Group VIII metals peroxo complexes

As shown by Table 3, most of the Group VIII metal-peroxo complexes are obtained from the direct interaction of dioxygen with the corresponding reduced forms. A considerable effort has been devoted to this subject in the last decade with the hope that selective oxidations of hydrocarbons could be achieved by the activation of molecular oxygen under mild conditions12,56 133,184 and several such examples have actually been shown to occur. 

With the notable exception of rhodium, Group VIII metal-peroxo complexes are generally reluctant to react with simple alkenes by nonradical pathways. However, such an oxygen transfer has been shown to occur in the reaction of 180-labeled [(AsPh3)4Rh02]+C104" with terminal alkenes under 02-free, anhydrous conditions, producing lsO-labeled methyl ketone (equation 52).131... 

Stable five-membered dioxametallacyclic adducts are obtained from the reaction of Group VIII metal-peroxo complexes (Rh, Ir, Pd, R) with nucleophilic ketones and electrophilic alkenes or ketones. The adducts of R02(PPh3)2 with acetone and l,l-dicyano-2-methylpropene have been characterized by X-ray crystallography. The reaction of (Ph3P)2R02 with nucleophilic ketones involves, as the major pathway, the precoordination of the carbonyl compound to the vacant axial site of platinum prior to intramolecular 1,3-dipolar cycloaddition (equation 42). ... 

Dioxygen binds to metal porphyrins in the three expected modes, i.e. u-superoxo, peroxo and bridging peroxo. In contrast to die simple complexes discussed previously, dioxygen coordination occurs with a wide range of transition metals from titanium and niobium through to the Group VIII metals. 

Despite the different mode of preparation and the greater influence of the metal environment in the case of Group VIII dioxygen adducts than in the case of early d° metal-peroxo complexes, no distinction can be made between these two types of dioxygen complex. In fact, several peroxo complexes can be obtained from both oxygen sources (equations 334 and 435,36). 

These complexes can exist in a triangular peroxo form (7a) for early d° transition metals, or in a bridged (7b) or linear (7c) form for Group VIII metals. They can be obtained from the reaction of alkyl hydroperoxides with transition metal complexes (equations 9 and 10),42-46 from the insertion of 02 into a cobalt-carbon bond (equation ll),43 from the alkylation of a platinum-peroxo complex (equation 12),44 or from the reaction of a cobalt-superoxo complex with a substituted phenol (equation 13).45 Some well-characterized alkylperoxo complexes are shown (22-24). 

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