B4 Organoboron routes to unsaturated hydrocarbons

Hydroboration of 1-haloalk-l-ynes followed by treatment with sodium methoxide and then acetic acid produces (E)-alkenes (Equation B4.1). 

1-Haloalk-l-ynes may be generated by halogenation of the corresponding lithium acetylides at low temperature. 

Direct displacement at an sp2 carbon centre is generally energetically disfavoured. 

Note that protonation of the alkenylborane proceeds with retention of stereochemistry - see Section B2.2. 

Only one alkyl group is transferred from boron to carbon in the reaction and so generation of the required dialkylborane from thexylborane prevents wastage of more valuable alkyl groups (Equation B4.2). Note that the migrating alkyl group involved in the reaction sequence depicted in Equation B4.2 retains its stereochemistry. 

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