Chapter 20, Problem 20.48P

(a)

Interpretation Introduction

Interpretation: For the chair-like transition state of a $\text{[3,3]}$- Sigmatropic shift, the analogous pictures for an allowed boat-like reaction geometry have to be shown.

Concept Introduction:

$[3,3]$- Sigmatropic shift:

It is a special class of pericyclic reaction in which a $\text{σ}$-bond moves on the phase of one or more $\text{π-bonds}$. However, there will be redistribution of six electrons as the characteristic of a pericyclic reaction. The numbering in the nomenclature of the shift indicates the formation of a new sigma bond in between the two numbered positions of any two allylic systems as shown in the following example:

Stereochemistry in $[3,3]$- Sigmatropic shift:

Two types of transition states are likely to occur in this shift such as chair-like and boat-like transition states. According to the Frontier-Molecular-Orbital theory, both the transition states are allowed transition states inspite of the stability difference. The product obtained from the chair-like transition state has trans-conformation whereas the product obtained from the boat-like transition state has cis-conformation.

(b)

Interpretation Introduction

Interpretation: The reason for why the products with boat-like conformation are formed to a lower extent than those with chair-like conformation.

Concept Introduction:

$[3,3]$- Sigmatropic shift:

It is a special class of pericyclic reaction in which a $\text{σ}$-bond moves on the phase of one or more $\text{π-bonds}$. However, there will be redistribution of six electrons as the characteristic of a pericyclic reaction. The numbering in the nomenclature of the shift indicates the formation of a new sigma bond in between the two numbered positions of any two allylic systems as shown in the following example:

Stereochemistry in $[3,3]$- Sigmatropic shift:

Two types of transition states are likely to occur in this shift such as chair-like and boat-like transition states. According to the Frontier-Molecular-Orbital theory, both the transition states are allowed transition states inspite of the stability difference. The product obtained from the chair-like transition state has trans-conformation whereas the product obtained from the boat-like transition state has cis-conformation.