Tiny amounts of palladium turn alkynes into trans alkenes for pheromone synthesis

Chemists have long struggled to convert internal alkynes into trans (E) alkenes without triggering unwanted side reactions, particularly for long, linear alkyl chains where the freshly formed double bond tends to migrate along the carbon skeleton. A new study reports that parts-per-million quantities of palladium chloride, one of the simplest and cheapest palladium salts on the market, can accomplish this transformation directly. The reaction proceeds through a cascade: first a conventional cis (Z) semi-hydrogenation of the alkyne, followed by an in-situ Z/E isomerization, all catalyzed by vanishingly small amounts of palladium with no ligands or added hydride sources. The work, published in the journal Catal, demonstrates the method on three industrial pheromone precursors using just 0.0015 to 0.003 mol% of PdCl2 under mild conditions, and in one case the semi-hydrogenation step was scaled to one kilogram of starting material.

The difficulty lies in the chemistry of the double bond itself. The classical Lindlar catalyst, palladium modified with lead on calcium carbonate, hydrogenates alkynes to Z alkenes because both hydrogen atoms insert on the same side of the metal-coordinated alkyne, following the well-established Horiuti-Polanyi mechanism. This geometry makes E alkenes essentially inaccessible by direct semi-hydrogenation. Moreover, internal alkyl alkenes are notoriously prone to skeletal isomerization, often called alkene walking, under metal-catalyzed conditions, so any catalyst strong enough to isomerize Z to E also tends to scramble the position of the double bond. Existing routes to E alkenes therefore rely on expensive stoichiometric reagents such as Birch reductions with sodium and ammonia, or on metathesis, cross-coupling and substitution chemistry, followed in some cases by non-catalytic Z/E isomerization with iodine or silver.

The research team, led by Antonio Leyva-Pérez, used as a model substrate the pheromone precursor 2-(pentadec-10-yn-1-yl)-1,3-dioxolane, a sixteen-carbon linear alkyl chain bristling with methylene positions vulnerable to chain walking. In ethanol solvent, ppm levels of PdCl2 outperformed the Lindlar catalyst, achieving full conversion to the Z alkene with no detectable E product and no skeletal isomers, thanks to the extremely low palladium loading. Switching the solvent to isopropanol changed everything: the reaction ran seven times faster, with an initial turnover frequency of 8,333 compared with 1,450 hours-1 in ethanol, and the desired E alkene began to appear. After optimizing concentration and reaction time, the team obtained a 53% yield of the E alkene after 24 hours at 0.8 M, with no skeletal isomers detected and only minor over-hydrogenation to the alkane.

Mechanistic experiments pointed to ultrasmall palladium species as the true catalysts. The initial reaction rate actually slowed as palladium concentration increased, and the turnover frequency dropped rapidly with higher palladium amounts, consistent with rapid aggregation into inactive nanoparticles. Fluorescence spectroscopy revealed the characteristic emission bands expected for sub-nanometer metal clusters at palladium levels of 30 ppm or below, with a Stokes shift of roughly 50 nanometers matching two-to-five-atom clusters, while no fluorescence appeared above 400 ppm, where high-resolution transmission electron microscopy instead showed nanoparticle formation. Aberration-corrected HAADF-STEM imaging visualized the difference directly: at 5000 ppm palladium formed aggregates of roughly one nanometer, whereas at 30 ppm only sub-nanometric clusters averaging 0.3 nanometers, individual atoms plus few-atom clusters, were observed.

X-ray absorption spectroscopy reinforced this picture. A sample trapped at 1000 ppm showed a XANES profile like metallic palladium with an average coordination number of nine, consistent with particles around 1.8 nanometers. The 30 ppm sample, by contrast, displayed a mixed profile: linear combination analysis suggested about 25% residual PdCl2 and 75% metallic Pd(0), and EXAFS fitting yielded 1.2 palladium-chloride interactions at 2.28 angstroms alongside five palladium-palladium interactions split over two shells, implying small, disordered particles below 0.75 nanometers. The authors note that some aggregation may have been induced by the carbon trapping process or the hard X-ray measurement itself. Together with the confirmed liberation of HCl when PdCl2 is reduced by hydrogen, the evidence strongly supports catalytically active ultrasmall Pd(0) species, sub-nanometer clusters and possibly individual atoms, formed in situ.

The isomerization step proved equally demanding of explanation. Pure beta-PdCl2, a more robust six-membered ring structure, showed four times lower isomerization activity than commercial gamma-PdCl2 despite also reducing under the reaction conditions, ruling out a simple Lewis acid mechanism. Tests with HCl alone, with or without palladium and hydrogen, gave no isomerization at all. Pd/C, remarkably, catalyzed both the semi-hydrogenation and the isomerization, delivering 19% of the E alkene with just 6% alkane, one of the first reported instances of Pd/C producing an E alkene in significant amounts by direct semi-hydrogenation. Reactive experiments showed that the active species form under the combined action of hydrogen and isopropanol, and that hydride sources are essential: adding a sacrificial, easily hydrogenated alkene such as 1-pentene, or hydride quenchers like sodium iodide, sodium methoxide or benzyl alcohol, completely shut down the isomerization, implicating Pd(0)-hydride species.

The selectivity for alkyl over aromatic substrates is unusual and mechanistically telling. Z-stilbene refused to isomerize under the optimized conditions, and adding it to the reaction mixture of the alkyl alkene completely stopped the latter’s isomerization, apparently by blocking productive interaction with the electron-rich Pd(0) species. In the scope study, 4-octyne gave a 70% yield of E-4-octene, while diphenylacetylene gave 96% of Z-stilbene with no isomerization, a contrast the authors attribute to the absence of allylic hydrogen atoms in the aromatic substrate, which are considered essential for alkene walking-type isomerization. A second and third pheromone precursors gave E alkenes in 55% and 50% yields, a long-chain alkyne gave 43%, and a highly branched alkyne bearing dehydration-sensitive tertiary propargylic alcohols yielded 24% of an E alkene that is otherwise difficult to obtain despite industrial interest as an emulsifier. Skeletal isomerization stayed below 15% throughout, and alkanes typically below 5%.

Scale-up brought the process into industrially relevant territory. In a 1-liter reactor, four attempts at varying hydrogen pressure culminated in a 79% yield of the Z alkene at 20 bar with 60 ppm of PdCl2, the higher loading compensating for the limited solubility of hydrogen, calculated at just 2.77 x 10-3 mol/L under these conditions. In a 10-liter pilot reactor with one kilogram of alkyne and a continuous hydrogen stream at 1.5 bar, the Z alkene yield reached 94%, with no alkane or skeletal isomerized products detected in any large-scale test. The team also demonstrated an electrochemical variant in an undivided cell with zinc electrodes at room temperature, achieving greater than 99% conversion and 67% selectivity to the E alkene, and a continuous-flow process for C10-C12 alkynes delivering at least 99% conversion with 95-96% Z-selectivity at Pd loadings down to 0.003 mol%.

An economic analysis underscored the practical stakes. Compared with the current industrial combination of Lindlar semi-hydrogenation and sodium-ammonia Z/E isomerization, the PdCl2 process was estimated to be 60% cheaper per kilogram of product in a model based on commercial prices, and an independent cost analysis by a representative pheromone company using real market prices found that the Lindlar-based process added 27 euros per kilogram in hydrogenation-related costs against only 6 euros per kilogram for the PdCl2 method, a 77.8% reduction that translated into a 6.5% lower overall production cost of the final pheromone. The catalyst can also be recovered and reused, retaining good activity through four cycles, with a hot filtration test showing that the active species remain attached to the MgCO3 solid vehicle used to weigh out the tiny palladium quantities.

The authors are candid about the method’s limits: the Z/E isomerization step does not yet deliver E pheromones purer than 90%, so the approach is best suited to applications where the Z isomer does not inhibit the targeted pest-control effect, or where E/Z mixtures enriched in the E isomer are commercially acceptable, since simple urea treatment can separate the isomers. Even so, the demonstration that a commodity palladium salt, used at parts-per-million levels and without any ligand, can steer the challenging cascade from internal alkyl alkyne to E alkene, with kilogram-scale validation and mechanistic evidence for few-atom Pd(0) hydride species, offers the synthetic community a genuinely viable and more sustainable route to a class of molecules that the pheromone industry has long had to buy at a premium.

Subject of Research: Catalytic semi-hydrogenation of alkynes to trans alkenes using parts-per-million palladium, applied to pheromone synthesis

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

Article References: Molina–García, M., Orlando, F., Manzorro, R., Hernández–Garrido, J. C., Martin-Diaconescu, V., Oliver-Meseguer, J., Mon, M., Aragón, J., & Leyva–Pérez, A. (2026). Synthesis of alkyl trans–alkenes from alkynes by catalytic semi–hydrogenation reaction with parts–per–million of palladium: application to pheromone synthesis. Catal, 2(1), Article 9. https://doi.org/10.1007/s44422-025-00015-5

Image Credits: AI Generated

DOI: 10.1007/s44422-025-00015-5

Keywords: palladium catalysis, semi-hydrogenation, trans alkenes, alkynes, Z/E isomerization, pheromones, single-atom catalysis, PdCl2, green chemistry, scale-up, flow chemistry, electrocatalysis