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Hydroboration of asymmetric internal alkynes and alkenes?

  1. Jul 4, 2016 #1
    How can you predict which product will be in excess if you have asymmetric hydroboration of internal alkynes and alkenes? For example, if I used 9-BBN or something sterically-hindered like R2BH...
     
  2. jcsd
  3. Jul 7, 2016 #2

    TeethWhitener

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    For a bulky borane, the excess product will be the one where addition of the boryl moiety is least sterically hindered. (Again, I haven't done these reactions myself, so I'm not a specialist) For example, if you have 2-methyl-2-butene, the 3 position is less sterically hindered than the 2 position. Therefore the BR2 moiety will add to the 3 position and the H (much smaller in size compared to the BR2) will add to the more sterically hindered 2 position. If you're following the hydroboration with a standard oxidation, the major product will ultimately be 3-methyl-2-butanol. Since hydroboration is a syn addition (both substituents add to the same side of the alkene), and the transition state is a 4-membered ring, the addition of the bulky BR2 group to an already sterically crowded position on an alkene is highly energetically unfavorable. This explains the anti-Markovnikov character of the addition.
     
  4. Jul 7, 2016 #3
    Thanks! But what if you have, say, 2-pentene? Which side is more sterically-hindered (the one with more carbons?)? And what about if you had 4-methyl-2-pentene? Does the methyl group that's farther away have any effect?
     
  5. Jul 8, 2016 #4

    TeethWhitener

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    My gut says probably. Remember that the ethyl group in 2-pentene can swing 360 degrees around its bond between the 3 and 4 positions, a motion which sweeps out a lot of volume and makes it harder for bulky substituents to attack at the 3 position. Keep in mind, though, that the more similar the groups are, the less difference I'd expect in yield of major vs. minor product. So for something like 3-heptene, where one side has an ethyl group and the other has a propyl group, you probably wouldn't see too much of a difference between major and minor products.

    The placement of the methyl group in this case ends up making the group pretty bulky, so I imagine the effect would be pretty pronounced here. In (for example) something like 10-methyl-2-dodecene, the extra methyl group probably wouldn't have much of an effect. Also, I should point out that there are electronic effects in hydroboration, but this is more often seen with groups that can push electrons around a little more (electronegative species, aryl groups, etc.). The effects from hydrocarbon groups on stereochemistry tend to be much more easily rationalized in terms of steric considerations.
     
  6. Jul 8, 2016 #5

    Thank you!
     
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