Cyclopropylmethyl cation bisected

The preferred conformation of the cyclopropylmethyl cations is one in which the p orbital at the cationic center is oriented for maximum overlap with the cyclopropane C-C bonds (bisected conformation). It has essentially the same energy as the cyclobutyl cation, and the barrier to interconversion is very low ( 2 kcal/mol). The low energy of cyclobutyl cation is due to a strong 1,3-interaction, forming a bicyclobutyl cation. The extreme of this sort of interaction is found with the bicyclo[l.l.l]pent-l-yl cation, in which the structure is found to have a 1.54 A cross-ring distance. The ion appears to correspond to a proton attached to the nonbonded charge density near the bridgehead carbons of [l.l.ljpropellane. 

Isotope labeling experiments and EPR spectroscopic studies have shown that the cyclopropylmethyl radical is a discrete chemical species with a finite lifetime = 7 x 10 s at 25°C in methylcyclopropane solution). Unlike the corresponding cyclopropylmethyl cation, 1 has no nonclassical or fluxional characteristics, and it does not rearrange to cyclobutane derivatives. Its rate of formation from diazenes, peroxides, and other precursors is slightly greater than that of model primary acyclic radicals, which indicates that it has a small thermodynamic stabilization. EPR spectroscopic studies have shown that rotation of the methylene group carrying the unpaired electron is not free and that the preferred conformation is bisected rather than perpendicular. 

The first cyclopropylmethyl cation to be directly observed was the tricyclo-propylmethyl cation (100) and the subsequent study of a variety of cyclopropylmethyl cations " led to the conclusion that the tertiary cations are static and, in the absence of constraining skeletal rigidity, adopt a bisected geometry rather than an eclipsed one (making the ex substituents on the carbenium ion... 

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. 

The initially formed cyclopropylmethyl cation 7.132 can rearrange into other isotopomeric cyclopropylmethyl cations (7.133 and 7.134) and cyclobutyl cation 7.135 (Scheme 7-39). Solvolysis of specifically labeled cyclopropylmethyl derivatives demonstrated that these rearrangements proceed stereospecifically (Majerski and Schleyer, 1971). NMR spectra in superacid (amorphous SbFs) at low temperature (5-17 K) were interpreted as being consistent with a mixture of the bisected cyclopropylmethyl cations 7.132-7.134 and the cyclobutyl cations 7.135 (Myhre et al., 1990b see also earlier work by Roberts and Olah s groups Staral et al., 1978 Prakash et al., 1985). 

Only the bisected conformation aligns the cyclopropyl C—C orbitals for effective overlap. Crystal structure determinations on two cyclopropylmethyl cations with additional stabilizing substituents, C and D, have confirmed the preference for the bisected geometry (Fig. 5.7). 

Studies in superacid media provided early information concerning the preferred conformation of the cyclopropylmethyl cation. The NMR spectrum of the 2-cyclopropyl-2-propyl cation revealed that the two methyl groups were nonequivalent. This is consistent with the bisected, but not the perpendicular, conformation. Furthermore, the existence of the two peaks requires a significant barrier to rotation since, otherwise, an averaged peak would be observed in the NMR spectrum. Later work established the rotational barrier to be about 14kcal/mol. ... 

In conclusion, most of the evidence and molecular orbital calculations (see p. 27) support the view that cyclopropylcarbinyl cations have a bisected conformation that is stabilized because of the increased p-character of the bent C—C cyclopropane bonds which increases greatly their C—C hyperconjugative ability. The most important evidence that has been interpreted as arguing against this formulation is the NMR spectra of the cyclopropylmethyl cation itself, and hence there must be certain reservations about the < bove conclusion concerning this ion. It must also be... 

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... 

As pointed out by the pioneer in those studies C4H has some of the characteristics expected for a very rapidly equilibrating mixture of classical cyclopropylmethyl, cyclobutyl, and 3-butenyl cations and yet other characteristics which wholly belie any description that implies conventional charge distributions or geometries derived from structural representations using solid lines representing two-electron bonds . The rapidly equilibrating ions referred to are shown in 59a, 59b and 59c respectively, and the structures with unconventional charge distribution are the bicyclobutonium ion (60) or the bisected structure (61). 

Ion 38 (Fig. 16) could be generated both from cyclobutyl and cyclopropylmethyl precursors. At lowest temperatures studied (= 140 °), ion 38 is still an equilibrating mixture of bisected o-delocalized cyclopropylcarbinyl cations 169 and the bicyclo-butonium ion 170. 

---