Chiral Bis (imidazolidine)-containing NCN Pincer Metal Complexes for Cooperative Asymmetric Catalysis

摘要

A chiral N,N,N-terdentate bis (imidazolidine) pyridine (PyBidine) has showed remarkable ligand acceleration effects in various metal catalyzed asymmetric reactions. For an example, PyBidine-Cu(OTf)(2) catalyzed a highly endo-selective [3+2] cycloaddition of nitroalkenes with imino esters. X-ray crystallographic analysis of the PyBidine-Cu(OTf)(2) complex and DFT calculations suggest that an intermediary generated Cu-enolate of the imino ester reacts with nitroalkenes, which are activated by NH-proton of imidazolidine ligand. Based on the intelligent role of imidazolidine, a new chiral imidazolidine-containing NCN palladium pincer complex (tBu-PhBidine-PdX) was prepared by a ligand introduction route. The neutral tBu-PhBidine-PdCl complex demonstrated significant catalytic activity for the reaction of nitroalkenes with malononitrile to give the products in good yields with high enantioselectivities. On the contrary, the cationic chiral imidazolidine-containing tBu-PhBidine-PdOTf catalyst promoted the nucleophilic addition of unprotected indoles to N-Boc imines. Using sulfinyl amines as the N-Boc imine precursors, the combined use of catalyst with K2CO3 activated the N-H-free indoles to give chiral 3-indolyl methanamines with up to 98% ee. Compared with conventional acid-catalyzed Friedel-Crafts reactions, this reaction proceeding under mildly basic conditions shows advantages for the use of acid sensitive substrates. Furthermore, chiral bis (imidazolidine)-containing PhBidine-Rhodium complexes (PhBidine-RhX2 and tBu-PhBidine-RhX2) were prepared by a C-H insertion method. The tBu-PhBidine-Rh(OAc)(2) smoothly catalyzed an asymmetric Mannich reaction of malononitrile with N-Boc imines to give products, which are useful for the synthesis of chiral alpha-amino acids.