Carbocations bridging

Much effort was put into studying whether certain carbocations represent rapidly equilibrating or static (bridged, delocalized) systems (more about this in Chapter 9). 

The differentiation of bridged nonclassical from rapidly equilibrating classical carbocations based on NMR spectroscopy was difficult because NMR is a relatively slow physical method. We addressed this question in our work using estimated NMR shifts of the two structurally differing ions in comparison with model systems. Later, this task... 

As mentioned, we also carried out IR studies (a fast vibrational spectroscopy) early in our work on carbocations. In our studies of the norbornyl cation we obtained Raman spectra as well, although at the time it was not possible to theoretically calculate the spectra. Comparison with model compounds (the 2-norbornyl system and nortri-cyclane, respectively) indicated the symmetrical, bridged nature of the ion. In recent years, Sunko and Schleyer were able, using the since-developed Fourier transform-infrared (FT-IR) method, to obtain the spectrum of the norbornyl cation and to compare it with the theoretically calculated one. Again, it was rewarding that their data were in excellent accord with our earlier work. 

The alkyl-bridged structures can also be described as comer-protonated cyclopropanes, since if the bridging C—C bonds are considered to be fully formed, there is an extra proton on the bridging carbon. In another possible type of structure, called edge-protonated cyclopropanes, the carbon-carbon bonds are depicted as fully formed, with the extra proton associated with one of the bent bonds. MO calculations, structural studies under stable-ion conditions, and product and mechanistic studies of reactions in solution have all been applied to understanding the nature of the intermediates involved in carbocation rearrangements. 

It is believed that this process involves migration through a pentacoordinate protonated cyclopropane in which an alkyl group acts as a bridge in an electron-deficient carbocation structure. The cyclohexyl- methylcyclopentyl rearrangement is postulated to occur by rearrangement between two such structures. 

A mercurinium ion has both similarities and differences as compared with the intermediates that have been described for other electrophilic additions. The proton that initiates acid-catalyzed addition processes is a hard acid and has no imshared electrons. It can form either a carbocation or a hydrogen-bridged cation. Either species is electron-deficient and highly reactive. 

Some years ago, we tackled (ref. 7) the particular question of bromine bridging, related mainly to stereochemistry, postulating that bromonium ions and bromo-carbocations are formed in separate pathways as shown in Scheme 3. The relative rates of reaction by these pathways depend on the olefin structure. As demonstrated later... 

When a substituent is able to resonantly stabilize the positive charge of the ionic intermediate, there is no bromine bridging and the intermediate is an open P-bromocarbocation. These carbocations have been shown to occur in the bromination of a-methylstilbenes (ref. 9), 1 and 2, and of a variety of enol ethers (ref. 10) and acetates (ref. 11). 

When the bromination of the unsubstituted P-methy 1-styrenes (ref. 19) is carried out in methylene chloride, the two diastereoisomeric dibromides are obtained in ratios of 72 threo/28 erythro and 20 threo/80 erythro for the cis and trans isomers, respectively. This result agrees fairly well with a partially bridged intermediate, since the corresponding benzylic carbocation leads to a 65 erythro/35 threo ratio (ref. 20). When the same reactions are carried out in methanol, the... 

Most of these results have been obtained in methanol but some of them can be extrapolated to other solvents, if the following solvent effects are considered. Bromine bridging has been shown to be hardly solvent-dependent.2 Therefore, the selectivities related to this feature of bromination intermediates do not significantly depend on the solvent. When the intermediates are carbocations, the stereoselectivity can vary (ref. 23) widely with the solvent (ref. 24), insofar as the conformational equilibrium of these cations is solvent-dependent. Nevertheless, this equilibration can be locked in a nucleophilic solvent when it nucleophilically assists the formation of the intermediate. Therefore, as exemplified in methylstyrene bromination, a carbocation can react 100 % stereoselectivity. 

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. ... 

A bridged carbocation with a two-electron, three-centre bond was proposed as early as 1939 (Nevell et al., 1939) for the 2-norbornyl cation [lO ] as a reactive intermediate in the solvolysis of 2-norbornyl system (see also Winstein and Trifan, 1949). It has now been isolated as the SbFe salt and the bridged structure is accounted for using solid-state nmr studies... 

Such species with a bridging phenyl group are known as phenonium ions. The neighbouring group effect is even more pronounced with an OH rather than an OMe substituent in the p-position. Solvolysis is found to occur % 106 times more rapidly under comparable conditions, and matters can be so arranged as to make possible the isolation of a bridged intermediate (5), albeit not now a carbocation ... 

This is clearly demonstrated in the pinacolinic deamination (cf. p. 114) of an optically active form of the amino-alcohol (50). Such reactions proceed from a conformation (antiperiplanar 50a or 50b) in which the migrating (Ph) and leaving (NH2 as N2 cf. p. 114) groups are TRANS to each other. Rearrangement via a bridged carbocation would necessarily lead to 100% inversion at the migration terminus in the product ketone (5lab), whichever initial conformation, (50a) or (50b), was involved ... 

A bridged intermediate exactly analogous to a bromonium ion cannot be formed as H has no electron pair available, but it may be that in some cases a n complex (21) is the intermediate. We shall, however, normally write the intermediate as a carbocation, and it is the relative stability of possible, alternative, carbocations (e.g. 23 and 24) that determines the overall orientation of addition, e.g. in the addition of HBr to propene (22) under polar conditions ... 

An unusual cationic domino transformation has been observed by Nicolaou and coworkers during their studies on the total synthesis of the natural product azadirachtin (1-105) [30]. Thus, exposure of the substrate 1-106 to sulfuric acid in CHjClj at 0°C led to the smooth production of diketone 1-109 in 80% yield (Scheme 1.27). The reaction is initiated by proto nation of the olefinic bond in 1-106, affording the tertiary carbocation 1-107, which undergoes a 1,5-hydride shift with concomitant disconnection of the oxygen bridge between the two domains of the molecule. Subsequent hydrolysis of the formed oxenium ion 1-108 yielded the diketone 1-109. 

Knolker and coworkers also used a domino [3+2] cycloaddition for the clever formation of a bridged tetracyclic compound 4-172, starting from a cyclopentanone 4-168 and containing two exocydic double bonds in the a-positions (Scheme 4.36) [57]. The reaction of 4-168 with an excess of allylsilane 4-169 in the presence of the Lewis acid TiCLj led to the spiro compound 4-170 in a syn fashion. It follows a Wag-ner-Meerwein rearrangement to give a tertiary carbocation 4-171, which acts as an electrophile in an electrophilic aromatic substitution process. The final step is the... 

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