Highly Selective Catalytic Synthesis of (E,E)-1,4-Diiodobuta-1,3-diene via Atom-Efficient Addition of Acetylene and Iodine: A Versatile (E,E)-1,3-Diene Building Block in Cross-Coupling Reactions

Abstract

The first practical procedure is reported for the synthesis of (E,E)-1,4-diiodobuta-1,3-diene from very simple starting materials (acetylene and I₂). A pure crystalline product was obtained in a green chemical procedure utilizing the key advantages of highly selective Pt-catalyzed transformation and 100% atom efficiency of the addition reaction. The Pt catalyst was recovered and re-used in the reaction without a noticeable loss of activity.

Key words

cross-coupling - Pt catalyst - green chemistry - natural product synthesis - total synthesis

References and Notes

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10

Activation barriers for both pathways are small in acetone and should be easy to overcome at r.t., thus the reaction is controlled thermodynamically (Scheme  [¹] ). Of course, the values of computed energies may be affected by known limitations of accuracy of DFT calculations, however, the relative reactivity should be correctly predicted.

11

As a proof of principle, complete E,E-stereoselectivity was observed in polar solvents (H₂O and MeOH), while some minor amounts of E,Z-isomer may be detected in acetone. Note, however, that double-bond isomerization was also observed in the synthesized product. In either case this has no influence on the purity of final product obtained after purification step.

12

Synthetic Procedure for the Preparation of (E,E)-1,4-Diiodobuta-1,3-diene
The reaction was carried out in 20 mL tube with PTFE-sealed screw cap. The catalyst precursor PtCl₄ (11.8 mg, 3.5˙10^-5 mol), NaI (0.6 g, 4.0˙10^-³ mol) and I₂ (1.0 g, 4.0˙10^-³ mol) were placed into the tube. Solution of acetylene in acetone (5 mL) was added to the tube. The tube was sealed with screw cap, and the reaction was carried out at r.t. for 20 h under stirring using a magnetic stirrer.^¹³

13

Detailed description of the synthetic procedure, isolation, and purification of (E,E)-1,4-diiodobuta-1,3-diene, as well as catalyst regeneration and spectral characterization of the E,E-, E,Z-, and Z,Z-isomers are provided in the Supporting Information.

14

For detailed description of the X-ray analysis and NMR characterization of intermediate Pt complex see Supporting Information.

15

Dissociation of iodide ligands and formation of acetylene
π-complexes were omitted for clarity (Scheme  [¹] ). Note also, that replacement of iodide ligands by solvent molecules may also take place at various stages of the catalytic cycle.