Norbornyl 7-bridged

Even more rigid donor-acceptor systems were constructed using a norbornyl bridge (systems 7 and 8, Scheme I). For 7, the fluorescence excitation spectrum... 

Figure 8. Norbornyl-bridged donor-acceptor complexes (a) all-trans linkages (b) with one cis linkage in the bridge (145).

Fully extended structures of 8 were induced by complexation of the compounds with methylated P-CD. The fluorescence intensities and lifetimes of the extended conformations were determined. From the lifetimes and those of model compounds of the isolated aromatic groups, the ket values of the extended conformations were calculated. The k t decreases exponentially with n and the p value was 0.89 A (1.09 per methylene unit), regardless of the nature of aromatic moiety. The p value is similar to that observed with the rigid norbornyl bridges [40]. Comparison of the effects of Me-j -CD on the steady-state fluorescence intensity and on the excited-state lifetime suggested that the... 

Winstein, one of the most brilliant chemists of his time, concluded that it is attractive to account for these results by way of the bridged (non-classical) formulation for the norbornyl cation involving accelerated rate of formation from the exo precursor [by anchimeric assistance His formulation of the norbornyl cation as a cr-bridged species stimulated other workers in the solvolysis field to interpret results in a variety of systems in similar terms of rr-delocalized, bridged carbonium... 

In 1965 he stated, On the other hand, the norbornyl cation does not possess sufficient electrons to provide a pair for all of the bonds required by the proposed bridged structures. One must propose a new bonding concept, not yet established in carbon structures (emphasis added). 

The Brown-Winstein nonclassical ion controversy can be summed up as differing explanations of the same experimental facts (which were obtained repeatedly and have not been questioned) of the observed significantly higher rate of the hydrolysis of the 1-exo over the 2-endo-norbornyl esters. As suggested by Winstein, the reason for this is participation of the Ci-Q single bond leading to delocalization in the bridged nonclassical ion. In contrast. Brown maintained that the... 

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. 

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 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... 

The structure of the 2-norbornyl cation has been a focal point of controversy in physical organic chemistry. Experimental NMR spectroscopy and computational methods have been the decisive tools, favoring the hypercoordinated symmetric bridged structure 30, a protonated nortricyclane.79 The tricoordinated 2-norbornyl cation 31 is not a local minimum (MP2/6-31G(d)) on the energy surface.80... 

Methyl-2-norbornyl (66) and 1,2-dimethyl-2-norbornyl (67) cations have partially bridged structures. 

Similarly to the 2-norbornyl cation, comparison of calculated (IGLO/DZ//MP2/ 6-31G(d)) and experimental 13C NMR chemical shifts allowed to differentiate between the hypercoordinated 70 and the trivalent form 71 of the bicyclo[2.1.1.]hexyl cation.85 The experimental (157.8 ppm) and calculated (158.5 ppm) values for Cl and C2 (averaged signal) are reported to be nearly identical for the symmetrically bridged... 

This interpretation has not been universally accepted, one reason being that the product ratios do not decline in like manner to the rate ratios. Another factor is that, even if the rate ratios are taken to indicate the existence of some participation of the electrons in the bond between C-1 and C-6 in the solvolysis of exo-2-norbornyl tosyl-ate, this is far removed from proving the existence of the symmetrical and hence fully delocalized ion at the transition state or as an intermediate in the reaction. Hence, if one concludes that the exo- and cndo-derivatives of the restricted systems are solvolysing to essentially unbridged or classical ions, the extent of bridging to be deduced from high exojendo rate ratios found with less restricted systems is unclear. 

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... 

When 2-norbornyl fluoride is dissolved in superacid solution, a carbo-cation is obtained as shown in Equation 6.42. This ion has been examined by a number of physical methods, and the data are consistent with its structure being the bridged ion, 82.112 For example, the ion has been examined by ESCA... 

The nmr and Raman spectra of the parent norbornyl cation in superacid is also most consistent with the bridged structure 82.115 The nmr spectra of a num-... 

Since evo-2-norbornyl arenesulfonates apparently do solvolyze with anchi-meric assistance, the less strained 2-bicyclo[2.2.2]octyl derivatives (92) are of interest. In this system no comparison of the. rates of exo and endo derivatives can be made, since a substituent exo to one bridge is endo to t fief other. Stereochemical studies are, however, possible. If the carbonium ion (93) were formed from chiral 92 by participation of the —C6 bond, it would be chiral. Scheme 5 shows that attack at C2 of 93 should give retained bicyclo[2.2.2.]octyl solvolysis products (92a), and attack at Cx should give chiral exo-2-bicyclo[3.2.1]octyl... 

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