Trans alkenes can be synthesized with organoboranes. The following sequence is represented in Smith's Organic Synthesis
How does the mechanism look like after the first step (hydroboration)?
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1 Answer
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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:
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.
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$\begingroup$ 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. $\endgroup$– JanDec 16, 2016 at 23:02
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$\begingroup$ 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. $\endgroup$– ZheDec 16, 2016 at 23:09
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$\begingroup$ Oh, you’re talking about a formal inversion! I thought you meant an actual inversion =D $\endgroup$– JanDec 17, 2016 at 18:55
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$\begingroup$ Here's the reference as promised. Somehow my email ended up in Greg Fu's spam folder... :( JACS 1998, 120, 2275 $\endgroup$– ZheDec 21, 2016 at 21:48