Carbonium ions bridgehead

Following the extensive investigation of bridgehead carbonium ion reactivities, which provides the most conclusive experimental evidence available for the preferred planarity of carbonium ions 187), similar studies of bridgehead free radical reactivity have been initiated. The results are equally instructive. 

Since a bridgehead carbonium ion would be expected to be shorter-lived than an a-phenethyl cation, the additional equilibration occurs at the stage of the correspondingly longer-lived diazonium ion pair. This work also provides the clearest demonstration that nitrogen may be lost from within an ion pair without causing its disruption. 

Figure 5. The eight structures, unique in terms of the numbering of their atoms, are produced by a single 1,2-alkyl shift of 70. Transitions involving bridgehead carbonium ions were allowed, but three- and four-membered rings were forbidden.

Figure 6. Complete interconversion map for CjoHie ing systems devoid of three- and four-membered rings. Pathways involving no bridgehead carbonium ions (solid lines) pathways involving bridgehead carbonium ions (dashed lines).

The apocamphyl case is an extreme one, and bridgehead carbonium ions in other systems are more accessible.Inclusion of more atoms in the bridge gives a more flexible molecule and allows carbonium ion formation to proceed with a somewhat lower activation energy. Thus, the relative solvolysis rates of the bridgehead bromides 1-bromoadamantane, l-bromobicyclo[2.2.2]octane, and 1-bromobicyclo[2.2.1]heptane in 80% ethanol at 25°C are 1,10, and Under the... 

Chemical reactivity differences may be calculated if for the transition state of a rate-determining step of a reaction a structural model can be given which is describable by a force field with known constants. We give only two examples. Schleyer and coworkers were able to interpret quantitatively a multitude of carbonium-ion reactivities (63, 111) in this way. Adams and Kovacic studied the pyrolysis of 3-homoadamantylacetate (I) at 550 °C and considered as transition state models the two bridgehead olefins II and III (112). From kinetic data they estimated II to be about 2 kcal mole-1 more favourable than III. 

All the amendments to Bredt s rule that have been presented in the past decade have been more or less violated. One reason for these failures is that the past rules ignored the strain in parts of the molecule other than at the bridgehead double bond. Schleyer defines the strain at the bridgehead double bond, or olefin strain (OS), as the difference in strain between the olefin and the parent hydrocarbon, analogous to for carbonium ions (293). He... 

More recently, other approaches to this interesting ring system have also been developed. These are illustrated in Eqs. (41), 29< 13°) and (42) 131 As indicated, photochlorination of the parent hydrocarbon occurs only at the methylene positions 13°). The correspondence between free radical and car-bonium ion reactivities at the bridgehead positions of polycyclic hydrocarbons suggests that the bridgehead position of bisnoradamantane should also be highly unreactive in carbonium ion processes 1321. 

The nmr spectrum of 1-chloroadamantane in SbFs-S02 s°) or of adaman-tane in SbFs-FS03H 157) is attributed to the 1-adamantyl cation. The essential characteristics of the spectrum are summarized in Table 4. Contrary to the normal observation where the hydrogens adjacent to the carbonium ion site experience the largest deshielding effect, the 7 (bridgehead) hydrogens of the... 

These variations in the ease of formation of bridgehead cations are attributed to steric effects 9S>105>187l Carbonium ions prefer planarity strongly 187>292). 

Since the solvolysis reactions of bridgehead substrates are mechanistically uncomplicated (i.e. competing elimination is highly unlikely187 ns-H9) an(j backside solvent participation is impossible), they provide ideal models for limiting carbonium ion behavior. Adamantyl derivatives have become the substrates of choice for this purpose due to their availability and convenient reactivity. 

That this mechanistic pathway is followed is of some interest since the 1-tri-quinacenyl cation (420) has been shown on the basis of semiempirical calculations to be a twofold allyl-stabilized bridgehead ion.389 This cation, as well as 421 and 422, could be generated by allowing the respective chlorides to react with antimony pentafluoride in S02C1F at —78 °C.388 The and 13C NMR spectra have been recorded and suggest that almost planar divinyl carbonium ion units are present in... 

The classic carbonium theory of this reaction (/3-rule) postulates the formation of a carbonium ion at one of the carbon atoms close to the bridgehead carbon. Among the possible Wagner-Meerwein transpositions, the ones leading to four-membered rings are not observed, and may, instead, produce ring breaking (Fig. 16). 

This molecule is trigonal planar around the central charged carbon. It is observed experimentally that carbonium ions are more stable if it is possible for them to adopt a planar configuration. One reason for this is that it allows for the maximum separation of the substituents, and so reduces the steric crowding to a minimum. Hence, carbonium ions at a bridgehead are less stable than would be expected if the considerations discussed earlier were the only factors involved in determining their stability. 

A confirmation of this mechanism was searched for by a tracer experiment with alcohol ll-d3, which upon ionization in FS03H/S02C1F was expected to yield via the intermediate cation 4I-d3 the cation 33-d 3, in which the CD3 group is located at the bridgehead position. However, the species observed was carbonium ion 33-d3 ( ), the H NMR spectrum of which lacked the signal due to the indicated allylic methyl group (Scheme 18). 

Fig. 19. a) Bridgehead derivatives used in the calculation b) An example of correlation between -log of the experimental tosylate acetolysis rate constants at 70° and the calculated hydrocarbon-carbonium ion strain energy... 

---