Addition Reactions of Radicals to Substituted Alkenes

Radicals for addition reactions can be generated by halogen-atom abstraction by stannyl radicals. The chain mechanism for alkylation of alkyl halides by reaction with a substituted alkene is outlined below. There are three reactions in the propagation cycle of this chain mechanism, shown in Fig. 10,3. The rates of each of these steps must exceed those of competing chain termination reactions in order for good yields to be obtained. The most important competitions are between the addition step and reaction of the intermediate R- with Bu3SnH and between the H-abstraction step ki and addition to another molecule of the alkene. If the addition step is not fast enough, the radical R- will 

No perturbing interaction with substituent—no strong stabilization. 

Electron donor on radical electron acceptor on alkene— stabilizing interaction between SOMO and alkene LUMO. 

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

Radicals formed by fragmentation of xanthate and related thiono esters can also be trapped by reactive alkenes.217 The mechanism of radical generation from thiono esters was discussed in connection with the Barton deoxygenation method in Section 5.4. 

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