Radicals decarboxylation

Radical Decarboxylation Barton esters AJdrichirnica Acta 1987, 20 (2), 35... 

The extent of decarboxylation primarily depends on temperature, pressure, and the stabihty of the incipient R- radical. The more stable the R- radical, the faster and more extensive the decarboxylation. With many diacyl peroxides, decarboxylation and oxygen—oxygen bond scission occur simultaneously in the transition state. Acyloxy radicals are known to form initially only from diacetyl peroxide and from dibenzoyl peroxides (because of the relative instabihties of the corresponding methyl and phenyl radicals formed upon decarboxylation). Diacyl peroxides derived from non-a-branched carboxyhc acids, eg, dilauroyl peroxide, may also initially form acyloxy radical pairs however, these acyloxy radicals decarboxylate very rapidly and the initiating radicals are expected to be alkyl radicals. Diacyl peroxides are also susceptible to induced decompositions ... 

Mercuric carboxylates, which decarboxylate by a chain mechanism when initiated by peroxides, also decarboxylate under UV irradiation (123,128,129,131-140,142,144-146,153-155). In addition, decarboxylation was observed for mercuric benzoate and mercuric a-naphthoate (123). Side reactions [Eqs. (24), (25), (109)] observed in peroxide initiated reactions also occurred on UV irradiation, and mercurous salt formation [Eq.(24)] was more extensive under the latter conditions. Decarboxylation giving methylmercuric acetate occurred on irradiation of mercuric acetate in aqueous solution and is considered to be of environmental significance (156,157). Stepwise decarboxylation giving (CF3)2Hg occurred on irradiation of solid mercuric trifluoroacetate at -196° C (158), but, at 20° C, trifluoromethyl radicals diffused from the solid and dimerized (158). No other diorganomercurial has been formed by radical decarboxylation, and the reaction is not preparatively competitive with the thermal decarboxylation synthesis of (CF3)2Hg (26,27) (Section III,A). 

Birch reduction-alkylation of 5 with 2-bromoethyl acetate was carried out with complete facial selectivity to give 57. This tetrafunctional intermediate was converted to the bicyclic iodolactone 58 ( > 99% ee) from which the radical cyclization substrate 59 was prepared. The key radical cyclization occurred with complete regio- and facial-selectivity and subsequent stereoselective reduction of the resulting tertiary radical gave 60 with the required trans BC ring fusion.The allylic alcohol rmit of (+)-lycorine was obtained by a photochemical radical decarboxylation, 62 63. 

Radical decarboxylation via the corresponding thiocarbonyl derivatives of the carboxylic acids. 

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... 

The use of hypervalent iodine reagents in carbon-carbon bond forming reactions is summarized with particular emphasis on applications in organic synthesis. The most important recent methods involve the radical decarboxylative alkylation of organic substrates with [bis(acyloxy)iodo]arenes, spirocyclization of para- and ortho-substituted phenols, the intramolecular oxidative coupling of phenol ethers, and the reactions of iodonium salts and ylides. A significant recent research activity is centered in the area of the transition metal-mediated coupling reactions of the alkenyl-, aryl-, and alkynyliodonium salts. 

Radical Decarboxylative Alkylation with [Bis(acyloxy)iodo]arenes 101... 

Recent laser flash photolysis (LFP) studies have provided absolute rates of addition of perfluoro-n-alkyl radicals to a variety of alkenes in solution [ 114,115]. In these studies, C2Fj, C3F7, and n-C7F,5 were generated instantaneously by photolysis of the respective diacyl peroxides. The initially-formed perfluoroacyloxyl radicals decarboxylated rapidly to yield the perfluoroalkyl radicals, after which the additions of these radicals to styrene, a-methylstyrene, etc. were monitored directly via observation of the growth of UV absorption due to the transient benzylic radicals. 

The radical decarboxylation of a Barton ester proceeds to the corresponding alkane after treatment with tributyltin hydride or t-butylmercaptan ... 

The use of the hypervalent iodine reagent [bis(trifluoroacetoxy)iodo]benzene has been reported to be effective in the synthesis of C-nucleoside-like compounds. Radical decarboxylation of a suitably protected uronic acid, initiated photochem-ically, followed by addition of a heterocyclic base provided the C-nucleoside in high yield.154 The mode of action involves initial radical formation of 122 (Scheme 33), followed by introduction of the base and radical coupling.155 The anomeric selectivity was high in some examples, and low in others—lepidine gave the highest proportion of the ( anomer. Isolated yields were poor to moderate. 

Carbonate and carbamate derivatives, prepared from the reaction of alcohols and amines with phosgene and /V-hydroxy-2-thiopyridone, provide the corresponding alkoxyl radicals and aminyl radicals respectively, via radical decarboxylation [71-81]. For example, photolytic treatment of carbamate (57) derived from 4-pentenylamine generates the corresponding 4-pentenylaminyl radical, as shown in eq. 8.24. Under neutral conditions in the presence of tert-BuSH, a direct reduction product (A) of 4-pentenylaminyl radical is formed, while, in acidic conditions, cyclized product (C), pyrrolidine, via 5-exo-trig manner from 4-pentenylaminyl radical is formed. Under the latter conditions, the real reactive species is an electrophilic 4-pentenylaminium radical which rapidly cyclized via 5-exo-trig manner. 

Generally, potassium persulfate in the presence of Ag+ is used for the Hunsdiecker type radical decarboxylation of carboxylic acids in water. (Bu4N+)2S208 (P) is soluble in THF, and a sulfate anion radical [i] is formed under refluxing conditions. Thus, refluxing treatment of / (tetrabutylammonium) persulfate (P) in the presence of alcohol in THF provides tetrahydrofuryl-protected alcohol (4), through the abstraction of a-H from THF by sulfate anion radical [I], followed by oxidation to a tetrahydrofuryl cation, as shown in eq.12.3 [28]. 

Radical decarboxylative alkylation of heteroaromatic bases mediated by [bis(acyloxy)iodo]arenes... 

Efficient preparative sequences involving radical decarboxylation followed by carbon-nitrogen bond formation are rare. Acyl nitrates decompose at elevated temperatures to give nitroalkanes (equation... 

Hexenyl radicals cyclize to cyclopentylmethyl radicals (see Volume 4, Chapter 4.2). Thus radical decarboxylation of 6-heptenoic acids, by whatever means, usually results in die formation of five-mem-v beied rings. Although this fact had been appreciated previously it is only recendy, widi the advent of the 0-acyl thiohydroxamates, that it has been exploited from a syndietic point of view. An example is provided by the synthesis of bicyclo[4.3.0]proline derivatives from aspartic acid carried out by the Barton group (equation 51). It will be noted that activation of die C—C double bond acting as a radical trap is not necessary in these intramolecular reactions. 

Barton, D.H.R., Herve, Y, Potier, R, and Thiery, J. 1984. Reductive radical decarboxylation of amino-acids and peptides. J Chem Soc Chem Commun 1298-1299. 

Efficient preparative sequences involving radical decarboxylation followed by carbon-nitrogen bond formation are rare. Acyl nitrates decompose at elevated temperatures to give nitroalkanes (equation 46), but are unfortunately explosive and have to be prepared in situ and stored in solution. A noteworthy exception is found in the thermal or photochemical decarboxylation of tetrahydro-l,2-oxazine-3,6-diones leading to -lactams (equation 47). Doubtless a key factor in this reaction, considered to proceed via a radical cage mechanism, is the intramolecular nature of the carbon-nitrogen bond formation. 

---