Norbornyl rearrangement

The solvolysis of 252 is one of the rare examples for a norbornyl-norpinyl rearrangement. While the ew-trimethylsilyl brosylate 251 yields mainly substitution and elimination products 253-255 with an intact norbomyl framework, 252 gives nearly 86% of norpinene 256 (equation 39). The bis-(trimethylsilyl)substituted compound 257 gives almost exclusively the norpinene derivative 258 (equation 40). While the trimethylsi-lyl group(s) in 252 and 257 exert no kinetic effect on the reaction rate, the /3-effect on the intermediate carbocations 252A and 259, respectively, determines the product distribution99. 

Less direct precedents are also available for the rearrangement of both exo-1,2-trimethylenenorbornane (32) and emfo-2,6-trimethylenenorbornane (33) to adamantane. Solvolysis of exo-1,6-trimethylene-exo-2-norbornyl tosylate in aqueous acetone at room temperature gives nearly quantitative yields of endo-2,6-trimethylene-ejco-2-norbornanol [Eq. (12)], which, when treated with sulfuric acid, produces 1-adamantanol43). 

The tunneling intramolecular rearrangement of 2-norbornyl cation was considered by Yannoni et al. [1982] and Myhre et al. [1985], The NMR spectra show that the structure is symmetric at temperatures as low as 4.2 K, so that the interconversion rate constant between the two asymmetric structures is greater than 109s-1 ... 

Transformations shown in Schemes 12-14 constitute the first examples of catalytic AROM reactions ever reported. Meso-triene 50 is converted to chiral heterocyclic triene 51 in 92% ee and 68% yield with 5 mol% 4a (Scheme 12) [21]. Presumably, stereoselective approach of the more reactive cydobutenyl alkene in the manner shown in Scheme 12 (II) leads to the enantioselective formation of Mo-alkylidene III, which in turn reacts with an adjacent terminal olefin to deliver 51. Another example in Scheme 12 involves the net rearrangement of meso-bicycle 52 to bicyclic structure 54 in 92% ee and 54% yield. The reaction is promoted by 5 mol% 4a and requires the presence of diallyl ether 53 [22], Mechanistic studies suggest that initial reaction of 53 with 4a leads to the formation of the substantially more reactive chiral Mo=CH2 complex (vs neophylidene 4a) which can react with the sterically hindered norbornyl alkene to initiate the catalytic cycle. 

A number of alkyl and aryl migrations, which may be tenuously regarded as similar to carbonium-ion rearrangements, have been summarized by P. Brown and Djerassi (1967). The close similarity of the mass spectra of norbornyl chlorides have been explained by rearrangement of carbonium ions (Bunton and Del Pesco, 1969). 

The 3-spirocyclopropyl-2-cyclopropyl-2-norbornyl cation 97, R = C-C3H5 is stable even up to -20 °C, whereas the phenyl and methyl analogues rearranged to the allylic cations 98 at -70 °C and -90 °C, respectively (equation 58). 

The norbornyl cation is at the heart of the nonclassical ion problem . The argument over the stable cation concerns whether it is a rapidly equilibrating pair of classical ions or rather a symmetrical ion (Brown, 1977). The isotopic perturbation studies by Saunders and Kates (1980) show that the postulated rapid Wagner-Meerwein rearrangement (4) is not consistent with the results but is a static structure like [4],... 

Some of these rearrangements have also been observed in the parent norbornyl cation [4] and the Jco-3,2-hydride shift (A(7 =11.4 kcal mol" ) and the e/idb-6,2-hydride shift AG = 5.8 kcal moh ) have been studied by dynamic H-nmr spectroscopy (Olah et al., 1970c). 

The 2-brexyl cation (2-tricyclo[4.3.0.0 - ]nonyl cation, [215]) has an intriguing structure containing two norbornyl moieties. Of the two obvious rearrangement paths possible for ion [215], Wagnei-Meerwein shifts (139) and 1,3-hydride shifts (140), only the former is degenerate (Nickon et ai, 1965). The latter gives the isomeric 4-brexyl cation [216], which can undergo further... 

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