Synthesis of β-chloro, β-bromo, and β-iodo α,β-unsaturated ketones

Abstract

A new, efficient method for the preparation of β-chloro, β-bromo, and β-iodo α,β-unsaturated ketones is described. The method involves the reaction of β-diketones or α-hydroxymethylenecycloalkanones with triphenylphosphine dihalides in the presence of triethylamine. With the dichloride and dibromide reagents, the reactions are conveniently carried out in benzene at room temperature, while with triphenylphosphine diiodide the reactions are best performed in refluxing acetonitrile (β-diketones) or in acetonitrile–hexamethylphosphoramide (α-hydroxymethylenecycloalkanones). The reaction of triphenylphosphine diiodide – triethylamine with a series of 4-alkyl-1,3-cyclohexanediones provides mainly or exclusively (depending on the size of the alkyl group) 6-alkyl-3-iodo-2-cyclohexen-1-ones, while reaction of this reagent with 2-hydroxymethylenecyclohexanone and 2-hydroxymethylenecyclopentanone affords stereoselectively and regioselectively (E)-2-iodomethylenecyclohexanone and (E)-2-iodomethylenecyclopentanone, respectively.