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 l-acetonyl-l,2-dihydro-2-methyl-5-nitroisoquinoline (2) have been evaluated over the pH range 10.0-1 1.3 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 1 I previously used for the synthesis of this adduct. Analogous data for the reaction of hexadeuteroacetone with this heterocyclic 1 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. Chem. 64,973 (1986) OpCrant en solutions aqueuses, a 25°C et dans un lntervalle de pH allant de 10,O 11,3, on a CvaluC les constantes de vitesse et les constantes d'kquilibre pour la rkaction entre lYacCtone et le catlon mkthyl-2 nitro-5 isoquinolkinium qui conduit a l'acktonyl-1

Rate constants for the addition of the enolate ion of acetone to quinolinium and acridinium cations

“…In such cases, even the structure assignments are sometimes uncertain. We have been unable to locate any report of a quantitative study of rate or equilibrium constants-for such reactions, other than our own earlier report (7) for the addition of the acetone enolate ion to the 2-methyl-5-nitroisoquinolinium cation.…”

“…The third-order rate constants were converted to second-order rate constants (k2 in Table 2) for the attack of the enolate ion on k , and k , are defined by eqs. [6] and [7].…”

“…the heterocyclic cation according to eq. [7], where pK, = 19.16 for acetone in aqueous solution (44), and K, is the ionic product of water.…”

“…For these two cations, kz can be apportioned using the ratio of 35% of C-2 adduct to 65% of C-4 adduct that is estimated above for these two cations. The true second-order rate constants (kNu) for enolate ion attack at individual sites in each of the cations under study are listed in Table 3 , along with literature data for the related hydroxide ion reactions, and also data from our earlier study (7) of acetone addition to the 2-methyl-5-nitroisoquinolinium cation (9). The 3-bromo-1-methylquinolinium cation is the only electrophile in the current study for which we were able to observe an equilibrium mixture containing significant amounts of both RC and R+.…”

“…We have demonstrated (7) that the rates and equilibria for the addition of the enolate ion of acetone to the 2-methyl-5nitroisoquinolinium cation can be readily measured in aqueous solution. As a follow up, we have now investigated the addition of this nucleophile to the 10-methylacridinium cation (I) and also to several C-3 substituted 1 -methylquinolinium cations (2).…”