New Convenient One-Pot Methods of Conversion of Alkynes to Cyclobutenediones or α,β-Unsaturated Carboxylic Acids Using Novel Reactive Iron Carbonyl Reagents

Reactions of NaHFe(CO)4/RX or [HFe3(CO)11]- reagents with alkynes lead to the formation of the corresponding α,β-unsaturated carboxylic acids and/or the cyclobutenediones. The reagent generated in situ using the NaHFe(CO)4/CH3I combination in THF, on reaction with alkynes followed by CuCl2·2H2O oxidation, gives the corresponding cyclobutenediones (27−42%) and α,β-unsaturated carboxylic acids (10−22%), whereas the reagent generated using CH2Cl2 in place of CH3I leads to α,β-unsaturated carboxylic acids (37−60%) and their derivatives (35−55%) at 25 °C. The same reagent system in the presence of acetic acid (4 equiv) yields the corresponding cyclobutenedione (33%). The reaction using Me3SiCl gives the corresponding α,β-unsaturated carboxylic acids (45−54%) at 25 °C and the corresponding cyclobutenediones (51−63%) at 60 °C. Interestingly, the reaction of the [HFe3(CO)11]- species generated using Fe(CO)5/NaBH4/CH3COOH, with alkynes at 25 °C, followed by CuCl2·2H2O oxidation gives the corresponding cyclobutenediones (60−73%). The possible intermediates and pathways for the formation of α,β-unsaturated carboxylic acids and cyclobutenediones are discussed.