Asymmetric Rhodium-Directed anti-Markovnikov Regioselective Boracyclopentannulation

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.