Bicyclobutonium cations

Carbocations on Surfaces Formation of Bicyclobutonium Cation via Ionization of Cyclopropylcarbinyl Chloride over NaY Zeolite... 

Recent progress in preparation and study of alkylated fullerene cations RC6o+ and RC o+ as long-lived species are examined by T. Kitagawa in Chapter 12. Chapter 13 by C. J. A. Mota and co-workers examines the formation of the bicyclobutonium cation via cyclopropylcarbinyl chloride over solid acid catalysts. 

The rearrangement of the cyclopropylcarbinyl chloride in solution is well known in the literature (//). In polar solvents three products, arisen from the nucleophilic substitution of the solvent to the chloride, are usually detected, which are formed via nucleophilic substitution of chloride by solvent. This chemistry can be explained by the formation of the bicyclobutonium cation (C4H7+), which acts as a tridentated ion, generating the three products shown in scheme 3. 

The l3C NMR spectrum of the C4H7+ cation in superacid solution shows a single peak for the three methylene carbon atoms (72) This equivalence can be explained by a nonclassical single symmetric (three-fold) structure. However, studies on the solvolysis of labeled cyclopropylcarbinyl derivatives suggest a degenerate equilibrium among carbocations with lower symmetry, instead of the three-fold symmetrical species (13). A small temperature dependence of the l3C chemical shifts indicated the presence of two carbocations, one of them in small amounts but still in equilibrium with the major species (13). This conclusion was supported by isotope perturbation experiments performed by Saunders and Siehl (14). The classical cyclopropylcarbinyl cation and the nonclassical bicyclobutonium cation were considered as the most likely species participating in this equilibrium. 

Rearrangement of the cyclopropylcarbinyl chloride takes place over NaY zeolite, indicative of the formation of the bicyclobutonium cation. Theoretical calculations show that the bicyclobutonium is an intermediate on the zeolite surface and might be in equilibrium with the alkyl-aluminumsilyl oxonium ion. 

The results of cyclopropylcarbinyl chloride rearrangement over NaY impregnated with NaBr suggest that there is an equilibrium between the bicyclobutonium cation and the alkyl-aluminumsilyl oxonium ion, explaining the preferred formation of the allylcarbinyl bromide in the rearranged products. It also suggests that zeolites may act as solid solvents, providing unsymmetrical solvation for the ions inside the cavities. 

From the large amount of 7r-electron participation it is evident that considerable charge delocalization is developed in the transition state of the assisted solvolysis. This may be represented as 89. The multiplicity of products ranging from linear to three- and four-membered cyclic derivatives may be accommodated by the idea of a non-identical bicyclobutonium cationic intermediate 90 (Jacobs and Macomber, 1969 Santelli and Bertrand, 1969c). Interconversion of various bicyclobutonium ions and possibly conversion of these to various other cations,... 

The "symmetrical ion (B), corresponding most nearly to the accepted symmetrical structure of the cyclopropylcarbinyl or bicyclobutonium cation (4) [22], represents delocalisation of the 4,5-bonding electrons as well as the sr-electrons. In its extreme form, 4,5-bond delocalisation is represented by the "unsymmetrical structure (C). Recent work indicates that structures involving 4,5-bond delocalisation may be important. Whitham [43] found that cholesteryl tosylate and 3jS-hy-droxymethyi A-norcholest-5-ene tosylate i8) solvolyse in buffered aqueous acetone to give i-cholesterol and cholesterol... 

The strain in bicyclo[1.1.0]butane results in decreased stability and enhanced reactivity. The strain energy is 63.9kcal/mol the central bond is nearly pure p in character, and it is associated with a relatively high HOMO. These structural features are reflected in enhanced reactivity toward electrophiles. In acid-catalyzed reactions, protonation gives the bicyclobutonium cation (see Section 4.4.5) and leads to a characteristic set of products. 

M. Franco, N. Rosenbach Jr., G. B. Ferreira, A. C. O. Guerra, W. B. Kover, C. C. Turci, C. J. A. Mota, Rearrangement, nucleophilic substitution, and halogen switch reactions of alkyl halides over NaY zeolite formation of the bicyclobutonium cation inside the zeolite cavity, J. Am. Chem. Soc., 2008, 130, 1592-1600. 

Scheme 1, Generation and rearrangement pathways for trialkylsilyl (SiRR 2)-substituted bicyclobutonium cations.

Hydrolysis (lime-water) of the chloride (207 X=C1) proceeds with retention of configuration to give alcohol (204) this would require that a-bridged ions (IS8) such as bicyclobutonium cation, are unimportant in this reaction (77). 

The isotopic perturbation method is very useful for studying fractional bonding and hypercoordination in coordinatively unsaturated and electron deficient compounds such as transition metal complexes or carbocations. The 2-norbornyl cation and the bicyclobutonium cation are the most prominent examples of carbocations whose structures have led to much controversial discussion (the so-called classical-nonclassical ion controversy). The application of the isotopic perturbation methed is likely to be the most decisive piece of nmr evidence for the hypercoordinated structure of these two cations in solution. 

Cyclopropyhnethyl, cyclobutyl and bicyclobutonium cations The fast rate of solvolysis and the rapid interconversion of cyclopropyl-methyl and cyclobutyl derivatives have been extensively investigated. The dominant structure in the dynamic equilibria of the cation [122] and... 

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