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

摘要

The geometry optimized structures and total energies of 3-substituted (R)2,5-dihydrofurans (1) 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=MoO to-30.5 kJ mol~(-1) for both R=COOMe and R=No_2.The reaction enthalpies appear to be controlled by the electronic effect of R of the strength of p-#pi# conjugation in b.The a isomer has a planar ring,independent of R (excluding NH_2),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=NO_2,but nonplanar for less conjugation-enhancing substituents.