Solid state and dynamic solution structures of O-carbamidine amidoximes gives further insight into the mechanism of zinc(II)-mediated generation of 1,2,4-oxadiazoles

Abstract

Three new iminium salts [H2N=C(R)ON=C(R’)NH2](p-TolSO3)•½H2O ([1–3](p-TolSO3)•½H2O; R/R’ = NMe2/PhCH2 1, NMe2/p-BrC6H4 2, N(CH2)5/p-BrC6H4 3) were synthesized via ZnII-mediated amidoxime-cyanamide coupling and their solid structures were studied by X-ray diffraction. Solution structure and conformational changes of [1–3](p-TolSO3)•½H2O were studied by dynamic NMR. The obtained quantitative data were supported by DFT calculations. All the obtained results help to understand the relative stability of the salts [H2N=C(R)ON=C(R’)NH2](X) (R = NAlk2, Alk, Ar) and give a further insight into the mechanism of ZnII-mediated generation of 1,2,4-oxadiazoles. The electron delocalization and sesquialteral bonds in the [H2N=C(NR2)ON=C(R’)NH2]+ system was recognized by estimation of values of activation energy barriers (14–18 kcal/mol by DNMR and 16–17 kcal/mol by DFT calculations) for the rotation around the CN bonds for the NR2 groups and inspection of the solid-state X-ray data along with the Wiberg bond indices (intermediate single/double bond order for the CN distances). This electron delocalization is responsible for the stabilization of the positively charged iminium cation. The moderate strength hydrogen bonding between the oxime N atom and the =NH2 group, which is verified from the X-ray, DNMR experiments, and by using quantum chemical calculations, stabilizes the iminium salt, but it is still weak to prevent the heterocyclization. Theoretical calculations of the heterocyclization of [H2N=C(R)ON=C(R’)NH2]+ to 1,2,4-oxadiazoles demonstrated that it is kinetically hindered to a greater extent for R = NAlk2 and this explains their lower reactivity as compared to the iminium salts with R = Alk, Ar.