The development of scalemic multidentate niobium complexes as catalysts for the highly stereoselective ring opening of meso-epoxides and meso-aziridines

The discovery and development of a new Lewis acid system based on a complex formed from niobium(V) methoxide and (R)-3,3'-bis(2-hydroxy-3-isopropylbenzyl)-1,1'-binaphthalene-2,2'-diol, a novel tetradentate BINOL derivative, is presented. The system was shown to be extremely effective in promoting the desymmetrative ring opening of linear and cyclic meso-epoxides using anilines as nucelophiles, delivering the corresponding (R,R) anti-amino alcohols in good to excellent yields (up to quantitative) and excellent enantioselectivity (up to 96% ee). Furthermore, the catalyst system displays a remarkable sensitivity to steric bulk at the beta-carbon of the epoxide, selectively facilitating ring opening of smaller epoxides in the presence of more sterically hindered epoxides. This property was confirmed by a series of competition reactions using a mixture of meso-2-butene oxide and another aliphatic meso-epoxide, with the result that the former, less encumbered epoxide reacted preferentially with up to 98% chemical selectivity. While it was found to be most convenient to conduct the reactions with 10 mol % catalyst loading at 0.16 M, at higher overall concentration the reaction still proceeded efficiently with as little as 0.25 mol % catalyst to give the desired products with no significant reduction in yields or enantioselectivities. In addition, the current catalyst system was also found to mediate the asymmetric ring opening of nonsymmetrical cis-2-alkene oxides with anilines to give preferentially the corresponding (2R,3R)-2-amino-3-ols arising from ring opening at the methyl terminus, in excellent yields (up to quantitative) and good to excellent regio- and enantioselectivities (up to 18:1 and > 99% ee, respectively). Intriguingly, it was discovered that the same catalyst system also promoted the ring-opening desymmetrization of aziridines with aniline nucleophiles to give the corresponding (S,S) vicinal diamines in good to excellent yields and enantioselectivity (up to 95% and 84% ee [> 99% ee following a single recrystallization]). Catalyst systems that promote closely related reactions with opposite stereochemical outcomes in high selectivity such as the current niobium system are extremely unusual. To the best of our knowledge, this report constitutes not only the first example of the catalytic desymmetrization of both meso-epoxides and meso-aziridines but also a rare example of such complementary stereoselectivity in a catalytic reaction.