Chiral tetranuclear and dinuclear copper(ii) complexes for TEMPO-mediated aerobic oxidation of alcohols: are four metal centres better than two?

Abstract

The one-pot reaction of 3,5-di-tert-butyl-2-hydroxybenzaldehyde, (R)-2-aminoglycinol and Cu(OAc)₂·2H₂O in a 1 : 1 : 1 ratio in the presence of triethylamine led to the isolation of X-ray quality crystals of the chiral complex (R)-1 in high yield. The single crystal structure of (R)-1 reveals a tetranuclear copper(II) complex that contains a {Cu₄(μ-O)₂(μ₃-O)₂N₄O₄} core. A reaction using (1S,2R)-2-amino-1,2-diphenylethanol as precursor under the same conditions generated the chiral complex (S,R)-2; its structure was determined by single crystal X-ray crystallography and was found to contain a {Cu₂(μ-O)₂N₂O₂} core. Both (R)-1 and (S,R)-2 have been used for catalytic aerobic oxidation of benzylic alcohols in combination with the TEMPO (2,2,6,6-tetramethylpiperidinyl-1-oxyl) radical. (R)-1 selectively catalyses the conversion of various aromatic primary alcohols to the corresponding aldehydes with high yields (99%) and TONs (770) in the air, while (S,R)-2 exhibits less promising catalytic performance under the same reaction conditions. The role of the cluster structures in (R)-1 and (S,R)-2 in controlling the reactivity towards aerobic oxidation reactions is discussed.