Transmetallation Grignard compound

In Grignard reactions, Mg(0) metal reacts with organic halides of. sp carbons (alkyl halides) more easily than halides of sp carbons (aryl and alkenyl halides). On the other hand. Pd(0) complexes react more easily with halides of carbons. In other words, alkenyl and aryl halides undergo facile oxidative additions to Pd(0) to form complexes 1 which have a Pd—C tr-bond as an initial step. Then mainly two transformations of these intermediate complexes are possible insertion and transmetallation. Unsaturated compounds such as alkenes. conjugated dienes, alkynes, and CO insert into the Pd—C bond. The final step of the reactions is reductive elimination or elimination of /J-hydro-gen. At the same time, the Pd(0) catalytic species is regenerated to start a new catalytic cycle. The transmetallation takes place with organometallic compounds of Li, Mg, Zn, B, Al, Sn, Si, Hg, etc., and the reaction terminates by reductive elimination. 

The last electrophilic substitution reaction of a Grignard compound we want to consider is a transmetalation, namely one that leads to an arylboronic ester (Figure 5.45). Arylboronic esters or their hydrolysis products, the arylboronic acids, are valuable reagents in modem aro-... 

Fig. 16.11. Nickel-catalyzed arylation of Grignard compounds. The Grignard compound can be prepared via a substituent-directed peri-lithia-tion of a substituted naphthalene (see Section 5.3.1 for the analogous ortho-lithiation) and subsequent transmetalation with MgBr2 (for the method cf.

Dlalkylzincs are stable, distillable liquids that can be made by transmetallating Grignard reagents with zinc bromide. They are much less reactive than organolithium or organomagnesium compounds, but they are still rather basic and react with water to give zinc hydroxides and... 

Organolithium and -magnesium compounds. Compared with extensive studies carried out on the Ni-catalyzed transmetallation reaction of Grignard reagents[43I,432], few examples of the Pd-catalyzed reactions of Mg are... 

Tin compounds can be produced by the Nalco-type reactions39 as well as by Cd and Zn mediated methods42,43. The use of organotin compounds in electrodically induced transmetallations has also been described in a study in which Grignard-type allylation of carbonyl compounds has been carried out by electrochemically recycled allyltin reagents44. 

Asymmetric deprotonation of the achiral oxazolidine iV-Boc-4,4-dimethyl-l,3-oxazolidine with s-BuLi-(—)-sparteine affords a lithium derivative that adds unselectively to aldehydes. However, the transmetalation from lithium to magnesium, and addition of the resulting Grignard to benzaldehyde occurs with 90% diastereoselectivity and 93% enantioselectivity. The authors speculate that deprotonation and lithiation occur stereoselectively to give the R organolithium compound, and subsequent transmetalation and addition to benzaldehyde proceed with retention (Scheme 40). 

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