Cyclopropylcarbinyl cation nonclassical

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. 

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). 

J. D. Roberts,seems to have first used the term nonclassical ion when he proposed the tricyclobutonium ion structure for the cyclopropylcarbinyl cation. See J. D. Roberts and R. H. Mazur, J. Am. Chem. Soc. 73, 3542 (1951). Winstein referred to the nonclassical structure of norbornyl, cholesteryl, and 3-phenyl-2-butyl cations. See S. Winstein and D. 

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). 

J. D. Roberts seem to have first used the term nonclassical ion when he proposed the tricyclobutonium structure for the cyclopropylcarbinyl cation [Roberts, J. D., Mazur,... 

Although the structure of the cyclopropylcarbinyl cation is not controversial, there is some disagreement whether this species should be considered to be classical or nonclassical 399. a-Bridging is clearly not involved. Yet the classical representation 388) is unsatisfactory in view of the abundant evidence for conjugation. The... 

In a number of cases 1,4-bishomotropyIium ions was generated to elucidate the three possible structures of the intermediate 9-barbaralyl cation 468 a-cyclopropylcarbinyl 468, nonclassical 482 and classical 483 ... 

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