Hexafluoroisopropanol As A Suitable Solvent For Rearrangements Via Zwitterionic Intermediates

Rearrangement of the tricarbonyl(cycloheptatriene)iron derivative 1 to 2 and 3 took place very readily in 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) at room temperature. This finding is in striking contrast to the stability of 1 in MeOH under otherwise similar conditions. The isomerization processes which give rise to an equilibrium among 1, 2 and 3 must involve zwitterionic intermediates of the type 4, whose formation, consequently, is highly favored by HFIP. Heating 9 in HFIP brought about its rearrangement to 15. We suggest that formation of the zwitterionic intermediate 12 triggers a rearrangement of the carbocyclic moiety which can take place as a result of the high polarity and low nucleophilicity of HFIP. The rearrangement 9 --> 15 could not be achieved by using other polar solvents such as methanol or nitromethane. This observation once again demonstrates how suitable HFIP is as a medium for isomerizations via zwitterionic intermediates. The role of traces of free acid, present even in purified HFIP, is discussed.