Unstable Carbon Radicals

A carbon radical is a highly reactive and unstable intermediate that has an unpaired electron. 

Because of the weak C-Te bond, organotellurides can serve as efficient carbon radical precursors [13]. However, they are generally unstable to handle, unlike the... 

The next step, addition (equation 83) of molecular oxygen to C1CO, involves the intermediacy of the unstable chloroformylperoxy radical, which may readily dissociate (equation 84) to chlorine monoxide and carbon dioxide. 

Carbon radicals, with only seven electrons in the valence shell for carbon, and carbocations, with only six electrons and a positive charge on the carbon, do not satisfy the octet rule and are quite unstable. These species are only encountered as highly reactive, transient intermediates in certain chemical reactions. 

These oxyradicals add to the alkene to give an unstable primary carbon radical that adds to another molecule of alkene, and so on. 

Generally, chiral tricoordinate centers are configurationally stable when they are derived from second-row elements. This is exemplified by sulfonium salts, sulfoxides and phosphines. In higher rows, stability is documented for arsines and stibines. In contrast, tricoordinate derivatives of carbon, oxygen, and nitrogen (first-row atoms) experience fast inversion and are configurationally unstable they must therefore be viewed as conformationally chiral (see Fig. 3, Section 3.b). Oxonium salts show very fast inversion, as do carbanions. Exceptions such as the cyclopropyl anion are known. Carbon radicals and carbenium ions are usually close to planarity and tend to be achiral independently of their substituents [21-23]. 

It would be expected that these trivalent carbon radicals would be very unstable and that they might undergo, spontaneously, one of the types of transformation which have been found to be characteristic for trivalent carbon radicals of the triarylmethyl type, namely, addition, mutual oxidation and reduction, and polarization. 

Stable radicals, such as nitroxides hydroxy-2,2,6,6-tetramethylpiperidinyloxy (TEMPO) [8,156], can be added to the polymerization medium to terminate all polymer radicals produced. For styrenes and acrylates [157], this mainly occurs through combination. Chambard et al. [157] showed this technique allows for the modification of poly( -butyl acrylate)-Br in the presence of an excess of hydroxy-TEMPO, resulting in hydroxy-functional poly( -butyl acrylate) with good functionality (f> 95%). This process is not desirable, because the polymer produced is thermally unstable (carbon nitroxide) and cannot be used at high temperature. 

The nitrosoperoxycarbonate anion is highly unstable and decomposes to form nitrogen dioxide and carbonate radical anions with the yield y 0.33 [21] ... 

The use of TEMPO to effect oxidative demercuration was originally demonstrated by Whitesides [16], and is attractive because it gives a functional handle for further structural elaboration. This technique was invoked by Kang in the syntheses of (-t-)-lactacystin and (-l-)-furanomycin [17]. For example, alkene 10 was subjected to mercurioamidation conditions to afford the cyclized organomercury intermediate 11 (Scheme 4). Treatment with lithium borohydride in the presence of TEMPO forms the unstable organomercury hydride. This fragments to release the primary carbon radical, which is trapped by TEMPO to yield the masked alcohol product 12, an intermediate in the synthesis of the neurotropic factor (-l-)-lactacystin. 

From the results that the coexistence of C3H6 and NO was necessary to form the reactive intermediate, the intermediate is suggested to be produced by the reaction between the unstable carbon species such as radical or 7C-aryl and NO. Thus, it is suggested that NO reduction proceeds via decomposition of the nitrogen containing... 

---