Racemization 2-norbornyl cation

Fig. 7.8 Pictorial representation of the celebrated 2-norbornyl solvolysis problem where both enantiomer-ically pure 2-enc/o and 2-exo substrates lead through a common symmetric intermediate to the more stable racemic 2-exo substitution product. Much mechanistic emphasis was placed on the central 2-norbornyl cation (cf. Fig. 7.11).

The description of the nonclassical norbornyl cation developed by Winstein implied that the bridged ion is stabilized relative to a secondary ion by C-C ct bond delocalization. H. C. Brown put forward an alternative interpretation,arguing that all the available data were consistent with describing the intermediate as a rapidly equilibrating classical secondary ion. The 1,2-shift that interconverts the two ions was presumed to be rapid, relative to capture of the nucleophile. Such a rapid rearrangement would account for the isolation of racemic product, and Brown suggested that the rapid migration would lead to preferential approach of the nucleophile from the exo direction. 

Additionally, the solvolysis behavior of the norbornyl system is itself subject to substituent effects. The 1,2-dimethylnorbornyl cation is conceptually similar to the norbornyl cation and should afford racemic products on attack by nucleophiles. It has been found, however, that hydrolysis of l,2-dimethyl-exo-2-norbornyl p-... 

For example, acetolysis of exo-2-norbornyl brosylate 254 produces exclusively exo-2-norbornyl acetate 255. The exo-brosylate 254 is more reactive than the endo-brosylate 256 by a factor of 350 and the acetolysis of optically active exo-brosyl ate gave completely racemic exo-acetate 255. Thus, the carbonium ion produced from exo-254 is more rapidly (thus more easily) formed than that from endo-256. These results were originally rationalized in term of a bridged (nonclassical) cation 257 (Winstein approach) (97) or as the rapidly equilibrating classical carbonium ions 258 and 259 (Brown approach (98, 99)). 

Figure 28.9. Conversion of optically active e c< -norbornyl brosylate into racemic x( >norbornyl acetate via nonclassical ion. Brosylate anion is lost with anchimeric assistance from C-6, to give bridged cation III. Cation III undergoes back-side attack at either C-2 (path a) or C-1 (path b). Attacks a and b are equally likely, and give racemic product.

The solvolysis of optically active exo-2-norbornyl brosylate (74) affords completely racemic products. The rate of racemization exceeds the rate of solvolysis, indicating return from an achiral ion pair89). Products of return to isomeric cations were isolated from the solvolysis of many substituted norbornyl sulfonates some... 

In contrast to (74), optically active l,2-dimethyl-2-norbornyl p-nitrobenzoate (79) gives active products the SN1 product (80) is formed with ca. 9% and the El product (81) with ca. 63% retention of configuration96. Interconversion of enantiomeric 1,2-dimethylnorbornyl cations apparently competes with the product forming steps. The different optical purities of (80) and (81) show that they are derived from different intermediates. The authors suggest that most, or all, of the El product is formed from an intimate ion pair and that the SN1 product is formed from a solvent-separated ion pair or a dissociated carbocation. Solvolysis is accompanied by ion pair return which results in racemization of (79) and equilibration of 180-labeled (79). The rate of racemization exceeds that of scrambling of 180 by a factor of ca. 2. Substrate re-formed by ion pair return must be at least as optically active as the El product (81). Therefore, and keq correspond to upper limits of 37% and 20% of the total return, respectively. Scrambling of 180 detects only a small fraction of the total ion pair return in the solvolysis of (79). 

Symmetry. Solvolysis of optically active exo-2-norbornyl brosylate (74) yields racemicexo-norbornylderivatives10,513. The rate of racemizationexceeds the titrimetric rate by a factor of 1.40 in 75% acetone, 2.94 in ethanol, and 3.46 in acetic acid10) (later revised to 4.6513 ). This is attributed to recapture of the anion by the racemic cation. Solvolysis of optically active endo-norbornyl brosylate (688) yields exo-norbornyl products with a small amount of retained activity (13% in 75% acetone, 7% in acetic acid, and 3% in formic acid)10,513). Solvent attack with inversion on (688), or the corresponding tight ion pair, must be involved. 

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