Nuclear Magnetic Resonance Study of the Effect of Hyperconjugation on Amide and Sulfenamide Rotational Barriers in Methyl N-Benzyl-N-(trihalomethanesulfenyl)carbamates

Article abstract of DOI:10.1021/jo00281a038

The title compounds CH3OCON(CH2C6H5)SCX3 have two conformational changes requiring substantial activation energies: rotation about the SN and the N-CO bonds.Both of these changes have been observed simultaneously and assigned by utilizing the 1H NMR signals of the prochiral benzyl methylene protons, which behave differently toward the stereochemically distinct processes.Since SN torsion is a T_C process and amide rotation is a T_A process, slow rotation of the former renders the methylene protons diastereotopic, giving riseto an AB quartet, while slow rotation about the N-CO bond gives rise to two unequally intense singlets represent ing the syn and anti amide isomers.Activation free energies for both processes were determined: 6a, X = CL, ΔG^%_CN = 11.6 kcal/mol, ΔG^%_SN = 14.3 kcal/mol; 6b, X = F, ΔG^%_CN = 11.6 kcal/mol, ΔG^%_SN = 12.5 kcal/mol.The syn and anti equilibrium constants at 210 K are 7.0 and 6.9 respectively.The strong ?-acceptor groups CX3 cause an increase in sulfenamide bariers and a decrease in amide bariers, relative to phenyl-substituted anlogues, indicating the effect of hyperconjugation in this system, which was not observed in the phenyl analogues.