2-Norbornyl brosylate

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)). 

Norbornyl acetate, 202 2-Norbornyl brosylate, 202 D-Normorphinan, 356 Nucleophilic additions, 209-242 Nucleophilic 1,4-additions, 221-242... 

In contrast, when R—X in Scheme 4.4 is enantiomerically enriched endo-2-norbornyl brosylate (2 in Fig. 4.5) under the same reaction conditions, solvolysis is still accompanied by racemisation, but the rate of racemisation is identical with that of product formation [14]. The achiral carbocation in this reaction, therefore, is captured by solvent much faster than it can undergo internal return, i.e. k2 i in Scheme 4.4. In this case, therefore, the initial ionisation is the rate-determining step. 

Fig. 4.5 exo- and encfo-2-Norbornyl brosylates (1 and 2) and the achiral nonclassical 2-norbornyl carbe-nium ion, 3. 

It is well known that acetolysis of both exo-2-norbornyl brosylate (2.17) and endo-2-norbornyl brosylate (2.18) produces exclusively exo-2-norbornyl acetate (2.19). However, 2.17 is 350 times more reactive than 2.18 (Scheme 2.13). Further, optically active 2.17 gives... 

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.

In 1952 Winstein reported a detailed study of the solvolysis of exo- and endo-norbornyl brosylates and observed fascinating phenomena which he interpreted in terms of a symmetrical a-bridged norbornyl intermediate8. Thus the exo isomer (11) underwent acetolysis (5) at a rate 350 times that of the endo isomer (13) (6). 

The optically active exo-norbornyl brosylate (11) yielded inactive e.w-nor-bomyl acetate (12). However, the optically active endo-norbornyl brosylate yielded ejconorbornyl acetate retaining 7 to 8% of the original activity. 

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... 

Secondary Deuterium Kinetic Isotope Effects. Deuterium substitution has been employed to probe for bridging in the transition state of 2-norbornyl brosy-late solvolyses. The secondary a-, (3-, and 7-deuterium kinetic isotope effects for exo-and endo-norbornyl brosylate are shown in formulas 721) and (722), respectively. The overall pattern for the mfo-compounds (722), with an a-effect close to the limiting value (1.22, cf. Section 7.2.3)506), with nil effect of C(1)-DS07 a modest... 

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. 

The solvolysis of exo-norbornyl brosylate in acetic acid gives 4% elimination products, 98% of which is nortricyclene and only 2% is norbornene. The larger yield of the cyclopropane than the olefin was quoted as support for a non-classical rather than a rapidly equilibrating pair of classical carbonium ions . ... 

In 1978 Harris and Brown independently concluded that the solvolysis of 2-endo-norbornyl brosylate proceeds mainly not by route k, with nucleophilic solvent... 

Brown has questioned whether the spatial structure of the solvolysis products of 2-exo-norbornyl brosylate is a proof of the intermediate nonclassical ion He believes the classical 2-norbornyl cation to yield, when attacked by the nucleophile, only exo compounds for two resons ... 

The benzene ring just as the double bond brings about anchimeric acceleration, but far less than the latter. The solvolysis rate of brosylate 247 is 6 10 times higher than that of 7-norbornyl brosylate but it is 10 times lower than that of 7-anti-norbornenyl brosylate. The authors consider this to be due to some destabilization of the aromatic ring in the intermediate ion 249. 

The rates of acetolysis of 7-norbornyl brosylate and 6-nitro-9-syn-benzobomenyl brosylate are practically equal, i.e. there is no n-participation only the latter compound is completely rearranged under solvolysis reaction. The driving force of rearrangement for 9-syn-brosylates seems to be the tendency to form a stable benzylic carbocation. The kocHj ratio for syn-isomers is only 43, i.e. much less... 

During the acetolysis of exo- and endo-norbornyl brosylates it is found that solvolysis of the exo-isomer is 350 times faster than for the endo-isomer both isomers give only the exo-acetate and optically pure exo-brosylate gives 100% racemic product while an optically pure endo-brosylate gives 93% racemic exoacetate (Scheme 2.37). 

The acetolyses of both exo-2-norbornyl brosylate and cndo-2-norbornyl brosylate produce exclusively eaco-2-norbornyl acetate. The exo-brosylate is more reactive than the endo-brosylate by a factor of 350, as measured by their first-order rate constants. Furthermore, acetolysis of optically active exo-brosylate gave completely racemic exo-acetate, and endo-brosylate gave exo-acetate that was at least 93% racemic. 

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