Cycloadditions: Supra-antara Cycloadditions

During (s+a) cycloadditions the stereochemistry of the antarafacially reacting component is being changed.

For steric reasons, (s+a) cycloadditions are rare. Substituents at alkenes prevent an approach of the components. Because of their linear character ketenes and heterocumulenes are sterically less demanding and undergo [2+2] cycloadditions following a (s+a) mechanism more readily than alkenes, for example:

3D Animation of [2+2] supra-antara cycloaddition

Though the approach shown above (R back, b above) is sterically most favored if R, a, and b are bulky substituents; they all will end up on the same side of the 4-membered ring thus generating an unfavorable steric configuration. The formation of the thermodynamically less favored product might be indicative of an (s+a) approach.

In summary, the Woodward-Hoffmann rules for thermally induced cycloadditions can be defined as follows:

Woodward-Hoffmann rules for thermally induced cycloadditions

• (4n+2) π Electrons: supra-supra allowed supra-antara forbidden
• 4n π Electrons: supra-supra forbidden supra-antara allowed

The opposite is true for photochemical reactions.