Versatile Syntheses of Optically Pure PCE Pincer Ligands: Facile Modifications of the Pendant Arms and Ligand Backbones

A series of chiral C-stereogenic PCP and PCN ligand precursors were prepared in situ from inexpensive achiral starting materials via a simple catalytic asymmetric P–H addition reaction in good overall yields. This facile catalytic method of preparing the ligand backbones renders easy and economical modifications of the electronically crucial <i>para</i>-substituent, chiral functionalities, and donor atoms for different transition metal ions. A one-pot synthetic procedure was used efficiently to prepare the corresponding optically pure pincer complexes. All the new complexes were characterized by NMR and mass spectroscopy. The molecular structures of several selected complexes have also been elucidated by X-ray crystallography. Preliminary studies indicated that minor structural changes on these novel pincer complexes affect their chemical properties significantly when they were applied as catalysts for the reaction between diphenylphosphine and chalcone

Asymmetric 1,4-Conjugate Addition of Diarylphosphines to α,β,γ,δ-Unsaturated Ketones Catalyzed by Transition-Metal Pincer Complexes

An enantioselective asymmetric 1,4-addition of diarylphosphines to linear α,β,γ,δ-unsaturated dienones was developed. A series of chiral PCP- and PCN-transition-metal (Pd, Pt and Ni) pincers, themselves prepared catalytically via asymmetric hydrophosphination, were sequentially screened to reveal the roles of backbone architecture and metal ion in catalyst design. The selected ester-functionalized PCP-palladium pincer afforded the chiral 1,4-phosphine adducts in excellent yields with up to >99% <i>ee</i>. The same catalyst when utilized for the hydrophosphination of an α,β,γ,δ-unsaturated malonate ester also revealed the critical role played by the ester functionality on the ligand backbone in dictating the enantioselectivity of the 1,6-adduct

Asymmetric 1,4-Conjugate Addition of Diarylphosphines to α,β,γ,δ-Unsaturated Ketones Catalyzed by Transition-Metal Pincer Complexes

An enantioselective asymmetric 1,4-addition of diarylphosphines to linear α,β,γ,δ-unsaturated dienones was developed. A series of chiral PCP- and PCN-transition-metal (Pd, Pt and Ni) pincers, themselves prepared catalytically via asymmetric hydrophosphination, were sequentially screened to reveal the roles of backbone architecture and metal ion in catalyst design. The selected ester-functionalized PCP-palladium pincer afforded the chiral 1,4-phosphine adducts in excellent yields with up to >99% <i>ee</i>. The same catalyst when utilized for the hydrophosphination of an α,β,γ,δ-unsaturated malonate ester also revealed the critical role played by the ester functionality on the ligand backbone in dictating the enantioselectivity of the 1,6-adduct

Synthesis of Stereoprojecting, Chiral N‑C(sp³)‑E Type Pincer Complexes

A synthetic strategy to generate chiral N-C­(sp³)-E (E = S, O) pincer complexes incorporating enhanced stereoprojecting groups at the N-arm site has been established. The synthesis of the tridentate pincer ligand was carried out via palladacycle-catalyzed asymmetric hydrophosphination of N-chelating enones. The chelation properties of the substrates were initially demonstrated on C­(sp²)-N type palladacycles. The extended substrate scope allows versatile structural modifications on the ligand backbone. Subsequent cyclometalation provided N-C­(sp³)-E complexes in a diastereoselective reaction