Arylation alkyl halides, cobalt-catalyzed

Copper- and Cobalt-Catalyzed Arylations of Alkyl (Pseudo]Halides... 

Although cobalt catalysis is somewhat less developed compared to other transition metal catalysis for cross-coupUngs, but has shown rather unconventional, novel reactivity profiles [50]. For example, Oshima reported a cobalt-catalyzed tandem radical cyclization/cross-coupling reaction between an aryl Grignard reagent and an alkyl halide bearing an (o-alkenyl group (Equation 5.44) [51]. 

Table 5.5 Cobalt-catalyzed cross-coupling (and tandem cyclization/arylation) reactions between unactIvated alkyl halides and aryl metal reagents.

A series of papers have also reported the coupling of alkyl and aryl electrophiles with aryl Grignard reagents catalyzed by iron (Equation 19.15) and cobalt complexes. These reactions build upon Kochi s and Molander s early results on coupling reactions catalyzed by complexes of these metals. The recent reactions have been conducted with simple metal salts in many cases and with discrete metal complexes as catalyst precursors in others. Although little recent mechanistic data is available on these reactions, they have earlier been shown to involve radical intermediates. Kochi concluded that the catalytic process occurs by an Fe(I)-Fe(III) couple reactions of optically active alkyl halides generate racemic coupled products and reactions of diastereomerically pure aUcyl halides generate equal ratios of diastereomeric products, as depicted in Equations 19.16 and 19.17. ... 

Cobalt has attracted attention in recent years as a cost-effective and sustainable alternative to palladium or nickel. As indicated earlier, functionalized arylzinc reagents can be prepared from aryl halides and sulfonates by zinc insertion in the presence of catalytic amounts of CoBtj [50, 51, 210). Interestingly, the presence of this cobalt salt suffices to catalyze in situ the cross-coupling of the arylzincs thus produced with aryl, alkyl, and heteroaryl electrophiles such as organic halides [211-214] or thioethers, as exemplified with the formation of 286 [214] (Scheme 4.63). 

The use of cobalt species to catalyze cross-coupling reactions of activated or unactivated alkyl halides with alkynyl Grignard reagents in order to form C(sp)-G(sp ) bonds is also of interest. Cobalt compleres promote the crossbenzyl chlorides and bromides react with 2-trimethylsilylethynyl- and 1-octyn-l-ylmagnesium halides to provide l-aryl-2-alkynes in the presence of a catalytic amount of a cobalt complex [100,101]. 

As a Generator of Alkyl Radicals from Alkyl Halides in the Presence of Cobalt Catalysts. A conceptually novel use of Me3SiCH2MgCl recently emerged in cobalt-catalyzed Heck-type transformations. The palladium-catalyzed Heck reaction always etT5)loys aryl- or alkenyl halides. In eontrast, alkyl halides are not available for use as starting material, due to predominant 0-hydride elimination from the corresponding alkylpalladium... 

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