Carbon migration

Step 3 Carbon migrates from boron to oxygen displacing hydroxide ion Carbon migrates with the pair of electrons m the carbon-boron bond these become the electrons m the carbon-oxygen bond... 

When unsymmetrical ketones were used in this reaction (with BF3 as catalyst), the less highly substituted carbon preferentially migrated. The reaction can be made regioselective by applying this method to the a-halo ketone, in which case only the other carbon migrates. The ethyl diazoacetate procedure has also been applied to the acetals or ketals of a, P-unsaturated aldehydes and ketones. ... 

E. Nitrogen-to-Carbon, Oxygen-to Carbon, and SulfuMo-Carbon Migration... 

For another reaction involving boron-to-carbon migration, see 10-109... 

Though the mechanism has not been investigated thoroughly, it has been shown to be intramolecular by the failure to find crossover products when mixtures of boranes are used, The following scheme, involving three boron-to-carbon migrations, has been suggested. 

The first example of a carbon migrating across a double bond of a vinyl cation generated by solvolysis was reported by Hanack, Schleyer, Stang, and co-workers (177) with the vinyl substrate 230. Solvolysis of 230 in 80% aqueous ethanol gave, besides a small amount of allene, exclusively the rearranged ketone... 

Scheme 9.1 shows several examples of one-carbon homologations involving boron to carbon migration. Entry 1 illustrates the synthesis of a symmetrical tertiary alcohol. Entry 2 involves interception of the intermediate after the first migration by reduction. Acid then induces a second migration. This sequence affords secondary alcohols. 

By studying the effect of various a-substituents, it has been shown that the bond to the most highly substituted a carbon is preferentially cleaved and that the more nucleophilic alkyl carbon migrates to the relatively electron-poor free radical to form the carbene intermediate,... 

Hydrogen and 1,2-carbon migrations are the most common singlet carbene rearrangements. They involve simple shifts of substituents from an adjacent carbon to the carbenic center, affording an alkene product Eq. 7. 

Benzocyclobutenylcarbenes (87) display a striking preference for carbon migration no hydride shift product (90) appears upon photolysis or thermolysis of... 

Figure 6.16 Oxygen-to-carbon migration of phosphorus in an aromatic system.

Iboga alkaloids devoid of the 19-hydroxy group are significantly more stable toward oxidation than are the corresponding hydroxy bases. Abstraction of the hydroxy proton of 7-hydroxyindolenines by bases leads to concomitant carbon migration and formation of pseudoindoxyls. In some cases the rearrangement is better accomplished by warm HC1. The interrelationship among indoles, 7-hydroxyindolenines, and pseudoindoxyls has been exhaustively treated by Cordell (see Ref. 6). 

Since the reactive substructure of cA-2-(l,3-butadienyl)cyclopropylidene is contained in the bicyclic carbene 124, there is a possibility that a carbene-carbene rearrangement occurs together with 1,5-carbon migration. Analysis of the probable reaction pathways allows one to conclude that 1,5-C-migration (124 - 132) in the fixed c -l,3-butadienyl fragment of structure 124 is impossible. The 1,3-carbon migration (124 - 130) which takes place instead is mechanistically analogous to the vihylcyclopropylidene-cyclopentylidene... 

The most electron-rich alkyl group (more substituted carbon) migrates first. The general migration order ... 

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