Homogeneous hydrogenation of dienes with rhodium complexes

TLDR: In this paper, different Rh complex catalysts were compared for the hydrogenation of methyl sorbate and linoleate in the absence of solvents, and the results indicated that 1,2-reduction was more than twice as important as 1,4-addition of H2 via conjugated diene intermediates.

Abstract: 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 H2 the following complexes, RhCl(Ph3 P)3 (Ph= phenyl), [RhClNBD]2 (NBD=norbornadiene) and RhH(CO)(Ph3P)3, produced mainly methyltrans-2-hexenoate (34 to 56%). Their diene selectivity was not particularly high as they produced 14 to 41% methyl hexanoate. With RhCl(Ph3 P)3 constant ratios between rates of methyl sorbate disappearance and formation of methyltrans-2- andtrans-3-hexenoate indicate approximately the same activation energy for 1,2-addition of H2 on the Δ4 double bond of methyl sorbate and for 1,4-addition to this substrate. In the hydrogenation of methyl linoleate with RhCl(Ph3 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 H2 via conjugated diene intermediates. Although the complexes RhCl(CO)(Ph3 P)3 and [Rh(NBD)(diphos)]+PF6 (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.