Jp. Pezacki et al., Lifetimes of dialkylcarbocations derived from alkanediazonium ions in solution: Cyclohexadienyl cations as kinetic probes for cation reactivity, J AM CHEM S, 121(28), 1999, pp. 6589-6598
The first estimates of the lifetimes of the 2-propyl cation (4a), cyclobuto
nium ion (4b), cyclopropylethyl cation (4c), and 2-adamantyl cation (4d) in
1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), trifluoroethanol (TFE), and acet
onitrile solvents have been determined using electrophilic aromatic additio
n to 1,3,5-trimethoxybenzene as a kinetic probe reaction in laser flash pho
tolysis (LFP) experiments. The lifetimes ranged from similar to 100 ps to s
imilar to 40 ns at 22 degrees C. Oxadiazoline precursors 1 were used to gen
erate 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 carbo
cationic intermediates 4. The cations were found to react with 1,3,5-trimet
hoxybenzene to form cyclohexadienyl cations. The latter were monitored by L
FP. Stern-Volmer kinetics were used to determine lifetimes and rate constan
ts 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 mechani
sm in order to effect alkylation.