Recyclable immobilized rhodium catalysts in the diastereoselective hydrogenation of unsaturated steroids

Macroreticular anion exchange resins, Amberlite IRA-900 and Amberlite IRA-96, were used to bind monosulfonated triphenylphosphine ligand via noncovalent electrostatic interactions. The resin-phosphine system was found to be very efficient at promoting the immobilization of [Rh2(μ-OMe)2(cod)2]. The resulting stable Rh/TPPMS immobilized catalyst was tested in the hydrogenation of unsaturated oxosteroids (4-androstene-3,17-dione and 3β-acetoxypregna-5,16-dien-20-one). These immobilized catalysts could selectively reduce CC bonds leading to the preferential formation of the α-diastereoisomer, as in the homogeneous systems. Furthermore, these new immobilized rhodium catalysts showed good performance, easy separation and recycling without loss of activity after at least three cycles.When the immobilization of [Rh2(μ-OMe)2(cod)2] was carried out in situ using either Amberlyst A27/TPPMS or the polystyrene-block-poly(m-vinyltriphenylphosphine) (PS-b-PPh3), the catalytic activity was lower and a significant decrease was observed on recycling.

A comparative study of the hydrogenation of unsaturated oxosteroids using immobilized rhodium catalysts onto ion exchange resins is described. These new immobilized catalysts show a high activity combined with easy product separation and catalyst reuse. This set of results makes the process here reported very attractive for performing the chemo, regio and diastereoselective hydrogenation of multifunctional steroid molecules.Figure optionsDownload as PowerPoint slide