Organomagnesiums, 154. halogens

The reaction of aryl electrophiles with organomagnesium compounds is known as Kumada or Kumada-Tamao-Corriu reaction. The most common leaving groups in the electrophile are halogen atoms and, among them, chlorine is the most wanted due to the good availability and the low price of aryl or heteroaryl chlorides. Unfortunately, the oxidative addition of a metal center to an aryl chloride is a difficult reaction and many efforts have been made to overcome existing limitations. 

Oshima, K. Selective halogen—magnesium exchange reaction via organomagnesium ate 483 complexes./. Org. Chem. 2001, 66, 4333—4339. 

The first alkyne cyclisations, from 377, 379 and 381, predate the early alkene cyclisations by a couple of years these three date from 1966173 and 1967,174 and illustrate the favourability of both exo and endo-dig cyclisation. All three generate benzylic vinyllithiums (378, 380 and 382), and both aryl (377, 379) and alkyl halides (381) are successful starting materials. Similar organomagnesium cyclisations were described at about the same time.175 However, it is not clear in these reactions how much of the product is due to participation of radicals in the mechanism - alkylbromides undergo halogen-metal exchange with alkyllithiums via radical intermediates (chapter 3).176 If it really is an anionic cyclisation, cyclisation to 378 is remarkable in being endo. Endo-dig anionic cyclisations are discussed below. 

Gilman s colour test 2, which depends on rapid metal-halogen exchange with 4-bromo-./V,7V-dimethylaniline, is positive for alkyllithium compounds [3], but not for organomagnesium compounds. 

Reactions of organomagnesium compounds with A-halogenoamines give variable proportions of halogen compounds and amines (see also Chapter 14) ... 

Formation of Carbon-Halogen Bonds via Organomagnesium Compounds... 

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