Alkyl halides organomagnesiums + halogens

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. 

Dimethylberyllium may be synthesized in Me2S solvation occurs, but free BeMe2 can be obtained by repeated sublimation . Using organomagnesium halides formed in benzene (alkyl halogen ratio >1) solutions of uncoordinated BeR2 (R = Et, n-Bu, n-CjHii, Ph, p-tolyl) are obtained, and from these solutions free dialkyl compounds should be isolable, although, as yet, this is realized only for Be(Bu-n) . 

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