Erika Martin

Chiral thioether ligands: coordination chemistry and asymmetric catalysis

Abstract This review covers the recent applications of chiral thioether ligands in asymmetric catalysis with transition metal complexes. S,S-Homo and S,X-heterodonor ligands are considered with special emphasis to their coordination chemistry. The results obtained with metal/thioether containing catalytic systems in asymmetric allylic substitution, hydrogenation, transfer hydrogenation, hydrosilylation, Michael addition, 1,4-conjugate addition and copolymerization reactions are reviewed including mechanistic considerations.

Recent advances in the application of chiral phosphine ligands in Pd-catalysed asymmetric allylic alkylation.

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.

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.

A smart palladium catalyst in ionic liquid for tandem processes

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.

Persulfurated complexes of palladium(II) and platinum(II) with fluorinated aryl-thiolates. X-ray structure of [Pt(SC6F5)2(CH3SCH(CH3)CH(CH3)SCH3)]

Abstract 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.

Palladium nanoparticles stabilised by cinchona-based alkaloids in glycerol: efficient catalysts for surface assisted processes

Palladium nanoparticles (PdNPs) were synthesised and fully characterised, both in solution and the solid state, using naturally-occurring cinchona-based alkaloids in neat glycerol. These nano-systems were stable under reaction conditions, finding applications in hydrogenation and hydrodehalogenation processes, as a result of their surface-like behaviour. Their reactivity was improved in relation to that involving PdNPs stabilised by phosphines and also by Pd/C as a heterogenous catalyst, mainly in terms of recyclability. In particular, the colloidal palladium catalyst stabilised by quinidine was highly efficient to promote the hydrodechlorination of aromatic compounds under low dihydrogen pressure. These original catalysts found applications in the synthesis of secondary and tertiary amines including N-substituted anilines, by means of one-pot tandem Pd-catalysed methodologies under smooth conditions. In all of these processes, glycerol performed a crucial function as a liquid support for the immobilisation of nanoparticle-based catalysts, allowing both the stabilisation of the nano-catalysts and easy recycling of the catalytic phase.

Polythiolated complexes. Part 2(1). Palladium(II) and platinum(II) derivatives of polyfluorophenylthiolates

SummaryThe 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.

Novel chiral dithioethers derived from l-tartaric acid

Abstract The synthesis of a new family of systematically modified chiral dithioethers to be used as ligands is described. Phenylthioether derivative 5 and fluorine-containing dithioether ligands 6 – 8 and 13 – 15 were prepared by direct reaction of phenylthiol and o -, m - or p -fluorophenylthiol with two different ditriflate derivatives based on the l -tartaric skeleton. The chiral ditriflate 12 containing a dioxolane moiety was reacted with ethane- and propanedithiol, producing cyclic dithioethers 16 and 17 , respectively, in good yields (≈50%). The analogous ditriflate 4 with benzyl ether protecting groups, having a skeleton without restricted rotation, gave the thiolane 9 as the main product.

Rh nanoparticles from thiolate dimers: selective and reusable hydrogenation catalysts in ionic liquids

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.