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

Novel P-stereogenic bis-phosphoramidite pincer palladium complexes 1 and 2 derived from (S)-(−)-α,α-diphenyl-2-pyrrolidinemethanol and (S)-(+)-indolinemethanol, respectively, were synthesized in reasonable yields (i.e., 55−62%) by using a flexible, modular synthetic approach and were characterized by X-ray crystal structure determination. Reacting (S)-(+)-indolinemethanol with PCl3 leads to the formation of a novel P-chiral building block, which exhibits good thermal stability of its stereochemical features. This enabled the design and development of new P-chiral bis-phosphoramidite pincer arene ligands and their corresponding metal complexes. The molecular structure of the new indolinemethanol-derived phosphoramidite pincer metal complex screens quadrants I and III, which is in contrast to the previously reported l-proline-derived P-chiral ligands that screen quadrants II and IV. The bis-phosphoramidite pincer palladium complexes 1 and 2 are active catalysts for asymmetric homoallylation of sulfonimines, where low (ee 33%) or no enantioselectivity was observed for reactions catalyzed by 2 and 1, respectively. Preliminary catalytic results revealed that enantiomeric excess values varied by using differently functionalized sulfonimines, suggesting that both electronic properties and steric congestion of sulfonimines affect the transition state of the electrophilic attack of the 1η-allyl Pd intermediate in the allylation.