Unlike most electrophilic additions to alkenes that occur in a stepwise manner via charged intermediates, this addition is concerted so that both new bonds are formed more or less at the same time. The result is a new borane in which one of the hydrogen atoms has been replaced by an alkene. This monoalkyl borane is now able to undergo addition with another molecule of the alkene to produce a dialkyl borane, which in turn undergoes further reaction to produce a trialkyl borane. All these boranes have a vacant p orbital and are flat so that repeated attack to produce the trialkyl borane is easy and normal if an excess of alkene is present.
Hydroboration is regioselective and the addition is syn across the alkene. The boron atom always adds to the end of the alkene and is attached to the carbon of the double bond which is less substituted. The two steps of the reaction are concerted, but formation of the C-Br bond goes ahead of formation of the C-H bond so that boron and carbon are partially charged in the four-centred transition state.
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