Dioxygen Exclusion

Hydroxylation and dioxygenation are not, however, mntnally exclusive, because the toluene dioxygenase from Pseudomonasputida FI hydroxylates both phenol and 2,5-dichloro-phenol with the introduction of only one atom of oxygen (Spain et al. 1989). Snbsequent degradation by dioxygenation follows established pathways in which chloride is eliminated from muconic acids after ring fission. 

The stereochemistry of dioxygen oxidation has been studied for the di-f-butyldimesityldisilene (3).93 Oxidation of (E)-3 produced ( )-59a, ( > 60a, and ultimately ( )-61a exclusively, showing that all the steps in the sequence are stereospecific with retention of configuration. (Similar oxidation of a mixture of ( > and (Z)-3 gave isomeric mixtures of 59a, 60a, and 61a having the same proportions of stereoisomers as in the starting material.) Oxidation of 3 was found to be stereospecific both in solution and in the solid state. 

The model shown in Scheme 2 indicates that a change in the formal oxidation state of the metal is not necessarily required during the catalytic reaction. This raises a fundamental question. Does the metal ion have to possess specific redox properties in order to be an efficient catalyst A definite answer to this question cannot be given. Nevertheless, catalytic autoxidation reactions have been reported almost exclusively with metal ions which are susceptible to redox reactions under ambient conditions. This is a strong indication that intramolecular electron transfer occurs within the MS"+ and/or MS-O2 precursor complexes. Partial oxidation or reduction of the metal center obviously alters the electronic structure of the substrate and/or dioxygen. In a few cases, direct spectroscopic or other evidence was reported to prove such an internal charge transfer process. This electronic distortion is most likely necessary to activate the substrate and/or dioxygen before the actual electron transfer takes place. For a few systems where deviations from this pattern were found, the presence of trace amounts of catalytically active impurities are suspected to be the cause. In other words, the catalytic effect is due to the impurity and not to the bulk metal ion in these cases. 

An obvious test of such a mechanism is use of standard free-radical traps (p-benzoquinone, TEMPO) indeed they block the reaction which, to us, also indicates that the reaction is not exclusively interfacial, but occurs also in the solution. We believe that the role of the nickel (II) centers is joint coordination of the sacrificial aldehyde and of dioxygen in the initiation step. 

Recently, the cobalt(II)-tetrasulfonatophthalocyanine system was reinvestigated for its catalytic activity while intercalated into a Mg5Al2 -layered double hydroxide. The intercalate exhibited catalytic properties in the activation of atmospheric dioxygen for the oxidation of a thiolate to a disulfide (97a) and for the oxidation of 2,6-di-tert-butylbenzene to (nearly exclusively) the 2,6,2, 6 -tetra-tert-butyldiphe-noquinone (97b). In marked contrast to the results reported for the homogeneous catalyst, this intercalated catalyst remained active for... 

Stereo- and regio-selective epoxidation of tricyclic trienes, such as (36), followed by Lewis acid-catalysed opening of the resulting monoepoxide with alcohols, has been reported to occur exclusively at the vinyl terminus of unsaturated system through a typical S 2 process, affording 1,6-dioxygenated derivatives (e.g. 37).49... 

Previously,(37) the introduction of a 6-Me substituent had shown that the 3,4-dioxygenated morphinan was the exclusive product, rather than the yield being dominated by the 2,3-isomer as in the 6-unsubstituted benzyl derivatives. The ketai of 22 (R=CHO) was brominated with N-bromoacetamide (NBA)... 

In the presence of excess HOOH, the Fe (MeCN)4 + catalyst forms a reactive adduct (14) that reacts with diphenylbenzofuran, arylalkenes, 9,10-diphenylanthracene, or rubrene to form exclusively dioxygenated products (Table 9). Such reactivities parallel those of dioxygenases with this group of substrates. 

These reactions are not mutually exclusive, however, and both may operate in sequence. Indeed, monooxygenation may, in some cases, be the result of dioxygenation followed by elimination. A review of the metabolism of aromatic compounds with N, O, and S hetero atoms has been given (Neilson and Allard 1998). 

---