Ferrocene and its derivatives play important roles in material chemistry due to their stability, aromaticity, low toxicity, and reversible electrochemistry. While exploring the method of extending carbocyclic rings on ferrocene, we noticed that the β-ferrocenoylpropionic acid and γ-ferrocenoylbutyric acids underwent intramolecular ring-closing between keto and carboxylic acid groups in the presence of trifluoroacetic anhydride (TFAA) giving unsaturated lactones. The unusual nucleophilicity of the carbonyl oxygen prompted us to explore the mechanism of lactone formation in related keto carboxylic acids. When we worked with 2-(ferrocenyl-carbonyl)benzoic acid under similar conditions, we isolated lactone dimers resulting from the coupling at the alpha position of the ferrocenyl moiety. We observed the same compounds as minor products when we performed Clemmensen reduction of 2-(ferrocenyl-carbonyl)benzoic acid using Zn(Hg) and HCl. However, when we used NaOH as reaction solvent using Zn(Cu) as reductant, we obtained 3-ferrocenylphthalide on acidic workup without any dimerization. In an attempt to reduce the keto group in γ-ferrocenoylbutyric acid using NaBH<sub>4</sub>, followed by acidic workup, we also observed the formation of a lactone. The synthesis, characterization, and mechanism of lactone formation will be discussed.

Lactones from ferrocene-bound keto-carboxylic acids

ACS Spring Meeting 2021

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