Kharasch addition reaction

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... 

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

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... 

There are only very few applications of osmium complexes in radical chemistry. Gasanov and coworkers studied the efficiency of M3(CO)12 (M=Fe, Ru, Os) in Kharasch addition reactions in the presence and absence of DMF as a ligand and found that the efficiency of the catalyst decreases in the order Fe>Ru>Os [93]. DMF activates all three systems. Its role was attributed to the generation of... 

The Kharasch addition reactions promoted by [RuCl2(PPh3)3] are believed to proceed through a redox chain mechanism (Eqs. 1-3) [ 16]. Their kinetics show a first-order dependence both on the ruthenium complex and on CC14. Whereas no clear-cut evidence for alkene coordination to the metal was found with catalyst precursor 1 (which readily loses one phosphine ligand), olefin coordination cannot be excluded because there is a saturation kinetic rate dependence on the alkene. This observation led to the proposal of a reversible step involving olefin coordination to the metal center [ 16,19,20]. Recent work with other ruthenium-based catalysts further supports olefin coordination (see later). 

Besides promoting the Kharasch addition reaction of polyhalogenated alkanes to MMA,the [RuCl2(PPh3)3] complex (1) also initiates the controlled polymerization of MMA, provided that the XCC13 concentration is kept low. Thus, the switch between the polymerization and the 1 1 Kharasch addition reaction depends solely on the relative concentration of the polyhaloalkane ( the initiator in polymerization reactions) to the metal catalyst. Using near-to-... 

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-... 

Scheme 2. Metal-Catalyzed Radical Addition Reaction (Kharasch Addition Reaction)...

As with ruthenium, iron belongs to the group 8 series of elements and can similarly take various oxidation states (—2 to +4), among which Fe(II), Fe-(I), and Fe(0) species have been reported to be active in Kharasch addition reactions.33 For metal-catalyzed living radical polymerizations, several Fe(II) and Fe-(I) complexes have thus far been employed and proved more active than the Ru(II) counterparts in most cases (Figure 2). The iron-based systems are attractive due to the low price and the nontoxic nature of iron. 

Among the various oxidation states of nickel (0— IV), Ni(II) and Ni(0) are the most stable. There have been only a limited number of examples of nickel-catalyzed Kharasch addition reactions, in contrast to those of ruthenium, iron, and copper, probably be-... 

Figure 10.2 Van Koten s Ni-containing metallodendrimer and urea-linked metallodendron catalysts for the Kharasch addition reaction.

Radical chemistiy has advanced tremendously since the discovery of triphenylmethyl radical in 1900 by Goomberg.l The early synthetic work started with Kharasch addition reaction (Scheme 1)2,3 in which halogenated methanes were directly added to olefinic bonds in the presence of free radical initiators or light. However, it was not until early 1980s that the full potential of... 

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