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Relevant academic research and scientific papers

Recoverable polystyrene-supported palladium catalyst for construction of all-carbon quaternary stereocenters via asymmetric 1,4-addition of arylboronic acids to cyclic enones

The development of recoverable catalysts based on the combination of synthetically demanding ligands with transition metals attracts a lot of attention, especially from the environmental point of view. In this paper, we describe the preparation of a recoverable polystyrene supported chiral palladium catalyst based on PyOx ligand suitable for asymmetric 1,4-addition of arylboronic acids to cyclic 3-substituted five- and six-membered enones. In the reaction, all?carbon quaternary stereocenters are formed with a high level of enantioselectivity (up to 91% ee) and conversion (up to 99%). The catalyst was used in 6 cycles with no loss of enantioselectivity and only a small decrease in conversion. A solution of the problems associated with the transition from homogeneous to heterogeneous catalytic systems is discussed.

Pyridine-Hydrazone Ligands in Asymmetric Palladium-Catalyzed 1,4- and 1,6-Additions of Arylboronic Acids to Cyclic (Di)enones

Catalysts generated by combinations of Pd(TFA)2 and enantiomerically pure pyridine-hydrazone ligands have been applied to the 1,4-addition of arylboronic acids to β-substituted cyclic enones, building all-carbon quaternary stereocenters in high

Metal alkylidene complexes comprising an unsymmetrical unsaturated NHC ligand

The present invention relates to metal alkylidene complex comprising a 1-aryl-3-alkyl-imidazol-2-ylidene ligand, wherein the aryl group of said 1-aryl-3-alkyl-imidazol-2-ylidene ligand is an aromatic and wherein the alkyl group of said 1-aryl-3-alkyl-imidazol-2-ylidene ligand is selected from the group consisting of a cyclic secondary aliphatic alkyl and a C2-C10-heteroalkyl, and wherein said metal is selected from the group consisting of Pd, Ag, Au, Ir, Rh, Cu, Ni, Mg, Zn, Ti, Pt and Fe, with the provision that the aryl group of said 1-aryl-3-alkyl-imidazol-2-ylidene ligand is other than an ortho-hydroxyaryl group when said metal is Pd and when said cyclic secondary aliphatic alkyl is cyclohexyl.

Multicomponent synthesis of chiral bidentate unsymmetrical unsaturated N-heterocyclic carbenes: Copper-catalyzed asymmetric C-C bond formation

A multicomponent strategy was applied to the synthesis of chiral bidentate unsaturated hydroxyalkyl- and carboxyalkyl-N-heterocyclic carbene (NHC) precursors. The newly developed low-cost chiral ligands derived from amino alcohols and amino acids were eva

Synthesis of diverse β-quaternary ketones via palladium-catalyzed asymmetric conjugate addition of arylboronic acids to cyclic enones

Abstract The development and optimization of a palladium-catalyzed asymmetric conjugate addition of arylboronic acids to cyclic enone conjugate acceptors is described. These reactions employ air-stable and readily-available reagents in an operationally si

Palladium-Catalyzed Conjugate Addition of Arylboronic Acids to β,β-Disubstituted Enones in Aqueous Media: Formation of Bis-benzylic and ortho-Substituted Benzylic Quaternary Centers

Palladium-catalyzed conjugate addition of arylboronic acids to β,β-disubstituted enones in aqueous media is reported. Additions of a wide range of arylboronic acids to β,β-disubstituted enones occur to form ketone products bearing benzylic all-carbon quaternary centers. These reactions are promoted by a simple catalyst prepared from palladium trifluoracetate and 2,2′-bipyridine. The use of aqueous sodium trifluoracetate as the reaction medium significantly enhances reactivity and enables the formation of challenging bis-benzylic and ortho-substituted benzylic all-carbon quaternary centers.

Mechanism and enantioselectivity in palladium-catalyzed conjugate addition of arylboronic acids to β-substituted cyclic enones: Insights from computation and experiment

Enantioselective conjugate additions of arylboronic acids to β-substituted cyclic enones have been previously reported from our laboratories. Air- and moisture-tolerant conditions were achieved with a catalyst derived in situ from palladium(II) trifluoroacetate and the chiral ligand (S)-t-BuPyOx. We now report a combined experimental and computational investigation on the mechanism, the nature of the active catalyst, the origins of the enantioselectivity, and the stereoelectronic effects of the ligand and the substrates of this transformation. Enantioselectivity is controlled primarily by steric repulsions between the t-Bu group of the chiral ligand and the α-methylene hydrogens of the enone substrate in the enantiodetermining carbopalladation step. Computations indicate that the reaction occurs via formation of a cationic arylpalladium(II) species, and subsequent carbopalladation of the enone olefin forms the key carbon-carbon bond. Studies of nonlinear effects and stoichiometric and catalytic reactions of isolated (PyOx)Pd(Ph)I complexes show that a monomeric arylpalladium-ligand complex is the active species in the selectivity-determining step. The addition of water and ammonium hexafluorophosphate synergistically increases the rate of the reaction, corroborating the hypothesis that a cationic palladium species is involved in the reaction pathway. These additives also allow the reaction to be performed at 40 C and facilitate an expanded substrate scope.

Palladium-catalyzed asymmetric conjugate addition of arylboronic acids to heterocyclic acceptors

Flava Flavanone: Asymmetric conjugate additions to chromones and 4-quinolones are reported utilizing a single catalyst system formed in situ from Pd(OCOCF3)2 and (S)-tBuPyOX. Notably, these reactions are performed in wet solvent under ambient atmosphere, and employ readily available arylboronic acids as the nucleophile, thus providing ready access to these asymmetric heterocycles (see scheme).

Palladium-catalyzed asymmetric quaternary stereocenter formation

An efficient palladium catalyst is presented for the formation of benzylic quaternary stereocenters by conjugate addition of arylboronic acids to a variety of β,β-disubstituted carbocyclic, heterocyclic, and acyclic enones. The catalyst is readily prepared from PdCl2, PhBOX, and AgSbF 6, and provides products in up to 99 % enantiomeric excess, with good yields. Based on this strategy, (-)-α-cuparenone has been prepared in only two steps. Copyright