Enhanced Catalyst Activity and Enantioselectivity with Chirality-Switchable Polymer Ligand PQXphos in Pd-Catalyzed Asymmetric Silaborative Cleavage of meso-Methylenecyclopropanes
摘要:
The poly(quinoxaline-2,3-diyl)-based helically chiral phosphine ligands PQXphos exhibited high enantioselectivities up to 97% ee in palladium-catalyzed desymmetrization of meso-1,2-dialkylsubstituted-3-methylenecyclopropanes through silaborative cleavage of the C C bond. The observed enantioselectivities were higher than those obtained with 2-diarylphosphino-1,1'-binaphthyl in our original report. Remarkable rate enhancement was also observed with a series of PQXphos in comparison with the corresponding low-molecular weight ligands.
Nickel-Catalyzed Regio- and Stereoselective Reductive Coupling between Methylenecyclopropanes, Aldehydes, and Triethylborane with Retention of the Cyclopropane Ring
Running rings: The first regio‐ and stereoselective reductive coupling between methylenecyclopropanes, aldehydes, and triethylborane with retention of the cyclopropanering was achieved using a nickel–phosphine catalyst (see scheme). The reductive coupling reaction constructed a stereo‐defined cyclopropanering with formation of a quaternary stereogenic carbon center.
Nickel-catalyzed alkylativecoupling of an enone or enal with methylenecyclopropane in the presence of triethylborane was achieved via stereospecific proximal C–C bond cleavage of methylenecyclopropane. With the use of methylenecyclopropane possessing an acyclic alkyl substituent, this reaction was also accompanied by the β-hydrogen elimination.