Two Faces of a Biomimetic Non‐Heme HOFe^VO Oxidant: Olefin Epoxidation versus cis‐Dihydroxylation

Abstract

The family of non-heme iron complexes-exemplified by [Fe II (tpa)(CH 3 CN) 2 ] 2+ , where tpa is the tetradentate tripodal tris(2-pyridylmethyl)amine ligand-efficiently utilizes H 2 O 2 to carry out stereospecific olefin epoxidation and cis-dihydroxylation. [1][2][3][4] The latter reaction was not previously known to be catalyzed by a synthetic iron complex and is precedented only in the chemistry of Rieske dioxygenases, [5] non-heme iron enzymes responsible for cis-dihydroxylation in the biodegradation of arenes by soil bacteria. Isotope-labeling experiments on the biomimetic reaction show the involvement of a highly selective metal-based oxidant capable of introducing water into the product. [1,2] This oxidant is proposed to result from the low-spin complex [Fe III (tpa)(H 2 *)(OOH)] 2+ (1) and be a formally H*ÀFe V =O species (2; * indicates an oxygen atom with isotope labeling), both shown in Figure 1.Previous DFT calculations have demonstrated the feasibility of an OÀO bond heterolysis mechanism that leads to an S = 3/2 cis-HOÀFe V =O species with a short FeÀO bond (1.66 ) and a longer FeÀOH bond (1.77 ).[6] The three unpaired electrons are mainly distributed on the iron atom (1.58), the oxo oxygen atom (1.0), and the hydroxo oxygen atom (0.44): a spin distribution not unlike that found for compounds I of heme peroxidases and cytochrome P450 and related model complexes. [7,8] The corresponding sextet and doublet states of HOÀFe V =O lie more than 10 kcal mol À1 higher in energy than the quartet ground state.[9]Further DFT calculations presented herein provide new insight into how this unique oxidant can carry out both olefin epoxidation and cis-dihydroxylation. Hence, the present study focuses on the subsequent reaction of 2 with olefins (see the Supporting Information) to reveal that olefin epoxidation and cis-dihydroxylation in fact represent different faces of the same oxidant.The key results of our DFT study are illustrated by the energy profiles in Figure 2, showing that 2 can react with olefins (namely, 2-butene) through two different channels. As shown on the left, the oxo group attacks the olefin to form the C1ÀO1 bond and intermediate A-Fe IV (path A). On the right, the hydroxo ligand attacks the olefin to form the C2ÀO2 bond and intermediate B-Fe IV (path B). Both intermediates have an iron(iv) center and a radical on the partially oxidized substrate.Intermediate A-Fe IV is an iron(iv) complex with a hydroxide (r Fe-O2 = 1.79 ) and the partially oxidized olefin as ligands. The formation of the new C1ÀO1 bond is exergonic by 2.5 kcal mol À1 and involves an activation energy of 7.8 kcal mol À1 ; solvent effects (+ 3.1 kcal mol À1 ), entropy effects (+ 0.6 kcal mol À1 ), and big-basis effects (+ 2.1 kcal mol À1 ) contribute to increase the barrier by 5.6 kcal mol À1 . Unpaired spin density can be found on the metal center (1.53), the partially oxidized olefin (0.97), and the remaining ligands (0.50). Antiferromagnetic coupling of the unpaired electron on the partially oxidized olefin with those ...

Bio-inspired nonheme iron catalysts for olefin oxidation

“…The question of olefin epoxidation and cis-dihydroxylation by a common HO-Fe V O oxidant as implied by the experimental data has also been addressed by DFT calculations [7]. These calculations reveal that epoxidation and cisdihydroxylation of cis-2-butene are both very exothermic and have comparable activation barriers, with the latter being favored by only about 1.6 kcal mol À1 (Fig.…”

“…2). Overall, the data accumulated over the last few years (Table 1) reveal a surprisingly complex reaction landscape on which olefin epoxidation is favored under some conditions and cis-dihydroxylation under others, suggesting that these two reactions are in fact closely related [6,7]. (1) and Fe(TPA) (2), derived, respectively, from a linear and a tripodal tetradentate ligand.…”

Ligand Topology Effects on Olefin Oxidations by Bio‐Inspired [FeII(N2Py2)] Catalysts

“…[35] However, given ligand backbones and topologies that are different from those of tpa, the activation barriers for epoxidation and cis-dihydroxylation do not necessarily have to be isoenergetic. In fact, experimental results suggest that for 2 and 8 epoxidation is more favored than cis-dihydroxylation by the putative [Fe V (L)(O)(OH)] oxidant generated from a wa pathway.…”

Biomimetic High‐Valent Non‐Heme Iron Oxidants for the cis‐Dihydroxylation and Epoxidation of Olefins

“…[21][22][23][24][25] Our proposal is based on {Fe IV (OH) 2 } (S = 1) and its tautomer {Fe IV =O(OH 2 )} (S = 2), which have not been considered and are derived from experiment and a DFT analysis. We suggest that the {Fe V = O(OH)} pathway is less probable but cannot be excluded with our L 1 -based system.…”