In the hydroboration of a typical alkene, the boron fragment ends up attached to the less substituted carbon:
What are the electronic or steric factors that lead to this regioselectivity?
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For the hydroboration of isobutene
there is a steric aspect:
And though sterics may play some part, it doesn't explain near 100% regioselectivity. More important in determining regiochemistry, I would say, are the electronics of the transition state:
Boron is electrophilic, and as such will draw the electron density of the alkene π-bond towards itself. This C–B bond formation occurs earlier than the C–H bond formation, which in turn imparts a partial positive charge to the other carbon of the double bond in the transition state of the reaction.
This somewhat resembles a carbocation, which you might remember are stabilized by additional substituents via hyperconjugation. Thus, hydroboration results in an anti-Markovnikov product for mostly the same reason that acid catalyzed hydration results in a Markovnikov product — stabilization of positive charge.
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1$\begingroup$ How boron will draw the electron density of the alkene π-bond towards itself while carbon is more electronegative than it? $\endgroup$– MuhammadCommented Feb 14, 2019 at 19:14
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$\begingroup$ How can hyperconjugation explain stability of partial charges.? There is no p orbital on carbon to hyperconjugate $\endgroup$ Commented May 21, 2019 at 3:51
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2$\begingroup$ Hyperconjugation doesn't need to go into a pure p orbital, it works with any kind of orbital that isn't full, as long as there is sufficient overlap. $\endgroup$ Commented Feb 26, 2021 at 2:28