Question

Propose a mechanism for the following pinacol rearrangement catalyzed by boron trifluoride ethyl etherate.

Short answer

Answer: Boron trifluoride ethyl etherate acts as a Lewis acid catalyst in the pinacol rearrangement reaction. The key steps involved in the proposed mechanism include the formation of a boron trifluoride-diol complex, a proton transfer, a migratory shift, rearrangement, release of boron trifluoride, and tautomerization to yield the final ketone product.

Step-by-step solution

Identify the pinacol substrate

In the pinacol rearrangement, the substrate is a 1,2-diol (two hydroxyl groups on adjacent carbons). We start by identifying the substrate molecule in the reaction, which will have the general structure: $$\mathrm{R_1-C(OH)-C(OH)-R_2}$$ Where $$\mathrm{R_1}$$ and $$\mathrm{R_2}$$ represent the alkyl or aryl groups connected to the two adjacent carbons.

Formation of boron trifluoride-diol complex

Boron trifluoride (BF3) acts as a Lewis acid catalyst in this reaction. It forms a complex with the diol by accepting a pair of electrons from one of the hydroxyl groups: $$\mathrm{R_1-C(OH)-C(OH)-R_2 + BF_3 \rightarrow [R_1-C(OH)-C(O^-)-R_2-BF_3^{+}]}$$ This complex formation step involves the formation of a coordinate covalent bond between the negatively charged oxygen atom and the positively charged boron atom.

Proton transfer

In this step, a proton is transferred from the oxygen atom bound to the boron to the adjacent hydroxyl group. This results in the formation of a carbocation and a boron trifluoride-alkoxide complex: $$\mathrm{[R_1-C(OH)-C(O^-)-R_2-BF_3^{+}] \rightarrow R_1-C^{+}(-OH)-C(-O^-)-R_2-BF_3}$$

Migratory shift

The migratory shift is the key step in the pinacol rearrangement. One of the alkyl or aryl groups connected to the carbocation migrates to the adjacent carbon bearing the negative charge. This results in a new carbocation and a 1,2-shift in the structure of the molecule: $$\mathrm{R_1-C^{+}(-OH)-C(-O^-)-R_2-BF_3 \rightarrow R_1-\overset{+}{C}(-O^-)-C(-OH)-R_2-BF_3}$$

Rearrangement and release of boron trifluoride

In this step, a proton is transferred from the oxygen atom bound to boron to the adjacent carbocation. This results in the rearranged ketone and the release of the boron trifluoride catalyst: $$\mathrm{R_1-\overset{+}{C}(-O^-)-C(-OH)-R_2-BF_3 \rightarrow R_1-C(O)-C(-OH)-R_2 + BF_3}$$

Tautomerization

Finally, the product undergoes keto-enol tautomerization to give the more stable ketone: $$\mathrm{R_1-C(O)-C(-OH)-R_2 \rightarrow R_1-C(=O)-C-H-R_2}$$ The proposed mechanism for the pinacol rearrangement reaction catalyzed by boron trifluoride ethyl etherate involves the formation of a boron trifluoride-diol complex, a proton transfer, a migratory shift, rearrangement, release of boron trifluoride, and tautomerization to give the final ketone product.