Nonclassical bicyclobutonium ions

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... 

Olah, Roberts and coworkers observed3 1 temperature-dependent chemical shifts for the C4H7+ ion, prepared from cyclopropylcarbinol-l-l3C. They suggested an equilibration involving nonclassical bicyclobutonium ion 2 and the bisected cyclopropylcarbinyl cation 3 (equation 12). 

The parent secondary cyclobutyl cation 47 undergoes threefold degenerate rearrangement viao-bond delocalization involving nonclassical bicyclobutonium ion-like system171,172 (see Section 3.5.2.5). Similar behavior is also observed for the 1-methylcyclobutyl cation 48a.173 176 The 1-phenylcyclobutyl cation 48b, on the other... 

CH2, Jqh - 189 Hz). Thus all three methylene carbons are identical, and the protons attached to each of them are nonequivalent. Equilibrating classical cyclopropylcarbinyl cations can be ruled out not only based on substantially shielded averaged methylene carbon NMR shift, but also due to the presence of nonequivalent geminal protons on each of the methylene carbons. A pentacoordinated nonclassical bicyclobutonium ion can account for the observed results. The proton NMR absorptions at 3 4.64 and 4.21 are assigned to the endo- and exo-methylene hydrogens, respectively, based on the NMR spectra of the stereospeciflcally deuteriated cyclopropylcarbinyl cations The endo-deuteriated cyclopropylcarbinyl cation was prepared from alcohol 5a, whereas a 1 1 mixture of endo- and exo-deuteriated cations were prepared from alcohol 5b (equations 10 and 11). 

In their study of structures, energies, and C NMR chemical shifts of C4H7 and C5H/ ions, Olah et al. have recently found - that the ko delocalized bisected cyclopropylmethyl cation 110 and the nonclassical bicyclobutonium ion 111 (both of G symmetry) are minima for C4H7 (MP2/cc-pVTZ level). At the MP4(SDTQ)/cc-pVTZ//MP2/cc-pVTZ + ZPE level, structure 111 is more stable by 0.4 kcal mol than structure 110. On the basis of C NMR chemical shift calculations [GIAO-CCSD(T) method] and relative energies, the equilibrium of the two structures in superacid solutions most likely account for the experimentally observed NMR chemical shifts. 

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... 

Olah et al. considered several possible structures [32]-[35] and [101]-[103] for the ion. Structures [33], [35] and [101] are equilibrating classical cations and [32], [34], [102] and [103] are nonclassical ones. Correlation of the observed spectral parameters of [100] with those estimated from suitable models and theoretical calculations favoured rapidly equilibrating bicyclobutonium ions (75) as structural representation of ion [100]. Some of the arguments are summarized below. 

It was suggested that the temperature dependence of the chemical shifts in the C NMR spectrum of ion 101 was due to an equilibrium between two or more energetically similar structural isomers of C4H7+ that interconvert rapidly, even at -155°C. Using chemical shift arguments, the major contributing isomer was assigned by Roberts and coworkers to the nonclassical bicyclobutonium structure 104. 2°... 

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. 

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