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

Carbanions in the Gas Phase

“…Efforts to generate 3-cyclopropenyl anions in the gas phase with less stabilizing substituents have been less successful. Methyl and aryl precursors were found to lead to rearranged products (eqs 55–). , The acidity of 3-(4-methylphenyl)cyclopropene at the allylic position was successfully measured nevertheless using the DePuy kinetic approach in which the product distribution of the reaction between hydroxide ion and the 3-aryl-3-trimethylsilylcyclopropene was used to obtain Δ H ° acid = 398.6 ± 1.4 kcal mol –1 . This value can be compared to that for phenylcyclopropane, and as expected for a cyclic 4π electron species, the saturated anion is less basic (eq ).…”

Rearrangements of 3-Aryl-Substituted Cyclopropenyl Anions and the Gas-Phase Acidity of 3-(4-Methylphenyl)cyclopropene

“…We were interested in generating a minimally stabilized cyclopropenyl anion in order to explore its thermochemistry and reactivity. An aryl group seemed to be a reasonable substituent since ab initio calculations predict that methyl (G2+) and vinyl (MP2/6-31+G(d) via isogyric reactions with cyclopropyl and allyl radicals) derivatives are unstable with regard to electron loss, and efforts to generate 3-methyl-3-cyclopropenyl anion were unsuccessful. , The same approach that was used to generate 3-methoxycarbonyl-3-cyclopropenyl anion ( 1 ) was attempted, and 3-(4-trifluoromethylphenyl)-3-trimethylsilylcyclopropene ( 2 ) was reacted with fluoride ion in a Fourier transform mass spectrometer (eq 3). A few initial experiments, however, revealed that the ion which corresponds to loss of the trimethylsilyl group, [ 2 -TMS] - , is not cyclopropenyl anion 3 as its proton affinity is too low.…”

“…In addition to generating highly stabilized cyclopropenyl anions in the gas phase, we sought to explore derivatives which are more basic and would better reflect the thermochemistry of the parent. High-level ab initio calculations indicate that cyclopropenyl and 3-methyl-3-cyclopropenyl anions are unstable with regard to electron loss (i.e., the anions are unbound). , Other simple alkyl derivatives also are apt to be unstable, so we decided to examine aryl-substituted cyclopropenes. Herein, we report the results of this study along with computed structures and energies of the anions.…”

Gas-Phase Acidities of Cycloheptatrienes: Effects of Alkyl Groups on the Stability of Carbanions

“…Although carbanions are the most widely used reactive intermediates in organic synthesis, the gas-phase acidities of the alkanes are not well understood [1][2][3] because, with few exceptions, simple alkyl anions cannot be detected in the gas phase. [4][5][6] These carbanions are unstable as isolated entities towards loss of an electron [7][8][9][10][11][12] and it has not been possible to apply the usual methods for determining their stabilities. DePuy et al 7,13 and other groups, 14,15 therefore, used indirect kinetic methods to deduce the relative acidities of simple alkanes in the gas phase.…”