Organoboranes protonolysis

The sterically demanding organoboranes provide a efficient tool for non-catalytic ds-hydrogenation of carbon-carbon triple bonds. Coupled with protonolysis, this sequence gives an elegant method for the synthesis of corresponding m-enynes or... 

As the second addition of disiamylborane is very solw, the dihydroboration can be carried out with sterically less hindered dicyclohexylborane. Protonolysis of the intermediate organoboranes with acetic acid affords the corresponding cis, m-dienes (Eq. 132). 

The mechanism of the reaction is not certain, but it iq)pears to have analogies with the protonolysis of organoboranes by carboxylic acids and on that basis codd be formulated as in equation (38), which is an example of general equation (7). 

Initially, protonolysis of the organoborane adducts was carried out with propionic acid. The adducts prepared from dichlorovinylborane were treated with two equivalents of sodium methoxide to avoid the formation of hydrogen chloride, and then refluxed with... 

The reactions of organoboranes are in general not discussed here, but are distributed throughout Comprehensive Organic Synthesis wherever appropriate reaction types are under consideration. However, protonolysis of organoboranes is treated in Section 3.10.7 because of its necessary role in the reduction of carbon-carbon multiple bonds via hydroboration (Scheme 1). Reviews of other synthetically useful organoborane reactions appear elsewhere. - ... 

Protonolysis with carboxylic acids differs from that with mineral acids in that it appears to involve coordination of the carbonyl oxygen atom of the carboxylic acid to the boron atom of the organoborane, followed by intramolecular proton transfer (Scheme 14). - This is presumably the reason for the relative ease of protonolysis with carboxylic acids. 

The protonolysis of alkenyldialkylboranes to give -alkenes can be conducted, in most cases, under neutral conditions using methanol. More hindered alkenyldisiamylboranes react less well, unless a small amount of a carboxylic acid is added. A variety of Z-alkenyl pheromones was prepared using this method. The synthesis of trans-alkenes and unsymmetrical ketones was also accomplished using vinylic organoborane chemistry.7... 

Syn addition of molecular hydrogen to alkenes can be accomplished by protonolysis of organoboranes with carboxylic acids. For a discussion of the mechanism of the reaction and examples of its use in S3mthesis, see reference 235. 

Amination and protonolysis are two important synthetically useful reactions of organoboranes ... 

The base, however, acts to abstract the proton from the a-haloketones to produce the a-halocarbanion. The latter is immediately removed by the reaction with the free, uncomplexed organoborane, and the transfer of alkyl group from boron occurs and product methyl ketone is obtained by the hydrolysis of the intermediate (Scheme 7.4) [2c]. It is significant to mention that protonolysis of the reaction intermediate by 2,6-di-f-butylphenol produced in the reaction is relatively difficult. Consequently, ethyl alcohol is added to liberate ketone. 

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