The discussion centers on predicting the products of asymmetric hydroboration of internal alkynes and alkenes, particularly focusing on the influence of steric hindrance and the nature of the borane used, such as 9-BBN or sterically hindered R2BH. Participants explore the implications of steric factors on product distribution and the resulting structures after oxidation.
Participants express varying opinions on the influence of steric hindrance and the effects of different substituents, indicating that multiple competing views remain on how these factors impact product formation. The discussion does not reach a consensus.
Participants acknowledge that steric hindrance is a significant factor but also point out that electronic effects may play a role, particularly with electronegative or aryl groups. The discussion remains open regarding the specific contributions of these factors in different scenarios.
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?TeethWhitener said: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.
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.fangrz said:Which side is more sterically-hindered (the one with more carbons?)
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.fangrz said:And what about if you had 4-methyl-2-pentene? Does the methyl group that's farther away have any effect?
Thank you!TeethWhitener said: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.