Bridged Nonclassical Carbocations

Both acetolyses were considered to proceed by way of a rate-determining formation of a carbocation. The rate of ionization of the cnrfo-brosylate was considered normal, since its reactivity was comparable to that of cyclohexyl brosylate. Winstein 

This intermediate serves to explain the formation of racemic product, since it is achiral. The cation has a plane of symmetry passing through C(4), C(5), C(6), and the midpoint of the C(l)—C(2) bond. The plane of symmetry is seen more easily in an alternative, but equivalent, representation. Carbon 6, which bears two hydrogens, serves as the bridging atom in the cation. 

Attack by acetate at C(l) or C(2) is equally likely and results in formation of equal amounts of the enantiomeric exo-acetates. The product is exo because reaction with acetate occurs from the direction opposite the bridging interaction. The bridged ion can be formed directly only from the exo-brosylate because it has the proper anti relationship between the C(l)-C(6) bond and the leaving group. The bridged ion can be formed from the enrfo-brosylate only after an unassisted ionization, which explains the rate difference between the exo and endo isomers. 

The description of the nonclassical norbornyl cation developed by Winstein implied that the bridged ion is stabilized relative to a secondary ion by C-C ct bond delocalization. H. C. Brown put forward an alternative interpretation,arguing that all the available data were consistent with describing the intermediate as a rapidly equilibrating classical secondary ion. The 1,2-shift that interconverts the two ions was presumed to be rapid, relative to capture of the nucleophile. Such a rapid rearrangement would account for the isolation of racemic product, and Brown suggested that the rapid migration would lead to preferential approach of the nucleophile from the exo direction. 

The Transition State, Chem. Soc. Spec. Publ, (1966) Tetrahedron, 32, 179 (1976). 

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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... 

The C—H—C bond is not linear, the angle being about 170° according to high-level MO calculations. Several bridged cycloalkyl carbocations of the type 2 have been prepared [236]. Complexes between a number of alkyl cations and alkanes have been detected in mass spectrometric experiments [235]. The nonclassical structure of the ethyl cation, 3, may be cited as another example of hydride bridging (for a discussion, see ref. 55). 

Superelectrophilic onium dications have been the subject of extensive studies and their chemistry is discussed in chapters 4-7. Other multiply charged carbocationic species are shown in Table 2. These include Hogeveen s bridging, nonclassical dication (14)26 the pagodane dication (15)27 Schleyer s l,3-dehydro-5,7-adamantane dication (16)28 the bis(fluroenyl) dication (18)29 dications (17 and 19) 19a trications (20-21)19a,3° and tetracations (22-23).31 Despite the highly electrophilic character of these carbocations, they have been characterized as persistent ions in superacids. 

The presence of a transition metal is not necessarily required for hydrocarbon insertion. Alkyne incorporation has been reported for boracyclobutenes, as well as metallacyclobutene complexes of the transition elements. Boracyclobutene 51, a reactive intermediate prepared in situ (Section 2.12.9.2.1), inserts an additional equivalent of trimethylsilylacetylene into the B-C(sp2) bond to yield boracyclohexadiene 52 (Scheme 7). This isomerizes to the interesting bridged compound 53, an analogue of a nonclassical carbocation <1994AGE2306>. The related boracyclobutene 7 inserts the alkyne to yield a persistent boracyclohexadiene 54, but this product clearly arises from insertion into the boracyclobutene carbon-carbon bond rather than a boron-carbon bond <1994AGE1487>. 

Schleyer et al. (1980) use another type of criterion for discrimination between classical and nonclassical carbocations. C-nmr chemical shift sums of carbocations and their respective hydrocarbon precursors are compared. A classical carbocation has a C-nmr chemical shift sum of at least 350 ppm higher than the sum for the corresponding hydrocarbon. This difference can be attributed partly ca 124 ppm) to the hybridization change, an increase of one sp -centre, and partly to the deshielding influence of the positive charge. Since nonclassical ions prefer bridged pentaco-ordinated structures to sp -hybridization their chemical shift differences, relative the hydrocarbons, are much smaller, often less than 200 ppm. 

With this tool [201] is found to show entirely classical behaviour with a i C-chemical shift sum of 977 ppm, 372 ppm more than for the corresponding hydrocarbon, 1,2,3,5,7-pentamethyladamantane. In the secondary system the corresponding shift difference is only 252 ppm, well within the range of nonclassical carbocations, supporting the cr-bridged structure [202]. 

For many years, a lively controversy centered over the actual existence of nonclassical carbocalions. " The focus of argument was whether nonclassical cations, such as the norbornyl cation, are bona fide delocalized bridged intermediates or merely transition states of rapidly equilibrating carbenium ions. Considerable experimental and theoretical effort has been directed toward resolving this problem. Finally, unequivocal experimental evidence, notably from solution and solid-state C NMR spectroscopy and electron spectroscopy for chemical analysis (ESCA), and even X-ray crystallography, has been obtained supporting the nonclassical carbocation structures that are now recognized as hypercoordinate ions. In the context of hypercarbon compounds, these ions will be reviewed. 

As should be the case with the establishment of any new principle, the concept of cr-bridged nonclassical intermediates was subjected to a searching analysis. An alternative explanation, based on classical carbocations, was put forth by H. C. Brown of Purdue University. Brown pointed out that the evidence in support of the nonclassical formulation for norbornyl cation consisted of ... 

The mechanism of this reaction and the structure of the intermediate carbocation were once the subject of intense controversy. When the double bond of norbornene is protonated, a carbocation is produced (Eq. 10.25). The question of whether this cation is 54a and is in rapid equilibrium with the isomeric cation 54b, or whether 54a and 54b are simply two contributing resonance structures to the resonance hybrid 55, has been the focus of the debate. Note that the contributing structures 54a and 54b dijfer in the location of an cr-bond, not in the location of Tr-bonds as is more commonly encountered in resonance structures. The delocalized structure 55 is thus referred to as a nonclassical carbocation, and most chemists now accept this formulation as the more likely representation of this intermediate. It is evident from empirical results that the sterically more accessible side of the carbocation is the one away from or exo to the bridging carbon atom as shown. Nucleophilic attack of water solely from this side leads to cxo-norborneol (53) rather than endo-norborneol (56). 

Using methods such as those discussed for the norbornyl cation, nonclassical structures have now been established for a number of carbocations. " Representative examples are shown below. The 7-phenyl-7-norbornenyl cation 19 exists as a bridged strucmre 20, in which the formally empty p orbital at C7 overlaps with the C2—C3 double bond. This example is of a homoallylic cation. The cyclopropyl-carbinyl cation 21, historically one of the first systems where nonclassical ions were proposed, has been shown to exist in superacids mainly as the nonclassical bicyclo-butonium ion 22, although it appears as if there is a small amount of the classical 21 present in a rapid equilibrium. Cations 23 and 24 are examples of p-hydridobridged... 

Cram s original studies287 established, based on kinetic and stereochemical evidence, the bridged ion nature of (3-phenylethyl cations in solvolytic systems. Spectroscopic studies (particularly1H and 13C NMR)288-291 of a series of stable long-lived ions proved the symmetrically bridged structure and at the same time showed that these ions do not contain a pentacoordinate carbocation center (thus are not nonclassical ions ). They are spiro[2.5]octadienyl cations 111 (spirocyclopropylbenzenium ions)—in... 

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