An Ab Initio Study of the Relative Stabilities and Molecular Structures of 3-Substituted 2,5-Dihydrofurans and 4-Substituted 2,3-Dihydrofurans

The geometry optimized structures and total energies of 3-substituted (R) 2,5-dihydrofurans (a) and their isomers, 4-substituted 2,3-dihydrofurans (b), have been determined by ab initio calculations at the MP2/6-31G*//HF/6-31G* level. The nature of the moiety R has a marked effect on the relative total energies of the isomeric forms: at the calculation level cited, the reaction enthalpies for the a → b isomerization range from +4.7 kJ mol−1 for R = MeO to −30.5 kJ mol−1 for both R = COOMe and R = NO2. The reaction enthalpies appear to be controlled by the electronic effect of R on the strength of p-π conjugation in b. The a isomer has a planar ring, independent of R (excluding NH2), whereas the planarity of b depends on the electronic nature of R: the 2,3-dihydrofuran ring is planar for both R = COOMe and R = NO2, but nonplanar for less conjugation-enhancing substituents.