Hydrogenation of dienes by a recyclable poly(ethylene oxide)–rhodium phosphineless catalytic system

The monocationic complex [(η5-Cp∗)2Rh2(μ2-Cl)3]PF6 in poly(ethylene oxide) (PEO) 3350/MeOH has proven to be a very efficient catalyst for 1,7-octadiene, 1,9-decadiene, and 1,5-cyclooctadiene hydrogenation. This system allows perfect product separation and catalyst phase recycling, resulting in thousands of catalytic cycles (TON ≈ 9000). Even at room temperature, turnover frequencies as high as 5000 h−1 are attained (50 bar), which makes this one of the most active catalytic systems for diene hydrogenation. Kinetic studies reveal that the reaction rate is second-order in H2 pressure and first-order in both rhodium and diene concentrations. Typical tests suggest that, despite the reductive reaction conditions, which could lead to colloidal metal dispersion, the reduction of dienes is catalyzed by molecular species.