Alkyne derivatives haloalkynes

Some of the most useful synthetic transformations of terminal alkynes involve intermolecular and intramolecular homo- and cross-coupling reactions between their. sp-carbon centers, leading to butadiyne or polyyne derivatives. The two most widely used and practical systems are (i) oxidative homocoupling reactions, i.e. Glaser and Eglington reactions and (ii) heterocoupling reactions, i.e. Chodkiewicz-Cadiot coupling of a terminal alkyne with a haloalkyne. 

Even internal alkynes like 3-hexyne undergoes dihydroboration to yield a gem derivative, whereas 1-haloakynes undergo only monohydroboration at the 1 position. This reveals that the electronic rather than the steric factor is more important in limiting the reaction of 9-BBN with 1-haloalkyne to the monohydroboration product only. 

The B-( )-l-alkenyl-9-BBN derivatives exhibit exceptional stereospecific reactivity [12] and are prepared via the regio- and stereospecific monohydrobora-tion [13] of 1-alkynes with 9-BBN or dehydroboration [14] of the 1,1-diboryl adduct. For the synthesis of B-(Z)-l-alkenyls-9-BBN, the hydroborylation of 1-haloalkynes with 9-BBN, followed by treatment with tert-butyllithium afford [15] a mixture of two products, due to indiscriminate migration of both the hydride and cyclooctyl ring. The controlled catalytic hydrogenation of B-l-al-kynyl-9-BBN derivatives under a variety of experimental conditions also results in a complex mixture of products [16]. 

Compounds containing halogen atoms directly joined to triply bonded carbon (haloalkynes) are formed either by (1) reaction between molecular halogen and a metallic derivative of the alkyne. 

Biphenyl derivatives bearing an alkyne unit at one of their ortho positions were converted into substituted phenanthrene derivatives with catalytic amounts of either PtCla, AuCla, GaCls, or InCls in toluene (Scheme 18.5) [5]. The reaction proceeds through the 6-endo-dig cyclization. In the case of biaryls with a lateral haloalkyne unit, InCls was found to affect the intramolecular cyclization with good selectivity. The resulting 10-halophenanthrene derivatives are suited for further elaboration. 

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