In the asymmetric Diels–Alder reactions, instead of using chiral ligands or chiral auxiliaries to differentiate the side selectivity of the dienolphiles, the differentiation of face selectivity of the dienes (especially for cyclopentadiene derivatives) using stereoelectronic effects have been reported by Woodward since 1955. A systematic research of facial selectivity using substituted cyclopentadiene or permethylcyclopentadiene derivatives have been conducted and the results can be listed as below.
The stereoelectronic effect affecting the outcome of the facial selectivity of the diene in the Diels–Alder reaction is the interaction between the σ(C(sp)–CH) (when σ(C(sp)–X) is a better acceptor than a donor) or σ(C(sp)–X) (when σ(C(sp)–X) is a better donor than an acceptor) and the σ* orbital of the forming bond between the diene and the dienophile.
If the two geminal substituents are both aromatic rings with different substituents tuning the electron density, the differentiation of the facial selectivity is also facile where the dienophile approaches the diene anti to the more electron-rich C–C bond where the stereoelectronic effect, in this case, is similar to the previous one.
The ring opening of cyclobutene under heating conditions can have two products: inward and outward rotation.
The inward rotation transition state of the structure shown below is relatively favored for acceptor R substituents (e.g. NO) but is especially disfavored by donor R substituents (e.g. NMe).