Monodentate Phosphine Ligands Research Articles

Nickel-catalyzed regiodivergent hydrosilylation of α-(fluoroalkyl)styrenes without defluorination

The fluoroalkyl-containing organic molecules are widely used in drug discovery and material science. Herein, we report ligand regulated nickel(0)-catalyzed regiodivergent hydrosilylation of α-(fluoroalkyl)styrenes without defluorination, providing an atom- and step-economical synthesis route of two types of fluoroalkyl substituted silanes with exclusive regioselectivity. The anti-Markovnikov addition products (β-fluoroalkyl substituted silanes) are formed with monodentate phosphine ligand. Noteworthy, the bidentate phosphine ligand promote the generation of the more challenging Markovnikov products (α-fluoroalkyl substituted silanes) with tetrasubstituted saturated carbon centers. This protocol features with easy available starting materials and commercially available nickel catalysis, a wide range of substrates and excellent regioselectivity. The structure divergent products undergo a variety of transformations. Comprehensive mechanistic studies including the inverse kinetic isotope effects demonstrate the regioselectivity controlled by ligand structure through α-CF3 nickel intermediate. DFT calculations reveal a distinctive mechanism involving an open-shell singlet state, which is crucial for generating intricate tetra-substituted Markovnikov products.

Photoluminescence of 3- and 4-coordinate platinum(0) phosphine complexes

Abstract Six platinum complexes bearing monodentate phosphine ligands were prepared and their structures and photophysical properties examined. The complexes were photoluminescent in the solid state, with a maximum quantum yield of 72%. The tris(phosphine) platinum complexes displayed more red-shifted emissions than the tetrakis complexes. The nature of the emissive excited states is discussed based on the results of density functional theory and time-dependent density functional theory calculations.

Isolation of Cationic η3-Allenylnickel(II) Key Intermediate Complexes: Origins of Enantioselectivity and Regioselectivity in Nickel(0)-Catalyzed Asymmetric Propargylic Substitutions.

Herein, we report the synthesis and isolation of cationic η3-allenylnickel(II) complexes that bear rac-BINAP as a bidentate ligand for the first time via Me3SiOTf-promoted C-O bond cleavage of propargylic tert-butyl carbonate. In contrast, in the presence of the monodentate phosphine ligand PEt3, treatment of propargylic tert-butyl carbonate with Ni(cod)2 resulted in a gradual C-O bond cleavage leading to η1-allenylnickel(II) complexes, i.e., trans-(PEt3)2Ni(η1-CPh═C═CHR)(OBoc). X-ray diffraction and NMR spectroscopy studies of [(η3-RCH-CCPh)Ni(rac-BINAP)](OTf) revealed that the complex adopts an η3-allenyl coordination mode both in the crystal lattice and in solution. A thorough structural comparison between [(η3-RCH-CCPh)Ni(rac-BINAP)](OTf) and palladium and platinum analogues revealed that the η3-allenyl moiety in the nickel complex is similar to that observed in palladium and platinum complexes, albeit that each Ni-C bond is shorter than the corresponding Pd-C and Pt-C bonds due to the smaller ionic radius of nickel to that of Pd or Pt. The reactions of either N-methylaniline or sodium N-methylanilide with [(η3-RCH-CCPh)Ni((R)-BINAP)](OTf) furnished (R)-PhC≡CCH(NMePh)Me as an asymmetric propargylic substitution (APS) product with excellent enantioselectivity. Furthermore, when the nickel-catalyzed APS reaction of propargylic tert-butyl carbonate with N-methylaniline was conducted in DMSO at 60 °C in the presence of 5 mol % of [(η3-RCH-CCPh)Ni((R)-BINAP)](OTf) and 7.5 mol % of sodium N-methylanilide as a catalytic precursor and an additive, respectively, (R)-PhC≡CCH(NMePh)Me was obtained in 79% yield with 90% ee. The experimental results and computational calculations strongly suggest that the nickel-catalyzed APS reaction might proceed via a cationic η3-allenylnickel(II) species as the key reaction intermediate.

Ligand-Controlled Regiodivergent Nickel-Catalyzed Hydroaminoalkylation of Unactivated Alkenes.

Ligand modulation of transition-metal catalysts to achieve optimal reactivity and selectivity in alkene hydrofunctionalization is a fundamental challenge in synthetic organic chemistry. Hydroaminoalkylation, an atom-economical approach for alkylating amines using alkenes, is particularly significant for amine synthesis in the pharmaceutical, agrochemical, and fine chemical industries. However, the existing methods usually require specific substrate combinations to achieve precise regio- and stereoselectivity, which limits their practical utility. Protocols allowing for regiodivergent hydroaminoalkylation from the same starting materials, controlling both regiochemical and stereochemical outcomes, are currently absent. Herein, we report a ligand-controlled, regiodivergent nickel-catalyzed hydroaminoalkylation of unactivated alkenes with N-sulfonyl amines. The reaction initiates with amine dehydrogenation and involves aza-nickelacycle intermediates. Tritert-butylphosphine promotes branched regioselectivity and syn diastereoselectivity, whereas ethyldiphenylphosphine enables linear selectivity, yielding regioisomers with inverse orientation. Systematic evaluation of diverse monodentate phosphine ligands reveals distinct regioselectivity cliffs, and % Vbur (min), a ligand steric descriptor, was established as a predictive parameter correlating ligand structure to regioselectivity. Computational investigations supported experimental findings, offering mechanistic insights into the origins of regioselectivity. Our method provides an efficient and predictable route for amine synthesis, demonstrating broad substrate scope, excellent tolerance toward various functional groups, and practical advantages. These include the use of readily available starting materials and cost-effective nickel(II) salts as precatalysts.

Insights into the Mechanism, Regio-/Diastereoselectivities and Ligand Role of Nickel-Initiated [3+2] Cycloadditions between Vinylcyclopropane and N-Tosylbenzaldimine

Density functional theory (DFT) was employed to explore the reaction mechanism, regio- and diastereoselectivities of nickel-initiated [3+2] cycloaddition between vinylcyclopropane (VCP) and N-tosylbenzaldimine assisted by phosphine ligands. Four different binding modes of the nickel center to VCP substrate were explored during the ring-opening of VCP, among which the C,C_anti and C,C_syn modes were verified to be the most accessible ones. Further explorations about four different phosphine ligand-assisted reactions based on the two most probable binding modes show that the difference in binding mode of bi- and monodentate phosphine ligands can vary the optimal reaction pathway, especially in the [3+2] cycloaddition process between the ring-opened intermediate and N-tosylbenzaldimine. The formation of C–C and C–N bonds between N-tosylbenzaldimine and the ring-opened intermediate through [3+2] cycloaddition is found to be stepwise, with the former acting as the rate-determining step (RDS) in most cases. Computed free energy barriers of RDS transition states on the optimal path I or II not only give out good predictions for reaction rates and half-lives, but also provide reasonable explanations for the major generation of cis-pyrrolidine. Noncovalent interaction analyses of key stationary points suggest the rate is influenced by both electronic effects and steric hindrance, while the diastereoselectivity is mainly controlled by electronic effects.

Synthesis of Chroman-2,4-diones via Ring-Opening/Ring-Closing Reaction Involving Palladium-Catalyzed Intramolecular Aryloxycarbonylation.

Palladium-catalyzed aminocarbonylation of 3-iodochromone was studied in the presence of primary and secondary amines using atmospheric pressure of carbon monoxide as a carbonyl source. This procedure successfully provided a library of chromone-3-carboxamides and 3-substituted chroman-2,4-diones in 40 to 92% isolated yields. The reaction proceeded via highly chemoselective aminocarbonylation (up to 100%) in the presence of secondary amines by using monodentate or bidentate phosphine ligands. The tendency of 3-iodochromone substrate to undergo ANRORC rearrangement with N-nucleophiles was crucial to shift the reaction toward an unprecedented chemoselective carbonylative transformation, where a late-stage carbonyl insertion is favored concomitantly to the last ring-closure step. The proposed aza-Michael addition/ring-opening/intramolecular aryloxycarbonylation sequence showed compatibility, uniquely, to primary amines when XantPhos was used as a ligand. The solid-state structures of chromone-3-carboxamide (2a) and chroman-2,4-dione (3s) were undoubtedly established by single-crystal XRD analysis. A catalytic cycle was proposed to rationalize the formation of the two types of carbonylated compounds.

Synthesis, Characterization and Electrochemical Studies of bis(Monothiolato) {FeFe} Complexes [Fe2(μ‐SC6H4‐OMe‐m)2(CO)5L] (L = CO, PCy3, PPh3)

AbstractReaction of Fe3(CO)12 with 3‐methoxybenzenethiol in toluene afforded a new bis(monothiolato) hexacarbonyl diiron complex [Fe2(μ‐SC6H4‐OMe‐m)2(CO)6] 1. Substitution of CO in complex 1 with monodentate phosphine ligands (PCy3 and PPh3) further afforded two new complexes [Fe2(μ‐SC6H4‐OMe‐m)2(CO)5(PCy3)] 2 and [Fe2(μ‐SC6H4‐OMe‐m)2(CO)5(PPh3)] 3. All the three complexes were characterized by FTIR, NMR, UV‐Vis, mass and elemental analysis. Further, complexes 1–3 were found to be electrocatalytically active for proton reduction in acetonitrile in the presence of both weak and strong acids as proton sources.

Influence of Achiral Phosphine Ligands on a Synergistic Organo- and Palladium-Catalyzed Asymmetric Allylic Alkylation.

An unusual diastereodivergent stereoselective allylation reaction is presented. It consists of a palladium-catalyzed allylation reaction of an organocatalytically generated amino isobenzofulvene, where the diastereoselectivity is controlled by the electronic properties of a monodentate, achiral ligand on palladium. One major diastereoisomer is formed using triarylphosphines substituted with neutral or electron-donating substituents of the aryl group, while those with electron-withdrawing substituents favor the other diastereoisomer. The diastereoselectivity correlates with the Taft inductive parameter of substituents on the triarylphosphine ligand on palladium. The synergistic reaction involves both a catalytic secondary amine catalyst for the indene-aldehyde activation and the monodentate phosphine ligands on palladium, affording a highly enantioselective reaction with up to 98 % enantiomeric excess. Based on computational investigations, the role of the monodentate phosphine ligand on the diastereoselectivity is discussed.

Hydroxyl-directed C–C bond activation of benzophenones for the O-acylation of phenols

We report that 2-hydroxyl benzophenones can be converted to phenyl salicylate esters by loss of an aryl group and O-acylation of the phenols. Iridium, cyclooctadiene, and bulky monodentate phosphine ligands catalyze the ketone C–C bond activation and fragmentation. We identified common side products in this process, which include C-acylation of the phenol reactant or trapping on cyclooctadiene. Several of the processes reported here appear to be under equilibrium control. Overall, our report provides insight into the activation and functionalization of unstrained C–C bonds with group 9 catalysts.

Parameterization of Phosphine Ligands Modified Rh Complexes to Unravel Quantitative Structure‐Activity Relationship and Mechanistic Pathways in Hydroformylation

AbstractThis study has established the quantitative structure‐activity relationship (QSAR) model to predict formaldehyde hydroformylation activity using a class of phosphine‐Rh complexes and computational mechanistic pathway analysis. A group of computational parameters (e. g., cone angle, G‐parameter, buried volume, CO vibration frequency, NBO charge, HOMO and LUMO energy, Rh−P distance) describing the complex structural (e. g., steric and electronic) features were achieved for descriptor database of monodentate phosphine ligands. Mathematical modelling of the catalytic results with the descriptors via multivariate linear regression reveals the hydroformylation rate is principally under electronic control within the investigated ligands. Computational mechanistic analysis demonstrates significant impact of electronic feature on TOF‐determining transition state/intermediate and energetic span. The H2 distortion energy analysis elucidates the variation of energetic span of the transition states related to H2 oxidation addition, rationally accounting for the reaction outcomes.

Ruthenium(II) Phosphine/Picolylamine Dichloride Complexes Hydrogenation and DFT Calculations

Treating [Ru(PPh3)3Cl2] with the amine 2-picolylamine (Picam) ligand in a 1:1 molar ratio, the Ru(II) complex trans-Ru(PPh3)2(Picam)Cl2 (1) is obtained in methylene chloride and can be isolated as a pure solid compound. The single-crystal structure of 1 was determined by X-ray crystallography. The geometry at the Ru metal center is a distorted octahedral environment with a trans arrangement of the two chlorides. A trans effect of the bond lengths was observed within the structure. Similarly, treating [Ru(PPh3)3Cl2] with 1:1:1 molar ratios of 2-picolylamine (Picam) and 1,1′-bis(diphenylphosphine)ferrocene (DPPF) ligands yielded the Ru(II) complex trans-Ru(DPPF)(Picam)Cl2 (2). In identical conditions, the homogeneous hydrogen transfer catalytic reactivity of complexes 1 and 2 has been tested in a basic 2-propanol solution and they indicate different catalytic activity. It was discovered that monodentate and bidentate phosphine ligands of Ru(II) complexes, as well as cis- and trans-chloro configuration display different catalytic properties from our experimental data, in agreement with literature data. Based on DFT calculations, the relative molecular catalytic reactivity of all available experimental data is understood from the relative calculated molecular energy.

Synthesizing Highly Fluorinated Oligophenyls via Negishi Coupling of Fluoroarylzinc Pivalates

AbstractPreviously established general synthetic methods for the synthesis of highly fluorinated biphenyls using Suzuki–Miyaura protocols require the use of organoboron compounds, which are not very stable under reactions conditions and thus need to be used in large excess. Herein, we report an improved general strategy for the synthesis of highly fluorinated biphenyls, terphenyls, and phenyl-substituted terphenyls using organozinc pivalates. The influence of several parameters was investigated: (1) in a series of monodentate phosphine ligands, X-Phos showed the best performance; (2) a higher yield was obtained for substrates bearing less steric hindrance or lower amount of fluorine substitution; (3) as iodinated substrates decomposed during the reaction, brominated electrophiles were found to be superior. The presented protocol is scalable, versatile, and works with commonly used and commercially available phosphine ligands (X-Phos) and palladium sources (Pd2dba3). Also, it does not require excess nucleophile for terphenyl synthesis and only a slight excess is needed for the preparation of phenyl substituted terphenyls.

Exploring the Effect of Phosphine Ligand Architecture on a Buchwald–Hartwig Reaction: Ligand Synthesis and Screening of Reaction Conditions

A laboratory project to introduce upper-level undergraduates to Pd-catalyzed C–N σ-bond forming reactions is described. The exercise involves evaluation of monodentate phosphine ligands and solvents for the Buchwald–Hartwig coupling of 4-chloro- or 4-bromo-anisole with morpholine. Students gain experience in the synthesis of a biarylphosphine ligand, performing reactions under anhydrous and anaerobic conditions, with systematic screening and analysis of reaction conditions, and with the use of GC–MS data to gauge the extent of each coupling reaction. Students develop arguments from spectroscopic evidence to support claims regarding the effectiveness of each set of reaction conditions and explore the impact of the ligand within the context of currently accepted mechanistic models. Optimal conditions are determined and used for the scaled-up synthesis of the coupling product, and electronic aspects of the product are explored via computational chemistry.

Photophysical Properties of Simple Palladium(0) Complexes Bearing Triphenylphosphine Derivatives.

Pd(0) complexes with monodentate phosphine ligands, [Pd(P)n] (n = 3, 4), are well-known catalysts. However, the nature of the Pd(0) complex, especially the basic photophysical properties of the Pd(0) complexes, has not been extensively explored. In this work, we measured the general photophysical properties and crystal structures of Pd(0)-bearing PPh3 derivatives in the solid state and in solution. In the solid state, four-coordinated Pd(0) complexes exhibited blue-yellow emission. On the other hand, three-coordinated Pd(0) complexes displayed yellow-orange emission. In solution, orange emission of three-coordinated complexes was observed, and prompt fluorescence was detected using time-resolved emission spectroscopy, which suggests a thermally activated delayed fluorescence mechanism. Density functional theory (DFT) and time-dependent DFT calculations show that the difference in the transition mechanism between the [Pd(PPh3)4] and [Pd(PPh3)3] complexes explains the different emission colors. The emitting states of both complexes have metal-to-ligand charge-transfer character, but the metal-centered d → p transition is considerably incorporated for emission of the tris complex.

Cu23(PhSe)16(Ph3P)8(H)6]·BF4: Atomic-Level Insights into Cuboidal Polyhydrido Copper Nanoclusters and Their Quasi-simple Cubic Self-Assembly

Polyhydrido copper nanoclusters are an emerging class of nanomaterials. Unfortunately, insights into the structural evolution and structure–property relationship of such copper nanoclusters are scant, because of the difficulty of synthesizing and crystallizing nanoclusters with high nuclearity and new morphologies. Here, we report an anisotropic cuboidal polyhydrido copper nanocluster, [Cu23(PhSe)16(Ph3P)8(H)6]·BF4, with a distorted cuboctahedral Cu13 core stabilized by two square protecting motifs and six hydrides. The cuboidal nanoclusters self-assemble into a quasi-simple cubic packing pattern with perfect face-to-face contact of neighboring nanoclusters and interdigitation of intercluster surface ligands. Atomic-level observations reveal the crucial role that subtle synergies between nanocluster geometry and intercluster noncovalent interactions play in guiding nanocluster self-assembly. In addition, a comparison with previously reported analogous metal nanoclusters points to bulky monodentate phosphine ligands as a potent inducing agent for the formation of rectangular hexahedral nanoclusters. These findings have significant implications for the controllable synthesis of polyhedral nanomaterials and their superstructures.

C-N, C-S and S-S Bond Cleavage by Rhodium PCcarbene P Pincer Complexes.

Rhodium PCcarbene P complexes 1-L {L=PPh3 , PPh2 (C6 F5 )} react with isothiocyanate, carbodiimide and disulphide to enable C-S, C-N and S-S bond cleavage. The cleaved molecules are sequestered by the metal center and the pincer alkylidene linkage, forming η2 -coordinated sulfide or imide centered pincer complexes. When a C-S or S-S bond is cleaved, the resulting complexes can bridge two rhodium centers through sulphur forming dimeric complexes and eliminating a monodentate phosphine ligand.

Investigations into the ligand steric and electronic effects of Ru-catalyzed C–H bond arylation directed by 8-aminoquinoline as a bidentate-directing group

In this study, we report an investigation into the steric (cone angle, θ) and electronic properties of ligands in Ru-catalyzed C–H arylation of aromatic benzamides bearing 8-aminoquinoline as an N,N’-bidentate-directing group. The study employs [RuCl2( p-cymene)]2 as a precatalyst, and a ligand, under study, as a cocatalyst. Various electronically and sterically different monodentate and bidentate phosphine ligands were examined. Other ligands such as phosphites and amines were also tested. The study reveals that while bidentate phosphines, phosphites, and aryl and alkyl amines were found to be ineffective, monodentate triarylphosphines represented by triphenylphosphine were found to be the most effective ligands in the Ru-catalyzed C–H arylation under the conditions specified. In addition, the study reveals that there is a correlation between the steric effects, cone angle (θ) and the reaction efficiency. Thus, for symmetrical phosphine ligands, as the cone angle increases, the yield of the CH arylation product gradually decreased. Moreover, the electronic properties of triarylphosphine ligands influenced the reaction as demonstrated by the decreased ability of electron-poor ligands to promote the reaction. The study also reveals a correlation between the electronic parameter, υCO, of the triarylphosphine ligand and the reaction efficiency. As the carbonyl stretching frequency increases, the reaction yield gradually decreased.

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.

Enantioselective hydrosilylation of unsaturated carbon-heteroatom bonds (C[double bond, length as m-dash]N, C[double bond, length as m-dash]O) catalyzed by [Ru-S] complexes: a theoretical study.

A detailed theoretical study on the mechanism of enanthioselective hydrosilylation of imines and ketones catalyzed by the ruthenium(ii) thiolate catalyst [Ru–S] ([L*-Ru(SDmp)]+[BAr4F]−) with a chiral monodentate phosphine ligand is carried out in this work. We elucidate all the pathways leading to the main products or by products mediated by the [Ru–S] complex in order to have deep understanding of the chemoselectivity and enantioselectivity. The DFT (Density Functional Theory) calculations show that the reaction mechanism including: (1) Si–H bond cleavage by the dual activity of Ru–S bond; (2) the generation of a sulfur-stabilized silane cation; (3) the electrophilic attack of silane cation to NC/OC; (4) hydrogen transfer from Ru to carbon cation. The hydrosilylation products are found to be the final products rather than the dehydrogenative ones, which is consistent with the experimental results. The dehydrogenative silylation reaction pathways which give N- or O-silylated enamine/enol ether are reversible according to our calculations. The computational results also show that the electrophilic attack of silicon to NC/OC is the rate-determining step and the ee value can be improved significantly with more bulky model phosphine ligand based on the same calculation methods.

Catalytic Hydrogenation of N‐protected α‐Amino Acids Using Ruthenium Complexes with Monodentate Phosphine Ligands

AbstractA ruthenium complex with a monodentate phosphine ligand was used to catalytically hydrogenate N‐protected α‐amino acids under essential retention of the configuration of their α‐chiral centers. Among the ligands tested for this hydrogenation, which proceeds at a relatively low temperature, tris(para‐fluorophenyl)phosphine exhibited the best performance. In comparison, electron‐rich monodentate, bidentate, and tridentate phosphines were far less effective. The precatalyst Ru(OAc)2[(p‐FC6H4)3P]2 was synthesized and isolated, and its structure was determined by a single‐crystal X‐ray diffraction analysis. N‐protected α‐amino acids with neutral alkyl side chains, including polar functional groups such as sulfides, indoles, ethers, phenols, pyrroles, and arenes, are compatible with the applied hydrogenation conditions, affording the corresponding optically active 2‐substituted‐2‐(1H‐pyrrol‐1‐yl)ethan‐1‐ol (2‐amino ethanol) derivatives in moderate to high yield.magnified image