Highly diastereoselective preparation of chiral NHC-boranes stereogenic at the boron atom† †Electronic supplementary information (ESI) available: Experimental details, spectroscopic data and theoretical investigations; X-ray data for compounds 4a, 4b and 4d. CCDC 1856403, 1857908 and 1857909. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9sc01454c

Stereogenic main group elements, a promising strategy for metal-free enantioselective catalysis. Stereogenic main group elements are clearly generating interest in the enantioselective catalysis field. Surprisingly, while chiral organoboron reagents are very useful in stereoselective transformations, few scaffolds stereogenic at boron and configurationally stable have been reported to date. Herein, we describe an original library of chiral NHC-boranes, stereogenic at the boron atom, that has been prepared in only a few steps and in good yields (up to 93%). Key steps involve a chlorination/arylation sequence in the presence of simple Grignard reagents from bicyclic NHC-boranes. The high and unprecedented diastereoselectivity observed during the second step (up to 99 : 1 dr) has been rationalized through a plausible S RN 1 mechanism thanks to EPR observations and DFT calculations.