Reactions Involving Free-Radical Intermediates

The fundamental mechanisms of free radical reactions were considered in Chapter 11 of Part A. Several mechanistic issues are crucial in development of free radical reactions for synthetic applications.285 Free radical reactions are usually chain processes, and the lifetimes of the intermediate radicals are very short. To meet the synthetic requirements of high selectivity and efficiency, all steps in a desired sequence must be fast in comparison with competing reactions. Owing to the requirement that all the steps be fast, only steps that are exothermic or very slightly endothermic can participate in chain processes. Comparison between addition of a radical to a carbon-carbon double bond and addition to a carbonyl group can illustrate this point. 

This comparison suggests that of these two similar reactions, only alkene additions are likely to be a part of an efficient radical chain sequence. Radical additions to carbon-carbon double bonds can be further enhanced by radical stabilizing groups. Addition to a carbonyl group, in contrast, is endothermic. In fact, the reverse fragmentation reaction is commonly observed (see Section 10.3.6) A comparison can also be made between abstraction of hydrogen from carbon as opposed to oxygen. 

The reaction endothermicity establishes a minimum for the activation energy whereas abstraction of a hydrogen atom from carbon is a feasible step in a chain process, abstraction of a hydrogen atom from a hydroxyl group is unlikely. Homolytic cleavage of an O-H bond is likely only if the resulting oxygen radical is stabilized, such as in phenoxy radicals formed from phenols. 

CHAPTER 10 REACTIONS INVOLVING CARBOCATIONS, CARBENES, AND RADICALS AS REACTIVE INTERMEDIATES 

SECTION 10.3. REACTIONS INVOLVING FREE-RADICAL INTERMEDIATES 

A fragmentation reaction occurs if one of the oxime substituents can give rise to a relatively stable carbocation. Fragmentation is very likely to occur if X is a nitrogen, oxygen, or sulfur atom. 

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The second mechanism is the one followed when addition occurs opposite to Markovmkov s rule Unlike electrophilic addition via a carbocation intermediate this alternative mechanism is a chain reaction involving free radical intermediates It is pre sented m Figure 6 7... 

Free radicals were for a long time believed to be too reactive and indiscriminate in their reactivity to be harnessed usefully for synthesis. However, because of increased understanding of the nature of their reactivity, synthetic chemists have learned to tame these highly reactive intermediates to such an extent that reactions involving free radical intermediates are now considered to be quite useful in synthesis, particularly with respect to carbon-carbon bond forming and carbocyclic ring forming processes. 

Development of adhesive bonding capabilities for rapid high energy electron or photoinduced polymerization reactions involving free radical intermediates requires knowledge of the following concepts. 

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