PREPARATION OF ALKYL AND ALKENYL BORANES

Diborane reacts with alkenes to form alkylboranes in ether solvents. This important finding apparently resulted from an observation by Brown that reduction of ethyl oleate, in ether solvents and in the presence of a Lewis acid, consumed 2.4—2.5 hydrides per mole and 3 equivalents when a larger excess of the reagent was used.3 Brown discovered that borane had added to the it bond of the alkene moiety generating a monoalkyl- 

An excellent in situ preparation of diborane results from the reaction of boron trichloride (BCI3) or boron trifluoride (BF3) with sodium borohydride in the presence of alkenes, as shown above. This reagent gave virtually instantaneous conversion of an alkene to a trialkylborane at ambient temperatures, when done in a ether solvent. When diborane was isolated and purified it also reacted with alkenes, rapidly and quantitatively in ether solvents. 

General Properties of Boranes. When borane reacts with alkenes there are several predictable characteristics.9 

Borane reacts with alkenes and alkynes rapidly and quantitatively (in most cases) hy addition of the hydrogen-horon moiety to the Jt- bond of an alkene or an alkyne, to give an alkylborane or an 

The reaction proceeds hy cis addition of hydrogen and horon, probably via a four-center transition state such as 4. 

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