Conformational Studies of Dodecanolactone by Dynamic NMR Spectroscopy and Computational Methods

Abstract

Low‐temperature carbon NMR spectra of dodecanolactone (1), the parent 13‐membered lactone, showed the presence of two conformations, with populations of 55.5 and 44.5 % at –164.2 °C. Populations were estimated at the coalescence temperature of –151.9 °C for the carbonyl carbon atoms, and rate constants of 172.1 and 141.1 s–1 were obtained by lineshape matching. The corresponding free‐energy barriers were 5.63 and 5.68 kcal/mol. A search of conformational space was made with Allinger's MM4 program, and the conformations found were numbered 1a–m in order of increasing steric energies. Ab initio calculations (HF/6‐311G* level) of chemical shifts and of free energies at –150 °C predict that the major conformation is 1b and the minor conformation is 1a.

Correlation between Polymerization Rate, Mechanism, and Conformer Thermodynamic Stability in Urea/Methoxide-Catalyzed Polymerization of Macrocyclic Carbonates

“…We postulated that the carbonate group of the MCs predominately would exist in two different conformer populations, ( E,Z ) and ( Z,Z ) (as the energy level of ( E , E ) would be too high), that rapidly interconvert (Chart ). − The calculations show that the ( Z,Z )-conformer in dimethyl carbonate is favored over the ( E,Z )-conformer with 7.5 kJ mol –1 , which agrees with the experimentally obtained enthalpy difference of 10.9 ± 2.1 kJ·mol –1 determined in a neat solution . The dipole moment differs substantially between the two conformers, where the ( Z,Z )-conformer has an overall dipole moment of 0.39 Debye, while the ( E,Z )-conformer has a larger dipole moment (4.76 Debye) (Chart ).…”