Behavior of aromatic cations and radicals in strong acid media

Nitroxides were shown to undergo other novel reactions in strong acid media. After three days in trifluoroacetic acid, and much more rapidly in trifluoromethanesulfonic acid, 4-methoxyphenyl-t-butylnitroxide gave a product, whose electron spin resonance (esr) spectrum is proposed to be that of the radical cation of 4-methoxyphenylhydroxylamine. This reaction could occur by protonation of the nitroxide oxygen, loss of t-butyl cation, and protonation of the resultant free radical. An identical esr spectrum was observed when bis(4-methoxyphenyl)nitroxide was dissolved in trifluoromethanesulfonic acid. Ipso protonation of one 4-methoxyphenyl ring and loss of anisole, followed by an electron transfer reaction and protonation would also produce the 4-methoxyphenylhydroxylamine radical cation. Additional studies of nitroxides gave the first evidence for the protonation of an aryl nitroxide, and for the ability of nitroxides to act as nucleophiles.

The former was evidenced by the esr spectra of 4-methoxyphenyl-t-butylnitroxide and bis(4-methoxyphenyl)nitroxide in various acids. The latter was shown by the S_N2 reaction of di-t-butylnitroxide with methyl triflate.

Aromatic cations were shown to undergo the seemingly unlikely EAS reactions with strong acids. 2,2',2",6,6',6"-Hexamethoxytriphenylmethyl cation was observed to exchange its meta hydrogens in trifluoromethanesulfonic acid rapidly enough to be observed by nuclear magnetic resonance (nmr) spectroscopy. Other aromatic cations were shown, by deuterium incorporation, to undergo similar but slower reactions. Such reactions require attack of one positively charged species with another of like charge, and must, therefore, go through a doubly charged intermediate. Several such dications were observed by nmr for the first time in FSO₃H-SbF₅ solutions, and were shown to be in equilibrium with their monocations. The effect of substituents on the reactivity and regioselectivity of these novel EAs reactions was investigated, and found to similar to that of neutral aromatic compounds. Reaction with NO₂⁺ as the electrophile showed the phenomenon is not limited to H⁺ and D⁺.