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Acyloxyl radicals

Three different mechanisms of perester homolytic decay are known [3,4] splitting of the weakest O—O bond with the formation of alkoxyl and acyloxyl radicals, concerted fragmentation with simultaneous splitting of O—O and C—C(O) bonds [3,4], and some ortho-substituted benzoyl peresters are decomposed by the mechanism of decomposition with anchimeric assistance [3,4]. The rate constants of perester decomposition and values of e = k l2kd are collected in the Handbook of Radical Initiators [4]. The yield of cage reaction products increases with increasing viscosity of the solvent. [Pg.119]

Formation of both adduct 106, mesitylene adduct 109 and heterocycle 104 can be ascribed to homolysis (Scheme 21, pathway (i)). Decarboxylation of acyloxyl radical gives radicals, R , which either combine with the persistent free radical 102 giving 106 or, by abstracting hydrogen from solvent, give 3,5-dimethylbenzyl radicals, which combine with 102 to give 109. [Pg.91]

Table 6 Acyloxyl radical adducts formed in the oxidation of tetrabutylammonium carboxylates and PBN [3] by Ph4POsvCl6 in dichloromethane at 20°C. ... Table 6 Acyloxyl radical adducts formed in the oxidation of tetrabutylammonium carboxylates and PBN [3] by Ph4POsvCl6 in dichloromethane at 20°C. ...
Peroxides are used when the reaction requires a more reactive initiating species. Thermolysis of dibenzoyl peroxide [PhC(0)0—OC(0)Ph], with a ti/2 of 1 h at 95 °C and 7 h at 70 °C, is the most familiar to synthetic chemists. It initially produces acyloxyl radicals that often decarboxylate prior to undergoing bimolecular reactions and affording the equally reactive phenyl radicals. [Pg.52]

The last point also favours acyloxyl radical as the abstracting radical X in pathway (i). [Pg.892]

The persistence of alkoxamidyls (164) probably facilitates rebound capture of acyloxyl radicals in competition with their decarboxylation and hydrogen abstraction reactions. [Pg.892]

Some homolytic fragmentation reactions are driven by formation of small, stable molecules. Alkyl acyloxyl radicals (RCOp decarboxylate rapidly (fe > 1 x 10 s ) to give alkyl radicals, and even aryl acyloxyl radicals (ArCOp decarboxylate to aryl radicals with rate constants in the 10 s range." Azo radicals produced in the homolysis of azo initiators eliminate nitrogen rapidly. Elimination of carbon monoxide from acyl radicals occurs but is slow enough (fe 10" -10 such that the acyl radical can be trapped in a bimolecular process,... [Pg.152]

Pyridine-2-thione-A-oxycarbonyl (PTOC) derivatives of carboxylic esters 53 were developed by Barton et al. and serve as a convenient source of acyloxyl radicals, which upon decarboxylation provide specific routes to free radicals (equation 82). This process can also proceed by a radical addition (equation 83). Acyl selenides (54) are a convenient source of acyl radicals, which can undergo decarbonylation also giving specific free radicals (equation 84). ° ... [Pg.35]

Table 7.1. Rate of decay of some acyloxyl radicals. (Hilborn and Pincock 1991)... Table 7.1. Rate of decay of some acyloxyl radicals. (Hilborn and Pincock 1991)...
From the practical point of view, alkylation of heteroaromatics with Barton decarboxylation of A-acyloxy-2-thiopyridones (17), prepared from carboxylic acids and Af-hydroxy-2-thiopyridone, is very useful, since it can be used for various kinds of carboxylic acids such as sugars and nucleosides [23-26]. This reaction comprises of the initial homolytic cleavage of the N-0 bond in A-acyloxy-2-thiopyridone to form an acyloxyl radical and PyS , (3-cleavage of the acyloxyl radical to generate an alkyl radical and C02, addition to the electron-deficient position of heteroaromatics by the alkyl radical to form the adduct, and finally, abstraction of a hydrogen atom from the adduct by PyS , as shown in eq. 5.10. [Pg.163]

In contrast to intramolecular cyclizations, the intermolecular addition of O-centered radicals to ti systems as initiating step in complex radical cyclization cascades has only recently attracted considerable attention. The reason for the low number of papers published on O radical addition to alkenes and alkynes could originate from the perception that O-centered radicals, such as alkoxyl radicals, RO, or acyloxyl radicals, RC(0)0", may not react with 7t systems through addition at rates that are competitive to other pathways, for example, allylic hydrogen abstraction and p-fragmentation in the case of RO" or decarboxylation in the case of RC(0)0" (Scheme 2.9). [Pg.16]

Hydroxylation Using Flydroxyl or Acyloxyl Radicals or Cations. 104... [Pg.99]

Fig. 16) If weak allylic or benzylic C—H groups are present, a lactone product may form through an acyloxyl radical, while amino acids may go on to form imines or aldehydes (99,100). [Pg.20]

More recently, Kochi studied the decarboxylation of acyloxyl radicals generated by PET in charge transfer ion pairs between benzilate anions (Ar2C(0H)C02 ) and a cationic acceptor (methylviologen di-cation (MV +) or A-methyl-4-cyano-pyridinium (NCP+)) in aqueous solution (Scheme 70) [232, 233]. [Pg.1219]

To our knowledge, only one report exists in which the formation of carboxylic acid by radical carboxylation with carbon dioxide has been documented. Curran and co-workers observed the formation of 9-anthracenecarboxylic acid in 10% yield, together with 71% yield of anthracene, when the radical reduction of 9-io-doanthracene with the ethylene-spaced fluorous tin hydride was run using supercritical CO2 (90°C, 280 atm) as the reaction media (Scheme 4-41) [69], As demonstrated in this example, the CO2 trapping reaction by radicals is not an efficient process and therefore is of limited synthetic utility. The rate constant for the addition is yet to be determined, but kinetic studies to date indicate that generally the decarboxylation of acyloxyl radicals is a rapid process [70]. [Pg.118]

The only possible explanation for these last by-products and for chlorobenzene is the formation of the acyloxyl radical [Eq. (6.5)], which reacts according to Eqs. (6.9a)-(6.9d). [Pg.223]

OT-Chlorobenzoic acid was the main reaction product because it was formed by hydrogen abstraction from NHPI by the acyloxyl radical [Eq. (6.10)]. [Pg.224]

The fundamental steps in the PTOC-thiol method are illustrated in Fig. 4 [10]. The radical source is one of Barton s PTOC esters or a related thiohydroxamic acid derivative [11, 12] these are made from the corresponding carboxylic acids. The acyloxyl radicals produced in the initiation step rapidly decarboxylate to give the radical of interest. This radical reacts with a hydrogen atom donor, a thiol in this... [Pg.323]

This section is subdivided according to the molecularity of the process (es) examined. Strictly speaking, no absolute rates are known for any acyloxyl radical CIDNP measurements, included in Section 8.2.1 approach this goal even if they do involve time related assumptions. [Pg.125]


See other pages where Acyloxyl radicals is mentioned: [Pg.90]    [Pg.92]    [Pg.94]    [Pg.19]    [Pg.86]    [Pg.94]    [Pg.892]    [Pg.895]    [Pg.142]    [Pg.8]    [Pg.1220]    [Pg.40]    [Pg.429]    [Pg.324]    [Pg.372]    [Pg.357]    [Pg.125]    [Pg.125]    [Pg.126]    [Pg.126]    [Pg.127]    [Pg.128]    [Pg.129]    [Pg.130]    [Pg.131]   
See also in sourсe #XX -- [ Pg.323 ]




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Acyloxylation

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