Migration 1,2-sigmatropic shifts

When an alkyl group migrates, there is an additional stereochemical feature to consider. The shift can occur with retention or inversion at the migrating center. The analysis of sigmatropic shifts of alkyl groups is illustrated in Fig. 11.7. The allowed processes include the suprafacial 1,3-shift with inversion and the suprafacial 1,5-shift with retention. Sigmatropic rearrangements of order [3,3] are very common ... 

These are suprafacial sigmatropic shifts of order [1,5] and should occur with retention of configuration at the migrating carbon. This stereochemical course has been established for the 1,5-alkyl shift that converts 16 to 17. The product which is isolated, 18, results from a subsequent 1,5-hydrogen shift, but this does not alter the stereochemistry at the migrating... 

Application of symmetry rules also yields important results for the class of reactions known as sigmatropic shifts, in which a double bond switches position as an atom or a group migrates ... 

Fig. 17. a) Some interesting molecules that undergo 1-3 sigmatropic shifts, b) Clockwise (CW) and counterclockwise (CCW) rotation at the migrating center, c) Mechanisms and parameters for the EHT study of the rearrangements of bicyclo [3.2.0] heptenes with and without inversion of configuration. 

So it is the number of electrons and not the number of atoms which determines the selection rule. Therefore, the selection rules for hydrogen migration in thermal sigmatropic shifts can be summarized as follows as given in the table ... 

Sigmatropic shifts are reversible reactions in which a o-bonded group (usually H) migrates from one point to another within a molecular system. Sigmatropic shifts may occur either thermally or photochemi-cally, but 1,3- and 1,7-shifts are the most commonly encountered for the photochemical reactions. 

Stereorigid radical cations may undergo stereospecific sigmatropic shifts for example, the puckered ions, anti- and syn- 5-methyl-19 ", undergo stereospecific hydride or methyl migration, respectively, forming the 1-methylcyclopentene ion (46 ) and the 3-methyl isomer (47 ). ... 

For example, CdS can mediate a symmetry forbidden [1,3] sigmatropic shift of hydrogen, Eq. (43) In the absence of oxygen, these cyclic derivatives of diphenyl-cyclobutene underwent a reversible double bond migration. Since the reaction was... 

Although evidence for thermally induced migrations points to a sigmatropic shift of the substituent, as discussed above, the photochemical conversion of 2-cyanopyrrole into 3-cyanopyrrole proceeds by a [1,3] sigmatropic walk of the heteroannular atom by route (a), as shown in Scheme 2. 2-Cyano-3-methylpyrrole and 2-cyano-4-methylpyrrole undergo similar phototranspositions to produce 3-cyano-4-methylpyrrole (by route a) and 4-cyano-2-methylpyrrole (by route a), respectively (75CC786), whilst 2-cyano-5-methylpyrrole produces... 

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