Carbocation-Forming Reactions in Ionic Liquids

Abstract

A number of trifluoroacetates, mesylates, and triflates have been studied in ionic liquids. Several lines of evidence indicate that all of these substrates react via ionization to give carbocationic intermediates. For example, cumyl trifluoroacetates give mainly the elimination products, but the Hammett rho+ value of -3.74 is consistent with a carbocationic process. The analogous exo-2-phenyl-endo-3-deutero-endo-bicyclo[2.2.1]hept-2-yl trifluoroacetate gives an elimination where loss of the exo-hydrogen occurs from a cationic intermediate. 1-Adamantyl mesylate and 2-adamantyl triflate react to give simple substitution products derived from capture of 1- and 2-adamantyl carbocations by the residual water in the ionic liquid. The triflate derivative of pivaloin, trans-2-phenylcyclopropylcarbinyl mesylate, 2,2-dimethoxycyclobutyl triflate, the mesylate derivative of diethyl (phenylhydroxymethyl)-thiophosphonate, and Z-1-phenyl-5-trimethylsilyl-3-penten-1-yl trifluoroacetate all give products derived carbocation rearrangements (kDelta processes). anti-7-Norbornenyl mesylate gives products with complete retention of configuration, indicative of involvement of the delocalized 7-norbornenyl cation. 1,6-Methano[10]annulen-11-yl triflate reacts in [BMIM][NTf2] to give 1,6-methano[10]annulen-11-ol, along with naphthalene, an oxidized product derived from loss of trifluoromethanesulfinate ion. Analogous loss of CF3SO2- can be seen in reaction of PhCH(CF3)OTf. Ionic liquids are therefore viable solvents for formation of carbocationic intermediates via kC and kDelta processes.