13C, 1H and two-dimensional NMR studies of charge distribution in sterically congested persistent cycloalkyl- and alkyl-pyrenium ions generated by protonation in superacid media

Abstract

In FSO₃H–SO₂CIF at low temperature, the symmetrical all α-substituted tetraisopropylpyrene 1, tetracyclohexylpyrene 2, and tetracyclopentylpyrene 3 monoprotonate at the ipso position to give 1H⁺, 2H ⁺ and 3H⁺, respectively. Whereas 1H⁺ is directly observable by low-temperature NMR spectroscopy before it rearranges to 1aH⁺, the sterically more congested pyrenium ions 2H⁺ and 3H⁺ are not persistent, rearranging rapidly to the NMR-observable 2aH⁺ and 3aH⁺. These rearranged pyrenium cations are the first examples of pyrenium ions of αβ attack. Ambient disproportionation of 2 in triflic acid ‘TfOH’ gives 1,3,6,8,7-pentacyclohexylpyrene.

The 1,3,6-triisopropylpyrene 4 is protonated at the available α position to give 4H⁺. The 2,7-di-tert-pentylpyrene 5 and 2,7-di-tert-butylpyrene 6 are α monoprotonated, and 2-tert-pentylpyrene 7 is exclusively monoprotonated at the α position ortho to the tert-pentyl group (→7H⁺). The 4-isopropylpyrene 8 is monoprotonated at the ‘remote’α positions to give 8H⁺ and 8aH⁺ in near equal amounts. The charge distribution in the resulting monopyrenium ions was probed with ¹³C, ¹H and two-dimensional NMR (HECTOR, COLOC and COSY) experiments. The positive charge is extensively delocalized away from the site of attack and exists at the alternating carbons of the periphery. Thus the phenalenium ion character of the pyrenium ion of α attack and phenanthrenium ion character of the pyrenium ion of αβ attack are shown. Low-temperature reaction of 2 with FSO₃H·SbF₅(4:1)–SO₂CIF gives a varying amount of sulfonylation σ-complex 2SO₂H⁺, the oxidation dication 2²⁺ and a 1,3-diprotonation species, 2D²⁺(D = dynamic). Likewise, protonation of 6 and 7 with FSO₃H·SbF₅(1:1 )‘magic acid^R’–SO₂CIF leads to diprotonation forming, 6D²⁺ and 7D²⁺ in addition to dealkylation to give R ⁺. The 1,3-diprotonation species consist of singly charged phenalenium ions and tertiary carbocations in which the alkyl group at the 2-position is dynamic and alkyl cation-like.