Sequential displacement of chloride in N-aryl-and N-aroyl-carbonimidoyl dichlorides

Abstract

Hydrolysis of carbonimidoyl dichlorides (1) occurs via base catalysed and uncatalysed pathways in 1:4 dioxanwater at 25°. Hammett ρ values measured under basic (ρ+1.26) and neutral (ρ–2.30) conditions indicate that the former involves direct attack by hydroxide ion on the substrate while in the latter unimolecular C–Cl bond cleavage (to give an azocarbonium ion intermediate) occurs. The observed solvent and common ion effects support this. The products of hydrolysis are the corresponding amines except at High pH where the decarboxylation of N-arylcarbamates formed as intermediate is slow. The reaction of (1) with morphine is S_N2 to give the N-inminomethylmorpholine (2; Y = morpholino)initially; however, the intermediate (2; Y = morpholino) undergoes further hydrolysis (by an S_N1 mechanism) more rapidly than the initial bimolecular step in all the dioxanwater solvent mixtures studied. N-Aroyl dichlorides (9) undergo nucleophilic displacement ca. 10³-fold more rapidly than (1) while the rate of the subsequent S_N1 step is reduced ca. 10⁶-fold due to the destabilization of azocarbonium ion formation by the neighbouring carbonyl group. In general then at high nucleophile concentration and in solvens of low dielectric constant the intermediates (14) may be isolated (Y = 2, NMe, or O). The dichloride (9) also shows a pH independent rate of hydrolysis at low pH but this is attributed to nucleophilic attack by solvent water, representing the first time that this has been observed in an imidoyl halide system.