Insights into Electrophilic Methane Activation with High-Valent Lead in Trifluoroacetic Acid and Trifluoromethanesulfonic Acid.

The formation of high-valent lead compounds has been investigated in trifluoroacetic acid (HTFA) and trifluoromethyl sulfonic acid (HOTf). Pb-(IV) species are formed in these acids, starting from Pb3O4 or other Pb-(IV) precursors (Pb-(OAc)4, PbPh4). Attempts to isolate Pb-(IV) compounds resulted in the isolation of the Pb-(II) salts Pb-(TFA)2(HTFA)2 and Pb-(OTf)2(HOTf), which have been structurally characterized. Pb-(TFA)4 and Pb-(OTf)4 can undergo reductive elimination in solution to give their respective Pb-(II) compounds and formation of CF3TFA and CF3OTf, together with loss of CO2 and SO3, respectively. A solution of Pb-(OTf)4 in HOTf/Tf2O activates methane at 5 bar at 60 °C to give MeTf, MeOTf and HOTf. The formation of PbMe-(OTf)3 is believed to be a key intermediate in these reactions, which reacts with Tf2O to form MeTf and Pb-(OTf)4, and undergoes reductive elimination to form MeOTf and Pb-(OTf)2.