Diphenylprolinol-Derived Symmetrical and Unsymmetrical Chiral Pincer Palladium(II) and Nickel(II) Complexes: Synthesis via One-Pot Phosphorylation/Metalation Reaction and C−H Activation

The symmetrical P-stereogenic bis-phosphoramidite PCP pincer Pd(II) complexes 1 and 2 were easily prepared by a “four-component, one-pot phosphorylation/palladation” procedure via C−H bond activation of the related ligands. In this synthetic procedure, (S)-diphenyl(pyrrolidin-2-yl)methanol was first phosphonated with PCl3 to afford the expected phosphorochloridate adduct, which then reacted in situ with resorcinol or disubstituted resorcinol, followed by treatment with PdCl2. The first examples of the unsymmetrical P-stereogenic phosphoramidite- and imidazoline-containing PCN pincer Pd(II) complex 3 and Ni(II) complex 4 could be obtained in a similar manner by using a chiral imidazoline-containing m-phenol derivative instead of resorcinol as a backbone. 31P NMR of the complexes confirmed the formation of a single diastereoisomer concerning the P-stereogenic center, and its absolute configuration was determined by an X-ray crystal structure determination. Preliminary investigations on the use of these complexes in the asymmetric allylation indicated that the unsymmetrical pincer Pd complex 3 exhibited higher catalytic activity than the related symmetrical ones. On the other hand, the more bulky Pd complex 2 gave better enantioselectivity, especially in the allylation of 4-nitrobenzenesulfonimine (69% ee).