13 C NMR study of the substituent effects on the internal steric hindrances in amidines

13 C NMR spectra of ten formamidines, five acetamidines and five butyramidines were recorded at low temperatures. The barrier to rotation around the CN 1 bond for N 2 ‐alkylformamidines is approximately 50 kJ mol −1 and for N 2 ‐benzylformamidines approximately 52 kJ mol −1 , and is thus ca. 10 kJ mol −1 lower than for N 2 ‐phenylformamidines. The height of the barrier is related to Taft's substituent constants at N 2 , δG* = 49.25 + 8.08σ* −3.21 E s . In the acetamidines the steric hindrance between the alkyl substituent at N 2 with C F CH 3 contributed to the decrease in δ* (lower than 40 kJ mol −1 ). The i ‐C 3 H 7 substituent at C F in butyramidines forces isomerization and these amidines become Z isomers. MNDO calculations indicate no significant changes of bond length on E / Z isomerization, and the low rotational barrier in butyramidines is probably caused by the steric interaction of the aromatic ring at N 2 with the N 1 (CH 3 ) 2 group.