Cross alkyl halides, cobalt-catalyzed

Transition metal-catalyzed cross-coupling reactions between vinyl organometallic compounds and unactivated alkyl halides that can be usually performed with palladium, nickel and cobalt are of particular synthetic interest [37-39]. Recently, the groups of Cahiez [48] and Cossy [49] concurrently reported the first iron-catalyzed reaction of alkenyl Grignard compounds with primary and secondary alkyl halides (X=Br, I) (Scheme 5.15). The two protocols basically differ in the iron source... 

Fig. 50 Cobalt-catalyzed cross-coupling of alkyl halides with Grignard reagents...

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.

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]. 

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). 

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