The gas-phase acidity of cyclopropene and simple alkyl derivatives: Can they be measured?

The gas-phase acidity of 3-methylcyclopropene (5) at the allylic position was explored computationally and experimentally. G2+ calculations indicate that ΔH°(acid) = 415.5 kcal/mol making 3-methyl-3-cyclopropenyl anion (6) an extremely strong base. This species is also predicted to be unstable with respect to electron loss (EA(3-methyl-3-cyclopropenyl radical) = -1.54 kcal/mol). A kinetic approach for determining the acidity of 5 using the hydroxide-induced desilylation of 3-methyl-3-trimethylsilylcyclopropene (the DePuy method) was employed but fails in this case because of an unanticipated rearrangement. This raises the question: Can the acidity of cyclopropene and its simple alkyl derivatives be measured? Positive and negative responses to this question are given and discussed. (C) 2000 Elsevier Science B.V.