Non-redox Oxy-Insertion via Organometallic Baeyer–Villiger Transformations: A Computational Hammett Study of Platinum(II) Complexes

Mechanism of Nitrous Oxide Activation in C(sp2)-O Bond Formation Reactions Catalyzed by Nickel Complexes

Recent groundbreaking experimental reports demonstrated that Ni complexes bearing a bidentate- or tridentate-bipyridine-based ligand can be used to activate N2O for use as an O-transfer agent in C(sp2)-O bond formation reactions under mild experimental conditions. In this work, quantum chemical calculations are used to shed light on the mechanism through which such metal complexes catalytically activate nitrous oxide, providing new fundamental insights into the development of novel catalysts for N2O revalorization. As a case study, we consider the recent work by Cornella and co-workers (Nature, 2022, 604, 677) concerning the synthesis of phenols from aryl halides at room temperature, which requires the use of an external reducing agent. Our results suggest that the metal center remains in its Ni(II) oxidation state throughout the whole catalytic cycle, despite the presence of various redox steps in the mechanism and the Ni ability to maneuver between a number of oxidation states. This counterintuitive behavior is made possible by the ligand redox activity in the catalytic process, which involves accepting electrons from the reducing agent. Several possible pathways are systematically investigated, each associated with distinct activation modes, kinetics, and reaction outcomes. The governing factors in dictating the preferred path lie in the electronic nature of the ligand (strong vs weak field) and its geometric structure (specifically, the number of coordinating arms). These characteristics play a pivotal role in determining whether the process follows a catalytic or stoichiometric route and can be in principle modulated for the design of new metal complexes with tailored redox properties and reactivity.

Catalytic synthesis of phenols with nitrous oxide

The development of catalytic chemical processes that enable the revalorization of nitrous oxide (N₂O) is an attractive strategy to alleviate the environmental threat posed by its emissions^1–6. Traditionally, N₂O has been considered an inert molecule, intractable for organic chemists as an oxidant or O-atom transfer reagent, owing to the harsh conditions required for its activation (>150 °C, 50‒200 bar)^7–11. Here we report an insertion of N₂O into a Ni‒C bond under mild conditions (room temperature, 1.5–2 bar N₂O), thus delivering valuable phenols and releasing benign N₂. This fundamentally distinct organometallic C‒O bond-forming step differs from the current strategies based on reductive elimination and enables an alternative catalytic approach for the conversion of aryl halides to phenols. The process was rendered catalytic by means of a bipyridine-based ligands for the Ni centre. The method is robust, mild and highly selective, able to accommodate base-sensitive functionalities as well as permitting phenol synthesis from densely functionalized aryl halides. Although this protocol does not provide a solution to the mitigation of N₂O emissions, it represents a reactivity blueprint for the mild revalorization of abundant N₂O as an O source.

A study demonstrates that nitrous oxide can act as the source of O in a catalytic conversion of aryl halides to phenols, releasing N₂ as by-product.

Development of highly efficient platinum catalysts for hydroalkoxylation and hydroamination of unactivated alkenes

Hydrofunctionalization, the direct addition of an X-H (e.g., X=O, N) bond across an alkene, is a desirable strategy to make heterocycles that are important structural components of naturally occurring molecules. Described here is the design and discovery of "donor-acceptor"-type platinum catalysts that are highly effective in both hydroalkoxylation and hydroamination of unactivated alkenes over a broad range of substrates under mild conditions. A number of alkene substitution patterns are accommodated, including tri-substituted, 1,1-disubstituted, (E)-disubstituted, (Z)-disubstituted and even mono-substituted double bonds. Detailed mechanistic investigations suggest a plausible pathway that includes an unexpected dissociation/re-association of the electron-deficient ligand to form an alkene-bound "donor-acceptor"-type intermediate. These mechanistic studies help understand the origins of the high reactivity exhibited by the catalytic system, and provide a foundation for the rational design of chiral catalysts towards asymmetric hydrofunctionalization reactions.

How to tame a palladium terminal oxo

The isolation of terminal oxo complexes of the late transition metals promises new avenues in oxidation catalysis like the selective and catalytic hydroxylation of unreactive CH bonds, the activation of water, or the upgrading of olefins. While terminal oxo ligands are ubiquitous for early transition metals, well-characterized examples with group 10 metals remain hitherto elusive. In search for palladium terminal oxo complexes, the relative stability/reactivity of such compounds are evaluated computationally (CASSCF/NEVPT2; DFT). The calculations investigate only well-known ligand systems with established synthetic procedures and relevance for coordination chemistry and homogeneous catalysis. They delineate and quantify, which electronic properties of ancillary ligands are crucial for taming otherwise highly reactive terminal oxo intermediates. Notably, carbene ligands with both strong σ-donor and strong π-acceptor properties are best suited for the stabilization of palladium(ii) terminal oxo complexes, whereas ligands with a weaker ligand field lead to highly reactive complexes. Strongly donating ligands are an excellent choice for high-valent palladium(iv) terminal oxo compounds. Low coordinate palladium(ii) as well as high-valent palladium(iv) complexes are best suited for the activation of strong bonds.

DFT study of the reaction of a two-coordinate iron(II) dialkyl complex with molecular oxygen

DFT studies are reported of a monomeric iron dialkyl for which oxygen atom insertion into metal-methyl bonds occurs with O2: FeMe2 + O2 → Fe(OMe)2. Computation of the reaction coordinate implicates the intermediacy of Fe(III)-peroxo, Fe(VI)-dioxo, and Fe(IV)-oxo intermediates, connected by O2 oxidative addition and two methyl migration steps. Analysis of the reaction of O2 with d(6)-Fe(Me)2 indicates that oxy-insertion for this iron complex occurs with lower free energy barriers than competing homolytic/radical pathways, exploiting "spin-flip" processes via minimum energy crossing points (MECPs).

Oxy-functionalization of Group 9 and 10 transition metal methyl ligands: use of pyridine-based hemi-labile ligands

Hemi-labile ligands (HLLs) are intriguing candidates for catalysts since they may facilitate bond activation and bond formation through facile ligand dissociation/association.

Computational Hammett analysis of redox based oxy-insertion by Pt(II) complexes

A computational Hammett analysis of oxy-insertion into platinum-aryl bonds is performed. Modeled transformations involve the two-step conversion of [((X)bpy)Pt(R)(OY)](+) (R = p- or m-X-C(6)H(4); Y = 4- or 3-X-pyridine; (X)bpy = 4,4'- or 5,5'-X-bpy; X = NO(2), H, OMe, NMe(2)) proceeding through a Pt-oxo intermediate to form aryloxide [((X)bpy)Pt(OR)(Y)](+), which contrasts a one-step non-redox (Baeyer-Villiger) oxy-insertion. A structural connection is proposed between redox and non-redox transition states, linked to, among other parameters, oxidant identity. The electronic impact of the catalytic components is compared to previous Hammett studies on OMBV transformations. The Hammett sensitivity for aryl migration is diminished for the migrating group (R) and leaving group (Y), components as compared to OMBV transitions, while the bipyridine supporting ligand (L(n)) has an increased impact. The Hammett impact of R, Y and L(n) upon the aryl migration transition state is small in a global sense, ca. 5 kcal mol(-1); therefore, we conclude that the metal and oxidant are the most important factors in controlling oxy-insertion kinetics for these late metal systems. These results also point to a possible mechanistic advantage for redox over non-redox functionalization of hydrocarbons to alcohols.