Alkylboranes reaction with alkynes

Internal alkynes induce olefin loss from alkylboranes to form alkenyl boranes. Thus, (C HsjaB reacts rapidly and reversibly with alkynes at 160-200°C to form trial-kenylborane and C2H4. With (r-C4H9)3B, the reaction proceeds under milder conditions. It is not known whether such reactions proceed via B-H intermediates produced by -hydride elimination, or by intramolecular hydride transfer from the alkyl group to a coordinated acetylene. This reaction is useful for reduction of acetylenic hydrocarbons. 

Catecholborane (34) reacts with alkynes but requires reaction temperatures that are higher than those used for reaction of alkynes and alkylboranes, as noted earlier. Phenylethyne, for example, reacts with 34 to give excellent yields of vinyl borane, but the reaction must be done in refluxing THF. Alkenylborinates derived from catecholborane are stable and can be isolated by distillation or recrystallization,... 

Like alkylboranes, dnylboranes can be converted into alcohols by treatment with basic peroxide (Fig. 10.75).The products are ends in this case, and they are in equilibrium with the corresponding carbonyl compounds. Here s an important point The regiochemistry of this reaction is the opposite of the regiochemistry of the hydration reaction of alkynes. This situation is exactly the same as the one that exists with the alkenes Hydration of an alkene gives overall Markovnikov addition, whereas the hydroboration/oxidation sequence gives the anti-Markovnikov product. 

Palladium-catalyzed hydroboration, haloboration, and similar reactions involving addition of organoboron compounds to alkynes have been reviewed.Alkylborane addition to alkynes followed by intra- or intermolecular cross-coupling with a vinyl halide or triflates according to the Suzuki-Miyaura protocol constitutes a very powerful synthetic route for the generation of two carbon-carbon bonds in cascade. This subject and its applications in synthesis have been reviewed. ... 

Protonolysis of alkenylboranes generates an alkene and this reaction is faster than the analogous protonolysis of alkylboranes.57 Once again, protonolysis proceeds stereospecifically with retention of configuration. In Negishi s synthesis of the sex pheromone of Lobesia botrana (65), hydroboration of 64 was followed by protonolysis with acetic acid at 0°C to give 65, for a net conversion of an alkyne to an alkene.58 As with alkylboranes, protonolysis of vinylboranes with deuteroacetic acid allows the synthesis of deuterated alkenes. 

By far the most widely used method for making boron-carbon bonds is by the addition of boranes to alkenes or alkynes. Much of the work in this area is due to H.C. Brown. This reaction is termed hydroboration . Although it is reversible the equilibrium normally lies in favour of product rather than reactants in ether solvents at room temperature, so that the addition goes to completion. On heating above ca. 100°C organoboranes with a j5-C—H function eliminate alkene, that is, the reverse reaction takes place. Isomerization of alkylboranes occurs by a series of these addition-elimination steps, leading eventually to terminal boranes. The alkene can then be displaced by a less volatile alkene. The direction of this... 

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