Pericyclic Reactions: Aromaticity of Transition States
A total of 6 p orbitals from butadiene and ethylene take part in the reaction. Two σ bonds and one π bond are formed from three π bonds during the reaction. The mechanism is formally described by three arrows indicating the flow of electrons with each arrow representing two electrons. Since the number of sign inversions in the transition state is even (in this case zero), the system is a Hückel system. The cyclic transition state with 4n+2 electrons (6 electrons in this case) is aromatic and, therefore, allowed.
The basis set consists of two p orbitals, one and one s orbital. Four electrons take part in the reaction indicated by two arrows that show the flow of electrons. The number of sign inversions in the transition state is even (zero in this case), therefore, the system is Hückel-antiaromatic, i.e., disallowed.
A methyl group is being transferred in this example. The participating p orbital from the methyl group can be shown to indicate conjugation (red line) passing through the origin of the p orbital. This does not count as sign inversion. Therefore, the number of sign inversions is odd (one in this case), and we are dealing with a Möbius system. The system is aromatic because four electrons are involved and the reaction is allowed.
Migration of the methyl group proceeds with inversion, similar to the reaction. This can be observed experimentally only when the migrating group is chiral, i.e., contains four different substituents. The following simple rule involving sterochemistry can be set up:
Inversion takes place if conjugation at a reaction center passes through the origin of the orbital.