Grignard reagents cobalt salts

By using this method, 4-methyl-2 -cyanobiphenyl, an important intermediate for the synthesis of the antihypertensive drug Losartan, can be easily prepared.132 2-(t>-Chlorophenyl)-oxazoline reacts with excess of arylmagnesium halide without catalysis, as it was shown by Meyers for 2-(o-methoxyphenyl)-oxazolines.133 Cobalt(ll) salts catalyze the coupling of Grignard reagents with a-chloroazines at —40°C in ether (Scheme 47).134 135... 

Michael additions of organolithium, Grignard, and diorganozinc reagents to enones and other rt, -unsaturated carbonyl compounds are catalyzed inter alia by copper, nickel and cobalt salts. The best results are obtained with cop-per(I) catalysts, especially those in which copper is bound to a soft , readily polarizable center (sulfur or phosphorus). 

Cobalt catalysts were used for substitution reactions of organic halides [442] and a small-ring enol lactone [Eq. (201) 443]. The cobalt salt seems to be the most effective one among a few transition metal salts involving copper, nickel, [see Eq. (130)], and iron. Without the catalyst, Grignard reagent attacked the carbonyl carbon of the enol lactone to give a mixture of a few ketonic compounds in a low yield. 

Another method of arylating tertiary phosphines is provided by the cobalt salt method , in which the aryl halide reacts in the presence of cobalt(n) chloride and a Grignard reagent. The process has radical character and the alkyl group of the Grignard reagent does not become attached to the phosphorus. 

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. ... 

In the presence of catalytic amounts of cobalt, nickel, or copper salts Grignard reagents react with propargyl chlorides to produce allenes [equation (13)]. ... 

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