title: Unexpected differences in the alpha-halogenation and related reactivity of sulfones with perhaloalkanes in KOH-t-BuOH.
abstract: Most alkyl phenyl sulfones are readily alpha-chlorinated with CCl(4) and alpha-brominated with CBrCl3 in KOH-t-BuOH via radical-anion radical pair (RARP) reactions. While isopropyl mesityl sulfone (4) is easily alpha-chlorinated with CCl(4), it was completely recovered when treated with the more reactive CBrCl3. Subsequent investigations showed the latter result to be due to the poor acidity of 4 together with the rapid depletion of CBrCl3 and KOH by their reaction with each other, and led to a variety of other important results. 4-Hydroxyphenyl isopropyl sulfone (6) is unreactive with either CCl4 or CBrCl3 in KOH-t-BuOH, its phenoxide anion strongly reducing the electronegativity of the sulfonyl group, thereby inhibiting alpha-anion formation. This effect is reversed by the electron-withdrawing influence of two alpha-phenyls, so that benzhydryl 4-hydroxyphenyl sulfone (8) is readily alpha-halogenated in KOH-t-BuOH with CCl4 or CBrCl3. On further contact with KOH-t-BuOH the alpha-halogenated sulfones from 8 are decomposed into benzophenone and phenol. While the alpha-halogenated derivatives of 4-methoxyphenyl benzhydryl sulfone (9) are stable to base, they are decomposed even under mildly acidic conditions into 4-methoxyphenyl 4-methoxybenzenethiolsulfonate (9c), phenol, and benzophenone. Mono-alpha-halogenation of benzyl phenyl sulfone (10) enhances the rate of the subsequent halogenation, so that alpha,alpha-dihalogenation is attained while much substrate is still present and the mono-alpha-halogenated product is not detected. The ease of reductive debromination of alpha-bromo sulfones with Cl3C- was correlated with the stability of the formed alpha-anions, explaining the success with alpha-bromobenzylic sulfones but failure with alpha-bromoalkyl sulfones. In the presence of air and the absence of competing halogenation, formation of the alpha-anions of alkyl aryl sulfones is quickly accompanied by oxidative cleavage by atmospheric O2, leading to the formation of arenesulfonyl alcohols, arenesulfonyl halides, and haloarenes.
authors.affiliation: Meyers Institute for Interdisciplinary Research in Organic and Medicinal Chemistry, Southern Illinois University, Carbondale 62901, USA. cal@chem.siu.edu
authors.forename: Cal Y
authors.initials: CY
authors.lastname: Meyers
authors.forename: Roch
authors.initials: R
authors.lastname: Chan-Yu-King
authors.forename: Duy H
authors.initials: DH
authors.lastname: Hua
authors.forename: Vera M
authors.initials: VM
authors.lastname: Kolb
authors.forename: Walter S
authors.initials: WS
authors.lastname: Matthews
authors.forename: Thomas E
authors.initials: TE
authors.lastname: Parady
authors.forename: Toyokazu
authors.initials: T
authors.lastname: Horii
authors.forename: Paul B
authors.initials: PB
authors.lastname: Sandrock
authors.forename: Yuqing
authors.initials: Y
authors.lastname: Hou
authors.forename: Songwen
authors.initials: S
authors.lastname: Xie