Cyclobutadienyl dication

Several doubly eharged ions are ineluded in Seheme 9.1 some have been observed experimentally. Ionization of 3,4-diehloro-l,2,3,4-tetramethyleyclobutene in Sbp5-S02 at —75 C results in an NMR spectrum attributed to the tetramethyl derivative of the cyclobutadienyl dication ... 

The cyclobutadienyl dication 79 (R = H) and the tetramethylcyclobutadienyl dication 79 (R = CH3) according to quantum chemical calculations at the MP4/ 6-31G(d)//HF/6-31G(d) level are predicted to have puckered structures which are more stable than the planar forms.88... 

P-silyl substituted benzyl cations, 151-152 cyclobutadienyl dication, 150... 

Problem 10.30 Design a table showing the structure, number of tt electrons, energy levels of tt MO s and electron distribution, and state of aromaticity of (a) cyclopropienyl cation, b) cyclopropenyl anion, (c) cyclobutadiene, (d) cyclobutadienyl dication, (c) cyclopentadienyl anion, (/) cyclopentadienyl cation, (g) benzene, (h) cycloheptatrienyl anion, (/) cyclooctatetraene, (/ ) cyclooctatetraenyl dianion. ... 

An isomeric form of the anion that is initially formed is converted to the all-cA system rapidly at room temperature. Data on the equilibrium acidity of the parent hydrocarbon are not available, so the stability of the anion cannot be judged quantitatively. The NMR spectrum of the anion, however, is indicative of aromatic character. Several doubly charged ions are included in Scheme 8.1 some have been observed experimentally. Ionization of 3,4-dichloro-l,2,3,4-tetramethylcyclobutene in Sbp5-S02 at -75°C results in an NMR spectrum attributed to the tetramethyl derivative of the cyclobutadienyl dication. 

SO2 at -75 C results in an NMR spectrum attributed to the tetramethyl derivative of the cyclobutadienyl dication " ... 

Similar applications have been reported, among others, for the cyclobutadienyl dication (4) (demonstrating the nonplanarity of this Hiickel-aromatic compound), and for the... 

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