Question

Is hydroboration syn or anti?

Answer 1

Explanation with Diagrams:

Hydroboration is an example of an alkene electrophilic addition reaction because alkanes have double bonds, they are nucleophilic partners, whereas borane is an electron-deficient species and thus an electrophile partner.

The following mechanism represents general alkene hydroboration:

Diagram:

1. We conclude that the addition of $\mathrm{H}$ and ${\mathrm{BH}}_2$ to alkenes occurs through the same face.
2. As a result, ${\mathrm{BH}}_3$ addition is a syn addition.
3. Hydroboration is a one-step reaction with four member transition states.
4. The hydration doesn't stop here; This continues with two more alkene molecules, resulting in a trialkyl borane containing all three hydrogen atoms${\mathrm{BH}}_3$.

Diagram:

Hence, hydroboration is syn, not anti.