Oxygen-18 Labelling

The reaction mechanism is supported by findings from experiments with 0-labeled benzophenone 6 after rearrangement, the labeled oxygen is found in the carbonyl group only ... 

Which bond of the ester is broken, the acyl—O or the alkyl—O bond The answer is found by the use of Hj O. If the acyl—O bond breaks, the labeled oxygen will appear in the acid otherwise it will be in the alcohol (see 10-10). Although neither compound is radioactive, the one that contains 0 can be determined by submitting both to mass spectrometry (MS). In a similar way, deuterium can be used as a label for hydrogen. In this case, it is not necessary to use mass spectrometry (MS), since IR and NMR spectra can be used to determine when deuterium has been substituted... 

The kinetic parameters are E = 6.3 kcal.mole" and AS = —38.4 eu, and at 25 °C the reaction exhibits a primary kinetic isotope effect of 6.6. When 0-labelled MnO was employed, no labelled oxygen appeared in the benzophenone. The mechanism involves abstraction of hydrogen, either as a hydride ion or a hydrogen atom, from the anion of the alcohol... 

The oxidation reactions were performed in a 25 mL ronnd bottom flask. In a typical reaction the catalyst (0.5 % Fe mol) was added to 0.125 M olefin solntion in acetone then dry TBHP (3.5 M in CH2CI2 or in PhCl) was added in one step, and the reaction mixture was stirred and heated in an oil bath at 40°C for 7 h. For the allyhc oxidation of cyclohexene with isotopically labelled oxygen ( 02) the following procedure was carried out the suspension of the catalyst (0.5% Fe mol) in cyclohexene (4 mL, 0.125 M) was frozen and the air in the reactor was evacuated and replaced by an oxygen (21% mol) - argon (79% mol) mixture. Then, the suspension was allowed to warm at room temperatnre and 1.3 mmol of degasified TBHP was added to the solution and the reaction mixtnre was stirred at 40°C for 3 h. 

The role of oxygen on the allyhc oxidation of cyclohexene over the FePcCli6-S/TBHP catalytic system was determined by using 2 labelled oxygen. Since more than 70% of the main cyclohexene oxidation products, 4,11, and 12, had labelled oxygen, we can assure that molecular oxygen acts as co-oxidant. However, under the reaction conditions the over-oxidation of 4 seems to be unavoidable. Labelled 2, 3- epoxy-l-cyclohexanone (13), 2-cyclohexen-l, 4-dione (14), and 4-hydroxy-2-cyclohexen-l-one (15) were detected as reaction products. 

Studies using labeled oxygen have shown that oxygen evolved during the reaction is a product of water oxidation and is not derived from carbon dioxide. The overall reaction of synthesis of glucose (C6H1206) can be written as [8,37,113,114]... 

Ozonolysis of styrene and ethylidenecyclohexane in the presence of [ 0]benzal-dehyde yields stable secondary ozonides incorporating 0. O NMR showed that labelled oxygen appeared as the ether oxygen, not the peroxo bridge, thus confirming the Criegee mechanism as opposed to the so-called unified concept. ... 

Relationships used in [ OJphosphate analysis to determine the probability that an enzyme s reaction with a phosphate containing m labeled oxygens in the presence of unlabeled water will result in the release of a phosphate that contains n labeled oxygens. 

S.H. Yee, K. Lee, P.A. Jerabek, P.T. Fox, Quantitative measurement of oxygen metabolic rate in the rat brain using microPET imaging of briefly inhaled 150-labelled oxygen gas, Nucl. Med. Commun. 27 (2006) 573-581. 

Mechanism A clearly predicts the formation of more double-labeled oxygen, in this case a 36/34 ratio of 0.0149, calculated from statistical considerations. Mechanism B, involving cage recombination of acetoxy radicals, predicts a more random distribution and a 36/34 ratio of 0.00828, closer to the experimentally determined ratio of 0.00835. 

For this mechanism to be confirmed by EPR, it is necessary to use 170-labeled oxygen to be certain that the Oad8 actually exchanges and this has been seen in only one case (212a) to date. Alternatively, such an exchange could involve an intermediate (0j-02) complex the possible existence of such complexes has been discussed in Section V,A. This may also be a major mechanism in the photo-induced exchange processes (see Section VI,C). 

In the majority of cases, the oxidizing species involved in the photooxidation reactions is not well known because the nature of the species is inferred from indirect experiments. The 02 ion has been invoked as the precursor of the oxidizing species in a number of reactions because it is often the only species observed at room temperature using EPR. However, a variety of other species such as O and OJ have been identified on surfaces when the low-temperature photoreactions are observed by EPR. In addition, O" formed on the surface may have a very short lifetime (as discussed in Ref. 1, p. 93) and can only be detected by its reactivity. With these points in mind, we conclude that O-, and particularly OJ in the presence of excess oxygen, may play a much more important role in photocatalysis than has yet been generally realized. In this connection, the paper of Kubokawa et al. (410) is of particular importance but the use of 170-labeled oxygen is necessary to confirm the nature of the species involved. In order to explore... 

M(OMe)5 undergoes peroxidation206 on treatment with H202. The use of labelled H20 indicated the fixation of the labeled oxygen between M and OMe. 

The allylic oxidation of propene is catalyzed by (compound) metal oxides, which essentially contain metal ions of variable valency. It is commonly accepted that a redox mechanism is operative in such a way that the catalyst acts as the oxidizer and that lattice oxygen is incorporated in the oxidation products. The assumptions have been proved for several catalysts by the analysis of cation valency changes and by experiments with labelled oxygen. 

With respect to the participation of lattice oxygen, some recent contributions concerning studies with labeled oxygen and experiments in the absence of gas phase oxygen must be mentioned. 

A second approach is to use [lsO]bicarbonate and to follow the incorporation of lsO into a carboxylated substrate. If C02 is the primary substrate only two labeled oxygen atoms enter the compound, whereas if HC03 is the reactant three are incorporated.287 A third technique is measurement of the rate of incorporation of C02 or bicarbonate in the carboxylated product. Over a short interval of time, e.g., 1 min, different kinetics will be observed for the incorporation of C02 and of bicarbonate.288 Using these methods, it was established that the product formed in Eq. 13-46 and the reactant in Eq. 13-47 is C02. However, the carboxylation enzymes considered in the next section use bicarbonate as the substrate. 

When [180]bicarbonate is a substrate, two labeled oxygen atoms enter the oxaloacetate, while the third appears in P . A concerted, cyclic mechanism could explain these results. However, study of kinetic isotope effects,291 use of a substrate with a chiral thio-phospho group,292 and additional lsO exchange studies293 have ruled out this possibility. A transient carboxyl phosphate (Eq. 13-53) is evidently an intermediate.294 295 The incorporation of the lsO from bicarbonate into phosphate is indicated by the asterisks. 

Figure 18-22 Some possible intermediates in the action of extradiol (left) and intradiol (right) aromatic dioxygenases. Although the steps depict the flow of pairs of electrons during the formation and reaction of peroxide intermediates, the mechanisms probably involve free radicals whose formation is initiated by 02. The asterisks show how two atoms of labeled oxygen can be incorporated into final products.

The isotopic 180 labelling experiments for the incorporation of oxygen into the epoxide, support mainly an Ag-O complex as the reactive species, because the incorporation of 180 labelled oxygen from H2180 into the epoxide is consistent with an independent Ag-O intermediate which undergoes isotopic exchange with 180 enriched water (ref. 10)... 

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