Halogenating agent-promoted transformation of enaminones into α,α-dihaloketones (halo = Cl, Br)

Abstract

Enaminone 1 was treated with an equivalent of a halogenating agent, specifically N-bromo- or N-chlorosuccinimide (NBS or NCS, respectively), forming the corresponding α-haloenaminone 3, and its reaction with an equivalent of NBS or NCS gave the corresponding α,α-dihaloketone 2 with high yield under mild reaction conditions (no additive, at room temperature). Furthermore, we investigated the direct transformation of enaminone 1 into the corresponding α,α-dihaloketone 2 treated with two equivalents of the halogenating agent. Concerning the probable reaction mechanism, we initially expected the isolation of only α-bromo-α-chloroketone 2-BrCl when treating α-chloroenaminone 3-Cl with NBS or α-bromoenaminone 3-Br with NCS. However, the resulting mixture containedα-bromo-α-chloro-,α,α-dichloro-, and α,α-dibromoketones 2. Through experimentation with the 1H NMR spectrum, it was discovered that the formation of α,α-dichloro- and α,α-dibromoketone 2 and the existence of other compounds as an intermediate.