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

Abstract

Abstract13C NMR spectra of ten formamidines, five acetamidines and five butyramidines were recorded at low temperatures. The barrier to rotation around the CN1 bond for N2‐alkylformamidines is approximately 50 kJ mol−1 and for N2‐benzylformamidines approximately 52 kJ mol−1, and is thus ca. 10 kJ mol−1 lower than for N2‐phenylformamidines. The height of the barrier is related to Taft's substituent constants at N2, δG* = 49.25 + 8.08σ* −3.21Es. In the acetamidines the steric hindrance between the alkyl substituent at N2 with CFCH3 contributed to the decrease in δ* (lower than 40 kJ mol−1). The i‐C3H7 substituent at CF 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 N2 with the N1 (CH3)2 group.