Acidity of cyclic sulfamates: Study of substituted 1,2,3-benzoxathiazole 2,2,-dioxides and theoretical investigation of the effect of conformation on acidity

Article abstract of DOI:10.1021/jo00112a022

Sulfamate 1b, 5-nitro-3-(4-toluenesulfonyl)-1,2,3-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-Cl; 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 (1a-h) with sodium azide or potassium fluoride. The pK(a) values for these compounds were determined by potentiometric titration in 60% v/v EtOH/H₂O. A Hammett plot using σ(m) for 2a-e, σ(p) for 2f, and both σ(m) and σ(p) for 2g gave a ρ = 2.74. Ab initio calculations were done using sulfamic acid as a simple sulfamate model to test the effect of the geometry changes on pK(a) The calculations showed that the sulfamate with the ringlike geometry should be 3.6 pK(a) units more 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.