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In the hydroboration of a typical alkene, the boron fragment ends up attached to the less substituted carbon:

Hydroboration of an alkene

What are the electronic or steric factors that lead to this regioselectivity?

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For the hydroboration of isobutene

enter image description here

there is a steric aspect:

sterics

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:

electronics

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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  • $\begingroup$ How boron will draw the electron density of the alkene π-bond towards itself while carbon is more electronegative than it? $\endgroup$ – Mason Feb 14 '19 at 19:14
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    $\begingroup$ It is a Lewis acid. $\endgroup$ – ringo Feb 14 '19 at 19:38
  • $\begingroup$ Why C-B bond forms faster ? $\endgroup$ – ado sar May 20 '19 at 15:36
  • $\begingroup$ How can hyperconjugation explain stability of partial charges.? There is no p orbital on carbon to hyperconjugate $\endgroup$ – user226375 May 21 '19 at 3:51
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    $\begingroup$ 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. $\endgroup$ – orthocresol Feb 26 at 2:28

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