Detritiation reactions in aromatic systems.

A review of the literature on electrophilic aromatic hydrogen-exchange reactions is presented. A number of mono-tritiated substituted benzenes, heterocyclic, and polynuclear compounds have been prepared and rates of detritiation have been measured, mainly in anhydrous trifluoroacetic acid. The results provide a measure of the reactivities of various aromatic positions of dibenzothiophen, dibenzofuran, benzothiophen, thiophen, diphenyl ether, diphenyl sulphide, 5-ethylcarbazole, naphthalene, biphenyl, fluorene, phenanthrene, and diphenylmethane. Conclusions are considered in relation to data available for the more familiar aromatic substitutions, such as bromination and nitration, and some anomalies are noted. An analogy has been shown to exist in the detritiation of [2-3H] fluorene and 9,9-dimethyl-[2-3H] fluorene on the one hand, and [p-3H] toluene and [p-3H]-t-butylbenzene on the other; the significance of this is discussed. Measurement of the rate of detritiation of o- and p-tritiated -toluene and -t-butylbenzene in a variety of media has shown that variations in the relative reactivities of ortho- and para- positions cannot be ascribed to steric effects. The rates of detritiation of the compounds, 4-X-[4'-3H] biphenyl (where x = 4-Me > 4-Meo > 4-C1 4-Br > 4-NO2) and 2-x-[7-3H] fluorine (where X = Me > Meo > H C1 > Br > CO2H > NO2 ) have been measured. The effects of substituents, in analogous positions, are very similar in the fluorene and biphenyl systems, but the results do not show whether tautomeric effects are transmitted from one ring to the other in these systems. Anomalous results for the 2-Meo- [7-3H] fluorene and 4-Meo-[4'-3H] biphenyl indicate that some interaction, possible hydrogen bonding, occurs between the methoxy- substituent and trifluoroacetic acid.