Carboxyl radicals, decarboxylation

Esters of (V-hydroxyphthalimide can also be used for decarboxylation. Photolysis in the presence of an electron donor and a hydrogen atom donor leads to decarboxylation. Carboxyl radicals are formed by one-electron reduction of the phthalimide ring. 

Diacyl peroxides are sources of alkyl radicals because the carboxyl radicals that are intitially formed lose CO2 very rapidly. In the case of aroyl peroxides, products may be derived from the carboxyl radical or the radical formed by decarboxylation. The decomposition of peroxides can also be accomplished by photochemical excitation. 

The anodic oxidation of the carboxylate anion 1 of a carboxylate salt to yield an alkane 3 is known as the Kolbe electrolytic synthesis By decarboxylation alkyl radicals 2 are formed, which subsequently can dimerize to an alkane. The initial step is the transfer of an electron from the carboxylate anion 1 to the anode. The carboxyl radical species 4 thus formed decomposes by loss of carbon dioxide. The resulting alkyl radical 2 dimerizes to give the alkane 3 " ... 

The current-potential relationship indicates that the rate determining step for the Kolbe reaction in aqueous solution is most probably an irreversible 1 e-transfer to the carboxylate with simultaneous bond breaking leading to the alkyl radical and carbon dioxide [8]. However, also other rate determining steps have been proposed [10]. When the acyloxy radical is assumed as intermediate it would be very shortlived and decompose with a half life of t 10" to carbon dioxide and an alkyl radical [89]. From the thermochemical data it has been concluded that the rate of carbon dioxide elimination effects the product distribution. Olefin formation is assumed to be due to reaction of the carboxylate radical with the alkyl radical and the higher olefin ratio for propionate and butyrate is argued to be the result of the slower decarboxylation of these carboxylates [90]. 

Stepwise decarboxylation also occurs, particularly in reactions in which the carboxylate radical (RC02 ) is formed. This radical usually decomposes to a hydrocarbon radical (R-) and C02- The overall decarboxylation product is determined by what R- reacts with If a good hydrogen donor is present, RH is formed if a halogen donor such as Br2 is present, RBr is formed ... 

Another method involves electrolysis of sodium or potassium carboxylate solutions, known as Kolbe electrolysis, in which carboxylate radicals are formed by transfer of an electron from the carboxylate ion to the anode. Decarboxylation may occur simultaneously with, or subsequent to, the formation of carboxylate radicals, leading to hydrocarbon radicals, which subsequently dimerize ... 

There is some competing decarboxylation of the ethanoic acid, but the conversions in this kind of reaction are usually good. The key steps in the reaction probably are exchange of carboxylic acid groups on tetravalent lead, cleavage of the Pb-O bond to give the carboxylate radical, decarboxylation, oxidation... 

Diacyl peroxides are another important source of free-radicals and, consequently, are also commonly used as initiators of free-radical reactions. There is a vast amount of data pertaining to the kinetics and mechanism of decomposition of these compounds in conventional solvents there are a number of side reactions, both radical and ionic in nature, that complicate the kinetics of their decomposition. Generally, these compounds decompose by initial 0-0 bond cleavage that generates carboxyl radicals (RC02 ), which subsequently decarboxylate yielding R (Scheme 4.7)... 

The utilization of perfluorodiacyl peroxides for this purpose has been more widely developed. The rate of decomposition of perfluorodiacyl peroxides in the presence of electron-rich benzene derivatives is enhanced by a significant factor via a process of electron-transfer [66, 280], As can be seen by the contrasting examples below [281], highly reactive arenes are capable of trapping the per-fluoroalkyl carboxyl radical before it decarboxylates to RF, a result which can diminish the synthetic utility of this process. 

The mode of cation formation in electrochemical decarboxylation appears not to be uniform. Skcll 204 found two discrete 1 e-steps for oxidation of car-boxylates by chronopotentiometry. He attributed the second electron transfer to oxidation of the alkyl radical (path b, Eq. (94) ) as the carboxylate radical RC02 is to shortlived (r 10 10 sec) to survive for the second oxidation step. 

In flavonoids acylated with aliphatic acids, the most common acids are acetic and malonic. In the MS fragmentation of the dicarboxylic acids (as malonic acid), a first loss of 44 mass units is observed (loss of the carboxylic radical, CO2), and this is due to decarboxylation. 

The main theme of this chapter is the cleavage of alkyK aryl- and vinyl-caiboxyl single bonds by means of the fragmentation (decarboxylation) of carboxyl radicals (equation 1). The fia entation of... 

The majority of reactions discussed are readily applicable to simple primary, secondary and tertiary aliphatic acids. The decarboxylation of aryl- and vinyl-carboxyl radicals is a much more difficult process which limits the application of many of the methods described to aliphatic acids. As such, particular attention is drawn in the text to examples of aryl and vinyl decarboxylations. 

A wide variety of thiophilic radicals induce fragmentation of 0-acyl thlohydroxamates by addition to the thiocarbonyl group, and in doing so generate carboxyl radicals (Scheme 1). After decarboxylation the... 

Alkyl radicals derived by decarboxylation of carboxyl radicals may be added m carbon-carbon multiple bonds resulting in an overall homologation of the starting acid. This reaction type is not stricdy a C— bond oxidation nevertheless, one of the key steps is C- bond cleavage by decarboxylation and it is appropriate to briefly consider the scope of such reactions here. A more complete description of inter- and intra-molecular radical C—C bond-forming reactions is given in Volume 4, Chapters 4.1 and... 

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