Selective head-to-tail coupling of methyl phenylpropynoate providing palladacycles with bidentate N-ligands

The complexes (1,3-dicarbomethoxy-2,4-diphenylbuta-1,3-dien-1,4-diyl)(N,N)palladium(II), where N,N = 2,2′-bipyridyl (1), tetramethylethelenediamine (2), 1,10-phenanthroline (3) have been obtained by reaction of Pd(dba)2 with the respective bidentate N-ligand and two equivalents of methyl phenylpropynoate via a completely regioselective head-to-tail coupling of the asymmetric acetylenes. Such regioselectivity, especially in conjunction with the high yield, is very unusual in the formation of palladacycles and has so far only been observed for head-to-head or tail to tail coupling. The compounds 1–3 have been characterized by elemental analyses, NMR spectra and single crystal X-ray diffraction studies for 2 and 3. The X-ray crystal structures reveal pseudo square planar metal centers, the palladacycles and chelate rings are essentially planar.

Graphical abstractUnexpected, complete regioselectivity has been observed in the high-yield formation of palladacycles from methyl phenylpropynoate with Pd(dba)2 in the presence of bidentate N-ligands(bpy, tmeda, phen), to provide exclusively the head-to-tail coupled palladacyclic compounds 1–3. Aromatic compounds from cyclotrimerization were not observed, even in the presence of excess alkyne.Figure optionsDownload full-size imageDownload as PowerPoint slide