Rearrangement and nucleophilic trapping of bicyclo[4.1.0]hept-2-yl derived nonclassical bicyclobutenium ions

Abstract

Here we describe the synthesis of two specifically labelled 13C isotopologues of cis-2-(4-nitrophenoxy)bicyclo[4.1.0]heptane and their solvolysis reactions in trifluoroethanol. By using one- and two-dimensional 1H- and 13C-NMR spectroscopy, we characterized the pathways for the rearrangement of these isotopologues to give 13C-labelled 4-(2,2,2-trifluoroethoxy)cycloheptene. We show that the initially formed cationic intermediate undergoes a degenerate rearrangement, which does not reach equilibrium before nucleophilic capture of the cation. Moreover, we show that the nonclassical carbocation, cyclohept-3-ene(3,1,4-deloc)ylium, gives an approximate 6:1 ratio of the cis- to trans-diastereomeric 2-(2,2,2-trifluoroethoxy)bicyclo[4.1.0]heptane as reaction intermediates that subsequently solvolyze to the 4-(2,2,2-trifluoroethoxy)cycloheptene product.