Syntheses and characterizations of methylpalladium complexes bearing a biphenyl-based bulky phosphine ligand: Weak interactions suggested by NBO and QTAIM analyses

Palladium-Catalyzed Direct α-C(sp3) Heteroarylation of Ketones under Microwave Irradiation

Heteroaryl compounds are valuable building blocks in medicinal chemistry and chemical industry. A palladium-catalyzed direct α-C(sp3) heteroarylation of ketones under microwave irradiation is developed and reported in this study. Under optimized conditions, twenty-eight (28) heteroarylated ketones were prepared in this study to demonstrate the substrate scope of this reaction. The ground-state optimized structure of Pd(0) active catalyst with 2-dicyclohexylphosphino-2',4',6'-triisopropylbiphenyl (XPhos) in toluene, and the products of its reaction with 3-bromopyridine and acetophenone were studied using all-atom density functional theory. This study provided insightful information for palladium catalytic system design to generate heteroaryl compounds.

Group 9 and 10 Metal Complexes of an Ylide-Substituted Phosphine: Coordination versus Cyclometalation and Oxidative Addition

Ylide-substituted phosphines (YPhos) have been shown to be excellent ligands for several transition metal catalyzed reactions. Investigations of the coordination behavior of the YPhos ligand Y_SPPh₂ (1) [with Y_S = (Ph₃P)(SO₂Tol)C] toward group 9 and 10 metals revealed a surprisingly diverse coordination chemistry of the ligand. With Ni(CO)₄, the formation of a di- as well as tricarbonyl complex is observed depending on the reaction conditions. In [(κP,η²-benzene-1)Ni(CO)₂] the phosphine ligand also coordinates via a phosphonium bound phenyl group to the metal leading to a unique nickel η²-arene interaction, which can be viewed as an intermediate state toward P–C bond activation. Full cleavage of the P–C bond takes place with [Rh(COD)Cl]₂ leading to a complex salt with [(κP,κO-1)Rh(COD)]⁺ as cation and a dirhodium trichloride complex anion. Here, Y_SPPh₂ underwent P–C bond cleavage to thus act as an anionic diphosphine ligand. In contrast, in [(κP,κO-1)Rh(COD)]⁺ as well as [(κP,κO-1)Rh(CO)Cl], formed from the reaction of 1 with [Rh(CO)₂Cl]_2, the YPhos ligand acts as bidentate ligand complexing the metal via the phosphine and sulfonyl moiety with an intact PPh₃ unit. A further type of coordination is observed with [Ir(COD)Cl]₂. Here, phosphine coordination is accompanied by C–H activation at one of the phosphonium bound phenyl groups leading to a cyclometalated complex.

Group 9 and 10 metal complexes with the ylide-substituted phosphine Y_SPPh₂ (Y_S = PPh₃=C−SO₂Tol) were prepared. The ligand exhibits a surprisingly versatile coordination behavior: simple coordinations as mono- or bidentate ligand as well as P−C and C−H bond activations are observed depending on the used metal precursor.

Intramolecular Arene C-H to C-P Functionalization Mediated by Ni(II) and Pd(II)

A trisphosphine ligand with a triarylbenzene backbone was employed to support mono-nickel(II) and palladium(II) complexes. Two phosphine arms coordinated to the metal center, while the third phosphine was found to form a C-P bond with dearomatization of the central arene. Deprotonation effected the rearomatization of the central ring and metal reduction from M(II) to M(0). The overall conversion corresponds to a functionalization of an unactivated arene C-H bond to a C-P bond. This transformation represents a rare type of mechanism of C-H functionalization, facilitated by the interactions of the group 10 metal with the arene π-system. This conversion is reminiscent of and expands the scope of recently reported intramolecular rearrangements of biaryl phosphine ligands common in group 10 catalysis.

Stereospecific cross-coupling of secondary organotrifluoroborates: potassium 1-(benzyloxy)alkyltrifluoroborates

Potassium 1-(alkoxy/acyloxy)alkyltrifluoroborates have been synthesized through a copper-catalyzed diboration of aldehydes and subsequent conversion of the resulting potassium 1-(hydroxy)alkyltrifluoroborates. The palladium-catalyzed Suzuki-Miyaura reaction employing the potassium 1-(benzyloxy)alkyltrifluoroborates with aryl and heteroaryl chlorides provides access to protected secondary alcohols in high yields. The β-hydride elimination pathway is avoided through use of the benzyl protecting group, which is proposed to stabilize the diorganopalladium intermediate by coordination of the arene to the metal center. This cross-coupling is stereospecific with complete retention of stereochemistry.

Biaryl phosphane ligands in palladium-catalyzed amination

Palladium-catalyzed amination reactions of aryl halides have undergone rapid development in the last 12 years, largely driven by the implementation of new classes of ligands. Biaryl phosphanes have proven to provide especially active catalysts in this context. This Review discusses the application of these catalysts in C-N cross-coupling reactions in the synthesis of heterocycles and pharmaceuticals, in materials science, and in natural product synthesis.