Selective Aromatic Hydroxylation with Dioxygen and Simple Copper Imine Complexes

The formation of a bis(-oxido)dicopper complex with the ligand 2-(diethylaminoethyl)-6-phenylpyridine (PPN) and its subsequent hydroxylation of the pendant phenyl group (studied earlier by Holland etal., Angew. Chem. Int. Ed. 1999, 38, 1139-1142) has been reinvestigated to gain a better understanding of such systems in view of the development of new synthetic applications. To this end, we prepared a simple copper imine complex system that also affords selective o-hydroxylation of aromatic aldehydes by using dioxygen as the oxidant: Applying the ligand N-benzylidene-N,N-diethylethylenediamine (BDED), salicylaldehyde was prepared in good yields and we show that this reaction also occurs through an intermediate bis--oxido copper complex. The underlying reaction mechanism for the PPN-supported complex was studied at the BLYP-D/TZVP level of density functional theory and the results for representative stationary points along reaction paths of the BDED-supported complex reveal a closely related mechanistic scenario. The results demonstrate a new facile synthetic way to introduce OH groups into aromatic aldehydes.