Exo-Norbornyl acetate

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)). 

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... 

Fig. 13. Apparent activation energies (Eg) for acid-catalyzed reactions of exo- and cndo-2-norbornyl acetate and AH for equilibration. (Reproduced from Goering, H. L., Schewene, C. B. J. Am. Chem. Soc. 87. 3516 (1965).]...

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. 

Table 1. Calculated and Experimental Isotopic Distribution for the 2-Norbornyl Acetate Formed on Acetolysis of 2-exo-Norbomyl [2,3- C]brosylate ...

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. 

FIGURE 21.48 Both endo- and exo-2-norbornyl tosylate react in acetic acid to give ca o-2-norbornyl acetate. The reaction of the exo isomer is faster. 

CJglycerol trinitrate On the other hand, equimolar amounts of H CHO reacted with nitromethane to give 2-nitro[l- C]ethanol (115). Dehydration of 115 furnished 1-nitro-[2- C]ethylene (116). a valuable Michael acceptor and Diels-Alder dienophile. Its reaction with cyclopentadiene, for example, gave predominantly e t/o-2-nitro[3- C]-norcamphane (117). Subsequent catalytic reduction of the double bond, Fe-reduction of the nitro group and concluding acyloxy-deamination converted 117 into exo-[ C]norbornyl acetate (118). ... 

Quadricyclane is a highly strained and reactive compound. It reacts readily with acetic acid to give a mixture of nortricyclyl acetate and exo-norbornyl acetate and with bromine to yield a mixture of 2,6-dibromonortricyclene and exo-5-anti-l-dihvo-monorbornene.3 Quadricyclane undergoes cycloaddition reactions with a variety of dienophiles to give 1 1 adducts.15 1 2 3 4 5 6 7... 

The first laboratory test of these predictions was the pyrolysis of deuterated enorbital symmetry rules, this thermal suprafacial [l,3]-sigmatropic reaction took place with complete inversion at C-7. Similar results have been obtained in a number of other cases. However, similar studies of the pyrolysis of the parent hydrocarbon of 121, labeled with D at C-6 and C-7, showed that while most of the product was formed with inversion at... 

Corey and Casanova found that oxidation of either meso- or dM, 2-diphenyl-succinic acid with lead tetraacetate in pyridine afforded tranr-stilbene (40-45% yield) cw-stilbene was shown not to be an intermediate. Similar oxidation of either endo- or exo-norbornane-2-carboxylic acid gave cjco-norbornyl acetate (24-67%). 

Corey showed 1963 that 2-exo-103 and 2-endo-norbornylamines 104 are deaminat-ed in acetic acid to yield practically identical mixtures both in the exo endo product ratio and in the retention degree of optical activity. He postulated both reactions to the same intermediate classical 2-norbornyl cation since no optically active products were obtained from a symmetrical nonclassical 2-norbornyl ion 5. Somewhat later Berson studied the composition of reaction mixtures from the deamination of amines 103 and 104 (Table 8) more carefully three essential difference in the compositions of reaction mixtures were pointed out ... 

These data can be interpreted as follows endo amine 104 gives a classical 2-norbornyl cation 6 which either reacts with a nucleophile or transforms — with a small energy barrier — into a nonclassical ion 5. The rate ratio of exo and endo addition to the classical ion 6 must equal the ratio of the optical purity of exo acetate 66 to that of endo acetate 105 obtained from endo amine 104. Deamination of exo amine 103 seems to follow two routes ... 

The results of the oxidation of other unsubstituted cycloaliphatic compounds are summarized in Table 4, which indicates that cyclopentane and cyclohexane are oxidized in high yield and selectivity. The medium-sized rings, however, afford only low yields of the trifluoroacetates in a complex product mixture. This lack of selectivity is probably due to extensive rearrangments of the intermediates and easy follow-up oxidations of the products. In dichloromethane/acetic acid the same non-selectivity is encountered. The electrolysis of bicyclo[2.2.1]heptane (3) leads in dichloromethane/trifluoroacetic acid to exo-bicyclo[2.2.1]heptan-2-ol (4) in 83% yield as single product after hydrolysis of the trifluoroacetate (equation 6). The stereochemistry of 4 indicates that a nonclassical norbornyl cation is involved as intermediate. In dichloromethane/20% acetic acid/0.05 M TBABF4 a slightly lower yield (61%) of exn-2-norbornyI acetate was obtained. 

Solvolysis of 7-isodeltacyclyl brosylate (134) is reported to give exo-8-deItacyclyl acetate (135) plus small amounts of exo-7-isodeltacyclyl acetate (136). When (134) deuteriated at C-5 is used, the deuterium ends up at both indicated sites in (135). It is suggested that normal 2-norbornyl a participation leads to (137) [the numbering is as in (134)] and so to (138), which has a two-fold rotation axis. ... 

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 . ... 

However, the relatively weak nucleophilic solvent assistance in the solvolysis of 2-endo-norbornyl sulphonate is corroborated, firstly, by the formation of about 8 % of optically active 2-exo-acetate 66 secondly, by a significant decrease in the solvolysis rate of 2-endo-tosylate on introducing 3-exo-substituents shielding the backside approach of solvent molecules... 

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). 

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