Q33 E - Organic Chemistry (Mcmurry)

Question

What stereochemistry—antarafacial or suprafacial—would you expect to observe in the following reactions?

 (a) A photochemical [1,5] sigmatropic rearrangement

 (b) A thermal [4 + 6] cycloaddition 

(c) A thermal [1,7] sigmatropic rearrangement

 (d) A photochemical [2 + 6] cycloaddition

Step-by-Step Solution

Answer

The stereochemistry observed would be:

1. A photochemical [1,5] sigmatropic rearrangement will be antrafacial.
2. A thermal [4 + 6] cycloaddition, means 5 double bonds so it will be suprafacial. 
3. A thermal [1,7] sigmatropic rearrangement will be antrafacial.
4. A photochemical [2 + 6] cycloaddition, means 4 double bonds so it will be suprafacial.

 

Step 1: Stereochemical Rules for Sigmatropic Rearrangements

The rules for stereochemicalsigmatropicrearrangements can be summarized as: 

 

Electron Pairs	Thermal Reaction	Photochemical Reaction
Even	Antarafacial	Superafacial
Odd	Superafacial	Antarafacial

 

Step 2: Superafacial or Antrafacial

1. A photochemical [1,5] sigmatropic rearrangement will be antrafacial.
2. A thermal [4 + 6] cycloaddition, means 5 double bonds so it will be suprafacial. 
3. A thermal [1,7] sigmatropic rearrangement will be antrafacial.
4. A photochemical [2 + 6] cycloaddition, means 4 double bonds so it will be suprafacial.