Orbital Interactions. 6. The Birch Reduction as a Tool for Exploring Orbital Interactions through Bonds. Through-Three-Bond Interactions

Article abstract of DOI:10.1021/ja00522a038

Product and relative rate data have been obtained for the Birch reduction (Li/liquid NH3/tert-butyl alcohol) of a number of 1,4-methanobiphenylenes.The rates of reduction of the double bond in exo- and endo-1,4,4a,8b-tetrahydro-1,4-methanobiphenylene (exo- and endo-16) are respectively 141 and 78 times greater than the rate of reduction of norbornene.However, the reduction of the aromatic rings in exo- and endo-16 showed little enhancement compared with the rates of reduction of exo- and endo-1,2,3,4,4a,8b-hexahydro-1,4-methanobiphenylenes (exo- and endo-19), respectively.The double bonds of exo- and endo-1,4,4a,5,8,8b-hexahydro-1,4-methanobiphenylenes (exo- and endo-18) are little affected compared with norbornene.Interactions between the vacant MOs in these and other molecules were explored with the aid of extended Hueckel calculations.The results of these calculations demonstrated the presence of sizable through-bond interactions in exo-16.However, net through-space interactions in the vacant MO manifold are predicted to prevail in endo-16.A causal connection between the presence of orbital interactions and the rates of Birch reductions of exo and endo compounds 16, 18, and 19 was established within the framework of the mechanism of the reaction.A linear relationship between ln (rate of reduction) of a substrate and its LUMO energy was observed.