Carbocation, 106 carbon radical

Carbocations, carbon radicals, and carbanions are important reactive carbon intermediates in organic chemistry and their interconversions could be effected, in principle, by redox processes. With the cation pool method at hand, we next examined the redox-mediated interconversions of such reactive carbon species. 

Scheme 5.26 Redox interconversions of carbocation, carbon radical, and carbanion...

On the other hand, ring strain reUef is triggered by reactive intermediates, and as a matter of fact, this is the alternative option. The reactive intermediates are carbocation, carbon radical and carbene, and their hetero-analogs. Once generated at the position adjacent to C-4, they mediate sequential ring expansion to a five-membered ring [23]. The similar story may be depicted by metal catalyses [73,74]. 

Thus, under general laboratory conditions, elimination of the hydroxide anion (OH"), hydroxyl radical ( OH), or the hydroxyl cation (OH" ) from an alcohol, enol, or phenol, leaving behind a carbocation, carbon radical, or carbanion, respectively, appears to be unknown. Indeed, even the loss of water, with the OH and H coming from adjacent atoms and, in this way, introducing unsaturation (or further unsaturation) into a compound in the laboratory and without involvement of an acid, base, or acidic, basic, or neutral metal catalyst is rare. 

The intermediates produced by cleavage of bonds to carbon give reactive intermediates with only three bonds to carbon carbocations, carbon radicals, and carbanions. These trivalent species are all highly reactive, and they rapidly react to give more stable, tetravalent carbons. 

Like carbocations, radicals are electron-deficient species. A carbon radical has seven electrons in the valence shell in contrast to six electrons for carbocations. Like carbocations, carbon radicals are stabilized by the inductive effect of groups bonded to the radical center. Because radicals are not as electron deficient as carbocations, the differences in the stability of radicals are smaller than for carbocations. The order of carbon radical stability parallels the order of carbocation stability. Like a carbocation, a carbon radical has a trivalent, sp -hybridized carbon atom. The methyl radical has a planar structure with H—C—H bond angles of 120°. The trivalent carbon atom also has an unhybridized 2p orbital that is perpendicular to the plane of the C-H bonds. It contains the unpaired electron (Figure 3.1b). 

Carbocations Carbon Radicals Methyl Radical Carbanions... 

Lee VY, Sekiguchi A (2007) Stable silyl, germyl, and stannyl cations, radicals, and anions heavy versions of carbocations carbon radicals, and carbanions. Acc Chem Res 40 410... 

Mechanistic Transform. A transform involving a sequence of reactive intermediates such as carbocations or carbon radicals which are generated in a stepwise mechanistic manner and which lead finally to stable predecessor structure(s). 

In the step above, Br attacked the alkene at the less substituted carbon, in order to form the more substituted carbon radical (C ). Tertiary radicals are more stable than secondary radicals, for the same reason that tertiary carbocations are more stable than secondary carbocations. Just as alkyl groups donate electron density to... 

In this chapter, we will consider examples of RIs characterized by a hypervalent or valency-deficient carbon, such as carbocations, carbenes, carbanions, and carbon radicals. In the first part, we will consider examples that take advantage of stabilization and persistence to determine their structures by single crystal X-ray diffraction. In the second part we will describe several examples of transient reactive intermediates in crystals. ... 

One electron oxidation of monocarbanions leads to carbon radicals and two electron oxidation gives carbocations. In most of these oxidations, the mechanism is not known, though progress is being made on some mechanisms. But there appears to be a parallelism between base strength and ease of oxidation of carbanions. 

An anodic azacyclization, producing tropane-related 11-substituted dibenzo[a,d]cycloheptimines 123, was recently developed by Karady et al. [136, 137]. This two-electron process is initiated by anodic oxidation of the O-substituted hydroxylamine 119 in nucleophilic solvent. It is proposed that the first one-electron oxidation leads to the aminium radical cation 120 which adds rapidly to the double bond. The electron-rich carbon radical 121 is readily oxidized to the carbocation 122. Selective nucleophilic attack on 122 from the less hindered exo-side yields the 11- substituted product 123. Depending on the... 

Oxidation of aliphatic ketones in trifluoroacetic acid leads to hydrogen abstraction by the carbonyl oxygen radical-cation fonning a carbon radical, then further oxidation of the radical to the carbocation and migration of this centre along the carbon chain by a series of hydride transfer steps. Long chain ketones yield a mixture of alcohol trifluoToacetates by reaction of the carbocation centres with the solvent [3]. 

Significantly slower rates are found only for compounds that do not exhibit any aromatic ring or carbon-carbon double bond, and for aliphatic compounds with no easily abstractable H-atoms. Such H-atoms include those that are bound to carbon atoms carrying one or several electronegative heteroatoms or groups. (Note that the stabilization of a carbon radical (R ) is similar to that of a carbocation.) We will come back to such structure-reactivity considerations in Section 16.3, when discussing reaction of HO" with organic pollutants in the gas phase (i.e., in the atmosphere). 

A carbon atom that is directly attached to a benzene ring is called a benzylic carbon (analogous to the allylic carbon of C=C—C). A phenyl group (C H —) is an even better conjugating substituent than a vinyl group (H2C=CH—), and benzylic carbocations and radicals are more highly stabilized than their allylic counterparts. The double bond of an alkenylbenzene is stabilized to about the same extent as that of a conjugated diene. 

An initial addition of a ruthenium-oxygen double bond to a a-C—H bond leads to an intermediate containing a carbon-ruthenium bond. This bond suffers a homolytic scission leading to a carbon radical, which is oxidized to a carbocation that provides a carbonyl group by deprotonation. 

Although radicals are not nearly so prone to rearrangement as are, for example, carbocations, there are a few such rearrangements which have become identified as characteristic of carbon radicals. These include radical cyclizations, particularly the 5-hexenyl radical cyclization, and radical C-C bond cleavages, particularly the cyclopropylcarbinyl to allyl carbinyl radical rearrangement. In hydrocarbon systems, as organic synthetic chemists have learned how to control rapid chain processes, such rearrangements have become important synthetic tools [176-179]. 

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. 

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