D<sub>4</sub>-Symmetric Dirhodium Tetrakis(binaphthylphosphate) Catalysts for Enantioselective Functionalization of Unactivated C–H Bonds
Ziyi Chen, Kristin Shimabukuro, John Bacsa, Djamaladdin G. Musaev, Huw M. L. Davies
Abstract
High Resolution Image Download MS PowerPoint Slide Dirhodium tetrakis(2,2′-binaphthylphosphate) catalysts were successfully developed for asymmetric C–H functionalization with trichloroethyl aryldiazoacetates as the carbene precursors. The 2,2′-binaphthylphosphate (BNP) ligands were modified by introduction of aryl and/or chloro functionality at the 4,4′,6,6′ positions. As the BNP ligands are C 2 -symmetric, the resulting dirhodium tetrakis(2,2′-binaphthylphosphate) complexes were expected to be D 4 -symmetric, but X-ray crystallographic and computational studies revealed this is not always the case because of internal T-shaped CH−π and aryl–aryl interactions between the ligands. The optimum catalyst is Rh 2 ( S -megaBNP) 4, with 3,5-di( tert -butyl)phenyl substituents at the 4,4′ positions and chloro substituents at the 6,6′ positions. This catalyst adopts a D 4 -symmetric arrangement and is ideally suited for site-selective C–H functionalization at unactivated tertiary sites with high levels of enantioselectivity, outperforming the best dirhodium tetracarboxylate catalyst developed for this reaction. The standard reactions were conducted with a catalyst loading of 1 mol % but lower catalyst loadings can be used if desired, as illustrated in the C–H functionalization of cyclohexane in 91% ee with 0.0025 mol % catalyst loading (29,400 turnover numbers). These studies further illustrate the effectiveness of donor/acceptor carbenes in site-selective intermolecular C–H functionalization and expand the toolbox of catalysts available for catalyst-controlled C–H functionalization.