Spirooctadienyl 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. 

Nucleophilic capture of the spirooctadienyl cation opens the 3-member ring. This behavior characterizes many reactions of many other cyclopropane-containing carbocations, as well, y-radiolysis of perdeuterated propane forms CsD ions, most of which either transfer D or form isopropyl adducts. As the propane pressure is raised from 1000 mbar to 2000 mbar, however, the isopropyl/ -propyl adduct ratio falls from 30 1 to about 5.5 1. This implies the formation of corner-protonated cyclopropane, which reacts with nucleophiles as though it were an -propyl cation. With increased pressure, vibrationally excited protonated cyclopropane experiences more frequent nonreactive collisions, which deactivate it and slow down its rate of unimolecular isomerization to isopropyl cation. 

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