Nonclassical bridged species

The nature of the intermediate cationic species, whether it is a classical methyl cation or a nonclassical bridged cation, has also been determined. The deamination of optically active e c/o-2-aminobicyclo[2.2.1]hcplane-exo-2-carbonitrile gave 2-hydroxybicyclo[3.1. l]-heptane-1-carbonitrile (19) in 9% yield with 92% retention of the optical activity. Similarly, deamination of e.vo-2-trifluoromethylbicyclo[2.2.1]heptyl-e Jo-2-amine gave 1-trifluoromethyl-bicyclo[3.1.1]heptan-2-ol (20) in 11 % yield with 88% retention of the optical activity. These results are interpreted in terms of intermediate bridged cations.87... 

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. 

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. 

Participation by remote double bonds has often been adressed as the tt route to a-bridged ( nonclassical ) cations. Detailed examination of the intermediates is deferred to Section 7.6 where the generation of such species from other sources is discussed. 

Although the structure of the cyclopropylcarbinyl cation is not controversial, there is some disagreement whether this species should be considered to be classical or nonclassical 399. a-Bridging is clearly not involved. Yet the classical representation 388) is unsatisfactory in view of the abundant evidence for conjugation. The... 

To summarize the situation at present, the nonclassical norbomyl cation seems to be the species observed in superacid media, but the reservation has been expressed that the species observed may not be the same as the intermediate in solvolytic reactions, in which much of the electrophilicity of the cationic center may be attenuated by solvation. The cationic intermediate generated in solvolysis reactions may also be the nonclassical ion, but the observable properties of rapid solvolysis, high exo/endo capture ratio, and high exo/endo rate ratio are not sufficient to require this conclusion. Similar properties can be shown to obtain in model systems that surely react through intermediates in which bridging is absent. 

The distribution of deuterium in the product was examined by NMR methods. The fact that unequal amounts of 1 and 2 are found can rule out a symmetrical nonclassical norbornyl ion as the only cationic species in the reaction. The excess of 1 over 2 indicates that some syn addition occurs by ion pair collapse before the bridged... 

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