π-Facial Stereoselectivity: Rates and Stereoselectivities of Cycloadditions of Hexachlorocyclopentadiene to 7-Substituted Norbornadienes, and Photoelectron Spectral and Molecular Orbital Computational Investigations of Norbornadienes

The partial rate factors for the Diels-Alder cycloadditions of hexachlorocyclopentadiene to 7-substituted norbornadienes in the four possible modes were measured. Syn-endo and anti-endo rates are essentially identical and decrease slightly as the effective electronegativity of the 7-substituent increases. The rates of the anti-exo cycloadditions decrease more rapidly in the same direction. Only the 7-tert-butoxy substituent gives significant acceleration of syn-endo attack. Both the ionization potentials (from photoelectron spectroscopy) of the π orbitals of norbornadienes and the rates and stereoselectivities of the cycloadditions are related to the effective electronegativities of the 7-substituents. Ab initio STO-3G calculations indicate that 7-substituents cause moderate shifts of the HOMO onto one of the norbornadiene double bonds but that reorientation of the directions in which the bonds point in space is also significantly influenced by these substituents. The ease of various distortions of the olefinic hydrogens out of plane, and perhaps alkene planarity itself, are influenced by the 7-substituents. A model for the influence of the 7-substituents on the orbitals of norbornadiene, and the stereochemistry of attack, is proposed which differs significantly from previous models but only partially accounts for -facial stereoselectivity in these systems.