Selective oxidation of [Rh-1(cod)](+) by H2O2 and O-2
de Bruin B, Brands JA, Donners JJJM, Donners MPJ, de Gelder , Smits JMM, Gal AW, Spek AL
CHEMISTRY-A EUROPEAN JOURNAL 5 (10): 2921-2936 OCT 1999

Abstract:
New, five-coordinate Z,Z-1,5-cyclooctadiene (cod) complexes ['N-3'Rh-1(cod)](+) have been structurally characterised by NMR spectroscopy and X-ray diffraction ('N-3' = tridentate cyclic triamine or podal pyridine-amine-pyridine ligand). Their electrochemical oxidation and their oxygenation by H2O2 and O-2 have been investigated. The sigma-donor capacity of ligand 'N-3' in ['N-3'Rh-1(cod)](+) strongly influences the electrochemical oxidation potential and the C-13 chemical shift of the cod double bond. The relative sigma-donor strength of the individual amine (N-amine(R)) and pyridine (N-Py) nitrogens in the pyridine-amine-pyridine ligands, N-amine(H) > N-Py > N-Py.(Me) > N-amine(Bu) congruent to N-amine(Bz), is largely determined by steric repulsions. The cod complexes are selectively oxygenated by H2O2, and in one case by O-2 to rhodium(III)oxabicyclononadiyl complexes which rearrange to rhodium(III)hydroxycyclooctenediyl complexes. Oxygenation of cod to an oxabicyclononadiyl fragment and subsequent rearrangement to a hydroxycyclooctenediyl fragment are both thought to proceed via a 2-rhodaoxetane intermediate. Oxygenation of ['N-3'Rh-1(cod)](+) by H2O2 is relatively independent of the ligand and the solvent, and proceeds instantaneously and selectively. Oxygenation of ['N-3'Rh-1(cod)](+) by O-2 is greatly influenced by both the ligand and the solvent. Entirely selective oxidation by O-2 could only be obtained for 'N-3'= N,N-di(2-pyridylmethyl)amine (BPA) in CH2Cl2. Oxygenation by O-2 in CH2Cl2 requires one mole of O-2 per mole of [(BPA)Rh-1(cod)](+), is catalysed by acid and is likely to proceed by mononuclear activation of dioxygen. For both the cyclic triamine ligands and the podal pyridine-amine-pyridine ligands, the cod complexes with the lowest oxidation potentials are the most reactive and the most selective in oxygenation by O-2. Oxidation of the analogous 1,5-hexadiene (hed) complexes ['N-3'Rh-1(hed)](+) by either H2O2 or O-2 results in elimination of hed.