| Abstract
| - 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 (ΔG⧧ = 6.8 ± 0.2 kcal/mol), CASSCF and CASPT2 calculations havebeen performed on ring inversion in this and other bicyclo[2.1.0]pentanes. The results of the calculationsshow that a cooperative interaction between the geminal fluorines at C2 and the fluorines at C1 and C3in the singlet cyclopentane-1,3-diyl transition structure (TS) contributes 3.7 kcal/mol to lowering thebarrier to ring inversion in the tetrafluoro compound. In contrast, a competitive substituent effect in theTS for ring inversion of 1,4-dicyano-5,5-difluorobicyclo[2.1.0]pentane is predicted to raise the barrierheight by 6.1 kcal/mol. The origin of these cooperative and competitive substituent effects is discussed.
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