Ambident neighbouring groups. Part V. Mechanism of cyclization of 2-halogenoethylsulphonamides to aziridines

Abstract

N-(2-Bromo- and 2-chloroethyl)sulphonamides are smoothly cyclized in basic solution to N-(arylsulphonyl)-aziridines. Kinetic studies [in water at 25°, µ= 1·0 (KCl)] imply that the sulphonamidate anion is the reactive species. Electron-withdrawing substituents in the aryl ring aid formation of the anion (ρ=+0·94, pK_a⁰= 10·59), while retarding the rate of cyclization of the anion (ρ=–0·58, log k₀=–1·30), with Br^– as leaving group. The chlorides behave similarly, but are ca. 50-fold less reactive than the corresponding bromides. In ethanol containing an excess of ethoxide ion, particularly at higher substrate concentrations, other products including N-2-(ethoxyethyl)arylsulphonamides and dimeric materials such as piperazines are also formed. In aqueous solution the N-arylsulphonylaziridines undero slow ring cleavage [to give N-(2-hydroxyethyl)arylsulphonamides], but only in strongly acidic or basic media.