Hydrogen Exchange Reaction of Arenium Ions and their Precursors

3 Hydrogen Exchange Reaction of Arenium Ions and Their Precursors 

In compounds containing non-equivalent ring hydrogen atoms the most basic positions are primarily substituted (see, e.g., the data on 1,3-dimethoxybenzene but with a sufficiently long time of reaction and with stronger acids it is possible to exchange the less basic positions as well 

Alkylarenium ions possess one more centre for the hydrogen isotope exchange the a-CH (n = 1, 2, 3) groups located at the positions para and ortho to the ring sp -hybridized carbon since for such ions the conjugate bases are not only aromatic compounds 118, R2 = H), but also alkylidencyclohexadienes (7/9) and (720). 

When substituents Ri and R2 in ion (777) are not hydrogen, alkylidencyclohexadienes of types (779) and (720) can be obtained by interaction of an ion salt with a base. From 4-alkylated ions the para-semibenzoid isomers (779) are preferably formed, e.g. l-R-l,2,3,5,6-pentamethyl-4-methylenecyclohexa-2,5-dienes from the salts of l-R-l,2,3,4,5,6-hexamethylbenzenium ions  

Dissolution of 9,9-dimethyl-10-methylene-9,10-dihydroanthracene (727) in CF3COOD yields ion (722) in which at 70 °C all protons of the 10-methyl group 

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