The Search for Persistent Cyclobutylmethyl Cations in Superacidic Media and Observation of the Cyclobutyldicyclopropylmethyl Cation1a

Abstract

Primary and secondary cyclobutylmethyl cations were found to be elusive in superacidic media even at low temperatures. They give thermodynamically more stable rearrangement products, i.e., the dimeric bicyclo[4.4.0]dec-1-yl cation and substituted cyclopentyl cations, respectively. In the search for a persistent cyclobutylmethyl cation, the more stabilized cyclobutyldicyclopropylmethyl cation was prepared from its corresponding alcohol in FSO3H/SO2ClF at −90 °C. Using variable-temperature 13C NMR studies and theoretical calculations, the ion was shown to exist preferentially in its bisected conformation at −80 °C. The ion is predominantly classical, with expected significant delocalization into the neighboring cycloalkyl rings.