Synthesis of Hemilabile Phosphine−Phosphine Oxide Ligands via the Highly Selective Pd-Catalyzed Mono-oxidation of Bidentate Phosphines: Scope, Limitations, and Mechanism

Phosphine-incorporated Metal-Organic Framework for Palladium Catalyzed Heck Coupling Reaction

As an emerging material with the potential to combine the high efficiency of homogeneous catalysts and high stability and recyclability of heterogeneous catalysts, metal-organic frameworks (MOFs) have been viewed as one of the candidates to produce catalysts of the next generation. Herein, we heterogenized the highly active mono(phosphine)-Pd complex on surface of UiO-66 MOF, as a catalyst for Suzuki and Heck cross coupling reactions. The successful immobilization of these Pd-monophosphine complexes on MOF surface to form UiO-66-PPh2-Pd was characterized and confirmed via comprehensive set of analytical methods. UiO-66-PPh2-Pd showed high activity and selectivity for both Suzuki and Heck Cross Coupling Reactions. This strategy enabled facile access to mono(phosphine) complexes which are challenging to design and require multistep synthesis in homogeneous systems, paving the way for future MOF catalysts applications by similar systems.

Palladium-Catalyzed Amination of Aryl Halides with Aqueous Ammonia and Hydroxide Base Enabled by Ligand Development

The conversion of aryl halides to primary arylamines with a convenient and inexpensive source of ammonia has been a long-standing synthetic challenge. Aqueous ammonia would be the most convenient and least expensive form of ammonia, but such a palladium-catalyzed amination reaction with a high concentration of water faces challenges concerning catalyst stability and competing hydroxylation, and palladium-catalyzed reactions with this practical reagent are rare. Further, most reactions with ammonia to form primary amines are conducted with tert-butoxide base, but reactions with ammonium hydroxide would contain hydroxide as base. Thus, ammonia surrogates, ammonia in organic solvents, and ammonium salts have been used under anhydrous conditions instead with varying levels of selectivity for the primary amine. We report the palladium-catalyzed amination of aryl and heteroaryl chlorides and bromides with aqueous ammonia and a hydroxide base to form the primary arylamine with high selectivity. The palladium catalyst containing a new dialkyl biheteroaryl phosphine ligand (KPhos) suppresses both the formation of aryl alcohol and diarylamine side products. Mechanistic studies with a soluble hydroxide base revealed turnover-limiting reductive elimination of the arylamine and an equilibrium between arylpalladium amido and hydroxo complexes prior to the turnover-limiting step.

Phosphonium-Catalyzed Monoreduction of Bisphosphine Dioxides: Origin of Selectivity and Synthetic Applications

Controlling product selectivity in successive reactions of the same type is challenging owing to the comparable thermodynamic and kinetic properties of the reactions involved. Here, the synergistic interaction of the two phosphoryl groups in bisphosphine dioxides (BPDOs) with a bromo-phosphonium cation was studied experimentally to provide a practical tool for substrate-catalyst recognition. As the eventual result, we have developed a phosphonium-catalyzed monoreduction of chiral BPDOs to access an array of synthetically useful bisphosphine monoxides (BPMOs) with axial, spiro, and planar chirality, which are otherwise challenging to synthesize before. The reaction features excellent selectivity and impressive reactivity. It proceeds under mild conditions, avoiding the use of superstoichiometric amounts of additives and metal catalysts to simplify the synthetic procedure. The accessibility and scalability of the reaction allowed for the rapid construction of a ligand library for optimization of asymmetric Heck-type cyclization, laying the foundation for a broad range of applications of chiral BPMOs in catalysis.

Donor Ability of Bisphosphinemonoxide Ligands Relevant to Late-Metal Olefin Polymerization Catalysis

Late-transition-metal catalysts for polymerization of olefins have drawn a significant amount of attention owing to their ability to tolerate and incorporate polar comonomers. However, a systematic way to experimentally quantify the electronic properties of the ligands used in these systems has not been developed. Quantified ligand parameters will allow for the rational design of tailored polymerization catalysts, which would target specific polymer properties. We report a series of platinum complexes bearing bisphosphinemonoxide ligands, which resemble those used in the polymerization catalysts of Nozaki and Chen. Their electronic properties are investigated experimentally, and trends are rationalized by using computed spectral properties. Benchmarking computational data with known experimental parameters further enhances the utility of both methods for determining optimal ligands for catalytic application.

Significant cell uptake of Gd(III)-diphenylphosphoryl-diphenylphosphonium complexes: evidence for a new conformationally-dependent tumour cell targeting vector

The synthesis, characterisation, and tumour cell uptake of six novel Gd(III)-diphenylphosphoryl-diphenylphosphonium complexes are reported. The propyl-linked Gd(III) complexes can accumulate inside human glioma cells at prodigious levels, approaching 1200%, over the parent triphenylphosphonium salts. DFT and quantum chemical topology analyses support a new type of conformationally-dependent tumour cell targeting vector.

Transition-metal complexes of group 12 with 1,1'-bis(phosphanyl)ferrocene ligands

The syntheses of four new cadmium and zinc complexes with 1,1'-bis(phosphanyl)ferrocene ligands and their phosphine chalcogenide derivatives are reported. The complexes were characterized by elemental analyses and IR, ¹H NMR, ³¹P NMR and electronic absorption spectroscopy. The crystal structures of dichlorido[1-diphenylphosphinoyl-1'-(di-tert-butylphosphanyl)ferrocene-κ²O,P]cadmium(II), [CdCl₂{(C₁₇H₁₄OP)(C₁₃H₂₂P)Fe}] or CdCl₂(κ²P,O-dppOdtbpf) (1), bis[μ-(tert-butyl)(1'-diphenylphosphinoylferrocen-1-yl)phosphinato-κ³O,O':O'']bis[chloridozinc(II)], [Zn₂{(C₉H₁₃O₂P)(C₁₇H₁₄OP)Fe}₂Cl₂] or [ZnOCl{κ²O,O'-Ph₂POFcPO₂(t-Bu)}]₂ (2), 1,1'-bis(di-tert-butylthiophosphinoyl)ferrocene, [Fe(C₁₃H₂₂PS)₂] or dtbpfS₂ (3), and [1,1'-bis(dicyclohexylphosphanyl)ferrocene-κ²P,P'][chlorido/cyanido(0.25/1.75)]zinc(II), [Zn(CN)_1.75Cl_0.25{(C₁₇H₂₆P)₂Fe}] or Zn(CN)₂(κ²-dcpf) (4), were determined crystallographically. Compound 1 has tetrahedral geometry in which the Cd^II centre is coordinated by one dppOdtbpf ligand in a κ²-manner and by two Cl atoms, while compound 2 displays a centrosymmetric dimeric unit in which two oxide atoms bridge the two Zn atoms to generate an eight-membered ring. Compound 3 revealed a sandwich structure with both phosphane groups sulfurized. In compound 4, the Zn^II atom adopts a tetrahedral geometry by coordinating to the 1,1'-bis(dicyclohexylphosphanyl)ferrocene ligand in a κ²-manner and to two cyanide ligands.

Enantioselective α-Arylation of Ketones via a Novel Cu(I)-Bis(phosphine) Dioxide Catalytic System

A novel catalytic system based on copper(I) and chiral bis(phosphine) dioxides is described. This allows the arylation of silyl enol ethers to access enolizable α-arylated ketones in good yields and enantiomeric excess up to 95%. Noncyclic ketones are amenable substrates with this method, which complements other approaches based on palladium catalysis. Optimization of the ligand structure is accomplished via rational design driven by correlation analysis. Preliminary mechanistic hypotheses are also evaluated in order to identify the role of chiral bis(phosphine) dioxides.

Dual roles of bisphosphines and epoxides: Rh-catalyzed highly chemoselective and diastereoselective (3 + 2) transannulations of 1,2,3-thiadiazoles with cyanoepoxides

Dual roles of bisphosphines and epoxides are demonstrated in highly chemoselective and diastereoselective (3 + 2) transannulations.

Next-Generation Total Synthesis of Vancomycin

A next-generation total synthesis of vancomycin aglycon is detailed that was achieved in 17 steps (longest linear sequence, LLS) from the constituent amino acid subunits with kinetically controlled diastereoselective introduction of all three elements of atropisomerism. In addition to new syntheses of three of the seven amino acid subunits, highlights of the approach include a ligand-controlled atroposelective one-pot Miyaura borylation-Suzuki coupling sequence for introduction of the AB biaryl axis of chirality (>20:1 dr), an essentially instantaneous and scalable macrolactamization of the AB ring system nearly free of competitive epimerization (>30:1 dr), and two room-temperature atroposelective intramolecular SNAr cyclizations for sequential CD (8:1 dr) and DE ring closures (14:1 dr) that benefit from both preorganization by the preformed AB ring system and subtle substituent effects. Combined with a protecting group free two-step enzymatic glycosylation of vancomycin aglycon, this provides a 19-step total synthesis of vancomycin. The approach paves the way for large-scale synthetic preparation of pocket-modified vancomycin analogues that directly address the underlying mechanism of resistance to vancomycin.

Reactive metallocene cations as sensitive indicators of gas-phase oxygen and water

Gas-phase oxidation of air-sensitive organometallic compounds does not proceed to a significant extent in mass spectrometric analysis unless a vacant coordination site is generated, making nitrogen generators a suitable source of desolvation gas.

A rational pre-catalyst design for bis-phosphine mono-oxide palladium catalyzed reactions

Significant catalyst loading reduction and increased reaction robustness have been achieved for a Pd-catalyzed asymmetric intramolecular C-N coupling through comprehensive mechanistic studies. Detailed kinetic, spectroscopic, and crystallographic analyses revealed that the mono-oxidation of the bis-phosphine ligand is critical for a successful transformation. 31P NMR studies provided an understanding of the inefficient activation of the Pd(OAc)2/(R,R)-QuinoxP* pre-catalyst to form the active bis-phosphine mono-oxide-Pd(0) catalyst with competitive formation of a less active (R,R)-QuinoxP*·PdBr2 complex. Based on these detailed mechanistic studies, a new series of bis-phosphine mono-oxides (BPMO)-ligated Pd(ii) pre-catalysts have been rationally developed that allow for reliable and complete catalyst activation which should have general utility in academic and industrial settings.

Mononuclear bis(dithiolene) Mo(iv) and W(iv) complexes with P,P; S,S; O,S and O,O donor ligands: a comparative reactivity study

Comparative redox reactions of eight Mo^IV/W^IVcomplexes with P,P; S,S; S,O and O,O donor ligands are presented.

Access to Cyclopropyl-Fused Azacycles via a Palladium-Catalyzed Direct Alkenylation Strategy

Palladium-catalyzed direct alkenylation of cyclopropyl C-H bonds proceeds in high efficiency. This transformation provides access to novel cyclopropyl-fused azacycles. Ligand studies suggest that bisphosphine monoxide analogues of dppf and rac-BINAP are the active ligand species. Preliminary results support that both BozPhos and IPrMonophos ligands can achieve high enantioinduction for this novel direct alkenylation reaction. To date, this represents the first example of enantioselective C-H functionalization employing a bisphosphine monoxide ligand.

Palladium-Catalyzed Enantioselective Intermolecular Carboetherification of Dihydrofurans

A novel enantioselective Pd-catalyzed intermolecular carboetherification of dihydrofurans is reported. The in situ generation of chiral bis-phosphine mono-oxide ligands is crucial, and a general catalytic system has been identified based on this approach. It provides access to a variety of fused tetrahydrofurobenzofurans in consistently high yield and enantiomeric excess.

Mono-Oxidation of Bidentate Bis-phosphines in Catalyst Activation: Kinetic and Mechanistic Studies of a Pd/Xantphos-Catalyzed C-H Functionalization

Kinetic, spectroscopic, crystallographic, and computational studies probing a Pd-catalyzed C-H arylation reaction reveal that mono-oxidation of the bis-phosphine ligand is critical for the formation of the active catalyst. The bis-phosphine mono-oxide is shown to be a hemilabile, bidentate ligand for palladium. Isolation of the oxidative addition adduct, with structural elucidation by X-ray analysis, showed that the mono-oxide was catalytically competent, giving the same reaction rate in the productive reaction as the Pd(II)/xantphos precursor. A dual role for the carboxylate base in both catalyst activation and reaction turnover was demonstrated, along with the inhibiting effect of excess phosphine ligand. The generality of the role of phosphine mono-oxide complexes in Pd-catalyzed coupling processes is discussed.

Unexpected synthesis of an Au2In2 tetrametallatricyclic complex from α-aminophosphines and formation of Au-In-P and Ag-In-P nanomaterials

Four Au-(μ-phosphinite)-In units assemble to form an unprecedented Au2In2 12-membered metallacycle which intersects at the In centres an 8-membered ring containing two In-μ-phosphinate linkages, resulting in a tricyclic structure. Thermal treatment of this complex and of its Ag(I) analog affords Au-In-P and Ag-In-P nanomaterials, respectively.

Mechanistic significance of the si-o-pd bond in the palladium-catalyzed cross-coupling reactions of arylsilanolates

Through the combination of reaction kinetics (both stoichiometric and catalytic), solution- and solid-state characterization of arylpalladium(II) arylsilanolates, and computational analysis, the intermediacy of covalent adducts containing Si-O-Pd linkages in the cross-coupling reactions of arylsilanolates has been unambiguously established. Two mechanistically distinct pathways have been demonstrated: (1) transmetalation via a neutral 8-Si-4 intermediate that dominates in the absence of free silanolate (i.e., stoichiometric reactions of arylpalladium(II) arylsilanolate complexes), and (2) transmetalation via an anionic 10-Si-5 intermediate that dominates in the cross-coupling under catalytic conditions (i.e., in the presence of free silanolate). Arylpalladium(II) arylsilanolate complexes bearing various phosphine ligands have been isolated, fully characterized, and evaluated for their kinetic competence under thermal (stoichiometric) and anionic (catalytic) conditions. Comparison of the rates for thermal and anionic activation suggested, but did not prove, that intermediates containing the Si-O-Pd linkage were involved in the cross-coupling process. The isolation of a coordinatively unsaturated, T-shaped arylpalladium(II) arylsilanolate complex ligated with t-Bu3P allowed the unambiguous demonstration of the operation of both pathways involving 8-Si-4 and 10-Si-5 intermediates. Three kinetic regimes were identified: (1) with 0.5-1.0 equiv of added silanolate (with respect to arylpalladium bromide), thermal transmetalation via a neutral 8-Si-4 intermediate; (2) with 1.0-5.0 equiv of added silanolate, activated transmetalation via an anionic 10-Si-5 intermediate; and (3) with >5.0 equiv of added silanolate, concentration-independent (saturation) activated transmetalation via an anionic 10-Si-5 intermediate. Transition states for the intramolecular transmetalation of neutral (8-Si-4) and anionic (10-Si-5) intermediates have been located computationally, and the anionic pathway is favored by 1.8 kcal/mol. The energies of all intermediates and transition states are highly dependent on the configuration around the palladium atom.

Synthesis, structure and catalytic activity of a gold(i) complex containing 1,2-bis(diphenylphosphino)benzene monoxide

A bisphosphine monoxide gold(i) complex, synthesized by a new protocol, is a highly active catalyst for the hydroarylation of ethylpropiolate.

Hemilabile N-xylyl-N'-methylperimidine carbene iridium complexes as catalysts for C-H activation and dehydrogenative silylation: dual role of N-xylyl moiety for ortho-C-H bond activation and reductive bond cleavage

Direct dehydrogenative silylation of pyridyl and iminyl substrates with triethylsilane was achieved using (L)Ir(cod)(X) (1) (L = a perimidine-based carbene ligand, X = OAc and OCOPh) complexes as catalysts under toluene refluxing conditions in the presence of norbornene as a hydrogen scavenger, and the silylated products were obtained in good yields. The isolated bis(cyclometalated)iridium complexes, (C(∧)C:)(C(∧)N)IrOAc (2) (C(∧)C: = a cyclometalated perimidine-carbene ligand and C(∧)N = a cyclometalated pyridyl- and iminyl-ligated aromatic substrate), were key intermediates, where cyclometalated five-membered metallacycles of substrates such as phenylpyridine were selectively formed before yielding mono-ortho-silylation products. The bis(cyclometalated)iridium complex ((Xy)C(∧)C:)(C(∧)N)IrOAc (2d) ((Xy)C(∧)C: = a cyclometalated N-xylyl-N'-methylperimidine-carbene ligand and C(∧)N = a 2-pyridylphenyl ligand), reacted with 2 equiv of Et3SiH to give an iridium hydride complex, (L(4))(C(∧)N)Ir(H)(SiEt3) (8d) (L(4) = N-CH3, N-3,5-(CH3)2C6H3 perimidine), via demetalation of a N-3,5-xylyl ring of the carbene ligand of 2d. The formation of 8d was confirmed by isolating the corresponding chloro complex (L(4))(C(∧)N)Ir(Cl)(SiEt3) (8d-Cl) by treatment with CCl4. The N-methyl moiety of the carbene ligand coordinated to 8d was cyclometalated in the presence of norbornene at room temperature to afford ((Me)C(∧)C:)(C(∧)N)Ir(SiEt3) (10d) ((Me)C(∧)C: = a cyclometalated N-xylyl-N'-methylperimidine-carbene), while at high temperature 8d reacted with norbornene and Et3SiH to afford the silylated product, 2-(2-triethylsilyl)phenylpyridine (3a) and norbornane. A deuterium labeling experiment using 2d and Et3SiD (excess) revealed the incorporation of deuterium atoms at two ortho-positions of the N-xylyl group (>90%) and at the 3-position of 2-pyridylphenyl ligand (ca. 40%) within 3 h at room temperature, indicating that the cyclometalation/demetalation of the N-xylylperimidine carbene and 2-phenylpyridine ligands were reversible processes. Isolation of these cyclometalated iridium complexes under controlled conditions and D-labeling experiments thus revealed a dual function of the N-aryl group bound to the perimidine-carbene ligand, which acted as both a neutral carbene ligand and a monoanionic ortho-metalated aryl-carbene ligand through reversible C-H bond activation and Ir-C bond cleavage of the N-aryl group during the catalytic cycle.

Homonuclear SQ-DQ correlations in solids: applications of the broadband BaBa scheme to reveal 31P-31P spatial correlations in two-, three-, four-, and five-spin systems

In this paper, we report on the use and limitations of the popular double-quantum recoupling sequence back-to-back in studies of (31)P-(31)P spatial proximities in Pd-phosphine complexes at medium field (9.38 T) under the conditions of fast MAS. The effects of internuclear distances in different spin systems and the impact of isotropic chemical shift and chemical shift anisotropy (CSA) offsets on the detectibility of SQ-DQ correlations were of particular interest to us. Selected model compounds with these requirements in mind were synthesized. By optimization of the excitation times of SQ-DQ correlations up to 4.6 Å could be obtained even in four-spin systems; however, certain long-range correlations may be weak or missing. Although under fast MAS, CSA values up to 270 ppm are well tolerated, in multi-spin systems in cases of isotropic chemical shift offsets larger than 50 ppm, the DQ coherences cannot be properly excited.