Carbocations norbornyl

One of the major contemporary chemical controversies in which I was inadvertently involved developed in the 1950s, surprisingly over the structnre of a deceptively simple seven carbon-containing bicyclic carbocation, the 2-norbornyl (bicyclo[2.2.1]heptyl) cation. The in-... 

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. 

Clearly there was no lack of devoted adversaries (perhaps a more proper term than enemies) on both sides of the norbornyl ion controversy. It is to their credit that we today probably know more about the structure of carbocations, such as the norbornyl cation, than about most other chemical species. Their efforts also resulted not only in rigorous studies but also in the development or improvement of many techniques. Although many believe that too much effort was expended... 

The most studied hypercoordinate carbocation is the 2-norbornyl cation, around which the nonclassical ion controversy centered (Chapter 9). 

SECTION 5.12. THE NORBORNYL CATION AND OTHER NONCLASSICAL CARBOCATIONS... 

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... 

The behavior of members of the bicyclo[2.2.1]heptene family is also different from that of other common 1,2-disubstituted alkenes.230 The parent bicy-clo[2.2.1]heptene gives bicyclo[2.2.1]heptane in only 3.5% yield when it is treated with Et3SiH/TFA. The major product is reported to be a 2-bicyclo[2.2.1]heptyl trifluoroacetate of unspecified configuration (Eq. 70).230 The carbocation intermediate is presumably the 2-norbornyl cation. Addition of small amounts of boron trifluoride etherate to the reaction mixture causes the yield of hydrocarbon product to rise to 22% after a reaction time of 24 hours at room temperature. Further... 

The cyclobutyl/cyclopropylmethyl cation system (C4II7 ) has probably been the focus of more studies than any other carbocation system except the 2-norbornyl cation. Bridged cyclobutyl cations 16 are called bicyclobutonium ions. Bicyclobutonium... 

For historical overviews on the 2-norbornyl cation, the bicyclobutonium ion and related hypercoordinated carbocations see (a) Nonclassical Ions, Reprints and Commentary, P. D. Bartlett, W.A. Benjamin, Inc, New York and Amsterdam 1965 (b) The Nonclassical Ion Problem, Brown, H.C. with comments by Schleyer, P.v.R. Plenum Press, New York, 1977... 

Bicyclic and polycyclic carbocations, NMR spectroscopy, 145-150 1, 2-dimethyl-2-norbornyl cation, 149 bicyclobutonium ions, 145-146, 146/ 3- /<7o-trialkylsilylbicyclobutonium ions, 147-148... 

Similarly, trityl cation in aromatic hydrocarbons initiates the fragmentation of simple tetraalkyl plumbanes and stannanes yielding the plumbyl or stannyl cationic species, e.g. 11, and alkenes. The reaction is thought to proceed via plumbyl-or stannyl-substituted carbocations 12, which in a second step eliminate the al-kene. This approach was used in the synthesis of norbornyl cations of the elements tin and lead, e.g. 13, (Scheme 5). ... 

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... 

Further evidence for the SnI mechanism is that reactions run under SnI conditions fail or proceed very slowly at the bridgehead positions10 of [2.2.1] (norbornyl) systems25 (e.g. 1-chloroapocamphane, 8). If SnI reactions require carbocations and if carbocations must... 

Certain nucleophilic substitution reactions that normally involve carbocations can take place at norbornyl bridgeheads32 (though it is not certain that carbocations are actually involved in all cases) if the leaving group used is of the type that cannot function as a nucleophile (and thus come back) once it has gone, e.g.,... 

Carbenes (Continued) substituents on, 115 Carbocations, 105-108 acids and electrophiles, 97 dialkoxy, 105 ethyl, 84 fluoro, 105 norbornyl, 84 reactions, 107 substituents on, 106... 

The solvolysis of 252 is one of the rare examples for a norbornyl-norpinyl rearrangement. While the ew-trimethylsilyl brosylate 251 yields mainly substitution and elimination products 253-255 with an intact norbomyl framework, 252 gives nearly 86% of norpinene 256 (equation 39). The bis-(trimethylsilyl)substituted compound 257 gives almost exclusively the norpinene derivative 258 (equation 40). While the trimethylsi-lyl group(s) in 252 and 257 exert no kinetic effect on the reaction rate, the /3-effect on the intermediate carbocations 252A and 259, respectively, determines the product distribution99. 

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. 

Many more cyclic and polycyclic equilibrating carbocations have been reported. Some representative examples, namely, the bisadamantyl (499),859 2-norbornyl (500),40 7-perhydropentalenyl (501),188 9-decalyl (502),188 and pentacylopropylethyl (503)860 cations, are given in Scheme 3.19. All these systems again involve hypercoordinate high-lying intermediates or transition states. 

The methods that were developed in the early 1960s to generate and observe stable carbocations in low-nucleophilicity solutions18 were successfully applied to direct observation of the norbomyl cation (C7H114"). Preparation of the ion by the c route from 2-norbornyl halides, by the n route from 4-(2-haloethyl)-cyclopentenes, and by the protonation of nortricyclene ( bent o route ) all led to the same 2-norbomyl cation. 

If the classical structure were correct, the 2-norbornyl cation would be a usual secondary carbocation with no additional stabilization provided by c-delocalization (such as the cyclopentyl cation). The facts, however, seem to be to the contrary. Direct experimental evidence for the unusual stability of the secondary 2-norbomyl cation comes from the low-temperature solution calorimetric studies of Arnett and Petro.75 In a series of investigations, Arnett and Hofelich76 determined the heats of ionization (AHi) of secondary and tertiary chlorides in SbF5-SC>2ClF [Eq. (3.131)] and subsequently alcohols in HS03F-SbF5-SC>2ClF solutions [Eq. (3.132)]. 

In contrast, when R—X in Scheme 4.4 is enantiomerically enriched endo-2-norbornyl brosylate (2 in Fig. 4.5) under the same reaction conditions, solvolysis is still accompanied by racemisation, but the rate of racemisation is identical with that of product formation [14]. The achiral carbocation in this reaction, therefore, is captured by solvent much faster than it can undergo internal return, i.e. k2 i in Scheme 4.4. In this case, therefore, the initial ionisation is the rate-determining step. 

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