Search

Palladium(0) nanoparticles have been widely used in cross coupling reactions, including Heck reactions. For this study, we synthesized palladium(0) nanoparticles in colloidal suspension using different combinations of solvents and reducing methods under aerobic conditions. The variation in systems used to synthesize palladium(0) nanoparticles resulted in different nanoparticle sizes. To investigate whether the particle size had an effect on catalysis, we first used common Heck C–C cross-coupling reaction conditions (200 °C and 18 h). In addition, we omitted the use of stabilizing agents, other than the solvent and/or the anions in the initial nanoparticle synthesis, since the use of stabilizing agents adds cost and processing time to catalysis. All of the catalysts investigated worked in the cross coupling C–C Heck reaction, but yields did not show appreciable differences, as high temperature and long reaction times promote a high reduction of palladium(II). Therefore, we decided to work with a temperature and reaction time in which conversion would start to be observed (minimum reaction conditions). The experiments to determine minimum reaction conditions showed that this would be 120 °C and 10 h, therefore we used these conditions in Heck C–C cross-coupling reactions and all the palladium nanoparticle systems. The best C–C catalysis conversion was observed when N,N-dimethylformamide was used as solvent in the absence of reducing agent. This catalyst system resulted in the largest possible nanoparticles, which were kept in dispersion (did not precipitate out), showing that size is important in obtaining good yields in C–C Heck catalysis (where cocktail-type catalysis could explain the conversion). Nanoparticles of this size also act as a reservoir of soluble palladium species that behave as the true catalyst. The second best conversion was observed in N,N-dimethylformamide with sodium citrate, where citrate may have added extra protection, and since the palladium(0) nanoparticles were small, cocktail-type catalysis was not involved in obtaining high yields.

We report the study of a series of Ru-bipyridine (Ru-Bpy) complexes to correlate their photophysical and electrochemical properties to their performance in [4 + 2] cycloadditions, as a model reaction for electron transfer catalysis. Redox potentials, absorption and emission spectra, quantum yields and DFT calculations are presented to understand the catalytic transformation. The study shows that complex [Ru(Bpy) 3 ](PF 6 ) 2 is the more active photocatalyst, giving complete conversion to the product after 2 h reaction under white light. Any substitution in the para position of the Bpy ring decreases the reaction conversion. The [4 + 2] cycloaddition can be performed with different substrates and dienes, using either acetonitrile or nitromethane as solvents. Mechanistic studies suggest that the active catalyst for the transformation is [Ru(Bpy) 3 ] 3+ and that the rate determining step is the oxidation of [Ru(Bpy) 3 ] 2+ to generate the corresponding Ru 3+ species.

Palladium nanoparticles (PdNPs) prepared in neat glycerol containing TPPTS (tris(3-sulfophenyl)phosphine trisodium salt) or cinchona-based alkaloids (cinchonidine, quinidine) as capping agents, were applied as catalysts in fluoride-free Hiyama couplings and conjugate additions with the aim of evaluating the influence of the stabilizer in the catalytic reactivity. Therefore, PdNPs stabilized by phosphine favored C C cross-couplings, whereas those containing alkaloids showed enhanced suitability for C C homo-couplings and conjugate additions. The metal/stabilizer coordination mode, i.e. Pd P dative bond and π-π interaction between quinoline moiety and palladium surface, is certainly key for the stabilization of different active metallic species and then promoting distinctive catalytic pathways.

Thiolate-capped rhodium nanoparticles were synthesized by decomposition of the organometallic precursors [Rh(μ-SC12H25)(COD)]2 (I) and [Rh(μ-SC6H11)(COD)]2 (II) under hydrogen pressure, in imidazolium-based ionic liquids and organic solvent, without the external addition of ligands as stabilizers and were compared to RhNPs synthesized from [Rh(μ-OMe)(COD)]2. The use of ionic liquids during the synthesis of rhodium nanoparticles resulted in homogeneously well-dispersed systems as observed by TEM, where the thiolate ligand remains intact as demonstrated by XPS analysis and gas chromatography. The thiolate-capped rhodium nanoparticles generated from II were active catalysts in hydrogenation reactions with high selectivity towards alkene, nitro and imine groups. The absence of thiolate ligand generated very active catalytic systems and the hydrogenation of carbonyl groups was possible. The effect of thiol stabilizer was studied in the reductive N-alkylation to produce N-benzylaniline through one pot synthesis from benzaldehyde and nitrobenzene, where only the thiolate-capped RhNPs showed complete conversion and high selectivity. Additionally, the thiolate-capped RhNPs permitted the reuse of the catalytic systems for ten catalytic cycles for selected hydrogenation reactions after product separation.

article i nfo Palladium nanoparticles stabilized by thioether-phosphine ligands (1-3) were used as catalytic precursors in Heck-Mizoroki cross-coupling and C_C reduction reactions in ionic liquids. The ionic liquid (EMI)(MeHPO3) (A) exhibited an important hydrogen donor/acceptor behavior depending on the reaction conditions. Tandem or parallel pathways for C-C coupling and H-transfer reactions were observed, leading to the coupled product trans-4-phenyl-3-buten-2-one (I) and the corresponding reduced partner 4-phenylbutan-2-one (II) pointing toamolecular-likecatalyticreactivityoftheprocess.Whenthereactionwascarriedoutunderhydrogenpressure and low temperature, the reduction to II was inhibited by the presence of thioether-phosphine ligands.

International audience; The asymmetric allylic alkylation of rac-1,3-diphenyl-3-acetoxyprop-1-ene (I) catalysed by palladium and diverse phosphorus containing ligands [(S)-BINAP, (R,R)-Chiraphite and (R,R)-Et-Duphos] in an ionic liquid [HDBU][OAc] was successfully performed, achieving full conversions and up to 96% ee of the (S)-product when (R,R)-Et-Duphos was used as a ligand. The reaction could be performed using an equimolar amount of substrate, malonate and base DBU, in which case the total products sum to the desired alkylated product and the ionic pair [HDBU][OAc]; this system thus produces its own IL solvent as the only co-product. These catalytic systems were active in recycling experiments for up to four cycles, albeit with a loss of activity due to the poor retention of palladium in the ionic liquid. The catalytic performance of each Pd/ligand system was optimized by varying the ratio of the substrate and malonate. Systems based on [HDBU][OAc] were found to be the best.

A Pd/(S)-BINAP system was successfully applied to the asymmetric allylic alkylation of rac-1,3-diphenyl-3-acetoxyprop-1-ene () using imidazolium-based ionic liquids (ILs) attaining up to 225 h(-1) TOF and 88% ee of the (R)-product. Although the system was barely active in the recycling experiments, the catalyst life was confirmed after recharging the system with substrate/reactants resulting in an alkylated product. In the latter case, the conversion rates and enantiomeric excesses were similar or lower compared to those in the first cycle. In order to explain the observed catalyst performance in the recycling as well as in the recharging experiments, we investigated the reactivity between the catalyst precursors, substrate and reactants in ILs. We were able to identify the species involved in the catalytic reactions under various conditions by means of (31)P NMR analyses. Allylpalladium intermediates () were found to be the active and selective species at a high substrate concentration. When the substrate was consumed, competing reactions took place leading to different palladium complexes. [PdCl(NHC(Bu,Me))((S)-BINAP)]Cl (), together with [Pd((S)-BINAP)2] (), were recognised as the species responsible for the loss of activity, meanwhile, the decrease in enantioselectivity was accounted for by the formation of mixed (NHC)(monophosphine)-palladium species.

One of the most powerful approaches for the formation of simple and complex chiral molecules is the metal-catalysed asymmetric allylic alkylation. This reaction has been broadly studied with a great variety of substrates and nucleophiles under different reaction conditions and it has promoted the synthesis of new chiral ligands to be evaluated as asymmetric inductors. Although the mechanism as well as the active species equilibria are known, the performance of the catalytic system depends on the fine tuning of factors such as type of substrate, nucleophile nature, reaction medium, catalytic precursor and type of ligand used. Particularly interesting are chiral phosphines which have proved to be effective asymmetric inductors in several such reactions. The present review covers the application of phosphine-donor ligands in Pd-catalysed asymmetric allylic alkylation in the last decade.

We prepared new thioether-arylphosphines bearing alkyl and fluoroalkyl chains at the sulfur atom in order to use them in metal-catalyzed reactions in new reaction media, such as a fluorous biphasic system. The characterization of the new compounds is discussed and the partition coefficients in the biphasic system perfluoromethylcyclohexane-cyclohexane were determined. Fluorous thioether-phosphines reacted with [Rh(acac)(CO) 2 ] under fluorous biphasic conditions to produce rhodium species retained in the fluorous phase with

This review covers reports in the literature on the use of thioether ligands in Pd-catalysed asymmetric allylic substitution reactions from 1996 to June 2006. In the first section, we will cover the results using (S,S)-homo and (S,S)-heterodonor ligands. In the next section, we will present the results using heterodonor (S,X)-ligands, emphasizing the results obtained with (S,N)- and (S,P)-donor ligands. We also discuss any reported mechanistic aspects according to the allylic substitution results.

Palladium allylic systems containing modular chiral dithioether ligands were chosen as catalysts for a systematic study of homodonor ligands in allylic alkylation reactions. For this purpose, new t...

A series of cationic IrI complexes containing chiral dithioether ligands have been prepared in order to study the influence of the sulfur substituents and the metallacycle size on the acetamidoacrylate hydrogenation reaction. In the case of complexes 6, 7 and 10, a mixture of diastereomers is observed in solution due to the sulfur inversion processes. In contrast, this fluxional behaviour is efficiently controlled by using bicyclic ligands which inhibit the S-inversion in complexes 8 and 9. The solid-state structure of complex 10b shows only one diastereomer with the sulfur substituents in a relative anti disposition and in an overall configuration of SCSCSSSS at the coordinated dithioether ligand. Iridium complexes containing seven- and six-membered metallacycles (6b–d, 7b,c, 10a,b) react with the substrate through S-ligand substitution, and the rate of this substitution is related to the position of the fluorine atom on the aromatic ring. On the contrary, complexes containing a bismetallacycle (8 and 9) are not displaced by the substrate. The catalytic hydrogenation activity of complexes 8 and 9 is analysed in terms of the high stability of the corresponding dihydride complexes (13 and 14). In both cases, only two of the four possible diastereomeric dihydride species are formed in solution. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Thioether-phosphines, 1 and 2, were applied for the stabilisation of palladium nanoparticles (PdNPs) synthesised by a bottom-up methodology, using [Pd2(dba)3] as an organometallic precursor. For the phenyl containing ligand 1, small (d(mean) = 1.6 nm), well-defined and dispersed nanoparticles were obtained; however, ligand 2 involving a long alkyl chain led to agglomerates. NMR and GC-MS analyses throughout the synthesis of the nanomaterials revealed partial cleavage of ligands by C-S and C-P bond activations, and XPS spectra of the isolated nanoparticles indicated the presence of both thioether-phosphines and their fragments on the metallic surface. Reactivity studies of molecular palladium systems as well as on extended palladium surfaces pointed out that cluster entities are responsible for C-heteroatom activations, triggering structure modifications of stabilisers during the synthesis of PdNPs.

New catalytic systems based on in situ and preformed palladium nanoparticles in ionic liquids (characterised by TEM) starting from palladium acetate or dipalladiumtris(dibenzylideneacetone) have been applied in the synthesis of 4-phenylbutan-2-one (II), a model compound for the preparation of fragrances. Imidazolium-based ionic liquid containing a methyl hydrogenophosphonate anion leads to an efficient Pd-catalyzed tandem coupling/reduction process, taking advantage of the multi-role of this solvent (nanoparticles stabiliser, base, hydrogen transfer agent). The influence of the mono-phosphine ligands (1-3) on the catalyst has been evaluated, showing that the ligand-free palladium system turns into the most appropriate for the formation of II using Pd(OAc)(2) as precursor. Fine-tuning conditions involved in this multi-parameter process have led us to propose a plausible mechanism based on the hydrogen transfer coming from the methyl hydrogenophosphonate anion.

By reacting [Pt(SR)2COD] with K2[Pt(SR′)4] (R and R′C6F5 or p-C6HF4) platinum(II) homoleptic binuclear compounds with bridging fluorothiolate groups are obtained. Compounds bearing mixed ligands show a variety of isomers in solution.

The preparation and characterization of the new thiolate complexes [M(SR)2(SEt2)2] (M=Pt, R=C6F5 orp-C6HF4) and [M(SR)2]n (M=Pd, R=C6F5,p-C6HF4 orp-C6H4F; M=Pt, R=p-C6H4F) is discussed. The tendency to form polymeric, rather than monomeric species, varies as follows: Pd>Pt; C6H4F>C6HF4> C6F5. [Pt(SC6F5)2(SEt2)2] has atrans square planar coordination.

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.

The syntheses of the persulfurated [M(SRF)2(CH3SCH(CH3)CH(CH3)SCH3)], M=Pd or Pt; RF=C6F5 or C6F4H-4 compounds allow the separation of two diastereomers and the NMR detection of five conformers (three anti and two syn) of each compound. 1H, 19F and 195Pt NMR data have been used to assign each conformer and to obtain their relative populations as well as to follow their equilibrium at high temperatures. The X-ray diffraction molecular and crystalline structure of [Pt(SC6F5)2(CH3SCH(CH3)CH(CH3)SCH3)] has been determined.