Kharasch catalysts

Van Koten et al. reported on a negative dendritic effect in the Kharasch addition reaction. [3 9,40] A fast deactivation for the carbosilane dendrimer supported NCN pincer catalyst (Figures 4.28 and 4.29) was observed by comparison with a mononuclear analogue. This deactivation is expected to be caused by irreversible formation of inactive Ni(III) sites on the periphery of these dendrimers. 

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). 

The allylic acyloxylation of alkenes, the Kharasch-Sosnovsky reaction, Eq. 81, would be an effective route to nonracemic allylic alcohol derivatives, if efficient, enantioselective catalysts were available. The reaction is mediated by a variety of copper salts, and as such, has been the target of considerable research in an attempt to render the process enantioselective. The original reaction conditions described by Kharasch require high temperatures when CuBr is used as the catalyst (93). However, the use of CuOTf (PhH)0 5 allows the reaction to proceed at temperatures as low as -20°C. Unfortunately, long reaction times are endemic in these processes and the use of excess alkene (2-100 equiv) is conventional. Most yields reported in this field are based on the oxidant. 

The use of bis(oxazoline) ligands in the Kharasch-Sosnovsky reaction proved to be beneficial, affording well-behaved catalysts. The original investigations were communicated independently and concurrently by Pfaltz and co-workers (108) and Andrus et al. (109). 

The application of dinuclear metal catalysts to the Kharasch-Sosnovsky reaction is mechanistically intriguing due to their illustrated role in mediating biological oxidations (119). Fahmi (120) examined a variety of dinucleating ligands with Cu(MeCN)4PF6 as catalysts in the allylic oxidation of cyclohexene, Eq. 102. In these studies, early results have been inferior to those obtained from bis(oxa-zoline)-copper catalysts. 

Figure 2.45. ATRP and Kharasch addition with nickel catalyst...

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

Fig. 10. The application of 7c as catalyst for the Kharasch addition of methyl methylacrylate and carbon tetrachloride in a CFMR using a SelRO-MPF-50 nanofiltation membrane. (Residence time (or cycle) is 43 min) (26).

An early example of a dendritic catalyst was reported by Knapen et al. 24), who functionalized GO and G1 carbosilane dendrimers with up to 12 NCN pincer-nickel(II) groups (7a) and applied them as catalysts in the Kharasch addition of organic halides to alkenes (Scheme 3). 

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). 

Undoubtedly, the most notable feature of these new dendrimeric organometallic molecules is their ability to act successfully as effective homogeneous catalysts for the Kharasch addition reaction of polyhalogenoalkanes to olefinic C=C double bonds. Indeed, they show catalytic activity and clean regiospecific formation of 1 1 addition products in a similar way to that observed in the mononuclear compounds. Likewise, the nanoscopic size of these first examples of soluble dendritic catalysts allows the separation of such macromolecules from the solution of the products by ultrafiltration methods. 

The first attempts to modify the course of the reaction of Grignard reagents with various substrates by using iron salts as a catalyst were reported by Kharasch and coworkers ... 

Kharasch Sosnovsky (Ref 10) reinvestigated the reaction and concluded that, using aq H202, cyclohexanone always gave conqid II in the absence of an acid catalyst. In the presence of an acid, compd III was always formed treatment of III with perchloric acid gave the peroxide VI, in addn to high-boiling acids... 

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). 

Scheme 4 Proposed deactivation pathway for Gi-2 and G2-2 catalysts in the Kharasch addition...

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... 

Nesmeynov s and Freidlina s groups studied Kharasch additions of methyl dibromoacetate 31 to olefins 30 furnishing 2,4-dibromoesters 32 thoroughly in the 1960s and 1970s (Fig. 6) (review [89]). The preferred catalyst was iron... 

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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