Cooperative and Competitive Effects of Substituents at C1 and C4 on the Barriers to Ring Inversion of 5,5-Difluorobicyclo[2.1.0]pentanes

摘要

To identify the reasons for the very low barrier that has been measured for ring inversion of 1,4,5,5-tetrafluorobicyclo[2.1.0]pentane(DELTA G~(not =)= 6.8 +-0.2 kcal/mol),CASSCF and CASPT2 calculations have been performed on ring inversion in this and other bicyclo[2.1.0]pentanes.The results of the calculations show that a cooperative interaction between the geminal fluorines at C2 and the fluorines at C1 and C3 in the singlet cyclopentane-1,3-diyl transition structure(TS)contributes 3.7 kcal/mol to lowering the barrier to ring inversion in the tetrafluoro compound.In contrast,a competitive substituent effect in the TS for ring inversion of 1,4-dicyano-5,5-difluorobicyclo[2.1.0]pentane is predicted to raise the barrier height by 6.1 kcal/mol.The origin of these cooperative and competitive substituent effects is discussed.