Mixtures of Configurationally Stable and Fluxional Atropisomeric Monodentate P Ligands in Asymmetric Rh-Catalyzed Olefin Hydrogenation

摘要

Several years ago it was reported that monodentate phosphites,[1] phosphonites,[2] and phosphoramidites[3] derived from 2,2-dihydroxy-1,1-binaphthyl (BINOL) are efficient ligands in various Rh-catalyzed asymmetric olefin-hydrogenation reactions (90-99 % ee). These observations were surprising, because it had been accepted that chelating bidentate P ligands are generally necessary for high levels of enantioselectivity, probably due to restricted rotation in the Rh complexes. Examples of other types of monodentate P ligands are also known.[4] Preliminary mechanistic results show that two monodentate phosphites (or phosphonites) are bonded to Rh in the transition state of hydrogenation[5] when the precatalyst is [Rh(cod)(1)2]BF4 (cod=cycloocta-1,5-diene). Based on these results we subsequently demonstrated that the use of a mixture of two different monodentate P ligands constitutes a new and efficient approach to combinatorial asymmetric transition-metal catalysis.[6] Once a library of cheap ligands has been prepared, mixtures result in high catalyst diversity and new ligands are not needed. Although each system actually contains three (pre)catalysts in an (as yet) unpredictable ratio, namely the two homocombinations [Rh(cod)LaLa]BF4 and [Rh(cod)LbLb]BF4, as well as the heterocombination [Rh(cod)LaLb]BF4, enantioselectivities of 95-99 % ee are often possible, even though the respective homocombinations in pure form result in lower enantioselectivities. Mixtures of BINOL-derived P ligands in combination with a phosphinine (phosphabenzene) or triphenylphosphine cause reversal of enantioselectivity in a few cases.[7]