Kharasch reaction

The addition of halocarbons (RX) across alkene double bonds in a radical chain process, the Kharasch reaction (Scheme 9.29),261 has been known to organic chemistry since 1932. The overall process can be catalyzed by transition metal complexes (Mt"-X) it is then called Atom Transfer Radical Addition (ATRA) (Scheme 9.30).262... 

Polymer formation during the Kharasch reaction or ATRA can occur if trapping of the radical (123), by halocarbon or metal complex respectively, is sufficiently slow such that multiple monomer additions can occur. Efficient polymer synthesis additionally requires that the trapping reaction is reversible and that both the activation and deactivation steps are facile. 

Andrus et al.44 employed a C2-symmetric bis(oxazoline) copper catalyst in the Kharasch reaction. When cyclohexene was used as the reaction substrate, yields ranging from 34% to 62% and ee from 30% to 81% were observed (Scheme 8-15). 

Chiral Lewis acid promoted atom transfer reaction (Kharasch reaction) of a-halo oxazolidinone imide 90 and 1-octene 92 has been reported by Porter et al. (Scheme 23) [78]. The enantioselective atom transfer utilizing Zn(OTf)2 and phenyl bisoxazoline ligand 93 as a chiral Lewis acid. The yields of the products, however, were quite low ranging from 5-15% and only moderate enantioselectivities were achieved (up to 40%). 

Pincer ligand Carbosilane Den- Ni Kharasch reaction 0.03 mol% 1-79 Ultrafiltration Negative dendritic 68, 70,... 

Fig. 14. Dendritic dodeca-nickel catalyst, 47, for Kharasch reaction...

Another application of ruthenium indenylidene complexes was the atom transfer radical addition of carbon tetrachloride to vinyl monomers reported by Verpoort [61]. This Kharasch reaction afforded good yields for all substrates tested, especially with the catalyst VIII (Equation 8.11, Table 8.8). 

The Kharasch reaction has also been employed by Gennari and coworkers to synthesize, from (i)-carvone, a potential functionalized precursor of sarcodictyins and eleuther-obin (Scheme 62). 

Addition and substitution reactions of heteroatom-centered radicals with multiple bonds have been extensively studied and are sometimes preparatively useful.11 This section will briefly consider the addition reactions of H—Y and X—Y reagents (Kharasch reactions) and substitution reactions (Scheme S6).245... 

Recently, immobilized metal ion-containing ionic liquids were presented for the Kharasch reaction [85]. Whereas copper salts proved to be suitable catalysts in the addition of CCI4 to styrene, FeCl2 gave poor results (12% product yield). 

The most prominent reactions catalyzed by low-valent iron species involving radical intermediates are cross-coupling reactions of alkyl halides (recent reviews [32-35]) and atom transfer radical reactions. In cross-coupling reactions the oxidation state of the catalytically active species can vary significantly depending on the reaction conditions very often it is not known exactly. To facilitate a summary, all iron-catalyzed cross-coupling reactions are treated together and involved oxidation states, where known, are mentioned at the example. In contrast, iron-catalyzed Kharasch reactions will be treated at the oxidation state of the iron precursors. 

The development of ruthenium complexes for other applications in radical chemistry is still in its infancy, but seems well suited to future expansion, thanks to the versatility of ruthenium as a catalytically active center. Large avenues have not been explored yet and remain open to research. For instance, the development of methodologies for the asymmetric functionalization of C-H bonds remains a challenge. The Kharasch-Sosnovsky reaction [51,52],in which the allylic carbon of an alkene is acyloxylated, its asymmetric counterpart, and the asymmetric version of the Kharasch reaction itself are practically terra incognita to ruthenium chemistry, and await the discovery of improved catalysts. 

Selective trapping of alkyl radicals from the alkyl halide component during the course of the catalytic disproportionation is the same as the previous observation with silver, and it indicates that the prime source of radicals in the Kharasch reaction lies in the oxidative addition of alkyl halide to reduced iron in Equation 47. Separate pathways for reaction of i-propyl groups derived from the organic halide and the Grignard reagent are also supported by deuterium labelling studies which show that they are not completely equilibrated.(49) Furthermore, the observation of CIDNP (AE multiplet effect) In the labelled propane and propene... 

This catalytic sequence is known as Kharasch addition or atom transfer radical addition (ATRA) [4]. Various polyhalogenated compounds such as CCI4 and CCI3CO2R are used as the organic halides, and transition metal salts or complexes are used as the catalyst [3]. Intramolecular version of the Kharasch addition reaction (atom transfer radical cyclization, ATRC) has opened novel synthetic protocols to the synthesis of carbocyde or heterocyles catalyzed by transition metals [5-7], and this has become a very important field in free radical cydization in organic synthesis. Transition metal-catalyzed Kharasch reactions sometimes afford telomers or poly-... 

The addition of polyhaloalkanes and related halo-genated compounds to alkenes can occur via a classical radical chain process (Scheme 13), which is often called the Kharasch reaction.38 In 1961, Minisci et al.39 and Asscher and Vofsi40 discovered that this reaction is catalyzed by transition metal ions in their lower valent state such as Cu+ and Fe2+, and they formulated the mechanism in Scheme 14. The catalysis of the additions by simple metal salts or complexes such as Cu(I)-2,2 -bipyridyl41a and ruthe-... 

Radicals are postulated to be important intermediates in a variety of chemical reactions, e.g. the Kharasch reaction and mechanism,... 

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