Effect of the Ligand Backbone on the Reactivity and Mechanistic Paradigm of Non‐Heme Iron(IV)‐Oxo during Olefin Epoxidation

Abstract

The oxygen atom transfer (OAT)r eactivity of the non-heme [Fe IV (2PyN2Q)(O)] 2+ (2)c ontaining the sterically bulky quinoline-pyridine pentadentate ligand (2PyN2Q) has been thoroughly studied with different olefins.T he ferryl-oxo complex 2 shows excellent OATreactivity during epoxidations. The steric encumbrance and electronic effect of the ligand influence the mechanistic shuttle between OATpathway Iand isomerization pathwayI I(during the reaction stereo pure olefins), resulting in amixture of cis-trans epoxide products.In contrast, the sterically less hindered and electronically different [Fe IV (N4Py)(O)] 2+ (1)p rovides only cis-stilbene epoxide. A Hammett study suggests the role of dominant inductive electronic along with minor resonance effect during electron transfer from olefin to 2 in the rate-limiting step.Additionally, ac omputational study supports the involvement of stepwise pathwaysduring olefin epoxidation. The ferryl bend due to the bulkier ligand incorporation leads to destabilization of both d z 2 and d x 2 Ày 2 orbitals,l eading to av ery small quintet-triplet gap and enhanced reactivity for 2 compared to 1.Thus,the present study unveils the role of steric and electronic effects of the ligand towards mechanistic modification during olefin epoxidation Scheme 1. Olefin epoxidation by [Fe IV (2PyN2Q)(O)] 2+ (2), quinolinepyridine containing ligand backbone. [

Inspiration from Nature: Influence of Engineered Ligand Scaffolds and Auxiliary Factors on the Reactivity of Biomimetic Oxidants

“…255 These systems also gave dramatic rate enhancements in oxygen atom transfer and C−H abstraction reactions and affect selectivities. [256][257][258]265 In P450 chemistry, substrate hydroxylation is typically a stepwise process with an initial hydrogen atom abstraction to form an iron(IV)-hydroxo(heme) or iron(III)-hydroxo(heme cation radical) intermediate. 12−21 During the hydrogen atom abstraction step, an electron is transferred to the heme, which can fill an iron-type π* xz orbital with a second electron or fill the a 2u orbital with a second electron.…”

“…Further studies that introduced bulkier quinoline groups instead of pyridine rings to the ligand system enabled the synthesis of a high-spin ( S = 2) iron­(IV)-oxo model complex . These systems also gave dramatic rate enhancements in oxygen atom transfer and C–H abstraction reactions and affect selectivities. − , …”

Recent Advances in First‐Row Transition‐Metal‐Mediated C−H Halogenation of (Hetero)arenes and Alkanes

“…Among these functionalizations, the bio-mimetic metal-oxo mediated epoxidations of alkenes and CÀ H bond hydroxylations made significant advancement. [61][62][63][64][65] However, the progress in metal-oxo mediated halogenation is very less compared to the CÀ H hydroxylation. This can be rationalized on the basis of high reactivity of metal-oxo species facilitating the competitive hydroxylation reaction by inhibiting the halogenation reaction.…”

“…This led to the development of several bio‐mimetic model systems for C−H functionalization reaction that proceeds through the formation of high‐valent metal‐oxo species analogous to the naturally occurring enzymatic systems. Among these functionalizations, the bio‐mimetic metal‐oxo mediated epoxidations of alkenes and C−H bond hydroxylations made significant advancement [61–65] . However, the progress in metal‐oxo mediated halogenation is very less compared to the C−H hydroxylation.…”

Experimental and theoretical investigation of the tuning of electronic structure in SnO2viaCo doping for enhanced styrene epoxidation catalysis

“…Epoxides are a class of oxirane group-containing compounds, which are vital functional intermediates and building blocks for the synthesis of a variety of pharmaceuticals, perfumes, polymers, pesticides and other fine chemicals via versatile transformations in the organic reaction community. 1–6 Styrene oxide (SO), as an important member of the epoxides family, is traditionally synthesized by the non-catalytic chlorohydrination of styrene followed by dehydrochalorination of styrene chlorohydrin with a base, or by the organic peracids based catalytic oxidation of styrene. 7–9 However, these processes suffer from several limitations, such as production of a great amount of toxic waste, severe equipment corrosion, high production cost and difficulty in product separation.…”