Non‐Heme Iron Catalysts for Olefin Epoxidation: Conformationally Rigid Aryl–Aryl Junction To Support Amine/Imine Multidentate Ligands

Abstract

Atom-transfer chemistry represents an important class of reactions catalyzed by metalloenzymes. As a functional mimic of non-heme iron enzymes that deliver oxygen atoms to olefins, we have designed monoiron complexes supported by new N-donor chelates. These ligands take advantage of heme-like conformational rigidity of the π-conjugated molecular backbone, and synthetic flexibility of tethering non-heme donor groups for additional steric and electronic control. Iron complexes generated in situ can be used to carry out catalytic epoxidation of a wide range of olefin substrates by using mCPBA as a terminal oxidant. The fate of initial iron-peracid adduct and the involvement of iron-oxo species in this process were investigated further by mechanistic probes and isotope exchange studies. Our findings suggest that anilidopyridyl-derived [N,N]-bidentate motif could serve as a versatile structural platform to build non-heme ligands for catalytic oxidation chemistry.

Proton Switch in the Secondary Coordination Sphere to Control Catalytic Events at the Metal Center: Biomimetic Oxo Transfer Chemistry of Nickel Amidate Complex

“…The dpap ligand system is based on the N3 ligand 2 (Figure 2 a) which we recently reported for biomimetic iron chemistry [15] . The first‐generation tridentate ligand 2 was prepared by installing a flexible pyridyl pendant to the rigid and π‐conjugated anilido‐pyridyl [ N , N ]‐bidentate motif 3 .…”

“…As shown in Figure 2 b, the amide carbonyl group of the dpap ligand is not directly coordinated to the metal, but protonation or metal‐binding to this secondary coordination sphere can reversibly modulate the relative contribution of the amido vs. iminol/iminolate Lewis structure descriptions in resonance, thereby enabling remote control of the ligand donor ability [16] . The ligand H‐dpap was conveniently prepared by an amide coupling reaction between 3 [15] and 4 [17] in excellent yield (>95 %) (Scheme 1).…”

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

“…[13,[61][62][63][64][65][66] Further,t he small negative value of the reaction constant is similar to some of the reported high-valent iron-oxo and iron(III)-acylperoxo complexes. [17,49,62] Thereafter,w eh ave carried out kinetics of the different internal olefins.W eh ave studied the second-order kinetics of cis and trans-stilbene.Itwas found that the secondorder rate constant, (k 2 % 1.12 M À1 S À1 )o ft he trans isomer is almost ten times of the cis-stilbene (k 2 % 0.141 M À1 S À1 ). The higher olefin epoxidation rate of the trans-stilbene can be rationalized by the lower one-electron oxidation potential of the trans-stilbene compared to cis-stilbene.…”

Making Waxy Salts in Water: Synthetic Control of Hydrophobicity for Anion‐Induced and Aggregation‐Enhanced Light Emission

“…As a first step to realize the idea outlined in Figure 1 b, we designed the simplest molecular prototype [ 1‐Me ]Cl (Scheme 1 b). This compound was prepared by following the general synthetic protocol outlined in Scheme 1 a, starting with methylation of the anilido‐pyridine precursor A , [13] followed by acid‐catalyzed condensation reaction with formaldehyde in HCl/1,4‐dioxane (Scheme S1) [14] . The resulting yellow precipitate was isolated by simple filtration.…”