ANIONIC ETHER CLEAVAGE OF TETRAHYDROFURAN IN THE GAS-PHASE

The gas phase reactivity of tetrahydofuran (THF) towards a series of anionic bases (B- = NH2-, C6H5-, OH-, CH2=CH-CH2-) has been investigated under the low pressure conditions of a Fourier Transform Ion Cyclotron Resonance (FT-ICR) mass spectrometer. It appears that under the low pressure conditions the anionic base-induced reactions of THF predominantly proceed by 1,2-elimination. This elimination initially results in rovibrationally excited HB solvated but-3-ene-1-oxide ions. As a result of the absence of thermal interaction with the environment, HB is vaporized from these excited solvated ions. Depending on the nature of the base used, a proportion of the excited but-3-ene-1-oxide is found to eliminate molecular hydrogen or formaldehyde leading to overall exothermic formation of the resonance stabilized buta-1,3-diene-1-oxide ions and allyl anions, respectively. Interestingly, reaction between the but-3-ene-1-oxide ion and water results in the very efficient formation of hydrated hydroxide ions, [H2O, OH-]. Isotopic labeling reveals that formation of this water solvated hydroxide ion is initiated by an endothermic proton transfer from water to but-3-ene-1-oxide which is followed by a reversible hydroxide-induced E1cb process, leading to the elimination of 1,3-butadiene and formation of the hydrated hydroxide ion. To our knowledge, this is the first example of a gas-phase anionic base-induced elimination in which hydroxide is involved as a leaving group.