Iron Lewis Acid Catalyzed Reactions of Aromatic Aldehydes with Ethyl Diazoacetate: Unprecedented Formation of 3-Hydroxy-2-arylacrylic Acid Ethyl Esters by a Unique 1,2-Aryl Shift

The iron Lewis acid [η 5 -(C 5 H 5 )Fe(CO) 2 (THF)]BF 4 ( 1 ) was found to catalyze reactions of ethyl diazoacetate (EDA) and aromatic aldehydes, yielding 3-hydroxy-2-arylacrylic acid ethyl esters and the corresponding β-keto esters. According to the literature, this is the first report of the formation of enol esters from EDA and aromatic aldehydes. The yield of the enol esters increased with electron-rich aldehydes. With 2,4-dimethoxybenzaldehyde the only product isolated was the corresponding enol ester in 80% yield. However, in the presence of electron-deficient aldehydes such as p -nitrobenzaldehyde, formation of enol ester decreased to 32%. The most unique feature of these reactions is that enol esters are formed by an unusual 1,2-aryl shift, from a possible intermediate 8, which in turn is formed from the reaction of the iron aldehyde complex 7 and EDA.