Complex Kharasch

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. 

This chapter will begin with a discussion of the role of chiral copper(I) and (II) complexes in group-transfer processes with an emphasis on alkene cyclo-propanation and aziridination. This discussion will be followed by a survey of enantioselective variants of the Kharasch-Sosnovsky reaction, an allylic oxidation process. Section II will review the extensive efforts that have been directed toward the development of enantioselective, Cu(I) catalyzed conjugate addition reactions and related processes. The discussion will finish with a survey of the recent advances that have been achieved by the use of cationic, chiral Cu(II) complexes as chiral Lewis acids for the catalysis of cycloaddition, aldol, Michael, and ene reactions. 

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). 

SCHEME 128. Catalytic enantioselective Kharasch-Sosnovsky reaction catalyzed by different Cu-oxazohne chiral complexes... 

The pertinent part of the story thus lies between 1915 and 1940. In order to appreciate it, we must detail some aspects of Gomberg s discovery, know a deal about the Nobel committee for chemistry and its decision-making procedures as laid down by the statutes and by internal rules, and see how Gomberg s work was analyzed and judged in the light of this complex system of rules. In the process, we will also deal with a few other pioneers of radical chemistry and their relationship to the Nobel institution, namely W. Schlenk, F. Paneth and, briefly, M.S. Kharasch. 

Recently, Filrstner and coworkers have prepared a super-ate complex of iron(II) as shown in Scheme 5. The structure was fully characterized by X-ray crystallography. They have shown that methylmagnesium bromide reacts with pulegone in the presence of this complex to give the corresponding endocyclic silyl dienol ether. Consequently, they have proposed that a similar ate-complex is probably involved when the reaction is performed under the Kharasch conditions. 

Another example is the palladium-catalyzed oxidation of ethylene to acetaldehyde in the presence of oxygen and cupric salts, the so-called Wacker reaction. This catalytic cycle combines two stoichiometric processes, which involve first the reduction of Pd11 to Pd°, followed by reoxidation with Cu11. The understanding of the first step of this process came from the earlier work of Kharasch et al., who showed that the stoichiometric dinuclear complex shown in Figure 2.14 decomposed in the presence of water to acetaldehyde (ethanal), Pd° and HC1 [38]. 

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... 

Fig. 7 Catalytic cycle of Kharasch additions catalyzed by iron(0) complexes...

Susuki and Tsuji reported the first Kharasch addition/carbonylation sequences to synthesize halogenated acid chlorides from olefins, carbon tetrachloride, and carbon monoxide catalyzed by [CpFe(CO)2]2 [101]. Its activity is comparable to or better than that of the corresponding molybdenum complex (see Part 1, Sect. 7). Davis and coworkers determined later that the reaction does not involve homolysis of the dimer to a metal-centered radical, which reduces the organic halide, but that radical generation occurs from the dimeric catalyst after initial dissociation of a CO ligand and subsequent SET [102]. The reaction proceeds otherwise as a typical metal-catalyzed atom transfer process (cf. Part 1, Fig. 37, Part 2, Fig. 7). 

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