16.16 : 环加成反应：热活化的分子轨道要求

Why are some cycloaddition reactions thermally allowed and others forbidden?

Let's begin by examining the interaction between the frontier orbitals of the reacting components — HOMO and LUMO.

Under thermal conditions, cycloadditions proceed via the ground state HOMO and LUMO.

A simultaneous bonding overlap is possible only when the terminal lobes of the two π components are in phase.

In a [4 + 2] cycloaddition, the reacting ends have matching symmetries for a face-to-face bonding interaction. The two π components are said to interact suprafacially on both ends, making it a concerted symmetry-allowed process.

However, in a [2 + 2] cycloaddition, the symmetry mismatch gives rise to one bonding and one antibonding interaction.

Here, bond formation occurs on the same face, suprafacially, on one end and the opposite face, antarafacially, on the other. Although the interaction is symmetry allowed, the geometric constraint makes this a thermally forbidden process.