Structural evidences for the oxidation of α-hydroxycarboxilic acids to oxalate assisted by copper(II) ions

The mono-, di- or poly-nuclear copper(II)-oxalate complexes [Cu(phen)(ox)(H2O)]center dot H2O (1), {[Cu(mu-ox)(bpy)]center dot H2O]center dot 1/2EtOH}(n) (2), {[Cu(mu-ox)(bpy)]center dot M2O}(n) (3), [{Cu(NO3)(bpy)-(H2O)}(2)(mu-ox)] (4) and [{Cu(NO3)(phen)(H2O)}(2)(mu-ox)] (5) (ox = oxalato, phen = 1,10-phenanthroline y bpy = 2,2'-bipyridine) were prepared by reacting Cu2CO3(OH)(2)center dot 2H(2)O or Cu(NO3)(2)center dot 3H(2)O salts with glycolic (H(2)GLYO) or lactic (H(2)LACO) acids or D-gluconic acid delta-lactone (delta-gluconolactone, H-2 delta-Glc) in presence of diimine ancillary ligands. Their crystal structures are reported. The copper(II) atom exhibits a 4+1 five-coordinate CuN2O2O' environment in 1, a 4+2 elongated rhombic octahedral CuN2O2O'(2) stercochemistry in 2 or 3, and a 4+1+1 tetragonally elongated octahedral Cu2N2O2O'O '' surrounding in 4 or 5. The ligand arrangement and the packing of 5 seem to be strongly influenced by intra- and intermolecular hydrogen bonding as well as pi,pi-stacking interactions between C-6-rings from adjacent phen ligands. The most striking feature now reported is the Cu-II-mediated oxidation of GLYO(2-), LACO(2-) or delta-Glc(2-) leading to the formation of corresponding Cu-II-oxalate derivatives. For the glycolate case a mechanism involving a cleavage of the alpha-C-H bonds is proposed, whereas for lactate or D-gluconate related systems an oxidative degradation through C-C bond cleavage are assumed.