1- bicyclobutonium ion

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... 

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. 

Figurel. Equilibrating bicyclobutonium ions and bisected cyclopropylmethyl...

Die Ladungsverteilung im Bicyclobutonium-Ion und damit die Zusam-mensetzung der Reaktionsprodukte soUte sich durch Substituenten be-einflussen lassen [22). Tatsachlich tritt, wie die Beispiele (9) bis (14) zei-gen, die Substitution durch das Solvens jeweils am Substitutionsort einer... 

II, 4 kcal/Mol. Die Geometric des Bicyclobutonium-Ions ist dabei mit ungleichen 1,3- und 1,4-Uberlappungen und -Abstanden leicht asym-metrisch, der Winkel p ist gegeniiber dem Tetraederwinkel um 35° komprimiert. 

Die Reaktions- und Umlagerungsfahigkeit von bicyclischen exo- und endo-Cyclopropylmethyl-Verbindungen der Formel (152) und (153) ist besonders von K. B. Wiberg und Mitarbeitern untersucht worden, um Aufschlusse fiber die struktureUen Voraussetzungen zu gewinnen, die ffir die BUdung eines Bicyclobutonium-Ions notwendig sind. 

Unusual cationic species, e.g. nonclassical bicyclobutonium ions, are discernible in the solvolytic rearrangements, depending on the structure of the substrate.3,4 Computational studies using ab initio at the STO 4-31G level of theory indicate that the molecular symmetry of the unsubstituted parent cation C4H, is a bisected form of the cyclopropylmethyl system.5... 

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