P-oxo dimer

In a limited amount of watei, the fiist pioduct of hydiolysis is the monohydioxy estei, which cannot be isolated because it immediately leacts with anothei alkyl titanate to give a p.-oxo dimer ... 

More recently 233) it has been reported that cross-linked polystyrene containing imidazole ligands did not provide a support rigid enough to prevent dimerization, and that the p-oxo dimer was benzene extracted from oxygenated tetraphenyl porphyrin iron(ll), Fe(TPP), which had been attached to the modified polystyrene. A discussion of model synthetic porphyrins, from which definitive structural and other physical data are obtained, is given in section V.C. 

The heterogeneous catalytic system iron phthalocyanine (7) immobilized on silica and tert-butyl hydroperoxide, TBHP, has been proposed for allylic oxidation reactions (10). This catalytic system has shown good activity in the oxidation of 2,3,6-trimethylphenol for the production of 1,4-trimethylbenzoquinone (yield > 80%), a vitamin E precursor (11), and in the oxidation of alkynes and propargylic alcohols to a,p-acetylenic ketones (yields > 60%) (12). A 43% yield of 2-cyclohexen-l-one was obtained (10) over the p-oxo dimeric form of iron tetrasulfophthalocyanine (7a) immobilized on silica using TBHP as oxidant and CH3CN as solvent however, the catalyst deactivated under reaction conditions. 

The ligand group can be introduced either on the meso or on the /5-pyrrole position of the porphyrin ring, but the synthesis of the meso-functionalized derivatives is easier and has been more widely exploited. Balch (50-53) reported that the insertion of trivalent ions such as Fe(III) (32) and Mn(III) (33) into octaethyl porphyrins functionalized at one meso position with a hydroxy group (oxophlorins) leads to the formation of a dimeric head-to-tail complex in solution (Fig. 11a) (50,51). An X-ray crystal structure was obtained for the analogous In(III) complex (34), and this confirmed the head-to-tail geometry that the authors inferred for the other dimers in solution (53) (Fig. lib). The dimers are stable in chloroform but open on addition of protic acids or pyridine (52). The Fe(III) octaethyloxophlorin dimer (52) is easily oxidized by silver salts. The one-electron oxidation is more favorable than for the corresponding monomer or p-oxo dimer, presumably because of the close interaction of the 7r-systems in the self-assembled dimer. 

Magnetic susceptibility measures the strength of interaction of a sample with a magnetic field. Information obtained includes identification of spin state and spin coupling—ferromagnetic or antiferromagnetic. An example would be identification of the Fe(III)-0-Fe(III) p-oxo dimer moiety. 

In the case of iron-containing small molecule analogs of Mb and Hb a much rockier road to successful model compounds was encountered. Even though the syntheses of iron porphyrin complexes were carried out in analogous manner to the cobalt species described above, their irreversible oxidation to the p-oxo dimer upon... 

Figure 4.14 Formation of p-oxo dimer. (Adapted with permission from Figure 9 of Suslick, K. S. Reinert, T. J. J. Chem. Ed., 1985, 62(11), 974—983. Copyright 1985, Division of Chemical Education, Inc.)...

A simple molecular orbital scheme showing why p-oxo dimer formation is more likely to happen for Fe(III), d5, than for Co(III), d6, is shown in Figure 4.15. For d6 Co(III), the last electron goes into a 71 antibonding orbital, making reactions 4.16 and 4.17 less likely. 

In summary, researchers have found a number of methods for avoiding p-oxo dimer formation and preserving a five-coordinate Fe(II) in iron-containing model compounds, through ... 

In alkaline solution (pH 11), the complex is present as a p-oxo dimer and ascorbic acid is fully deprotonated. In the absence of oxygen, kinetic traces show the reduction of Fe(III) to Fe(II) with a reaction time on the order of an hour at [H2A] =5xlO-3M. The product [Fen(TPPS)] is very sensitive to oxidation and is quickly transformed to Fe(III) when 02 is added. This leads to a specific induction period in the kinetic traces which increases with increasing [02]. The net result of the induction period is the catalytic two-electron autoxidation of ascorbic acid in accordance with the following kinetic model (23) ... 

The orthogonal pitching of the mesityl rings (Fig. 1.25) may inhibit formation of p-oxo dimers (likely to inhibit catalytic oxidations) which can be formed by flat porphyrin rings in their metal complexes, e.g. (TPP) ligands [583]. 

The remaining two hepta-coordinate structures of Fe(III) are p-oxo dimers. The first of them [(H20)BFe-0-FeB(H20)](C104)4 includes a macrocyclic ligand (Table I), and the other one [Fe20(N03)4(bpy)2] (bpy = 2,2 -bipyridine), bidentate ligands (100,101). The PBP geometry of the p-oxo-diiron(III) complexes is unusual since these complexes have square-pyramidal or octahedral structures with the exception of the above-mentioned two complexes. 

However, there is another potentially fatal haw in the binding of dioxygen by heme irreversible oxidation. If free heme in aqueous solution is exposed to dioxygen, it is converted almost immediately into a p.-oxo dimer known as hematin. The mechanism of this reaction has been worked out in detail.18 The reactions are as follows, where the heme group is symbolized by the circle about an iron atom. The first step is the binding of the dioxygen molecule, as in hemoglobin 19... 

Interestingly, the analogous tetra-p-tolyl porphyrinato complex, which is known to form a p-oxo dimer upon oxygenation, was inactive as an oxygenation catalyst. This led the authors to conclude that inhibition of p-oxo dimer formation, via steric hindrance from bulky substituents in the porphyrin ring, is essential for catalytic activity. 

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