Grignard with halides

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. 

Grignard reagents react with halides of less electropositive metals to give other organometallic compounds. The less electropositive (more electronegative) metals include Hg, Zn, Cd, Si, and the nonmetal P. An example of this type of reaction is... 

Monosubstitution of acetylene itself to prepare terminal alkynes is not easy. Therefore, trimethylsilylacetylene (134) is used as a protected acetylene. After the coupling, the silyl group is removed by the treatment with fluoride anion. The hexasubstitution of hexabromobenzene (135) with 134 afforded hexaethynylbenzene (136) after desilylation in total yield of 28%. The product was converted to tris(benzocyclobutadieno)benzene (137) using a Co catalyst (see Section 7.2.1). Hexabutadiynylbenzene was prepared similarly [60], As another method, terminal alkynes 139 are prepared in excellent yields by the coupling of commercially available ethynyl Grignard (138) or ethynylzinc bromide with halides, without protection and deprotection [61]. 

THF is higher-boiling and more Lewis-basic than ether, and Grignard reagent preparations in this solvent are often successful with halides which react only with difficulty or not at all in ether, notably vinyl halides and... 

In the reaction with halides of the less electropositive metals such as mercury,79 arsenic,80- 81 antimony,82 and indium,83 a stannane provides nucleophilic hydrogen and gives the new metal hydride (equation 15-27). Free radical inhibitors may be added to repress radical reaction of vinyl reactants. The preparation of vinylmercury hydride by this route involves the unusual use of tributyltin chloride as a solvent (equation 15-28).79 On the other hand, in the reactions of tributyltin hydride with Grignard reagents, RMgX, the hydrogen behaves as an electrophile towards R, and the Sn-Mg bonded compounds are formed.84... 

Indolyl A-Grignards, "" or even better their zinc analogues, undergo reaction predominantly at C-3 with a variety of carbon electrophiles such as aldehydes, ketones and acid halides, or reactive halo-hetero-cycles. Including aluminium chloride in the zinc reactions produces high yields of 3-acyl-indoles. The copper-catalysed reactions of indolyl-A-Grignards with A-t-butoxycarbonyl-aziridines also proceed well at C-3. ... 

Activation of Mg in Grignard preparations by addition of HgCl2. Magnesium turnings activated in this manner react at temperatures in the region of 0°C with halides, e.g. 

Mannich reaction of acetophenone with formaldehyde and piperidine results into the formation of an aminoketone, which on grignardization with bicyclic halide and subsequent neutralization with HCl affords the offieial eompound biperiden hydrochloride. 

Stereospecific coupling of 1,2-disubstituted vinyl groups is attained by reaction of a vinyl Grignard with CrClg, CoClj, PdClj, and NiBrz catalysts (50, 51). The stereospecificity of the products appears to result from the selective configuration of the transition metal halide (Fig. 20). 

Coupling reactions between alkyl Grignards and aryl halides are usually successful using nickel-phosphine catalysts however, if the aromatic halide has an alkoxy substituent this does not work very well and it is better to couple an aromatic Grignard with an alkyl halide as shown above. 

Allyl complexes of the transition metals are most commonly synthesized by reactions of Grignard compounds or organolithium derivatives with halides or acetylacetonates of the transition metals. Dependent upon the starting material, complexes containing exclusively allyl groups (homoleptic complexes) may be obtained, or derivatives also possessing other ligands (heteroleptic complexes) may be prepared ... 

The Grignard reagents CgH (CH2MgCl)2-o react with MCI4O with halide elimination and formation of either 25 or W(CH2CgH CH2 )3,... 

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