Nachweis innermolekularer beweglichkeit durch NMR-spektroskopie-XV. Untersuchungen zur inversion am doppelt gebundenen stickstoffatom am beispiel der tetramethylguanidine

The influence of substituents Z on the rates of syn-anti-isomerization has been studied using guanidines as typical examples of imines X₂C = NZ. In the series: quinone imine < ketimime < C-aryl-imin < iminocarbonate < iminodithiocarbonate < guanidine the isomerization rate increases. The inversion mechanism was also proved by variation of Z. The inversion in imines is easier in the series: Z = OCH₃ ≈ N(CH₃)₂ < Alkyl < Aryl < Acyl. The same influence was observed on the inversion barrier of amines (such as aziridines). In p-substituted phenylguanidines (Z = C₆H₄R) a linear Hammett correlation of the rates and σ_p- of the p-substituent R was observed (ρ{variant} = +2·95 ± 0·30 at -50°) Large substituents in o-position of the phenyl ring make the inversion easier.