Hydride and Methyl Shift

Hydride and methyl shifts are responsible for changes in the position of carbenium ions. The net effect is generally to create a stable ion, e.g., tertiary ion, from a less stable ion, e.g. seeondary ion. The methyl shift can also change the location of a branch position, which creates isomers. 

The rate of hydride shift is considered to be much faster than alkyl shift due to the ease of moving as compared to the alkyl group. The hydride shift reaction test required a C - C - H string in the molecule that for the methyl shift reaction required a C - C - (CH3) string. 

The mles are summarized in Table 2. The reactions were allowed for all ions. The number of reactions allowed was constrained as a function of number of branches on the ions. This provided the proper spectrum of isomers and also kept the number of species and reactions manageable and in alignment with available analytical chemistry. 

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Butene as the feed alkene would thus—after hydride transfer—give 2,2,3-TMP as the primary product. However, with nearly all the examined acids, this isomer has been observed only in very small amounts. Usually the main components of the TMP-fraction are 2,3,3-, 2,3,4-, and 2,2,4-TMP, with the selectivity depending on the catalyst and reaction conditions. Consequently, a fast isomerization of the primary TMP-cation has to occur. Isomerization through hydride- and methyl-shifts is a facile reaction. Although the equilibrium composition is not reached, long residence times favor these rearrangements (47). The isomerization pathways for the TMP isomers are shown schematically in Fig. 3. 

Studies with sulfated zirconia promoted with Pt309 and industrial chlorinated Pt on AI2O3 isomerization catalysts310 led to the same conclusion, namely, the intermolecular mechanism operative for M-butane isomerization. A significant difference, however, is that on the industrial catalysts extensive hydride and methyl shifts taking place in the intermediates prior to P scission do not lead to a random distribution of the labels. Instead, a binomial distribution with one and three 13C atoms is observed.310 This is indicative of the involvement of the 31 carbocationic intermediate. 

This was explained by the intermediate 16 reacting with a second molecule of ethylene to form a new carbocation that undergoes a series of hydride and methyl shifts to yield the end product. 

Carbocation 1 can undergo hydride and methyl shifts to yield 2,3,4-trimethyl-pentenes ... 

An intensive study of R2AlCl-induced cyclization of unsaturated carbonyl substrates was made by Snider and co-workers. Their results show the advantage of alkyl-aluminum chloride over AICI3 in Lewis acid-catalyzed reactions, because these reagents can act as proton scavengers and Lewis acids. The reaction is interpreted as a Me2AlCl-promoted cyclization of the y,(5-unsaturated ketone then sequential hydride and methyl shift, as depicted in Sch. 25 [49],... 

Treatment of the des-A-A -unsaturated compound (406) with hydrogen fluoride gave the 9a-fluoro-product (407) and the backbone-rearranged ketone (409). Migration of a deuterium atom from the 17a- to the 13a-position, and failure to incorporate any deuterium other than at C(ll) when the reagent was deuterium fluoride, established a mechanism of the type illustrated (408), presumably with rapid sequential hydride and methyl shifts, not involving any olefinic... 

The studies of MeAlCh-induced cyclization of unsaturated ketones indicate the advantage of alkylaluminum chloride over AlCI in Lewis acid catalyzed reactions, since these reagents are capable of acting as proton scavengers as well as Lewis acids [33]. The reaction is interpreted as a MeAlCL-promoted cyclization of the y,b-unsaturated ketone followed by the sequential hydride and methyl shift as illustrated below. 

Cyclization. Piperidines iluorinated at C-3 and C-4 are accessible from diallylamine derivatives. Hydride and methyl shifts can intervene prior to capture of the carbocations by fluoride ion. ... 

Both computational and solvolysis studies have also been done to characterize the [ 5119] series of carbocations. The barrier to the hydride and methyl shifts that interconvert the methyl groups in the f-pentyl cation is 10-15 kcal/mol. This rearrangement must pass through a secondary ion or related bridged species. 

Surprisingly, when the acetal is cyclized in the presence of stannic chloride in nitromethane (conditions which had been so successful with the dienic acetal) the reaction takes a completely abnormal course involving rearrangement. The main product is a tricyclic substance. This product could have arrived through a consecutive 1,2-hydride and methyl shifts of a bicyclic cationic intermediate ... 

Rearranged labdane skeletons arise via a series of associated hydride and methyl shifts, usually involving concerted tra/ts-migrations of the participating substituents. 

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