Cyclopalladation of a ferrocene acylphosphine and the reactivity of the C-H activated products

Acylphosphines are an attractive subclass of phosphine ligands with specific reactivity and ligating properties. This study describes the synthesis of a ferrocene-based acylphosphine, FcC(O)PPh₂ (1, Fc = ferrocenyl), the corresponding phosphine chalcogenides FcC(O)P(E)Ph₂ (E = O, S and Se), and palladium(ii) complexes resulting from the orthometallation of 1, viz. [Pd(μ-X)(1- H)]₂ (2-X, where X = Cl, Br and I), which in turn serve as a convenient entry point to a range of monopalladium complexes. Thus, the cleavage of 2-X with phosphines, 4-(dimethylamino)pyridine and in situ-generated 1,3-dimesitylimidazolin-2-ylidene (L) provided complexes [PdX(L)(1- H)] (3-5), which are typically obtained as single isomers by crystallisation but undergo spontaneous isomerisation in solution leading to equilibrium mixtures of cis and trans isomers. Upon treatment with sodium acetylacetonate (Na(acac)), 2-Cl was transformed into [Pd(acac)(1- H)] (6), which reacted with (diphenylphosphino)acetic acid under proton transfer to give [Pd(Ph₂PCH₂CO₂-κ²O,P)(1- H)] (7), while the reaction with allylating agents produced the π-allyl complex [Pd(η³-C₃H₅)(1- H)] (9). Compound [PdCl(PMe₃)(1- H)] was further used to prepare a series of diorganopalladium complexes [Pd(R)(PMe₃)(1- H)] (8; R = Me, 4-C₆H₄Me, 4-C₆H₄CF₃, CH[double bond, length as m-dash]CHC₆H₄Me-4 and C[triple bond, length as m-dash]CC₆H₄Me-4). All compounds were structurally characterised using spectroscopic methods and, in most cases, also by X-ray diffraction analysis. The structural data indicated the rigidity of the Pd(1- H)⁺ fragment and revealed variations in the Pd-donor distances dictated by an interplay between the trans influence and steric demands of the ligands surrounding Pd(ii). Moreover, some compounds were studied by DFT to rationalise their isomerisation equilibria and electrochemical properties, which were examined by cyclic voltammetry.

Diphospha-Ureas from the Phosphaketene Ph3 GePCO

The phosphaketene Ph₃ GePCO is shown to react with the phosphide KP(tBu)₂ to generate the anion [Ph₃ GePC(O)P(tBu)₂ ]^- 1. This species reacts with CH₃ I or ClGePh₃ to give the dissymmetric diphospha-ureas (tBu)₂ PC(O)P(GePh₃ )(CH₃ ) 2 and (Ph₃ Ge)₂ PC(O)P(tBu)₂ 3 respectively. Sequential treatment of 2 with a base and CH₃ I affords a route to (tBu)₂ PC(O)P(CH₃ )₂ 5. These species are products of the first modular diphospha-urea synthesis. The subsequent thermal and photochemical reactivity of these species was also probed and described.

Non-Metal-Catalyzed Heterodehydrocoupling of Phosphines and Hydrosilanes: Mechanistic Studies of B(C6F5)3-Mediated Formation of P-Si Bonds

Non-metal-catalyzed heterodehydrocoupling of primary and secondary phosphines (R¹R²PH, R² = H or R¹) with hydrosilanes (R³R⁴R⁵SiH, R⁴, R⁵ = H or R³) to produce synthetically useful silylphosphines (R¹R²P-SiR³R⁴R⁵) has been achieved using B(C₆F₅)₃ as the catalyst (10 mol %, 100 °C). Kinetic studies demonstrated that the reaction is first-order in hydrosilane and B(C₆F₅)₃ but zero-order in phosphine. Control experiments, DFT calculations, and DOSY NMR studies suggest that a R¹R²HP·B(C₆F₅)₃ adduct is initially formed and undergoes partial dissociation to form an "encounter complex". The latter mediates frustrated Lewis pair type Si-H bond activation of the silane substrates. We also found that B(C₆F₅)₃ catalyzes the homodehydrocoupling of primary phosphines to form cyclic phosphine rings and the first example of a non-metal-catalyzed hydrosilylation of P-P bonds to produce silylphosphines (R¹R²P-SiR³R⁴R⁵). Moreover, the introduction of PhCN to the reactions involving secondary phosphines with hydrosilanes allowed the heterodehydrocoupling reaction to proceed efficiently under much milder conditions (1.0 mol % B(C₆F₅)₃ at 25 °C). Mechanistic studies, as well as DFT calculations, revealed that PhCN plays a key mechanistic role in facilitating the dehydrocoupling reactions rather than simply functioning as H₂-acceptor.

Silylphosphido complexes of gold(I) coordinated with NHC ligands

The N-heterocyclic carbenes IPr (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) and iPr2-bimy (iPr2-bimy = 1,3-di-isopropylbenzimidazole-2-ylidene) were utilized as a basis for the preparation of four gold–silylphosphido complexes: [(IPr)AuP(Ph)SiMe3] (1), [(IPr)AuP(SiMe3)2] (2), [(iPr2-bimy)AuP(Ph)SiMe3] (3), and [(iPr2-bimy)AuP(SiMe3)2] (4). These complexes represent rare examples of terminally bonded Au–PR2 and the first examples where phosphorus retains reactive P-SiMe3 moieties. The reactivity of the P–Si bonds in 1 and 3 was explored via the addition of PhC(O)Cl. The products of these reactions were the formation of the phosphido-bridged [(IPrAu)2(μ-PPhC(O)Ph)][AuCl2] (5) and, in the case of the smaller N-heterocyclic carbenes, the tertiary phosphine PPh(C(O)Ph)2 (6) was isolated together with the known gold complex [(iPr2-bimy)AuCl]. Both reactions proceed via the elimination of ClSiMe3.

Synthesis of stable phosphomide ligands and their use in Ru-catalyzed hydrogenations of bicarbonate and related substrates

New benzoyl- and naphthoyl-substituted phosphines have been synthesized, which are stable to air and moisture. Testing these so-called phosphomide ligands in the presence of different ruthenium precursors, the hydrogenation of sodium bicarbonate (NaHCO(3)) to sodium formate (NaHCO(2)) proceeded with good catalyst turnover numbers in the range of 1300-1600 at 80°C and a total pressure of hydrogen of 60 bar in the absence of amines or other additives. Similarly, catalytic hydrogenations of carbon dioxide, cinnam-, and benzaldehyde were possible with these new ruthenium complexes. As an intermediate of the catalytic cycle the defined ruthenium complex [(η(6)-C(6)H(6))-RuCl(2)(Cy(2)P(1-naphthoyl)] (Cy=cyclohexyl) was prepared and characterized by X-ray crystallography.

Synthesis of a new chiral bisphospholane ligand for the Rh(I)-catalyzed enantioselective hydrogenation of isomeric beta-acylamido acrylates

The highly stereoselective synthesis of a chiral silylphospholane has been described, which can be advantageously used as a building block under base-free conditions for the construction of diphosphines related to DuPHOS. The utility of silylphospholane is shown in the synthesis of a new bisphospholane ligand 1 (MalPHOS), which is characterized by a maleic anhydride backbone. The ligand forms with Rh(I) a complex with a larger bite angle P-Rh-P than the analogue Me-DuPHOS complex. Both complexes have been tested in the asymmetric hydrogenation of unsaturated alpha- and beta-amino acid precursors of pharmaceutical relevance. In several cases, the new catalyst was superior in comparison to the Me-DuPHOS complex, in particular when (Z)-configured beta-acylamido acrylates were used as substrates.