Isotope effects with oxygen

The results of Litt and Nickon (27) show small but reportedly significant differences in isotope effects with different photosensitizers and in chemical oxygenation. Further study is obviously desirable. The formation of a sensitizer-oxygen complex which dissociates before reacting 13, 14,17, 23, 34) would be entirely consistent with our results. 

The mechanism of aliphatic hydroxylation by cytochrome monoxygenases would appear to Involve a direct oxygen-insertion with retention of configuration. For exan le, when S-(+)-l- H-ethylbenzene was incubated with liver microsomes, the 1-phenylethanol produced in the reaction consisted of 92% of the R isomer and only 87. of the S isomer.83 pheno-barbital pretreatment diminished the stereoselectivity of this reaction. 63, 64 ihe reaction of the a a-dideutero-derivative exhibited an isotope effect of 1.8 when compared to the undeuterated substrate, consistent with an oxygen-insertion mechanism. In certain examples of aliphatic hydroxylation, however, it appears from the absence of an isotope effect with labeled substrates that the insertion reaction is not the rate-limiting step.85-67... 

Dediazoniation does not show a significant solvent isotope effect ( h2o/ d2o = 0.98 0.01 Crossley et al., 1940 Swain et al., 1975 a). This result is definitely not consistent with a mechanism in which charge is built up on oxygen in the rate-limiting transition state, as expected for an ANDN-like process. 

The oxidations of formic acid by Co(III) and V(V) are straightforward, being first-order with respect to both oxidant and substrate and acid-inverse and slightly acid-catalysed respectively. The primary kinetic isotope effects are l.Sj (25°C)forCo(IU)and4.1 (61.5 C°)for V(V). The low value for Co(lII) is analogous to those for Co(IIl) oxidations of secondary alcohols, formaldehyde and m-nitrobenzaldehyde vide supra). A djo/ h20 for the Co(III) oxidation is about 1.0, which is curiously high for an acid-inverse reaction . The mechanisms clearly parallel those for oxidation of alcohols (p. 376) where Rj and R2 become doubly bonded oxygen. 

One-step hydroxylation of aromatic nucleus with nitrous oxide (N2O) is among recently discovered organic reactions. A high eflSciency of FeZSM-5 zeolites in this reaction relates to a pronounced biomimetic-type activity of iron complexes stabilized in ZSM-5 matrix. N2O decomposition on these complexes produces particular atomic oj gen form (a-oxygen), whose chemistry is similar to that performed by the active oxygen of enzyme monooxygenases. Room temperature oxidation reactions of a-oxygen as well as the data on the kinetic isotope effect and Moessbauer spectroscopy show FeZSM-5 zeolite to be a successfiil biomimetic model. 

Relatively detailed study has been done for the reaction pathways over Au/Ti02 catalysts mainly because of simplicity in catalytic material components. The rate of PO formation at temperatures around 323 K does not depend on the partial pressure of C3H6 up to 20vol% and then decreases with an increase, while it increases monotonously with the partial pressure of O2 and H2 [57]. A kinetic isotope effect of H2 and D2 was also observed [63]. These rate dependencies indicate that active oxygen species are formed by the reaction of O2 and H2 and that this reaction is rate-determining [57,63,64]. 

The reaction proceeds via a cyclic TS involving coordination of both the alcohol and ketone oxygens to the aluminum. Computational (DFT) and isotope effect studies are consistent with the cyclic mechanism.190 Hydride donation usually takes place from... 

Reactions of eh with H and OH were once considered diffusion-controlled see, however, Elliot et al. (1990). The rate constants, 2.5—3.0 x 1010 M-1s 1 (see Table 6.6), are high. In both cases, a vacancy exists in the partially filled orbitals of the reactants into which the electron can jump. Thus, hydrogen formation by the reaction eh + H may be visualized in two steps (Hart and Anbar, 1970) eh + H—H, followed by H + H20— OH" This reaction has no isotope effect, which is consistent with the proposed mechanism. The rate of reaction with OH is obtained from the eh decay curve at pH 10.5 in the absence of dissolved hydrogen or oxygen, where computer analysis is required to take into account some residual reactions. At higher pH (>13), OH exists as O- and the rate of eh + O—"02 has been measured as 2.2 x 1010 M-1s-1. 

This is the same mechanism as that given above for esters, in equation (42). The difference between esters and amides is apparent from a comparison of the two tetrahedral intermediates [5] and [17], The former contains three oxygens, any of which can be protonated, resulting in much lsO exchange being observed when the reaction takes place in 180-enriched water,275,276 but [17] contains a much more basic nitrogen, which will be protonated preferentially and lead to much less 180 exchange, as observed.274 277,278 Also, ammonium ion formation makes the overall reaction irreversible, unlike ester hydrolysis. The calculated solvent isotope effect for the Scheme 15 process is 1.00,280 exactly in accord with experimental observation.278,279... 

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