Migrating center

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 ... 

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. 

Cationic Fp (olefin) complexes [Fp = f/5-C5H5Fe(CO)2] undergo regio-specific addition of heteroatomic nucleophiles.32 Subsequent ligand transfer (carbonyl insertion) occurs with retention of configuration at the migrating center (R—Fe—CO -> RCOFe).33 A combination of these processes has provided a novel stereospecific azetidinone synthesis which can also be applied to condensed systems.34... 

Aldol derivative synthesis Kitagawa and Taguchi group have shown a rearrangement of a-alkoxycarboxamides 41 which proceeds with a high level of retention at the migrating center to give optically active /3-hydroxycarboxamides 42 (equation 22). ... 

Classify as sigmatropic reactions and as cycloadditions each of the following, and predict stereochemistry at the migrating center for the ground-state transformation. 

Woodward and Hoffmann (1965b) also pointed out the possibility of a sigmatropic change involving a w orbital. Figure 20 indicates the frontier orbital viewpoint. This sigmatropic shift can involve any migrating center which possesses a p orbital. We should expect, therefore, that in systems of the type 10 in which X has an accessible a or p orbital, that... 

These results strongly suggest that the migrating center... 

The Baeyer-Villiger reaction occurs with retention of stereochemistry at die migrating center. This stereoselectivity has been utilized in a practical method for the preparation of isotopically chiral metiiyl acetic acid (5) ftom [2- H]cyclohexanone (4) prepared by enzyme-catalyzed stereoselective exchange of the pro-R a -proton and enantioconvergent exchange of the a-proton with deuterium (Scheme 2). As a cautionary note, prior epimerization of an acyl group prior to oxidation has been observed. ... 

Hnally, a useful, although not strictly a radical, meAod of effecting decarboxylative oxygenation is Ae so-called carboxy inversion reaction. The activated acid is transformed into a mixed all l aryl diacyl peroxide which suffers decarboxylative rearrangement to Ae alkyl ester of the aryl acid. This reaction is particularly useful as it takes place wiA retention of configuration at the migrating center (equation... 

The first step of the process is the protonation of one of the hydroxyl groups, which results in the loss of a water molecule to give a carbocation intermediate. This intermediate undergoes a /"f,27-shift to give a more stable carbocation that upon the loss of proton gives the product. The pinacol rearrangement was shown to be exclusively intramolecular, and both inversion and retention were observed at the migrating center. 

It appears that the stereochemistry at the migrating center has not been explicitly examined in a pinacol rearrangement but, as seen in related 1,2-migrations, retention of configuration is expected. 

Homolytic fission of the chiral ammonium ylide (143) generates two diastereomeric radical pairs which evolve by two different routes (a) cage recombination leads to the two diastereomeric ketoamines (161) and (162), where the migrating center has retained its absolute stereochemistry (b) radical escape gives achiral free radicals that combine to yield racemic products (Scheme 35). 

TTie nonracemic secondary ethers (58) and (60) rearrange with over 90% retention of stereochemistry at the migrating center (equations 16 and 17). A lithium-bridged diradical species (63 Scheme 2) is postulated to account for the observed diastereoselectivity. Recombination of the radical pair must occur more rapidly than inversion of the radical center, judging from the high degree of retention observed with (58) and (60). A two-directional application of this rearrangement was used to prepare the syn-skipped polyols (66) and (68 Scheme 3). 

XOCHCCLR in support, added methylfuran will trap some of this ion, and if the migrating center is optically active, the products are racemic.222... 

Therefore, if we derive or remember one rule for a pericyclic reaction, then any time an MO phase change is added the rule will reverse. Two reversals cancel each other. For example, 4n face to face (supra-supra) cycloadditions are not thermally allowed. If we add two electrons, we fill the next highest MO, which has a phase reversal. This means An+2 cycloadditions are thermally favored. Thermal electrocyclic reactions of 4n species go conrotatory, whereas thermal 4n+2 electrocyclic reactions go disrotatory. Thermal sigmatropic reactions of 4n species go supra-inversion or antara-retention. Count arrows to tell whether the pericyclic reaction is 4n or 4n + 2. Phase reversals occur between retention/inversion at the migrating center, between antarafacial/suprafacial migration, with 4n vs. 4n+2 electrons, and between thermal and photochemically excited species. 

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