Asymmetric Rhodium(I)-Catalyzed C–C Activations with Zwitterionic Bis-phospholane Ligands

Abstract

The development of unconventional ligand scaffolds is an important aspect to alter reaction pathways of transition-metal-catalyzed reactions. The nature of the counterion of cationic metal complexes plays an important role in the catalyst reactivity. We herein report a chiral anionic bidentate bis-phosphine ligand based on the popular phospholane scaffold. Subsequently, zwitterionic rhodium(I) complexes with no external counterion were synthesized, and their potential was evaluated in asymmetric carbon−carbon bond activation of cyclobutanones. This type of rhodium complex allows for a significantly lower reaction temperature than analogous cationic rhodium complexes and enables, for the first time, asymmetric transformations with up to 93.5:6.5 enantiomeric ratio.

Cooperative activation of cyclobutanones and olefins leads to bridged ring systems by a catalytic [4 + 2] coupling

“…3b , 37% ee). 48 – 49 This result suggests that the “cofactor”-assisted C–C activation mode is amenable to asymmetric catalysis, and work on this topic is ongoing.…”

Cleavage of Carbon–Carbon σ-Bonds of Four-Membered Rings

“…The enantioselective variant of this process appeared in 2014, where DTBM-SEGPhos was used as the chiral ligand for rhodium (Scheme a) . An analogous intramolecular insertion reaction of aldehydes was also developed (Scheme b) …”

Synthesis and applications of rhodacyclopentanones derived from C–C bond activation

“…50,51 An additional substituent (R 1 ) was incorporated at the 3-position of the cyclobutanone ring ( 86 ) and, consequently, cyclisation delivered complex unsymmetrical bicyclic ketones 87 . Cramer demonstrated that chiral bisphosphine-ligated zwitterionic Rh(I)-complexes provided significantly enhanced efficiencies in comparison with phosphine-ligated cationic Rh(I)-complexes, however, only moderate enantioselectivities were obtained (42-87% ee) 50. In a subsequent publication, a neutral DTBM-Segphos-ligated Rh(I)-catalyst system was disclosed which delivered bicyclic ketones 87 in good to excellent yields and with high levels of enantiocontrol (Scheme 27).…”