NMR Studies of the Species Present in Cross-Coupling Catalysis Systems Involving Pd(η3-1-Ph-C3H4)(η5-C5H5) and [Pd(η3-1-Ph-C3H4)Cl]2 Activated by PBut3, XPhos, and Mor-Dalphos: Nonexistence of Pd(XPhos)n and Pd(Mor-Dalphos)n (n = 1, 2) at Moderate Temperatures

A Robust HPLC Method for Easily Oxidizable Phosphine Ligand Analysis

Phosphine ligands are widely used in the manufacture of small molecule active pharmaceutical ingredients, for they play a key role in transition-metal-catalyzed cross-coupling. However, chromatographic analysis of phosphine ligands can be challenging because of the easily oxidizable nature of this class of compounds. This manuscript describes an out-of-specification (OOS) investigation study of XPhos raw material analysis by high performance liquid chromatography (LC). It is concluded that on-column degradation/oxidation is the culprit behind this OOS result. In addition, a slightly modified yet much improved new LC method is developed by adding a trace amount of tris(2-carboxyethyl)phosphine (TCEP) into the aqueous mobile phase. TCEP is also a phosphine compound and is commonly used as a reducing reagent in molecular biology. The trace amount of TCEP serves as a surrogate reagent to passivate the LC column and eliminate the on-column degradation/oxidation. As a result, a much more robust performance is achieved with greatly improved method precision and sensitivity. This is a general approach and can be applied to the LC analysis of many other phosphine ligands in the same manner.

Differences in the Performance of Allyl Based Palladium Precatalysts for Suzuki-Miyaura Reactions

Palladium(II) precatalysts are used extensively to facilitate cross-coupling reactions because they are bench stable and give high activity. As a result, precatalysts such as Buchwald's palladacycles, Organ's PEPPSI species, Nolan's allyl-based complexes, and Yale's 1-tert-butylindenyl containing complexes, are all commercially available. Comparing the performance of the different classes of precatalysts is challenging because they are typically used under different conditions, in part because they are reduced to the active species via different pathways. However, within a particular class of precatalyst, it is easier to compare performance because they activate via similar pathways and are used under the same conditions. Here, we evaluate the activity of different allyl-based precatalysts, such as (η³-allyl)PdCl(L), (η³-crotyl)PdCl(L), (η³-cinnamyl)PdCl(L), and (η³-1-tert-butylindenyl)PdCl(L) in Suzuki-Miyaura reactions. Specifically, we evaluate precatalyst performance as the ancillary ligand (NHC or phosphine), reaction conditions, and substrates are varied. In some cases, we connect relative activity to both the mechanism of activation and the prevalence of the formation of inactive palladium(I) dimers. Additionally, we compare the performance of in situ generated precatalysts with commonly used palladium sources such as tris(dibenzylideneacetone)dipalladium(0) (Pd₂dba₃), bis(acetonitrile)dichloropalladium(II) (Pd(CH₃CN)₂Cl₂), and palladium acetate. Our results provide information about which precatalyst to use under different conditions.

Formation of XPhos-Ligated Palladium(0) Complexes and Reactivity in Oxidative Additions

Understanding the nature of the intermediate species operating within a palladium catalytic cycle is crucial for developing efficient cross-coupling reactions. Even though the XPhos/Pd(OAc)₂ catalytic system has found numerous applications, the nature of the active catalytic species remains elusive. A Pd⁰ complex ligated to XPhos has been detected and characterized in situ for the first time using cyclic voltammetry and NMR techniques. In the presence of XPhos, Pd(OAc)₂ initially associates with the ligand to form a complex in solution, which has been characterized as Pd^II (OAc)₂ (XPhos). This Pd^II center is then reduced to the Pd⁰ (XPhos)₂ species by an intramolecular process. This study also sheds light on the formation of Pd^I -Pd^I dimers. Finally, a kinetic study probes a dissociative mechanism for the oxidative addition with aryl halides involving Pd⁰ (XPhos) as the reactive species in equilibrium with the unreactive Pd⁰ (XPhos)₂ . Remarkably, the reportedly poorly reactive PhCl reacts at room temperature in the oxidative addition, which confirms the crucial role of the XPhos ligand in the activation of aryl chlorides.

Evidence for a Cooperative Mechanism Involving Two Palladium(0) Centers in the Oxidative Addition of Iodoarenes

Oxidative addition of iodoarenes (ArI) to Pd⁰ ligated by 1-methyl-1H-imidazole (mim) in polar solvents leads to cationic arylpalladium(II) complexes [ArPd(mim)₃ ]⁺ . Kinetic analyses evidence that this reaction is second order with respect to the concentration of Pd⁰ , and a mechanism involving the cooperative intervention of two Pd⁰ centers has been postulated to explain this finding. This unusual behavior is also observed with other nitrogen-containing ligands and it is general for iodobenzenes substituted with electron-donating or weakly electron-withdrawing groups. In contrast, bromoarenes and electron-poor iodoarenes display first-order kinetics with respect to Pd⁰ . Theoretical calculations performed at the density functional theory (DFT) level suggest the existence of mim-ligated ArI-Pd⁰ complexes, in which the iodoarene is bound to the metal in a halogen-bond-like fashion. Coordination weakens the C-I bond and facilitates the oxidative insertion of another Pd⁰ center across this C-I bond. This conceptually novel mechanism, involving the cooperative participation of two distinct metal centers, allows a full explanation of the experimentally observed kinetics.

Palladium complexes of ferrocene-based phosphine ligands as redox-switchable catalysts in Buchwald–Hartwig cross-coupling reactions

Redox-switchable catalysis: Palladium(ii) complexes of two differently substituted [1]phosphaferrocenophanes FcPR (R = Mes, biaryl) and of diphenylferrocenyl phosphine Ph₂PFc were applied in redox-switchable Buchwald–Hartwig cross-coupling reactions.

The synthesis of planar chiral pseudo-gem aminophosphine pre-ligands based on [2.2]paracyclophane

We have synthesized three pseudo-gem [2.2]paracyclophane-derived P,N-ligands and report preliminary activity studies for the amination of aryl bromides and chlorides.

Dinuclear Pd(I) complexes with bridging allyl and related ligands

There are many important synthetic methods that utilize palladium catalysts. In most of these reactions, the palladium species are proposed to exist exclusively in either the Pd(0) or Pd(II) oxidation states. However, in the last decade, dinuclear Pd(I) complexes have repeatedly been isolated from reaction mixtures previously suggested to involve only species in the Pd(0) and Pd(II) oxidation states. As a consequence, in order to design improved catalysts there is considerable interest in understanding the chemistry of dinuclear Pd(I) complexes. A significant proportion of the known dinuclear Pd(I) complexes are supported by bridging allyl or related ligands such as cyclopentadienyl or indenyl ligands. This review provides a detailed account of the synthesis, electronic structure and stoichiometric reactivity of dinuclear Pd(I) complexes with bridging allyl and related ligands. Additionally, it describes recent work where dinuclear Pd(I) complexes with bridging allyl ligands have been detected in catalytic reactions, such as cross-coupling, and discusses the potential implications for catalysis.

Highly Efficient C-SeCF3 Coupling of Aryl Iodides Enabled by an Air-Stable Dinuclear Pd(I) Catalyst

Building on our recent disclosure of catalysis at dinuclear Pd(I) sites, we herein report the application of this concept to the realization of the first catalytic method to convert aryl iodides into the corresponding ArSeCF3 compounds. Highly efficient C-SeCF3 coupling of a range of aryl iodides was achieved, enabled by an air-, moisture-, and thermally stable dinuclear Pd(I) catalyst. The novel SeCF3 -bridged dinuclear Pd(I) complex 3 was isolated, studied for its catalytic competence and shown to be recoverable. Experimental and computational data are presented in support of dinuclear Pd(I) catalysis.

Generating Active "L-Pd(0)" via Neutral or Cationic π-Allylpalladium Complexes Featuring Biaryl/Bipyrazolylphosphines: Synthetic, Mechanistic, and Structure-Activity Studies in Challenging Cross-Coupling Reactions

Two new classes of highly active yet air- and moisture-stable π-R-allylpalladium complexes containing bulky biaryl- and bipyrazolylphosphines with extremely broad ligand scope have been developed. Neutral π-allylpalladium complexes incorporated a range of biaryl/bipyrazolylphosphine ligands, while extremely bulky ligands were accommodated by a cationic scaffold. These complexes are easily activated under mild conditions and are efficient for a wide array of challenging C-C and C-X (X = heteroatom) cross-coupling reactions. Their high activity is correlated to their facile activation to a 12-electron-based "L-Pd(0)" catalyst under commonly employed conditions for cross-coupling reactions, noninhibitory byproduct release upon activation, and suppression of the off-cycle pathway to form dinuclear (μ-allyl)(μ-Cl)Pd2(L)2 species, supported by structural (single crystal X-ray) and kinetic studies. A broad scope of C-C and C-X coupling reactions with low catalyst loadings and short reaction times highlight the versatility and practicality of these catalysts in organic synthesis.