How does the mechanism for hydroborations with #BH_3# in alkene happen?
The hydroboration of alkenes is a single-step concerted mechanism.
The π electrons act as the nucleophile with the electrophilic B atom, and the H is transferred to the C with syn stereochemistry.
Note that B is more electronegative than H, so B is the positive end of the B-H bond.
Like H⁺ in a Markovnikov addition, the B atom adds to the C with more H atoms.
The boron adduct can then be oxidized to an alcohol.
Here's a video on the hydroboration of alkenes.