Trans alkenes can be synthesized with organoboranes. The following sequence is represented in Smith's Organic Synthesis enter image description here

How does the mechanism look like after the first step (hydroboration)?


I think this might be a alkyl shift that results in an $\mathrm{S}_{N}2$ type substitution at an $sp^{2}$ center. Cool. I've only ever seen $\mathrm{S}_{N}2$ type substitution at an $sp^{2}$ center for vinyl iodides. Unfortunately, I don't have my teaching materials on me at work, so I can't find the reference for this for you.

EDIT: Mechanism

Note the "inversion" at the $sp^{2}$ center.

Greg Fu had this great example of a substitution on a vinyl iodide, like I mentioned above. I can't seem to find this reference, but I just emailed him, so I'll update when he writes me back.

  • $\begingroup$ I don’t think it’s a substitution: I would assume addition of methanolate to the boron centre which then induces a migration/insertion of some sort. But I don’t know myself, unfortunately. $\endgroup$ – Jan Dec 16 '16 at 23:02
  • $\begingroup$ Notice that even in that case, you need inversion of the $sp^2$ center. I am in fact proposing the exact same thing as you are. $\endgroup$ – Zhe Dec 16 '16 at 23:09
  • $\begingroup$ Oh, you’re talking about a formal inversion! I thought you meant an actual inversion =D $\endgroup$ – Jan Dec 17 '16 at 18:55
  • $\begingroup$ Here's the reference as promised. Somehow my email ended up in Greg Fu's spam folder... :( JACS 1998, 120, 2275 $\endgroup$ – Zhe Dec 21 '16 at 21:48

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