Immobilized rhodium hydrogenation catalysts

The showcase catalyst for olefin hydrogenations is Wilkinson's complex CIRh (PPh3)(3) (1). Here, the Wilkinson-type catalyst ClRh[Ph2P(CH2)(3)Si(OEt)(3 )](3) (2), trans-ClRh(CO)CPh2P(CH2)(3)Si(OEt)(3)](2) (3), and the chelate c omplex ClRh(PPh3)[Ph2PCH2-CHOHCH2PPh2] (4) have been synthesized using the bifunctional ligands Ph2P(CH2)(3)Si(OEt)(3) (5) and Ph2PCH2CHOHCH2PPh2 (6), and fully characterized. The dihydride complex 4(H)(2) has been generated in situ and characterized with NMR. Complexes 2, 3, and 4 have been immobil ized on silica, giving 2i, 3i, and 4i, respectively, which have been studie d by solid-state NMR. The catalytic activities of 2i-4i for the hydrogenati on of 1-dodecene (7), 2-cyclohexen-1-one (8), and 4-bromostyrene (9) are co mpared with those of the homogeneous analogs 2 and 3, as well as with 1. Wh ile there are no substantial differences between 1 and 2 concerning TON and TOF, 2i shows reversed activity for the three test samples: dodecene gives the lowest TOF, followed by bromostyrene and cyclohexenone. However, TON a nd maximal yield are the same for 1, 21 and 21. In contrast to 1 and 2, the immobilized catalyst 21 can be recycled seven times, 4i three times. Catal ysts 3 and 3i give maximal TOF for the hydrogenation of 7, followed by 8 an d 9. The yields are below 100% for the homogeneous catalyst 3, but immobili zation (3i) gives maximum yields for all substrates. (C) 2001 Elsevier Scie nce B.V. All rights reserved.