A-Bridged ion

Fig. 5.11. Contrasting potential energy diagrams for stable and unstable bridged norbomyl cation. (A) Bridged ion is a transition state for rearrangement between classical structures. (B) Bridged ion is an intermediate in rearrangement of one classical structure to the other. (C) Bridged nonclassical ion is the only stable structure.

The C4H7+ cation shown as the first entry in Scheme 5.5 is a particularly interesting example. It is a bridged ion which can be reached from isomeric cyclopropylmethyl and cyclobutyl irms. 

Although alkyl groups in general increase the rates of electrophilic addition, we have already mentioned (p. 974) that there is a different pattern depending on whether the intermediate is a bridged ion or an open carbocation. For brominations and other electrophilic additions in which the first step of the mechanism is rate determining, the rates for substituted alkenes correlate well with the ionization potentials of the alkenes, which means that steric effects are not important. Where the second step is rate determining [e.g., oxymercuration (15-3), hydroboration (15-17)], steric effects are important. ... 

Just as intriguing are the studies on the exo-compounds. Two sets of data are available, those having a low v due of 1 12 and the higher result of Humski, Malojcic, Borcic and Sunko, T20. The former value can be rationalized with a non-classical transition state in the A-B process but does not seem to require it. The larger value appears to be consistent with the presence of a normal ion-pair like transition state (B-C) involving the classical ion and is difficult to rationalize with the presence of a bridged ion. 

To account for the stereospecificity of bromine addition to alkenes, it has been suggested that in the initial electrophilic attack of bromine a cyclic intermediate is formed that has bromine bonded to both carbons of the double bond. Such a bridged ion is called a bromonium ion because the bromine formally carries the positive charge ... 

The curved-arrow notation clearly shows the electron flow needed to effect the rearrangement. What curved-arrow notation does not show is die timing of these events—that is, whether loss of a leaving group precedes or is concerted with 1,2-phenyl migration or if a bridged ion is an intermediate. Such considerations, if known, can be included in more detailed mechanistic sequences. 

The use of the 2R,3R isomer led to formation of only 2R,3R-2-ethoxy-3-phenylbutane. Thus the configuration at each chiral center was retained in the product. These stereochemical data rule out simple ionization and solvent capture as a reaction mechanism since this would lead to a mixture of 2R and 2S configurations. From these observations it has been postulated that the phenyl group assists ionization of the leaving group by electron donation to produce a bridged ion. 

This ion appears to have a classical structure, which is in equilibrium with a bridged ion under conditions of low nucleophilicity, but undergoes extremely fast 1,2-hydride shifts which have not been frozen out. It also shows complete hydrogen and carbon degeneracies which most probably involve corner protonated cyclopropane intermediates. At higher temperatures it rearranges to t-butyl cation. 

The effect of a 1-cyclopropyl substituent on the rate of solvolysis of 2-adamantyl tosylates (129) has been compared to a variety of other 1-substituents. These rate effects were attributed to electronic substituent effects on formation of a bridged ion, but steric effects are probably also involved and a detailed interpretation of the conjugative effect of cyclopropyl is not straightforward. 

IV. A CYCLOPROPYL RING AS AN INTERMEDIATE OR A BRIDGED-ION IN ELECTROCHEMICAL PROCESSES... 

Several alternatives were proposed one, that participation by phenyl in expulsion of tosylate occurs, but is weak another, that bridging occurs, not in the rate-determining step, but rapidly, following formation of an open cation. H. C. Brown (p. 507) suggested that—for unsubstituted phenyl, at least—the intermediate is not a bridged ion at all, but a pair of rapidly equilibrating open car-bonium ions phenyl, now on one carbon and now on the other, blocks back-side attack by the solvent and thus gives rise to the observed stereochemistry. 

To test these alternative hypotheses, a tremendous amount of work has been done, by Brown and by others. For example, camphene hydrochloride is known to undergo ethanolysis 6000 times as fast as rer/-butyl chloride, and this had been attributed to anchimeric assistance with formation of a bridged ion. Brown pointed out that the wrong standard for comparison had been chosen. He showed that a number of substituted (3°) cyclopentyl chlorides (examine the structure of camphene hydrochloride closely) also react much faster than rerr-butyl chloride. He attributed these fast reactions—including that of camphene hydrochloride—to relief of steric strain. On ionization, chloride ion is lost and the methyl group on the ap hybridized carbon moves into the plane of the ring four non-bonded interactions thus disappear, two for chlorine and two for methyl. For certain systems at least, it became clear that one need not invoke a nonclassical ion to account for the facts. 

This product distribution implies the intermediacy of a bridged ion, and it was shown that the rate of the reaction of TsiSiMe2I with CF3COOH was not significantly affected by the presence of NaOOCCF3. The mechanism thus appears to be of the SN type. 

Nucleophilic solvents compete with bromide, but anti stereoselectivity is still observed, except when ERG substituents are present. It is proposed that anti stereoselectivity can result not only from a bridged ion intermediate, but also from very fast capture of a carbocation intermediate. Interpretation of the ratio of capture by competing nucleophiles has led to the estimate that the bromonium ion derived from cyclohexene has a lifetime on the order of 10 ° s in methanol, which is about 100 times longer than for secondary carbocations. ... 

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