Boranes protonolysis

In another procedure, the addition of a dialkyIborane to a 1-haloalkyne produces an a-halo vinylic borane (82). Treatment of this with NaOMe gives the rearrangement shown, and protonolysis of the product... 

Alkynes are reactive toward hydroboration reagents. The most useful procedures involve addition of a disubstituted borane to the alkyne, which avoids complications that occur with borane and lead to polymeric structures. Catechol borane is a particularly useful reagent for hydroboration of alkynes.212 Protonolysis of the adduct with acetic acid results in reduction of the alkyne to the corresponding cw-alkene. Oxidative workup with hydrogen peroxide gives ketones via enol intermediates. 

Other disubstituted boranes have also been used for selective hydroboration of alkynes. 9-BBN can be used to hydroborate internal alkynes. Protonolysis can be carried out with methanol and this provides a convenient method for formation of a disubstituted Z-alkene.217... 

Diborane also has a useful pattern of selectivity. It reduces carboxylic acids to primary alcohols under mild conditions that leave esters unchanged.77 Nitro and cyano groups are relatively unreactive toward diborane. The rapid reaction between carboxylic acids and diborane is the result of formation of a triacyloxyborane intermediate by protonolysis of the B-H bonds. The resulting compound is essentially a mixed anhydride of the carboxylic acid and boric acid in which the carbonyl groups have enhanced reactivity toward borane or acetoxyborane. 

The dialkenylchloroboranes undergo the usual reactions of vinylic boranes, e.g., protonolysis with acetic acid gives olefins, oxidation with alkaline hydrogen peroxide provides the corresponding carbonyl compounds. However, the most useful reactions of these compounds are their ready conversion to the stereochemically pure (E, Z)-1,3-dienes by the Zweifel reaction with I2-NaOH 37>107.108> and into the symmetrical (E,E)- 1,3-dienes, 09 0), mono-olefins 1U) and 1,4-dienes (Chart 10). 

Attack of diborane occurs at the /2-carbon atom with the exception of enamines of acetaldehyde for which a-borane product is formed. When isomerism is possible, as in the case of a 2-methylcyclohexanone enamine, the less substituted isomer is the more reactive form, for steric reasons. Protonolysis with propionic acid in diglyme afforded alkenes, as a consequence of a trans elimination of the base and the borane group166. 

Other useful reactions of the boranes include protonolysis of the borane with a carboxylic acid, which leads to hydrogenation of the alkene. 

Conjugated cis,cis-d ieney. Dihydroboration of a conjugated diyne, for example dodcca-5,7-diyne (1), with dicyclohexylborane followed by protonolysis of the organo-borane intermediate with acetic acid leads to a conjugated di.cu-diene (rw.t is-dodeca-... 

Conjugated diynes can be reduced to cis enynes by the well-known hydroboration-protonolysis sequence . cw-5-Dodecen-7-yne (215) is obtained from the diyne 213 in 76% yield as shown in equation (23). The orientation of addition of the disiamyl-borane was established by using CH3CO2D in the second step. Addition of a second mole of disiamylborane to 214 is very slow, but reduction to the cis,cis diene can be accomplished by using dicyclohexylborane instead . 

The hydroboration of 1-bromoalkynes with alkyl(l,l,2-trimelhylpropyl)boranes, followed by treatment with sodium methoxide and protonolysis, provides a-chiral ( )-alkenes9. 

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