Condensation of 2-methylquinoline with benzaldehydes in acetic anhydride

Condensation of 2-methylquinoline with benzaldehydes to give trans-β-(2-quinolyl)styrenes has been studied kinetically in acetic anhydride containing acetic acid at 130°. The reaction proceeds via addition (k ca. 10–4 l mol–1 s–1 at 130°), esterification (k ca. 10–4 l mol–1 s–1 at 55°), and elimination (k ca. 10–4 s–1 at 30°). The intermediates, four 1-aryl-2-(2-quinolyl)ethanols and one acetate, were isolated and their rates of elimination were measured separately at 55 and 30°. The addition is rate-determining and the elimination is the most rapid step in the presence of acetic acid. The acetic acid-catalysed addition is promoted by electron-withdrawing groups on the aldehyde, with a Hammett ρ value of +1·42. The esterification of the intermediate alcohols with acetic anhydride is also accelerated slightly by electron-withdrawing groups, with a ρ value of +0·4. The elimination of the acetate is catalysed by carboxylic acids in the order chloroacetic > benzoic > acetic acid. The dependence of the rate of elimination on concentration of acetic acid changes from first order at lower concentrations to zero order at higher concentrations. A plausible mechanism involving 2-methylene-1,2-dihydroquinoline as an intermediate is suggested and discussed.