Radical Addition Reactions to Double Bonds

We now turn our attention to the second common reaction of radicals, addition to double bonds. Because an alkene contains an electron-rich, easily broken n bond, it reacts with an electron-deficient radical. 

Radicals react with alkenes via a radical chain mechanism that consists of initiation, propagation, and termination steps analogous to those discussed previously for radical substitution. 

HBr adds to alkenes to form alkyl bromides in the presence of light, heat, or peroxides. 

Problenri 15.28 Draw the products formed in each reaction. 

Problem 15.26 Draw the product(s) formed when each alkene Is treated with either [1] HBr alone or [2] HBr In the presence of peroxides. 

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Figure 7.9 Pathway for the biosynthesis of prostaglandins from arachidonic acid. Steps 2 and 5 are radical addition reactions to 02 steps 3 and 4 are radical additions to carbon-carbon double bonds.

As we stated earlier, the majority of methods to form C-C bonds in a total synthesis are based on heterolytic processes involving the participation of carbanion- or carbocation-like species or imply the utilization of various cycloaddition reactions. The main reason radical reactions are generally less suited for this purpose can be easily understood if one takes into account the mechanisms involving homolytic scission/formation of covalent bonds. Typically these reactions proceed as a sequence of discrete steps initiation, chain propagation, and termination, as is shown for the radical addition at the double bond in Scheme 2.139. 

Solution The chemistry in this problem involves the reactions of the carbon-carbon double bond in alkenes. Alkenes usually undergo electrophilic and free-radical addition across the double bond one-half of the double bond is broken and two new groups are attached to give a saturated compound. 

Problem 7.6. Provide a justification for the observation that vinyl (C=C-X X=C1, Br) and allyl (C=C-C-X X = Cl,Br) halides undergo free radical addition reactions to the carbon-carbon double bond with about equal facility. 

Where the diene provides terminal unsaturation, radical addition at the double bond may occur successively in a chain reaction akin to polymerization so that one peroxide radical could produce many cross-links, i.e. to give a cross-linking efficiency greater than 1. Such results have been observed with EPDM rubbers based on methylene norbornene and on vinyl norbornene. 

Addition to double bonds is not the only kind of reaction that converts an achiral molecule to a chiral one Other possibilities include substitution reactions such as the formation of 2 chlorobutane by free radical chlorination of butane Here again the prod uct IS chiral but racemic... 

Other nonpolymeric radical-initiated processes include oxidation, autoxidation of hydrocarbons, chlorination, bromination, and other additions to double bonds. The same types of initiators are generally used for initiating polymerization and nonpolymerization reactions. Radical reactions are extensively discussed in the chemical Hterature (3—15). 

Toluene, an aLkylben2ene, has the chemistry typical of each example of this type of compound. However, the typical aromatic ring or alkene reactions are affected by the presence of the other group as a substituent. Except for hydrogenation and oxidation, the most important reactions involve either electrophilic substitution in the aromatic ring or free-radical substitution on the methyl group. Addition reactions to the double bonds of the ring and disproportionation of two toluene molecules to yield one molecule of benzene and one molecule of xylene also occur. 

Other limitations of the reaction are related to the regioselectivity of the aryl radical addition to double bond, which is mainly determined by steric and radical delocalization effects. Thus, methyl vinyl ketone gives the best results, and lower yields are observed when bulky substituents are present in the e-position of the alkene. However, the method represents complete positional selectivity because only the g-adduct radicals give reductive arylation products whereas the a-adduct radicals add to diazonium salts, because of the different nucleophilic character of the alkyl radical adduct. ... 

Alternatively, a reactant radical might add to a double bond, taking one electron from the double bond and yielding a new radical. The net result is a radical addition reaction ... 

It is established that the initial reaction involves predominantly tail addition to monomer.473 There is no evidence that abstraction competes with addition. It should be noted that the addition of arenethiyl radicals to double bonds is readily reversible. 

Kolbe electrolysis is a powerful method of generating radicals for synthetic applications. These radicals can combine to symmetrical dimers (chap 4), to unsymmetrical coupling products (chap 5), or can be added to double bonds (chap 6) (Eq. 1, path a). The reaction is performed in the laboratory and in the technical scale. Depending on the reaction conditions (electrode material, pH of the electrolyte, current density, additives) and structural parameters of the carboxylates, the intermediate radical can be further oxidized to a carbocation (Eq. 1, path b). The cation can rearrange, undergo fragmentation and subsequently solvolyse or eliminate to products. This path is frequently called non-Kolbe electrolysis. In this way radical and carbenium-ion derived products can be obtained from a wide variety of carboxylic acids. 

By the radical pathway l, -diesters, -diketones, -dienes or -dihalides, chiral intermediates for synthesis, pheromones and unusual hydrocarbons or fatty acids are accessible in one to few steps. The addition of the intermediate radicals to double bonds affords additive dimers, whereby four units can be coupled in one step. By way of intramolecular addition unsaturated carboxyhc acids can be converted into five raembered hetero- or carbocyclic compounds. These radical reactions are attractive for synthesis because they can tolerate polar functional groups without protection. 

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