2130
J. Org. Chem. 1997, 62, 2130-2138
r-Vinylation of 1,3-Dicarbonyl Compounds with Alkenyl(aryl)iodonium Tetrafluoroborates: Effects of Substituents on the Aromatic Ring and of Radical Inhibitors Masahito Ochiai,* Toshifusa Shu, Takema Nagaoka, and Yutaka Kitagawa Faculty of Pharmaceutical Sciences, University of Tokushima, 1-78 Shomachi, Tokushima 770, Japan Received October 28, 1996X
Direct R-vinylations of enolate anions derived from 1,3-dicarbonyl compounds with 4-tert-butyl-1cyclohexenyl(aryl)iodonium 2 and 1-cyclopentenyl(aryl)iodonium tetrafluoroborates 3 are reported. Frequently, R-phenylations compete with the vinylations in the reaction of 1,3-dicarbonyl compounds with alkenyl(phenyl)iodonium salts 2a and 3a. Use of alkenyl(p-methoxyphenyl)iodonium salts 2b and 3b, however, leads to selective R-vinylation at the expense of the competing arylation of 1,3-dicarbonyl compounds. Use of an efficient aryl radical trap, 1,1-diphenylethylene, inhibits radical-induced decomposition of the alkenyl(aryl)iodonium salts, thereby improving the yields of R-vinylations of enolate anions derived from 1,3-dicarbonyl compounds. Simple unactivated vinyl halides tend to be nonreactive in bimolecular displacement reactions and, consequently, fail to react with metal enolates under standard experimental conditions.1,2 Multistep R-vinylations of metal enolates using a masked vinyl cation equivalent as an electrophile are now possible3 because of the availability of such electrophiles as R-trimethylsilyl aldehydes,4 R-phenylseleno aldehydes,5 and enol ethers.6 Recently, Pinhey and co-workers demonstrated that alkenyllead tricarboxylates, generated in situ by a metal (Hg, Sn, or Zn)-lead (Pb) exchange reaction, served as useful reagents for the R-vinylation of carbonyl compounds.7 Alkenyllead triacetates are highly labile and undergo relatively fast reductive elimination of lead(II) acetate, yielding an alkyne or an enol acetate.8,9 Rapid generation of alkenyllead triacetates (
