Alkyl halides halogens + alkynes

Just as It IS possible to prepare alkenes by dehydrohalogenation of alkyl halides so may alkynes be prepared by a double dehydrohalogenation of dihaloalkanes The dihalide may be a geminal dihalide, one m which both halogens are on the same carbon or it may be a vicinal dihalide, one m which the halogens are on adjacent carbons... 

The negative charge and unshared electron pair on carbon make an acetylide anion strongly nucleophilic. As a result, an acetylide anion can react with an alkyl halide such as bromomethane to substitute for the halogen and yield a new alkyne product. 

Halogenation of alkenyl organometallic compounds Addition of hydrogen halides to triple bonds Halogenation of alkynes or allenes Addition of alkyl halides to triple bonds Addition of acyl halides to triple bonds... 

Scheme 5.7 illustrates these and other applications of the hydride donors. Entries 1 and 2 are examples of reduction of alkyl halides, whereas Entry 3 shows removal of an aromatic halogen. Entries 4 to 6 are sulfonate displacements, with the last example using a copper hydride reagent. Entry 7 is an epoxide ring opening. Entries 8 and 9 illustrate the difference in ease of reduction of alkynes with and without hydroxy participation. 

Halogenation of alkynes or allenes 5-33 Addition of alkyl halides to triple bonds... 

The reactivity order also appears to correlate with the C-X bond energy, inasmuch as the tertiary alkyl halides both are more reactive and have weaker carbon-halogen bonds than either primary or secondary halides (see Table 4-6). In fact, elimination of HX from haloalkenes or haloarenes with relatively strong C-X bonds, such as chloroethene or chlorobenzene, is much less facile than for haloalkanes. Nonetheless, elimination does occur under the right conditions and constitutes one of the most useful general methods for the synthesis of alkynes. For example,... 

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. 

Pd(ll) catalysts can be employed as well. Compared to the standard Pd(ll) catalysts, the palladacycle catalyst 7 shows a broader compatibility with functional groups. Terminal alkynes, alkyl halides, and a-halogen ketones are tolerated <1997CB1449>. It is worth mentioning that other catalysts of Pd(ll) preferentially give dimerization products. 

Hydroalumination of alkenes. Hydroalumination of alkynes is a well-known reaction, but hydroalumination of alkenes has been achieved only recently under catalysis by TiCU or ZrCU, (8, 288). As expected hydroalumination affords a convenient, high-yield route to primary alkanes (by hydrolysis), terminal primary alcohols (by oxygenation), and primary alkyl halides (reaction with halogens, N-halosuccinimides, or CuXa). ... 

High yields of alkynes can be obtained from the reaction of 1,1-dichloromethyl-lithium with alkyl halides and subsequent dehydro-halogenation. The alkyne anion (40) with alkyl halides gives a-substituted prop-2-ynylamines and with CO2 gives a-acetylenic amino-acids, in good yields. 

Alkyl The general name for a radical of an alkane an alkyl halide is a halogenated hydrocarbon whose hydrocarbon backbone originated from an alkane. Aligmes An analogous series of unsaturated hydrocarbons with the general fomiula C H2 .2 the alkynes all contain just one triple bond between carbon atoms. 

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