Primary carbon radicals

Eisch, Behrooz and Galle196 give compelling evidence for the intervention of radical species in the desulphonylation of certain acetylenic or aryl sulphones with metal alkyls having a lower oxidation potential at the anionic carbon. The primary evidence presented by these workers is that the reaction of 5-hexenylmagnesium chloride outlined in equation (85) gives a mixture of desulphonylation products, in accord with the known behaviour of the 5-hexenyl radical, in which the cyclopentylmethyl radical is also formed. 

Ethyl l-cyano-2-methylcyclohexanecarboxylate has been prepared by catalytically hydrogenating the Diels-Alder adduct from butadiene and ethyl 2-cyano-2-butenoate3 and by the procedure described in this preparation.4 8 This procedure illustrates a general method for the preparation of alicyclic compounds by the cyclization of <5-ethylenic carbon radicals l.6 Whereas the primary 5-hexen-l-yl radical 1... 

The bromine atom then adds to the alkene, generating a new carbon radical. In the case of propene, as shown, the bromine atom bonds to the terminal carbon atom. In this way, the more stable secondary radical is generated. This is preferred to the primary radical generated if the central carbon were attacked. The new secondary radical then abstracts hydrogen from a further molecule of HBr, giving another bromine atom that can continue the chain reaction. 

Previously, it was mentioned that the steric characteristics of the carbon atom (primary, secondary, tertiary) should be considered it becomes clear with the new classes of compounds studied that the electronic characteristic of the substituent groups—aryl, hydroxyl, alkoxyl— must also be taken into account. Thus, whereas trisalkylperoxy radicals have weak recombination constants, dialkylhydroxy or dialkylalkoxy have much higher recombination constants, occasionally similar to values observed for secondary alkyl radicals. 

From C-H bond-dissociation energies of alkanes (see Table 4-6), the ease of formation and stabilities of the carbon radicals is seen to follow the sequence tertiary > secondary > primary. By analogy, the secondary l-bromo-2-propyl radical, 5, is expected to be more stable and more easily formed than the primary 2-bromo-1-propyl radical, 6. The product of radical addition should be, and indeed is, 1-bromopropane ... 

Previous work in our laboratory (3) and in others (4) has established that the primary photoprocess in a variety of excited carbanions involves electron ejection. This photooxidation will generate a reactive free radical if recapture of the electron is inhibited. Parallel generation of these same carbon radicals by electrochemical oxidation reveals an irreversible anodic wave, consistent with rapid chemical reaction by the oxidized organic species (5). Little chemical characterization of the products has been attempted, however (6). 

How the enthalpy AH of the substrate/reagent pair R—H/Cl changes when R and H—Cl are produced from it is plotted for four radical chlorinations in Figure 1.23 (left). These afford carbon radicals, the methyl, a primary, a secondary, and a tertiary radical. The reaction enthalpies A77 for all four reactions are known and are plotted in the figure. Only the methyl radical is formed slightly endothermically (AH = +2.0 kcal/mol). The primary radical, which is more stable by 4.3 0.7 kcal/mol (cf. Table 1.2), is formed exothermically with A77 = -2.3 kcal/mol. The formation of the more stable secondary and tertiary radicals are more exothermic by -4.7 and -7.6 kcal/mol, respectively. 

Dehydrodimerization. On excitation with a mercury vapor lamp, mercury is converted to an excited state, Hg, which can convert a C—H bond into a carbon radical and a hydrogen atom. This process can result in dehydrodimerization, which has been known for some time, but which has not been synthetically useful because of low yields when carried out in solution. Brown and Crabtree1 have shown that this reaction can be synthetically useful when carried out in the vapor phase, in which the reaction is much faster than in a liquid phase, and in which very high selectivities are attainable. Secondary C—H bonds are cleaved more readily than primary ones, and tertiary C—H bonds are cleaved the most readily. Isobutane is dimerized exclusively to 2,2,3,3-tetramethylbutane. This dehydrodimerization is also applicable to alcohols, ethers, and silanes. Cross-dehydrodimerization is also possible, and is a useful synthetic reaction. 

Diisopropyl tellurium intercepted primary carbon radicals, generated by the photolysis of acyl derivatives of iV-hydroxy-2-thiopyridone, and released isopropyl radicals that were trapped by olefins2. 

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

Carbon radicals are classified as primary (1°), secondary (2°), or tertiary (3°) by the number of R groups bonded to the carbon with the unpaired electron. A carbon radical is sp hybridized and trigonal planar, like sp hybridized carbocations. The unhybridized p orbital contains the unpaired electron and extends above and below the trigonal planar carbon. 

In the presence of dioxygen, the carbon radical R- produced by reactions (201) and (202) ar transformed into alkylperoxy radicals ROO, reacts with Co or Mn species to regenerate th Co " or Mn " oxidants, and produce primary oxygenated products (alcohol, carbonyl compounds which can be further oxidized to carboxylic acids. This constitutes the basis of several Industrie processes such as the manganese-catalyzed oxidation of n-alkenes to fatty acids, and the cobal catalyzed oxidation of butane (or naphtha) to acetic acid, cyclohexane to cyclohexanol-on mixture, and methyl aromatic compounds (toluene, xylene) to the corresponding aromatic monc or di-carboxylic acids. ... 

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