Radicals perbenzoate

Scheme 15. Cyclic diagram for free-radical perbenzoic decomposition in cyclohexane...

An example of this reaction is the reaction of cyclohexene with t-butyl perbenzoate, which is mediated by Cu(I). " The initial step is the reductive cleavage of the perester. The t-butoxy radical then abstracts hydrogen from cyclohexene to give an allylic radical. The radical is oxidized by Cu(II) to the carbocation, which captures benzoate ion. The net effect is an allylic oxidation. 

The allylic position of olefins is subject to attack by free radicals with the consequent formation of stable allylic free radicals. This fact is utilized in many substitution reactions at the allylic position (cf. Chapter 6, Section III). The procedure given here employs f-butyl perbenzoate, which reacts with cuprous ion to liberate /-butoxy radical, the chain reaction initiator. The outcome of the reaction, which has general applicability, is the introduction of a benzoyloxy group in the allylic position. 

The oxidation is initiated (a) by Fe3 to yield the benzoyl radical (99) which adds on a molecule of oxygen to form the perbenzoate radical... 

It should, however, be emphasized that the relaxation from (X ) -l-i to X (+) does not always require a negligible energy. Counterexamples where the homolytic cleavage of an ion radical appears to be endowed with a sizable internal reorganization energy can be found in the electrochemical reduction of perbenzoates,60 where the observation of a transition between a concerted and a stepwise mechanism fits with an exothermic cleavage of the anion radicals. 

Having a weak O—O bond, peroxides split easily into free radicals. In addition to homolytic reactions, peroxides can participate in heterolytic reactions also, for example, they can undergo hydrolysis under the catalytic action of acids. Both homolytic and heterolytic reactions can occur simultaneously. For example, perbenzoates decompose into free radicals and simultaneously isomerize to ester [11]. The para-substituent slightly influences the rate constants of homolytic splitting of perester. The rate constant of heterolytic isomerization, by contrast, strongly depends on the nature of the para-substituent. Polar solvent accelerates the heterolytic isomerization. Isomerization reaction was proposed to proceed through the cyclic transition state [11]. 

The higher the viscosity of the solvent, the longer the period of time of radical pair existence in the cage and the higher the observed value of scrambling rate constant [3,80-82]. The same phenomenon was observed during the photolysis of benzoyl peroxide and 1,1-dimethylethyl perbenzoate [3]. 

The study of benzaldehyde and cyclohaxanone co-oxidation showed the formation of s-caprolactone as the main product of cyclohexanone oxidation [5]. Cyclohexanone was found not to react practically with peroxyl radicals under mild conditions. The oxidation of benzaldehyde produces perbenzoic acid. The latter oxidizes the benzaldehyde to benzoic acid and cyclohexanone to s-caprolactone. 

Catalysis is demonstrated by the process that the radicals are generated by the oxidized form of the catalyst in the reaction with aldehyde, and the reduced form of the catalyst is rapidly oxidized by perbenzoic acid formed in the chain reaction. Data on the catalytic oxidation of aldehydes of different structures are found in Refs. [50,51]. 

The mechanism of the Kharasch-Sosnovsky reaction remains unclear. The generally accepted version, as proposed by Kochi and co-workers (94-96) and later improved by Beckwith and Zavitsos (97), is illustrated in Scheme 8. Cuprous ion reduces the perbenzoate to Cu(II)OBz (Bz = benzoyl) and free /-BuO radical. The radical abstracts an allylic hydrogen atom generating an allyl radical that combines with the cupric salt to form an allylcopper(III) species. Reductive elimination with... 

Sodium sulfonate 1 has previously been prepared from NaHS03 and vinyltrimethylsilane using sodium nitrite/sodium nitrate as the radical initiator.3 In the submitters hands this protocol resulted in salt 1 as a pale tan powder in only 15-53% yield if 50% (v/v) aqueous methanol is employed as solvent. The yield of 1 could be increased to 63% if 22% (v/v) aqueous methanol is employed. An advantage of this method is the elimination of a potentially explosive perester as radical initiator. However, lower yields of 1 and the subsequent lower yield of the sulfonyl chloride 2 (53% for the sulfonylation, 35% overall from vinyltrimethylsilane) make this procedure less desirable than the method presented. The use of tert-butyl perbenzoate as the radical initiator4 not only provides 1 in a higher yield, but the subsequent conversion to 2 also proceeds in better yield. 

The conversion of benzaldehyde in the presence of air to benzoic acid was reported in 1832 by Wohler and Liebig, and in 1900 Baeyer and Villiger proposed perbenzoic acid as an intermediate in the reaction. The currently accepted free radical chain mechanism for the process was proposed by Backstrom in 1934 (equation 34). Bates and Spence already in 1931 had proposed that photolysis of CH3I forming CHs in the presence of O2 led to peroxyl radicals CHaOO-. ... 

The oxidation is initiated (a) by Fe to yield the benzoyl radical (99) which adds on a molecule of oxygen to form the perbenzoate radical (1(X)), this reacts with benzaldehyde (97) to yield perbenzoic acid (101) and another benzoyl radical (99)—these two steps constituting the chain reaction (b). The actual end-product is not perbenzoic acid (101), however, as this undergoes a rapid acid-catalysed, non-radical reaction (c) with more benzaldehyde (97) to yield benzoic acid (98). This latter reaction (c), being acid-catalysed, speeds up as the concentration of product benzoic acid (98) builds up, i.e. it is autocatalytic. That benzoyl radicals (99) are involved is borne out by the observation that carrying out the reaction at higher temperatures ( 100°), and at low oxygen concentrations, results in the formation of CO, i.e. by PhCO-> Ph- + CO. 

Diarylpyrylium ions also are dehydrogenating agents.232,250 Thus 2,2 6,6 -diaryl-4,4 -bis-4//-pyrans 163 were dehydrogenated by such agents to 4,4 -dipyranylidene derivatives 164 by a one-electron mechanism involving radical cation intermediates like 378.232 2,2 4,4 6,6 -Hexamethyl derivative 163a was oxidized with perbenzoic acid, but the reaction products were not identified.218... 

When nonactivated peracids such as peracetic acid or perbenzoic acid are used in oxidation of alkanes, the formation of methane and benzene, respectively, as well as evolution of C02 are observed. This indicates a radical mechanism 68,78... 

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