TADDOL-Derived Phosphites and Phosphoramidites for Efficient Rhodium-Catalyzed Asymmetric Hydroboration

Two simple TADDOL-derived monodentate ligands, the (1R,2S)-2-phenylcyclohexanol-derived phosphite and the N,N-(phenylbenzyl)phosphoramidite, give comparably high levels of enantioselectivity (90−96% ee) in the rhodium-catalyzed hydroborations of substituted styrenes bearing either electron-donating or electron-withdrawing substituents. Rhodium(I) chloride and tetrafluoroborate catalyst precursors give comparable results. Pinacolborane is superior to catecholborane in these reactions.