“Ipso” Aromatic Nitration in the Gas Phase

Abstract

Nitration of selected dialkylbenzenes, including o-and p-cymene, p-tert-butyltoluene, p-diisopropylbenzene, and p-di-tert-butylbenzene, by protonated methyl nitrate, (CH 3 OH-NO 2 ) + , has been investigated in the gas phase in the pressure range from 10 -8 to 720 Torr by Fourier transform ion cyclotron resonance (FT-ICR) and collisional activated dissociation (CAD) mass spectrometry and by the radiolytic technique. The results provide the first demonstration of gas-phase electrophilic aromatic nitrodealkylation and a quantitative evaluation of its efficiency relative to the competing nitration process. Dealkylation occurs exclusively following attack of the nitrating cation at the "ipso" position, which promotes intramolecular migration of the alkyl substituent to the nitro group, yielding O-alkylated or O-protonated ions. The positional selectivity of the nitration, and the relative rate of the "ipso" attack by (CH 3 OH-NO 2 ) + are discussed, especially as regards the role of the intrinsic steric requirements of the alkyl group, which in the gas phase is unaffected by the many complicating factors, e.g., solvation, ion pairing, etc., operative in solution. The results are compared with those concerning condensed-phase nitrodealkylation and the related nitrodesilylation reaction.

Unified Mechanistic Concept of Electrophilic Aromatic Nitration: Convergence of Computational Results and Experimental Data

“…With highly reactive electron-rich aromatic substrates, however, the energetics can become favorable. Another question to be raised relates to the low observed ipso -substitution, which contrasts with the high spin density on that position …”

ipso‐Nitration of Arenes

“…Further transformation of such ipso ‐nitroarenium species can occur by intramolecular isomerization pathways (rarely involving migration of nitro groups) leading to a variety of products 14. 15 Evidence of ipso attack by the nitrating species has been studied in the gas phase by pulse radiolytic methods 16. Nitration can also be carried out without acid catalysis under homolytic (radical) nitration conditions 3.…”

Ketosteroid profiling using Girard T derivatives and electrospray ionization tandem mass spectrometry: direct plasma analysis of androstenedione, 17‐hydroxyprogesterone and cortisol

“…Weitere Umwandlungen solcher ipso ‐Nitroareniumspezies können über intramolekulare Isomerisierungen erfolgen (die in seltenen Fällen eine Migration von Nitrogruppen einschließen), was zu einer Vielfalt an Produkten führt 14. 15 Hinweisen auf einen ipso ‐Angriff durch die nitrierende Spezies wurde mithilfe von Pulsradiolyse in der Gasphase nachgegangen 16. Nitrierungen können außerdem ohne Säurekatalyse unter homolytischen (radikalischen) Bedingungen durchgeführt werden 3.…”