O-centered radicals

Scheme 35 Barton carbonate PTOC-OMe, a radical chain transfer reagent able to convert a C-centered radical into an O-centered radical (Eq. 35.1) and a radical initiator (Eq. 35.2)...

Many O-centered radicals undergo facile P-fragmentation. For example, acyl-oxyl radicals which are intermediates in the electrolytic oxidation of acids (Kolbe electrolysis), rapidly decompose into alkyl radicals and C02 [reaction (1)]. The rate of these reactions is in the order of 109 s 1 and increases with increasing branching of the alkyl substituent, i.e., decreasing C-C02 bond energy (Table 7.1). 

Examples for frequently encountered intermediates in organic reactions are carbocations (carbenium ions, carbonium ions), carbanions, C-centered radicals, carbenes, O-centered radicals (hydroxyl, alkoxyl, peroxyl, superoxide anion radical etc.), nitrenes, N-centered radicals (aminium, iminium), arynes, to name but a few. Generally, with the exception of so-called persistent radicals which are stabilized by special steric or resonance effects, most radicals belong to the class of reactive intermediates. 

Cascade Reactions Initiated by Addition of O-Centered Radicals to Alkynes (Self-Terminating Radical Oxygenations)... 

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). 

However, oxidations based on addition of O-centered radicals to unsaturated compounds appear to be a highly desirable synthetic goal, especially when the new C—O bond could be formed under the mild conditions that are typical for radical reactions. If this radical addition would involve C = C triple bonds, the resulting reactive vinyl radical would be highly suitable for the promotion of intramolecular cascade reactions. 

TABLE 2.1 Representative Results for the Reaction of Cyclodecyne (52) with Various O-Centered Radicals XO ... 

The hydrogen atom on the a-carbon is particularly prone to abstraction by O-centered radicals [189], According to the co-reactant, a carbonyl compound and either alcohol or carboxylic acid are formed besides the nitroxide. None of the co-reactants yielded peroxide proposed originally [199] for pathway /, Scheme 17. The mechanism exemplified in Scheme 20 was confirmed using labeled compounds [196,197]. 

Table 29.3.2 (continued) Heats of Formation of O-Centered Radicals... 

Figure 6.115 Competition between O-centered radical cyclizatlon onto a silyl enol ether vs. a terminal alkene.

This whole domino sequence becomes a chain reaction, which is not absolutely required for a domino reaction. For efficiently functionahzing domino products, individual rearrangements must be faster than termination of combination, disproportionation, and redox reactions of R , and faster than their reactions with the solvent, precursor, and initiator molecules. Also, the final intermediate R"" should react selectively with S while the rest of the radical species do not Hence, if the final rearrangement results in changes in polarity or reactivity of the propagating radical species, the generation of versatile O-centered radicals or vinyl-type radicals for efficient domino synthesis can be made. 

O-centered radicals, and subsequently is referred to as the O-neophyl rearrangement [84]. 

Interestingly, direct irradiation of ester- or carbo g lic acid-bearing VCP afforded five-membered lactones in low yields with unreacted VCP fScheme 11.39. Eq. 2). Formation of the cyclic lactone was presumably due to resonance delocalization of the biradical intermediate into the carbonyl group, revealing an O-centered radical species 53, which recombines to afford dihydrofuran 54. This dihydrofuran undergoes addition of water, followed by elimination of alcohol, to yield the cyclic lactone product 56 fScheme 11.401. ... 

Several tris(pyrazol-l-yl)metliane complexes of V(in, IV, or V), Fe(II), Cu(II), and Re(III or VII) (Fig. 22.2) have been found to be catalyst precursors for the peroxidative oxidation of cyclohexane (and cyclopentane in the cases tested) to give, in a single pot, the corresponding alcohols and ketones (Scheme 22.1). The reactions are usually carried out in acetonitrile, with aqueous H2O2 as the oxidizing agent, in acidic medium, at room temperature. They proceed via radical mechanisms with possible involvement of both C-centered and O-centered radicals as indicated by radical trap experiments. 

Mainly C-centered radical, oxidizing action, however, likely to occur through mesomeric O-centered radical. Immediate protonation. k > s" from ESR experiment. 

Mesomelic form between C- and O-centered radical, oxidizing action likely through 0-centered radical. 

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