Nickelacyclic carboxylates derived from 3-hexyne and CO2 and their application in the synthesis of a new muconic acid derivative

The mononuclear unsaturated nickelalactones [Ni{C(Et)C(Et)–COO}(DBU)2] (1) and [Ni{C(Et)C(Et)–COO}(dcpe)] (2) were synthesized by oxidative coupling of CO2 and 3-hexyne at zero-valent nickel in presence of DBU or dpce, respectively. Attempts to use Cy3P or dppe as ligands in this reaction gave the zero-valent complex [Ni(cy3P)2(η2-3-hexyne)] (3) and the well known [Ni(dppe)2] instead. However, the dppe stabilized unsaturated nickelalactone [Ni{C(Et)C(Et)–COO}(dppe)] (4) was synthesized from the corresponding 2,2′-bipyridine complex by ligand displacement with dppe. Complex 1 is a useful synthon for the synthesis of carboxylic acid derivatives, as shown by its reaction with NBS leading to 3-bromo-2-ethyl-pent-2-enoic acid in good yield. On the contrary, the reaction between [Ni{C(Et)C(Et)–COO}(bipy)] and NBS gave the hitherto unknown 2-(2,3,4-triethyl-5-oxo-2,5-dihydrofuran-2-yl)butyric acid (5) as major product. Compounds 1–5 were structurally investigated by NMR and IR spectroscopy and in the solid state by X-ray diffraction analysis of single crystals.

Three new unsaturated nickelalactones of the type [Ni{C(Et)C(Et)–COO}(L)] (L = 2 DBU, dcpe, dppe) have been synthesized and their structures have been elucidated by X-ray diffraction. Such nickelalactones were successfully employed in the synthesis of substituted carboxylic acid derivatives by the reaction with NBS. The influence of the neutral ligand on the constitution of the product formed has been investigated.Figure optionsDownload as PowerPoint slideHighlights
► Synthesis of new unsaturated nickelalactones by oxidative coupling of 3-hexyne and CO2.
► Strong influence of the neutral ligand (dbu or bipy) on the product formed by the reaction of the nickelalactones with NBS.
► Straightforward approach to highly substituted muconic acid derivatives from alkynes and CO2.