An NMR Study of intramolecular shielding in Alkyl N (Arylsulfonyl)Methyl]‐Carbamates, ‐Thiolcarbamates, ‐Thionocarbamates, and ‐Dithiocarbamates

The chemical shifts of the ester alkyl groups of alkyl N ‐[(arylsulfonyl)methyl]‐carbamates are displayed as doublets of unequal intensity at temperatures where rotation around the nitrogen to carbonyl bond is slow on the NMR time scale. Comparison with NMR data of simple alkyl N ‐substituted carbamates indicates that the ester alkyl absorption of at least one of the rotamers shows an appreciable upfield shift. These upfield shifts are dependent on the ring current in the aromatic nucleus of the molecule. An explanation for these observations is presented in terms of preferred conformations (with minimized lone pair‐lone pair repulsion) in which the ester alkyl group is situated above (or below) the plane of the aromatic ring. Minor structural changes in the molecule eliminate the shielding effect which supports the proposed shielding mechanism. Further evidence for intramolecular shielding was obtained from the observed solvent and temperature dependence of the chemical shifts of the ester alkyl protons. No evidence could be obtained for inter‐ or intramolecular association between the aromatic ring and the ester function. Chemical shift data for alkyl N ‐[(arylsulfonyl)methyl]thiolcarbamates, ‐thionocarbamates and ‐dithiocarbamates also indicate intramolecular shielding of the ester alkyl protons.