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

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 the¹H NMR signals of the prochiral benzyl methylene protons, which behave differently toward the stereochemically distinct processes. Since SN torsion is a T_cprocess and amide rotation is a T_Aprocess, slow rotation of the former renders the methylene protons diastereotopic, giving rise to an AB quartet, while slow rotation about the N-CO bond gives rise to two unequally intense singlets representing 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 σ-acceptor groups CX₃cause an increase in sulfenamide barriers and a decrease in amide barriers, relative to phenyl-substituted analogues, indicating the effect of hyperconjugation in this system, which was not observed in the phenyl analogues.