Phosphine-Free Palladium-Catalyzed Direct Arylation of Imidazo[1,2-a]pyridines with Aryl Bromides at Low Catalyst Loading

Ligand-free Pd­(OAc)₂ was found to catalyze very efficiently the direct arylation of imidazo­[1,2-a]­pyridines at C3 under very low catalyst concentration. The reaction can be performed employing as little as 0.1–0.01 mol % catalyst with electron-deficient and some electron-excessive aryl bromides

Cyclometalations on the Imidazo[1,2‑<i>a</i>][1,8]naphthyridine Framework

Cyclometalation on the substituted imidazo­[1,2-<i>a</i>]­[1,8]­naphthyridine platform involves either the C₃-aryl or C₄′-aryl <i>ortho</i> carbon and the imidazo nitrogen N₃′. The higher donor strength of the imidazo nitrogen in comparison to that of the naphthyridine nitrogen aids regioselective orthometalation at the C₃/C₄′-aryl ring with Cp*Ir^III (Cp* = η⁵-pentamethylcyclopentadienyl). A longer reaction time led to double cyclometalations at C₃-aryl and imidazo C₅′-H, creating six- and five-membered metallacycles on a single skeleton. Mixed-metal Ir/Sn compounds are accessed by insertion of SnCl₂ into the Ir–Cl bond. Pd­(OAc)₂ afforded an acetate-bridged dinuclear ortho-metalated product involving the C₃-aryl unit. Metalation at the imidazo carbon (C₅′) was achieved via an oxidative route in the reaction of the bromo derivative with the Pd(0) precursor Pd₂(dba)₃ (dba = dibenzylideneacetone). Regioselective C–H/Br activation on a rigid and planar imidazonaphthyridine platform is described in this work

Cyclometalations on the Imidazo[1,2‑<i>a</i>][1,8]naphthyridine Framework

Cyclometalation on the substituted imidazo­[1,2-<i>a</i>]­[1,8]­naphthyridine platform involves either the C₃-aryl or C₄′-aryl <i>ortho</i> carbon and the imidazo nitrogen N₃′. The higher donor strength of the imidazo nitrogen in comparison to that of the naphthyridine nitrogen aids regioselective orthometalation at the C₃/C₄′-aryl ring with Cp*Ir^III (Cp* = η⁵-pentamethylcyclopentadienyl). A longer reaction time led to double cyclometalations at C₃-aryl and imidazo C₅′-H, creating six- and five-membered metallacycles on a single skeleton. Mixed-metal Ir/Sn compounds are accessed by insertion of SnCl₂ into the Ir–Cl bond. Pd­(OAc)₂ afforded an acetate-bridged dinuclear ortho-metalated product involving the C₃-aryl unit. Metalation at the imidazo carbon (C₅′) was achieved via an oxidative route in the reaction of the bromo derivative with the Pd(0) precursor Pd₂(dba)₃ (dba = dibenzylideneacetone). Regioselective C–H/Br activation on a rigid and planar imidazonaphthyridine platform is described in this work

Palladium-Catalyzed Direct Arylation of 5‑Chloropyrazoles: A Selective Access to 4‑Aryl Pyrazoles

The use of a temporary protection by a chloro group at C5 of pyrazoles allows the synthesis of the 4-arylated pyrazoles, which were previously inaccessible by palladium-catalyzed direct arylation, with complete regioselectivity and in high yields using in most cases as little as 0.5–0.1 mol % Pd­(OAc)₂ as the catalyst with electron-deficient aryl bromides. Moreover, from 5-chloro-1,3-dimethylpyrazole, sequential catalytic C4 arylation, dechlorination, catalytic C5 arylation reactions allowed the synthesis of a 4,5-diarylated pyrazole derivative

Palladium-Catalyzed Direct Arylation of Heteroaromatics with Activated Aryl Chlorides Using a Sterically Relieved Ferrocenyl-Diphosphane

The palladium-catalyzed direct arylations at C3 or C4 positions of heteroaromatics are known to be more challenging than at C2 or C5 positions. Aryl chlorides are also challenging substrates for direct arylation of heteroaromatics. We observed that in the presence of a palladium-catalyst combining only 0.5 mol % of Pd­(OAc)₂ with the sterically relieved new ferrocenyl diphosphane <b>Sylphos</b>, the direct arylation at C3 or C4 of oxazoles, a benzofuran, an indole, and a pyrazole was found to proceed in moderate to high yields using a variety of electron deficient aryl chlorides. Turnover numbers up to 176 have been obtained with this catalyst. Assessment of the electron-donating properties of <b>Sylphos</b> from electrochemical studies and ¹<i>J</i>_PSe measurement on its selenide derivative indirectly indicated that the influence of steric properties of <b>Sylphos</b>–and in particular a less sterically congested environment at phosphorus due to a methylene spacer–are certainly dominant in its catalytic performance

Palladium-Catalyzed Regioselective C–H Bond Arylations of Benzoxazoles and Benzothiazoles at the C7 Position

We report herein, a very simple catalytic system for the direct arylation of benzoxazole and benzothiazole derivatives at C7 position, namely, phosphine-free PdCl₂ associated with PivOK in NMP at 150 °C. (Thio)­phenoxy chelation-assisted Pd-catalyzed C–H bond cleavage, from an opened intermediate, was proposed to explain this unique regioselectivity. This reaction allows the synthesis of 2-amino-6-arylphenols through the ring opening of the benzoxazole

Palladium Complexes with Tetrahydropyrimidin-2-ylidene Ligands: Catalytic Activity for the Direct Arylation of Furan, Thiophene, and Thiazole Derivatives

The synthesis and characterization of novel 1,3-benzyl-3,4,5,6-tetrahydropyrimidin-2-ylidene-based N-heterocyclic carbene palladium­(II) complexes (<b>1a</b>–<b>d</b>) were described. The crystal structure of <i>trans-</i>dichlorobis­[1,3-bis­(4-methylbenzyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene]­palladium­(II) was presented. Pd­(II) complexes <b>1a</b>–<b>d</b> were tested as catalysts in the direct C5 or C2 arylation of furans, thiophenes, and thiazoles, with various aryl bromides at 150 °C for 1 h. These complexes exhibited moderate to high catalytic activities under the given conditions

Isoquinoline Derivatives via Stepwise Regioselective sp² and sp³ C–H Bond Functionalizations

Efficient and practically attractive stepwise ruthenium- and palladium-catalyzed regioselective C–H bond functionalizations were achieved to produce 4-substituted tetrahydroisoquinoline derivatives featuring various heteroaromatic substructures in moderate to good yields. Both ruthenium- and palladium-based catalytic processes generated nontoxic and easily separable side products

From Molecules to Devices: Effect of Pd-Catalyzed C–H Bond Arylation to Improve Metrics

In this study, we detail advancements in the design and application of neutral, arylated, cyclometalated iridium(III) complexes in the use of OLEDs. We utilized the “catalysis-on-the-complex” approach to directly arylate (C^N)2Ir(acac) [where C^N represents either 2-(2,4-difluorophenyl)pyridine or 2-(2,4-difluorophenyl)-5-methoxypyridine] through Pd-catalyzed C–H bond arylation with 4-bromobenzotrifluoride. The reaction regioselectivity occurred on both cyclometalated ligands at the C–H bonds flanked by two fluorine atoms. Photophysical measurements in solution indicated that while the emission energy remained unchanged (blue colors, λmax EL = 482–490 nm), a significant enhanced quantum yield was observed for arylated complex. Impressively, devices incorporating these complexes as green-blue emitters revealed that arylation also boosts device performance metrics in terms of luminance efficiency, external quantum efficiency (EQE), and brightness. In contrast, the introduction of methoxy substituent at the meta-position of the nitrogen–iridium bond yielded modest metrics. This study illustrates the crucial role of ligand design in the rational development of blue emitters

Kinetic and Electrochemical Studies of the Oxidative Addition of Demanding Organic Halides to Pd(0): the Efficiency of Polyphosphane Ligands in Low Palladium Loading Cross-Couplings Decrypted

Oxidative addition (OA) of organic halides to palladium(0) species is a fundamental reaction step which initiates the C–C bond formation catalytic processes typical of Pd(0)/Pd­(II) chemistry. The use of structurally congested <i>polyphosphane</i> ligands in palladium-catalyzed C–C bond formation has generated very high turnover numbers (TONs) in topical reactions such as Heck, Suzuki, Sonogashira couplings, and direct sp²C–H functionalization. Herein, the OA of aryl bromides to Pd(0) complexes stabilized by ferrocenylpolyphosphane ligands <b>L1</b> (tetraphosphane), <b>L2</b> (triphosphane), and <b>L3</b> (diphosphane) is considered. The investigation of kinetic constants for the addition of Ph–Br to Pd(0) intermediates (generated by electrochemical reduction of Pd­(II) complexes coordinated by <b>L1</b>–<b>L3</b>) is reported. Thus, in the OA of halides to the Pd(0) complex coordinated by <b>L1</b> the series of rate constants <i>k</i>_app is found (mol^–1 L s^–1): <i>k</i>_app(Ph–Br) = 0.48 > <i>k</i>_app(ClCH₂–Cl) = 0.25 ≫ <i>k</i>_app(<i>p</i>-MeC₆H₄–Br) = 0.08 ≈ <i>k</i>_app(<i>o</i>-MeC₆H₄–Br) = 0.07 ≫ <i>k</i>_app(Ph–Cl). Kinetic measurements clarify the influence that the presence of four, three, or two phosphorus atoms in the coordination sphere of Pd has on OA. The presence of supplementary phosphorus atoms in <b>L1</b> and <b>L2</b> unambiguously stabilizes Pd(0) species and thus slows down the OA of Ph–Br to Pd(0) of about 2 orders of magnitude compared to the diphosphane <b>L3</b>. The electrosynthesis of the complexes resulting from the OA of organic halides to [Pd(0)/<b>L</b>] is easily performed and show the concurrent OA to Pd(0) of the sp³C–Cl bond of dichloromethane solvent. The resulting unstable Pd/alkyl complex is characterized by NMR and single crystal X-ray structure. We additionally observed the perfect stereoselectivity of the OA reactions which is induced by the tetraphosphane ligand <b>L1</b>. Altogether, a clearer picture of the general effects of congested polydentate ligands on the OA of organic halides to Pd(0) is given