Wagner-Meerwein hydride migrations

The acetylated products derived from higher alkenes are susceptible to protonation or solvolysis which produces carbenium ions that undergo Wagner-Meerwein hydride migrations. For example, on subjection of cyclohexene to standard Friedel-Crafts acetylation conditions (AcCl/AlCb, CS2—18 °C), products formed include not only 2-chlorocyclohexyl methyl ketone (in 40% yield) but also 4-chlorocyclohexyl methyl ketone. If benzene is added to the crude acetylation mixture and the tenperature is then increased to 40-45 °C for 3 h, 4-phenylcyclohexyl methyl ketone is formed in 45% yield (eq 3). ... 

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. 

The formation of sesquiterpenes by a carbocation mechanism means that there is considerable scope for rearrangements of the Wagner-Meerwein type. So far, only occasional hydride migrations have been invoked in rationalizing the examples considered. Obviously, fundamental skeletal rearrangements will broaden the range of natural sesquiterpenes even further. That such processes do occur has been proven beyond doubt by appropriate labelling experiments, and... 

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. 

Successive Wagner-Meerwein rearrangements have been utilized in triterpenoid syntheses. A unique and fascinating example of this can be foimd in Corey s synthesis of the triterpenoid oleanene. When 3P-friedelanol (35) is treated with acid, a total of seven 1,2-alkyl and 1,2-hydride migrations occur leading to the formation of 37. The stereospecific shifts are driven by the apparent decrease in steric strain due to the original location of the axial substituents. Intermediate products have been isolated, supporting the assertion that at least some of the steps are not concerted. 

Sdieme 6,59. Representations of the 1,2-hydride migration and Wagner-Meerwein rearrangements on a bicyclic carbocation. 

Thus, in Scheme 6.59, l,7,7-trimethylbicyclo[2.2.1]hept-2-ene ( bornene ) is protonated to yield a secondary carbocation, capable of undergoing 1,2-enrfo-hydride migration and/or the migration of a bond in a Wagner-Meerwein rearrangement to a new bicyclic (but now tertiary) carbocation and thence, by proton loss, to 3,3-dimethyl-2-methylenebicyclo[2.2.1]heptane ( camphene ). 

The initial carbocation formation bears the potential for various reactions including intramolecular attack of an olefinic double bond, hydride migrations, or Wagner-Meerwein rearrangements. The transformation of a linear polyisoprenoid... 

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