Unusual Anion Effects in the Iron‐Catalyzed Formation of 3‐Hydroxyacrylates from Aromatic Aldehydes and Ethyl Diazoacetate

Due to the lability of one of the CO ligands in trans ‐[Fe(PNP)(CO) 2 Cl] + this compound is an efficient catalyst for the coupling of a series of aromatic aldehydes with ethyl diazoacetate (EDA), which give, in most cases, selectively 3‐hydroxyacrylates rather than β‐oxo esters. This reaction is strongly dependent on the nature of the counterion. Whereas with BF 4 – the reaction proceeds with conversions up to 90 %, in the case of the counterions NO 3 – , CF 3 COO – , CF 3 SO 3 – ,SbF 6 – , and BAr′ 4 – [Ar′ = 3,5‐(CF 3 ) 2 C 6 H 3 ] no reaction took place. In the case of PF 6 – only up to 20 % conversion was achieved. A conceivable mechanism for the coupling of aromatic aldehydes with EDA was established by means of DFT/B3LYP calculations, which allowed the rationalization of both the chemoselectivity and the role of the counterions.