Other free-radical additions

This reaction proceeds through a chain mechanism. Free-radical additions to 1-butene, as in the case of HBr, RSH, and H2S to other olefins (19—21), can be expected to yield terminally substituted derivatives. Some polymerization reactions are also free-radical reactions. 

When double bonds are reduced by lithium in ammonia or amines, the mechanism is similar to that of the Birch reduction (15-14). ° The reduction with trifluoro-acetic acid and EtsSiH has an ionic mechanism, with H coming in from the acid and H from the silane. In accord with this mechanism, the reaction can be applied only to those alkenes that when protonated can form a tertiary carbocation or one stabilized in some other way (e.g., by a OR substitution). It has been shown, by the detection of CIDNP, that reduction of a-methylstyrene by hydridopenta-carbonylmanganese(I) HMn(CO)5 involves free-radical addition. ... 

The mechanistic picture is further simplified by the fact that free-radical additions to carbon-hetero double bonds are rare. The principal question remaining is which attacks first, the nucleophile or electrophile. In most cases it is the nucleophile that forms the first new bond to carbon, and these reactions are regarded as nucleophilic additions, which can be represented thus (for the C=0 bond, analogously for the others) ... 

Several other types of addition reactions of alkenes are also of importance and these are discussed elsewhere. Nucleophilic additions to electrophilic alkenes are covered in Section 2.6 and cycloadditions involving concerted mechanisms are encountered in Sections 6.1 to 6.3. Free radical addition reaction are considered in Chapter 11. 

Homolytic annulation of Schiff bases 56, via an intermolecular free-radical addition of its arylimidoyl radicals 57 to the azo group of diethyl azodicarboxylate (89CC757) in the presence of diisopropylperoxy dicarbonate (DPDC), gave 1,2-diethoxycarbonyl pyrido[l,2,4]triazines 60 via 58 and 59. No trace of the other isomer could be detected. 

Attachment of Hydroxycinnamic Acids to Structural Cell Wall Polymers. Peroxidase mediation may also result in binding the hydroxycinnamic acids to the plant cell wall polymers (66,67). For example, it was reported that peroxidases isolated from the cell walls of Pinus elliottii catalyze the formation of alkali-stable linkages between [2-14C] ferulic acid 1 and pine cell walls (66). Presumably this is a consequence of free-radical coupling of the phenoxy radical species (from ferulic acid 1) with other free-radical moieties on the lignin polymer. There is some additional indirect support for this hypothesis, since we have established that E-ferulic acid 1 is a good substrate for horseradish peroxidase with an apparent Km (77 /tM), which is approximately one fifth of that for E-coniferyl alcohol (400 /iM) (unpublished data). 

The addition of phosphine to olefins provides today a generally applicable method for the syntheses of organophosphines. Stiles, Rust and Vaughan were the first to study the reaction systematically. It is catalysed by organic peroxides such as, for example, di-t-butyl peroxide, by a, a -azobis-isobutyro-nitrile by other free radical sources or by exposing the reaction mixture to UV- or X-radiation. The PHj radicals, produced according to Eq. (77), react further with olefins thus producing PH2 radicals continually. 

Free-radical additions to cyclopropanes have been studied much less, but it is known that Br2 and Cl2 add to cyclopropanes by a free-radical mechanism in the presence of uv light. The addition follows Markovnikov s rule, with the initial radical attacking the least-substituted carbon and the second group going to the most-substituted position. Several investigations have shown that the reaction is stereospecific at one carbon, taking place with inversion there, but nonstereospecific at the other carbon.130 A mechanism that accounts for this behavior is131... 

The addition reactions of It with amines are also presumed to occur via exciplex or radical-ion pair intermediates however, exciplex fluorescence is observed only under conditions where chemical reactions do not occur. Transfer of hydrogen from the amine a-C-H (tertiary amine) or N-H (secondary amine) bond results in the formation of a radical pair which ultimately gives rise to stilbene amine adducts and other free-radical derived products. The radical-ion pairs can also be intercepted by external electrophiles and nucleophiles, leading to formation of radical-ion-derived products. 

Addition to n bonds is a second very common reaction of free radicals. Interaction of die free radical widi die 7r -electron pah causes one of die n electrons to pair up widi die unpaired electron of the free radical to produce a new bond to one of die r-bonded atoms. The remaining n electron is now unpaired and dius forms a new free-radical species. The process is often very favorable since the new a bond (70-90 kcal/mol) formed in die addition process is normally much stronger than die jt bond (60 kcal/mol) which is broken in the reaction. In the above example a new carbon-carbon a bond is formed by free-radical addition to produce a new carbon-centered free radical however, a wide variety of other free-radical species add readily to olefins. 

Resonance effects, on the other hand, can significantly affect the regiochem-istry of the cyclizadon. Resonance delocalization of the unpaired electron of a free radical stabilizes that radical. This is why the allyl radical is much more stable than the //-propyl radical. Thus, if a double bond is substituted with a group capable of providing resonance stabilization to a free radical, it undergoes free-radical addition much more readily than a double bond which cannot provide such resonance stabilization. 

The composition of the reaction products (1 1 adducts) needs further clarification. In the case of terminal olefins the anti-Markovnikov 1 1 addition product is almost the only 1 1 adduct, whereas the isomeric amide is formed in minute amounts only. Markovnikov-additions of free radicals to olefins have been observed in other cases too as side products (28). The point of initial attack in the free radical addition to an olefin of the type RCH=CH2 is at the terminal carbon. The intermediate radical (I) produced by this process (anti-Markovnikov) has a higher degree of resonance stabilization than the alternative radical (II) (4, 78). This means that in the present reaction,... 

Termination reactions include other reactions in addition to Reaction (5) of Table IV. There are numerous free radicals present in addition to ethyl and methyl radicals. These other free radicals can also combine or couple. The coupling reactions in the gas phase, including Reaction (5), are highly exothermic. To promote such coupling in the gas phase, a relatively heavy molecule (or third body) is likely needed to help dissipate the exothermic heat of reaction. 

Dowbenko (1964) has shown that carbon tetrachloride, chloroform and several other compounds react with a s-cis-l,5-cyclo-octadiene by a free-radical transannular 1,5-cyclo-addition to give derivatives of bicyclo-[3,3,0]octane. He suggested that the cyclo-addition may occur by a concerted reaction in which the configuration of the diene is such that the two double bonds lie in close proximity, or alternatively by a step-wise reaction which involves a free-radical addition across the ring. 

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