Boranes, with alkynes, reduction acids

Diborane or alkylboranes are used for reduction of alkenes and alkynes via hydrobora-tion (see pp. 37f., 47f., 130f.) followed by hydrolysis of the borane with acetic acid (H.C. Brown, 1975). 

Alkynes are reactive toward hydroboration reagents. The most useful procedures involve addition of a disubstituted borane to the acetylene. Catechol borane (l,3>2-benzodioxaborole), which is prepared from equimolar amounts of catechol (1,2-dihydroxybenzene) and borane, is a particularly useful reagent for hydroboration of acetylenes.Protonolysis of the adduct with acetic acid results in reduction of the original alkyne to the corresponding c/5-alkene. Oxidative workup with hydrogen peroxide gives ketones via an enol intermediate. Treatment of the vinyl borane with bromine and base leads to the vinyl bromide. The net anh-addition has been rationalized on the basis of anh-addition of bromine followed by a second z/tr/-elimination of bromide and boron but there are exceptions to this generalization. 

Like the double bond, the carbon-carbon triple bond is susceptible to many of the common addition reactions. In some cases, such as reduction, hydroboration and acid-catalyzed hydration, it is even more reactive. A very efficient method for the protection of the triple bond is found in the alkynedicobalt hexacarbonyl complexes (.e.g. 117 and 118), readily formed by the reaction of the respective alkyne with dicobalt octacarbonyl. In eneynes this complexation is specific for the triple bond. The remaining alkenes can be reduced with diimide or borane as is illustrated for the ethynylation product (116) of 5-dehydro androsterone in Scheme 107. Alkynic alkenes and alcohols complexed in this way show an increased structural stability. This has been used for the construction of a variety of substituted alkynic compounds uncontaminated by allenic isomers (Scheme 107) and in syntheses of insect pheromones. From the protecting cobalt clusters, the parent alkynes can easily be regenerated by treatment with iron(III) nitrate, ammonium cerium nitrate or trimethylamine A -oxide. ° ... 

Again, as seen with alkenes, both borane (B2H6) (Equation 6.22) and diimide (H-N=N-H) (Scheme 6.15) can be used to reduce alkynes. Indeed, the reaction of internal alkynes with borane is apparently more facile than that with the alkene that results from consummation of the reduction. Further, as would be expected, suprafacial addition of hydrogen and boron obtains and Z- (or cis-) alkene is the only product. The use of deuterated boron compounds (e.g., the hindered 9- H-9-borabicyclo[3.3.1]nonane [9- H-9-BBN]), commercially viable since it contains only one deuterium ( H), followed by use of deuterated acetic acid (CH3C02, i.e., reductive workup) produces Z- (or cw-)-dideuteroalkene of high stereospecificity and in high yield (Scheme 6.65). 

---