Dimerization of aldehydes to carboxylic esters catalyzed by K2[Fe(CO)4]–crown ether system

K₂[Fe(CO)₄] (1) with a crown ether was found to be an efficient catalyst for the dimerization of aldehydes to carboxylic esters. Several aromatic aldehydes including furfural gave the corresponding esters in good yields. This reaction also proceeded intramolecularly to give phthalide from phthalaldehyde. However, aliphatic aldehydes gave aldol-condensation products instead of the corresponding esters. In the reactions of p-substituted benzaldehydes with 1, the reactivity decreased with increase of the electron-releasing ability of the substituents. On the basis of these results, the reaction mechanism including the nucleophilic attack of tetracarbonylferrate dianion to the carbonyl carbon is discussed.