A relative approach for determining ring strain energies of heterobicyclic alkenes

Ring strain energies (RSEs) of heterocyclic compounds are predicted compared to their analogous homocyclic compounds using the G3 method. Suitable reference compounds are devised for row 2 and row 3 heterobicyclic alkenes. Any difference in energy between the ring and the reference is due to RSE. Row 3 heterobicyclic alkene RSEs are less than those of row 2. As the electronegativity of the heteroatom increases, the RSE increases. Substitutents at the bridgehead carbons cause a decrease in RSE. [3.2.1] Heterobicyclic alkenes are significantly less strained than their [2.2.1] counterparts. Relative and absolute RSEs are reported for heterobicyclic alkenes and their derivatives.