Dediazoniations of Arene Diazonium Ions in Homogeneous Solution. Part III: Heterolytic arylations in 2,2,2-trifluoroethanol - an SN1-or an SN2-reaction?

Benzenediazonium tetrafluoroborate in 2,2,2-trifluoroethanol decomposes to give fluorobenzene and phenyl 2,2,2-trifluoroethyl ether. In the presence of benzene, toluene, trifluoromethyl-benzene or anisole, the respective biphenyl derivatives are formed in addition to fluorobenzene and the ether. The distribution of the isomeric substituted biphenyls is consistent with an electrophilic substitution. No homolytic products (diazo tars, benzene) are formed. The reaction kinetics clearly show that ether formation and aryl-dediazoniations are of second-order type, i.e. that trifluoroethanol and the benzene derivatives mentioned above are rate-determining factors. It is shown that these results exclude the S_N1-mechanism which is usually assumed for heterolytic dediazoniations; free aryl cations are therefore not involved in these reactions. An S_N2-like mechanism seems to be the most likely, but one involving an encounter complex containing the dissociated benzenediazonium ion is also consistent with the experimental data.