ABSTRACT
A small library of highly tunable chiral Clickphine P,N-ligands has been prepared in an enantioselective fashion by Cu(I)-catalyzed asymmetric propargylic amination using a single chiral complex and a subsequent in situ cycloaddition click reaction. The scope of the propargylic amination to yield optically active triazolyl amines is described. The amines are transformed in a one-pot procedure to the corresponding Ir-Clickphine complexes, which serve as catalysts for the asymmetric hydrogenation of di-, tri-, and tetrasubstituted unfunctionalized alkenes. Enantioselectivities of up to 90% ee were obtained in these hydrogenations, which are among the best reported in the case of the tetrasubstituted substrate 2-(4'-methoxyphenyl)-3-methylbut-2-ene (9) (87% ee). This is a demonstration of the effective use of the chiral pool, as from one chiral catalyst a library of chiral Ir complexes has been synthesized that can hydrogenate various alkenes with high selectivity.
Subject(s)
Butanes/chemistry , Coordination Complexes/chemical synthesis , Iridium/chemistry , Amination , Catalysis , Coordination Complexes/chemistry , Copper/chemistry , Hydrogenation , Ligands , Molecular Structure , StereoisomerismABSTRACT
In this perspective the development of chiral phosphorus ligands for asymmetric catalysis is discussed, with a special focus on hybrid bidentate phosphorus ligands, in particular phosphine-phosphoramidites. An attempt is made to compare privileged ligand and combinatorial approaches to ligand development--for which the class of phosphine-phosphoramidite ligands is well suited--highlighting differences, similarities and their complementary use.
Subject(s)
Phosphorus/chemistry , Catalysis , Combinatorial Chemistry Techniques , Hydrogenation , Ligands , Molecular Structure , Nitrogen/chemistry , StereoisomerismABSTRACT
The high-yielding synthesis and detailed characterization of two well-defined, linkage isomeric tripodal, tetradentate all-phosphorus ligands 1-3 is described. Coordination to Cu(I) resulted in formation of complexes 4-6, for which the molecular structures indicate overall tridentate coordination to the copper atom in the solid state, with one dangling peripheral phosphine. The solution studies suggest fast exchange between the three phosphine side-arms. For these new Cu(I) complexes, preliminary catalytic activity in the cyclopropanation of styrene with ethyldiazoacetate (EDA) is disclosed. The anticipated well-defined tetradentate coordination in a C(3)-symmetric fashion was achieved with Rh(I) and Ir(I), leading to the overall five-coordinated complexes 7-12. Complex 11 has the norbornadiene (nbd) ligand coordinated in an unprecedented monodentate 2,3-eta(2) mode to Rh. Furthermore, unexpected but very interesting redox-chemistry and reactivity was displayed by the Rh(Cl)-complexes 7 and 8. Oxidation resulted in the formation of stable Rh(II) metalloradicals [7]PF(6) and [8]PF(6) that were characterized by X-ray crystallography, magnetic susceptibility measurements, cyclic voltammetry, and electron paramagnetic resonance (EPR) spectroscopy. Subsequent redox-reactivity of these metalloradicals toward molecular hydrogen is described, resulting in the formation of Rh(III) hydride compounds.
Subject(s)
Coordination Complexes/chemistry , Copper/chemistry , Phosphines/chemistry , Quantum Theory , Rhodium/chemistry , Ligands , Models, Molecular , Molecular StructureABSTRACT
Facile oxidation of a sterically encumbered Rh(I) complex generates a stable Rh(II) metalloradical species; the latter is able to activate H(2) under formation of the corresponding Rh(III) complex.
ABSTRACT
The power of natural selection through survival of the fittest is nature's ultimate tool for the improvement and advancement of species. To apply this concept in catalyst development is attractive and may lead to more rapid discoveries of new catalysts for the synthesis of relevant targets, such as pharmaceuticals. Recent advances in ligand synthesis using combinatorial methods have allowed the generation of a great diversity of catalysts. However, selection methods are few in number. We introduce a new selection method that focuses on the stability of catalytic intermediates measured by mass spectrometry. The stability of the intermediate relates inversely to the reactivity of the catalyst, which forms the basis of a catalyst-screening protocol in which less-abundant species represent the most-active catalysts, 'the survival of the weakest'. We demonstrate this concept in the palladium-catalysed allylic alkylation reaction using diphosphine and IndolPhos ligands and support our results with high-level density functional theory calculations.
Subject(s)
Mass Spectrometry/methods , Acetates/chemistry , Alkylation , Allyl Compounds/chemistry , Catalysis , Ligands , Models, Molecular , Molecular Structure , Palladium/chemistry , Phosphines/chemistryABSTRACT
The mechanism of the IndolPhos-Rh-catalyzed asymmetric hydrogenation of prochiral olefins has been investigated by means of X-ray crystal structure determination, kinetic measurements, high-pressure NMR spectroscopy, and DFT calculations. The mechanistic study indicates that the reaction follows an unsaturate/dihydride mechanism according to Michaelis-Menten kinetics. A large value of K(M) (K(M) = 5.01+/-0.16 M) is obtained, which indicates that the Rh-solvate complex is the catalyst resting state, which has been observed by high-pressure NMR spectroscopy. DFT calculations on the substrate-catalyst complexes, which are undetectable by experimental means, suggest that the major substrate-catalyst complex leads to the product. Such a mechanism is in accordance with previous studies on the mechanism of asymmetric hydrogenation reactions with C(1)-symmetric heteroditopic and monodentate ligands.
Subject(s)
Alkenes/chemistry , Organophosphorus Compounds/chemistry , Rhodium/chemistry , Catalysis , Crystallography, X-Ray , Hydrogenation , Kinetics , Ligands , Magnetic Resonance Spectroscopy , Molecular Structure , Stereoisomerism , TemperatureABSTRACT
The scope of the IndolPhos-Rh-catalyzed asymmetric hydrogenation of enamides, alpha-enol and alpha-enamido ester phosphonates, has been investigated. In addition, Taddol-based IndolPhos ligands are introduced. High activities and good to excellent enantioselectivities up to 99% ee are obtained for a broad range of structurally diverse substrates, giving important chiral products such as alpha, beta(2), and beta(3) amino acid derivatives, arylamines, and amino and hydroxy phosphonates.
Subject(s)
Amides/chemistry , Hydrogen/chemistry , Organophosphonates/chemistry , Rhenium/chemistry , Catalysis , EstersABSTRACT
Confined nanospaces in which reactions can take place, have been created by various approaches such as molecular capsules, zeolites and micelles. In this tutorial review we focus on the application of self-assembled nanocapsules with well-defined cavities as nanoreactors for organic and metal catalysed transformations. The self-assembly of nanocapsules based on noncovalent bonds such as hydrogen bonds and metal-ligand interactions is discussed to introduce the properties of the building blocks and capsules thereof. We will elaborate on the encapsulation effects that can be expected when reactions are carried out in a capsule-protected environment. Subsequently, literature examples will be described in which self-assembled nanocapsules are applied as nanoreactors, for various types of organic and metal catalysed reactions.
Subject(s)
Nanostructures/chemistry , Nanotechnology/methods , Catalysis , Ligands , Models, Molecular , Molecular Structure , Organometallic Compounds/chemical synthesis , Organometallic Compounds/chemistryABSTRACT
Hybrid bidentate phosphine-phosphoramidite ligands are prepared in a modular 2-step sequence and their rhodium complexes display high selectivity in rhodium catalysed hydrogenation and hydroformylation reactions.
