Addition of radicals to alkenes

Chapter 7, Alkenes Reactions and Synthesis—Alkene epoxidation has been moved to Section 7.8, and Section 7.11 on the biological addition of radicals to alkenes has been substantially expanded. 

Curran2 has reviewed recent applications of the tin hydride method for initiation of radical chain reactions in organic synthesis (191 references). The review covers intermolecular additions of radicals to alkenes (Giese reaction) as well as intramolecular radical cyclizations, including use of vinyl radical cyclization. 

As an illustration of the performance of various levels of theory in determining such barriers, we examine the addition of radicals to alkenes, beginning with methyl radical addition to ethylene,... 

The addition of radicals to alkenes is used to assess the performance of various levels of theory in the prediction of radical reaction enthalpies. Results for the addition of methyl radical to ethylene (Table 6.24) [41] show that the higher-level methods perform well in predicting the reaction enthalpy values range from -105.6 to -111.5 kJ/mol compared with the corrected experimental value of -113.1 kJ/mol. The AMI method greatly overestimates the exothermicity while the UB3LYP/6-311+G(3df,2p) level of theory, which performs well for the reaction barrier, significantly underestimates the exothermicity. The RB3LYP values... 

B. Giese, Formation of carbon-carbon bonds by addition of radicals to alkenes, Angew. Chern. Int. Ed Engl. 22 153 (1983). 

C-Alkylations have been performed with both support-bound carbon nucleophiles and support-bound carbon electrophiles. Benzyl, allyl, and aryl halides or triflates have generally been used as the carbon electrophiles. Suitable carbon nucleophiles are boranes, organozinc and organomagnesium compounds. C-Alkylations have also been accomplished by the addition of radicals to alkenes. Polystyrene can also be alkylated under harsh conditions, e.g. by Friedel-Crafts alkylation [11-16] in the presence of strong acids. This type of reaction is incompatible with most linkers and is generally only suitable for the preparation of functionalized supports. Few examples have been reported of the preparation of alkanes by C-C bond formation on solid phase, and general methodologies for such preparations are still scarce. 

Figure 1 Transition state for addition of radicals to alkenes...

Steric effects are used to explain the regioselective addition of radicals to alkenes. The radical preferentially attacks the less substituted (hindered) end of the double bond to give a less strained transition state with lower AH0 in an anti-Markovnikov-type reaction. If we introduce substituents on to the double bond, the rate of addition is lowered because of greater steric interaction. Thus, for example, the methyl radical will add three times more quickly to ethene (CH2CH2) than to the disubstituted alkene ( )-2-butene. 

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