A General Method for alpha-Functionalization of Enol Derivatives through the Umpolung Cross-Nucleophile Coupling Mediated by a Single Iodine(III) Reagent

In this paper, we present an unprecedented and general umpolung protocol that allows the functionalization of silyl enol ethers and of 1,3-dicarbonyl compounds with a large range of heteroatomic nucleophiles, including carboxylic acids, alcohols, primary and secondary amines, azide, thiols, and also anionic carbamates derived from CO2. The scope of the reaction also extends to carbon-based nucleophiles. The reaction relies on the use of 1-bromo-3,3-dimethyl-1,3-dihydro-1delta3[d][1,2]iodaoxole, which provides a key alpha-brominated carbonyl intermediate. The reaction mechanism has been studied experimentaly and by DFT, and we propose formation of an unusual enolonium intermediate with a halogen-bonded bromide.