Migrations, electron-deficient centres

In (IV) the group R migrates to an electron-deficient centre, and it might, therefore, be expected that electron release by the group R would aid reaction. The sequence in (rel.) of the Me < Bu" < Bus < Bu could thus be due to the inductive ( + 1) effect of the alkyl groups. If there were any steric strain in the four-coordinate... 

The most curious feature of these reactions is that the observed products can only be rationalized by preferential migration of the ester-bearing carbon to the electron-deficient centre of a cyclopropylcarbinyl cation. This is not at all the expected substituent effect for such an electron-withdrawing group. 

B. Migrations of Silyl Groups to Electron-deficient Centres. 466... 

The per acid first adds to the ketone to give adduct (30) which rearranges via a transition state (31) which is electron-deficient around the former carbonyl group. Consequently,the group which can best supply electrons to combat the deficiency migrates best. It does so with retention as it is a one step reaction in which the chiral centre ( in 30) never becomes detached. 

The rearrangements that we have considered to date all have one feature in common the migration of an alkyl or aryl group, with its electron pair, to a carbon atom which, whether it be a carbocation or not, is electron-deficient. Another atom that can similarly become electron-deficient is nitrogen in, for example, R2N or RN (a nitrene, cf. carbenes above), and it might be expected that alkyl or aryl migration to such centres would take place, just as it did to R3C and R2C this is indeed found to be the case. 

The migration of alkyl groups to carbene centres has much in common with the migration of alkyl groups to cationic centres discussed in Chapter 37—after all, both carbenes and carbocations are electron-deficient species with a carbon atom carrying only six electrons in its outer shelJ. 

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