Guillermo Muller

Coordination chemistry of oxazoline ligands

Abstract 4,5-Dihydro-1,3-oxazole ligands (commonly known as 2-oxazolines or simply oxazolines) have been used by many research groups as chiral auxiliaries in transition metal-catalyzed asymmetric organic syntheses. The oxazolines show a number of attractive characteristics: versatility of ligand design, straightforward synthesis of ligands from readily available precursors, and modulation of the chiral centers, which are located near the donor atoms. This review focuses on the transition metal coordination chemistry of oxazolines. It surveys data describing structural characterization in both solid state (X-ray diffraction) and solution (NMR spectroscopy).

Synthesis, characterization and catalytic reactivity of ruthenium nanoparticles stabilized by chiral N-donor ligands

The decomposition of the organometallic precursor [Ru(cod)(cot)] (cod = 1,5-cyclooctadiene; cot = 1,3,5-cyclooctatriene) under mild conditions (room temperature, 3 bars H2) and in the presence of optically pure ligands, L*, namely (R)-2-aminobutanol 1, amino(oxazolines) (2, 3), hydroxy(oxazoline) (4) and bis(oxazolines) (5–8), leads to stable ruthenium nanoparticles exhibiting a mean diameter between 1.6–2.5 nm. These nanoparticles can be isolated and re-dispersed. They display different mean sizes, shapes and dispersions depending on the stabilizer nature. These new colloids (Ru1–Ru18) have been characterized by both solid state and molecular chemistry techniques, including TEM/HRTEM, WAXS, elemental analysis, and IR and NMR spectroscopy. To further characterize the surface state of these particles, their catalytic behaviour has been examined in the reduction of organic prochiral unsaturated substrates. Although the asymmetric induction obtained is modest, it reveals the influence of the asymmetric ligand coordinated at the surface of the particles.

Modular bis(oxazoline) ligands for palladium catalyzed allylic alkylation: Unprecedented conformational behaviour of a bis(oxazoline) palladium η3-1,3-diphenylallyl complex

New families of enantiopure bis(oxazolines) with 4,5-trans (5 a-g) or 4,5-cis (6 c) stereochemistry at the individual rings have been prepared in high yield. Their eta(3)-allyl palladium complexes (8 a-g, 9 c and 10) have been used as catalytic precursors in allylic alkylation reactions with excellent enantioselectivities (up to 96 %) for the trans oxazoline derivatives, while Pd/6 c system was inactive. NMR studies on palladium eta(3)-1,3-diphenylallyl intermediates (11 a, c and e) showed the presence of syn/syn- and syn/anti-allyl isomers in solution; this resembles the first example of eta(3)-eta(1)-eta(3) isomerism in Pd allylic complexes containing bis(oxazolines) derived from malonic acid.

CHIRAL CYCLOPALLADATED COMPOUNDS FOR ENANTIOMERIC PURITIES OF FUNCTIONALIZED PHOSPHINES BY MEANS OF MULTINUCLEAR NMR

Abstract The application of the cyclopalladated compound [Pd(2-{Z-( R )-CHMeN=CH-2′,6′-Cl 2 C 6 H 3 }C 6 H 4 )Cl] 2 for the enantiomeric excess determination of functionalized phosphines is presented. The X-ray structure determination of diastereomeric complex [Pd(2-{Z-( R )-CHMeN=CH-2′,6′-Cl 2 C 6 H 3 }C 6 H 4 )Cl{( R , R )-PPh 2 (2-OHC 6 H 1 0 )}] is also reported.

A general approach to fabricate fe3O4 nanoparticles decorated with Pd, Au, and Rh: Magnetically recoverable and reusable catalysts for Suzuki C-C cross-coupling reactions, hydrogenation, and sequential reactions

A facile strategy has been explored for loading noble metals onto the surface of ferrite nanoparticles with the assistance of phosphine-functionalized linkers. Palladium loading is shown to occur with participation of both the phosphine function and the surface hydroxyl groups. Hybrid nanoparticles containing simultaneously Pd and Au (or Rh) are obtained by successive loading of metals. Similarly, ferrite nanoparticles decorated with Pd, Au, and Rh have also been formed by using the same strategy. The catalytic properties of the new nanoparticles are evidenced in processes such as reduction of 4-nitrophenol or hydrogenation of styrene. Besides, the sequential process involving a cross-coupling reaction followed by reduction of 1-nitrobiphenyl has been successfully achieved by employing Pd/Au decorated nanoferrite particles.

Chiral S,S-donor ligands in palladium-catalysed allylic alkylation

Abstract Chiral dithioether ligands have been tested in the model Pd-catalysed allylic alkylation reaction of (±)-3-acetoxy-1,3-diphenyl-1-propene with dimethyl malonate, giving high enantioselectivity (up to 81% e.e.) for the first time in this type of system. Pd(II)-allylic intermediates, [Pd(η3-1,3-Ph2-C3H3)(dithioether)]PF6 were prepared and characterised both in solution by NMR spectroscopy and solid state. The X-ray structure for [Pd(η3-1,3-Ph2-C3H3)(L)]PF6 (L=(R,R)-7,8-O-isopropylidene-1,5-dithia-cyclononane) was determined.

SYNTHESIS OF MONOHYDROXY -METHYL- AND -ETHYL-PHOSPHINES PPH2CHROH

Abstract The preparation of α-hydroxyphosphines PPh 2 CHROH with R  H( a ), Ph( b ) and Et( c ), and β-hydroxyphosphines PPh 2 CHR′HROH ( e-i ) has been achieved in high yield by reaction of HPPh 2 or LiPPh 2 with RCHO or the oxides of cyclohexene ( e ), limonene ( f ), styrene ( g, h ) and pinene ( i ). With the exception of styrene oxide, the reactions with the oxides are very regioselective, giving only one of the possible isomers. Cis- and trans -limonene oxides react with LiPPh 2 in different conditions selectively, giving trans -phosphino derivatives from commercial mixtures of limonene oxide.

Palladium catalyzed Suzuki C–C couplings in an ionic liquid: nanoparticles responsible for the catalytic activity

A new family of functionalized ligands derived from norborn-5-ene-2,3-dicarboxylic anhydride has been used in Suzuki C-C cross-couplings between aryl boronic acids and aryl bromide derivatives in [BMI][PF(6)] (BMI=1-n-butyl-3-methyl-imidazolium), using palladium acetate as catalytic precursor. High conversions and yields are obtained when amine ligands containing hydroxy groups are involved. TEM analyses after catalysis show the formation of small nanoparticles, in contrast to the agglomerates observed when nanoparticles are intentionally preformed, with a consequent decrease in the catalytic activity in the latter case. Some tests, including the correlation effect between solvent and ligand, are carried out to try to identify the true nature of the catalyst. All the results obtained suggest that formation of nanoparticles is required to lead to a catalytically active system.

Novel super-structures resulting from the coordination of chiral oxazolines on platinum nanoparticles

The reaction of Pt2(dba)3 with CO in toluene leads rapidly to the formation of platinum nanoparticles which can be further stabilized by addition of aminoalcohol or oxazoline ligands. The coordination of the oxazoline ligands at the surface of the particles is demonstrated by IR after purification of the colloids. The fcc structure of the particles is evidenced by WAXS investigations, a statistical method which also gives information about the degree of crystallinity of the particles. TEM and HREM micrographs reveal surprising organisations of the particles into super-structures depending on the nature and/or position of the substituents present on the ligands. In addition, these particles are catalytically active in the hydrogenation of ethyl pyruvate, even if the preliminary results are modest.

Synthesis and resolution of benzylisopropylphenylphosphine, a monodentate P-chiral ligand

Abstract The synthesis of benzylisopropylphenylphosphine by reaction between the benzylphenylphosphide anion and isopropyl chloride and its subsequent resolution by means of an optically active palladium metallacycle is reported. The synthesis of the organometallic complex [Pd(η 3 -2-MeC 3 H 4 )Cl(PBn i PrPh)] is also described, as well as the assignment of the absolute configuration of the coordinated phosphine by mono- and bidimensional proton NMR spectra, using the homochiral palladacycle as a reference point. In order to estimate the height of the energy barrier corresponding to the rotation of the phosphine ligand around the PdP bond, several calculations were performed at the semiempirical PM3(tm) level.