In the book Hydroboration and Organic Synthesis, on p. 99, it is written that:

hydroboration [of enol ethers] proceeds regioselectively, placing essentially all of the boron atoms at the β position

On p. 100 of the book, a specific example reaction is provided to illustrate the point:

enter image description here

The book does not explain the chemistry behind this specific regioselectivity for enol ethers and in this post, I would like to ask if anyone has a plausible explanation to offer. My hypothesis is that mesomeric electron pair donation from the oxygen atom adjacent to the $\ce {C=C}$ bond enhances the electron density on the β carbon, thus the electron-deficient boron atom preferentially attaches to it.


Dhillon, R. S. Hydroboration and Organic Synthesis. Springer-Verlag Berlin Heidelberg. Germany, 2007.

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    $\begingroup$ There is this chemistry.stackexchange.com/q/50311/16683 which covers pretty much the same thing, but I'll warn that I think my answer there needs some updating. The idea is correct but the devil is in the details... $\endgroup$
    – orthocresol
    Nov 29 '21 at 16:40
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    $\begingroup$ Just follow the polarity of the reacting bonds: Me2C=CHOEt and H2B-H. For the former the double bond is polarized with the negative charge on the carbon bearing the methyl groups and the positive charge on the other carbon stabilized by resonance fro the oxygen. Boron is more electropositive than hydrogen. Do a cycloaddition such that boron is delivered to the tertiary site. Your intuition is correct. Follow Markovnikov rules with the understanding that hydrogen is not always positive. $\endgroup$
    – user55119
    Nov 29 '21 at 17:30
  • $\begingroup$ @orthocresol Thanks, I think your answer is what I'm looking for $\endgroup$ Nov 30 '21 at 1:00
  • $\begingroup$ I think you can self answer it. This will make the post complete. $\endgroup$ Nov 30 '21 at 2:10

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