Oxygen scrambling

We have examined the competing isomerization and solvolysis reactions of 1-4-(methylphenyl)ethyl pentafluorobenzoate with two goals in mind (1) We wanted to use the increased sensitivity of modern analytical methods to extend oxygen-18 scrambling studies to mostly aqueous solutions, where we have obtained extensive data for nucleophilic substitution reactions of 1-phenylethyl derivatives. (2) We were interested in comparing the first-order rate constant for internal return of a carbocation-carboxylate anion pair with the corresponding second-order rate constant for the bimolecular combination of the same carbocation with a carboxylate anion, in order to examine the effect of aqueous solvation of free carboxylate anions on their reactivity toward addition to carbocations. 

For the oxygen migration test for oxirene participation to be valid, it must be shown (80ZN(B)1040) that intermolecular oxygen transfer does not occur, and that oxygen scrambling... 

Photolysis of the peroxide 62 gives the same product with complete oxygen scrambling 64, presumably via a free radical intermediate (equation 97). 

Table 6. Oxygen scrambling and racemization results for p-substituted benzhydryl p-nitro-benzoates (54 )76)...

Identical reactivity of and could result from rapid oxygen scrambling ... 

It should be noted that the aforementioned conclusions are based on the results of transient state experiments, that they contain the implicit assumption that the total surface coverage in reactive intermediates was close to unity (essential only for the kinetic analysis), and that the inference that rapid oxygen scrambling (Reaction 42b) does not take place is crucial for the interpretation of the results (59, 65). 

To understamd such chemical properties of the benzoyloxy radical as the rates of decarboxylation and of oxygen scrambling one should know its electronic structure. We had hoped by studying the epr spectra of pairs including this radical to determine its ground state experimentally. 

At low temperatures unstable adsorption products or reaction intermediates could be trapped. Thus, carbonite CO, ions arise on CO interaction with basic oxygen ions which account for catalytic reaction of isotopic scrambling of CO or thiophene on activated CaO. 

Two techniques, electrochemical reduction (section IIl-C) and Clem-mensen reduction (section ITI-D), have previously been recommended for the direct reduction of isolated ketones to hydrocarbons. Since the applicability of these methods is limited to compounds which can withstand strongly acidic reaction conditions or to cases where isotope scrambling is not a problem, it is desirable to provide milder alternative procedures. Two of the methods discussed in this section, desulfurization of mercaptal derivatives with deuterated Raney nickel (section IV-A) and metal deuteride reduction of tosylhydrazone derivatives (section IV-B), permit the replacement of a carbonyl oxygen by deuterium under neutral or alkaline conditions. 

Three different methods have been discussed previously (sections III-C,III-D and IV-A) for the replacement of a carbonyl oxygen by two deuteriums. However, in the conversion of a 3-keto steroid into the corresponding 3,3-d2 labeled analog, two of the three methods, electrochemical reduction (section ni-C) and Raney nickel desulfurization of mercaptal derivatives (section IV-A), lead to extensive deuterium scrambling and the third method, Clemmensen reduction (section III-D), yields a 2,2,3,3,4,4-dg derivative. 

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