Carbon-carbon double bonds react with B-H bonds. Boron is added to one end of the double bond, and hydrogen to the other end. The reaction proceeds through a four-membered ring transition state, which means that the boron and the hydrogen must add on the same face of the double bond. A partial positive charge develops during the reaction on the carbon which will become attached to the hydrogen. The boron, therefore, becomes attached to the carbon that is less able to stabilise a positive charge.
