1,2-Wagner-Meerwein shift

The firmest conclusion that can be drawn with regard to the structural problem is that both the exo- and endo-amines yield a substantial amount of classical norbomyl ions during deamination. If stabilization by non-classical bridging is in competition with classical ion formation, one might deduce from the optical purity of the exo-acetate produced from the exo-amine that it amounts to less than T4 kcal mole . On the other hand all the data could be reconciled without invoking the non-classical species provided that a small barrier is proposed for the Wagner-Meerwein shift. 

Table 7 contains shift information for a number of other compounds that have been used to analyse the norbomyl spectra. Included among these are data for the isopropyl ion which has been used as the principal model for the secondary ion, and for the 1,2-dimethylnorbomyl cation which has been reported to be an example of an equilibrating classical cation, the Wagner-Meerwein shift being too fast to be stopped at —140°. In the table it may also be noticed that the t-butyl cationic centre experiences a shift of about 11 p.p.m. on being transferred from neat SbFs to SbFs—SO2. 

Perhaps the most spectacular of the natural carbocation rearrangements is the concerted sequence of 1,2-methyl and 1,2-hydride Wagner-Meerwein shifts that occurs during the formation oflanosterol from squalene. Lanosterol is then the precursor of the steroid cholesterol in animals. 

With a variety of different mechanisms available, it is not surprising that the characteristics of hydrochlorination depend on the reaction conditions. Thus, in nitromethane, a medium that gives extensive Wagner-Meerwein shifts during hydrochlorination, olefins react according to the third-order rate law,... 

The photoreactions of saturated five-membered heterocycles are generally characterized by initial carbon-heteroatom bond homolysis. Tetrahydro-furans150 and 1,3-dioxolans151 behave in this way, and the major photoproducts of 2,2-dimethyl-1,3-dioxolan, for example, are acetone, propyl acetate, ethylene, acetaldehyde, methyl acetate, and oxiran. The vinyltetrahy-drofuran (180) is converted on irradiation in methanol to the ketal (181) and the ketone (182) by way of a Wagner-Meerwein shift in the carbocation... 

Kirmse and Sollenbohmer69 studied the trifluoroethanolysis of the norbomyl p-nitrobenzoate 57, and observed the formation of the intermediate 58, followed by a 6,2 migration of silicon and Wagner-Meerwein shifts to give a mixture of products. The solvolysis of the norbornyl system 57 is accelerated by a factor of 3 x 104 compared to unsubstituted 2-norbornyl p-nitrobenzoate. 

In the 2-exo isomer 260 the measured rate enhancement of 23 000 compared with 250 indicates a large /-silicon effect. The intermediate 261 undergoes a fast 2,6 migration and Wagner-Meerwein shifts to give the products via ion 262 (equation 41)100. 

The reaction path depicted in Scheme 5.14 involves Wagner-Meerwein shifts of the methyl group prior to cyclization followed by hydride shift to a number of cationic intermediates. The second scheme (Scheme 5.15) depicts ring closure before methyl migration. The first step involves protolysis of the C—H bond next to the methyl-bearing carbon. The corresponding ion can then rearrange by a 1,2-methyl shift and yield 1,16-dimethyldodecahedrane 28 by hydride abstraction from a hydride donor. 

An asymmetric [1,2]-Wagner-Meerwein shift has been achieved under Pd catalysis, allowing ring expansion of l-(alkyloxyallenyl)cyclobutanol and simple derivatives to ... 

The sulfuric acid catalyzed transformation of pinanic acid into abietic acid shown in Figure 11.6 includes a Wagner-Meerwein shift of an alkyl group. The initially formed carbenium ion, the secondary carbenium ion A, which is a localized carbenium ion, is generated by protonation of one of the C=C double bonds. A [l,2]-sigmatropic shift... 

The tabulation shows that the t-butyl system is a reasonable model for some equilibrating ions. It fails badly, however, when applied to the norbornyl compounds. The isopropyl system is a poor model for sec-butyl and cyclopentyl ions and is a very poor model for the norbornyl cation. The failure of the models to provide reasonable estimates of the shifts in the tertiary norbornyl cations which are undergoing either Wagner-Meerwein shifts or hydride migration makes it clear that the experimental shifts in the secondary system cannot be used as structural proofs. Rather they should be regarded as fascinating results to be rationalized in terms of the structure, whatever it may be. 

Wagner-Meerwein shift of the central bond occurred with covalent capture of the counterion, even perchlorate (see 562 and 563). The most unusual structural features of562 have been confirmed by X-ray analysis.426 Although 564 could be obtained readily, it too proved sensitive to carbonium ion rearrangement. The conversion of lactol 560 to bis-iodoformate 565 when treated with lead tetraacetate and iodine with irradiation is also notable.427 ... 

The acetate formed from [395] showed 26.1 3% D at C(9) while that produced from the mixture of [395] and [396] showed only 12.3 3%. The scrambling process under these conditions is clearly stereoselective if not entirely stereospecific. The amount of deuterium found at C(9) was concluded to indicate that about four Wagner-Meerwein shifts had occurred in the overall solvolysis. 

It was mentioned above that even alkanes undergo Wagner-Meerwein rearrangements if treated with Lewis acids and a small amount of initiator. Catalytic asymmetric Wagner-Meerwein shifts have been observed. An interesting application of this reaction is the conversion of tricyclic molecules to adamantane and its derivatives. It has been found that all tricyclic alkanes containing 10 carbons are converted to adamantane by treatment with a Lewis acid, such as AICI3. If the substrate contains >10 carbons, alkyl-substituted adamantanes are produced. The lUPAC name for these reactions is Schleyer adamantization. Two examples are... 

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