Modeling reactions involving singlet molecular oxygen (O2 [1Δg]) is challenging because the degeneracy of the highest occupied molecular orbital and lowest unoccupied molecular orbital (HOMO and LUMO) orbitals of oxygen causes a significant multireference character. Within the limit that singlet-singlet near-degeneracy disappears in the transition state, it would be possible to bypass singlet oxygen's multireference character by simply adding the experimentally determined singlet/triplet splitting (22.5 kcal/mol) to the energy of the triplet ground state of molecular oxygen. This method is tested by calculating rate constants for the reactions of singlet molecular oxygen with furan, 2-methylfuran, 2,5-dimethylfuran, pyrrole, 2-methylpyrrole, 2,5-dimethylpyrrole, and cyclopentadiene using transition state theory. We find that the reaction rate coefficients are within a factor of 15 of experimentally determined rate constants, indicating an error in the barrier energy of roughly 3 kcal/mol. Furthermore, we find that energy refinement at the CCSD(T)-F12 level of theory is crucial to achieving accurate results. We conclude that, based on a comparison with an experiment, this approximation is valid to some degree and can be used for other systems involving the 1,4-cyclo-addition of singlet oxygen.