1H and 13C NMR study of hindered rotation in N1,N1‐dimethyl‐N2‐substituted phenylacetamidines

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AbstractBarriers to rotation about the partial double CN1 bond were determined from line shape analysis of 1H and 13C dynamic NMR spectra of N1,N1‐dimethyl‐N2‐substituted phenylacetamidines with twelve different substituents on the phenyl ring. The values of ΔG are 51.2–58.7 kJ mol−1 and the correlation with Hammett's σ values indicates an important contribution from the substituent effects to the barrier height. The decrease of the rotational barrier of approximately 10 kJ mol−1 in acetamidines in comparison with the respective formamidines is probably mainly due to a steric interaction of the CCH3 group with aromatic protons; this leads to non‐planarity and a decrease of the conjugation of the aromatic ring with the acetamidine moiety.