Effect of Internal Ligand Strain on Coordination Behavior of PSP Pincer Ligands

Chelating ligands and most specifically pincer ligands, with their characteristic co-planar binding, usually undergo deformations upon coordination, resulting in a significant ligand strain. Such an effect on the properties of the so formed complex has rarely been explored. This study is an attempt to analyze this strain and its contribution to the overall binding energy and coordination behavior of PSP pincer ligands. Hence, we designed a rigid thioxanthone-based PSP pincer ligand (I) and studied the difference in the coordination properties with the more flexible thioxanthene and thioether-based PSP pincer ligands (II and III). Although with one equivalent of Pd(II) precursor, the three ligands exhibited a similar coordination behavior leading to similar κ³-P,S,P pincer complexes, an in-depth computational analysis pointed out the different contributions of the internal strain energy in lowering the binding energy of these complexes. This effect was clearly reflected when we calculated the enthalpy change of these ligand-exchange reactions. As these exchange reactions are enthalpy-driven, these results could also be confirmed experimentally. With two equivalents of Pd(II), the three ligands diverged in their coordination behavior. Specifically, ligands I and III gave each a binuclear complex, with different coordination modes, whereas the pincer complex of ligand II remained unaffected by excess of Pd(II). Our calculations suggest that the driving force for the formation of binuclear Pd(II) complexes is the relief of the internal ligand strain. With Pt(II), only the mononuclear κ³-P,S,P pincer complexes were obtained irrespectively of the amount of the Pt(II) precursor. In these cases, we assume that kinetic inertness of the formed mononuclear pincer Pt(II) complexes prevents binding of an additional Pt(II) nucleus. This study points out the important role of the internal ligand strain in PSP pincer ligand coordination behavior. We believe that our findings can be extended to other pincer ligands systems as well.

Synthesis of 2-Alkylaryl and Furanyl Acetates by Palladium Catalysed Carbonylation of Alcohols

The one-pot alkoxycarbonylation of halo-free alkylaryl and furanyl alcohols represents a sustainable alternative for the synthesis of alkylaryl and furanyl acetates. In this paper, the reaction between benzyl alcohol, chosen as a model substrate, CH3OH and CO was tested in the presence of a homogeneous palladium catalyst, an activator (isopropenyl acetate (IPAc) or dimethyl carbonate (DMC)) and a base (Cs2CO3). The influence of various reaction parameters such as the CO pressure, ligand and palladium precursor employed, mmol% catalyst load, temperature and time were investigated. The results demonstrate that decreasing the CO pressure from 50 bar to 5 bar at 130 °C for 18 h increases yields in benzyl acetate from 36% to over 98%. Further experiments were performed in the presence of piperonyl and furfuryl alcohol, interesting substrates employed for the synthesis of various fine chemicals. Moreover, furfuryl alcohol is a lignocellulosic-derived building block employed for the synthesis of functionalized furans such as 2-alkylfurfuryl acetates. Both the alcohols were successfully transformed in the corresponding acetate (yields above 96%) in rather mild reaction conditions (5–0.01 mol% catalyst, 5–2 bar CO pressure, 130 °C, 4–18h), demonstrating that the alkoxycarbonylation of alcohols represents a promising sustainable alternative to more impactful industrial practices adopted to date for the synthesis of alkylaryl and furfuryl acetates.

Synthesis and reactivity of PC(sp3)P-pincer iridium complexes bearing a diborylmethyl anion

Novel PCP-pincer iridium complexes bearing a diborylmethyl anion were synthesized. Strong σ-electron-donation to the metal and significant π-backdonation from the metal to boron atoms at the β-position were observed both experimentally and computationally. H/D exchange of the aromatic C-H bond proceeded smoothly and, in addition, the α-methine-hydrogen between boron atoms was found to be replaced with deuterium in benzene-d₆ solution possibly through diborylcarbene metal complexes as intermediates.

A sustainable route for the synthesis of alkyl arylacetates via halogen and base free carbonylation of benzyl acetates

The Pd-catalysed carbonylation of benzyl acetates for the synthesis of 2-alkylbenzyl acetates in the absence of base and halogen sources was investigated.

Catalytic Carbochlorocarbonylation of Unsaturated Hydrocarbons via C-COCl Bond Cleavage*

Here we report a palladium‐catalysed difunctionalisation of unsaturated C−C bonds with acid chlorides. Formally, the C−COCl bond of an acid chloride is cleaved and added, with complete atom economy, across either strained alkenes or a tethered alkyne to generate new acid chlorides. The transformation does not require exogenous carbon monoxide, operates under mild conditions, shows a good functional group tolerance, and gives the isolated products with excellent stereoselectivity. The intermolecular reaction tolerates both aryl‐ and alkenyl‐substituted acid chlorides and is successful when carboxylic acids are transformed to the acid chloride in situ. The reaction also shows an example of temperature‐dependent stereodivergence which, together with plausible mechanistic pathways, is investigated by DFT calculations. Moreover, we show that benzofurans can be formed in an intramolecular variant of the reaction. Finally, derivatisation of the products from the intermolecular reaction provides a highly stereoselective approach for the synthesis of tetrasubstituted cyclopentanes.

Application of 4-pyridylselenolate palladium macrocycles in Suzuki couplings

Pd macrocycles of diphosphine were constructed employing 4-pyridylselenolate. They act as excellent catalysts in the Suzuki coupling reaction.

Ranking Ligands by Their Ability to Ease (C6F5)2NiIIL → Ni0L + (C6F5)2 Coupling versus Hydrolysis: Outstanding Activity of PEWO Ligands

The Ni^II literature complex cis-[Ni(C₆F₅)₂(THF)₂] is a synthon of cis-Ni(C₆F₅)₂ that allows us to establish a protocol to measure and compare the ligand effect on the Ni^II → Ni⁰ reductive elimination step (coupling), often critical in catalytic processes. Several ligands of different types were submitted to this Ni-meter comparison: bipyridines, chelating diphosphines, monodentate phosphines, PR₂(biaryl) phosphines, and PEWO ligands (phosphines with one potentially chelate electron-withdrawing olefin). Extremely different C₆F₅-C₆F₅ coupling rates, ranging from totally inactive (producing stable complexes at room temperature) to those inducing almost instantaneous coupling at 25 °C, were found for the different ligands tested. The PR₂(biaryl) ligands, very efficient for coupling in Pd, are slow and inefficient in Ni, and the reason for this difference is examined. In contrast, PEWO type ligands are amazingly efficient and provide the lowest coupling barriers ever observed for Ni^II complexes; they yield up to 96% C₆F₅-C₆F₅ coupling in 5 min at 25 °C (the rest is C₆F₅H) and 100% coupling with no hydrolysis in 8 h at -22 to -53 °C.

Silylpalladium Cations Enable the Oxidative Addition of C(sp3)-O Bonds

The synthesis and characterization of the room-temperature and solution-stable silylpalladium cations (PCy₃)₂Pd-SiR₃⁺(C₆F₅)₄B^- (SiR₃ = SiMe₂Et, SiHEt₂) and (Xantphos)Pd-SiR₃⁺(BAr^f₄) (SiR₃ = SiMe₂Et, SiHEt₂; Xantphos = 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene; BAr^f₄ = (3,5-(CF₃)₂C₆H₃)₄B^-) are reported. Spectroscopic and ligand addition experiments suggest that silylpalladium complexes of the type (PCy₃)₂Pd-SiR₃⁺ are three-coordinate and T-shaped. Addition of dialkyl ethers to both the PCy₃ and Xantphos-based silylpalladium cations resulted in the cleavage of C(sp³)-O bonds and the generation of cationic Pd-alkyl complexes. Mechanistically enabling is the ability of silylpalladium cations to behave as sources of both electrophilic silylium ions and nucleophilic L_nPd(0).

Featuring Xantphos

This review highlights the use of the bisphosphine ligand group in homogeneous catalysis.

Monodentate phosphine substitution in [Pd(κ3-dppf)(PR3)][BF4]2 (dppf = 1,1′-bis(diphenylphosphino)ferrocene) compounds

The synthesis and dissociative monodentate phosphine substitution of [Pd(κ³-dppf)(P(p-C₆H₄R)₃)][BF₄]₂ compounds is presented. DFT calculations further examines the nature of the Fe–Pd interaction of the κ³-dppf ligand.

Coordination of bis(azol-1-yl)methane-based bisphosphines towards RuII, RhI, PdII and PtII: synthesis, structural and catalytic studies

This paper describes the transition metal chemistry of three bis(X-diphenylphosphino-azol-1-yl)-based bisphosphines: bis(2-Ph₂P-imidazol-1-yl)methane (1), bis(5-Ph₂P-pyrazol-1-yl)methane (2) and bis(5-Ph₂P-1,2,4-triazol-1-yl)methane (3). These show versatility in their coordination behaviour.

Steric and Electronic Influences of Buchwald-Type Alkyl-JohnPhos Ligands

The electron-donating and steric properties of Buchwald-type ligands ([1,1'-biphenyl-2-yl]dialkylphosphine; R-JohnPhos, where R = Me, Et, (i)Pr, Cy, (t)Bu) were determined. The π-acidity and σ-donating properties of the R-JohnPhos ligands were quantified using a Cotton-Kraihanzel analysis of the Cr(0)(CO)5(R-JohnPhos) complexes. Somewhat surprisingly, the σ-donating abilities of the R-JohnPhos ligands follow the trend (t)Bu-JohnPhos < Et-JohnPhos < (i)Pr-JohnPhos < Cy-JohnPhos ≪ Me-JohnPhos. This ordering is proposed to arise from competition between the intrinsic electron-donating ability of the R groups (Me < Et < (i)Pr ≈ Cy < (t)Bu) and steric interactions (front and back strain) that decrease the electron-donating ability of the phosphine. X-ray crystallographic data of 22 metal complexes (general forms: trans-Cr(0)(CO)4(PR3)2, Pd(0)(PR3)2(η(2)-dba), and trans-Pd(II)(Cl)2(PR3)2) were also analyzed to help explain the electronic trends measured for the R-JohnPhos ligands. The R-JohnPhos ligands are exceptionally sensitive to back strain in comparison to typical phosphines, and the strong σ-donating ability of the Methyl-JohnPhos ligand is attributed to its ability to avoid both front strain and back strain. Consequently, the -PMe2 moiety allows for very short phosphorus-metal bond distances. Because of the sterically dominating o-biphenyl and close phosphorus-metal bond distances, MeJPhos maintains a large overall steric profile that is actually larger than that of CyJPhos as measured by percent buried volume (%V(bur)). Overall, the -PMe2 moiety is a powerful way to incorporate strong σ-donation into "designer" phosphines while retaining other advantageous structural and reactivity properties.

Competing amination and C–H arylation pathways in Pd/xantphos-catalyzed transformations of binaphthyl triflates: switchable routes to chiral amines and helicene derivatives

A Pd(OAc)₂/xantphos catalyst system can be tuned to promote either amination or C–H arylation of hindered binaphthyl 2-triflates, with xantphos's hemilability playing a key role.

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.

Palladium catalyzed oxidative carbonylation of alcohols: effects of diphosphine ligands

The best catalytic performance was exhibited by a Lewis base P∩P ligand with a relatively wide bite angle capable of maintaining a cis geometry.

Heterobimetallic complexes with highly flexible 1,1′-bis(phospholanoalkyl)ferrocene ligands

Flexible 1,1′-bis(phospholanoalkyl)ferrocenes react selectively with platinum(ii), gold(i) and rhodium(i) to give macrocyclic chelate complexes.

Tuning the coordination properties of phenothiazine by regioselective introduction of diphenylphosphanyl groups

New phenothiazinyl-phosphines and their palladium and platinum complexes have been synthesized and characterized and their biological properties have been discussed.

NiXantphos: a deprotonatable ligand for room-temperature palladium-catalyzed cross-couplings of aryl chlorides

Although the past 15 years have witnessed the development of sterically bulky and electron-rich alkylphosphine ligands for palladium-catalyzed cross-couplings with aryl chlorides, examples of palladium catalysts based on either triarylphosphine or bidentate phosphine ligands for efficient room temperature cross-coupling reactions with unactivated aryl chlorides are rare. Herein we report a palladium catalyst based on NiXantphos, a deprotonatable chelating aryldiphosphine ligand, to oxidatively add unactivated aryl chlorides at room temperature. Surprisingly, comparison of an extensive array of ligands revealed that under the basic reaction conditions the resultant heterobimetallic Pd–NiXantphos catalyst system outperformed all the other mono- and bidentate ligands in a deprotonative cross-coupling process (DCCP) with aryl chlorides. The DCCP with aryl chlorides affords a variety of triarylmethane products, a class of compounds with various applications and interesting biological activity. Additionally, the DCCP exhibits remarkable chemoselectivity in the presence of aryl chloride substrates bearing heteroaryl groups and sensitive functional groups that are known to undergo 1,2-addition, aldol reaction, and O-, N-, enolate-α-, and C(sp²)–H arylations. The advantages and importance of the Pd–NiXantphos catalyst system outlined herein make it a valuable contribution for applications in Pd-catalyzed arylation reactions with aryl chlorides.