2-Bicyclo hexyl cation

Similarly to the 2-norbornyl cation, comparison of calculated (IGLO/DZ//MP2/ 6-31G(d)) and experimental 13C NMR chemical shifts allowed to differentiate between the hypercoordinated 70 and the trivalent form 71 of the bicyclo[2.1.1.]hexyl cation.85 The experimental (157.8 ppm) and calculated (158.5 ppm) values for Cl and C2 (averaged signal) are reported to be nearly identical for the symmetrically bridged... 

Brexyl cation 296 8-Deltacyclyl cations 297 Bicyclo[2.1.1]hexyl cations 298... 

Another controversial ion, the bicyclo[2.1.1]hexyl cation, has also been restudied by Saunders et al. (1977a). This ion was first observed by Seybold et al. (1973) and was concluded to be a group of interconverting bridged... 

The first observation of the parent 2-bicyclo[2.1.1]hexyl cation [230] under stable ion conditions was made by Seybold et al. (1973) who prepared [230] and its 2-deuterio analogue [232] from the corresponding chlorides [231]. The H-nmr data showed that all six methylene protons in [230] were equivalent... 

The 2-Bicyclo[2.1.1]hexyl Cation Another bicyclic cation that is a close relative of the 2-norbornyl cation is the 2-bicyclo[2.1.1]hexyl cation... 

The 2-bicyclo[2.1.1]hexyl cation (148) was first observed by Wiberg and coworkers in superacidic media. Based upon the observed H NMR chemical shift data they suggested a static, nonclassical symmetrically bridged ion... 

It is interesting to compare the 2-norbornyl cation with its lower homologue — the 2-bicyclo[2,l,l]hexyl cation. Calculations with the MINDO/3 method show that the steric strain in the bicyclo[2,l,l]hexyl ion makes it a particularly favourable model for exhibitingCT-participation... 

The difference in the free energies for the secondary and the tertiary 2-methyl-bicyclo[2,l,l]hexyl cations is 7.0-9.8 kcal/mole while that for 2-norbornyl ions it is 5.5 kcal/mole 236,298) fgj, cydopentyl it is g 12.5 kcal/mole... 

The difference between the sum of the chemical shifts of all the atoms of the 2-bicyclo[2,l,l]hexyl ion and the corresponding hydrocarbon is 297 ppm, i.e. by 100 ppm less than in the case of the 2-methyl-2-bicyclo[2,l,l]hexyl cation this points to a considerable nonclassical delocalization in the secondary ion... 

The 2-bicyclo[2.1.1]hexyl cation [113] or [114] shows only three carbon signals, one for C-4 and two averaged signals for C-l/C-2 and C-3/C-5/C-6 respectively. The fast dynamic process leading to the observed symmetry could be either a Wagner-Meerwein shift between trivalent cations (83) or a three-fold degenerate rearrangement of hypercoordinated cations [114] as shown in (84). 

The l-methyl-2-trideuteriomethyl-2-bicyclo[2.1.1]hexyl cation [116] has a definite equilibrium isotope effect. The averaged C-1 and C-2 carbons show an isotope splitting of 46.5 ppm at -128°C. The carbon bonded to the CDs-group is shifted to higher field. The enthalpy difference (AH° = — 50 cal mol" per D) for the isotopic equilibrium was calculated from the temperature dependence of the equilibrium constant between... 

C AH° = — 26 cal mol per D] on the other shows that the dimethyl-bicyclo[2.1.1]hexyl cation falls in the intermediate range. The decreasing size of the CDj-isotope effect on (85)-(87) indicates a decreasing demand for C—H hyperconjugative stabilisation due to increasing a-stabilisation in... 

Trishomoaromatic Cations. Following Winstein s proposal1058 1061 of the formation of a trishomoaromatic cation in the solvolysis of s-bicyclo[3.1.0] hexyl tosylate, extensive effort was directed toward its generation under stable ion... 

The C-nmr spectrum of [230] recorded by Olah et al. (1976b) was very much like its proton spectrum in that it consisted of only three signals, with a two carbon resonance at S 157.8 ppm. It was not possible to freeze out the static ion [30] or [233]. The 2-methyl- [234] and 2-phenyI-2-bicyclo[2.l.l]hexyl [235] cations were prepared from the corresponding alcohols. The H-nmr spectrum of [234] was found to be temperature dependent. At low temperature (—100°C) three non equivalent types of methylene protons were observed, but at higher temperatures they equilibrate with each other via 1,2-carbon shifts. By comparison of the C-chemical shifts of [234] with those of the... 

Paquette and coworkers have carried out a solvolytic study of a tricyclic system in which a 3,4-ethano group is attached to the 2-bicyclo[3.1.0]hexyl system. This was done to assess the importance of cyclopropyl versus cyclobutyl neighboring group involvement. Some relative rate data are given in Scheme 10 for 3,5-dinitrobenzoate hydrolyses in 80 % aqueous acetone at 115 °C. Product results are also shown. Deuterium labeling revealed the lack of any cyclopropylcarbinyl-cyclopropylcarbinyl cation rearrangements. Also, the results showed that cyclopropyl participation predominates over initial involvement of the cyclobutyl group. 

It is finally of interest to consider the solvolytic chemistry of 3,4-unsaturated and 3,4-benzo 2-bicyclo[3.1.0]hexyl systems. These are of special interest because they are sources of homologs of the antiaromatic cyclopentadienyl and inden-l-yl cations and as such should be destabilized antihomoaromatic systems. 

The initial report by Winstein and coworkers " in 1959 that cis-3-bicyclo[3.1.0]hexyl tosylate (108) undergoes solvolysis with cyclopropyl participation to give a homoaromatic trishomocyclopropenyl cation (109), opened up a large new area of... 

The proposal that cyclopropyl participation in solvolysis of the cis-3-bicyclo[3.1.0]hexyl tosylate (108) proceeds via a trishomocyclopropenyl cation intermediate (109) has been firmly established by product, deuterium scrambling and salt effect studies However, because the cyclopropyl participation in this system requires an unfavorable conformation, it is not apparent from simple kinetic studies. Thus, the cis-3-bicyclo[3.1.0]hexyl tosylate (108) has an acetolysis rate which is only nine times faster than that of the non-participating trans isomer, 110. However, when the 3-bicyclo[3.1.0]hexyl moiety is incorporated into a tricyclic or tetracyclic structure which locks it into a chair form, dramatically large rate enhancements are found. Some examples from the literature which are provided in the review by Haywood-Farmer are given below. These... 

The trishomocyclopropenium ion (CeH/, 199) was first proposed by Winstein and coworkers as an intermediate in the solvolysis of czs-bicyclo[3.1.0] hexyl tosylate and extensive efforts were directed toward its generation under stable ion conditions. The persistent cation 199 was first prepared by Masamune et al. by the ionization of czx-bicyclo[3.1.0]hexane in superacid media and it has since been generated from the corresponding alcohol [Eq. (5.29)]. The NMR spectra of structure 199 are consistent with an ion of Csv symmetry. The three equivalent C-H groups are found at high field in the C NMR spectrum (8 C 4.9,7c h = 195.4Hz) in accordance with their hypercoordinate environments. 

It is of interest to compare the chemical shifts in the PMR spectra of the 2-norbomyl 5 and the 2-bicyclo[2,l,l]hexyl 143 cations. While the bridge-head protons... 

The partial cr-bridging seems to be a general phenomenon in organic chemistry and is suggested by quantum chemical calculations Olah et al. tried to record the a-participation in stable bicyclo[2,l,l]hexyl ions by increasing the electron demand of the cation rentre. When they studied the NMR... 

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