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

Daniel Kost, Hana Egozy

Research output: Contribution to journalArticlepeer-review

18 Scopus citations

Abstract

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 Tcprocess and amide rotation is a TAprocess, 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.

Original languageEnglish
Pages (from-to)4909-4913
Number of pages5
JournalJournal of Organic Chemistry
Volume54
Issue number20
DOIs
StatePublished - 1 Sep 1989

ASJC Scopus subject areas

  • Organic Chemistry

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