16.16:
Cycloaddition Reactions: MO Requirements for Thermal Activation
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.
16.16:
Cycloaddition Reactions: MO Requirements for Thermal Activation
Thermal cycloadditions are reactions where the source of activation energy needed to initiate the reaction is provided in the form of heat. A typical example of a thermally-allowed cycloaddition is the Diels–Alder reaction, which is a [4 + 2] cycloaddition. In contrast, a [2 + 2] cycloaddition is thermally forbidden.
The reaction occurs between the highest occupied molecular orbital (HOMO) of one π component and the lowest unoccupied molecular orbital (LUMO) of the other. These are known as the frontier molecular orbitals. Under thermal conditions, the reaction proceeds via the ground state HOMO and LUMO. For cycloadditions to take place in a concerted manner, the terminal lobes of the interacting systems must have the same symmetry.
In a [4 + 2] cycloaddition, the terminal lobes of the 4 π and the 2 π components are in phase and interact suprafacially. Consequently, the reaction is thermally allowed. However, in a [2 + 2] cycloaddition, the symmetry mismatch leads to a suprafacial interaction on one end and antarafacial on the other. The geometric constraint makes it thermally forbidden.
