Alkene-Activating Effect in Molybdenum-Catalyzed Asymmetric Hydrogenation of Arenes: Insights into Activity and Selectivity.

The asymmetric hydrogenation of substituted benzenes with oxazoline imino(pyridine) (OIP) molybdenum precatalysts is described. A comparison of the hydrogenation activities between alkylbenzenes and the corresponding styrenes revealed an alkene-activating effect, where the introduction of an alkene substituent resulted in the hydrogenation of both the alkene and the arene. Distinct reaction pathways for alkene and arene hydrogenation were observed with the outcomes dependent on the hydrogen pressure, the substitution pattern of the arene, and the placement of the alkene tethered to the arene. The most efficient arene hydrogenation occurred when the alkene was in conjugation with the aromatic ring and the reaction was conducted with 80 atm of H2. Hydrogenation of a series of disubstituted styrenes established trends in chemo-, diastereo-, and enantioselectivity and provided insights into the nature of the catalyst-substrate interaction. To gain insight into the mechanistic origins of these trends, OIP-supported molybdenum η6-arene complexes with substituted benzenes were synthesized and characterized by X-ray diffraction and NMR spectroscopy. Subsequent hydrogenation demonstrated a correlation between the site selectivity of arene coordination and the diastereo- and enantioselectivity of the hydrogenation reaction. The data informed a stereochemical model for arene coordination and established that coordination of the substrate to the molybdenum center is the stereodetermining step in the hydrogenation reaction.