Textbook Question
Predict the products you would get when the following alkenes undergo (ii) oxymercuration–reduction [1. Hg(OAc)2 , H2O 2. NaBH4 ].
(b)
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10. Addition Reactions
Hydroboration
Multiple Choice
Which of the following best describes the first step in the hydroboration mechanism when an alkene reacts with (diborane) in the presence of tetrahydrofuran (THF)?
A
The boron atom adds to the more substituted carbon of the alkene while a hydrogen adds to the less substituted carbon in an anti addition.
B
The boron atom adds to the less substituted carbon of the alkene while a hydrogen adds to the more substituted carbon in a concerted syn addition.
C
The alkene undergoes a carbocation rearrangement before boron adds to the more substituted carbon.
D
The alkene is first protonated to form a carbocation, which is then attacked by borohydride.
Verified step by step guidance1
Recognize that hydroboration of alkenes with diborane (B\_2H\_6) in THF proceeds via a concerted mechanism, meaning bonds form simultaneously without intermediates like carbocations.
Understand that in this mechanism, the boron atom adds to the less substituted carbon of the alkene because boron is electrophilic and prefers to attach where it can form the most stable transition state with less steric hindrance.
Note that the hydrogen atom from the borane adds to the more substituted carbon, which is opposite to the regioselectivity seen in acid-catalyzed hydration reactions.
Remember that the addition is syn, meaning both the boron and hydrogen add to the same face of the alkene, leading to syn stereochemistry in the product.
Summarize that the first step involves a concerted syn addition where boron attaches to the less substituted carbon and hydrogen to the more substituted carbon, without carbocation formation or rearrangement.
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