An ab initio study of the relative stabilities and molecular structures of3-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/6-31 G* level. The nature of the moiety R has a marked effect on the relative to tal energies of the isomeric forms: at the calculation level cited, the rea ction enthalpies for the a --> b isomerization range from +4.7 kJ mol(-1) f or 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 st rength of p-pi conjugation in b. The a isomer has a planar ring, independen t of R (excluding NH2), whereas the planarity of b depends on the electroni c nature of R: the 2,3-dihydrofuran ring is planar for both R = COOMe and R = NO2, but nonplanar for less conjugation-enhancing substituents.