Substituent effects on the gas-phase ring-chain tautomerism of 3,4,5,6-tetrahydro-2H-1,3-oxazines

• Published in: Rapid communications in mass spectrometry Vol. 21; no. 22; pp. 3701 - 3710
• Main Authors: Juhász, Márta, Fülöp, Ferenc, Pihlaja, Kalevi
• Format: Journal Article
• Language: English
• Published: Chichester, UK John Wiley & Sons, Ltd 01.01.2007
• Subjects: Gases - chemistry; Oxazines - chemistry; Spectrometry, Mass, Electrospray Ionization - methods; Stereoisomerism
• ISSN: 0951-4198; 1097-0231
• DOI: 10.1002/rcm.3260

The electron ionization mass spectra of five series of seven 2‐aryl,4‐R‐substituted (R = Me, Et, i‐Pr, t‐Bu or Ph) 3,4,5,6‐tetrahydro‐2H‐1,3‐oxazines were recorded at 14 and 70 eV in order to study the ring‐chain tautomeric equilibria in the gas phase. Certain fragment ions were associated with the ring or with the open‐chain forms of the compounds. As in chloroform solution, the electron‐withdrawing effect of the aryl substituent (p‐NO2, m‐Br, p‐Cl, H, p‐Me, p‐OMe and p‐NMe2) shifts the equilibrium towards the ring form. The correlation of ring‐chain equilibria (log K = [ring]/[chain]) with the Hammett σ+ constants of the aryl substituents was in general good or satisfactory although in some cases the p‐NMe2 did not fit these correlations. Copyright © 2007 John Wiley & Sons, Ltd.