Chapter 28.5, Problem 17P

Interpretation Introduction

Interpretation:

[1,3]sigmatropic migrations of hydrogen cannot occur under thermal conditions but [1,3] sigmatropic migration of carbon can.  Reason for this should be explained.

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 reaction “ one new sigma-bond is formed as another breaks.”

Sigmatropic rearrangement reactions are named with digits.  For example a [1, 3] sigmatropic rearrangement describe a reaction in which the residue migrates from position 1 to position 3.  Migration of carbon and hydrogen will occur in a sigmatropic rearrangement reaction.  When hydrogen migrates in a sigmatropic rearrangement, the s orbital of the hydrogen is partially bonded to both the migration origin and the migration terminus in the transition state.  Migration of hydrogen in suprafacial and antarafacial rearrangement can be represented as follows,

Migration of carbon occurs through two ways because it has a two lobed p orbital. Carbon can simultaneously interact with the migration origin and the migration terminus using one lobe of its p orbital.

Migration of carbon in suprafacial and antarafacial rearrangement can be represented as follows,

Carbon migrating with one lobe of its p orbital interacting

Carbon migrating with both lobe of its p orbital interacting