Acidity of Cyclic Sulfamates: Study of Substituted 1,2,3-Benzoxathiazole 2,2-Dioxides and Theoretical Investigation of the Effect of Conformation on Acidity

Sulfamate lb, 5-nitro-3-(4-toluenesulfonyl)-l,2,3-benzoxathiazole 2,2- dioxide, was treated with various nucleophiles: imidazole, benzylamine, tert-butylamine, sodium azide, potassium fluoride, pyridine, and sodium hydroxide. The first five attacked the exocyclic (tosyl) sulfur atom. No reaction was observed with the pyridine. The hydroxide ion attacked the endocyclic sulfur atom leading to opening of the benzoxathiazole ring. Several N-unsubstituted cyclic sulfamates, X-3H-1,2,3-benzoxathiazole 2,2-dioxides (2a, X = 5-H; 2b, X = 5-NO₂; 2c, X = 5-Me; 2d, X = 5-Br; 2e, X = 5-C1; 2f, X = 6-NO₂; 2g, X = 5,6-Cl,Cl) and the naphtho-fused cyclic sulfamate (2h), were prepared by treatment of the respective N-tosyl compounds (la—h) with sodium azide or potassium fluoride. The pK_a values for these compounds were determined by potentiometric titraiion in 60% v/v EtOH/ H₂O. AHammett plot using om for 2a-e, σ_P for 2f, and both σm and a_p for 2g gave ao = 2.74. Ab initio calculations were done using sulfamic acid as a simple sulfamate model to test the effect of the geometry changes on pKa, The calculations showed that the sulfamate with the ringlike geometry should be 3.6 pK_a units moee acidic than the acyclic sulfamate. This overall change was broken down into three factors affecting the pK_a. The N-S bond rotation accounted for a change of 0.22 units, O—S bond rotation for 2.03 units, and ring strain for 1.36 units.