Rates of tritium exchange in a series of N-phenyl substituted pyrroles

Abstract

Rate constants k^H₃O+_exch for the H⁺-catalysed protiodetritiation of a number of N-phenyl substituted 2,5-dimethyl-3,4-[³H₂]pyrroles have been measured in aqueous solution. For the series of N-(para-X-phenyl substituted) derivatives studied (1a–h), the rate constants are found to obey a satisfactory linear Hammett relationship with a negative reaction constant ρ of –0.90. The low ρ value is attributed to reduced transmission of the electronic influence of the X substituent caused by steric hindrance to conjugation between the phenyl and pyrrole rings. While decreasing the thermodynamic basicity at β-C of the pyrrole ring, ortho-dimethylation of the phenyl ring increases appreciably the rate of exchange, consistent with the idea that the resulting tetramethyl-substituted pyrrolium cation but not the transition state of reaction, is subject to inhibition of solvation of positive charge which develops on the nitrogen atom. Methoxy substitution of the phenyl ring gives results which reflect a balance between the opposite resonance and inductive effects of the OCH₃ group, as well as steric effects around the methoxy-bearing positions.