A rate and equilibrium study of the addition of acetone enolate ion to the 2-methyl-5-nitroisoquinolinium cation in aqueous solution

Abstract

Rate and equilibrium constants for the reaction between acetone and the 2-methyl-5-nitroisoquinolinium cation to give 1-acetonyl-1,2-dihydro-2-methyl-5-nitroisoquinoline (2) have been evaluated over the pH range 10.0–11.3 in aqueous solutions at 25 °C. This reaction is shown to occur under much milder conditions (temperature, pH) and in much shorter reaction times than previously used for the synthesis of this adduct. Analogous data for the reaction of hexadeuteroacetone with this heterocyclic cation are also presented. The formation of 2 is shown to be first order in each of acetone, isoquinolinium cation, and hydroxide ion, and is not catalyzed by carbonate buffer species. These data are consistent with the rate-determining attack of the acetone enolate anion upon the isoquinolinium cation. The microscopic reverse of this reaction is the uncatalyzed decomposition of 2, which is consistent with the observed pH independence of the first-order rate constant for this process. Quantitative comparisons of rates and equilibria for the addition of hydroxide ion and acetone enolate ion to this isoquinolinium cation and to substituted benzaldehydes are now available.