine the effects of the structure of the N-halo-compound and the halogen.