C-H Activation and Hydrogen Isotope Exchange of Aryl Carbamates Using Iridium(I) Complexes Bearing Chelating NHC-Phosphine Ligands

Hydrogen isotope exchange (HIE) via C-H activation constitutes an efficient method for the synthesis of isotopically-enriched compounds, which are crucial components of the drug discovery process and are extensively employed in mechanistic studies. A series of iridium(I) complexes, bearing a chelating phosphine-N-heterocyclic carbene ligand, was designed and synthesized for application in the catalytic HIE of challenging N- and O-aryl carbamates. A broad range of substrates were labeled efficiently, and applicability to biologically-relevant systems was demonstrated by labeling an ʟ-tyrosine-derived carbamate with excellent levels of deuterium incorporation. Combined theoretical and experimental studies unveiled intriguing mechanistic features within this process, in comparison to C-H activation and hydrogen isotope exchange catalyzed by monodentate Ir(I) NHC/phosphine complexes.

meta-C-H Arylation of Aniline Derivatives via Palladium/ S,O-Ligand/Norbornene Cooperative Catalysis

Aromatic amines are ubiquitous moieties in organic molecules and their direct functionalization is of great interest in many research areas due to their prevalence in pharmaceuticals and organic electronics. While several synthetic tools exist for the ortho- and para-functionalization of anilines, the functionalization of the less reactive meta-position is not easy to achieve with current methods. To date, the meta-C-H arylation of aniline derivatives has been restricted to either the use of directing groups & templates, or their transformation into anilides & quaternary anilinium salts. Herein, we report the first general and efficient meta-C-H-arylation of non-directed aniline derivatives via cooperative catalysis with a palladium-S,O-ligand-norbornene system. The reaction proceeds under mild conditions with a wide range of aniline derivatives and aryl iodides, while being operationally simple and scalable. Our preliminary mechanistic investigation-including the isolation of several palladium complexes and deuterium experiments-reveal useful insights into the substituent-effects of both the aniline-substrate and the norbornene-mediator during the meta-C-H activation step.

Access to C4-arylated benzoxazoles from 2-amidophenol through C-H activation

A Pd-catalyzed aerobic approach to access C4-aryl benzoxazoles by tandem C-H ortho-arylation and acid-mediated annulation of 2-amidophenol has been presented. The directing potential of the -NHCOR group over the -OH group was exploited for selective arylation adjacent to the amide group. Deuterium labeling experiments suggest that palladation predominantly occurs adjacent to the -NHCOR group and is the key step during benzoxazole formation. One-pot hydrolysis of the resulting C4-arylated benzoxazole was also accomplished to access structurally challenging 3-aryl aminophenols for further applications.

Mechanisms for C(sp2)-Si activation of aryltrimethylsilyl groups in palladium-catalysed couplings

Silyl-substituted aromatic compounds can participate as the electrophilic component in palladium-catalysed cross-couplings, and reactivity is enhanced by a neighbouring silyl-group. Products analogous to those obtained from C-H activation chemistry are accessible by this means with the additional benefit of regiochemistry defined by the site of silyl substitution. DFT studies described here show that the mechanism of C-Si cleavage is distinct from previously recognised mechanisms for C-H cleavage, with a cascade of silyl intermediates en route to a stable product. The amide directing-groups are involved only in the stabilisation of palladacyclic intermediates, and are never disposed to activate silicon directly. 5-Membered and 6-membered palladacycles are known to behave differently in coupling reactions and the calculations reveal underlying reasons in the cationic pathways studied here.

Hydroxamic Acids as Chemoselective (ortho-Amino)arylation Reagents via Sigmatropic Rearrangement

The use of readily available hydroxamic acids as reagents for the chemoselective (ortho-amino)arylation of amides is described. This reaction proceeds under metal-free, mild conditions, displays a very broad scope, and constitutes a direct approach for the metal-free attachment of aniline residues to carbonyl derivatives.

Rhodium(iii)-catalyzed and MeOH-involved regioselective mono-alkenylation of N-arylureas with acrylates

A new and versatile Rh(iii)-catalyzed C–H activation/alkenylation/MeOH-involved ammonolysis/esterification cascade reaction has been disclosed.

One-pot synthesis of 2,3-difunctionalized indoles via Rh(iii)-catalyzed carbenoid insertion C–H activation/cyclization

A new and versatile Rh(iii)-catalyzed carbenoid insertion C–H activation/cyclization for one-pot synthesis of 2,3-difunctionalized indoles has been developed.

Cationic Pd(II)-catalyzed C-H activation/cross-coupling reactions at room temperature: synthetic and mechanistic studies

Cationic palladium(II) complexes have been found to be highly reactive towards aromatic C-H activation of arylureas at room temperature. A commercially available catalyst [Pd(MeCN)4](BF4)2 or a nitrile-free cationic palladium(II) complex generated in situ from the reaction of Pd(OAc)2 and HBF4, effectively catalyzes C-H activation/cross-coupling reactions between aryl iodides, arylboronic acids and acrylates under milder conditions than those previously reported. The nature of the directing group was found to be critical for achieving room temperature conditions, with the urea moiety the most effective in promoting facile coupling reactions at an ortho C-H position. This methodology has been utilized in a streamlined and efficient synthesis of boscalid, an agent produced on the kiloton scale annually and used to control a range of plant pathogens in broadacre and horticultural crops. Mechanistic investigations led to a proposed catalytic cycle involving three steps: (1) C-H activation to generate a cationic palladacycle; (2) reaction of the cationic palladacycle with an aryl iodide, arylboronic acid or acrylate, and (3) regeneration of the active cationic palladium catalyst. The reaction between a cationic palladium(II) complex and arylurea allowed the formation and isolation of the corresponding palladacycle intermediate, characterized by X-ray analysis. Roles of various additives in the stepwise process have also been studied.

Ru-Catalyzed selective C–H oxidative olefination with N-heteroarenes directed by pivaloyl amide

Ruthenium-catalysed C7-position alkynylation for indoline derivatives with various olefins was reported.

Palladium-catalysed directed C–H activation by anilides and ureas; water participation in a general base mechanism

A general-base pathway for CH-activation in anilides and ureas en route to palladacycles was developed through DFT calculations and fits available evidence.

Storable N-phenylcarbamate palladacycles for rapid functionalization of an alkyne-encoded protein

Here we report the synthesis of storable N-phenylcarbamate palladacycles that showed robust reactivity in the cross-coupling reaction with an alkyne-encoded protein with a second-order rate constant approaching 19 770 ± 930 M(-1) s(-1).

DFT studies on the directing group dependent arene–alkene cross-couplings: arene activation vs. alkene activation

Two competing reaction pathways for the dehydrogenative Heck couplings have been found. The properties of the directing groups on reactants will determine the dominance of these two pathways.

Transition metal-catalyzed C–H bond functionalizations by the use of diverse directing groups

In this review, a summary of transition metal-catalyzed C–H activation by utilizing the functionalities as directing groups is presented.

Storable palladacycles for selective functionalization of alkyne-containing proteins

We report the facile preparation of palladacycles as storable arylpalladium(II) reagents from acetanilides via cyclopalladation. The palladacycles exhibit good stability in PBS buffer and are capable of functionalizing a metabolically encoded HPG-containing protein, thus providing a new type of biocompatible organometallic reagent for selectively functionalizing the alkyne-encoded proteins.

Regioselective orthopalladation of (Z)-2-aryl-4-arylidene-5(4H)-oxazolones: scope, kinetico-mechanistic, and density functional theory studies of the C-H bond activation

Orthopalladated complexes derived from (Z)-2-aryl-4-arylidene-5(4H)-oxazolones have been prepared by reaction of the oxazolone with palladium acetate in acidic medium. The reaction is regioselective, only the ortho C-H bond of the arylidene ring being activated, producing a six-membered ring. The scope and reaction conditions of the orthopalladation are dependent on the acidity of the solvent. In CF(3)CO(2)H a large number of oxazolones can be metalated under mild conditions. As acidity decreases a lesser number of oxazolones can be efficiently palladated and harsher conditions must be used to achieve similar yields. The C-H bond activation in acidic medium agrees with an ambiphilic mechanism, as determined from kinetic measurements at variable temperature and pressure for different oxazolones substituted at the arylidene ring. The mechanism has been confirmed by density functional theory (DFT) calculations, where the formation of the six-membered ring is shown to be favored from both a kinetic and a thermodynamic perspective. In addition, the dependence of the reaction rate on the acidity of the medium has also been accounted for via a fine-tuning between the C-H agostic precoordination and the proton abstraction reaction in the overall process occurring on coordinatively saturated [Pd(κ(N)-oxazolone)(RCO(2)H)(3)](2+).