Disiamylborane adds only once to alkynes by virtue of its two bulky secondary isoamyl groups. Disiamylborane is prepared by the reaction of BH₃·THF with an alkene.
a. Draw the structural formulas of the reagents and the products in the preparation of disiamylborane.
b. Explain why the reaction in part (a) goes only as far as the dialkylborane. Why is Sia₃B not formed?

Verified step by step guidance

1. Step 1: Understand the preparation of disiamylborane. Disiamylborane is synthesized by reacting borane-tetrahydrofuran (BH₃·THF) with an alkene, specifically 2-methyl-2-butene (an isoamyl group). The reaction involves the addition of two isoamyl groups to the boron atom, forming disiamylborane (Sia₂BH). Write the structural formulas for BH₃·THF, 2-methyl-2-butene, and the resulting disiamylborane.
2. Step 2: Represent the reaction mechanism. The reaction proceeds via hydroboration, where the boron atom in BH₃ adds to the less substituted carbon of the alkene, and the hydrogen adds to the more substituted carbon. This process occurs twice, as BH₃ has three hydrogens, but steric hindrance prevents the addition of a third isoamyl group.
3. Step 3: Explain why the reaction stops at the dialkylborane stage. The bulky isoamyl groups attached to boron create significant steric hindrance, preventing a third alkene molecule from approaching the boron atom. This steric hindrance ensures that the reaction stops at the disiamylborane (Sia₂BH) stage, and a trialkylborane (Sia₃B) is not formed.
4. Step 4: Highlight the role of steric hindrance in the reaction. Steric hindrance refers to the physical crowding around the boron atom caused by the two bulky isoamyl groups. This crowding makes it energetically unfavorable for a third alkene to react with the boron atom, thus limiting the reaction to the formation of disiamylborane.
5. Step 5: Summarize the structural formulas and reasoning. The reagents are BH₃·THF and 2-methyl-2-butene, and the product is disiamylborane (Sia₂BH). The reaction stops at the dialkylborane stage due to steric hindrance, which prevents the formation of Sia₃B. This property of disiamylborane makes it a useful reagent for selective hydroboration of alkynes.

Key Concepts

Disiamylborane Structure and Preparation

Disiamylborane is a dialkylborane compound formed from the reaction of borane (BH3) with an alkene, specifically using tetrahydrofuran (THF) as a solvent. The structure features two isoamyl groups attached to the boron atom, which significantly influences its reactivity. Understanding the structural formula of both the reagents (BH3·THF and the alkene) and the product (disiamylborane) is crucial for visualizing the reaction mechanism.

Steric Hindrance

Steric hindrance refers to the prevention of reactions due to the spatial arrangement of atoms within a molecule. In the case of disiamylborane, the two bulky isoamyl groups create significant steric hindrance, which limits further reactions with additional alkynes. This concept is essential for understanding why disiamylborane adds only once to alkynes and does not form the trialkylborane (Sia3B).

Reactivity of Alkynes with Boranes

The reactivity of alkynes with boranes involves the formation of a boron-alkyne complex, which can lead to the addition of boron across the triple bond. However, the specific steric and electronic properties of disiamylborane dictate that it can only add once to the alkyne, resulting in a stable dialkylborane. This understanding is key to explaining why further addition to form Sia3B does not occur in this reaction.