Pericyclic Reactions: Cycloadditions and Diels-Alder Reaction
Cycloadditions: Examples for Facial Selectivity
Reaction of ethylene with maleic anhydride
With 4 electrons the reaction could proceed thermally through an (s+a) mechanism but no cycoaddition product is being observed. Obviously, steric reasons hinder any (s+a) approach. Photochemically, an (s+s) addition is allowed and can be observed in 71% yield after an extended irradiation.
Irradiation of butadiene and acrylonitrile
Both stereochemically possible cyclobutenes are obtained during irradiation of butadiene and acrylonitrile in a [2+2] cycloadition following an (s+s) mechanism. In contrast to this finding, the [4+2] adduct is formed only in miniscule amounts. These results are reversed in the thermal reaction affording nearly exclusively the [4+2] adduct (Diels-Alder product).
Diels-Alder reactions with maleic and fumaric esters
In a Diels-Alder reaction, maleic ester reacts in (s+s) fashion to yield the syn product while fumaric ester produces the anti adduct. The relative position of substituents in the starting material is being retained in the product during an (s+s) approach.
Dimerisation of anthracene
Anthracene dimerizes to dianthracene during irradiation. This reaction is a photochemically allowed [4+4] cycloaddtion with (s+s) approach. Reacting bonds are shown in red.
The reaction of tetracyanoethylene (TCNE) with heptafulvene represents a [14+2] cycloaddition. A total of 16 electrons (8 electron flow arrows) are participating in the reaction. Because 4n electrons are involved, the reaction has to pass through a Möbius transition state, i.e., we are dealing with an (s+a) approach. Heptafulvene represents the antarafacial component in the reaction since the two H atoms at the positions of attack are in anti configuration.