The chemistry of copper-dioxygen complexes is relevant
to copper
enzymes in biology as well as in (ligand)Cu–O2 (or
Cu2–O2) species utilized in oxidative
transformations. For overall energy considerations, as applicable
in chemical synthesis, it is beneficial to have an appropriate atom
economy; both O-atoms of O2(g) are transferred to the product(s).
However, examples of such dioxygenase-type chemistry are extremely
rare or not well documented. Herein, we report on nucleophilic oxidative
aldehyde deformylation reactivity by the peroxo-dicopper(II) species
[Cu2
II(BPMPO–)(O2
2–)]1+ {BPMPO-H = 2,6-bis{[(bis(2-pyridylmethyl)amino]methyl}-4-methylphenol)}
and [Cu2
II(XYLO–)(O2
2–)]1+ (XYLO– = a
BPMPO– analogue possessing bis(2-{2-pyridyl}ethyl)amine
chelating arms). Their dicopper(I) precursors are dioxygenase catalysts. The O2(g)-derived peroxo-dicopper(II) intermediates
react rapidly with aldehydes like 2-phenylpropionaldehyde (2-PPA)
and cyclohexanecarboxaldehyde (CCA) in 2-methyltetrahydrofuran at
−90 °C. Warming to room temperature (RT) followed by workup
results in good yields of formate (HC(O)O–) along
with ketones (acetophenone or cyclohexanone). Mechanistic investigation
shows that [Cu2
II(BPMPO–)(O2
2–)]1+ species initially reacts
reversibly with the aldehydes to form detectable dicopper(II) peroxyhemiacetal
intermediates, for which optical titrations provide the K
eq (at −90 °C) of 73.6 × 102 M–1 (2-PPA) and 10.4 × 102 M–1 (CCA). In the reaction of [Cu2
II(XYLO–)(O2
2–)]1+ with 2-PPA, product complexes characterized by single-crystal
X-ray crystallography are the anticipated dicopper(I) complex, [Cu2
I(XYLO–)]1+ plus a
mixed-valent Cu(I)Cu(II)-formate species. Formate was further identified
and confirmed by 1H NMR spectroscopy and electrospray ionization
mass spectrometry (ESI-MS) analysis. Using 18O2(g)-isotope labeling the reaction produced a high yield of 18-O incorporated
acetophenone as well as formate. The overall results signify that
true dioxygenase reactions have occurred, supported
by a thorough mechanistic investigation.
