Dynamic NMR studies of N-benzoyl pyrrolidine and N-benzoyl piperidine derivatives

Variable-temperature 1H and 13C NMR spectroscopy are used to investigate barrier of C–N rotation in series of amides, such as N-benzoyl pyrrolidine, N-(4-chlorobenzoyl) pyrrolidine, N-(4-methoxybenzoyl) pyrrolidine, N-(4-chlorobenzoyl) piperidine, and N  -(4-methoxybenzoyl) piperidine. ΔG298≠ of 65.2, 60.6, 58.8, 60.1, and 57.1 kJ mol−1are found for the above compounds, respectively. Theoretical studies were done for these compounds and the results showed good agreement with the experimental results. Theoretical and experimental results showed that substituted methoxy group on the para position of benzene ring decreases the rotational barrier. Increasing steric effect and therefore unstabilizing the ground state (GS) in piperidine compare to pyrrolidine ring also decreases the rotational barrier. It was found out that the rotational barrier in the polar solvent is higher than non-polar solvent.