Transfer Hydrogenation from 2-propanol to Acetophenone Catalyzed by [RuCl2(eta6-arene)P] (P = monophosphine) and [Rh(PP)2] (PP = diphosphine, X = Cl-,BF4-) Complexes

The reduction of ketones through homogeneous transfer hydrogenation catalyzed by transition metals is one of the most important routes for obtaining alcohols from carbonyl compounds. The interest of this method increases when opportune catalytic precursors are able to perform the transformation in an asymmetric fashion, generating enantiomerically enriched chiral alcohols. This reaction has been extensively studied in terms of catalysts and variety of substrates. A large amount of information about the possible mechanisms is available nowadays, which has been of high importance for the development of systems with excellent outcomes in terms of conversion, enantioselectivity and Turn Over Frequency. On the other side, many mechanistic aspects are still unclear, especially for those catalytic precursors which have shown only moderate performances in transfer hydeogenation. This is the case of neutral [RuCl2(η6-arene)(P)] and cationic [Rh(PP)2]X (X = anion; P and PP = mono- and bidentate phosphine, respectively) complexes. Herein, a summary of the known information about the Transfer Hydrogenation catalyzed by these complexes is provided with a continuous focus on the more relevant mechanistic features

Cyclometallated ruthenium complexes with P-stereogenic monophosphines containing a polycyclic aromatic substituent

Reactions of optically pure P-stereogenic ortho-tolyl substituted phosphines with [RuCl2(p-cymene)]2 afforded the corresponding kP-coordinated ruthenium(II) dichlorides (C1′, C2') even in the presence of sodium acetate. In contrast, the ruthenium cyclometallated (k2-C,P) complexes (C3eC9) were obtained with phosphines containing a polycyclic aromatic substituent (L3-L9), namely 1-naphthyl, 9-phenanthryl or 1-pyrenyl. Some diastereoselectivity in the cyclometallation process has been observed for the most bulky ligands. The new compounds have been used as catalytic precursors in the reduction of acetophenone to 1-phenylethanol by transfer hydrogenation

P-Stereogenic monophosphines with the 2-p-terphenylyl and 1-pyrenyl substituents. Application to Pd and Ru asymmetric catalysis

tThe synthesis of five optically pure P-stereogenic monophosphines of the type PPhArR (Ar = 2-p-terphenylyl (a), 1-pyrenyl (b); R = OMe, Me, i-Pr) is described. The ligands were fully characterisedand the absolute configurations of PPh(1-pyrenyl)R (3b and 5b; R = OMe and Me respectively) wereconfirmed by X-ray diffraction. The complexation of the monophosphines to Pd and Ru organometal-lic units yielded the neutral complexes [PdCl( 3-2-Me-allyl)P] (10-12) and [RuCl2( 6-p-cymene)P](16-18). Complete characterisation, including the crystal structure determination of [RuCl2( 6-p-cymene)(PMePh(2-p-terphenyl))] (17a) is provided. Neutral palladium complexes appeared as mixturesof two diastereomers in solution according to NMR. The synthesis and characterisation of four cationic[Pd( 3-2-Me-allyl)(P)2]PF6(13 and 14) is also described. The application of neutral Pd complexes tocatalytic styrene hydrovinylation afforded moderate conversions, high chemoselectivities (>92%) to 3-phenyl-1-butene and up to 43% ee with precursor 12a. Cationic Pd complexes were tested as catalyticprecursors in allylic substitution of rac-3-acetoxy-1,3-diphenyl-1-propene (rac-I), with the anion ofdimethylmalonate and benzylamine as nucleophiles, obtaining full conversions and up to 80% ee in alkyl-ation and 60% ee in amination with precursor 13a. Finally, ruthenium complexes were used as catalyticprecursors in transfer hydrogenation of acetophenone, with complete conversions after several hoursbut low enantioselectivities

Coordination chemistry and catalysis with secondary phosphine oxides

Secondary phosphine oxides present a tautomeric equilibrium between the pentavalent oxide form (SPO) and the trivalent phosphinous acid (PA). This dichotomy is the origin of the rich coordination chemistry of this class of compounds. As the pentavalent oxide form usually predominates, SPOs are air-stable but at the same time metal coordination can shift the tautomerism towards the PA form, making the ligand act as an ordinary trivalent phosphine. For this reason this class of ligands have found application in numerous homogeneously catalysed reactions, including some enantioselective transformations. This review aims to give an up to date account on the synthesis, coordination chemistry and homogeneous catalysis with SPOs

Diphosphorus Ligands Containing a P-Stereogenic Phosphane and a Chiral Phosphite or Phosphorodiamidite - Evaluation in Pd-Catalysed Asymmetric Allylic Substitution Reactions

The synthesis of 14 new optically pure C 1‐symmetric phosphane-phosphinite (1 -4 ), phosphane-phosphite (5 -9 ) and phosphane-phosphorodiamidite (10 -14 ) ligands is reported. The ligands were prepared through the condensation of (2‐hydroxyphenyl)phenylphosphanes PPh(2‐PhOH)R (R = Me, t Bu and Ph) with chlorodiisopropylphosphane (1 and 2 ), chlorodiphenylphosphane (3 and 4 ), the chlorodioxaphosphepine derived from both enantiomers of 1,1′‐bi‐2‐naphthol (5 -9 ) and the chlorodiazaphosphepine derived from both enantiomers of N ,N′ ‐dimethyl‐1,1′‐binaphthyl‐2,2′‐diamine (10 -14 ) in the presence of a base. With these ligands, cationic Pd complexes of the type [Pd(η3‐C4H7)(PP′)]PF6 (Pd1 -Pd14 ) were obtained and characterised; the crystal structures of Pd1 , Pd2 and Pd13 were obtained. In solution, the complexes are present as mixtures of two diastereomers because of the lack of symmetry of the ligand and the presence of the methallyl group. The Pd complexes catalyse the allylic alkylation with dimethyl malonate and the amination with benzylamine of the model substrate rac ‐3‐acetoxy‐1,3‐diphenyl‐1‐propene (I ). For the alkylation, full conversions and good enantioselectivities (up to 96 % ee with Pd14 ) were observed

Cyclopalladated Compounds with Polyhalogenated Benzylphosphanes for the Mizoroki-Heck Reaction

Nine partially halogenated benzylphosphanes ArXCH2PR2 (ArX = 3,6-dichlorophenyl, 3,6- difluorophenyl and 3,4,5-trifluorophenyl; R = Ph, Cy, iPr) have been prepared and reacted with palladium acetate to obtain the cyclometallated dimers [Pd(μ-OAc)(κ2-C,P-ArXCH2PR2)]2. The acetate bridge has been exchanged by bromide using lithium bromide and the obtained dimers have been thoroughly characterised. The dimers with the non-halogenated phosphanes PhCH2PR2 (R = Ph, iPr) have also been prepared. Treatment with norbornadiene in the presence of silver tetrafluoroborate has furnished the cationic mononuclear complexes [Pd(κ2-C,P-ArXCH2PR2)(nbd)]BF4 as stable solids. These complexes and some of the bromide dimers have been used as catalytic precursors in the Mizoroki- Heck reaction between bromobenzene and butyl acrylate. The complexes efficiently catalyse this transformation and important differences of activity are found depending on the ligand. In general, fluorinated phosphanes give more active systems than chlorinated analogues

Ruthenium complexes of P-stereogenic phosphines with a heterocyclic substituent

The synthesis via phosphine-boranes of 13 new optically pure P-stereogenic diarylphosphines P(Het)PhR (Het = 4-dibenzofuranyl (DBF), 4-dibenzothiophenyl (DBT), 4-dibenzothiophenyl-S,S-dioxide (DBTO2) and 1-thianthrenyl (TA); R = OMe, Me, i-Pr, Fc (ferrocenyl)) following the Jugé-Stephan method is described. The ligands were designed with the aim of having a heteroatom in a position capable of interacting with a metal upon coordination. The ligands and their precursors have been fully characterised, including the determination of two crystal structures of phosphine-boranes. Ru neutral complexes of the type [RuCl2(η6-arene)(κP-P)] (arene = p-cymene and methyl benzoate) have been prepared and characterised, including three crystal structure determinations. Treatment of solutions of the complexes with TlPF6 allowed the preparation of well-defined cationic complexes [RuCl(η6-arene)(κ2P,S-P)]PF6 for DBT- and TA-based phosphines. The complexes possess a stereogenic Ru atom and in most of the cases they are present as a single isomer in solution. All the Ru complexes have been used in the asymmetric transfer hydrogenation of acetophenone in refluxing 2-propanol, with good activities and up to 70% ee

Neutral and Cationic Palladium Complexes of P-Stereogenic Phosphanes Bearing a Heterocyclic Substituent

The coordination chemistry of 13 optically pure P ‐stereogenic diarylmonophosphanes P(Het)PhR [Het = 4‐dibenzofuranyl (DBF), 4‐dibenzothiophenyl (DBT), 4‐dibenzothiophenyl S ,S ‐dioxide (DBTO2) and 1‐thianthrenyl (TA); R = OMe, Me, i Pr, Fc (ferrocenyl)] to Pd‐allyl moieties is described. Both neutral [PdCl(η3‐(2‐methylallyl)(κP ‐P )] and cationic [Pd{η3‐(2‐methylallyl)(κP ‐P )2}]PF6 complexes have been prepared. Coordination of the heteroatom of the heterocycle was only possible in the case of TA‐based phosphanes; these furnished complexes of the type [Pd{η3‐(2‐methylallyl)(κ2P,S ‐P )}]PF6 after chloride abstraction with TlPF6. The crystal structure of the complex [Pd(η3‐2‐methylallyl)(κ2P,S ‐PPh(OMe)(1‐TA)]PF6 is reported. The neutral Pd complexes were found to be highly active in the hydrovinylation of styrene after activation with AgBF4, except for the TA‐based phosphanes. The cationic Pd complexes were evaluated in allylic alkylation and amination with the model substrate rac ‐trans ‐1,3‐diphenylprop‐2‐enyl acetate (rac ‐I ), achieving total conversions and up to 70 % ee

Borane as an efficient directing group. Stereoselective 1,2-addition of organometallic reagents to borane P-stereogenic N-phosphanylimines

In non-coordinating solvents, borane was shown to be an efficient directing group for the stereoselective 1,2-addition of organolithium reagents to P-stereogenic N-phosphanylimines. Selectivity was reversed in coordinating solvents. This process can lead to novel ligand scaffolds for asymmetric catalysis

Rhodium-Catalyzed Pauson−Khand Reaction Using a Small-Bite-Angle P‑Stereogenic C1‑Diphosphine Ligand

The asymmetric Pauson−Khand reaction catalyzed by [Rh(COD)(MaxPHOS)]BF4 is described. Several 1,6-enynes have been chosen as model substrates affording moderate yields and selectivities of up to 86% ee. Besides binap-type ligands, we have demonstrated that the Pstereogenic C1-symmetry small-bite-angle ligand MaxPHOS is a viable ligand in this process. The formation of [2+2+2] cycloaddition compounds has shown to be a competitive process. A mechanism is proposed to account for the observed results. The intermediate rhodium dicarbonyl complex 6 was synthesized, and its solid-state structure was elucidated by X-ray crystallography