Concerted displacement mechanisms at trigonal carbon: the aminolysis of 4-aryloxy-2,6-dimethoxy-1,3,5-triazines

Abstract

4-Aryloxy-1,3,5-triazines undergo bimolecular nucleophilic displacement reactions with amines and pyridines to yield the 4-substituted triazine and aryloxide ion. Rate constants in aqueous solution for the bimolecular reaction of morpholine and 4-dimethylaminopyridine with the title ethers obey the Hammett σ equations with Hammett ρ_lg values 1.65 and 0.82, respectively. Comparison of the ρ_lg values with the Hammett ρ_eq for the equilibrium constants indicates that build-up of effective charge on the departing ether oxygen in the transition structure is less than half of that for complete bond fission. Rate constants for the reaction of substituted pyridines with the 4-nitro- and 3,4-dinitro-phenyl ethers obey Brønsted equations with exponents β_nuc of 0.68 and 1.06, respectively. The build-up of effective charge in bond formation is greater than half of that expected for complete bond formation. Variation in β_nuc and ρ_lg as a function of leaving group and nucleophile structure, respectively, is consistent with substantial coupling between bond forming and bond breaking. The ratio of the Leffler exponents in the pyridinolysis reactions, α_nuc/α_lg, is greater than unity, consistent with an imbalance between bond fission and bond formation and indicating an accumulation of negative charge in the heteroaromatic nucleus in the transition structure 29% of that expected for adduct formation.