Model calculations of chemical interactions. Part 5.—Diastereofacial selectivity in 1,3-dipolar cycloaddition of formonitrile oxide to norbornene and cis-3,4-dichlorocyclobutene

A detailed quantitative analysis of the transition structures and energy barriers of syn/anti cycloadditions of norbornene and cis-3,4-dichlorocyclobutene with formonitrile oxide discloses the roles of a number of factors presumed to control facial diastereoselectivity. Steric and electrostatic repulsions, deformation energies of dipolarophiles and 1,3-dipoles, incipient bond energies and vicinal delocalizations between the incipient bonds and the bonds at the allylic positions participate in the complex energy balance which determines the stereochemical outcome. Classical steric and electrostatic repulsions and vicinal delocalization involving the forming bonds and the allylic antiperiplanar CC bonds of the ethano bridge in norbornene and the allylic synperiplanar CCl bonds in dichlorocyclobutene cycloaddition have been shown to be the driving forces of facial stereoselectivity in these examples.