Metal-porphyrin peroxo complex

The low trans selectivity and increased propensity for 1,4-m addition is thought to arise from a direct interaction with the metal-porphyrin peroxo complex similar to that suggested for the 2,5-dimethoxyphenyl derivatives shown in Scheme 8-13. It was demonstrated that the 2,5-dimethoxyphenyl derivatives worked in the aerobic oxidation, even without p-benzoquinone being present [64], Interestingly, in this case the 1,4-cis addition product predominates. It was proposed that the (jr-allyl)palladium complex is activated as shown in Scheme 8-13. 

Deprotonated peroxide coordinates to metals in low- and intermediate-oxidation states tend to bind side-on and display nucleophilic properties. For example, iron(III) porphyrin-peroxo complexes are nucleophilic they did not transfer an oxygen atom to electron-rich substrates (such as electron-rich olefins), but brought about epoxidation of electron-poor olefins or oxidative deformylation of aldehydes.130 Dinuclear... 

Secondly, there is an indication that metal(III)-peroxo side-on complexes are in general in equilibrium with corresponding metal(II)-superoxo end-on species. The position of such equilibrium could depend on various factors as structural and electronic properties of the porphyrin ligand, coordination of an axial ligand trans to peroxide/superoxide, solvent medium, temperature and involvement of coordinated peroxide/superoxide in possible hydrogen bonding or electrostatic interactions. These are interesting questions which should be addressed in future studies. 

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. 

Miksztal, A.R. and J.S. Valentine (1984). Reactivity of the peroxo hgand in metal-loporphyrin complxes Reaction of sulfur dioxide with iron and titanium porphyrin peroxo complxes to give sulfato complexes or sulfate. Inorg. Chem. 23(22), 3548-3552. 

The 2nd-row transition metal porphyrin complexes of Rh(II) and Ru(II) also react with O2 but the resulting species are usually quite different in character. Rhodium octaethyl- and tetraphenylporphyrin oxygen adducts are like the Co analogues with an end-on superoxide however, the octaethylporphyrin (OEP) species is only stable below -80°C and on warming to 20° C converts to a Rh(in) p.-peroxo complex, analogous to the Co systems . ... 

In specific examples, a preliminary coordination of the ligand to the metal is required before addition of the hydrogen peroxide solution. This is the case of cyclopentadienyl Nb, or Ti and Mo porphyrins derivatives , or for the dinuclear Ti(IV) peroxo citrate complexes . ... 

This type of complex is derived from the mononuclear superoxo species via a further one-electron reduction of the dioxygen moiety. Cobalt is the only metal to form these complexes by reaction with dioxygen in the absence of a ligating porphyrin ring. Molybdenum and zirconium form peroxo-bridged complexes on reaction with hydrogen peroxide. In most cases the mononuclear dioxygen adducts of cobalt will react further to form the binuclear species unless specific steps are taken to prevent this. 

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