The aminolysis and hydrolysis of N-(4,6-diphenoxy-1,3,5-triazin-2-yl) substituted pyridinium salts: Concerted displacement mechanism

Reaction of the title pyridinium salts in aqueous buffers containing amines involves nucleophilic displacement of the pyridine by the amine nitrogen in addition to background hydrolysis. The second-order rate constants for attack of a series of unhindered primary amines on the (4,6-diphenoxy-1,3,5-triazin-2-yl)pyridinium cation obey a Bronsted-like equation with slope beta(nuc) = 0.68. Six-membered-ring cyclic secondary amines have a similar Bronsted selectivity but are approximately 10-fold more reactive than primary amines of similar pK(a). The reactivity of ethanolamine against the title pyridinium salts obeys a Bronsted-like equation with beta(1g) = -0.47. In aminolysis of the triazinyl pyridinium salts the changes in bond fission and bond formation signalled by the polar substituent effects give Leffler alpha(1g) and alpha(nuc) values of 0.38 and 0.54, respectively, which indicate strong coupling between the bonding changes consistent with a concerted process for the displacement reaction. The adduct in the putative stepwise mechanism is considered too reactive to possess a significant barrier for it to exist as a discrete molecule.