Synthesis, Structure, and Reaction of 1-Alkynyl(aryl)-λ³-bromanes

Reported here for the first time are the synthesis, structure, and reaction of hypervalent 1-alkynyl(aryl)-λ3-bromanes. BF3-catalyzed ligand exchange on bromine(III) of p-trifluoromethylphenyl(difluoro)-λ3-bromane with 1-alkynyl(trimethyl)stannanes in dichloromethane at −78 °C afforded 1-alkynyl(aryl)-λ3-bromanes in good yields. Trimethyl(trimethylsilylethynyl)stannane gave silylethynyl-λ3-bromane selectively. 13C NMR chemical shifts of acetylenic carbon atoms of alkynylbromanes are compared with those of alkynyl-λ3-iodanes and explained in terms of the spin−orbit-induced heavy atom effects. The structure of tert-butylethynylbromane was established by a single-crystal X-ray analysis of its crown ether complex. The 1-alkynyl(aryl)-λ3-bromanes serve as highly electron-deficient Michael acceptors and undergo tandem Michael-carbene rearrangements by the reaction with sulfonate anions, yielding 1-alkynyl sulfonates.