Alkene 5:

The hydroboration of the structurally isomeric alkenes 1 and 3 is a stereospecific reaction. Each alkene affords a single product of hydroboration -- namely 1 ---> 6 and 3 ---> 10. The transition states for these reactions (5 and 9, respectively) are said to be concerted. Bonds are being formed and broken in concert. If bonds were formed stepwise, as in 1 ---> 7 and 3 ---> 8, then intermediates 7 and 8 could interconvert by rotation about the C -C bond (see the dotted blue arrow) prior to a second event, transfer of hydride (H-) from the boron atom. Under such conditions, both olefins would be expected to give the same borane (6 or 10) or the same mixture of the two boranes. The pathways designated by the red arrows do not occur. The monoalkyl substituted boranes 6 and 10 are racemic because they are formed from achiral starting materials. The remaining B-H bonds in 6 and 10 are available for reaction with their respective olefins although steric factors can inhibit the formation of a trialkylborane. What is the mechanism by which borane 6 is converted into alcohol 2 with retention of configuration? Click here.