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

•Syntheses and characterizations of 4 derivatives of [(bpmen)Fe(OTf)2] reported.•[(L)Fe(OTf)2] used as catalysts for oxidation of alkenes with carboxylic acid group.•Oleic, undecylenic, acrylic acid formed epoxide, hexenoic and pentenoic acid lactone.•Acid functional group enhanced conversion and selectivity.•Substitution at the 6-position of the pyridine ring reduced activity and selectivity.

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.

Graphical abstract[(bpmen)Fe(OTf)2] and related [(L)Fe(OTf)2] complexes are employed as catalysts for the oxidation of alkenes with carboxylic acid functional groups with hydrogen peroxide. Comparisons with the esters demonstrate the benefit of the acid group on conversion and selectivity. Epoxide and lactone products are formed with moderate to high conversions and selectivities.Figure optionsDownload full-size imageDownload as PowerPoint slide