Cyclopropylmethyl cation studies

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... 

The carbocations so far studied are called classical carbocations in which the positive charge is localized on one carbon atom or delocalized by resonance involving an unshared pair of electrons or a double or triple bond in the allylic positions (resonance in phenols or aniline). In a non-classical carbocation the positive charged is delocalized by double or triple bond that is not in the allylic position or by a single bond. These carbocations are cyclic, bridged ions and possess a three centre bond in which three atoms share two electrons. The examples are 7-norbomenyl cation, norbomyl cation and cyclopropylmethyl cation. 

The cyclopropylmethyl cation has recently been generated in the gas phase from both homoallyl chloride and cyclopropylmethyl chloride and studied using collisional activated dissociation mass spectrometry.216 Interestingly, cyclobutyl chloride, which yields the cyclopropylmethyl cation in the condensed phase, gives a substantial amount of the bicyclobutonium ion in the gas phase. 

NMR studies in super acid media and theoretical studies have suggested that the bicyclo-butonium ion and the classical cyclopropylmethyl cation should be close in energy. 

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 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. 

Schmitz and Sorensen have prepared the nortricyclylmethyl cation (117), a static primary cyclopropylmethyl cation. The and NMR spectroscopic studies of 117 indicate that the system has features of both a bridged... 

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... 

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. ... 

Allylmethyl-Cyclopropylmethyl-Cyclobutyl Systems. The cyclopropylmethyl cation has been the subject of semi-empirical natural orbital analysis, and the bonding analysis supports the suggestion that there is delocalization of charge throughout the species. MINDO/3 geometries have been obtained for both planar and non-planar homotropylium cation, and the results of the study show the ion to be best represented as an open species and not as a delocalized bicyclo[5,l,0]octadienyl cation, as previously suggested. ... 

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... 

The cyclobutyl to homoallyl rearrangement was studied above all for mechanistic reasons, especially to differentiate between different kinds of cationic intermediates.7 In general, mixtures of cyclopropylmethyl, cyclobutyl, and homoallyl derivatives are formed, depending on the type of substitution in the substrate and stability of the precursor ions. 

From solvolytic studies of iso topically labeled substrates it was shown that cyclopropyl-carbinyl-cyclobutyl interconversion is stereospecific51 52. The stereospecific interconversion of cyclobutyl cations to the corresponding cyclopropylcarbinyl cation was also cleanly observed in superacid medium, and was used to prepare otherwise unstable cis-(a-methylcyclopropyl)carbinyl cation 1753. Thus ionization of d.s-2-chloro- or cw-3-chloro-l-methylcyclobutane in SbF5-S02ClF at -135 °C yielded the ris-isomer which rapidly rearranged irreversibly into the trans-isomer 18 at about -100 °C. The trows-isomer 18 is the only cation obtained when the preparation was carried out at -80 °C, or when prepared from the cyclopropylmethyl carbinol20b 38 50ac (equation 24). 

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). 

Extensive experimental and theoretical work has been devoted to study the nature of cationic intermediates involved in cyclopropylmethyl, cyclobutyl and homoallylic... 

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. 

The cationic rearrangement of homoallylic compounds to cyclopropylmethyl derivatives is a well-known reaction which has received considerable attention, mainly due to studies of the homoallylic methyl ion intermediate. ... 

The cationic rearrangement of cyclobutyl compounds to cyclopropylmethyl derivatives is a well-studied reaction which, like the homoallylic rearrangement, has been mainly investigated in connection with the fate and nature of the cyclobutyl cation.Again the synthetic value of this rearrangement is limited because generally mixtures of cyclobutanes, cyclopropanes and alkenes are obtained. 

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