Isotopic oxidation studies

In 1965, the Dubna workers found a longer-lived lawrencium isotope, 256Lr, with a half-life of 35 s. In 1968, Thiorso and associates at Berkeley used a few atoms of this isotope to study the oxidation behavior of lawrencium. Using solvent extraction techniques and working very rapidly, they extracted lawrencium ions from a buffered aqueous solution into an organic solvent — completing each extraction in about 30 s. 

Thus we shall be concerned with properties that furnish information about the nature of the ligands, the oxidation state of the metal, and the geometry of the field of ligands. Techniques such as radio-isotope tracer studies, neutron-activation analysis, and electron microscopy are powerful methods for locating a metal within constituents of the cell and are particularly suited to heavy-metal rather than organic drugs but since they do not provide information about the chemical environment of the metal they will not concern us here. After each section below we shall give an example, not necessarily from platinum chemistry, where the method has been used with success in biochemistry. 

The complexities associated with these reactions were also evident during an intrazeolite product isotope effect study of 2,2,6,6-tetradeutero-l,4-dithiane97 (Fig. 29). The absence of an isotope effect during oxidation with m-chloroperbenzoic is consistent with the lack of C-H(D) abstraction on the reaction surface. The substantial isotope effect of 1.15 + 0.02 for the reaction of singlet oxygen is consistent with the... 

A methodology that has turned out to be a very powerful tool in trying to unravel the intricacies of the mechanism of cytochrome P450-catalyzed oxidation reactions has been the use of deuterium isotope effects. The use of intramolecular deuterium isotope effects have been particularly important in this regard as will be described in chapter 4 where a number of such studies are presented. But, before describing the specific technique that intramolecular isotope effect studies entail, a quick mini review on the nature of deuterium isotope effects is probably in order. 

Ye, R. W., Toro-Suarez, 1., Tiedje, J. M., and Averill, B. A. (1991). H2 0 isotope exchange studies on the mechanism of reduction of nitric oxide and nitrite to nitrous oxide by denitrifying bacteria. J. Biol. Chem. 266, 12848-12851. 

I-Ph, or LNiIH-0-NiIUL) have been proposed as the active oxidant (92). In the reaction, E olefins are more reactive than the corresponding Z isomers, and a strong correlation was observed between the electron-donating effect of the para substituents in styrene and the initial reaction rate (91). Isotope labeling studies have shown that the epoxide oxygen is derived from PhIO. 

The use of isotopic tracers has demonstrated that the selective oxidation of propylene proceeds via the formation of a symmetrical allyl species. Probably the most convincing evidence is presented by the isotopic tracer studies utilizing, 4C-labeled propylene and deuterated propylene. Adams and Jennings 14, 15) studied the oxidation of propylene at 450°C over bismuth molybdate and cuprous oxide catalysts. The reactant propylene was labeled with deuterium in various positions. They analyzed their results in terms of a kinetic isotope effect, which is defined by the probability of a deuterium atom being abstracted relative to that of a hydrogen atom. Letting z = kD/kH represent this relative discrimination probability, the reaction paths shown in Fig. 1 were found to be applicable to the oxidation of 1—C3He—3d and 1—QH —1 d. 

Deuterium tracer and isotope effect study of Mn(lll)-based oxidative free-radical cyclizations... 

A simple Co(m) complex (32) whose structure closely resembles those of the above-mentioned enzymes (29-31) has been developed [95], lsO labeling studies showed that an oxide group that is bridging the two metal centers cleaves the bridging phosphate monoester in 32 (Figure 6.21). Kinetic isotope effect studies found that p-nitrophenyl... 

Since the uranyl ion is so obviously its own category, it is very interesting to compare with the analogous species formed by transuranium elements. M = Np, Pu and Am form all three MO 72 and MO which are, by no means, the most stable oxidation states of their elements, and which tend toward reduction by the radiochemical products concomitant with the high specific radioactivity of the isotopes normally studied of plutonium and americium (whereas e.g. 244Pu with the half-life 82 million years would not present this problem). Contrary to some reports in literature, it does not seem that curium (and the subsequent elements) form such dioxo complexes. 

In an inner sphere process, the coordination sphere of one complex is substitute by a ligand bound to the other complex which then acts as a bridge and may be transferred during the redox process. For example, isotopic labelling studies show that to oxidation of aqueous Cr2+ with [Co(III)(NH3)5C1]2+ proceeds via a bridges species Cr CI Co, the chlorine not exchanging with free labelled Cl in solution but remaining attached to the kinetically inert Cr(III) product. 

Attempts to use molecular oxygen as the oxidant failed except in solvents that undergo efficient autoxidation to the corresponding hydroperoxide (e.g., THF). Mechanistic studies, including isotopic labeling studies, indicate that fBuOOH is the source of the oxygen atom incorporated into the product, and the reaction proceeds via a hydride-shift pathway that avoids formation of an enol intermediate (Scheme 12). 

Evidence has been presented for both these mechanistic postulates. Such evidence has come from kinetic and structural data as well as isotope effect studies. Theoretical calculations aimed at explaining the observed kinetic isotope effects support the (3+2) cycloaddition mechanism. Two questions, however, remain to be answered. The mechanistic details of oxidation of an alkene by 0s04 in the absence of L have not been studied in great detail. Also the changes, if any, that are effected by the introduction of L to such a mechanism are not known. 

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