Oxidative functionalization of organic compounds promoted by nonheme Fe and Mn complexes

Nonheme iron and manganese complexes are versatile and environmentally sustainable catalysts able to promote the oxidation of a broad range of organic compounds with interesting synthetic application. Careful simple modification in the design of this catalytic systems, can contribute to change significantly their selectivity and efficiency. In this Ph.D. thesis, some aspects related to the mechanistic analysis and catalytic activity of three different nonheme catalytic systems: aminopyridine iron complexes, imine iron complexes and aminopyridine supramolecular iron and manganese complexes, have been investigated in detail. The occurrence of an electron transfer-oxygen rebound process in the oxidation of aryl sulfides promoted by two tetradentate ([(PDP)FeII] and [(BPMCN)FeII]) and two pentadentate ([(N4Py)FeII] and [(Bn-TPEN)FeII]) aminopyridine iron complexes, has been highlighted through detailed product and kinetic studies of the oxidation of two series of aromatic sulfides (aryl 1-methyl-1-phenylethyl sulfides and aryl diphenylmethyl sulfides) whose corresponding radical cations are characterized by high C-S fragmentation rate constants. Using aryl 1-methyl-1-phenylethyl sulfides as substrates, it has been possible, moreover, to estimate the rate constants of the oxygen rebound process (kOT) from the reduced nonheme iron-oxo complexes to the sulfide radical cations. A bioinspired supramolecular version of the aminopyridine iron and manganese complexes, [(PDP)FeII] and [(PDP)MnII] containing benzo-18-crown-6 ether as receptor that reversibly bind the substrate, has been prepared. The ditopic catalysts have been applied with success to achieve high site-selective C-H functionalization on C8 and C9 methylene positions of a series of linear protonated primary amines thanks to reversible pre-association between ammonium moiety and the benzo-18-crown-6 ether recognition sites. Imine based iron complexes represent another interesting class of nonheme catalysts. Iminopyridine iron(II) complex, easily prepared in situ by self-assembly of cheap and commercially available 2 picolylaldehyde, 2 picolylamine, and Fe(OTf)2, in a 2:2:1 ratio, displayed good-to-moderate efficiency in the oxidation of aliphatic alcohols with H2O2. Quite surprisingly, benzylic alcohols are oxidized to aromatic ketones with low efficiency due to strong competitive aromatic hydroxylation. The remarkable selectivity for the oxidation of the aromatic ring in benzylic alcohols, has been exploited in the oxidation of alkyl aromatic compounds. In the presence of an aliphatic chain, the catalyst is highly selective for the aromatic nucleus hydroxylation with a selectivity pattern that closely matched that of electrophilic aromatic substitutions with a metal–based SEAr pathway, without a significant involvement of free diffusing radical pathways. A low efficiency in the oxidation of electron rich benzylic alcohols has been also observed, moreover, for the MnII/pyridine carboxylic acid/butanedione catalytic system. The analysis of the inhibition process of this catalytic system by catechol and guaiacol substrates indicated that the Mn/inhibitor ratio is a key parameter for the activity/inactivity of the system. The right proportion between them may prevents an irreversible deactivation of the complex. Finally, the fundamental importance of the identification of the active intermediates involved in the catalytic cycles, has prompted us to project an innovative approach consisting in the simultaneously use of time-resolved Energy Dispersive X-Ray Absorption (EDXAS) and UV/Vis spectroscopies with millisecond resolution to follow the time evolution of the catalyst activation processes. The excellent results obtained for the evolution of [(TPA)FeII(CH3CN)2]2+ with H2O2 and CH3COOOH, could give the opportunity to follow the evolution of less know systems and to detect and quantitatively monitor some elusive species, such as FeV intermediates.