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.
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