Carbocations bicyclobutonium

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... 

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... 

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

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 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. 

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

That maximum acceleration occurs when the vacant p orbital is parallel to the plane of the cyclopropyl ring can be seen from the solvolysis of spiro[cyclo-propane-l,2 -adamantyl] chloride (71). The carbocation formed by departure of Cl is unable to adopt the geometry of the bisected cyclopropylcarbinyl cation, but can orient its empty p orbital properly to form the bicyclobutonium ion. This compound solvolyzes 103 times more slowly than 1-adamantyl chloride.82 On the other hand, 72 solvolyzes 10s times faster than 73. The cation from 72 does have its p orbital parallel to the plane of the ring as in the bisected cyclopropylcarbinyl cation.83... 

Summary The l-(trimethylsilyl)bicyclobutonium ion and the 3-e c fe>-(/er/-butyldi-methylsilyl)bicyclobutonium ion were investigated by NMR spectroscopy in superacid solution and by quantum chemical ab initio calculations. The l-(trimethyl-silyl)bicyclobutonium ion undergoes a threefold degenerate methylene rearrangement. The 3-e /o-(/ert-butyldimethylsilyl)bicyclobutonium ion is the first static bicyclo-butonium ion. The NMR spectra of this carbocation are a direct proof for the hypercoordinated and puckered structure of bicyclobutonium ions. 

Silyl-substituted bicyclobutonium ions are also accessible from direct ionization of 3-silyl-sub-stituted cyclobutyl chlorides. Matrix co-condensation of cw//ra s-3-(trimethylsilyl)cyclobutyl chloride (Scheme 1, D (R = R = Me)) with SbFs onto a surface of SO2CIF/SO2F2 at -196°C yields after homogenization at -130°C a yellow solution of carbocation 12. The C-NMR spectrum obtained for cation 12, except for the alkyl groups at silicon, is very similar to the C-NMR spectrum of carbocation 9. The differences of the signals for the Co-, Cp/Cp -carbons are smaller than 1 ppm. 

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. 

Despite having made these analogies to other systems, the exact nature of the C4H7 system is less clear than that of norbornyl, and it is still under investigation. Considerable evidence exists in support of both the bicyclobutonium and cyclopropylcarbinyl carbenium ions as important contributors to C4H7. Under stable ion conditions ese two appear to be in equilibrium. As with so many carbocations, a very flat potential energy surface is implied, with structures of similar energy and low barriers to interconversion. 

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