Norbomyl carbocations

Certain nucleophilic substitution reactions that normally involve carbocations can take place at norbomyl bridgeheads (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, for example. 

CH2), and norbomyl (e.g., 4) type systems are especially prone to carbocation rearrangement reactions. It has been shown that the rate of migration increases with the degree of electron deficiency at the migration terminus. ... 

Bicycloalkylidenes. a-Delocalization in bicyclic carbocations provides the opportunity to detect the protonation of bicycloalkylidenes. An obvious choice was the 2-norbomyl cation (66), for which symmetrical bridging (equivalence... 

The carbocations so far studied are called classical carbocations in which the positive charge is localized on one carbon atom or delocalized by resonance involving an unshared pair of electrons or a double or triple bond in the allylic positions (resonance in phenols or aniline). In a non-classical carbocation the positive charged is delocalized by double or triple bond that is not in the allylic position or by a single bond. These carbocations are cyclic, bridged ions and possess a three centre bond in which three atoms share two electrons. The examples are 7-norbomenyl cation, norbomyl cation and cyclopropylmethyl cation. 

The non-classical carbocations can be generated if proper substrate is chosen. For example the norbomyl carbocation can be generated by the departure of a leaving group from an exosubstitued substrate... 

The "classical" - "non-classical" carbocation controversy concerned the Wagner-Meerwein rearrangement of norbomyl systems ... 

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. 

The method should work better for adamantyl chlorides or norbomyl chloride for which Sn2, and, presumably, preassociation processes are precluded. Speculative values of pATr1 and pATR for the relevant carbocations 54-56 are listed below based on a solvolysis rate constant in water for the... 

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. 

Applying the additivity of chemical shift analysis55 to the 2-norbomyl cation also supports the bridged nature of the ion. A chemical shift difference of 168 ppm is observed between the ion (C7Hn+) and its parent hydrocarbon [i.e., norbornane (507)], whereas ordinary trivalent carbocations such as the cyclopentyl cation (33) reveal a chemical shift difference of =360 ppm.55... 

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)]. 

Rearrangements which take place in carbocation sulfinate ion pairs, C ArSO, may involve re-bonding to the same sulfinate oxygen or bonding to the other one, according to labelling studies.147 The processes studied included the norbomyl norbomyl and... 

G. M. Kramer, C. G. Scouten, The 2-Norbomyl Carbonium Ion Stabilizing Conditions An Assessment of Structural Probes", in Advances in Carbocation Chemistry (X. Creary, Ed.) 1989, 1, JAI Press,... 

MP2-FC/6-31G calculations reveal the symmetrically bridged 6-sila-2-norbomyl cation 1 not only to be a local minimum (Fig. 18), but also to be 17.2 kcal mof more stable than the 2-norbomyl cation (Eq. 3) at MP2-FC/6-31G + AZPE(SCF/6-31G 0.89). However, the inherently greater stability of silyl cations contributes to this difference. The Si NMR chemical shift of the bridging silicon atom, ca 1 ppm vs TMS (IGL0/H //MP2-FC/6-31G ), is very strongly shielded in comparison with ca 300 ppm expected for a free RSiH2 species [38]. Thus, the sila congener of the 2-norbomyl carbocation also possesses a nonclassical stracture which is reflected by its stracture as well as its NMR properties. 

However, recent studies on long-lived carbocations in super acid solutions are beginning to give additional data. Thus Olah 6) has obtained evidence, by the use of a number of spectroscopic techniques, that the norbomyl cation is best represented as a corner protonated cyclopropane 31). 

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