Quaternary Nitrogen Heterocycles. V. Substituent Effects on the Equilibrium Constants for Pseudobase Formation from Quinolinium and Isoquinolinium Cations

Abstract

Equilibrium constants (pKROH) have been measured for pseudobase formation from the 1-methyl-x-nitroquinolinium cations (x = 5–8), the N,N′-dimethyl-1,5-, -1,6-, and -2,7-naphthyridinium dications and various N-substituted quinolinium, isoquinolinium, 5-nitroisoquinolinium, and 1,8-naphthyridinium cations. The pKROH values for N-substituted 5-nitroisoquinolinium and 1,8-naphthyridinium cations are correlated by the equations pKROH = −3.7σ* + 11.6 and pKROH = −4.9σ* + 12.5, respectively (σ* is Taft's substituent constant for the substituent on nitrogen).Proton magnetic resonance and u.v. spectral data have been used to assign the structures of the pseudobases formed from each of the above cations. In several cases ylide formation rather than pseudobase formation has been observed. The N,N′-dimethylnaphthyridinium dications are shown to form zwitterionic alkoxide ions in strongly basic aqueous solution, rather than undergoing attack by a second hydroxide ion.Equilibration between cation and pseudobase occurs at rates near or beyond the limit of the stopped-flow technique for all the above cations, except the 2-cyanomethyl-5-nitroisoquinolinium cation. An analysis of the pH-rate profiles for reversible pseudobase formation from this latter cation is given.