Mechanism of C-F reductive elimination from palladium(IV) fluorides

Organo-palladium(ii) complexes bearing unsymmetrical N,N,N-pincer ligands: synthesis, structures and oxidatively induced coupling reactions

Oxidation of the palladium-phenyl N,N,N_mipp-pincer with Selectfluor leads to C–C coupling to give biphenyl; neither C–C nor C–F coupling is observed using the more sterically bulky N,N,N_dipp-pincer analogue.

A practical and general ipso iodination of arylboronic acids using N-iodomorpholinium iodide (NIMI) as a novel iodinating agent: mild and regioselective synthesis of aryliodides

A mild and efficient protocol for the ipso-iodination of aryl boronic acids using N-iodomorpholinium iodide (NIMI) generated in situ from morpholine and molecular iodine as a novel iodinating agent has been developed.

Mechanism of Pd-catalyzed C(sp3)–H activation of aliphatic amines: an insight from DFT calculations

DFT studies on Pd-catalyzed C(sp³)–H activation of aliphatic amines have been performed using the B3LYP functional. The rate- and regio-determining step of the catalytic cycle is deprotonation of the C_methyl–H bond through a six-membered cyclopalladation transition state.

Combining Pd(π-allyl)Cp and PPh3 as a unique catalyst for efficient synthesis of alkyliodo indoles via C(sp3)–I reductive elimination

Alkyl iodides were formed via reductive elimination of alkylpalladium iodides possessing syn-β-hydrogens.

O,N,N-Pincer ligand effects on oxidatively induced carbon–chlorine coupling reactions at palladium

The structural properties of a series of ONN-Pd(ii) chloride pincer complexes have been shown to influence the conversion to 4-chlorotoluene upon oxidation with di-p-tolyliodonium triflate.

Studying regioisomer formation in the Pd-catalyzed fluorination of aryl triflates by deuterium labeling

Isotopic labeling has been used to determine that a portion of the desired product in the Pd-catalyzed fluorination of electron-rich, non-ortho-substituted aryl triflates results from direct C–F cross-coupling. In some cases, formation of a Pd-aryne intermediate is responsible for producing undesired regioisomers. The generation of the Pd-aryne intermediate occurs primarily via ortho-deprotonation of a L·Pd(Ar)OTf (L = biaryl monophosphine) species by CsF and thus competes directly with the transmetalation step of the catalytic cycle. Deuterium labeling studies were conducted with a variety of aryl triflates.

Anion-Dependent Switch in C-X Reductive Elimination Diastereoselectivity

Reaction of a complex Pt organometallic species with electrophilic halogen sources in the presence of X- ligands changes the mechanism of reductive elimination from a concerted reductive coupling type to an SN2 type reductive elimination. In the absence of the added X- ligand the reductive elimination is stereoretentive; in its presence, the process is stereoinvertive. This selectivity hinges on the reactivity of a key five-coordinate Pt(IV) intermediate with the X- ligand.

PhenoFluor: Practical Synthesis, New Formulation, and Deoxyfluorination of Heteroaromatics

We report a practical synthesis method of the reagent PhenoFluor on decagram scale, provide a new formulation of PhenoFluor as a toluene solution, which should decrease challenges associated with the moisture sensitivity of the reagent, and expand the substrate scope of deoxyfluorination with PhenoFluor to heteroaromatics.

Radical Pd(III)/Pd(I) reductive elimination in palladium sequences

Open-shell mechanisms are often at work in catalytic sequences involving first-row transition metals while usually not considered in palladium chemistry. Herein a computational study suggests their possible relevance in catalytic methods involving paramagnetic Pd(iii) intermediates. Indeed C-C bond forming reductive elimination previously thought to occur in Pd(iv) complexes has lower barriers in neutral, radical Pd(iii) intermediates instead. These species could form upon addition on Pd(ii) of an aryl radical generated via single electron transfer from a photo-active ruthenium complex and have the perfect stereoelectronic arrangement to smoothly undergo the coupling process.

Late-Stage Fluorination: From Fundamentals to Application

In this brief account, we review work from our lab with a focus on late-stage introduction of fluorine and fluorinated functional groups into small molecules. We attempt to highlight practical developments, which we believe may have potential for industrial applications, and critically reflect on developments that may not yet meet the bar for practical use.

Asymmetric palladium-catalyzed directed intermolecular fluoroarylation of styrenes

A mild catalytic asymmetric direct fluoro-arylation of styrenes has been developed. The palladium-catalyzed three-component coupling of Selectfluor, a styrene and a boronic acid, provides chiral monofluorinated compounds in good yield and in high enantiomeric excess. A mechanism proceeding through a Pd(IV)-fluoride intermediate is proposed for the transformation and synthesis of an sp(3) C-F bond.

Competition between sp³-C-N vs sp³-C-F reductive elimination from Pd(IV) complexes

This communication describes the design of a model system that allows direct investigation of competing sp(3)-C-N and sp(3)-C-F bond-forming reductive elimination from a Pd(IV) fluoro sulfonamide complex. The reductive elimination selectivity varies dramatically as a function of reaction additives. A mechanism is proposed that provides a rationale for these effects.

Copper-mediated C(sp2)–H amination using TMSN3 as a nitrogen source: redox-neutral access to primary anilines

A Cu-mediated C–H amination for the preparation of primary anilines under redox neutral conditions is developed.

Recent advances in C–H fluorination

Highlighted in this article are recent advances in C(sp²)–H and C(sp³)–H fluorination reactions.

Mechanism of Electrophilic Fluorination with Pd(IV): Fluoride Capture and Subsequent Oxidative Fluoride Transfer

Electrophilic fluorinating reagents derived from fluoride are desirable for the synthesis of 18F-labeled molecules for positron emission tomography (PET). Here, we study the mechanism by which a Pd(IV)-complex captures fluoride and subsequently transfers it to nucleophiles. The intermediate Pd(IV)-F is formed with high rates even at the nano- to micromolar fluoride concentrations typical for radiosyntheses with 18F due to fast formation of an outer-sphere complex between fluoride and Pd(IV). The subsequent fluorine transfer from the Pd(IV)-F complex is proposed to proceed through an unusual SET/fluoride transfer/SET mechanism. The findings detailed in this manuscript provide a theoretical foundation suitable for addressing a more general approach for electrophilic fluorination with high specific activity 18F PET imaging.

Dioxygen activation by an organometallic Pd(ii) precursor: formation of a Pd(iv)–OH complex and its C–O bond formation reactivity

An isolable Pd(iv)–OH complex formed upon aerobic oxidation of an organometallic Pd(ii) precursor exhibits selective C(sp²)–O bond formation reactivity.

Pd(OAc)2-catalyzed regioselective aromatic C-H bond fluorination

A novel Pd(OAc)2-NFSI-TFA system was developed for the highly selective ortho-monofluorination directed by diverse aryl-N-heterocyclic directing groups e.g., quinoxaline, pyrazole, benzo[d]oxazole, and pyrazine derivatives. A Pd(II/IV) catalytic cycle was proposed based on the ESI-MS/MS studies.

An improved catalyst system for the Pd-catalyzed fluorination of (hetero)aryl triflates

The stable Pd(0) species [(1,5-cyclooctadiene)(L·Pd)2] (L = AdBrettPhos) has been prepared and successfully evaluated as a precatalyst for the fluorination of aryl triflates derived from biologically active and heteroaryl phenols, challenging substrates for our previously reported catalyst system. Additionally, this precatalyst activates at room temperature under neutral conditions, generates 1,5-cyclooctadiene as the only byproduct, and leads to overall cleaner reaction profiles.

Palladium(III)-catalyzed fluorination of arylboronic acid derivatives

A practical, palladium-catalyzed synthesis of aryl fluorides from arylboronic acid derivatives is presented. The reaction is operationally simple and amenable to multigram-scale synthesis. Evaluation of the reaction mechanism suggests a single-electron-transfer pathway, involving a Pd(III) intermediate that has been isolated and characterized.

Unprecedented 1,3-migration of the aryl ligand in metallacyclic aryl α-naphthyl Pt(IV) difluorides to produce β-arylnaphthyl Pt(II) complexes

Electrophilic fluorination of aryl α-naphthyl Pt(II) complexes leads to an unprecedented 1,3-migration of the aryl ligand to the β-position of the naphthyl group. The reaction proceeds via the initial oxidative addition of two fluoro ligands to the Pt center followed by C(sp(2))-C(sp(2)) coupling and aryl migration.

Introduction of fluorine and fluorine-containing functional groups

Over the past decade, the most significant, conceptual advances in the field of fluorination were enabled most prominently by organo- and transition-metal catalysis. The most challenging transformation remains the formation of the parent C-F bond, primarily as a consequence of the high hydration energy of fluoride, strong metal-fluorine bonds, and highly polarized bonds to fluorine. Most fluorination reactions still lack generality, predictability, and cost-efficiency. Despite all current limitations, modern fluorination methods have made fluorinated molecules more readily available than ever before and have begun to have an impact on research areas that do not require large amounts of material, such as drug discovery and positron emission tomography. This Review gives a brief summary of conventional fluorination reactions, including those reactions that introduce fluorinated functional groups, and focuses on modern developments in the field.