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. 2024 Dec 9;16(6):2573-2580.
doi: 10.1039/d4sc06686c. eCollection 2025 Feb 5.

Palladium-catalyzed aerobic homocoupling of aliphatic olefins to dienes: evidence for rate-limiting concerted metalation-deprotonation

Sam Van Minnebruggen  1 , Harry Poels-Ryckeboer  1 , Hendrik Van Dessel  1 , Frederick Martens  1 , Wouter Stuyck  1 , Tom Nelis  1 , Igor Beckers  1 , Aram Bugaev  2 , Dirk De Vos  1
Affiliations

Palladium-catalyzed aerobic homocoupling of aliphatic olefins to dienes: evidence for rate-limiting concerted metalation-deprotonation

Sam Van Minnebruggen et al. Chem Sci. .

Abstract

Palladium(ii)-catalyzed dehydrogenative coupling of aliphatic olefins would enable an efficient route to (conjugated) dienes, but remains scarcely investigated. Here, 2-hydroxypyridine (2-OH-pyridine) was found to be an effective ligand for Pd(ii) in the activation of vinylic C(sp2)-H bonds. While reoxidation of Pd(0) is challenging in many catalytic oxidations, one can avoid in this reaction that the reoxidation becomes rate-limiting, even under ambient O2 pressure, by working in coordinating solvents. Via kinetic studies the elementary steps governing this reaction were elucidated, resulting in enhanced performance (turnover frequency) of the Pd(ii)/2-OH-pyridine system. The diene product is formed via a consecutive activation of two olefins on the same Pd atom, followed by a β-hydride elimination. The first olefin activation, viz. the C-H activation, determines the overall reaction rate under these conditions. The catalytic complex was studied by ESI-MS and X-ray absorption spectroscopy, revealing that the coordination sphere of the working palladium complex contains two 2-OH-pyridine ligands.

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Conflict of interest statement

There are no conflicts to declare.

Figures

Fig. 1
Fig. 1. Overview of different pathways to obtain diene moieties via coupling reactions.
Fig. 2
Fig. 2. Product selectivity (left) and linear free-energy relationship between reaction rate and σ-parameter of substituents on the 2-OH-pyridine ligand (right). Reaction conditions: 15 μmol Pd(OAc)2, 7 eq. ligand, 1.0 mL 1-octene, 1.4 mL dimethylacetamide (DMA) and 50 μL tetradecane at 90 °C under O2 atmosphere (sparged) after 10 min. The ligands used in this screening and the corresponding σ-values are shown in Table S2 (σpara is used for the substituent in the para position with respect to the OH-group, as in e.g. 5-NO2-2-OH-pyridine).
Fig. 3
Fig. 3. Influence of individual reaction parameters on the kinetics. (a) Variation of O2 pressure; (b) effect of the ratio of 2-OH-pyridine ligand to Pd; (c) effect of the pKa of the added carboxylic acid; (d) effect of the amount of acetic acid. Reaction conditions: 15 μmol Pd(OAc)2, 7 eq. 2-OH-pyridine (varied for (b)), 100 eq. acetic acid (or 0.7 M AcOH) (amount varied in (d), other acids (R-COOH) used in (c) are shown in Table S3†), 1.4 mL DMA, 0.6 mL 1-octene and 50 μL tetradecane at 100 °C under O2 atmosphere (sparged, varied for (a)) after 60 min.
Fig. 4
Fig. 4. Effect of the olefin concentration on turnover frequency. Reaction conditions: 15 μmol Pd(OAc)2, 7 eq. 2-OH-pyridine, 100 eq. AcOH, 1.4 mL DMA, x M 1-octene and 50 μL tetradecane at 100 °C under O2 atmosphere (sparged) after 30 min.
Fig. 5
Fig. 5. Mono- (left) and bimetallic (right) pathway for both olefin activation steps in the dehydrogenative olefin homocoupling (top) and the effect of the Pd(OAc)2 concentration on product formation (bottom). Reaction conditions: x μmol Pd(OAc)2, 7 eq. 2-OH-pyridine, 100 eq. AcOH, 2.5 mL 1-octene, 1.4 mL DMA and 50 μL tetradecane at 100 °C under O2 atmosphere (sparged) after 30 min.
Scheme 1
Scheme 1. Proposed mechanism for the Pd-catalyzed dehydrogenative homocoupling of simple olefins to (non-) conjugated dienes.
Fig. 6
Fig. 6. Effect of the temperature on reaction rates. Reaction conditions: 15 μmol Pd(OAc)2, 7 eq. 2-OH-pyridine, 100 eq. AcOH, 1.4 mL DMA, 0.6 mL 1-octene and 50 μL tetradecane at 70–130 °C under O2 atmosphere (sparged) after 30 min.
Fig. 7
Fig. 7. Kinetic isotope effect for the dehydrogenative homocoupling of styrene vs. styrene-d8 (left). Linear free-energy relationship between reaction rate and σ-parameter of substituted functionalized styrene reactants (right). Reaction conditions: 15 μmol Pd(OAc)2, 3 eq. 2-OH-pyridine, 100 eq. AcOH, 1.4 mL DMA, 2 mL olefin, and 50 μL tetradecane at 80 °C under O2 atmosphere (sparged) after 10 min. σp values were obtained from literature.
Fig. 8
Fig. 8. Effect of vinyl substitution in the olefin substrate on the turnover frequency (TOF), normalized for the number of vinylic protons (e.g. 3 for 1-octene). Reaction conditions: 15 μmol Pd(OAc)2, 3 eq. 2-OH-pyridine, 492 μL AcOH, 1 mL olefin, 1.4 mL dimethylacetamide and 50 μL tetradecane at 90 °C under O2 atmosphere (sparged) after 10 min.
Fig. 9
Fig. 9. Olefin competition experiment. Reaction conditions: 15 μmol Pd(OAc)2, 7 eq. 2-OH-pyridine, 500 eq. AcOH, 1.4 mL DMA, 0 or 2.5 mL 1-octene, 0 or 1 mL t-butyl acrylate and 50 μL tetradecane at 100 °C under O2 atmosphere (sparged) after 45 min.
Fig. 10
Fig. 10. Characterization of the active complex by ESI-MS and XAS. ESI-MS measurement of Pd(OAc)2 dissolved in CH3CN (a) and with additional 2-OH-pyridine (b). Conditions: 0.89 μmol Pd(OAc)2, 0 or 10 eq. 2-OH-pyridine and 1 mL CH3CN; positive mode. (c) XANES spectra for the Pd-(2-hydroxypyridine) and Pd(OAc)2 compared to some reference Pd precursors. (d) Experimental κ2-weighted Fourier transformed EXAFS for the Pd-(2-hydroxypyridine) and Pd(OAc)2 complexes compared to some reference Pd-precursors. Conditions: Pd(OAc)2 dissolved in DMA (with and without ligand) benchmarked with reference Pd-precursors (PdO and metallic Pd foil).
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