Demonstration of NC(sp²)‐centered stereofluctuations in a guanidine derivative and its cation

Abstract

NMR line shapes and coalescences of pairs of diastereotopic reporter nuclei (1H and 13C) at suitable molecular positions were used to measure the stereofluctuation rate constants of an N‐aryl cycloguanidine model compound of the type aryl—N═C[N(CH3)CH2]2 (7 in Scheme ) in toluene and in CDCl3 as the solvents. The key criterion is coincidence or noncoincidence of the rate constants, which revealed that both free N═C double‐bond rotation and free N—aryl single‐bond rotation occur much more slowly than in‐plane, lone‐electron pair inversion at nitrogen, which is fast even at −99°C. The related cycloguanidinium model compound (6 in Scheme ) cannot perform such an N‐inversion but simulates it at roughly 35°C in CDCl3 and in Cl2CD—CDCl2 through a less rapid N═C half‐rotation that is coupled with a correlated, counter‐sense N—aryl half‐rotation, whereas both free N═C double‐bond rotation and free N—aryl rotation are again significantly slower.