Chapter 17, Problem 46DSP

Interpretation Introduction

Interpretation:

The best described rearrangement step in the given reaction is to be predicted.

Concept introduction:

In some epoxide ring-opening reactions, $\text{C-O}$ bond cleavage is accompanied by the development ofcarbocation character at carbon (${}^{\text{δ+}}\text{C-O}$) to allow rearrangement to occur.

Epoxide opening reactions are facilitated by Lewis acids such as boron trifluoride (${\text{BF}}_{\text{3}}$) and aluminum chloride (${\text{AlCl}}_{\text{3}}$).

In the first step, the $\text{C-O}$ bond in the epoxide breaks and positive charge develops on the carbon atom that is more substituted.

In the second step, the migration of a group takes place which is assisted by electron-pair donation from the epoxide oxygen.

In the next step, subsequent deprotonation occurs to yield an aldehyde or ketone.

For an equally substituted epoxide, the aryl group prefers to migrate over the hydrogen atom.