Ter-body intermediates in the gas phase: reaction of ionized enols with tert-butanol

Abstract

In the gas phase, the CH 2CHOH ·+ enol radical cation 1 as well as its higher homologues CH 3CHCHOH ·+ 2 and (CH 3) 2CCHOH ·+ 3, undergo exactly the same sequence of reactions with tert-butanol, leading to the losses of isobutene, water and water plus alkene. Fourier transform ion cyclotron resonance (FT-ICR) experiments using labeled reactants as well as ab initio calculations show that independent pathways can be proposed to explain the observed reactivity. For ion 1, taken as the simplest model, the first step of the reaction is formation of a proton bound complex which gives, by a simple exothermic proton transfer, the ter-body intermediate [CH 2CHO ·, H 2O, C(CH 3) 3 +]. This complex, which was shown to possess a significant lifetime, is the key intermediate which undergoes three reactions. First, it can collapse to yield tert-butylvinyl ether with elimination of water. Second, by a regiospecific proton transfer, this complex can isomerize into three different ter-body complexes formed of water, isobutene and ionized enol. Within one of these complexes, which does not interconvert with the others, elimination of isobutene leads to the formation of a solvated enol ion. Within the others, a cycloaddition–cycloreversion process can proceed to yield the ionized enol 3 (loss of water and ethylene channel).