Addition Kharasch

Figure 4.12. Continuous Kharasch addition. (Reprinted with permission from ref. 40. Copyright 2000...

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. 

Carbon-carbon bond formation is a fundamental reaction in organic synthesis [1, 2,3,4], One way to form such a bond and, thus, extend a carbon chain is by the addition of a polyhalogenated alkane to an alkene to form a 1 1 adduct, as shown in Scheme 1. This reaction was first reported in the 1940s and today is known as the Kharasch addition or atom transfer radical addition (ATRA) [5,6], Historically, Kharasch addition reactions were conducted in the presence of radical initiators or... 

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

An early example of a dendritie eatalyst was reported by Knapen et al. 24), who functionalized GO (generation zero) and G1 earbosilane dendrimers with up to 12 NCN pincer-nickel(II) groups (7a). These dendrimers were applied as eatalysts in the Kharasch addition of organic halides to alkenes (Seheme 3). 

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).

Remarkably, an increase in steric congestion ongoing from dendrimer 84a to 84b, and finally to 84c did not affect the catalytic performance in this reaction, which is in contrast with the results representing the Kharasch addition reaction. 

The nature of the chlorinated reagent is crucial for promoting the Kharasch addition reaction (Equation 8.11). The results showed that carbon tetrachloride could be added to various olefins in a regioselective way. Under these reaction conditions, no polymerization products were detected. In contrast, when chloroform was used as the halide source the methyl methacrylate and styrene conversions reached only 33% and 40% with the best performing system (VIII), and a significant fraction of polymers was observed [61]. 

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. 

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

Scheme 8.11. Aryl Ni(II) surface modified dendrimers were demonstrated1441 to be effective at catalyzing the Kharasch addition reactions.

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... 

Susuki and Tsuji reported the first Kharasch addition/carbonylation reactions of olefins 126 with CCI4 and carbon monoxide catalyzed by [CpMo(CO)3]2 127 and proposed a carbometalation mechanism (Fig. 37) [229]. A kinetic study disproved... 

Mo(CO)6 was also used to catalyze Kharasch addition reactions of tetrachloro-methane or trichloroacetates to terminal olefins in acetonitrile (Fig. 38) [218]. A precomplexation like for Cr(CO)6 was not necessary. The ligand exchange... 

Fe(II)-Fe(III) Catalysis Kharasch Additions and Atom Transfer Cyclizations 210... 

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... 

Fig. 6 Kharasch additions catalyzed by low-valent iron species...

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). 

Freidlina et al. used 10 mol% of FeCl3 and 40 mol% of /V,/V-dimethylaniline to promote Kharasch additions of methyl dibromoacetate, methyl 2,2-dibromopropionate, methyl tribromoacetate, or dibromomalonate to acceptor-substituted olefins (cf. Fig. 6) [89]. Either linear addition products 32 or lactones were obtained in 23-60% and 43 -8% yield, respectively. 

By use of chelating tri- and tetramines 59 or 60 Kharasch additions can be performed in 65-75% yield using 0.3-10 mol% FeCl2 as the catalyst (Fig. 13) [116, 117]. The same catalysts are very efficient to promote otherwise difficult ATRC of co-alkenyl trichloroacetates 58 providing five-membered lactone 61 by 5-exo cycli-zation in 55% yield and eight- to ten-membered lactones 62 by 8-10-endo cyclizations in 34-50% yield, respectively. Even oo-allyl oligo(ethyleneoxy) trichloroacetates underwent radical macrolactonization reactions in 56-60% yield with 10 mol% of catalyst. 

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