Electrophilic Halogenation of Allenoates and 3-Alkynoates: Syn-thesis of 1,4-Dicarbonyl 3-(E)-Haloalkenes and Mechanistic Investigations

Herein, an electrophilic halogenation of di- or trisubstituted allenoates and 3-alkynoates is demonstrated with NBS and NIS reagents under DABCO catalysis. The protocol affords densely functionalized 1,4-dicarbonyl 3-haloalkenes in good yields with excellent (E)-stereoselectivity (up to 83% yield, >20:1 dr). The role of the Brønsted base in the unusual transformation is demystified through control experiments and computational studies. The deliverables are identified as good synthons for various synthetic transformations and valuable targets for biologically active pyridazine scaffolds.