An efficient palladium(II)-catalyzed oxidative carbohydroxylation of allene-substituted conjugated dienes in aqueous media has been developed. The reaction is conducted employing p-benzoquinone as the stoichiometric reoxidant for the palladium or under aerobic conditions with essentially the same outcome. It is worth mentioning that this is the first example of a palladium(II)-catalyzed oxidation where a carbon−carbon bond is formed in water and also one of the few cases of nucleophilic attack by water on a (π-allyl)palladium complex.
A photoredox strategy for the synthesis of a wide range of allylic amines and ethers from carboxylic acids and alkynes has been developed. This approach relies on the perturbation of the ground-state electronic properties of terminal alkynes through the formation and photoexcitation of copper acetylide intermediates. This process takes place through cooperative copper and organic photoredox catalysis and can be carried out in a stereodivergent manner. Thus, a systematic multivariate HTE screening spotlighted that a switch in the stereochemical outcome can be provoked by choosing an appropriate combination of ligand and base. The developed methodology has been applied to the stereoselective coupling of primary, secondary, and tertiary alkyl radicals with (hetero)aromatic terminal alkynes. As an additional practicality, similar reaction conditions allowed for the use of aromatic amines as radical precursors in a cross dehydrogenative coupling for the direct vinylation of inactivated C−H bonds.
A modular approach employing indene as common starting material, has enabled the straightforward preparation in three reaction steps of P-thioether ligands for the Pd-catalyzed asymmetric allylic substitution. The analysis of a starting library of Pthioether ligands based on rational design and theoretical calculations has led to the discovery of an optimized anthracenethiol derivative with excellent behavior in the reaction of choice. Improving most approaches reported to date, this ligand presents a broad substrate and nucleophile scope. Excellent enantioselectivities have been achieved for a range of linear and cyclic allylic substrates using a large number of C-, N-and O-nucleophiles (40 compounds in total). The species responsible for the catalytic activity have been further investigated by NMR in order to clearly establish the origin of the enantioselectivity. The resulting products have been derivatized by means of ring-closing metathesis or Pauson-Khand reactions to further prove the synthetic versatility of the methodology for preparing enantiopure complex structures. Scheme 1. Three-step synthesis of phosphite/phosphinite-thioether ligands L1-L7a-g from indene. (i) (R,R)-Mn-salen catalyst, 4-PPNO, aq. NaClO, CH2Cl2; 10 (ii) RSH, NaOH, dioxane/H2O (10:1); 11 (iii) ClP(OR 1 R 2 )2; (OR 1 R 2 )2= a-c, Py, toluene and (iv) ClPR 3 2; R 3 = d-g, NEt3, toluene.All ligands were characterized by 31 P{ 1 H}, 1 H and 13 C{ 1 H} NMR spectroscopy and HRMS. All data were in agreement with assigned structures. 13 See experimental section for purification and characterization details.
A library of modular iridium complexes derived from thioether-phosphite/phosphinite ligands has been evaluated in the asymmetric iridium-catalyzed hydrogenation of minimally functionalized olefins. The modular ligand design has been shown to be crucial in finding highly selective catalysts for each substrate. A DFT study of the transition state responsible for the enantiocontrol in the Ir-catalyzed hydrogenation is also described and used for further optimization of the crucial stereodefining moieties. Excellent enantioselectivities (enantiomeric excess (ee) values up to 99 %) have been obtained for a range of substrates, including E- and Z-trisubstituted and disubstituted olefins, α,β-unsaturated enones, tri- and disubstituted alkenylboronic esters, and olefins with trifluoromethyl substituents.
A new family of modular P,S-ligands has been prepared from enantiopure arylglycidols. These ligands have been iteratively optimized with respect to four different structural parameters for use in Pd-catalyzed allylic substitutions. As a final output, highly active and enantioselective ligands for these synthetically important transformations have been developed, and the factors controlling their catalytic behavior have been rationalized. From a methodological point of view, a convenient procedure for the regioselective ring-opening of cis-glycidic esters with bulky thiols to yield the corresponding beta-alkylthio-alpha-hydroxy carboxylic acids has been developed.
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