Bicyclobutonium intermediate

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). 

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 geometric and electronic properties of the 1-trialkylsilylbicyclobutonium cations, as concluded from a comparison of the experimental data for 422 and 424 and the calculated data for 425, are intermediate between that of the parent bicyclobutonium ion 418 and that of the 1-methylbicyclobutonium ion 420. 

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,... 

In fact, a cyclopropylcarbinyl ion is formed as the result of the attack by the cationic initiator. This carbo cation may rearrange, through intermediate bicyclobutonium ions, to give cyclobutylium or allylcar-binyl ion. The basic units obtained from such rearrangements have a structure that is either cyclobutenic or allylic. The predominant structures of the polymers are P17a and Pi7b of Equations 28 and 29. 

From the above considerations, it follows that the observed formation of cyclobutanol (cyclobutylamine) and cyclopropylcarbinol (cyclopropyl-carbinylamine) requires that cyclobutyl and cyclopropylcarbinyl cations, or at least some cyclic precursor of these ions, such as a bicyclobutonium ion, must exist in the dilute gas state at least for the time necessary to encounter a molecule of nucleophile, with lifetimes in excess of 10 s. Accordingly, cyclic C4H7 ions must be regarded as fully legitimate ionic intermediates, characterized by significant minima on the C4H7" ... 

Below we lay out the key carbocationic intermediates that could be involved in these solvolysis reactions. Cyclobutyl, cyclopropylcarbinyl, and 3-butenyl structures are conventional carbenium ions. Less conventional are the bicyclobutonium and tricyclobutonium ions. Although part of the earlier analyses of the solvolysis reactions, the tricyclobutonium... 

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