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The influence of multiple fluorination on alleged valence isomerisation of [2.2.2]propellanes

The study of the bond-stretch isomerisation in [2.2.2]propellane [ ], as a classic example suggested by R. Hofmann, was extended to its multiple fluorinated derivatives. Namely, the parent [2.2.2]propellane was not synthesised in spite of the large barrier along the reaction pathway of ca. 20 kcal mol-1, which leads to the more stable open diradical isomer that easily rearranges to 1, 4-dimethylene-cyclohexane [ ]. The existence of stable heavily fluorinated derivative 4 suggests a strong effect of fluorination on the central bond elongation reaction energy profile. Indeed, CASPT2/cc-pVDZ//CAS(2, 2)/cc-pVDZ calculations show that perfluoro[2.2.2]propellane 3 and multiply fluorinated derivatives 2 and 4 have a single minimum potential energy well, which hampers rearrangement to significantly more stable fluorinated 1, 4-dimethylenecyclohexane analogues.

bond-stretch isomerism; Grob-type fragmentation; propellanes; multiple fluorination

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