Substituted Norbomyl Cations

One can assume that at the start of the cation centre development at C ionization precedes a-participation at this moment the developing p-orbital at C is well oriented for interaction with the C -6-exo-H orbital but not with the C C bond. 

As the cation centre develops the C C bonding formed with delocalization of the C —C bond because the barrier to the Wagner-Meerwein rearrangement in the 

The PMR spectra of 2-methyl- 130 and 2-phenylnorbornyl ions 131 at —60 °C are those of static species where there are no 6,2,1-shifts. In the temperature range —20 to -t-20 °C such hydride shifts for ion 130, judging by the PMR spectrum, do occur the energy barrier is about 15 kcal/mole, by 9 kcal/mole higher than for the 2-norbomyl ion. In ion 131, even at -t-35 °C, there are no 6,2,1-hydride shifts. In the NMR spectrum of ion 130 the signal of the carbon is located at —76 ppm i.e. it is shifted by 66 ppm to the high field relative to the signal of C of the model ion 132. 

The close similarity of chemical shifts of in ion 131 and of in 1-phenyl-cyclopentyl ion 133 suggests that the a-delocalization is very smaU or absent in ion 131. Thus, the latter under stable ion conditions is a classical ion. This is also 

In 1977 Olah studied the characteristics of the NMR spectra of the same series of ions. Fig. 7 presents a graph of 5C depending on 8C the observed break in the slope is accounted for by an increase in the contribution of the C —C bond a-participation to the charge delocalization with increasing electron demand at C. Of similar character is the plot of 80 vs The character of the latter dependence does not change qualitatively when use is made ofa constants reflecting an increased demand of stable ions in comparison with solvolysis transition states. 

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Fig. 5.12. Crystal structures of substituted norbomyl cations. (A) 1,2,4,7-Tetramethylnorbomyl cation (reproduced from Ref. 154 by permission of Wiley-VCH). (B) 2-Methoxy-l,7,7-trimethyl-norbornyl cation (reproduced from Ref 155 by permission of the American Chemical Society).

By observation of symmetrical 1,2-bis-substituted norbomyl cations, Nickon and Lin have shown the l,2-dimethoxy-2-norbornyl cation to be classical and studied its degenerate rearrangement. Later Olah using the NMR and H spectra, as well as Raman laser spectra studied the 1,2-dimethylnorbomyl ion 134 and showed that in the latter, just as in the 2-methylnorbomyl ion, o-delocalization of C -6-exo-H-C occurs. But while in the 2-methylnorbomyl ion the barrier to the Wagner-Meerwein is rather high (the tertiary-secondary ion transition), in ion 134... 

The norbomyl cation and substituted nortricyclenes were shown to have five relatively weak bands in the C—H stretching region ( 3000 cm ). Only three were present in norbomane and its derivatives, and these were generally at lower frequency than those in the ion. 

More recently, Auner s and Schleyer s groups in a joint effort reported the experimental observation of an alkyl-substituted 6-sila-2-norbomyl cation110. The 6,6-Dimethyl-5-neopentyl-6-sila-2-norbomyl cation 41 was prepared by a hydride transfer reaction via the jr-route 111 from a suitable cyclopentenyl silane derivative at room temperature (equation 10). The toluene solution of the tetrakis(pentafluorotetraborate) salt of 41... 

The X-ray structure of a number of alkoxycarbenium ions has been determined.66 An interesting example is 2-methoxy-l,7,7,-trimethylbicyclo[2.2.1]hept-2-ylium tetrafluroborate 326.630 It is a substituted 2-norbomyl cation and, indeed, the C(2)-C(l)-C(6) bond angle (98.8°) and the C(l)-C(6) bond distance (1.603 A) indicate G-bond charge delocalization, that is, the contribution of the 326b resonance form. 

Figure 5.50 shows three related molecules, the 7-methyl substituted (the visual orbital progression explained here is not quite as smooth for the unsubstituted molecules) derivatives of the 7-norbomyl cation (a), the neutral alkene norbomene (b), and the 7-norbomenyl cation (c). For each species an orbital is shown as a 3D region of space, rather than mapping it onto a surface as was done in Fig. 5.49. In (a) we see the LUMO, which is as expected essentially an empty p atomic orbital on C7, and in (b) the HOMO, which is, as expected, largely the n molecular orbital of the double bond. The interesting conclusion from (c) is that in this ion the HOMO of the double bond has donated electron density into the vacant orbital on C7 forming a three-center, two-electron bond. Two n electrons may be cyclically delocalized, making the cation a bishomo (meaning expansion by two carbons) analogue of the aromatic cyclopropenyl cation [326], This delocalized bishomocyclopropenyl structure for 7-norbomenyl cations has been controversial, but is supported by NMR studies [327].

The electronic nature of the 3c-2e bonding of the 2-norbomyl cation and its 2,2-difluoro-substituted derivative (144) has been studied by Esteves and coworkers (B3LYP/6-311-I-I-G level). " The delocalization index (DI) and the C(l)-C(6) and C(l)-C(2) bond lengths (DI = 0.5 and 1.895 A, respectively, for each bond) clearly show the delocalized 3c-2e bonding. Data for the... 

The structure of the 2-norbomyl cation 5 is indirectly proved by the solvolytic cyclization of 2-(3-cyclopentenyl)ethanol. Thus the ar tolysis rate of nosylate 36 exceeds that of a saturated analogue by a factor of 95 This fact has been interpreted in favour of direct participation of a double bond in the substitution of an arylsulphonate anion. As shown by Sargent and Bartlett the substitution of... 

In 1981 Farcasiu applied the molecular mechanics technique to calculate the relative stability of secondary methyl-2-norbomyl cations and showed the most stable isomers to be those with methyl in the bridge-head (cf. Therefore he, as well as Schleyer substituted Arnett s models by others deprived of secondary effects ... 

Olah has discussed the extent of norbornyl cations and studied the l,2-diphenyl-2-norbomyl cation (102) by H and n.m.r. He concludes that it behaves as a rapidly equilibrating carbenium ion undergoing fast alkyl shift at -78°C. The cation (103) has also been prepared from several precursors in acidic media at —120 °C. Its spectra indicate rapid equilibration between carbenium forms. At — 60°C it rearranges to (104). The 8,9-dehydro-2-adamantyl cations (105 R = H or Me) have n.m.r. spectra which show that the tertiary cation is a static classical ion, whereas in the secondary case there is rapid equilibration making carbons 1, 8, and 9 equivalent. The analogous 2-substituted 2-adamantyl cations with R = Me, Et, Ph, OH, or halogen are simple ions which do not isomerize under the strong acid conditions used. " ... 

Norbomyl Cations.— The kinetics of solvolysis of 1-substituted 2-exo-norbornyl bromides (121) in 20% ethanol are reported. The rate constants, relative to that when R = H, are R = COjMe, 6.2 x 10 R = CN, 2.1 x lO"" and R = CH2OH, 2.0 X 10 These can be correlated with Taft substituent constants with correlation coefficient 0.999 and p = — 3.62. It is suggested that there is no 1—6 bond participation and that an unbridged cation is formed. Extrapolation from these three data predicts (if the mechanism were unaltered) a rate for the case of R = H 100 times lower than that found. It is concluded that this factor of 100 is due to participation and that all existing evidence supports a bridged structure for the intermediate in the solvolysis of unsubstituted 2-exo-norbornyl derivatives as well as for the long-lived ion obtainable in superacid media. ... 

The arguments discussed to this point, both for and against the nonclassical structure, rest on indirect evidence derived from interpretation of the kinetic and stereochemical characteristics of the substitution reactions. When techniques for direct observation of carbocations became available, the norbomyl cation was subjected to intense study from this perspective. 

Since reactivity of alkenes increases with increasing alkyl substitution, hydration is best applied in the synthesis of tertiary alcohols. Of the isomeric alkenes, cis compounds are usually more reactive than the corresponding trans isomers but strained cyclic isomeric olefins may exhibit opposite behavior. Thus, for example, raras-cyclooctene is hydrated 2500 times faster than cis-cyclooctene (98). Similar unusually high protonation rates were also measured for strained bridgehead alkenes. This may be attributed to the high groundstate strain (in the case of bicyclo[3.3.1]non-l-ene) or the formation of the nonclassical norbomyl cation when norbornene is protonated (95). 

As depicted in the following scheme, a homoallyl alcohol derived from a norbomyl a-diketone underwent a lead(IV) acetate reaction in MeOH, resulting in the formation of a novel methoxy substituted spirocyclic tetrahydrofuran <07CC4239>. It is believed that the addition of the methoxylead(IV) acetate species across the alkene from the less sterically hindered side to form a plumbonium cation leads to the major product after subsequent cyclization and reductive elimination. Moreover, construction of tetrahydrofurans by a Pd(II)/Pd(IV)-catalyzed aminooxygenation of homoallyl alcohols was also reported <07AGE5737>. 

Can the data on the structure of a stable 2-norbomyl ion be applied to the intermediates in the solvolysis of 2-substituted norbornanes Brown says no Indeed, in superacids the solvation of the cation, though feeble, is unspecific. Nucleophilic media exhibit specific interactions with the counter-ion and one or more solvent molecules, which may alter the structure, energies and properties of intermediates. Nevertheless, if we compare the same substrates in solvolytic media and in superacids, we obtain related information provided we really compare carbocations in different media, and not processes of S 2 type and stable ions. 

Another system, which has attracted much interest, is that of the cyclopropyl-methyl cation, for similar reasons to the 2-norbomyl systems namely, the great ease of solvolysis of cyclopropyhnethyl substrates accompanied by formation of rearrangement products. Of particular interest is that regardless of the method of generation of the cationic species the ratio of products is always very similar. Both hydrolysis of cyclopropylmethyl chloride under conditions favoring 1 substitution and diazotization of cyclopropylamine produce cyclobutanol and but-3-en-l-ol as well as cyclopropyhnethanol in approximately 47%, 5%, and 48% yields respectively. Moreover, diazotization of cyclobutylamine also produces the same three alcohols in the same ratios (Scheme 2.38). 

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