Asymmetric Rhodium-Directed anti-Markovnikov Regioselective Boracyclopentannulation

摘要

A Shimoi-type activation of B-H bond of NHC-boranes by a diphosphane-ligated cationic Rh complex was applied in an unprecedented intramolecular hydroboration reaction of simple olefins. The use of NHC-boranes as hydroborating reagents is still undisclosed due to their nonreactivity toward alkenes which could be explained by the high stability of this complex rendering it unable to provide a "free" borane hydroborating reagent. B-H bond Rh activation of NHC-borane circumvents this limitation, and asymmetric Rh-directed anti-Markovnikov boracyclopentannulation reaction led to a library of enantioenriched cyclic boranes in high yield (up to 94%) with high regio- (up to 100%) and enantioselectivity (er up to 99.2:0.8). This new activation mode of NHC-boranes highlights their use in organometallic chemistry and offers a very good approach to access chiral cyclic NHC-boranes.