Norbornyl cation formation

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... 

During my studies 1 realized that the formation of the cr-delocalized 2-norbornyl cation from 2-norbornyl precursors represented the equiv-... 

The formation of the (X-delocalized norbornyl cation via ionization of 2-norbornyl precusors in low-nucleophilicity, superacidic media, as mentioned, can be considered an analog of an intramolecnlar Friedel-Crafts alkylation in a saturated system. Indeed, deprotonation gives nortricyclane,... 

The 7-Norbornyl Cation. 7-Norbomyl derivatives were found to be extremely unreactive in solvolysis studies and product formation was shown to occur with predominant retention of configuration.917 920 These observations led to the suggestion by Winstein et al.917 that the cationic intermediate is a nonclassical ion. Attempts to isolate the 7-norbomyl cation under stable ion conditions in superacid... 

Unequivocal evidence for the existence of the 7-norbornyl cation, however, was subsequently provided by Sunko and coworkers by using the cryogenic matrix isolation technique. They codeposited 7-chloronorbornane 146 and a large excess of SbFj at -263°C then slowly warmed it to -173°C. The IR spectrum was in very good agreement with the calculated frequencies [MP4(sdq,fc)/6-31G //MP2(full)6-31G + ZPE level] for the 7-norbornyl cation 80 (Scheme 5.11). Further warming gave the 2-norbornyl cation 126 at -73°C. Tlie possibility of the formation of a complex of 146 with Sbf. was ruled out when similar experiments with both 146 and 147 resulted in identical spectra. Recent ab initio calculations support the nonclassical bent structure of the cation. 

In this case, the participation leads to the formation of the norbornyl cation, which is captured as the acetate. More is said about this important cation in Section 4.4.5. 

The IR spectrum published by Koch et al. (1990) was recorded using Vancik and Sunko s technique (1989) in which 2-chloronorbornane and SbF are deposited simultaneously from the gas phase on a Csl window at 77 K. During a slow temperature increase to 150 K, formation of the carbocation could be observed by the change in the IR spectrum. Koch et al. (1990) compared the frequencies and intensities of 21 absorption bands observed in the range 878-3110 cm with results that were calculated on the basis of a structure of the nonclassical 2-norbornyl cation obtained by Koch et al. (1989) using the 6-31G basis set and dynamic electron correlation on the MP2 level. The calculated IR bands correlated very well with the experimental frequencies and intensities. The calculated structure of this cation was. 

Brown notes that the formation in solvolysis of tight ion pairs rather than of free carbocations turns the rate of reactions of ions with nucleophiles to be lower than that of reaching an equilibrium between epimers. Therefore in order to trap the classical 2-norbornyl cation it is necessary not to use solvolysis, but other reactions in which more free carbocations are formed. The electrophilic addition of acids to norbomene belong to these reactions. Brown considers results obtained to be due to the capture of the classical 2-norbomyl cation before a complete equilibrium is established ... 

Reviews this year include the collation of evidence in favour of the o-bridged 2-norbornyl cation,the use of aliphatic diazonium ions as means for the production of carbocations, terpene rearrangements in superacid media,and the formation of carbocations by the silver-assisted reactions of chloroformates. A convenient procedure, using syringe techniques, has been described for the preparation of ca. IM solutions of alkyl cations in SbF5-S02ClF from suitable precursors that are soluble in SO2CIF at —78 Since conversions are essentially quantitative, the... 

Here the dotted line shows the postulated solvolysis course in the absence of synartesis, involving first the formation of the secondary 1,7,7-trimethyl-2-norbornyl cation, which rearranges after the rate-determining transition... 

Nevertheless, it is not surprising to find that rearrangements of this kind proceed very easily in solution and that in cases where the balance between the classical ion and the n complex is disturbed, the latter may exist as a stable entity even in solution. The classic example of this is the norbornyl cation (51), which rearranges on formation to the n complex (52). The con-... 

Since they are difficult to form, bridgehead cations are highly reactive. Irradiation of hahdes 8 in a variety of media provided a convenient means of exploring the remarkable reactivity of the 1-norbornyl cation (18). It was efficiently trapped by diethyl ether and THE to afford the ethers 19 and 20. Even in aqueous THE, a substantial portion of the butenyl ether 20 was obtained. Trapping by t-butyl alcohol afforded some of the ditertiary ether 21 but was accompanied by formation of 1-norbornanol (22) and 2-methylpropene. Cation 18 was trapped by CHjClj to afford chloride 23, and in aqueous CH3CN the Ritter product 24 was formed. It should be noted that radical abstraction from CH2CI2 occurs at H and ionic abstraction at CL... 

Another cation that, hke bridgehead cations, is difficult to form by conventional methods is the 7-norbornyl cation. The problem in this case is angle strain. Thus, extended treatment of 7-iodonorbornane (52) with methanohc AgN03 afforded only a trace of ether 53 (R = CH,). By contrast, irradiation in CH3OH readily afforded ether 53 (R = CHj), along with smaher amounts of the reduction product norbornane (9), the unprecedented 1,3-ehmination product 54 and the rearrangement products 55-57. The 2-chloro derivatives 58 underwent syn,anti interconversion in competition with formation of the reduction product 60, the ethers 61 and 63, and the 1,3-elimination product 62. By contrast, treatment... 

See Hrovat, D.A. and Borden, W.T., Afo initio calculations find that formation of cubyl cation requires less energy than formation of 1-norbornyl cation, /. Am. Chem. Soc., 112, yin, 1990. 

Compounds (5J)—(55)" 102) represent some more substrates which afford rearranged ion pair return products in an apparently stereospecific manner, with inversion at the origin of the 1,2 alkyl shift. Formation of endo-2-norbornyl p-nitroben-zoate from (87) is noteworthy because ion pair return occurs contrary to the natural" exo preference of the norbornyl cation101 The stereochemistry of return may be explained in terms of bridged carbocations. Alternatively the anion may rebond to the same side of the carbon framework from which it departed. Although return to the opposite side was clearly established with allyl cations (Section 4.2.1), reorien-... 

Subsequently the same authors established that the solvolysis of optically active 6,7-dimethoxy-l,2-dimethyl-2-exo-norbornyl chloride or -p-nitrobenzoate is accompanied by anchimeric acceleration as distinct from the unsul tituted analogues 275 and 276 the resulting products are optically active Thus, anchimeric assistance leads to the formation of an unsymmetrical cation which may be the homobenzylic ion 278 while the partial loss of activity is due to the interconversion of enantiomeric ions. The authors have also shown that the accelerating effect of the homopara-methoxyl group is practically the same for the solvolysis in methanol, 50% acetone... 

The question of the high exolendo rate ratio was examined with exo- and endo-2-phenyl-2-norbornanol. The tertiary and benzylic 2-phenylnorbornyl cation is classical, as evidenced by its NMR spectrum. Since the relative rates of hydrolysis of exo- and endo-2-phenylnorbornyl p-nitrobenzoate in 60% aqueous dioxane at 50°C are 140, and the reactions proceed by rate-determining formation of classical carbonium ions. Brown concluded that the exo-endo rate ratio of 340 for norbornyl brosylates does not require assisted ionization of exo-norbornyl brosylate. ... 

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