Migration terminus

This process is sometimes called the Whitmore 1,2 shift Since the migrating group carries the electron pair with it, the migration terminus B must be an atom with only six electrons in its outer shell (an open sextet). The first step therefore is creation of a system with an open sextet. Such a system can arise in various ways, but two of these are the most important ... 

CH2), and norbomyl (e.g., 4) type systems are especially prone to carbocation rearrangement reactions. It has been shown that the rate of migration increases with the degree of electron deficiency at the migration terminus. ... 

This information tells us about the degree of concertedness of the three steps of the rearrangement. First consider the migration terminus B. If racemization is found at B, it is probable that the first step takes place before the second and that a positively charged carbon (or other sextet atom) is present at B ... 

The stereochemistry at the migration origin A is less often involved, since in most cases it does not end up as a tetrahedral atom but when there is inversion here, there is an Sn2 type process at the beginning of the migration. This may or may not be accompanied by an Sn2 process at the migration terminus B ... 

Reactions in which the migration terminus is on an aromatic ring have been treated under aromatic substitution. These are 11-30-11-35, 11-39, 13-21-13-24,... 

Several alternative procedures have been developed in which other reagents replace carbon monoxide as the migration terminus.11 The most generally applicable of these methods involves the use of cyanide ion and trifluoroacetic anhydride (TFAA). In this reaction the borane initially forms an adduct with cyanide ion. The migration is induced by N-acylation of the cyano group by TFAA. Oxidation and hydrolysis then give a ketone. 

Rearrangement of diol monosulfonates can also be done using Lewis acids. These conditions lead to inversion of configuration at the migration terminus, as would be implied by a concerted mechanism.66... 

This is clearly demonstrated in the pinacolinic deamination (cf. p. 114) of an optically active form of the amino-alcohol (50). Such reactions proceed from a conformation (antiperiplanar 50a or 50b) in which the migrating (Ph) and leaving (NH2 as N2 cf. p. 114) groups are TRANS to each other. Rearrangement via a bridged carbocation would necessarily lead to 100% inversion at the migration terminus in the product ketone (5lab), whichever initial conformation, (50a) or (50b), was involved ... 

It might reasonably be expected that similar rearrangements would also occur in which the migration terminus was an electron-deficient oxygen atom such rearrangements are indeed known. 

Spectator substituents, bonded to the carbene s migration terminus (Ci), directly influence the lifetime and philicity of the carbene, but they do not primarily alter the migratory aptitudes of migrants on C2. Oxa spectator substituents stabilize singlet carbenes by electron donation to the vacant carbenic p orbital (LUMO) cf. resonance hybrid 69. 

Placing the electron deficient migration terminus within the original carbonyl partner converts the 1,1-cyclobutanone annulation into a 1,2- or lateral cyclo-pentenone annulation as summarized in Eq. 77. In the ring enlargement of 65 to 66,... 

Good diastereoselectivity is observed with respect to the migration terminus (Eq. 89)113). 

The most popular method for generation of a-thio-carbanion (migration terminus) is direct lithiation (deprotonation) with alkyllithium or lithium amide. These deprotonation methods are widely applicable to various substrates, not only benzyl or allyl sulfides , but also dithioacetals 142 which form 143 (equation 83), and a phosphonate substituted system 144 which gives 145 (equation 84). ... 

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