Rearrangements Wagner-Meerwein shift

One of the most characteristic properties of carbonium ions is their great tendency to undergo rearrangements. These rearrangements include 1,2-alkyl shifts, hydride shifts, cyclopropylcarbinyl rearrangements, Wagner-Meerwein rearrangements, and others. 

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. 

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. 

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 ... 

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... 

Closely related to the Wagner-Meerwein reaction is the [1,2] rearrangement of vicinal diols and related substrates collectively referred to as pinacolic rearrangements2,319. Like the Wagner-Meerwein shift, the pinacol rearrangement proceeds via formation of an intermediate cation, followed by alkyl migration and concurrent generation of a carbonyl system. 

The mechanistic controversies have not diminished the synthetically interesting transformations possible with controlled cation rearrangements." Mehta and Kapoor exploited the Wagner-Meerwein shift to synthesize himachalene via treatment of 79 with trifluoroacetic acid at 35°C, initially forming the norbornyl cation 80." 2 xbis cation gave the usual norbornyl cation rearrangement to 81, which was trapped by trifluoroacetate to give the final product... 

This particular type of Wagner-Meerwein shift has special recognition due to its importance in the field of terpene chemistry. For example, the conversion of a-methylcamphene into 4-methylisoborneol involves both a Nametkin and a Wagner-Meerwein rearrangement. In the Meinwald rearrangement both 1,2- and 1,3-shifts occur to give different products (Scheme 2.24). 

The 2-bicyclo[2.1.1]hexyl cation [113] or [114] shows only three carbon signals, one for C-4 and two averaged signals for C-l/C-2 and C-3/C-5/C-6 respectively. The fast dynamic process leading to the observed symmetry could be either a Wagner-Meerwein shift between trivalent cations (83) or a three-fold degenerate rearrangement of hypercoordinated cations [114] as shown in (84). 

Whenever a carbocation is an intermediate in a mechanism, rearrangements are possible. Besides the study of carbocation structures and reactivity in stable ion media, the majority of the information chemists have on carbocation rearrangements comes from SnI solvolysis reactions. A hydrogen, alkyl, or aryl group on a carbon adjacent (p) to the cationic carbon can shift to form a different carbocation. This is called a Wagner-Meerwein shift. Eq. 11.30 shows a thermoneutral example. 

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