Epoxidation of alkenes bearing a carboxylic acid group by iron complexes of the tetradentate ligand N,N′-dimethyl-N,N′-bis(2-pyridylmethyl)-1,2-diaminoethane and its derivatives

The addition of carboxylic acids enhances the rate and selectivity of alkene oxidations catalyzed by [(bpmen)Fe(OTf)2] (bpmen = N,N′-dimethyl-N,N′-bis(2-pyridylmethyl)-1,2-diaminoethane). The syntheses and characterizations of four derivatives of this iron complex with varying substitutions on the pyridine ring and with a substituted piperazine backbone are reported. These [(L)Fe(OTf)2] complexes and [(bpmen)Fe(OTf)2] are employed as catalysts for the oxidation of alkenes bearing a carboxylic acid functional group, namely oleic acid, undecylenic acid, 5-hexenoic acid and 4-pentenoic acid, with hydrogen peroxide as the oxidant. Comparisons with the analogous ester substrates demonstrate the beneficial impact of the acid functional group on conversion and selectivity when using [(bpmen)Fe(OTf)2] as a catalyst. For the oleic and undecylenic acids, epoxide product is formed with moderate to high conversions and high selectivities. Under the conditions employed, 4-pentenoic acid is oxidized to a γ-lactone, most likely via the epoxide intermediate, and 5-hexenoic acid to a mixture of epoxide and δ-lactone. Of the iron complexes with bpmen derivatives as ligands, only the N,N′-dimethyl-N,N′-bis(5-chloropyridin-2-ylmethyl)-1,2-diaminoethane variant shows appreciable activity. The effect of solvent choice is also investigated.