Ethylenebenzenium cations

Other problems dealt with by the Hehre-Pople method are internal rotation in vinylcyclopropane and vinylcyclobutane 153>, the structure of homoallyl cation 154> and ethylenebenzenium cation 155>, torsional barriers in -substituted phenols 156), inversion barriers in -substituted anilines 157>, the effects of a-substitution in keto-enol tautomerism 158> and the circumambulatory rearrangement in bicyclo [3.1.0] hex-3-en-2-yl cation 159> ... 

The parent spirocyclopropylbenzenium ion 102 can be prepared by the ionization of -phenylethyl chloride in HF-SbF5-S02ClF at -90 °C followed by warming to -60 °C. At higher temperatures (-27 to -5 C), the ion isomerizes to the a-methylbenzyl cation 103 with an activation energy of 13 kcalmol as shown by the NMR kinetic study (equation 61). The rearrangement probably proceeds through a partially delocalized primary 2-phenylethyl cation 104 ". The NMR spectrum of the ethylenebenzenium ion 102 showed (5 C 68.8 (Cl), 171.8 (C2, C6), 133.4 (C3, C5), 155.4 (C4), 60.7 (CH2). The equivalence of the C3, C5 and C2, C6 carbons as well as the methylene carbons show that the ion has Qv symmetry. The deshielded absorptions for the CH2 carbons are similar to those in other cyclopropylcarbinyl cations. 

Equation 16 illustrates a celebrated example where ring closure competes with vicinal hydride shift (a common form of atom transfer in cations, which does not take place in free radicals or anions). The gas phase reaction was explored by preparing the dimethylfluoronium ion, (CH3)2F" , by y-radiolysis of fluoromethane. Exothermic methylation of a sample of C-yd-phenethyl chloride (where the asterisk in equation 16 symbolizes the labeled position) in the gas mixture gives a vibrationally excited ion that loses chloromethane to form two isomeric ions, a-phenethyl cation and spirooctadienyl cation (sometimes called ethylenebenzenium). Nucleophilic attack by methanol in the reaction mixture yields PhCH(CH3)OCH3, whose isotopic label remains almost entirely at the methyl group. The recovered PhCH2CH20CH3 contains equally distributed between the two methylene positions. The spirooctadienyl ion does not isomerize to a-phenethyl cation, even though DFT calculations predict the latter to be 55 kJ/mol more stable. 

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