Synthesis and Use in Asymmetric Hydrogenations of Solely Planar Chiral 1,2-Disubstituted and 1,2,3-Trisubstituted Ferrocenyl Diphosphines: A Comparative Study

A total of 12 enantiopure 1,2-disubstituted and 1,2,3-trisubstituted ferrocenyl diphosphines [1-Ph2P-2-R2PCH2-3-R‘-Fc, R = Cy, t-Bu, 3,5-(CH3)2C6H3), R‘ = H, CH3, Ph, 3,5-(CH3)2C6H3)] have been synthesized, characterized, and tested in asymmetric rhodium- and ruthenium-catalyzed hydrogenations of four alkenes and two ketones. The performance of these ferrocene derivatives in catalytic hydrogenations was compared to those of catalysts based on Josiphos, PPF-t-Bu2, and Xyliphos [1-Ph2P-2-R2P(CH3)CH-Fc, R = Cy, t-Bu, 3,5-(CH3)2C6H3)]. Dichloro palladium(II) complexes of Xyliphos and its analogues lacking the stereogenic center were synthesized as model compounds, and their molecular structures were studied in solution and, for four complexes, in the solid state. In hydrogenation reactions the replacement of Xyliphos as the catalyst ligand with its analogues lacking the stereogenic center leads to changes in product configurations or significant increases or decreases in ee values.