Sustainable metal complexes for organic light-emitting diodes (OLEDs)

Pioneering studies by Kuivila, published more than 50 years ago, suggested ipso protonation of the boronate as the mechanism for base-catalyzed protodeboronation of arylboronic acids. However, the study was limited to UV spectrophotometric analysis under acidic conditions, and the aqueous association constants (Ka) were estimated. By means of NMR, stopped-flow IR, and quenched-flow techniques, the kinetics of base-catalyzed protodeboronation of 30 different arylboronic acids has now been determined at pH > 13 in aqueous dioxane at 70 °C. Included in the study are all 20 isomers of C6HnF(5-n)B(OH)2 with half-lives spanning 9 orders of magnitude: <3 ms to 6.5 months. In combination with pH-rate profiles, pKa and ΔS⧧ values, kinetic isotope effects (2H, 10B, 13C), linear free-energy relationships, and density functional theory calculations, we have identified a mechanistic regime involving unimolecular heterolysis of the boronate competing with concerted ipso protonation/C-B cleavage. The relative Lewis acidities of arylboronic acids do not correlate with their protodeboronation rates, especially when ortho substituents are present. Notably, 3,5-dinitrophenylboronic acid is orders of magnitude more stable than tetra- and pentafluorophenylboronic acids but has a similar pKa.

/pdf/base-catalyzed-aryl-b-oh-2-protodeboronation-revisited-from-2ky9b74z6k.pdf

Base-catalyzed Aryl-B(OH)2 Protodeboronation Revisited: from Concerted Proton-Transfer to Liberation of a Transient Arylanion.

Within the wide family of gold-catalyzed reactions, gold photocatalysis intrinsically features unique elementary steps. When gold catalysis meets photocatalysis, a valence change of the gold center can easily be achieved via electron transfer and radical addition, avoiding the use of stoichiometric sacrificial external oxidants. The excellent compatibility of radicals with gold catalysts opens the door to a series of important organic transformations, including redox-neutral C-C and C-X coupling, C-H activation, and formal radical-radical cross-coupling. The photocatalysis with gold complexes nicely complements the existing photoredox catalysis strategies and also opens a new avenue for gold chemistry. This review covers the achieved transformations for both mononuclear gold(I) catalysts (with and without a photosensitizer) and dinuclear gold(I) photocatalysts. Various fascinating methodologies, their value for organic chemists, and the current mechanistic understanding are discussed. The most recent examples also demonstrate the feasibility of both, mononuclear and dinuclear gold(I) complexes to participate in excited state energy transfer (EnT), rather than electron transfer. The rare applications of gold(III) photocatalysts, both homogeneous and heterogeneous, are also summarized.

Light in Gold Catalysis.

The reluctance of gold to achieve oxidative addition reaction is considered as an intrinsic limitation for the development of gold-catalyzed cross-coupling reactions with simple and ubiquitous aryl halide electrophiles. Here, we report the rational construction of a Au(I)/Au(III) catalytic cycle involving a sequence of Csp2–X oxidative addition, Csp2–H auration and reductive elimination, allowing a gold-catalyzed direct arylation of arenes with aryl halides. Key to this discovery is the use of Me-Dalphos, a simple ancillary (P,N) ligand, that allows the bottleneck oxidative addition of aryl iodides and bromides to readily proceed under mild conditions. The hemilabile character of the amino group plays a crucial role in this transformation, as substantiated by density functional theory calculations. Catalysis involving Au(I)/Au(III) cycles are notoriously hampered by the reluctance of Au(I) towards oxidative addition. Here, the authors show that an hemilabile bidentate ligand promotes oxidative addition of aryl halides to Au(I) and the catalytic formation of biaryl coupling products.

/pdf/rational-development-of-catalytic-au-i-au-iii-arylation-4utsgaulwx.pdf

Rational development of catalytic Au(I)/Au(III) arylation involving mild oxidative addition of aryl halides

Gold-catalyzed reactions have witnessed an exponential growth in the past decade. When the unique modes of activation exhibited by gold species meet species with either fluorinated building blocks or fluorinating reagents, new opportunities arise for the development of new methodologies in fluoroorganic chemistry. Indeed, gold and fluorine truly formed a very fruitful partnership, and different types of reactivity emerged from their combination. This review gives an overview of such endeavors. The special properties imparted by fluorine to organic molecules have been exploited in gold-catalyzed processes, allowing for the generation of unprecedented fluorinated chemical entities. Thus, the interaction of gold salts with fluorinated building blocks has been revised. In a second section, recent developments in gold-catalyzed nucleophilic fluorinations have been covered. The development of new gold catalysts that stabilize the Au–F bond as well as recent mechanistic studies in the field raised the interest o...

Fluorine and Gold: A Fruitful Partnership

This Review showcases the ability of bi- and tridentate ligands to stabilize gold in high oxidation states through the formation of mono- and biscyclometalated gold(III) complexes. In-depth studies on the synthesis, intrinsic reactivity, catalytic relevance, and photophysical properties of stabilized gold(III) species have been carried out, setting the stage for exciting developments in various research areas, such as catalysis, inorganic and bioinorganic chemistry, ligand design, and materials science.

Cyclometalated Gold(III) Complexes: Synthesis, Reactivity, and Physicochemical Properties

We report the design, synthesis, and application of a (N^C^C)-ligand framework able to stabilize highly electron-deprived gold(III) species. This novel platform enabled the preparation of C(sp(2))-gold(III) fluorides for the first time in monomeric, easy-to-handle, bench-stable form by a Cl/F ligand-exchange reaction. Devoid of oxidative conditions or stoichiometric use of toxic Hg salts, this method was applied to the preparation of multiple [C(sp(2))-Au(III)-F] complexes, which were used as mechanistic probes for the study of the unique properties and intrinsic reactivity of Au-F bonds. The improved photophysical properties of [(N^C^C)Au(III)] complexes compared to classical pincer (C^N^C)-Au systems paves the way for the design of new late-transition-metal-based OLEDs.

Luminescent (N^C^C) Gold(III) Complexes: Stabilized Gold(III) Fluorides

An efficient synthesis of biaryls through a gold-catalyzed oxidative cross-coupling of arenes with strong electron-deprived aryl boronates is presented herein. Regio- and chemocontrol are achieved by the selective activation of these coupling partners by gold at different oxidation states. Under reaction conditions devoid of basic additives or directing groups, the role of acetato ligand as an internal base has been revealed as a key parameter for expanding the reaction scope in these transformations.

Gold‐Catalyzed Direct Oxidative Arylation with Boron Coupling Partners

The ubiquity of tertiary alkylamines in pharmaceutical and agrochemical agents, natural products and small-molecule biological probes1,2 has stimulated efforts towards their streamlined synthesis3–9. Arguably the most robust method for the synthesis of tertiary alkylamines is carbonyl reductive amination3, which comprises two elementary steps: the condensation of a secondary alkylamine with an aliphatic aldehyde to form an all-alkyl-iminium ion, which is subsequently reduced by a hydride reagent. Direct strategies have been sought for a ‘higher order’ variant of this reaction via the coupling of an alkyl fragment with an alkyl-iminium ion that is generated in situ10–14. However, despite extensive efforts, the successful realization of a ‘carbonyl alkylative amination’ has not yet been achieved. Here we present a practical and general synthesis of tertiary alkylamines through the addition of alkyl radicals to all-alkyl-iminium ions. The process is facilitated by visible light and a silane reducing agent, which trigger a distinct radical initiation step to establish a chain process. This operationally straightforward, metal-free and modular transformation forms tertiary amines, without structural constraint, via the coupling of aldehydes and secondary amines with alkyl halides. The structural and functional diversity of these readily available precursors provides a versatile and flexible strategy for the streamlined synthesis of complex tertiary amines. The synthesis of tertiary amines is achieved through a carbonyl alkylative amination reaction facilitated by visible light, in which an aldehyde and an amine condense to form an iminium ion that subsequently reacts with alkyl radical.

A general carbonyl alkylative amination for tertiary amine synthesis.

The underlying reactivity of AuIII–F species with aryl boronic acids has been studied in detail taking advantage of four novel, stable difluoro-[(C^N)AuF2], arylmonofluoro-[(C^N)AuArF], and alkylmonofluoro-[(C^N)AuAlkF] gold(III) complexes, prepared and isolated in monomeric form. We provide the first experimental evidence for a direct AuIII–F/B transmetalation preceding the Csp2–Csp2 or Csp3–Csp2 bond formation.

Roopender Kumar

Papers

Cyclometalated Gold(III) Complexes: Synthesis, Reactivity, and Physicochemical Properties

Luminescent (N^C^C) Gold(III) Complexes: Stabilized Gold(III) Fluorides

Gold‐Catalyzed Direct Oxidative Arylation with Boron Coupling Partners

A general carbonyl alkylative amination for tertiary amine synthesis.

Evidence for Direct Transmetalation of AuIII–F with Boronic Acids