MULTIPLE PROTONATION SITES IN ARYL ETHERS

Molecules containing a benzene ring and an oxygen atom typically have two types of protonation sites: on the ring (where facile intramolecul ar hydrogen transposition from carbon to carbon probably takes place) or on an oxygen lone pair. Four aryl ethers are compared: the isomers phthalan (1, a cyclic benzylic ether) and coumaran (2, a cyclic phenyl ether), as well as isochroman (3) and isopropyl phenyl ether (iPrOPh) . The proton affinities of 1-3 have been measured using FT-ICR techniq ues as 830, 855 and 838 kJ/mol, respectively. Comparison with model co mpounds and Hartree-Fock-based SCF calculations indicate that protonat ed phthalan (1H) and protonated isochroman (3H) have O-protonated stru ctures. By contrast, the conjugate acids of coumaran and iPrOPh prefer ring-protonated structures. Acidification/neutralization experiments in the ICR, as well as MIKE spectra, demonstrate that chemical ionizat ion of iPrOPh produces noninterconverting O- and ring-protonated forms . Metastable ion decompositions of protonated phthalan and protonated isochroman give evidence of separate decomposition pathways for both t ypes of tautomers. Protonated coumaran exhibits complete randomization of hydrogen between oxygen and the ring, which is attributed to high barriers for expulsion of neutral fragments. (C) 1998 Elsevier Science B.V. All rights reserved.