Carbanions cyclobutene

Carbocation-carbanion zwitterionic intermediates were proposed for the thermal cleavage of several cyclic compounds. In most of these reactions the ionically dissociating bond belongs to one of four strained ring systems, i.e. cyclopropane (13), cyclobutane (14), cyclobutene (15) or norbornadiene (16). The mechanism is distinguished from the formation of a diradical intermediate through homolysis in terms of solvent and substituent effects... 

Subsequently, Kametani and coworkers observed a similar allylic sulfoxide-sulfenate-sulfoxide rearrangement. These authors reported the exceptionally facile ringopening reaction of condensed cyclobutenes facilitated by arylsulfinyl carbanion substituents. For example, treatment of sulfoxide 68 with butyllithium in tetrahydrofuran at — 30°C for 10 min, followed by normal workup, results in the formation of product 71, which can be explained by the intervention of a double [2,3]-sigmatropic rearrangement of the initial product 69 via 70 (equation 32). A similar double [2,3]-sigmatropic rearrangement of 1,4-pentadienylic sulfoxides has also been reported by Sammes and coworkers. ... 

However, perfluoro-cyclobutene (15) and -cyclopentene (18) behave quite differently [28] because electropolymerisation occurs at the cathode, to give interesting conducting materials (17), (20) respectively, whose properties have not been explored. It seems most likely that the propagation process in each case involves nucleophilic attack on the perfluorocycloalkene by an intermediate carbanion, (16) or (19). The likely propagating processes are outlined in Scheme 8. In contrast, cathodic reductive dimerisation of the trifluoromethyl derivative (21) occurs [29]. 

Cyclobutenes prepared by the Ramberg-BScklund reaction are almost always unsubstituted at the vinylic positions. In 1974, Paquette introduced a related and complementary ring contraction which is particularly suitable for the synthesis of 1,2-dialkylcyclobutenes. The reaction takes place when carbanions of five-membered cyclic sulfones are treated with LAH in refluxing dioxane (Scheme 19). The mechanism of the reaction is not clear. Good yields are obtained only when the sulfone is 2,5-dial-kylated in the absence of these groups, simple reduction of sulfone to sulfide becomes a serious side reaction. Dibenzyl sulfone is converted into a mixture of stereoisomeric stilbenes (56%) and dibenzyl sulfide (23%) under the same conditions. ... 

The second example refers to a-sulfonyl and a-sulfinyl carbanion substituted cyclobutenes and their facile ring-opening reaction 174). Treatment, e.g., of the cyclobutene sulfone 215 with n-butyllithium in THF/hexane between —78 and —30 °C led after 10 min (protonation with an aqueous solution of ammonium chloride) to the diene sulfone 218 in 90% yield. If the reaction mixture is treated at —78 °C with methyl iodide, the carbanion 216 is trapped before it rearranges to the carbanion 217. If the methylation is performed at —30 °C, 217 alone reacts. 

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