Bicyclobutonium

In 1967 he again wrote, The second subclass consists of ions such as the bicyclobutonium and the norbornyl cation in its cr-bridging form, which do not possess sufficient electrons to provide a pair for all of the bonds required by the proposed structures. A new bonding concept not yet established in carbon structures is required (emphasis added). 

The -conformation 13 is lower in energy than the Z-isomer 14. These are the smallest cyclopropyl substituted carbocations which can be investigated in solution by high resolution NMR. The corresponding primary cyclopropylmethyl cation 15 cannot directly be observed by high resolution NMR in solution because it is energetically less favorable than the bicyclobutonium ion 16 and thus only a minor isomer in the fast dynamic equilibrium of the cations 15 and 16. 13C- and H... 

The cyclobutyl/cyclopropylmethyl cation system (C4II7 ) has probably been the focus of more studies than any other carbocation system except the 2-norbornyl cation. Bridged cyclobutyl cations 16 are called bicyclobutonium ions. Bicyclobutonium... 

Scheme 3 Rearrangement of [C4H6R] + cations, for R = H a threefold degenerate interconversion of bicyclobutonium ion (54) and cyclopropylmethyl cations (55).

The 1-methylbicyclobutonium ion (56) and the l-(trimethylsilyl)bicyclobutonium ion (57) also undergo fast threefold methylene rearrangements (Scheme 3, R = CH3 or Si(CH3)3). 

For historical overviews on the 2-norbornyl cation, the bicyclobutonium ion and related hypercoordinated carbocations see (a) Nonclassical Ions, Reprints and Commentary, P. D. Bartlett, W.A. Benjamin, Inc, New York and Amsterdam 1965 (b) The Nonclassical Ion Problem, Brown, H.C. with comments by Schleyer, P.v.R. Plenum Press, New York, 1977... 

Bicyclic and polycyclic carbocations, NMR spectroscopy, 145-150 1, 2-dimethyl-2-norbornyl cation, 149 bicyclobutonium ions, 145-146, 146/ 3- /<7o-trialkylsilylbicyclobutonium ions, 147-148... 

Bisoxo substitution, 44, 48, 58 Bridged cyclobutyl cations, 146, see also Bicyclobutonium ions... 

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. 

Bicyclobutonium ions, are bridged carbocations with a pentacoordinated y-carbon which were first discussed as short lived intermediates in the solvolysis reaction of cyclopropylmethyl and cyclobutyl compounds (56, 57, 58, 59, 60, 61). 

The cartoon-like drawing of the structure of the parent bicylobutonium ion C4H7+ 36 is adopted from an ingenious forward-looking paper of Olah and coworkers in 1972, 61) long before routine 13C-FT-NMR spectroscopy and routine ab initio quantum chemical calculations were available, which envisaged correctly the stabilization mode of the parent bicyclobutonium ion to arise from the interaction of the backside lobe of the Cy-Hendo sp3 orbital with the empty carbenium carbon p-orbital at Ca. 

The endo-SiR/j bicyclobutonium structure of cations 40 was confirmed by MP2/6-31G(d) calculations, which revealed that the model structure 3-endo-SiH3-bicyclobutonium 44 (Figure 11) is an energy minimum, 7.9 kcal/mol lower in energy than the 3-exo-silylbicyclobutonium ion 45, which is not an energy-minimum structure. 

Figurel. Equilibrating bicyclobutonium ions and bisected cyclopropylmethyl...

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. 

On the other hand, many theoretical methods have been employed to elucidate the potential energy surface of the C4H7+ in gas phase (15,16) and in solution (77). High-level ab initio calculations suggest that, in gas phase, there are three C4H7+ structures as minima on the potential energy surface (14). These calculations pointed to bicyclobutonium and cyclopropylcarbinyl as the most stable structures. 

Figure 2 Calculated structure of the bicyclobutonium carbocation over zeolite Ysurface at B3LYP/6-31++G(d,p) MNDO.

However, this hypothesis does not explain the higher distribution of the cyclobutyl bromide as compared to cyclopropylcarbinyl bromide, since a distribution near 1 1 would be expected, if nucleophilic attack to the bicyclobutonium occurs in the same way as in solution. The different distribution, favoring the cyclobutyl bromide, may suggest that the bromide ion is not uniformly dispersed on the zeolite cavity, preferentially occupying certain positions on the zeolite surface, where it can better attack the bicyclobutonium at one of the three positions. 

To check this possibility we performed experiments with different concentrations of NaBr in the NaY zeolite. Table 2 presents the results. It can be seen that upon increasing the amount of NaBr impregnated on NaY, there is preference to formation of the cyclobutyl bromide over allylcarbinyl bromide, indicating that the relative position between the bromide ions and bicyclobutonium governs the product distribution. Hence, zeolites may act as solid solvent, favoring ionization of alkyl halides and nucleophilic substitution reactions. In contrast to liquid solvents, where solvation is mostly uniform, the zeolite surface seems to provide unsymmetrical solvation of the cations, leading to product distribution that is different from solution. 

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. 

Calculations showed that allylcarbinyl aluminumsilyl oxonium ion is the most stable species having 4.5 and 4.7 kcaLmol 1 lower energy than the cyclobutyl and cyclopropylcarbinyl aluminumsilyl oxonium ions, respectively. The bicyclobutonium and cyclopropylcarbinyl ion are 36.2 and 39.2 kcaLmol 1 higher in energy than the allylcarbinyl 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. 

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