Nickelacyclic carboxylates derived from 3-hexyne and CO 2 and their application in the synthesis of a new muconic acid derivative

Abstract

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 CO 2 and 3-hexyne at zero-valent nickel in presence of DBU or dpce, respectively. Attempts to use Cy 3P or dppe as ligands in this reaction gave the zero-valent complex [Ni(cy 3P) 2(η 2-3-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.