Enantioselective transannular reactions by palladium-catalysed conjugate addition of aryl boronic acids

A palladium-catalyzed asymmeric conjugate addition of aryl boronic acids to medium-sized cycloalkenones followed by intramolecular aldol trapping is reported. The use of in situ formed [Pd/(QuinoxP*)] as the catalyst enables the synthesis of arylbicyclic scaffolds in good yields and with excellent stereocontrol (up to 7 : 1 dr, up to 99% ee). The reaction is applicable to a range of medium size ketoenone substrates and funcionalized aryl boronic acids, including heterocyclic compounds.

The first N-ligand assisted Pd catalyzed asymmetric synthesis of 3-arylsuccinimides as novel antifungal leads

The first palladium/chiral nitrogenous ligand-catalyzed enantioselective addition of aryl boronic acids to various maleimides was reported.

Importance of Favourable Non-Covalent Contacts in the Stereoselective Synthesis of Tetrasubstituted Chromanone

Automated transiton state (TS) structure computations for a recently reported Pd-catalysed conjugated addition of arylboronic acids to 2-substituted chromones (Chem Sci, 2020, 11, 4602) reveal unexpected conformations of the key...

Stereoselectivity Predictions for the Pd-Catalyzed 1,4-Conjugate Addition Using Quantum-Guided Molecular Mechanics

The conjugate addition of aryl boronic acids to enones is a powerful synthetic tool to introduce quaternary chiral centers, but the experimentally observed stereoselectivities vary widely, and the identification of suitable substrate-ligand combinations requires significant effort. We describe the development and application of a transition-state force field (TSFF) by the quantum-guided molecular mechanics (Q2MM) method that is validated using an automated screen of 9 ligands, 38 aryl boronic acids, and 22 enones, leading to a MUE of 1.8 kJ/mol and a R² value of 0.877 over 82 examples. A detailed error analysis identified the structural origin for the deviations in the small group of outliers. The TSFF was then used to predict the stereoselectivity for 27 ligands and 59 enones. The vast majority of the virtual screening results are in line with the expected results. Selected results for 6-substituted pyrox ligands, which were not part of the training set, were followed up by density functional theory and experimental studies.

Palladium-catalyzed dehydrogenative coupling of cyclic enones with thiophenes: a rapid access to β-heteroarylated cyclic enones

A dehydrogenative coupling reaction of cyclic enones with thiophenes through palladium-catalyzed C–H functionalization is reported herein. Further molecular transformations of the coupling products to synthetically useful meta-heteroarylated phenols are also demonstrated.

Pd(ii)-Catalyzed asymmetric 1,6-conjugate addition of arylboronic acids to Meldrum's acid-derived dienes

A Pd(ii)-catalyzed asymmetric 1,6-conjugate addition of arylboronic acids to Meldrum's acid-derived dienes has been realized using a new pyridine–oxazoline ligand bearing an extended arm.

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.

Ligand- and base-free Pd(II)-catalyzed controlled switching between oxidative Heck and conjugate addition reactions

A simple change of solvent allows controlled and efficient switching between oxidative Heck and conjugate addition reactions on cyclic Michael acceptor substrates, catalyzed by a cationic Pd(II) catalyst system. Both reactions are ligand- and base-free and tolerant of air and moisture, and the controlled switching sheds light on some of the factors which favor one reaction over the other.

Mild and ligand-free Pd(II)-catalyzed conjugate additions to hindered γ-substituted cyclohexenones

Ligand-free cationic Pd(II) catalyst with NaNO3 as an additive is a highly active catalytic system for conjugate additions to sterically hindered γ-substituted cyclohexenones. More challenging γγ- and βγ-substrates also react well to produce products with quaternary centers in good dr. The conjugate additions occur in a diastereoselective fashion under mild, practical and air-stable conditions, using readily available commercial reagents.

Rh-catalyzed asymmetric 1,4-addition of arylboronic acids to α,β-unsaturated ketones with DIFLUORPHOS and SYNPHOS analogues

Applications of electron-deficient DIFLUORPHOS and SYNPHOS analogues in the rhodium-catalyzed asymmetric conjugate addition of boronic acids to α,β-unsaturated ketones afford the 1,4-addition adducts in yields up to 92% and with 99% ee. Particularly, a Rh-catalyzed asymmetric 1,4-addition of arylboronic acids to nonsubstituted maleimide substrates using the (R)-3,5-diCF(3)-SYNPHOS ligand is also reported. This protocol provides access to various enantioenriched 3-substituted succinimide units of biological interest, in high yields and good to excellent ee up to 93%, which could be upgraded up to 99% ee, after a single crystallization.

1,4-Additions of arylboron, -silicon, and -bismuth compounds to α,β-unsaturated carbonyl compounds catalyzed by dicationic palladium(II) complexes

An enantioselective synthesis of cyclic and acyclic β-aryl ketone and aldehydes via Pd(II)-catalyzed 1,4-addition of Ar-m [m = B(OH)2, BF3K, Si(OMe)3, SiF3, BiAr2] to α,β-unsaturated ketones or aldehydes is described. The catalytic cycle involves transmetallation between Ar-m and Pd complexes as a key process, the mechanism of which is discussed on the basis of characterization of the transmetallation intermediate and electronic effect of the substituents. The enantioselection mechanism and efficiency of a chiraphos ligand for structurally planar α,β-unsaturated ketones are discussed on the basis of the X-ray structure of the catalyst and results of density functional theory (DFT) computational studies on the model of coordination of the substrates to the phenylpalladium(II)/(S,S)-chiraphos intermediate.