Preparation of Alkynes and Alkyne Anions

As the chain length of the vinyl halide increases, the yield of terminal alkyne product generally decreases. 3,29 

Vinyl bromide 22 (n = 0) gave 61% of acetylene 23 (n = 0), for example, but 1-bromo-l-hexene (22, n = 4) gave only 18% of the corresponding alkyne (23, n = 4).29 Alkynes can also be formed from nonterminal (internal) vinyl halides (such as 3-bromo-3-heptene), but when alkoxide bases are used, the vigorous conditions usually required for elimination of the halogen can cause the triple bond of the product to migrate to the terminal position.30 

A useful source of terminal alkynes are 1,1-dibromoalkenes such as 24. When this was treated with n-butyl-lithium in THF at -78°C, alkyne 25 was formed in 83% yield.3 This example was taken from Evans synthesis of (+)-miyakolide. Dibromides such as 24 can be formed from aldehydes or ketones via a variation of the Wittig olefination reaction (see sec. 8.8.A). 

Coupling is an important reaction of alkynes that leads to di-ynes. It occurs under a variety of condi-tions,32 but is particularly important when alkynylcopper derivatives are involved. Two classical alkyne coupling reactions involve copper derivatives. In the Glaser reaction,33 where an alkyne such as phenylacetylene reacts with basic cupric chloride (CuCl2), subsequent air oxidation gives a diyne (in this case 26 in 90% yield). 

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