Position shift Migration, Rearrangement

F ritsch-Buttenberg-Wiechell — homoallyl — migration nitroso groups rearrangement position shift retroallyl rearrangement ring — santonin —... 

Wiechell rearrangement homoallyl rearrangement migration position shift retroallyl rearrangement snatoin —... 

While the example illustrated in Scheme 5.8 shows equilibrium between two chemically identical carbocations, there are factors influencing the direction of these transformations when applied to more complex systems. If we consider Scheme 5.9, we notice that the positive charge migrates exclusively to the tertiary center, reflecting the increased stability of tertiary carbocations over primary carbocations. In general, where 1,2-hydride shifts are possible, rearrangement of less stable carbocations to more stable carbocations is expected. 

Sigmatropic rearrangements are also non-1,2 shifts. This rearrangement consists of a migration of a a bond adjacent to one or more n systems to a new position in the molecule, with a new reorganized n system. Examples are... 

Polysacdiarides s. Disacdiarides Polysolfides s. Trisulfides Position shift (s. a. Migration, Rearrangement)... 

BUTENE. As shown in Figure 38, a group attached to C-1 can migrate from position 1 to 3 (1,3 shift) to produce an isomer. If it is a methyl group, we recover a 1-butene. If it is a hydrogen atom, 2-butene is obtained. A third possible product is the cyclopropane derivative. The photochemical rearrangement of 1-butene was studied extensively both experimentally [88]... 

The dienone intermediate (53a), as well as enolising to the phenol (52a), is itself capable of undergoing a Cope rearrangement to yield a second dienone (cf. 56a), whose enol is the p-substituted phenol (c/ 57a). Enolisation normally predominates, but where (51) has o-substituents, i.e. (54a), o-enolisation cannot take place, and only the p-phenol (57a) is then obtained. That this product is indeed formed not by direct migration of the allyl group, but by two successive shifts, is suggested by the double inversion of the position of the, 4C label in the allyl group that is found to occur ... 

Although formation of primary vinyl cation was disproved by the chirality probe approach, a vinyl cationic intermediate can be generated from a primary substrate via participation if a more stable cation could result. Unsymmetrically substituted 2,2-dialkylvinyl iodonium salt 24 gave mainly rearranged products on solvolysis.15 The products involve those of the 1,2-shift of either of the alkyl groups on the p position (Scheme 4). Those formed from migration of the alkyl... 

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