7-Norbornenyl cation

The consideration of HO—LU interaction is useful also in the interpretation of the stability of "nonclassical carbonium ions. For instance, the 7-norbornenyl cation would be stabilized by the symmetry-allowed... 

A5 C(C1) = —29.8, (A5 C(C1) = —54.5) are observed and a relatively large /(C2H) coupling constant of 165.9 Hz is detected. This counter-intuitive low-frequency shift of the C NMR resonance of Cl and C2 as well as the large scalar CH coupling constant was rationalized for similar bishomoaromatic carbon cations like the 7-norbornenyl cation, 79, by the hypercoordinated nature of the vinylic C atoms and was put forward as spectroscopic evidence for bishomoaromaticity. " ... 

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... 

As shown below, the 2,3-dimethyl-7-phenyl-7-norbornenyl cation (29) shows signihcant distortion of the bridging C7 toward the C2—C3 double bond." The C7-C3 distance is only 1.87 A, and the C2—C3 bond has elongated to 1.41 A. This structure is in agreement with the nonclassical (bis-homoaromatic) character of the non-methylated analogue 20. 

Compound 2 is also formed by photolysis of the tosylhydrazone sodium salt of bicyclo[2.2.1]hept-2-ene-7-one. In methanol with sodium methoxide, the initially formed norbornenylidene accepts a proton to give the norbornenyl cation and subsequently gives 2 and 3 in a 16 84 ratio.9... 

Structural information on these systems was recently provided by two groups. Laube reported the X-ray structure determination of the norbornenyl cation 86202 while Schleyer and colleagues published the results of high-level calculations on 84 and 85203. 

TABLE 8. Structures of 7-norbornenyl cations and related compounds ... 

Mechanism a involves a corner-protonated cyclopropane (28) we have already seen examples of such ions in the 2-norbornyl and 7-norbornenyl cations (pp. 453, 460). Mechanism b involves an edge-protonated cyclopropane (29). Mechanism c... 

Long-lived ions derived from the substrates 89 and 90 have also been studiedand a topic of particular interest has been the 7-norbornenyl cation (98a) which, as shown in 98b, is a cyclopropylcarbinyl system in which 7r-donation from one of the ring C-C bonds is effective. These ions may be derived from cyclopropylcarbinyl precursors (93) or, more usually, from 7-norbornenyl substrates and have been exhaustively studied, as summarized in some recent reports ". ... 

The carbonium centers in the norbomyl cation, 7-norbornenyl cation and 7-norbomadienyl cation10,207 are similar and related to the ethenemethonium ion,... 

An example of a bishomocyclopropenyl system is the 7-norbornenyl cation, 71 (see Table 15) 212-214) ( whose chemical shifts and coupling constants are not entirely incompatible with the formulation in which C(7) interacts equally with C(2) and C(3). However, the high chemical shift of the proton on the bridging carbon atom (H(7), t 6.76), although explained in terms of the tendency of the bridging carbon atom to rehybridize from sp3 to sp3 212,213 suggests that ring current effects are not operative in this system. 

The symmetrical ion structure was supported by the PMR data of the 2-methyl-7-norbornenyl cation 426... 

Other Systems.— The addition of formic, acetic, and chloroacetic acids to quadri-cyclane at 20 °C affords the corresponding esters of exo-2-norbornenol and nortri-cyclenol. Solvent effects and the results obtained using the 0-deuteriated carboxylic acids are interpreted in terms of the six-centre addition mechanism (97) the minor pathway to nortricyclyl products, in the absence of added mineral acid, is the result of leakage from (97) to the norbornenyl cation. ... 

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