Nonclassical cyclopropylmethyl

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... 

In contrast to classical tertiary and secondary cyclopropylmethyl cations (showing substantial charge delocalization into the cyclopropane ring but maintaining its identity), primary cyclopropylmethyl cations show completely o-delocalized nonclassical carbonium ion character (see Section 3.5.2.5). Also, some of the secondary cyclopropylmethyl cations undergo rapid degenerate equilibrium (see later discussion). 

The striking influence of y-substitution on product formation is readily explained in terms of the relative charge densities at the cyclopropylmethyl and homoallylmethyl centers in the incipient nonclassical carbocation intermediates. Starting from y-di- and monosubstituted homoallylic iodides such carbocations do have considerable cyclopropyl carbocationic character, which, as a consequence, leads to cyclopropylmethyl derivatives on substitution. In the nonsubstituted case, the intermediate carbocation has predominant homoallylic character which under these conditions results in elimination. 

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. 

J. D. Roberts was the first to use the term nonclassical ion when he proposed the tricyclobutonium ion structure for the cyclopropylmethyl cation. Winstein referred to the nonclassical structure of norbomyl, cholesteryl, and... 

In contrast to the classical secondary and tertiary systems, primary cyclopropylmethyl cations are delocalized and the nonclassical nature of both the parent cyclopropylmethyl cation 101 and the 1-methylcyclopropylmethyl cation 102 is now firmly established (Scheme 5.6) Ion 101 can be generated from alkenyl, cyclobutyl, and cyclopropylmethyl precursors under stable ion conditions. Even at the lowest temperatures studied by NMR spectroscopy, that is, -155°C, ion 101 gives rise to a spectrum indicating a structure of threefold symmetry or a set of rapidly interconverting structures with average threefold symmetry as shown by Olah et al. ... 

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. 

On the basis of several studies, including the application of the tool of increasing electron demand,ion 167 can be best described as a nonclassical symmetrical bridged 27i-bishomocyclopropenium cation as opposed to a rapidly equilibrating pair of cyclopropylmethyl cations, for example 171. The observed unusually large coupling constants at the C(7) position of 167... 

One very interesting structure that might at first glance appear to be a 1° carbocation is the cyclopropylmethyl cation. Calculations indicated, however, that this species is best described by a pair of rapidly equilibrating nonclassical structures (62 and 63, Figure 5.46) that are approximately equal in energyAttempts to prepare the 1° cyclobutylmethyl cation were unsuccessful, however, with cyclopentyl cations being formed instead. ... 

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