Determination of equilibrium constants and computational interaction energies for adducts of [Rh2(RCO2)(4-n)(PC)n] (n = 0-2) with Lewis bases

Properties of dirhodium catalysts with cyclometalated aryl phosphine ligands have been studied. We report here the study of the acid-base reaction of Rh2(RCO2)2(PC)2(H2O)2 catalysts (PC = cyclometalated aryl phosphine) with different Lewis bases. The determination of the equilibrium constants of these reactions can be used to study to which extent the properties of the axial coordination site of the catalyst, considered the active site, are affected by modification of the metalated phosphines, the carboxylate ligands, or the incoming axial ligand. The trends in the computational density functional theory interaction energies show good agreement with the major trends in the equilibrium constants, thus enabling a further study of the influence of the modification of the ligand core.     

In Situ Generation of Ruthenium Carbonyl Phosphine Complexes as a Versatile Method for the Development of Enantioselective C−O Bond Arylation

We report here a method for in situ generation of various ruthenium carbonyl phosphine catalysts for arylation via cleavage of inert aromatic carbon–oxygen bonds. The use of catalyst systems consisting of [RuCl₂(CO)(p-cymene)], CsF, styrene, and phosphines enabled facile screening of phosphine ligands. Asymmetric C–O arylation was also achieved for atropo-enantioselective biaryl synthesis using a chiral monodentate phosphine ligand.     

Evaluation of ruthenium‐based catalytic systems for the controlled atom transfer radical polymerisation of vinyl monomers

Only [RuCl₂(p‐cymene)(PR₃)] complexes where the phosphine ligand, PR₃, is both strongly basic and bulky proved to be effective catalysts for the controlled atom transfer radical polymerisation (ATRP) of methyl methacrylate and styrene. The best phosphine ligands were typically P(i‐Pr)₃, P(cyclohexyl)₂Ph, P(cyclohexyl)₃, and P(cyclopentyl)₃. Less basic and/or bulky phosphines led to ineffective systems for ATRP. Tricyclohexylarsine gave rise to a highly efficient catalyst system. However, related complexes in which the phosphine ligand was replaced by tricyclohexylstibine, nitrogen (piperidine and 4‐cyanopyridine) and carbon ligands (alkyl isocyanides) proved to be inefficient. The observation of a direct relationship between the p‐cymene lability (measured by TGA) and catalyst activity suggests that p‐cymene release is a prerequisite for the polymerisation process.     

The AZARYPHOS Family of Ligands for Ambifunctional Catalysis: Syntheses and Use in Ruthenium‐Catalyzed anti‐Markovnikov Hydration of Terminal Alkynes

The family of AZARYPHOS (aza–aryl–phosphane) phosphane ligands, containing a phosphine unit and sterically shielded nitrogen lone pairs in the ligand periphery, is introduced as a tool for developing ambifunctional catalysis by the metal center and nitrogen lone pairs in the ligand sphere. General synthetic strategies have been developed to synthesize over 25 examples of structurally diverse (6‐aryl‐2‐pyridyl)phosphanes (ARPYPHOS), (6‐alkyl‐2‐pyridyl)phosphanes (ALPYPHOS), 4,6‐disubsituted 1,3‐diazin‐2‐ylphosphanes or 1,3,5‐triazin‐2‐ylphosphanes, quinazolinylphosphanes, quinolinylphosphanes, and others. The scalable syntheses proceed in a few steps. The incorporation of AZARYPHOS ligands ( L ) into complexes [RuCp( L )₂(MeCN)][PF₆] (Cp=cyclopentadienyl) gives catalysts for the anti‐Markovnikov hydration of terminal alkynes of the highest known activities. Electronic and steric ligand effects modulate the reaction kinetics over a range of two orders of magnitude. These results highlight the importance of using structurally diverse ligand families in the process of developing cooperative ambifunctional catalysis by a metal and its ligand.     

Selective arylation of 1,1-disubstituted olefins using a biphenyl-based phosphine in Heck coupling reactions

The biphenyl-based phosphine, 2-diphenylphosphino-2′-methylbiphenyl is an effective ligand for palladium-catalyzed terminal arylation of 1,1-disubstituted olefins with aryl bromides in DMF and K₂CO₃ as base. The yields of products are independent of the electronic properties of the aryl bromides, however, the nature of the olefin has a major effect.     

Efficient Cluster‐Based Catalysts for Asymmetric Hydrogenation of α‐Unsaturated Carboxylic Acids

The new clusters [H₄Ru₄(CO)₁₀(μ‐1,2‐P‐P)], [H₄Ru₄(CO)₁₀(1,1‐P‐P)] and [H₄Ru₄(CO)₁₁(P‐P)] (P‐P=chiral diphosphine of the ferrocene‐based Josiphos or Walphos ligand families) have been synthesised and characterised. The crystal and molecular structures of eleven clusters reveal that the coordination modes of the diphosphine in the [H₄Ru₄(CO)₁₀(μ‐1,2‐P‐P)] clusters are different for the Josiphos and the Walphos ligands. The Josiphos ligands bridge a metal–metal bond of the ruthenium tetrahedron in the “conventional” manner, that is, with both phosphine moieties coordinated in equatorial positions relative to a triangular face of the tetrahedron, whereas the phosphine moieties of the Walphos ligands coordinate in one axial and one equatorial position. The differences in the ligand size and the coordination mode between the two types of ligands appear to be reflected in a relative propensity for isomerisation; in solution, the [H₄Ru₄(CO)₁₀(1,1‐Walphos)] clusters isomerise to the corresponding [H₄Ru₄(CO)₁₀(μ‐1,2‐Walphos)] clusters, whereas the Josiphos‐containing clusters show no tendency to isomerisation in solution. The clusters have been tested as catalysts for asymmetric hydrogenation of four prochiral α‐unsaturated carboxylic acids and the prochiral methyl ester (E)‐methyl 2‐methylbut‐2‐enoate. High conversion rates (>94 %) and selectivities of product formation were observed for almost all catalysts/catalyst precursors. The observed enantioselectivities were low or nonexistent for the Josiphos‐containing clusters and catalyst (cluster) recovery was low, suggesting that cluster fragmentation takes place. On the other hand, excellent conversion rates (99–100 %), product selectivities (99–100 % in most cases) and good enantioselectivities, reaching 90 % enantiomeric excess (ee) in certain cases, were observed for the Walphos‐containing clusters, and the clusters could be recovered in good yield after completed catalysis. Results from high‐pressure NMR and IR studies, catalyst poisoning tests and comparison of catalytic properties of two [H₄Ru₄(CO)₁₀(μ‐1,2‐P‐P)] clusters (P‐P=Walphos ligands) with the analogous mononuclear catalysts [Ru(P‐P)(carboxylato)₂] suggest that these clusters may be the active catalytic species, or direct precursors of an active catalytic cluster species.     

Computational Study of the Palladium‐Catalyzed Carbonylative Synthesis of Aromatic Acid Chlorides: The Synergistic Effect of PtBu3 and CO on Reductive Elimination

We describe herein computational studies on the unusual ability of Pd(PtBu₃)₂ to catalyze formation of highly reactive acid chlorides from aryl halides and carbon monoxide. These show a synergistic role of carbon monoxide in concert with the large cone angle PtBu₃ that dramatically lowers the barrier to reductive elimination. The tertiary structure of the phosphine is found to be critical in allowing CO association and the generation of a high energy, four coordinate (CO)(PR₃)Pd(COAr)Cl intermediate. The stability of this complex, and the barrier to elimination, is highly dependent upon phosphine structure, with the tertiary steric bulk of PtBu₃ favoring product formation over other ligands. These data suggest that even difficult reductive eliminations can be rapid with CO association and ligand manipulation. This study also represents the first detailed exploration of all the steps involved in palladium‐catalyzed carbonylation reactions with simple phosphine ligands, including the key rate‐determining steps and palladium(0) catalyst resting state in carbonylations.     

The Effect of Metal‐Ligand Affinity on Fe3O4_Supported Co–Rh Catalysts for Dicyclopentadiene Hydroformylation

The catalytic performances of Co‐Rh/Fe₃O₄ catalysts modified with phosphine ligands (PPh₃) and its analogues on dicyclopentadiene hydroformylation were evaluated. Among these catalysts, Co‐Rh/Fe₃O₄ modified with tris(p‐trifluoromethylphenyl)phosphine was determined to be effective for monoformyltricyclodecanes production, whereas Co‐Rh/Fe₃O₄ modified with PPh₃ or tri‐p‐tolylphosphine was effective for the diformyltricyclodecanes production. To investigate the ligand effects, the complex catalyst system (Co‐Rh/Fe₃O₄ and phosphine ligand) was subjected to pretreatment with syngas and then characterized by thermogravimetry and differential thermal analysis (TG‐DTA). It was determined that the threshold decomposition temperature reflected the corresponding Rh‐phosphine interaction strength, affecting the catalytic selectivity toward different products. A weak Rh‐phosphine interaction was desirable to produce monoformyltricyclodecanes with fast reaction kinetics, whereas a strong Rh‐phosphine complex was required for the synthesis of diformyltricyclodecanes. In addition to the selectivity rule shown in the PPh₃ series, experiments with other ligands also demonstrated similar selectivity trends.     

Palladium Complexes Based on Ylide-Functionalized Phosphines (YPhos): Broadly Applicable High-Performance Precatalysts for the Amination of Aryl Halides at Room Temperature

Palladium allyl, cinnamyl, and indenyl complexes with the ylide-substituted phosphines Cy₃P⁺−C⁻(R)PCy₂ (with R=Me ( L1 ) or Ph ( L2 )) and Cy₃P⁺−C⁻(Me)PtBu₂ ( L3 ) were prepared and applied as defined precatalysts in C−N coupling reactions. The complexes are highly active in the amination of 4-chlorotoluene with a series of different amines. Higher yields were observed with the precatalysts in comparison to the in situ generated catalysts. Changes in the ligand structures allowed for improved selectivities by shutting down β-hydride elimination or diarylation reactions. Particularly, the complexes based on L2 (joYPhos) revealed to be universal precatalysts for various amines and aryl halides. Full conversions to the desired products are reached mostly within 1 h reaction time at room temperature, thus making L2 to one of the most efficient ligands in C−N coupling reactions. The applicability of the catalysts was demonstrated for aryl chlorides, bromides and iodides together with primary and secondary aryl and alkyl amines, including gram-scale applications also with low catalyst loadings of down to 0.05 mol %. Kinetic studies further demonstrated the outstanding activity of the precatalysts with TOF over 10.000 h⁻¹.     

Automated Quantum Mechanical Predictions of Enantioselectivity in a Rhodium‐Catalyzed Asymmetric Hydrogenation

A computational toolkit (AARON: An automated reaction optimizer for new catalysts) is described that automates the density functional theory (DFT) based screening of chiral ligands for transition‐metal‐catalyzed reactions with well‐defined reaction mechanisms but multiple stereocontrolling transition states. This is demonstrated for the Rh‐catalyzed asymmetric hydrogenation of (E )‐β‐aryl‐N ‐acetyl enamides, for which a new C ₂‐symmetric phosphorus ligand is designed.     

Robust and Electron‐Rich cis‐Palladium(II) Complexes with Phosphine and Carbene Ligands as Catalytic Precursors in Suzuki Coupling Reactions

A new imidazolinium ligand precursor [L²H]Cl ( 2 ) was prepared in 86 % yield. Compared with its imidazolium counterpart, [L¹H]Cl ( 1 ), 2 is very sensitive to moisture and can undergo ring‐opening reactions very readily. Palladium complexes with the ring‐opened products from imidazolinium salts were isolated and characterized by X‐ray crystallography. Theoretical studies confirmed that the imidazolinium salt has a higher propensity for the ring‐opening reaction than the imidazolium counterpart. New mixed phosphine/carbene palladium complexes, cis‐[PdCl₂(L)(PR₃)] (L=L¹ and L²; R=Ph, Cy), were successfully prepared. These complexes are highly robust as revealed by variable‐temperature NMR spectroscopic studies and thermal gravimetric analysis. The structural and electronic properties of the new complexes on varying the carbene group (imidazol‐2‐ylidene group (unsaturated carbene) vs. imidazolin‐2‐ylidene (saturated carbene)) and the phosphine group (PPh₃ vs. PCy₃) were studied in detail by X‐ray crystallography, X‐ray photoelectron spectroscopy, and theoretical calculations. The catalytic study reveals that cis‐[PdCl₂(L²)(PCy₃)] is a competent Pd^II precatalyst for Suzuki coupling reactions, in which unreactive aryl chlorides can be applied as substrates.     

Influence of the ligand structure on SLP-catalysed hydroformylation of propene

Hydroformylation of propene was studied at 90–120°C and 3–10 atm. The catalyst was hydrido-(carbonyl)tris(triphenylphosphine)rhodium [H(CO)Rh(PPh₃)₃] supported on silica, in an excess of a liquid phosphine (P) ligand as solvent. The following series of ligands (P) was synthesized and studied in this application: CH₃(CH₂)_nPPh₂ (n = 3, 7, 17), (c – C₆H₁₁)_xPPh_3?x (x = 0, 1, 2) and also unsaturated allyl- and poly(butadienyl)-diphenylphosphines. The activity and regioselectivity of the catalysts are discussed in terms of the mobility and coordination ability of the ligands used. With the same electron density of the phosphorus atom, the activity of the catalysts increases with the mobility of the ligands. On the other hand, given the same mobility of the ligand, a lower electron density on phosphorus results in increased catalytic activity.     

Copper-Catalyzed Dynamic Kinetic Asymmetric P−C Coupling of Secondary Phosphine Oxides and Aryl Iodides

Transition-metal-catalyzed enantioselective P−C cross-coupling of secondary phosphine oxides (SPOs) is an attractive method for synthesizing P-stereogenic phosphorus compounds, but the development of such a dynamic kinetic asymmetric process remains a considerable challenge. Here we report an unprecedented highly enantioselective dynamic kinetic intermolecular P−C coupling of SPOs and aryl iodides catalyzed by copper complexes ligated by a finely modified chiral 1,2-diamine ligand. The reaction tolerates a wide range of SPOs and aryl iodides, affording P-stereogenic tertiary phosphine oxides (TPOs) in high yields and with good enantioselectivity (average 89.2 % ee). The resulting enantioenriched TPOs were transformed into structurally diverse P-chiral scaffolds, which are highly valuable as ligands and catalysts in asymmetric synthesis.     

Preparation and Catalysis of a Tri‐coordinated Copper(I) Complex with Bulky P˜N Ligand

Coordination of a bulky pyridinyl‐phosphine ( P?N ) ligand toward CuBr was investigated. However, this P?N donor behaves as a monodentate via the coordination of phosphine to form a bromide bridged dimeric [( P?N )Cu(μ‐Br)₂Cu( P?N )], which was characterized by spectral and crystal structural analysis. It appeared that the “PCu(μ‐Br)₂CuP” unit is planar with a short distance between Cu…Cu' [2.7585(9) Å]. The catalytic activity on Sonogashira coupling of phenylacetylene with aryl halides was studied.     

Preparations of Saturated N‐P‐N Type Secondary Phosphine Oxides and their Applications in Cross‐Coupling Reactions

A series of cyclohexane‐1,2‐diamine ( 3a – 3d ) and benzene‐1,2‐diamine derivatives ( 3e – 3h ) were pre‐ pared. Followed by hydrolysis, the reaction of 3a – 3c with PCl₃ successfully led to the formation of cor‐ responding metastable saturated heteroatom‐substituted secondary phosphine oxides (HASPO 4a – 4c ), a tautomer of the saturated heteroatom‐substituted phosphinous acid (HAPA). Whereas ambient‐stable diamine‐coordinated palladium complexes were obtained, HAPA‐coordinated palladium complexes were not successfully synthesized. The molecular structures of HASPO 4c , Pd(OAc)₂(3a) , PdBr₂(3b) and Pd(OAc)₂(3c) and [Cu(NO₃)(3d)⁺][NO₃ ^? ] were determined by single‐crystal X‐ray diffraction method. Catalysis of in‐situ Suzuki‐Miyaura cross‐coupling reactions for aryl bromides and phenylboronic acid using diamine 3a as ancillary ligand showed that the optimized reaction condition at 60 °C is the combination of 2 mmol % 3a /3.0 mmol KOH/1.0 mL 1,4‐dioxane/1 mmol % Pd(OAc)₂. Moreover, moderate reactivity was observed when using aryl chlorides as substrates (supporting infor‐ mation). When diamine 3d was employed in Heck reaction, good tolerance of functional groups of aryl bromides were observed while using 4‐bromoanisole and styrene as substrates. The optimized condi‐ tion for Heck reaction at 100 °C is 3 mmol % 3d /3.0 mmol CsF/1.0 mL toluene/3 mmol % Pd(OAc)₂. In general, cyclohexane‐1,2‐diamine derivatives exhibited better catalytic properties than those of benzene‐1,2‐diamines.     

ortho‐C−H Arylation of Benzoic Acids with Aryl Bromides and Chlorides Catalyzed by Ruthenium

A system consisting of catalytic amounts of [(p‐cym)RuCl₂]₂/PEt₃?HBF₄, K₂CO₃ as the base, and NMP as the solvent efficiently mediates the ortho‐C?H arylation of benzoic acids with aryl bromides at 100 °C. Replacing the phosphine ligand with the amino acid dl ‐pipecolinic acid enables the analogous transformation with aryl chlorides. The key advantage of this broadly applicable transformation is the use of an inexpensive ruthenium catalyst in combination with simple carboxylates as directing groups, which can either be tracelessly removed or used as anchor points for decarboxylative ipso substitutions.     

Structure and chromotropism properties of an unsymmetrical alkoxo-bridged nickel(II) cubane-like complex

A new tetranuclear cubane-like complex, [Ni₄(L)₄Cl₄(H₂O)₃(EtOH)]·(H₂O), has been synthesized from the reaction of a metal salt with the bidentate ligand 2-hydroxymethylpyridine (LH), and its crystal structure, spectroscopic and chromotropic properties have been studied. The complex has a [Ni₄O₄] core comprising a distorted cubane arrangement, of which four nickel(II) ions were bridged by μ₃-alkoxo moieties. Each nickel(II) was coordinated to three μ₃-alkoxo oxygens, one pyridine nitrogen and one chloride. The peripheral ligation was completed by an oxygen atom of water or ethanol ligand, which participated in intramolecular hydrogen bonding. Chromotropism properties of the complex including solvato-, thermo-, and ionochromism were investigated. The complex displayed strong ionochromism and shows high-sensitive and selective activity toward CN^? and SCN^? anions in the presence of other halides and pseudo-halides. The solvatochromic property of the complex was analyzed by a multi-parametric equation using SPSS/PC software. The stepwise multiple linear regression method demonstrated that the acceptor power of the solvent plays the most important role in the observed negative solvatochromism of the compound. It shows reversible thermochromism in solution due to loss of the weakly coordinated water and ethanol from the nickel(II) units.     

Tethering versus Non‐Coordination of Hydroxy and Methoxy Side Chains in Arene Half Sandwich Dichloro Ruthenium Complexes

We are reporting on the hydroxyalkyl appended arene ruthenium half sandwich complexes [{η⁶‐C₆H₅(CH₂)_nOH}RuCl₂] (n = 2, 3) and the methyl ether of the hydroxypropyl derivative. Most significantly, a structural comparison between the hydroxypropyl complex 1a and its methyl ether 2a reveals, that the latter adopts the conventional dichloro bridged dimeric structure while 1a is a monomer. Coordinative saturation of the ruthenium centre is achieved by intramolecular coordination of the appended hydroxy function, thus rendering the functionalized arene an eight electron donor chelate ligand. The structure is further stabilized by intermolecular OH···Cl hydrogen bridges between a terminal chloride ligand of one and the coordinated hydroxy group of a neighbour molecule, resulting in a sheet structure. These intermolecular interactions appear to be even stronger in the hydroxyethyl analogue. Several phosphine adducts have been prepared from the hydroxy or alkoxy functionalized [(η⁶‐arene)RuCl₂]_n precursors, including water soluble P(CH₂OH)₃ adducts. Electrochemical properties of the phosphine adducts and of the dichloro bridged aryl ether complex 2a are also discussed.     

Application of polymer-supported triphenyl phosphine in the palladium-catalyzed cyanation reaction under microwave conditions

A variety of aryl nitriles were prepared in excellent yields from the palladium acetate catalyzed coupling of aryl halides with Zn(CN)₂ using polymer-supported triphenyl phosphine as the ligand and dimethylformamide as solvent under microwave irradiation conditions.     

Copper‐Catalyzed Radical/Radical C?H/P?H Cross‐Coupling: α‐Phosphorylation of Aryl Ketone O‐Acetyloximes

The selective radical/radical cross‐coupling of two different organic radicals is a great challenge due to the inherent activity of radicals. In this paper, a copper‐catalyzed radical/radical C H/P H cross‐coupling has been developed. It provides a radical/radical cross‐coupling in a selective manner. This work offers a simple way toward β‐ketophosphonates by oxidative coupling of aryl ketone o‐acetyloximes with phosphine oxides using CuCl as catalyst and PCy₃ as ligand in dioxane under N₂ atmosphere at 130 °C for 5 h, and yields ranging from 47 % to 86 %. The preliminary mechanistic studies by electron paramagnetic resonance (EPR) showed that, 1) the reduction of ketone o‐acetyloximes generates iminium radicals, which could isomerize to α‐sp³‐carbon radical species; 2) phosphorus radicals were generated from the oxidation of phosphine oxides. Various aryl ketone o‐acetyloximes and phosphine oxides were suitable for this transformation.