Synthesis and Characterization of β-Haloalkenyl-λ~3-bromanes: Stereoselective Markovnikov Addition of Difluoro(aryl)-λ~3-bromane to Terminal Acetylenes

摘要

Hypervalent 1-alkenyl(phenyl)-λ~3-iodanes enjoy their rich chemistry in modern organic synthesis. Because of the very high leaving group ability of phenyl-λ~3-iodanyl groups, they undergo unusual vinylic S_N2 displacement by the reaction with a wide range of nucleophiles. They also serve as excellent progenitors for generation of alkylidene carbenes. In a marked contrast, little is known concerning the chemistry of the closely related group 17 1 -alkenyl-λ~3-bromanes because a method for their syntheses is not available and no well-established 1-alkenyl-λ~3-bromanes are known. In 1985, Olah and co-workers reported the preparation of vinyl(methyl)(hexafluoroantimonato)-λ~3-bromane in chlorosulfonyl fluoride solution; alkylation of vinyl bromide with a large excess of methyl fluoride—antimony pen-tafluoride complex at -78℃ in SO_2ClF afforded a light-yellow colored solution of the vinyl(methyl)-λ~3-bromane, whose ~(13)C NMR spectrum at -90℃ showed three absorptions at δ 132.9(C_β), 120.9 (C_α), and 44.1 (Me) ppm. The vinyl(methyl)-λ~3-bromane was found to be stable at -78℃ for only several hours (ca. 4 h), after which polymerization sets in. We report herein, for the first time, the synthesis, isolation, and characterization of β-fluoro- and β-chLoroalkenyl(aryl)-λ~3-bromanes.