First steps of the reactions of alkanes and arenes with adamantyl cations in sulfuric acid in comparison with other reagents

Solutions of 1-adamantanol in sulfuric acid at T < 100 °C interact with alkanes (RH, H₂SO₄] > 85%) and arenes (ArH, H₂SO₄] > 59%). The data on the kinetics, kinetic isotope effect (KIE), effects of the structure of RH and ArH and acidity of the medium, and the observation of 1,4-cis-dimethylcyclohexane isomerization indicate that adamanyl cations (Ad⁺) serve as reactive species. In the reactions with alkanes, the Ad⁺ cation abstracts the hydride ion from RH in the rate-determining step. Compensation dependences appear between the activaion parameters for the KIE and “effect 5/6” (ratio of the rate constants for the C–H bond cleavage in cyclopentane and cyclohexane) in the reactions of cycloalkanes with Ad⁺ and other electrophilic reagents, such as “anthracene” (An₂)H⁺ and hydroxymethyl (CH₂OH)⁺ cations and Hg^II ions, including the points of the lower selectivity limit (k _H/k _D) = 1.4, (“5/6”) = 1. In the reactions with the Ad⁺ cation, the bond selectivity 3⁰: 2⁰ of alkanes is higher, while 2⁰: 2⁰ is lower compared to other reagents. In the first case, the selectivity is probably determined predominantly by the energies of the cleaved C–H bonds, whereas in the second case it is determined by steric hindrances. Judging by the kinetic and selectivity data in the series benzene—toluene—o-xylene—m-xylene and the absence of the reaction with p-xylene, mesitylene, and pseudocumene, it can be concluded that the main contribution to the Ad⁺ + ArH interaction is made by adamantylation to the para- and meta-positions of the benzene ring, whereas the ortho-positions are inaccessible to the attack because of steric hindrances. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1581–1596, August, 2008.