Steric control and the mechanism of benzaldehyde oxidation by polypyridyl oxoiron(iv) complexes: aromatic versus benzylic hydroxylation of aromatic aldehydes

Mechanisms of Sulfoxidation and Epoxidation Mediated by Iron(III)-Iodosylbenzene Adduct: Electron-Transfer vs. Oxygen-Transfer Mechanism

The mechanisms of sulfoxidation and epoxidation mediated by previously synthesized and characterized iron(III)-iodosylbenzene adduct, Fe^III(OIPh) were investigated using para-substituted thioanisole and styrene derivatives as model substrates. Based on detailed kinetic reaction experiments, including the linear free-energy relationships between the relative reaction rates (logk_rel) and the σ_p (4R-PhSMe) with ρ = -0.65 (catalytic) and ρ = -1.13 (stoichiometric), we obtained strong evidence that the stoichiometric and catalytic oxidation of thioanisoles mediated by Fe^III(OIPh) species involves direct oxygen transfer. The small negative slope -2.18 from log k_obs versus E_ox for 4R-PhSMe gives further clear evidence for the direct oxygen atom transfer mechanism. On the contrary, with the linear free-energy relationships between the relative reaction rates (logk_rel) and total substituent effect (TE, 4R-PhCHCH₂) parameters with slope = 0.33 (catalytic) and 2.02 (stoichiometric), the stoichiometric and catalytic epoxidation of styrenes takes place through a nonconcerted electron transfer (ET) mechanism, including the formation of the radicaloid benzylic radical intermediate in the rate-determining step. On the basis of mechanistic studies, we came to the conclusion that the title iron(III)-iodosylbenzene complex is able to oxygenate sulfides and alkenes before it is transformed into the oxo-iron form by cleavage of the O-I bond.

Stoichiometric Alkane and Aldehyde Hydroxylation Reactions Mediated by In Situ Generated Iron(III)-Iodosylbenzene Adduct

Previously synthesized and spectroscopically characterized mononuclear nonheme, low-spin iron(III)-iodosylbenzene complex bearing a bidentate pyridyl-benzimidazole ligands has been investigated in alkane and aldehyde oxidation reactions. The in situ generated Fe(III) iodosylbenzene intermediate is a reactive oxidant capable of activating the benzylic C-H bond of alkane. Its electrophilic character was confirmed by using substituted benzaldehydes and a modified ligand framework containing electron-donating (Me) substituents. Furthermore, the results of kinetic isotope experiments (KIE) using deuterated substrate indicate that the C-H activation can be interpreted through a tunneling-like HAT mechanism. Based on the results of the kinetic measurements and the relatively high KIE values, we can conclude that the activation of the C-H bond mediated by iron(III)-iodosylbenzene adducts is the rate-determining step.

Change of Selectivity in C-H Functionalization Promoted by Nonheme Iron(IV)-oxo Complexes by the Effect of the N-hydroxyphthalimide HAT Mediator

A kinetic analysis of the hydrogen atom transfer (HAT) reactions from a series of organic compounds to the iron(IV)-oxo complex [(N4Py)Fe^IV(O)]²⁺ and to the phthalimide N-oxyl radical (PINO) has been carried out. The results indicate that a higher activating effect of α-heteroatoms toward the HAT from C-H bonds is observed with the more electrophilic PINO radical. When the N-hydroxy precursor of PINO, N-hydroxyphthalimide (NHPI), is used as a HAT mediator in the oxidation promoted by [(N4Py)Fe^IV(O)]²⁺, significant differences in terms of selectivity have been found. Product studies of the competitive oxidations of primary and secondary aliphatic alcohols (1-decanol, cyclopentanol, and cyclohexanol) with alkylaromatics (ethylbenzene and diphenylmethane) demonstrated that it is possible to modify the selectivity of the oxidations promoted by [(N4Py)Fe^IV(O)]²⁺ in the presence of NHPI. In fact, alkylaromatic substrates are more reactive in the absence of the mediator while alcohols are preferably oxidized in the presence of NHPI.

Stability and Catalase-Like Activity of a Mononuclear Non-Heme Oxoiron(IV) Complex in Aqueous Solution

Heme-type catalase is a class of oxidoreductase enzymes responsible for the biological defense against oxidative damage of cellular components caused by hydrogen peroxide, where metal-oxo species are proposed as reactive intermediates. To get more insight into the mechanism of this curious reaction a non-heme structural and functional model was carried out by the use of a mononuclear complex [Fe^II(N4Py*)(CH₃CN)](CF₃SO₃)₂ (N4Py* = N,N-bis(2-pyridylmethyl)- 1,2-di(2-pyridyl)ethylamine) as a catalyst, where the possible reactive intermediates, high-valent Fe^IV=O and Fe^III–OOH are known and spectroscopically well characterized. The kinetics of the dismutation of H₂O₂ into O₂ and H₂O was investigated in buffered water, where the reactivity of the catalyst was markedly influenced by the pH, and it revealed Michaelis–Menten behavior with K_M = 1.39 M, k_cat = 33 s⁻¹ and k₂(k_cat/K_M) = 23.9 M⁻¹s⁻¹ at pH 9.5. A mononuclear [(N4Py)Fe^IV=O]²⁺ as a possible intermediate was also prepared, and the pH dependence of its stability and reactivity in aqueous solution against H₂O₂ was also investigated. Based on detailed kinetic, and mechanistic studies (pH dependence, solvent isotope effect (SIE) of 6.2 and the saturation kinetics for the initial rates versus the H₂O₂ concentration with K_M = 18 mM) lead to the conclusion that the rate-determining step in these reactions above involves hydrogen-atom transfer between the iron-bound substrate and the Fe(IV)-oxo species.

Interplay Between Steric and Electronic Effects: A Joint Spectroscopy and Computational Study of Nonheme Iron(IV)-Oxo Complexes

Iron is an essential element in nonheme enzymes that plays a crucial role in many vital oxidative transformations and metabolic reactions in the human body. Many of those reactions are regio- and stereospecific and it is believed that the selectivity is guided by second-coordination sphere effects in the protein. Here, results are shown of a few engineered biomimetic ligand frameworks based on the N4Py (N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) scaffold and the second-coordination sphere effects are studied. For the first time, selective substitutions in the ligand framework have been shown to tune the catalytic properties of the iron(IV)-oxo complexes by regulating the steric and electronic factors. In particular, a better positioning of the oxidant and substrate in the rate-determining transition state lowers the reaction barriers. Therefore, an optimum balance between steric and electronic factors mediates the ideal positioning of oxidant and substrate in the rate-determining transition state that affects the reactivity of high-valent reaction intermediates.

Catalytic and stoichiometric flavanone oxidation mediated by nonheme oxoiron(iv) complexes as flavone synthase mimics: kinetic, mechanistic and computational studies

The present study describes the first example of the stoichiometric and catalytic oxidation of flavanone by synthetic nonheme oxoiron(iv) complexes and their precursor iron(ii) complexes with m-CPBA as the terminal oxidant. These models, including detailed kinetic, mechanistic and computational studies, may serve as the biomimics of flavone synthase (FS) enzymes.

Dramatic rate-enhancement of oxygen atom transfer by an iron(iv)-oxo species by equatorial ligand field perturbations

The reactivity and characterization of a novel iron(iv)-oxo species is reported that gives enhanced reactivity as a result of second-coordination sphere perturbations of the ligand system.