Carbon-oxygen bond cleavage by bis(imino)pyridine iron compounds: Catalyst deactivation pathways and observation of acyl C-O bond cleavage in esters

Investigations into the substrate scope of bis(imino)pyridine iron-catalyzed hydrogenation and [2π + 2π] diene cyclization reactions identified C-O bond cleavage as a principal deactivation pathway. Addition of diallyl or allyl ethyl ether to the bis(imino)pyridine iron dinitrogen complex, (^iPrPDI)Fe(N₂)₂ (^iPrPDI = 2,6-(2,6-ⁱPr₂-C₆H₃N=CMe) ₂C₅H₃N, 1-(N₂)₂), under a dinitrogen atmosphere resulted in facile cleavage of the C-O bond and yielded a mixture of the corresponding paramagnetic iron allyl and alkoxide complexes. For ethyl vinyl ether, clean and selective formation of the iron ethoxide was observed with concomitant loss of the vinyl fragment. In situ monitoring of the catalytic hydrogenation of trans-methyl cinnamate established ester C-O bond cleavage as a competing process. Stoichiometric reactions between 1-(N ₂)₂ and allyl and vinyl acetate also produced facile C-O oxidative addition. For the latter, a six coordinate diamagnetic bis(imino)pyridine acetatoxy iron vinyl compound was obtained and characterized by X-ray diffraction. Phenyl acetate undergoes exclusive acyl C-O bond cleavage, while alkyl-substituted esters such as ethyl, pentyl, benzyl, isopropyl, cyclohexyl, and tert-butyl acetate undergo competing ester and acyl C-O bond cleavage accompanied by iron-promoted decarbonylation. Deuterium labeling studies established that reversible C-H activation and chelate cyclometalation occur prior to, but are not a prerequisite for, carbon-oxygen bond oxidative addition of ethyl acetate. The molecular and electronic structures of the ether and ester C-O bond cleavage products have been established and demonstrate that ligand- rather than metal-based oxidation accompanies substrate activation.