Lactone Enols Are Stable in the Gas Phase but Highly Unstable in Solution

Abstract

2-Hydroxyoxol-2-ene (C(5)-1), the enol tautomer of gamma-butyrolactone, was generated in the gas phase as the first representative of the hitherto elusive class of lactone enols and shown by neutralization-reionization mass spectrometry to be remarkably stable as an isolated species. Ab initio calculations by QCISD(T)/6-311+G(3df,2p) provided the enthalpies of formation, proton affinities, and gas-phase basicities for gaseous lactone enols with four- (C(4)-1), five- (C(5)-1), and six-membered rings (C(6)-1). The acid-base properties of C(4)-C(6) lactones and enols and reference carboxylic acid enols CH(2)=C(OH)(2) (3) and CH(2)=C(OH)OCH(3) (4) were also calculated in aqueous solution. The C(4)-C(6) lactone enols show gas-phase proton affinities in the range of 933-944 kJ mol(-)(1) and acidities in the range of 1401-1458 kJ mol(-)(1). In aqueous solution, the lactone enols are 15-20 orders of magnitude more acidic than the corresponding lactones, with enol pK(a) values increasing from 5.6 (C(4)-1) to 14.5 (C(6)-1). Lactone enols are moderately weak bases in water with pK(BH) in the range of 3.9-8.1, whereas the lactones are extremely weak bases of pK(BH) in the range of -10.5 to -17.4. The acid-base properties of lactone enols point to their high reactivity in protic solvents and explain why no lactone enols have been detected thus far in solution studies.

1,1‐Ethenediol: The Long Elusive Enol of Acetic Acid

“…More generally speaking, most enols are short‐lived reactive species and they readily isomerize to their keto‐forms; enols can be stabilized through resonance, steric factors, and electronic effects . Experimental methods such as time‐resolved spectroscopy, neutralization‐reionization mass spectrometry, NMR as well as IR spectroscopy, and other spectroscopic techniques along with computational methods have played a significant role in understanding the structures and reactivities of enols. The simplest enol, ethenol (vinyl alcohol), was generated in the gas phase through the pyrolysis of ethylene glycol .…”

The Expanding Role of Electrospray Ionization Mass Spectrometry for Probing Reactive Intermediates in Solution

“…But for most of its history, MS has long served as a leading tool for gas-phase ion chemistry. By classic hard ionization techniques (such as electron impact, chemical ionization, atom bombardment) ions are created under vacuum, in which some highly reactive small (volatile) molecules could be generated and monitored in the gas phase, e.g., femtosecond real-time observation of benzyne intermediates in a molecular beam [ 16 ], neutralization-reionization mass spectrometry for neutral intermediates [ 1 , 17 , 18 ]. Indeed, a few “softer” vacuum ionization mass spectrometers, including secondary ion MS (SIMS) and matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), could be used to produce intact molecular ions from condensed-phase samples.…”

1,1‐Ethendiol – Das lange Zeit schwer fassbare Enol der Essigsäure

“…Enole können durch Resonanz, sterische Faktoren und elektronische Effekte stabilisiert werden. Experimentelle Methoden wie zeitaufgelöste Spektroskopien, Neutralisations‐Ionisations‐Massenspektrometrie, NMR‐ und IR‐Spektroskopien, als auch weitere spektroskopische Techniken zusammen mit computerchemischen Berechnungen spielen eine bedeutende Rolle im Verständnis der Struktur und Reaktivität von Enolen. Das einfachste Enol, Ethenol (Vinylalkohol), wurde in der Gasphase durch Pyrolyse von Ethylenglykol bereits dargestellt .…”

“…Das Enol wird hierbei durch eine intermolekulare Wechselwirkung mit einem zweiten Molekül Acetophenon in Form eines Keto‐Enol Dimers stabilisiert . Turecek et al . konnten das Enol von γ‐Butyrolacton in der Gasphase darstellen und mittels Neutralisations‐Ionisations‐Massenspektrometrie charakterisieren.…”