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 [Rh-2(mu-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 3 beta-acetoxypregna-5,16-dien-20-one). These immobilized catalysts could selectively reduce C=C bonds leading to the preferential formation of the alpha-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 [Rh-2(mu-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. (C) 2009 Elsevier B.V. All rights reserved.