Lead tetraacetate oxidative decarboxylation

The most common P-substituted model porphyrin is octaethylporphyrin, which is available by a Knorr pyrrole synthesis, followed by hydrogenation of a P-acetyl group, lead tetraacetate oxidation of the a-methyl group, acid-catalyzed decarboxylation, and subsequent cyclization and dehydrogenation (Scheme 6.3.4). Octaethylporphyrin can thus be made routinely on a 10-g scale within 3 weeks (Inhoffen et al., 1966a). 

One-electron oxidation of carboxylate ions generates acyloxy radicals, which undergo decarboxylation. Such electron-transfer reactions can be effected by strong one-electron oxidants, such as Mn(HI), Ag(II), Ce(IV), and Pb(IV) These metal ions are also capable of oxidizing the radical intermediate, so the products are those expected from carbocations. The oxidative decarboxylation by Pb(IV) in the presence of halide salts leads to alkyl halides. For example, oxidation of pentanoic acid with lead tetraacetate in the presence of lithium chloride gives 1-chlorobutane in 71% yield ... 

The Cg-amine, originally obtained by the methanolysis of kasugamycin, on treatment with lead tetraacetate or sodium periodate afforded a nitrile amine, with evolution of carbon dioxide, showing a maximum at 2200 cm.-1. This reaction is explained only by the structure (13). The -N-C=N group of the product can be formed by oxidative decarboxylation and can be easily rationalized by the present understanding of such reagents (2, 13) as shown below. On the other hand, the treatment... 

A cursory inspection of key intermediate 8 (see Scheme 1) reveals that it possesses both vicinal and remote stereochemical relationships. To cope with the stereochemical challenge posed by this intermediate and to enhance overall efficiency, a convergent approach featuring the union of optically active intermediates 18 and 19 was adopted. Scheme 5a illustrates the synthesis of intermediate 18. Thus, oxidative cleavage of the trisubstituted olefin of (/ )-citronellic acid benzyl ester (28) with ozone, followed by oxidative workup with Jones reagent, affords a carboxylic acid which can be oxidatively decarboxylated to 29 with lead tetraacetate and copper(n) acetate. Saponification of the benzyl ester in 29 with potassium hydroxide provides an unsaturated carboxylic acid which undergoes smooth conversion to trans iodolactone 30 on treatment with iodine in acetonitrile at -15 °C (89% yield from 29).24 The diastereoselectivity of the thermodynamically controlled iodolacto-nization reaction is approximately 20 1 in favor of the more stable trans iodolactone 30. 

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. 

The decarboxylation of carboxylate anions is carried out chemically by a variety of one-electron oxidants such as lead tetraacetate, uranyl nitrate, peroxides, quinones, pyridinium cations, etc.199 Importantly, the carboxylate anion (as... 

Many of the early reports of spin-trapping experiments were focused on mechanistic investigations, and some of these feature in the early reviews (see p. 4). Unfortunately, it is in this application that inferences drawn may be most suspect. For example, the inability of the method to differentiate between radical trapping on the one hand, and a combination of nucleophile trapping with one-electron oxidation on the other, is a serious shortcoming. An early example of this was the tentative conclusion that acetoxyl radicals were spin-trapped by PBN competitively with their decarboxylation in reactions of lead tetraacetate. In view of the rapidity of the decarboxylation reaction, trapping of acetate ion and subsequent oxidation seems a likely alternative. 

Periodic acid reacts well in aqueous solution. Usually, if the reactant has to be run in organic solvents, lead tetraacetate is used as the reagent. Interestingly, periodic acid will not act on a-keto acids or a-hydroxy acids whereas lead tetraacetate wiU. The corresponding reactions are actually oxidative decarboxylations. 

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

Anodic dehydrogenations, e.g., oxidations of alcohols to ketones, have been treated in Sect. 8.1 and formation of olefins by anodic elimination of C02 and H+ from carboxylic acids was covered in Sect. 9.1. Therefore this section is only concerned with anodic bisdecarboxylations of v/odicarboxylic acids to olefins. This method gives usually good results when its chemical equivalent, the lead tetraacetate decarboxylation, fails. Combination of bisdecarboxylation with the Diels-Alder reaction or [2.2] -photosensitized cycloadditions provides useful synthetic sequences, since in this way the equivalent of acetylene can be introduced in cycloadditions. 

The rest of the synthesis (Scheme 13) is completely stereospecific and most of the steps are known (20). The bicyclic acid was oxidatively decarboxylated with lead tetraacetate and copper acetate (21). The resulting enone was alkylated with methyllithium giving a single crystalline allylic tertiary alcohol. This compound was cleaved with osmium tetroxide and sodium periodate. Inverse addition of the Wittig reagent effected methylenation in 85% yield. Finally, the acid was reduced with lithium aluminum hydride to grandisol. 

OXIDATIVE DECARBOXYLATION Lead tetraacetate. Sodium hypochoorite. 

Oxidative decarboxylation. Oxidation of [n.2.2]propellanecarboxylic acids (1) with lead tetraacetate in pyridine at 80 gives bicyclic acetates 2 and/or tricyclic acetates 3. The latter products are converted into 2 on vapor-phase thermolysis."... 

Another widely used decarboxylation procedure involves the use of lead tetraacetate. Depending on the nature of the substrate and the reaction conditions, this reagent may transform a carboxylic acid into an alkane or alkene, or into the respective acetoxy derivative (Scheme 2.144). The most favorable conditions for alkane formation utilize a good hydrogen donor as the solvent. Usually this transformation is carried out as a photochemically induced oxidative decarboxylation in chloroform solution, as is exemplified in the conversion of cyclobutanecarboxylic acid in cyclobutane.In contrast, the predominant formation of alkenes occurs in the presence of co-oxidants such as copper acetate. ... 

In the case of vicinal dicarboxylic acids, the interaction with lead tetraacetate in the presence of co-oxidants (O2 or Cu " ) invariably leads to the formation of an alkene. The decarboxylation of vicinal dicarboxylic acids is an especially... 

Oxidative deearboxyUttian of acids (1, 554-557 2, 235-237 3, 168-169). The oxidative decarboxylation of acids by lead tetraacetate has been reviewed by Sheldon and Kochi. ... 

The 1,4-dihydrobenzoic acids derived from reductive alkylation may undergo facile rearomatization with either loss of the carboxylic acid group or the alkyl group. The gibberellin synthesis intermediate (82), for example, was found to be especially labile, forming (83) simply on exposure to air." Oxidative decarboxylation may be deliberately achieved with lead tetraacetate or electrochemically." Loss of the 1-alkyl group is likely to be a problem when the alkyl moiety can form a reasonably stable free radical, since a chain reaction may then be sustained." ... 

Dihydrooxazoles such as 263 have also been prepared <2004T7591> via an oxidative decarboxylation and elimination of the corresponding oxazolidine -carboxylic acid 262 (Scheme 77). The oxazolidine-4-carboxylic acids were in turn derived from L-serine. The thermal oxidative decarboxylation using lead tetraacetate was reported to be higher yielding and more practical than the analogous electrochemical version. Dihydrooxazoles 263 have been extensively used as chiral olefmic components in cycloaddition reactions and these reactions are discussed in Section 4.04.6.2.1. 

Oxidative decarboxylation of cyclopropane derivative 679 by lead tetraacetate in the presence of copper salts mainly gives tricyclic compounds (680) in low yields (equation 240) . 

Sheldon, R. A., Kochi, J. K. Oxidative decarboxylation of acids by lead tetraacetate. Org. React. 1972,19, 279-421. 

---