Homogeneous hydrogenation of dienes with rhodium complexes

Different Rh complex catalysts were compared for the hydrogenation of methyl sorbate and linoleate in the absence of solvents. At 100 C and 1 atm H 2 the following complexes, RhCl(Ph 3 P) 3 (Ph= phenyl), [RhClNBD] 2 (NBD=norbornadiene) and RhH(CO)(Ph 3 P) 3 , produced mainly methyl trans ‐2‐hexenoate (34 to 56%). Their diene selectivity was not particularly high as they produced 14 to 41% methyl hexanoate. With RhCl(Ph 3 P) 3 constant ratios between rates of methyl sorbate disappearance and formation of methyl trans ‐2‐ and trans ‐3‐hexenoate indicate approximately the same activation energy for 1,2‐addition of H 2 on the Δ4 double bond of methyl sorbate and for 1,4‐addition to this substrate. In the hydrogenation of methyl linoleate with RhCl(Ph 3 P) 3 , the kinetic curves were simulated by a scheme in which 1,2‐reduction was more than twice as important as 1,4‐addition of H 2 via conjugated diene intermediates. Although the complexes RhCl(CO)(Ph 3 P) 3 and [Rh(NBD)(diphos)] + PF 6 (diphos=diphosphine) were inactive in the hydrogenation of methyl sorbate, they catalyzed the hydrogenation of methyl linoleate at 100 C and 1 atm. Catalyst inhibition apparently was caused by stronger complex formation with methyl sorbate than with the conjugated dienes formed from methyl linoleate.