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(cy(3)P)(2)(eta(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. (c) 2011 Elsevier Ltd. All rights reserved.