Reactions of carbonyl-conjugated alkynes with N-bromosuccinimide and N-iodosuccinimide in DMF/H2O and methanol sulfuric acid:: Syntheses of dihalo diketones, dihalo ketoesters, and dihalo acetals

The following terminal, carbonyl-conjugated alkynes were reacted with N-bromosuccinimide (NBS) and N-iodosuccinimide (NIS) in MeOH/H2SO4 to give dibromo and diiodo acetals in the indicated yields: 3-butyn-2-one, 1: NBS(75%), NIS (95%); 1-phenyl-1-propyn-1-one, 2: NBS(90%), NIS (40%); 1-hexyn-3-one, 3: NBS(90%), NIS (70%); methyl propiolate, 4: NBS (20%, not isolated), NIS (95%). 4,4-Dimethyl-1-pentyn-3-one (5) gave only a trace of dibromo acetal and no diiodo acetal; tribromide and tetrabromide were the major products. NBS and NIS reactions required, respectively, 20% and 33 wt % of H2SO4. The reaction was unsuccessful with internal alkynes 4-phenyl-3-butyn-2-one and 3-hexyn-2-one which gave only complex mixtures of products. Alkyne 2 gave a significant yield of acetal-ketal in addition to the dihalo acetals. Both the dibromo acetal-ketal and diiodo acetal-ketal were isolated, but only the former could be hydrolyzed to the dibromo acetal. Internal, carbonyl-conjugated alkynes reacted with NBS and NIS in H2O/DMF (40:60) to give the following products in the indicated yields: 4-phenyl-3-butyn-2-one (6): 1-phenyl-3,3-dibromo-1, 3-butanedione (17, 70%), 1-phenyl-3,3-diiodo-1,3-butanedione (21, 95%); 3-hexyn-2-one (7): 3,3-dibromo-2,4-hexanedione (18, 80%), 3,3-diiodo-2,4-hexanedione (22, 95%); methyl 3-phenyl-2-propynoate (8): methyl 2,2-dibromo-3-keto-3-phenylpropanoate (19, 43%), methyl 2,2-diiodo-3-keto-3-phenylpropanoate (23, 95%); methyl 2-pentynoate(9): methyl 2,2-dibromo-3-ketopentanoate (20, 80%), methyl 2,2-diiodo-3-ketopentanoate (24, 95%). All reactions, except for 6 and 8 with NBS, required H2SO4. The terminal, carbonyl-conjugated alkyne, 3-butyn-2-one, did not give products, possibly because of oxidation of the intermediate aldehyde by NBS and NIS. Mechanisms involving electrophilic attack by halogen on the triple bond and an acid-catalyzed mechanism are discussed.