Backbone-Modified <i>C</i><sub>2</sub>-Symmetrical Chiral Bisphosphine TMS-QuinoxP*: Asymmetric Borylation of Racemic Allyl Electrophiles
Hiroaki Iwamoto, Yu Ozawa, Yuta Takenouchi, Tsuneo Imamoto, Hajime Ito
Abstract
A new C2-symmetrical P-chirogenic bisphosphine ligand with silyl substituents on the ligand backbone, (R,R)-5,8-TMS-QuinoxP*, has been developed. This ligand showed higher reactivity and enantioselectivity for the direct enantioconvergent borylation of cyclic allyl electrophiles than its parent ligand, (R,R)-QuinoxP* (e.g., for a piperidine-type substrate: 95% ee vs 76% ee). The borylative kinetic resolution of linear allyl electrophiles was also achieved using (R,R)-5,8-TMS-QuinoxP* (up to 90% ee, s = 46.4). An investigation into the role of the silyl groups on the ligand backbone using X-ray crystallography and computational studies displayed interlocking structures between the phosphine and silyl moieties of (R,R)-5,8-TMS-QuinoxP*. The results of DFT calculations revealed that the entropy effect thermodynamically destabilizes the dormant dimer species in the catalytic cycle to improve the reactivity. Furthermore, in the direct enantioconvergent case, detailed calculations indicated a pronounced enantioselective recognition of carbon–carbon double bonds, which is virtually unaffected by the chirality at the allylic position, as a key for the borylation from both enantiomers of racemic allyl electrophiles.