Addition of radicals to olefins

These reactions can be divided into two types, according to the method, oxidation or reduction used for the generation of the radical. 

Lead tetraacetate (LTA) reacts with olefins via several mechanisms.23 The first one implies a methyl radical intermediate generated by the decomposition of LTA and results in the formation of a monoacetate after addition of the methyl group. Polar mechanisms give the gem- and pfc-diacetate. Sonochemical enhancement of the first process was predicted by Ando et ah, who indeed found an example of sonochemical switching when the reaction was performed on styrenes (Fig. 3). 24.25 

21 March, J. Advanced Organic Chemistry J. Wiley, New York, N.Y., 4 Edn., 1992. 

22 Comprehensive Organic Synthesis (Trost, B.M. Ed.), Pergamon Press, Oxford, 1991. 

23 House, H.O. Modem Synthetic Reactions Benjamin, Menlo Park, Ca., 1972, 379 

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The effect of substituents on the overall rate of addition of radicals to olefins. 54... 

A study of the overall rate of addition of radicals to olefins shows us that polar forces, familiar to chemists working with ionic species in solution, are apparent in free radical reactions, i.e. in reactions involving uncharged species in the gas phase. The effects are smaller than in solution but are none the less clearly apparent. The results also show that polarity is not the whole story and that some of the trends observed in Tables 1 to 7 may be partly due to steric... 

Anodic addition of nucleophiles to olefins can be achieved via oxidation of the alkene to a radical cation.This means that a nucleophile can be added to a nucleophilic alkene by reversing its polarity to an electrophilic radical cation... 

Radical addition of polyhalomethanes to olefins can be initiated by electro-chemically generated Mn(lll) species. The procedure offers an alternative method of a radical addition reaction with an easy control of the initiation via the current... 

A regiospecific concerted syn addition (path (a)) competing with a radical chain reaction (path (b)) has been proposed to rationahze the addition of TeCl4 to olefins. 

Protonated heteroaromatic bases are therefore more reactive than simple olefins toward acyl radicals. The radical addition of pivalaldehyde to olefins is, in fact, characterized by a radical chain, whose propagation is determined by decarbonylation of the pivaloyl radical and addition of <-butyl radical to the olefin. The synthetic interest is great in the case of substrates with only one reactive position, such as benzothiazole, ... 

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. 

Because the addition steps are generally fast and consequently exothermic chain steps, their transition states should occur early on the reaction coordinate and therefore resemble the starting alkene. This was recently confirmed by ab initio calculations for the attack at ethylene by methyl radicals and fluorene atoms. The relative stability of the adduct radicals therefore should have little influence on reacti-vity 2 ). The analysis of reactivity and regioselectivity for radical addition reactions, however, is even more complex, because polar effects seem to have an important influence. It has been known for some time that electronegative radicals X-prefer to react with ordinary alkenes while nucleophilic alkyl or acyl radicals rather attack electron deficient olefins e.g., cyano or carbonyl substituted olefins The best known example for this behavior is copolymerization This view was supported by different MO-calculation procedures and in particular by the successful FMO-treatment of the regioselectivity and relative reactivity of additions of radicals to a series of alkenes An excellent review of most of the more recent experimental data and their interpretation was published recently by Tedder and... 

This species is similar to the bromonium ion that is responsible for stereospecific anti addition in the electrophilic mechanism. Further evidence for the existence of such bridged radicals was obtained by addition of Br to olefins at 77 K. Esr spectra of the resulting species were consistent with bridged structures.54... 

Addition of alkylcopper reagents to unsaturated carboxylic acids 5-22 Free-radical addition of acids to olefins... 

Although very few studies of free-radical reactions of sulfenyl compounds have as yet been reported, irradiation with ultraviolet light enhances the rate of addition of CI3CSCI to olefins.218 Evidence for free-radical behavior is found in the easy decompositions which are observed for RSC1 under irradiation, e.g., the conversion of methanesulfenyl chloride to chloromethanesulfenyl chloride287 under the influence of sunlight. In the photoinitiated chlorination reaction with trichloro-... 

This process is quite common for carbon-centered free radicals because die carbon-carbon a bond which is formed is stronger by about 30 kcal dian die n bond which is broken. Other radical species, however, are well known to undergo olefin additions as well. The addition of bromine to olefins is die key step in the anti-Markovnikov addition of HBr to olefins. 

A classic example is the free-radical addition of chloroform to olefins initiated by benzoyl peroxide. 

The addition of thiols to olefins (thiolene reaction), to form thioethers, is a well-known reaction. The process can occur by either free-radical or ionic mechanisms. The free-radical reaction can be initiated thermally via a peroxide or by UV irradiation with benzophenone. The initiation step involves the formation of a thiyl radical by hydrogen atom abstraction. Both of these species are capable of starting polymer chains (Table 2.30). 

We referred above to the large substituent effect on the heterolytic addition of bromine to olefins, which is a polar process actually involving intermediate ionic species. By contrast, the addition of alkyl radicals to olefins involves... 

Since the addition of formamide to olefins is induced photochemically as well as by peroxides at elevated temperatures, it may be safe to assume that we deal here with a free radical reaction. Let us apply this assumption to interpret the results. A reasonable free radical derived from formamide would be a carbamoyl radical CONH2 which can be formed by loss of a hydrogen atom from formamide. Experimental data show that irradiation of formamide in the presence of acetone and in the absence of an olefin leads to the formation of considerable amounts of oxamide. 

The reaction of formamide with aromatic compounds under ultraviolet irradiation is still unexplored and only preliminary results have so far been obtained. In the cases already studied it has been found that this reaction must be sensitized with a ketonic sensitizer, usually acetone, in order to take place. The mechanism of the photoamidation of aromatic compounds certainly differs from the one of simple olefins. The detailed mechanism still awaits further experimental evidence, and in some cases involves, most probably, radical combinations and not addition of radical to unsaturated systems. Interactions of the excited sensitizer with aromatic compounds, having in some cases triplet energies similar or just a bit higher than those of the sensitizers used, must be brought into consideration. Experimentally it has been shown that the photosensitized amidation of benzene leads to benzamide (11),... 

The addition of cyclohexanone to olefins involves an alpha ketone radical o... 

A few Cr(0) complexes were reported to catalyze the Kharasch addition of polyhalocarbons to olefins. (Naphthalene)chromium tricarbonyl exhibited low to moderate activity in additions of tetrachloromethane to olefins. The reactions were proposed to occur by a non-radical mechanism [215]. A later kinetic study showed that a radical mechanism operates (see Sect. 7) [216, 217]. Shvo et al. used 5 mol% Cr(CO)6 in acetonitrile to add tetrachloromethane to 1-octene [218]. It was necessary to transform the precatalyst first to the active catalyst... 

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