So we are currently covering a variety of reactions in my ochem class, one of which is hydroboration-oxidation. I am very confused by the regioselectivity of hydroboration.
So far, we were given the two possible transition states of the first step:https://ibb.co/jRjYtQ3
The two main reasons for the regioselectivity they told us about were
1) Charge stabilization (the most substituted carbon is able to stabilize the partial + charge better through hyperconjugation)
2) Lower steric interactions between the big BH2
and alkyl groups.
Now, here's what doesn't make sense to me: why does the carbon on the left have to have a + charge in order to stabilize it through hyperconjugation?? Why can't it have a partial negative charge? Other than the sterics of the top transition state, wouldn't it be able to stabilize the negative charge just as well as the positive charge through hyperconjugation? Why is this a reason for the regioselectivity?
Also, another question I have is about the charge dipoles on the transition states. Shouldn't the boron technically have a partial negative charge because it is acting as the electrophile, and the hydrogen should have a partial positive charge because it is acting as the nucleophile? I understand that the hydrogen is slightly more electronegative in this scenario (and that's why it acts as the nucleophile), but I remember drawing transition states for electrophilic additions with HBr and the Br had a partial negative charge (since it acts as the electrophile) and the H had a partial positive charge.
I have seen something similar to what I am describing in our textbook (funnily enough, it is different from our lecture), and it looks like this:https://ibb.co/19kbNh9
Can someone explain the charge dipoles in this? Are they technically the same as the ones above or no? How do they explain the induced dipole on the carbon? Thanks a ton!!!