Hydrogen tunneling avoided: enol-formation from a charge-tagged phenyl pyruvic acid derivative evidenced by tandem-MS, IR ion spectroscopy and theory

Abstract

A charge-tagged hydroxycarbene formed via tandem-MS delivers exclusively enol-tautomers and avoids quantum mechanical hydrogen tunneling in the gas phase.

Breslow Intermediates (Amino Enols) and Their Keto Tautomers: First Gas‐Phase Characterization by IR Ion Spectroscopy

“…Due to our quite satisfactory experience with ammonium charge tags in our earlier ESI‐MS IR ion spectroscopy studies, [15, 17] we decided to also employ the NMe 3 + substituent in the current investigation. Scheme 3 summarizes the NHCs/azolium salts and aldehydes used in this study.…”

“…The investigations summarized above concerned Breslow intermediates both in solution and in the crystalline state. We have recently extended our mechanistic investigations in the field of carbene chemistry to the gas phase, by using ESI‐MS IR ion spectroscopy in combination with quantum‐chemical calculations, as a powerful tool to determine tautomeric ion structures of charge‐tagged analytes, and to even follow the kinetics of their interconversion [15–19] . For IR ion spectroscopy, selected ions are exposed to IR photons provided from an appropriate‐wavelength tunable light source, for example, a free‐electron laser (FEL; wavenumber range 600–1900 cm −1 ).…”

“…As the density of ions in typical storage devices used in these experiments renders transmission‐based IR spectroscopy inefficient and difficult, the extent of precursor ion depletion and product ion formation is recorded as the IR radiation energy is tuned (FEL: 600–1800 cm −1 , optical parametric oscillator: 2800–3700 cm −1 ). In IR ion spectroscopy, the energy of many resonantly absorbed photons leads to global vibrational excitation of all oscillators in the irradiated ion, by virtue of intramolecular vibrational redistribution [15, 17–20] . Ultimately, the photoactivated ion reaches internal energies at which one or more dissociation pathways become accessible; this is why IR ion spectroscopy is referred to as an “action spectroscopy” approach.…”

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 ketoforms; enols can be stabilized through resonance, [23] steric factors, [1] and electronic effects. [2,24] Experimental methods such as time-resolved spectroscopy, [5,25] neutralization-reionization mass spectrometry, [26,27] NMR as well as IR spectroscopy, [28] and other spectroscopic techniques [29] along with computational methods [30][31][32][33] 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.…”

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

“…Enole kçnnen durch Resonanz, [23] sterische Faktoren [1] und elektronische Effekte [2,24] stabilisiert werden. Experimentelle Methoden wie zeitaufgelçste Spektroskopien, [5,25] Neutralisations-Ionisations-Massenspektrometrie, [26,27] NMR-und IR-Spektroskopien, [28] als auch weitere spektroskopische Techniken [29] zusammen mit computerchemischen Berechnungen [30][31][32][33] 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.…”