Reduction hydroboration-protonolysis

Three types of reactions are used to convert alkynes to alkenes and alkanes catalytic reduction, hydroboration-protonolysis, and dissolving-metal reduction. 

Steps a-c (hydroboration-protonolysis-oxidation) represent a method for the selective reduction of a terminal alkyne in the presence of an alkene. 

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 net effect of hydroboration of an internal alkyne followed by treatment with acetic acid is reduction of the allgme to a ds-alkene. Thus, hydroboration-protonolysis and catalytic reduction over a Lindlar catalyst provide alternative schemes for conversion of an alkyne to a ds-alkene. 

Thus, by the proper choice of reagents and reaction conditions, it is possible to reduce an alkyne to either a ds-alkene (by catalytic reduction or hydroboration-protonolysis) or to a frans-alkene (by dissolving-metal reduction). 

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. 

Although the term hydroboration is most commonly employed2-3 16 to denote the addition of a boron-hydrogen linkage to carbon-carbon multiple bonds, it has also been used for the two-step oxidative process to distinguish it from the process of reduction involving H-B addition and protonolysis. 18... 

Reduction of tertiary kaiides. Tertiary halides are reduced to hydrocarbons by. sodium borohydride in tetramethyicne sulfone (sulfolane, 1, 1144-1145 2, 402 -403). The reaction proceeds by way of elimination, hydroboration, and protonolysis. Halides... 

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. - ... 

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