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

Short 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-by-step solution

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

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.