Synthesis and Reactions of Alkylboranes

Alkenes react with borane, BHj, and a number of its derivatives to give synthetically useful alkylboranes. Borane is an electron-deficient molecule and in its pure form exists as a hydrogen-bridged dimer. 

Borane reacts with electron pair donors to form Lewis acid-base complexes. The most common forms for use in synthesis are the THF and dimethyl sulfide complexes. Stronger bases, particularly amines, form less reactive adducts. 

The addition of borane to alkenes is an electrophilic process, but it is concerted. The alkene donates electron density to the borane but a hydrogen shifts to carbon. 

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