Synthesis of alkyl trans–alkenes from alkynes by catalytic semi–hydrogenation reaction with parts–per–million of palladium: application to pheromone synthesis

The direct catalytic synthesis of trans (E) alkenes from internal alkynes remains a challenge in organic synthesis, particularly for alkyl alkenes where the double bond can easily migrate by skeletal isomerization (alkene walking) under metal–catalyzed reaction conditions. Here we show that parts–per–million (ppm) of PdCl2, the simplest Pd salt in the market, or even the solid Pd/C, catalyze the direct transformation of long–linear chain internal alkyl alkynes into the corresponding trans (E) alkenes, after cis (Z) semi–hydrogenation and in–situ Z/E isomerization reactions, with just minor skeletal isomerization (< 10%) and none over–hydrogenation reaction. Mechanistic experiments unveil that the catalytic active species for both the hydrogenation and Z/E isomerization are composed by ultrasmall species (clusters and individual atoms) of reduced Pd, which also enable the selective Z/E isomerization reaction of alkyl vs. aromatic alkynes. The synthesis of three representative industrial pheromone precursors is achieved with just 0.0015–0.003 mol% of PdCl2 under mild reaction conditions, in up to 1 kg scale in one case. These results bring to the synthetic community an industrially viable methodology to obtain E alkyl alkenes from alkyl alkynes.