HYDROGENATION OF TRANS-CINNAMALDEHYDE WITH HYDRIDO-CARBONYL OSMIUM(II) COMPLEXES OF CHELATING PHOSPHINE-LIGANDS

A series of new hydridocarbonyl osmium(II) complexes, OsHCl(CO)(PPh3)( L-L)[L-L=Ph2P(CH2)(n)PPh2 (n=1 (1), 2 (2), 3 (3), cis-Ph2PCH=CHPPh2 (4 ), and Fe(eta(5)-C5H4PPh2)(2) (5)] has been synthesized from OsHCl(CO) (PPh3)(3) and chelating diphosphines. These complexes have been charac terized by IR, H-1 NMR and elemental analysis. The catalytic activitie s of these complexes both for the transfer hydrogenation of trans-cinn amaldehyde with 2-propanol ag the hydrogen donor, and for the selectiv e hydrogenation of trans-cinnamaldehyde with H-2, have been examined. Complexes (1)-(5) were shown to have higher selectivities for the tran sfer hydrogenation of the C=O bond of aldehyde than for the transfer h ydrogenation of the C=C bond of aldehyde. The selectivities for the tr ansfer hydrogenation with 2-propanol as well as for the hydrogenation with H-2 have been found to decrease in the order 3>5>2>4>1. Complex ( 3) has shown to possess almost 90% of the selectivity to cinnamyl alco hol for transfer hydrogenation It is also found that there is a correl ation between the v(CO) of each complex and the hydrogenation, of the C=O bond of trans-cinnamaldehyde. Overall, the selectivities with the complexes (1)-(5) are greater far the transfer hydrogenation with 2-pr opanol than for the hydrogenation with H-2.