Carbonium ions bridged

The high solvolytic stereospecificity of the tosylate 91 together with the unexpectedly fast reaction rates was tentatively interpreted by Cram 88b> in terms of /8-phenyl participation in the ionization step to produce a highly strained bridged carbonium ion 96 which is opened in a second reaction step to give the final product. Both the formation of 96 and its opening must involve complete inversion in order to ensure retention of stereospecificity in the overall solvolytic process. 

Deuterium labeling experiments show that oxidation of 2-phenylethyl iodide proceeds through the bridged carbonium ion 11 [21]. Both iodocyclobutane and... 

The erythro compound, ET, gave the erythro acetate, EA, with almost quantitative retention of optical activity. On the other hand, the optically active three tosylate, TT, gave racemic threo acetate, TA. The explanation for these striking results is given in terms of the different, bridged carbonium ions formed in each case ... 

The inherent plausibility of metastable bridged carbonium ions as intermediates is supported by two independent types of observation. One is the extensive rearrangements which can occur in allylic systems under conditions in which ionic displacement reactions are possible. A second is the existence of stable bridged compounds, including, in the case of boron compounds, pentavalent atoms. Thus diborane and substituted diboranes have stable bridged structures. 

The first-order hydrolysis of neopentyl bromide gives mixtures of sec-amyl alcohol and some neopentyl alcohol. While this does not establish the stability of a bridged carbonium ion intermediate, it does indicate the passage through such a state. The reaction is otherwise very difficult to picture. The postulated carbonium ion mechanism is... 

Til contrast, the reaction in basic alcoholic solution with Na+OC Hs gives normal product, ethyl neopentyl ether, with a second-order rate law indicating a direct >Sjv2 displacement mechanism. Bridged carbonium ions are not always formed as metastable intermediates, other structures being sometimes more stable. ... 

However, two compounds which are potentially within this definition have been prepared and their spectra obtained (80 and 81, see Table 16). The 7-norbomadienyl cation, 80 220,227), definitely prefers not to avail itself of bicycloaromatic stabilization of the bridge carbonium ion by "both double bonds, as the expected C211 symmetry is ruled out by the n.m.r. spectrum which in fact resembles that of the 7-norbomenyl cation, 71. Thus, bishomoconjugation only is observed, and the barrier to bridge flipping is estimated to be >19.6 kcal/mole 228). 

Further support for the nonclassical structure of the 2-norbomyl cation came from an application of NMR spectroscopy that is based on the difference between the total chemical shift of a carbocation and that of the corresponding alkane. Differences in total chemical shift of 350 ppm or more are associated with classical carbocations, while differences of less than 200 ppm are thought to indicate nonclassical, bridged carbonium ions. For example, the sum of the total NMR chemical shift of propane is 47 ppm, while the sum for the 2-propyl cation is 423 ppm. The difference, 376 ppm, indicates that the 2-propyl cation is a classical ion. For the 2-norbomyl system the total of the shifts is 408 ppm, while the total for norbomane is 233 ppm. The difference, 175 ppm, was taken as evidence for a nonclassical structure. ... 

Solvolysis of 4-twistyl (145 both epimers) and 10-protoadamantyl (146) tosylates in 70% aqueous acetone, ethanol, or acetic acid affords a mixture of 4-exo-twistyl and the predominant 10-exo-protoadamantyl derivatives by way of the common bridged carbonium ion intermediate (147) kinetic data reveal that the reaction of exo-(145) is anchimerically assisted, and hence endo- l45) is converted into (147) by leakage from the initial classical carbocation. Additionally an energy difference of 4.4kcalmol between twistane and protoadamantane systems is estimated. ... 

The corner-protonated cyclopropane structure corresponds to a methyl-bridged carbonium ion. 

Although the synthesis and reactivity of propellanes, recently reviewed, is discussed later, it may be noted that symmetry considerations and calculations have been used to rationalize the observed high reactivity of strained propellanes to acid and to radical attack but their stability to nucleophilic attack. EH calculations have been used to probe the stabilization of bridged carbonium ions by through-bond interactions. 

Another subtlety in the mechanism of 1,3-hydrogen elimination is the possibility that the proton transfer occurs from a protonated cyclopropane intermediate (z.e., bridged carbonium ion). If 1,3-hydrogen elimination does take place from a protonated cyclopropane, the hydrogen which is eliminated should, for stereoelectronic reasons, lie in the plane of the three-membered ring in the transition state. 

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