Gas-phase base-catalyzed Claisen-Schmidt reactions of the acetone enolate anion with various para-substituted benzaldehydes

Substituent effects were determined for the gas-phase base-catalyzed Claisen-Schmidt reaction of the acetone enolate anion and various para-substituted benzaldehydes. Under chemical ionization conditions, the adduct for the reaction was detected and the fraction of adduct that is tetrahedral was determined. The Hammett constants for the substituents correlate the fraction of the adduct population that is tetrahedral. The fraction of tetrahedral intermediate is greatest for those systems in which the negative charge is most highly stabilized. The structures of the adducts are determined on the basis of collisionally activated decomposition mass spectra. These spectra show that both the adducts of the ion-molecule reactions and deprotonated reference compounds, which have a structure that is similar to the tetrahedral intermediate, decompose by elimination of water and by a retro-aldol reaction. The adducts formed from the ion-molecule reactions show a greater propensity to reform the acetone enolate, whereas the deprotonated reference compounds eliminate H₂O readily. The reaction constant ρ from the Hammett correlation is +1.6, which substantiates that the production of tetrahedral intermediates is facilitated by electron-withdrawing substituents.