Lead tetraacetate mechanism

The mechanisms of oxidation using bismuthate, periodate or lead tetraacetate, while still not completely understood, are probably similar, involving some type of cyclic ester formation as the first step. ... 

Thermal decomposition of lead tetraacetate gives rise to methyl radicals, again through the initial formation of acetoxy radicals. " An ionic mechanism for the decomposition has also been postulated, and it is possible that both mechanisms may occur, depending on the conditions. 

A possible mechanism for the formation of the furanones 6 and 7 is illustrated in Scheme 2. The initial alkoxy radical generated from the alcohol 5 and lead tetraacetate (LTA) undergoes /3-scission to produce the acyl radical intermediate 9. Subsequent cyclization to 10 proceeds through attack of the radical at the carbonyl oxygen. The resulting Pb(IV) intermediate 11 finally collapses via the reductive... 

However, the cyclic-intermediate mechanism cannot account for all glycol oxidations, since some glycols that cannot form such an ester (e.g., 12) are nevertheless cleaved by lead tetraacetate... 

The mechanism with lead tetraacetate is generally accepted to be of the free-radical type. First, there is an interchange of ester groups ... 

Compounds containing carboxyl groups on adjacent carbons (succinic acid derivatives) can be bisdecarboxylated with lead tetraacetate in the presence of O2 263 jjjg reaction is of wide scope. The elimination is stereoselective, but not stereospecific (both meso- and dl- 2,3-diphenylsuccinic acid gave trans- stilbene) a concerted mechanism is thus unlikely. The following mechanism is not inconsistent with the data ... 

A. endo-7-Iodomethylbicyclo[3.3.1]nonan-3-one. A 2-1., threenecked, round-bottomed flask equipped with an eflicient mechanical stirrer and a reflux condenser is charged with 600 ml. of dry ben2 ene (Note 1). The flask is immersed in a water bath, stirring is initiated, and 58.3 g. (0.132 mole) of lead tetraacetate (Note 2), 37.4 g. (0.147 mole) of iodine, and 10.0 g. (0.066 mole) of 1-adamantanol (Note 3) are added (Note 4). The bath temperature is gradually raised to 80° over a 20-minute period and is then allowed to cool to 70-75°. Stirring is continued for 2 hours at 70-75° (Note 5) and for an additional hour while the mixtiu e is cooled to room temperature. The inorganic salts are filtered and carefully washed with five 50-ml. portions of ethyl ether. The... 

A solution of lead tetraacetate in pyridine rapidly oxidises the most recalcitrant trans-diols, especially if a considerable excess of oxidant (3-4 moles) is used, implying yet a further mechanism for the action of this versatile oxidant ... 

Carboxylic acids are oxidized by lead tetraacetate. Decarboxylation occurs and the product may be an alkene, alkane or acetate ester, or under modified conditions a halide. A free radical mechanism operates and the product composition depends on the fate of the radical intermediate.267 The reaction is catalyzed by cupric salts, which function by oxidizing the intermediate radical to a carbocation (Step 3b in the mechanism). Cu(II) is more reactive than Pb(OAc)4 in this step. 

Banerjee and coworkers181-184 have been interested in elucidating the reaction mechanism of the oxidation of mandelic acid and its derivatives by lead tetraacetate [Pb(OAc)4]. 

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. 

Dihydrocorynantheine was obtained via similar steps from normal cyanoacetic ester 319 (172). Stereoselective transformation of the alio cyanoacetic ester 315 to the normal stereoisomer 319 was achieved by utilizing a unique epimerization reaction of the corresponding quinolizidine-enamine system (174). Oxidation of alio cyanoacetic ester 315 with lead tetraacetate in acetic acid medium, followed by treatment with base, yielded the cis-disubstituted enamine 317, which slowly isomerized to the trans isomer 318. It has been proved that this reversible eipmerization process occurs at C-15. The ratio of trans/cis enamines (318/317) is about 9 1. The sodium borohydride reduction of 318 furnished the desired cyanoacetic ester derivative 319 with normal stereo arrangement. The details of the C-15 epimerization mechanism are discussed by B rczai-Beke etal. (174). 

There have been examples of sonochemical switching in homogeneous reactions. The decomposition of lead tetraacetate in acetic acid in the presence of styrene at 50 °C generates a small quantity of diacetate via an ionic mechanism. Under otherwise identical conditions sonication of the mixture gives 1-phenylpropyl acetate predominantly through an intermediate methyl radical which adds to the double bond (Scheme 3.8) [55,56]. These results are in accord with the proposition that radical processes are favoured by sonication. 

B. 3 -Acetoxy-18-iodo- and 18-hydroxy-18,20 -oxido-5-pregnene. In a 5-1. three-necked flask fitted with a mechanical stirrer, a thermometer, and a reflux condenser are placed 31. of cyclohexane (Note 6), 180 g. ca. 0.37 mole) of commercial lead tetraacetate containing approximately 10% acetic acid (Note 7), 24 g. (0.095 mole) of iodine and 30 g. (0.083 mole) of 3j3-acetoxy-20j8-hydroxy-5-pregnene. The reaction mixture is stirred and heated to the boiling point by irradiation with a 1000-watt lamp (Note 8) from underneath. When the iodine color has disappeared (usually after about 60-90 minutes) (Note 9), the reaction mixture is cooled to room temperature, filtered with suction, and the... 

According to Ando et al. (2000), the sonolytic acetoxylation of styrene by lead tetraacetate follows the ion-radical mechanism. Lead tetraacetate was not subject to the sonication influence. The ultrasonic effect facilitates electron transfer from styrene (the nonmetallic donor) to lead tetraacetate. 

Oxidation of 3,5-diamino-4-arylhydrazonopyrazoles (802) with lead tetraacetate gives 2-aryl-4-cyano-2//-l,2,3-triazoles (803) in moderate yield (Equation (75)). A mechanism involving pyr-azolotriazole and/or nitrene intermediates is proposed <86JCS(P1)1379>. 

The oxidation of hydrazine derivatives with diethyl azodicarboxylate is of particular interest because it involves direct hydrogen abstraction. The oxidation of keto hydrazones with lead tetraacetate leads to azoacetates, presumably by a free radical mechanism. 

Phenylhydroperoxides 307a-h are converted into the corresponding 5-7-membered cyclic peroxides 308a-h, in variable yields, upon treatment with lead tetraacetate (Scheme 76)239 240. The best yields were observed for the endoperoxides 308d (76%) and 308e (66%)239. A plausible mechanism that may conform with the yield distribution of products 308 involves the transitory intermediates 309-311 (Scheme 76). Singlet... 

Olefins react with manganese(III) acetate to give 7-lactones.824 The mechanism is probably free-radical, involving addition of CH2COOH to the double bond. Lactone formation has also been accomplished by treatment of olefins with lead tetraacetate,825 with a-bromo carboxylic acids in the presence of benzoyl peroxide as catalyst,826 and with dialkyl malonates and iron(III) perchlorate Fe(C104)3-9H20.827 Olefins can also be converted to 7-lactones by indirect routes.828 OS VII, 400. 

Another is the oxidation of a secondary alcohol to a ketone (9-3), where A and B are alkyl or aryl groups and Z is also CrOjH. In the lead tetraacetate oxidation of glycols (9-7) the mechanism also follows this pattern, but the positive leaving group is carbon instead of hydrogen. It should be noted that the cleavage shown is an example of an E2 elimination. 

Lead tetraacetate oxidation of diethyl IV-alkylaminofumarates led to the formation of the unusual dihydropyrrolo[3,2-b ]pyrrole derivatives (343), along with the pyrroles (344) and pyridones (345). In addition, the oxidative dimers (346) and (347) were also isolated (Scheme 119). Although the pyrroles (344) and pyridones (345) can be derived from the dimers (346) and (347), the mechanism of formation of the pyrrolo[3,2-6]pyrroles from three moles of starting material is obscure (77CC854). 

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