Gas Phase Methylation of Polymethylbenzenes by Dimethylfluoronium Ions. A Radiolytic and Mass Spectrometric Study

Abstract

The alkylation of trimethyl-and tetramethylbenzenes by dimethylfluoronium ions, (CH 3 ) 2 F + , has been studied both by radiolytic techniques in gaseous CH 3 F at atmospheric pressure and by CH 3 F chemical ionization mass spectrometry.In particular, the substrate selectivity of the ionic methylation has been studied over an exceptionally wide pressure range. The mechanism of the gas-phase alkylation is discussed. The observed lack of substrate selectivity, and the appreciable positional selectivity, are rationalized with a kinetic model that could be of general value to aromatic substitution by very reactive gaseous cations.

Radiolytic approach to gas-phase ion chemistry

“…Thus, the yield of 1,3-dimethyl-4-íerí-butylbenzene, the "kinetic" product from the tert-butylation of m-xylene, decreases from 65% at 725 Torr to 8% at 20 Torr, in favor of the more stable l,3-dimethyl-5-tert-butylbenzene, unless the bulk gas (neopentane) contains strong bases. 21 Evidence for steric hindrance to deprotonation has been found in a study of the tert-butylation of crowded polymethylbenzenes primarily aimed at evaluating the intrinsic steric requirements of free t-C4H9+, whose salient features can be summarized as follows: (i) Activation of a ring position by one ortho Me group is insufficient to balance steric hindrance: e.g., no ortho substitution occurs in toluene, while p-xylene is unreactive. (ii) The activation by two Me groups, ortho and para to a ring position, overcomes steric hindrance by the ortho substituent, making the position nearly as reactive as position 4 of toluene, (iii) No teri-butylation occurs ortho to two Me groups•, e.g., mesitylene is unreactive.…”

Gas-Phase Aromatic Amination by Protonated Phenylazide. A Mass Spectrometric and Radiolytic Study

“…Owing to the lack of suitable charged reactants, gas-phase aromatic amination is conspicuously missing among the reactions investigated, which include alkylation by carbenium ions [2][3][4] and by dialkylhalonium ions [5,6], nitration by protonated alkyl nitrates [7,8], silylation by trimethylsilyl ions [9,10], etc.…”