Hindered rotation in five‐membered cyclic nitrosamines: A 13C NMR study

The equilibrium rotamer populations and NN rotational barriers of N‐nitrosopyrrolidine (1), N‐nitrosothiazolidine (2), N‐nitrosooxazolidine (3) and their 2‐methyl derivatives, 4, 5 and 6, were determined by 13C NMR spectroscopy. While equal rotamer populations occur in 1 and 2, the E rotamers predominate in the other four compounds, with the highest percentage (92%) of E rotamer occurring in N‐nitroso‐2‐methyloxazolidine (6). The average barrier to NN bond rotation varies over a range of 4.1 kcal mol−1in these compounds, decreasing in the order N‐nitrosopyrrolidine > N‐nitrosothiazolidine > N‐nitrosooxazolidine. The compounds which contain an exocyclic 2‐methyl group have average rotational barriers which are 0.1–0.9 kcal mol−1 higher than those of the corresponding unmethylated derivatives. The results are interpreted in terms of the relative effects of steric hindrance by the 2‐methyl substituents and electron induction by the heterocyclic sulfur and oxygen atoms on both the rotamer populations and the NN rotational barriers.