Catalysis by Palladium Pincer Complexes

Catalytic enantioselective allylation of carbonyl compounds and imines.

Chiral anion-mediated asymmetric ion pairing chemistry

Pincer ligands have become ubiquitous in organometallic chemistry and homogeneous catalysis. Recently, new varieties of pincer ligands with non-symmetrical backbones and/or ligating groups have been reported and their application in transition metal complexes has been exploited in a variety of catalytic transformations. This non-symmetric approach vastly increases the structural and electronic diversity of this class of ligand. This approach has proven beneficial in a variety of ways, such as the use of a single weakly coordinating moiety, which can dissociate and thereby create a vacant coordination site to increase the catalyst activity. Additionally, this provides further access to chiral ligands and complexes for asymmetric induction. This perspective highlights recent, important examples of non-symmetric pincer ligands, which feature aryl or pyridine backbones, and the synthesis and use of subsequent complexes in catalytic transformations, and discusses the future potential of this type of ligand system.

Non-symmetric pincer ligands: complexes and applications in catalysis

Aryl-based pincer metal complexes with anionic terdentate ligands have been widely applied in organic synthesis, organometallic catalysis and other related areas. Synthetically, the most simple and convenient method for the construction of these complexes is the direct metal-induced Caryl–H bond activation, which can be fulfilled by choosing the appropriate functional donor groups in the two side arms of the aryl-based pincer preligands. In this perspective, we wish to summarize some results achieved by our group in this context. Successful examples include symmetrical chiral bis(imidazoline) NCN pincer complexes with Ni(II), Pd(II) and Pt(II), bis(phosphinite) and bis(phosphoramidite) PCP pincer Pd(II) complexes, unsymmetrical (pyrazolyl)phosphinite, (amino)phosphinite and (imino)phosphinite PCN pincer Pd(II) complexes, chiral (imidazolinyl)phosphinite and (imidazolinyl)phosphoramidite PCN pincer complexes with Ni(II) and Pd(II) as well as unsymmetrical (oxazolinyl)amine and (oxazolinyl)pyrazole NCN′ pincer Pd(II) complexes. Among them, the P-donor containing complexes are efficiently synthesized by the “one-pot phosphorylation/metalation” method. The obtained symmetrical and unsymmetrical pincer complexes have been used as catalysts in Suzuki–Miyaura reaction (Pd), asymmetric Friedel–Crafts alkylation of indole with trans-β-nitrostyrene (Pt) as well as in asymmetric allylation of aldehyde and sulfonimine (Pd). In the Suzuki couplings conducted at 40–50 °C, some unsymmetrical Pd complexes exhibit much higher activity than the related symmetrical ones which can be attributed to their faster release of active Pd(0) species resulting from the hemilabile coordination of the ligands. Literature results on the synthesis of some related pincer complexes as well as their activities in the above catalytic reactions are also presented.

Symmetrical and unsymmetrical pincer complexes with group 10 metals: synthesis via aryl C-H activation and some catalytic applications.

Feel the pinch! Planar-chiral, cationic, ruthenium-palladium complexes based on eta(6),eta(1)-coordinated ECE' pincer ligands are synthesized as racemic mixtures by reacting ECE'-palladium complexes and [Ru(C(5)R(5))(MeCN)(3)](+) arenophiles (R=H or Me). Chiral resolution of the cationic complexes was achieved by using the chiral counterion [Delta-TRISPHAT](-), and solving the X-ray crystal structure of one diastereoisomer (shown here).

Synthesis and resolution of Planar-Chiral Ruthenium-Palladium complexes with ECE pincer ligands

Chiral Amino Alcohol Derived Bis-phosphoramidite Pincer Palladium Complexes and Their Applications in Asymmetric Allylation of Aldimines

Novel PCN- and PCS-pincer palladium complexes 2-(dimethylamino)methyl-5-methoxy-6- (diphenylphosphinoxy)phenylpalladium(II) bromide (1), 2-(phenylimino)-5-methoxy-6-(diphenylphos- ACHTUNGTRENUNGphinoxy)phenylpalladium(II) bromide (2), 2-(phenylthiomethyl)- 5-methoxy-6-(diphenylphosphinoxy)phenylpalladium( II) bromide (3), 2-(phenylthio)methyl- 5-methoxy-6-(diphenylphosphinoxy)phenylpalladi- ACHTUNGTRENUNGum(II) chloride (4) have been synthesized (55–95% yield) by using a flexible and straightforward synthetic route starting from isovanillin as the common precursor. The structures of complexes 1, 2 and, 4 in the solid state were determined using X-ray diffraction and showed a typical pincer-type geometry. The catalytic activities of 1–3 in the homoallylation reactions of aldehydes and allylACHTUNGTRENUNG(tributyl)stannane as well as of their corresponding cationic complexes 1a–3a in the tandem reaction of aldehydes or sulfonimines with allyl chlorides and hexamethyldistannane were investigated. It was found that the catalytic activities are very dependent on the combination of the Edonor moieties in the pincer ligand. Generally, PCSpincer complex 3 and its cationic complex 3a outperform PCN-pincer complexes 1 and 2 as well as their cationic complexes 1a and 2a in both the homoallylation and tandem reaction. PCS-pincer palladium complexes 3 and 3a seem to benefit in a positive sense from the combination and cooperativity of a paccepting phosphorus donor and a s-donating sulfur donor.

Jie Li

Papers

Synthesis and resolution of Planar-Chiral Ruthenium-Palladium complexes with ECE pincer ligands

Chiral Amino Alcohol Derived Bis-phosphoramidite Pincer Palladium Complexes and Their Applications in Asymmetric Allylation of Aldimines

PCN- and PCS-Pincer Palladium Complexes as Tandem Catalysts in Homoallylation Reactions

Novel phosphite palladium complexes and their application in C–P cross-coupling reactions

P′CP′-Pincer palladium complex-catalyzed allylation of N,N-dimethylsulfamoyl-protected aldimines