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

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+ ion s 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 pr ocess, occurring competitively at both basic sites (despite the large diffe rence between their proton affinities, -105-145 kJ mol(-1)), and the ion ab undance ratio [MH+] / [MH - MeOH](+) reflects the ratio of abundances of th e 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 t he 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 am ino group when the approach of the protonating reagent (C4H9+ ion in our me asurements) to the nitrogen atom is increasingly hindered by the N-substitu ents. Another effect of steric hindrance in the CI process involves enhance d formation of the molecular radical cations M+. (presumably by a charge ex change mechanism), in competition with the usual protonation, in aminoether s with bulky N-substituents.