Alkane functionalization radical mechanism

Carboxylic acids are oxidized by lead tetraacetate. Decarboxylation occurs and the product may be an alkene, alkane or acetate ester, or under modified conditions a halide. A free radical mechanism operates and the product composition depends on the fate of the radical intermediate.267 The reaction is catalyzed by cupric salts, which function by oxidizing the intermediate radical to a carbocation (Step 3b in the mechanism). Cu(II) is more reactive than Pb(OAc)4 in this step. 

In the cases of mesoporous silica, AMS and quartz chip, the 0.5 C3/C4 ratio being close to unity means that two reactions proceed with almost equal probability to each other. This is in accordance with the classical radical mechanism of alkane cracking supposing that the energy required to form tertiary radical is not so different from that required for secondary radical and that both radicals are cracked by P-scission mechanism shown below [13]. Thus, the results shown in Fig. 4 strongly suggest that isohexane is cracked via the radical mechanism on the mesoporous silica catalysts, or, in other words, MCM-41, both with and without aluminum impurity, and FSM-16 exhibit radical type catalytic function. 

Examples of alkane functionalization reactions of the type shown in equation (1) are first considered, in which the atom X to which the new C—X bond is form comes from metals in Group I, followed by subsequent groups in the Periodic Table. Within each section, radical, electrophilic and carbenoid mechanisms are (Uscussed. 

The photo-oxidation and other photoreactions promoted by soluble poly-oxometal clusters are also interesting methodologies to functionalize alkanes (eq. (52)) (78). The reactions proceed via a radical mechanism. 

The alkane functionalization reactions proceed via radical mechanisms, with a high chemoselectivity, although with low regio-, bond-, and stereo-selectivities as expected for the involvement of the hydroxyl radical, features that were not discussed in this chapter. 

Even with the limitation on yield implied by the statistical process, cross-dimerization is still useful when one of the reactants is an alkane, because the products are easy to separate, and because of the few other ways to functionalize an alkane. The cross-coupling of an alkane with trioxane is especially valuable, because hydrolysis of the product (10-6) gives an aldehyde, thus achieving the conversion RH RCHO. The mechanism probably involves abstraction of H by the excited Hg atom, and coupling of the resulting radicals. 

As previously mentioned, Davis (8) has shown that in model dehydrocyclization reactions with a dual function catalyst and an n-octane feedstock, isomerization of the hydrocarbon to 2-and 3-methylheptane is faster than the dehydrocyclization reaction. Although competitive isomerization of an alkane feedstock is commonly observed in model studies using monofunctional (Pt) catalysts, some of the alkanes produced can be rationalized as products of the hydrogenolysis of substituted cyclopentanes, which in turn can be formed on platinum surfaces via free radical-like mechanisms. However, the 2- and 3-methylheptane isomers (out of a total of 18 possible C8Hi8 isomers) observed with dual function catalysts are those expected from the rearrangement of n-octane via carbocation intermediates. Such acid-catalyzed isomerizations are widely acknowledged to occur via a protonated cyclopropane structure (25, 28), in this case one derived from the 2-octyl cation, which can then be the precursor... 

In 1983 Barton and co-workers reported the use of iron-based reagents in the functionalization of alkanes (equation 72). The elucidation of the mechanisms involved a long dispute, but the role of free radical reactions appears to be well established. ... 

The rate constants for oxidation of alkanes and alkenes with peroxynitrous acid in aqueous-gas phase are bell-shaped functions of the volume ratio between the liquid and gas phases. The kinetics of the generation of OH radicals and its importance in understanding the mechanism of lipid membrane oxidation has been stressed.236... 

In more recent years, monohalogenated alkanes have been added to alkenes via a radical chain mechanism. In order for these reactions to be synthetically useful, functional groups that are able to stabilize radicals (such as carbonyl) must be present on the a-carbon atom683,699-702. A good example is the addition of bromomalonates to enol silyl ethers687. After the addition, spontaneous elimination of HBr occurs (equation 109). 

This type of reaction is important industrially since it is one of the few that allows compounds containing functional groups to be made from alkanes. As you might guess, since it needs light for initiation, the process is another example of a radical chain reaction. As with the radical addition of HBr to alkenes, we can identify initiation, propagation, and termination steps in the mechanism. 

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