Unusual deactivation in the asymmetric hydrogenation of itaconic acid

Activation, Deactivation and Reversibility Phenomena in Homogeneous Catalysis: A Showcase based on the Chemistry of Rhodium/Phosphine Catalysts

In the present work, the rich chemistry of rhodium/phosphine complexes, which are applied as homogeneous catalysts to promote a wide range of chemical transformations, has been used to showcase how the in situ generation of precatalysts, the conversion of precatalysts into the actually active species, as well as the reaction of the catalyst itself with other components in the reaction medium (substrates, solvents, additives) can lead to a number of deactivation phenomena and thus impact the efficiency of a catalytic process. Such phenomena may go unnoticed or may be overlooked, thus preventing the full understanding of the catalytic process which is a prerequisite for its optimization. Based on recent findings both from others and the authors’ laboratory concerning the chemistry of rhodium/diphosphine complexes, some guidelines are provided for the optimal generation of the catalytic active species from a suitable rhodium precursor and the diphosphine of interest; for the choice of the best solvent to prevent aggregation of coordinatively unsaturated metal fragments and sequestration of the active metal through too strong metal–solvent interactions; for preventing catalyst poisoning due to irreversible reaction with the product of the catalytic process or impurities present in the substrate.

Synthetic Designs and Structural Investigations of Biomimetic Ni-Fe Thiolates

Described are the syntheses of several Ni(μ-SR)₂Fe complexes, including hydride derivatives, in a search for improved models for the active site of [NiFe]-hydrogenases. The nickel(II) precursors include (i) nickel with tripodal ligands: Ni(PS₃)^- and Ni(NS₃)^- (PS₃^3- = tris(phenyl-2-thiolato)phosphine, NS₃^3- = tris(benzyl-2-thiolato)amine), (ii) traditional diphosphine-dithiolates, including chiral diphosphine R,R-DIPAMP, (iii) cationic Ni(phosphine-imine/amine) complexes, and (iv) organonickel precursors Ni( o-tolyl)Cl(tmeda) and Ni(C₆F₅)₂. The following new nickel precursor complexes were characterized: PPh₄[Ni(NS₃)] and the dimeric imino/amino-phosphine complexes [NiCl₂(PCH═N^An)]₂ and [NiCl₂(PCH₂NH^An)]₂ (P = Ph₂PC₆H₄-2-). The iron(II) reagents include [CpFe(CO)₂(thf)]BF₄, [Cp*Fe(CO)(MeCN)₂]BF₄, FeI₂(CO)₄, FeCl₂(diphos)(CO)₂, and Fe(pdt)(CO)₂(diphos) (diphos = chelating diphosphines). Reactions of the nickel and iron complexes gave the following new Ni-Fe compounds: Cp*Fe(CO)Ni(NS₃), [Cp(CO)Fe(μ-pdt)Ni(dppbz)]BF₄, [( R,R-DIPAMP)Ni(μ-pdt)(H)Fe(CO)₃]BAr^F₄, [(PCH═N^An)Ni(μ-pdt)(Cl)Fe(dppbz)(CO)]BF₄, [(PCH₂NH^An)Ni(μ-pdt)(Cl)Fe(dppbz)(CO)]BF₄, [(PCH═N^An)Ni(μ-pdt)(H)Fe(dppbz)(CO)]BF₄, [(dppv)(CO)Fe(μ-pdt)]₂Ni, {H[(dppv)(CO)Fe(μ-pdt)]₂Ni]}BF₄, and (C₆F₅)₂Ni(μ-pdt)Fe(CO)₂(dppv) (DIPAMP = (CH₂P(C₆H₄-2-OMe)₂)₂; BAr^F₄^- = [B(C₆H₃-3,5-(CF₃)₂]₄^-)) Within the context of Ni-(SR)₂-Fe complexes, these new complexes feature new microenvironments for the nickel center: tetrahedral Ni, chirality, imine, and amine coligands, and Ni-C bonds. In the case of {H[(dppv)(CO)Fe(μ-pdt)]₂Ni}⁺, four low-energy isomers are separated by ≤3 kcal/mol, one of which features a biomimetic HNi(SR)₄ site, as supported by density functional theory calculations.

Preparing acid-resistant Ru-based catalysts by carbothermal reduction for hydrogenation of itaconic acid

Ru-based catalysts with good stability and acid-resistance for hydrogenation of itaconic acid to methylsuccinc acid were prepared via carbothermal reduction, in which CO generated in situ functioned as an efficient reducing species.

Rhodium diphosphine complexes: a case study for catalyst activation and deactivation

The present work provides an overview of possible activation and deactivation phenomena in homogeneous catalytic processes promoted by different types of rhodium complexes containing diphosphine ligands.

Homo-Roche ester derivatives by asymmetric hydrogenation and organocatalysis

Asymmetric hydrogenation routes to homologues of The Roche ester tend to be restricted to hydrogenations of itaconic acid derivatives, that is, substrates that contain a relatively unhindered, 1,1-disubstituted alkene. This is because in hydrogenations mediated by RhP2 complexes, the typical catalysts, it is difficult to obtain high conversions using the alternative substrate for the same product, the isomeric trisubstituted alkenes (D in the text). However, chemoselective modification of the identical functional groups in itaconic acid derivatives are difficult; hence, it would be favorable to use the trisubstituted alkene. Trisubstituted alkene substrates can be hydrogenated with high conversions using chiral analogs of Crabtree's catalyst of the type IrN(carbene). This paper demonstrates that such reactions are scalable (tens of grams) and can be manipulated to give optically pure homo-Roche ester chirons. Organocatalytic fluorination, chlorination, and amination of the homo-Roche building blocks was performed to demonstrate that they could easily be transformed into functionalized materials with two chiral centers and α,ω-groups that provide extensive scope for modifications. A synthesis of (S,S)- and (R,S)-γ-hydroxyvaline was performed to illustrate one application of the amination product.