Copper Complexes in the Promotion of Aldol Addition to Pyridine-2-carboxaldehyde: Synthesis of Homo- and Heteroleptic Complexes and Stereoselective Double Aldol Addition.

CuCl2·2H2O and Cu(ClO4)2·6H2O are able to promote aldol addition of pyridine-2-carboxaldehyde (pyca) with acetone, acetophenone, or cyclohexenone under neutral and mild conditions. The general and simple one-pot procedure for the aldol addition to Cu(II) complexes accesses novel Cu complexes with a large variety of different structural motifs, from which the aldol-addition ligand can be liberated by treatment with NH3. Neutral heteroleptic complexes in which the ligand acts as bidentate, or homoleptic cationic complexes in which the ligand acts as tridentate can be obtained depending on the copper salt used. The key step in these reactions is the coordination of pyca to copper, which increases the electrophilic character of the aldehyde, with Cu(ClO4)2 leading to a higher degree of activation than CuCl2, as predicted by DFT calculations. A regio- and stereoselective double aldol addition of pyca in the reaction of Cu(ClO4)2·6H2O with acetone leads to the formation of a dimer copper complex in which the novel double aldol addition product acts as a pentadentate ligand. A possible mechanism is discussed. The work is supported by extensive crystallographic studies.