Methyl group 1,2-shift

The introduction of a (3-methyl group shifts the stereoselectivity to anti, indicating a preference for TS E. There is some dependence on the Lewis acid. For example, the reaction below gives a high ratio of chelation control with MgBr2 and SnCl4, but not with TiCl4.165... 

Hydride or methyl group shift to form the more stable carbocation species occurs during catalytic cracking. For example, methyl group shift to form a tertiary carbocation from a secondary ion species is favored. The presence of relatively high concentrations of branched species in FCC products can be explained by this mechanism. 

Methyl group shift from another carbon atom in the molecule to the carbon atom deficient in electrons to form a new carbonium ion having a different carbon-to-carbon skeleton... 

This secondary carbocation can rearrange to a more stable tertiary carbocation by a methyl group shift. 

Once the carbonium ions are formed, the modes of interaction constitute an important means by which product formation occurs during catalytic cracking. For example, isomerization either by hydride ion shift or by methyl group shift, both of which occur readily. The trend is for stabilization of the carbonium ion by movement of the charged carbon atom toward the center of the molecule, which accounts for the isomerization of a-olefins to internal olefins when carbonium ions are produced. Cyclization can occur by internal addition of a carbonium ion to a double bond which, by continuation of the sequence, can result in aromatization of the cyclic carbonium ion. 

The clear preference for a methylene group shift over a methyl group shift explains the predominant ring contraction in Scheme 2.12. 

This is a mechanism of polyolefin cracking. The main polymer chains are reduced by reaction with protons or other carbonium ions, followed by chain scission giving C30-C50 oligomeric hydrocarbons [7], As a result of further, secondary cracking reactions by P-scission of C30-C50 hydrocarbons, gas and lower-molecular liquid C10-C25 hydrocarbons are produced. Other reactions are double bond and saturated hydrocarbon isomerization as well as methyl group shift . 

Isomerization the carbonium ion intermediates can undergo rearrangement by hydrogen or carbon atom shifts, leading to, e.g. a double-bond isomerization of an olefin. Other important isomerization reactions are methyl group shift and isomerization of saturated hydrocarbons. 

A third carbocation rearrangement occurs by shift of a methyl group A second methyl-group shift gives a final carbocation intermediate ... 

The venerable dienone-phenol rearrangement (3-1 3-2) offers another way for preparing steroids with an aromatic A-ring (Scheme 3.3). The simplest case of this method illustrates a serious drawback to this approach one of the geminal methyl groups shifts onto the adjacent position. 

Thus, once the appropriate orientation within the corresponding enzyme occurs, steroids, cucurbitacins, or other triterpenes may be formed. In the case of cucurbitacins, the squalene epoxide adopts a chair-boat-chair conformation, and the cyclization leads to a protostane cation, which is the immediate precursor of the cucurbitanes through a series of 1,2-proton and methyl group shifts, Fig. (4) [40]. 

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