Carbon radicals secondary

In the synthesis of molecules without functional groups the application of the usual polar synthetic reactions may be cumbersome, since the final elimination of hetero atoms can be difficult. Two solutions for this problem have been given in the previous sections, namely alkylation with nucleophilic carbanions and alkenylation with ylides. Another direct approach is to combine radical synthons in a non-polar reaction. Carbon radicals are. however, inherently short-lived and tend to undergo complex secondary reactions. Escheirmoser s principle (p. 34f) again provides a way out. If one connects both carbon atoms via a metal atom which (i) forms and stabilizes the carbon radicals and (ii) can be easily eliminated, the intermolecular reaction is made intramolecular, and good yields may be obtained. 

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... 

As is expected from these results, it is very difficult to control the polymerization of monomers other than St, e.g., that of MMA, because of the too small dissociation energy of the chain end of poly(MMA). In fact, the polymerization of MMA in the presence of TEMPO yielded the polymer with constant Mn irrespective of conversion, and the Mw/Mn values are similar to those of conventional polymerizations [216]. The disproportionation of the propagating radical and TEMPO would also make the living radical polymerization of MMA difficult. In contrast, the controlled polymerization of MA, whose propagating radical is a secondary carbon radical,has recentlybeen reported [217]. Poly(MA) with a narrow molecular weight distribution and block copolymers were obtained. 

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. 

Intermolecular addition of radicals, generated by photo-electrochemical catalysis, to activated alkenes can also be brought about. The reaction of 66 is used as a key step in one synthesis of the insect pheromone, brevicomin [219]. The reaction of a secondary radical from 67 occurs at low cathode potentials and without photochemical assistance [219]. This illustrates the equiibrium between a secondary al-kylcobalt(m) species and the radical - cobalt(ii) pair. The carbon radical is eventually captured by reaction with the alkene. Further steps in the synthesis lead to four isomers of the pheromone, multistriatin, each of which is a pure enantiomer since... 

Additional examples of alkyl substituents and their names are listed in Table 3-2. These are further classified according to whether they are primary, secondary, or tertiary. An alkyl group is described as primary if the carbon at the point of attachment is bonded to only one other carbon, as secondary if bonded to two other carbons, and tertiary if bonded to three other carbons. Thus, if R is any hydrocarbon radical, the different kinds of alkyl groups are... 

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 ... 

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. 

As indicated in Scheme VII/32, cyclononanone (VII/165) is transformed into hydroperoxide hemiacetal, VII/167, which is isolated as a mixture of stereoisomers. The addition of Fe(II)S04 to a solution of VII/167 in methanol saturated with Cu(OAc)2 gave ( )-recifeiolide (VII/171) in quantitative yield. No isomeric olefins were detected. In the first step of the proposed mechanism, an electron from Fe2+ is transferred to the peroxide to form the oxy radical VII/168. The central C,C-bond is weakened by antiperiplanar overlap with the lone pair on the ether oxygen. Cleavage of this bond leads to the secondary carbon radical VII/169, which yields, by an oxidative coupling with Cu(OAc)2, the alkyl copper intermediate VII/170. If we assume that the alkyl copper intermediate, VII/170, exists (a) as a (Z)-ester, stabilized by n (ether O) —> <7 (C=0) overlap (anomeric effect), and (b) is internally coordinated by the ester to form a pseudo-six-membered ring, then only one of the four -hydrogens is available for a syn-//-elimination. [111]. This reaction principle has been used in other macrolide syntheses, too [112] [113]. 

Bothe et al. OH and secondary carbon radical chemistry Peyton (1996) ... 

The photoreactions of aliphatic amines with aromatic hydrocarbons have also been reported by several groups. With tertiary amines, deprotonation occurs from the radical cations of amines at the a-carbon to generate carbon radicals which react with the radical anion of aromatic hydrocarbons. With secondary amines, deprotonation from the radical cations of amines occurs both at the a-carbon and at the nitrogen atom, so that the reaction becomes complicated [64-65]. 

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. 

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