Selective hydrogenation with tris(triphenylphosphine) chlororhodium (I) catalyst: Preparation of octadecenoate isomers

Fractionation of products obtained from partial catalytic hydrogenation of methylcis-9,cis-12-octadecadienoate (9c,12c-18:2) with tris(triphenylphosphine) chlororhodium RhCl(Ph₃P)₃] provided a facile method for preparation of a nearly equal molar mixture of methylcis-9- andcis-12-octadecenoate (9c-18∶1 and 12c-18∶1). Isolation of products was achieved by silver resin and C18 reverse phase liquid chromatography. Catalytic deuteration of 9c,12c-18∶2 yields a mixture of 9c-18∶1-12,13-d₂ and 12c-18∶1-9,10-d₂ with an isotopic purity of 85%. Final isolated yield of the mixture of 9c- and 12c-18∶1 products was 30%. Isolation of products from partial hydrogenation of conjugated octadecadienoates (9c,11t-18∶2 or 10t,12c-18∶2) provided a convenient method for synthesis of an almost equal molar mixture of methyltrans-10 andtrans-11-octadecenoate (10t-18∶1 and 11t-18∶1). Characterization of the reaction products from hydrogenation of 9c,12c-28∶2 indicates that the 9c- and 12c-18∶1 products are formed by the expected 1,2-hydride addition. The presence of small amounts of 10t- and 11t-18∶1 and conjugated octadecadienoates was evidence for a secondary isomerization-1,4-hydride addition pathway. Isolation and characterization of products from RhCl(Ph₃P)₃-catalyzed hydrogenation of 9c,11t-18∶2 and 10t,12c-18∶2 indicate that both 1,2- and 1,4-hydride addition to the conjugated diene isomers occurs at about equal rates, but only thecis bond is reduced by the 1,2-hydride addition pathway and the 1,4-hydride addition pathway yields only atrans-18∶1. Because of this unusual selectivity for acis bond conjugated with atrans bond, hydrogenation of both 9c,11t-18∶2 and 10t,12c-18∶2 yields the same mixture of t-18∶1 isomers.