Lifetimes of dialkylcarbocations derived from alkanediazonium ions in solution: Cyclohexadienyl cations as kinetic probes for cation reactivity

The first estimates of the lifetimes of the 2-propyl cation (4a), cyclobutonium ion (4b), cyclopropylethyl cation (4c), and 2-adamantyl cation (4d) in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), trifluoroethanol (TFE), and acetonitrile solvents have been determined using electrophilic aromatic addition to 1,3,5-trimethoxybenzene as a kinetic probe reaction in laser flash photolysis (LFP) experiments. The lifetimes ranged from similar to 100 ps to similar to 40 ns at 22 degrees C. Oxadiazoline precursors 1 were used to generate sec-diazoalkanes 2 by LFP which, in the presence of a proton source, give rise to sec-alkanediazonium ions 3 that lose N-2 rapidly to form carbocationic intermediates 4. The cations were found to react with 1,3,5-trimethoxybenzene to form cyclohexadienyl cations. The latter were monitored by LFP. Stern-Volmer kinetics were used to determine lifetimes and rate constants for reactions of cations 4a-d. Shea lifetimes of 4a-d imply that simple sec-alkanediazonium ions, from carcinogenic N-alkyl-N-nitrosamines, must be generated within contact distances of DNA through a preassociation mechanism in order to effect alkylation.