Bridged Peroxides

Bridged Peroxides. - EMscussions of the mechanism and solvent effects 

Studies on endoperoxide analogues of prostaglandins continue,and the paths that are followed on decomposition of bicyclic peroxides such as (132) in the presence of palladium(O) and ruthenium(ll) have been investigated, as has the breakdown of (133) in the presence of PPha. The thermal decomposition of (134) leads to (135) when = Me but gives 

Bridged Peroxides.—Routine procedures have been extended to the synthesis and characterization of photo-adducts of singlet oxygen with cyclo-octa-1,3,5-triene, cycloheptatrienes, 3,5-cycloheptadienone, cyclohepta-1,3-dienes, ° naphthalenes, anthracenes, a-pyrones, pyrazines and pyrimidines,imidazoles, and indenes. Transformation of some of these 

Rigaudy, A. Defoin, and J. Baranne-Lafont, Angew. Chem., Int. Ed. Engl., 1979, 18, 413. 

Interest in the prostaglandin endoperoxides continues to serve as a focus for synthetic studies. The formation of bicyclic peroxides by halide displacement from a monocyclic hydroperoxide proceeds in moderate yield. A full paper reports on an alternative procedure whereby a monocyclic diol can be closed, again in moderate yield, to a bicyclic peroxide by reaction with bis(tri-n-butyltin) peroxide. 

We also note studies concerning effects of magnetic fields on the kinetics of decomposition of endoperoxides, solvent effects on the decomposition of peroxides, and the acid-catalysed decomposition of ozonides.  

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It is possible that steric factors prevent the formation of the bridging peroxide species in the case of vitamin Bj2,. (55), however, other cases have been reported (59) in which oxygenation proceeds no further than the formation of mononuclear peroxo radicals (i.e. first stage above). 

A similar type of oxygen complex has been observed during the oxidation of [Con(CN) s]-3 but it was not possible to show that this species was formed in the initial reaction step since with this system, as with the cobaloxime(II) system, the 1 1 adduct apparently reacts very rapidly with another molecule of pentacyanocobaltate(II) to form a diamagnetic binuclear complex with a bridging peroxide ligand 116). It appears that in the Bi2-system the bulk of the corrin ring does not allow formation of the diamagnetic binuclear complex. 

That a bridged peroxide 68 (for a more detailed discussion, see 1>, p. 84) arises from the intermediate open-chain peroxide formed either by recombination of hydrazide radical ion with. 02( > radical ion, such as 66, or by nucleophilic attack of ion on a carbonyl group of a diazaquinone, such as 67, appears to be very plausible. One of the most important experimental reasons for this assumption has been put forward by E. H. White and M. M. Bursey 106> who, on oxidation of luminol with... 

The light output is visually distinctly lower than that of a dia-zaquinone, or a hydrazide of comparable structure. A bridged peroxide is not possible as intermediate for steric reasons. 

Thus maleic acid forms from the hydroquinone and oxalic acid forms from pyrocatechol. However, the intermediate compounds are triplets, so the intermediate steps are spin-resistant and may not proceed in the manner indicated. The intermediate maleic acid and oxalic acid are experimentally detected in this low-temperature oxidation process. Although many of the intermediates were detected in low-temperature oxidation studies, Benson [59] determined that the ceiling temperature for bridging peroxide molecules formed from aromatics was of the order of 300°C that is, the reverse of reaction (3.128) was favored at higher temperatures. 

Weatherburn and co-workers prepared the first and only trinuclear Mn3 complex, [Mn3O(OAc)2(O2)(dien)3]l3 H2O 0.33CH3OH (104), which has a bridging peroxide group (triangular c. Figure 21). This complex was prepared from the reaction of either Mn(OAc)2 or Mn(OAc)3 with dien in methanol under reflux. The Mn" Mn separations in (104) are 3.32(3) A and 3.14(4) A. 

It seems reasonable that the pathway for production of dioxygen would involve the formation of a bridged peroxide and superoxide. Oxidation could involve transient formation of Mn111. Various mechanisms have been put forward387 and reviewed.379,384... 

The fact that three peroxide-to-copper charge-transfer transitions are observed in oxyhemocyanin and oxytyrosinase led us to consider the spectral effects of bridging peroxide between two Cu(II) ions. A transition-dipole vector-coupling (TDVC) model was developed that predicts that each charge-transfer state in a Cu-peroxide monomer... 

It was of critical importance to evaluate experimentally this unusual electronic structure description for the side-on bridged peroxide and its relation to the spectral features of the p-rt2 rj2 model complex and oxyhemocyanin. This evaluation was accomplished through a series of... 

Fig. 14. Bridging peroxide mechanism. Fe and Cu represent Fe and Cub in the O2 reduction site, respectively. N represents the nitrogen of the fifth ligand of heme 33. The shaded rectangles represent a side view of the porphyrin plane.

Moenne-Loccoz, P., Krebs, C., Herlihy, K., Edmondson, D. E., Theil, E. C., Huynh, B. H., and Loehr, T., 1999, The ferroxidase reaction of ferritin reveals a diferric p-1,2 bridging peroxide intermediate in common with other 02-activating non-heme diiron proteins,... 

FIGURE 2. The bridging peroxide mechanism. Shaded rectangles denote porphyrin planes. Imidazole nitrogen, Fe,3 and Cub are shown by N, Fe and Cu respectively. 

Solomon et alf° have suggested that the peroxide is bound to a single copper at the binuclear type 3 site, and so diflers from the bridging peroxide found for hemocyanin and tyrosinase (Figure 57). Comparison with a laccase from which the type 2 copper has been removed has led to the suggestion that the type 2 copper is necessary to stabilize the type 3 Cu-hydroperoxide complex. The type 2 and type 3 centres appear to be close together, as rapid-quench ESR experiments with reduced laccase and have shown that the thus formed is bound equatorially at the... 

Thamann, Loehr and Loehr73) continued to examine the 0—0 stretch using unsymmetrically labelled 02 (1602 + 1802 + 160 180) by the resonance Raman method. They found three Raman peaks at 749,728 and 708 cm-1 for the labelled molluscan oxyhemocyanin and the peak intensities were the same as observed for the 02 gas mixture used. That requires that both oxygen atoms be equivalent in the bound form and strongly supports a bridging peroxide between the two Cu (II) ions. A terminal symmetrical peroxide (a) would be expected to have an... 

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