Lead tetraacetate, oxidation reactions

For the reaction carried out in the presence of pyridine, a-deuteration did not affect the second-order rate constant significantly. However, small isotope effects of 1.19 and 1.21 were observed for the lead tetraacetate oxidation of PhCH(OD)COOD and... 

The pyridine-catalysed lead tetraacetate oxidation of benzyl alcohols shows a first-order dependence in Pb(OAc)4, pyridine and benzyl alcohol concentration. An even larger primary hydrogen kinetic isotope effect of 5.26 and a Hammett p value of —1.7 led Baneijee and Shanker187 to propose that benzaldehyde is formed by the two concurrent pathways shown in Schemes 40 and 41. Scheme 40 describes the hydride transfer mechanism consistent with the negative p value. In the slow step of the reaction, labilization of the Pb—O bond resulting from the coordination of pyridine occurs as the Ca—H bond is broken. The loss of Pb(OAc)2 completes the reaction with transfer of +OAc to an anion. 

Other methods for the generation of cation-radicals should also be mentioned. Mesoionic 5,5 -azinobis(l,3-diphenyltetrazole) and related mesoionic compounds give cation-radicals on oxidation by lead tetraacetate. The reaction proceeds in the presence of sodium tetrafluoroborate. The tetra-fluoroborate cation-radical salt is stable and can be purified by column chromatography on silica gel and stored in air for several months without appreciable decomposition (Araki et al. 1999). The cation-radicals of A,A -dimethyldiazines are produced by usual oxidation of the parent diazine... 

Lead tetraacetate oxidation of 3-substituted 1- and 2-aminoindazoles 6 and 7, has been shown by Rees to result in rapid and almost quantitative conversion into 4-substituted 1,2,3-benzotriazines (8, R = Me, OMe, Ph). 1,2,3-Benzotriazine (8, R = H) was prepared similarly, and for the first time, by carefully controlled oxidation of either 1- or 2-aminoindazole (6, 7, R = H). These remarkable ring expansion reactions, which apparently proceed via the formation of intermediate nitrenes, appear to be a direct and logical extension of original work by Bamberger and Goldberger in 1898 on the oxidative ring expansion of... 

Enantiomerically pure nitrile imines (211) have also been generated by the lead tetraacetate oxidation of aldehydo sugar p-nitrophenyl hydrazones. Reaction with methyl acrylate gave the pyrazolines as a 1 1 mixture of the (55) and (5R) epimers, which were resolvable in some cases (116). 

Oxepane (seven-membered) rings (1,6-anhydrohexitols)52 have been prepared from the 3,4-isopropylidene acetals of D-mannitol, D-glucitol, and L-iditol, by way of alkaline hydrolysis of the corresponding 1,2 5,6-dianhydrides. The ring structures of the products were established through periodate oxidation and lead tetraacetate oxidation the requisite amount of formic acid was produced, and 3 equivalents of lead tetraacetate were consumed. No inversions at any of the asymmetric centers were involved in the reactions conducted, so the oxepanes had retained the configurations of the starting hexitols. 

The Diels-Alder reaction of 3-methoxyfuran with octyl vinyl ketone took place at room temperature in quantitative yield to afford exclusively the endo cycloadduct (27) (81CC221). Reduction of the carbonyl group with lithium tri-r-butoxyaluminum hydride produced a single alcohol (28). Ozonolysis of the double bond followed by Jones oxidation yielded the lactone ester (29). Hydrolysis of the ester and lead tetraacetate oxidation gave the lactone acetate. This was converted by further hydrolysis and Jones oxidation to the bis-lactone (30), a known intermediate in the synthesis of ( )-isoavenaciolide (Scheme 6). 

II. Functionalization of the Angular Methyl Groups / 237 General principles of substitution / 237 Lead tetraacetate oxidations / 240 Hypohalite reactions / 246 Photolysis of nitrites (Barton reaction) / 253 A-Chloroamine reactions (Hoffmann-Lofller reaction) / 257 Ketone irradiations / 260 Summary / 264... 

R. A. Sheldon and J. K. Kochi, Oxidative Decarboxylation of Acids by Lead Tetraacetate, Organic Reactions 19, 279 (1972). 

Oxidation of primary N-aminobenzimidazole 32 with PhI(OAc)2 4 in the presence of olefins gives aziridines 34 [54]. Similar oxidations are effected by lead tetraacetate. The reaction was initially proposed to involve the intermediacy of AT-nitrene as a reactive species, thought to be produced through reductive a-elimination of amino-A3-iodane 33. Recent mechanistic studies on lead tetraacetate oxidation, however, suggests that the acetoxyamine 35 instead of AT-nitrene is the aziridinating species, and the reaction proceeds through a transition state 36 similar to that of epoxidation using peracids [55]. 

These various possibilities for oxidation-resistant 1,2-diols demonstrate the need for caution in the interpretation of the results of periodate and lead tetraacetate oxidation. As in the studies of D-glucosan < 1,4 > 0-< 1,6> and D-galactosana, supporting evidence based on other chemical reactions and the formation of derivatives should be obtained for the choice between absence of adjacent hydroxyl groups and presence of resistant 1,2-diols. 

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