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

The one-pot reaction of 3,5-di-tert-butyl-2-hydroxybenzaldehyde, (R)-2-aminoglycinol and Cu(OAc)(2)-2H(2)O 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(s) complex that contains a {Cu-4(mu-O)(2)(mu(3)-O)(2)N4O4) core. A reaction using (1S,2R)-2-amino-1,2-diphenylethanot 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 {Cu-2(mu-O)(2)N2O2] 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.