Chapter 28.5, Problem 15P

Interpretation Introduction

Interpretation:

The differences in the products obtained under photochemical and thermal condition should be accounted for the given reaction.

Concept introduction:

Pericyclic reactions are “ any concerted reaction in which bonds are formed or brocken in a cyclic transition state”.  There is a single transition state from start to finish, in contrast to a stepwise reaction.

There are mainly three types of pericyclic reactions,

1. 1) Electrocyclic reactions
2. 2) Cycloaddition reactions
3. 3) Sigmatropic reactions

In a sigmatropic rearrangement reaction “ one new sigma-bond is formed as another breaks.”

Sigmatropic rearrangement reactions are designated with digits.  For example a [1, 3] sigmatropic rearrangement describe a reaction in which the residue migrates from position 1 to position 3.

Woodward – Hoffmann rules for sigmatropic rearrangement reactions are listed below

$\begin{array}{ccc}\begin{array}{l}Numberofpairsofelectronsinthe\\ reactingsystem\end{array}& \text{Reaction}\text{conditions}& \text{Allowed}\text{mode}\text{of}\text{bond formation}\\ \text{Even}\text{number}& \text{Thermal}& \text{Antarafacial}\\ & \text{Photochemical}& \text{Suprafacial}\\ \text{Odd}\text{number}& \text{Thermal}& \text{Suprafacial}\\ & \text{Photochemical}& \text{Antarafacial}\end{array}$