Electron-deficient centers rearrangement

This rearrangement is unusual because hydrogen usually migrates toward electron-deficient centers as calculated for vinylcarbene. 

Shifts in Neutral Species Shifts in Charged Species Shifts in Electron Deficient Centers Application of EMO Method Cope and Claisen Rearrangements Cope Rearrangement Oxy-Cope Rearrangement Claisen Rearrangement Some Other Examples Abnormal Claisen Rearrangement... 

Alkylboranes rearrange intramolecularly (on heating) to thermodynamically more stable alkylboranes, typically with the boron atom attached to the StericaUy least hindered position on the alkyl chain (the hydroboration rear-rangement). " 53 529.533 wjnjams proposed that this equilibrium occurs by a stepwise intramolecular attack of the electron-deficient center on the neighboring C-H o bonds. The transition state of the intramolecular migration incorporates a pentacoordinate carbon atom [Eq. (6.142)] ... 

Numerous examples of hypervalent iodine-promoted fragmentations or rearrangements at electron-deficient centers have been reported. Several examples of oxidative fragmentations are shown below in Schemes 3.139-3.142. A mild and efficient fragmentation reaction of p-amino alcohols 349 and a-amino acids 350 upon treatment with [bis(trifluoroacetoxy)iodo]pentafluorobenzene leading to N,0-acetals 351 has been developed (Scheme 3.139). This method has been utilized in an improved synthesis of the key intermediate of discorhabdins [455,456]. 

The carbon of the carbonyl is positive and thus represents an electron-deficient center toward which electron rich bonds migrate. The rearrangements encountered with aldehydes and ketones are generally acid or base catalyzed and thus other processes, such as condensation reactions vide supra) may intrude. 

As described in Section 11.5.12 with reference to SnI reactions, a hydride or alkyl shift in a carbenium ion is a common rearrangement. It entails the movement of a hydrogen, alkyl, or vinyl / aryl group from a carbon adjacent to a carbenium ion to the electrophilic center in order to create a more stable carbenium ion. Many rearrangements have a similar migration as a key step, with the additional feature of a heteroatom on the p-carbon that stabilizes the newly formed electron deficient center (Eq. 11.46). Two prototype examples are the pinacol rearrangement and the benzilic acid rearrangement. 

In the dehydration of alcohols, we recall that a methyl group can migrate to an adjacent electron-deficient center if a more stable carbocation results (Section 9.19). In this case, the rearrangement of a methyl group yields a carbocation that is formally a secondary ion. However, it is a stabilized hydroxy carbocation, or oxonium ion. The nonbonded electron pair on the oxygen atom is delocalized to the positive carbon atom. 

The reaction of the stable germylene 32 with a triarylmethylium ion gave not the expected germylium ion 33, but in a series of unexpected rearrangements, the germy-lium ion 34, which is intramolecularly stabilized by interaction of the electron-deficient germylium center with a remote aryl group (Scheme 16).80... 

Cp(Ph2PCH2CH2PPh2)Fe=CH-CMe3]+ does not rearrange, however, probably because the increased back donation by the more electron-rich phosphine-substituted iron center decreases the electron deficiency at the carbene carbon. ... 

The Hofmatm, Curtius, Schmidt and Lossen rearrangements generally involve nucleophilic migrations from a carbon to an electron deficient nitrogen center (Scheme 5), giving isocyanates as the initial products, which undergo further reactions as already shown in Scheme 2. 

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