PMR spectra of 2-substituted pyrazines and the corresponding pyrazine N-oxides and N,N′-dioxides

Abstract

The PMR spectra of pyrazine, its 2-substituted derivatives, and the corresponding N-oxides and N,N′-dioxides were investigated. The character of the change in the chemical shifts in the N-oxides and N,N′-dioxides of unsubstituted pyrazine indicates the electron-donor effect of the N→O group. An additive effect of the N→O groups on the chemical shifts of the ring protons is observed in pyrazine N,N′-dioxide. An appreciable interaction of the substituent with the N₍₁₎→O group and a weak interaction with the N₍₄₎→O group occur in N-oxides and N,N′-dioxides of 2-substituted pyrazines. A significant increase in the ortho and meta spin-spin coupling constants of the ring protons is noted when the nitrogen atoms are oxidized. The sign of the spin-spin coupling constant (J₃₅) through the N→O group was determined.