Proton Affinity and Ion-Molecule Reactions of a Simple Silyl Enol Ether

Article abstract of DOI:10.1021/ja00371a002

The gas-phase positive ion-molecule chemistry of the simple enol silyl ether, 2-(trimethylsilyloxy)propene (1), has been studied by using ion cyclotron resonance spectroscopy.A lower bound to the proton affinity (PA) of 1 is found to be 16 +/- 1 kcal/mol above PA(NH3).The experimental results are consistent with C-protonation forming an ion with a structure equivalent to that of the adduct of the trimethylsilyl cation with acetone.The reactivity of the protonated enol ether suggests that most of the positive charge is localized on the trimethylsilyl group.This is in agreement with molecular orbital calculations on a model complex.On the basis of thermochemical data, the protonated enol ether is calculated to be stable by 42 +/- 10 kcal/mol with respect to the trimethylsilyl cation and acetone.Trimethylsilyl cation transfer reactions to various bases B have been observed from both the adduct and protonated 1.A dual group transfer reaction involving transfer of a proton from BH⁺ to 1 and abstraction of the trimethylsilyl cation by the base to form the product B-SiMe3⁺ and acetone has been observed to occur in a single reactive encounter.A brief comparison with solution results is made.