Bridgehead carbocation

Since the central carbon of tricoordinated carbocations has only three bonds and no other valence electrons, the bonds are sp and should be planar. Raman, IR, and NMR spectroscopic data on simple alkyl cations show this to be so. In methylcycohexyl cations there are two chair conformations where the carbon bearing the positive charge is planar (9 and 10), and there is evidence that difference is hyperconjugation make 10 more stable. Other evidence is that carbocations are difficult to form at bridgehead atoms in [2.2.1] systems, where they cannot be planar (see p. 397). ° Bridgehead carbocations are known, however, as in [2.1.1]... 

C. The smaller retarding effect of the thioxo group when introduced into the more flexible system supports the applicability of the authors methodology to change the conjugative ability of bridgehead carbocations. 

For a review of organic synthesis using bridgehead carbocations, sec Kraus Hon Thomas Laramay Liras Hanson Chem. Rev. 1999, 89, 1591-1598. 

Bridgehead bicyclo[2.2.1]hept-l-yl cation (1-norbornyl cation) has not yet been directly observed 1-chloronorbomane yields the stable 2-norbornyl cation in SbF5-SO2 solution.192 Thus, ionization to the bridgehead carbocation must be followed by a fast shift of hydrogen from C( 1) to C(2) (either intramolecular or intermolecular), the driving force for which is obviously the tendency to relieve strain in the carbocation. 

We have imposed the restriction of using only secondary or tertiary carbenium ions and have avoided bridgehead carbocations, only because of their higher instability. This comment is in order in view of recent evidence of their existence. See, for example. Problem 44. 

The preferred structure of a carbocation is planar, and the nature of the bridgehead position is such that any carbocation formed at the bridgehead is unable to become planar. This raises the question of how easy is it to form a bridgehead carbocation The role of kinetics is significant... 

In a recent extensive computational study [BLYP/6-31G(d,p) level, atoms in molecules (AIM) and NBO theories], DuPre has found charge distribution in cation 86, which prevents the development of unstable bridgehead carbocation. Electron delocalization results in nearly neutral atoms across the C—H—C bonding with the bridging hydrogen essentially in the Is electron configuration, and thus the proton is highly shielded in the NMR spectrum. 

We have recently reported results of a systematic DPA study of bridgehead carbocations using as precursors alcohols, chlorides and bromides. Figure 1 is a plot of AfG°(17, X = Br, calc. Cl), i.e., values of ArG°(17, X = Br) obtained from experimental AjG°(17, X = Cl) and corrected as indicated above, versus the experimental AfG°(17, X = Br) and ArG°(17, X = Br, calc. OH). It shows that the self-consistency is excellent. 

This low resolution overview of the gas-phase stabilities of a variety of carbenium ions shows that they are by no means exotic data unrelated to solution reactivity. Quite on the contrary, they are the fundamental factor determining the relative stabilities of the same species in solution (as determined by the pA R and pAia scales, for example). Furthermore, the ranking of solvolysis rates of compounds R-X is largely dependent on the intrinsic stability of R" ". While this was already known in the case of bridgehead carbocations, we see now that this applies also to nonbridgehead tertiary cations as well as to a variety of secondary cations, even sometimes in cases wherein the access of solvent to the reaction center is not hindered. Intrinsic stability of carbocations thus appears as a very powerful unifying factor. 

The apocamphyl structure is particularly rigid, and bridgehead carbocations become accessible in more flexible structures. The relative solvolysis rates of the bridgehead bromides 1-bromoadamantane, l-bromobicyclo[2.2.2]octane, and l-bromobicyclo[2.2.1]heptane illustrate this trend. The relative rates for solvolysis in 80% ethanol at 25°C are shown. ... 

Hyperconjugative stabilization has also been considered to be important for bridgehead carbocations since 10-tricyclo[5.2.1.0 ]decyl tosylate (40) reacts nearly 10 times slower than expected on the basis of molecular mechanics calculations. The carbocation from this compound has none of the P-y C—C or C—H bonds antiperiplanar to the vacant p orbital, and so hyperconjugative stabilization should be absent, whereas with most... 

Other bridgehead cations this is not so. Two factors may lead to hyperconjugative stabilization of bridgehead carbocations being particularly favorable increased p character of C—C bonds as a result of increased strain and the cationic carbon having a tetrahedral geometry that was calculated... 

The various rearrangements of natural terpenoid compounds which occur in superacid media are reviewed. The stabilities of the bridgehead carbocations (97), (98), (99),... 

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