Steric hindrance and the kinetic nature of the protonation process in 4-amino-1-methoxycyclohexanes upon chemical ionization

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Eur. J. Mass Spectrom. 5, 449 - 454 (1999)

Steric hindrance and the kinetic nature of the protonation process in 4-amino-1-methoxycyclohexanes upon chemical ionization

V. Vais, A. Etinger and A. Mandelbaum^*
Department of Chemistry, Technion - Israel Institute of Technology, Haifa, Israel

ABSTRACT:

The abundant [MH - MeOH]⁺ ions in the isobutane-chemical ionization (CI) mass spectra of trans-4-amino-1-methoxycyclohexanes (where proton transfer between the two sites does not take place, in contrast to the cis-isomers) indicate protonation at the two basic sites, affording two isomeric MH⁺ ions in each case, one protonated at the dimethylamino group, MH⁺(N), and the other at the less basic oxygen function, MH⁺(O). This result shows that the isobutane-CI protonation of the aminoethers is a kinetically controlled process, occurring competitively at both basic sites (despite the large difference between their proton affinities, ~105-145 kJ mol^-1), and the ion abundance ratio [MH⁺] / [MH - MeOH]⁺ reflects the ratio of abundances of the isomeric MH⁺ ions, MH⁺(N) and MH⁺(O), initially protonated at one of the two sites. The latter ion abundance ratio decreases with the bulkiness of the N-substituents (by a factor of more than 10 between N,N-dimethyl- and N,N-diisopropyl-derivatives), indicating lower rates of protonation at the amino group when the approach of the protonating reagent (C₄H₉⁺ ion in our measurements) to the nitrogen atom is increasingly hindered by the N-substituents. Another effect of steric hindrance in the CI process involves enhanced formation of the molecular radical cations M^+" (presumably by a charge exchange mechanism), in competition with the usual protonation, in aminoethers with bulky N-substituents.

Keywords: steric hindrance, protonation upon CI, proton transfer, protonation site, CID, aminoethers, kinetic control of protonation

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