Model calculations of chemical interactions. Part 6.—Origin of diastereofacial selectivity in 1,3-dipolar cycloaddition of formonitrile oxide with bicyclo[2.1.0]pent-2-ene and 2,3-dioxabicyclo[2.2.2]oct-5-ene

Abstract

The previous paper in this series, Part 5, gave a novel theoretical analysis of the origin of diastereofacial selectivity in 1,3-dipolar cycloadditions and discussed the reactions of formonitrile oxide with norbornene and cis-3,4-dichlorocyclobutene. The present paper both deepens that analysis by defining new quantities related to facial interactions and describes the successful application of the theory to two new examples, the reactions of formonitrile oxide with bicyclo[2.1.0]pent-2-ene and 2,3-dioxabicyclo[2.2.2]oct-5-ene, which cover qualitatively different results from those considered in the earlier work. Total and facial repulsion, total and facial delocalization and forming-bond delocalization, together with the deformation energy of dipolarophiles and 1,3-dipoles and the stabilization energy of vicinal bond–bond interactions, form the conceptual framework for a detailed and coherent account of the geometry and energy differences between the syn and anti transition structures. Facial repulsions and vicinal interactions involving the forming bonds and the bonds at the allylic positions are the primary controlling factors of the differential activation energy of syn/anti cycloadditions of 1,3-dipoles with double bonds incorporated into rigid cyclic systems.