Nitrogen, electron-deficient, migration

Another important reaction involving migration to electron-deficient nitrogen is the Beckmann rearrangement, in which oximes are converted to amides.282... 

The Wolff rearrangement has a close formal resemblance to the Hofmann and related reactions (p. 122), in which migration takes place to an electron-deficient nitrogen atom to form an isocyanate, RN=C=0, intermediate. 

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. 

A simplified mechanism for the Beckmann rearrangements and important related reactions is shown hi Scheme 9. Summarizing the mechanism section, the key step of the reaction is the migration of an a-carbon group to the electronically deficient nitrogen atom of the oxime. A nitrilium ion in some cases or an imidate in others are key intermediates in the reaction. Their destiny determines the course of the transformation. Basically, three different pathways may be possible and can be synthetically exploited ... 

In contrast to the somewhat limited synthetic utility of nitrenes, there is an important group of reactions in which migration occurs to electron-deficient nitrogen. One of the most useful of these reactions is the Curtius rearrangement 16 This reaction has the same relationship to acylnitrene intermediates that the Wolff rearrangement does to acylcar-benes. The initial product is an isocyanate, which can be isolated or trapped by a nucleophilic solvent. 

Another important reaction involving migration to electron-deficient nitrogen is the... 

Our consideration of rearrangements to electron-deficient heteroatoms must be brief. In discussing migrations to electron-deficient nitrogen, we first discuss three rearrangements that occur in carbonyl derivatives, the Beckmann, Hofmann, and Schmidt rearrangements, and then consider rearrangements of nitrenium ions. 

It should be noted that amines may be formed by hydrolysis of amides arising from the intramolecular Beckmann rearrangement of ketoximes (p. 1047) this rearrangement is a further example of the migration of a nucleophilic carbon species from a carbon to an electron-deficient nitrogen. 

As the bromine leaves, the nitrogen becomes electron deficient, so the carbon group migrates to the nitrogen. The nitrogen uses a pair of electrons to stabilize the carbonyl carbon. 

The Hofmann rearrangement also involves a migration to an electron-deficient nitrogen. In this case, an amide is treated with Cl2 or Br2 in aqueous base, resulting in the formation of an amine with one less carbon. The original carbonyl carbon is lost as carbon dioxide. The mechanism is shown in Figure 22.7 and an example is provided by the following equation ... 

Nitrenes, like carbenes, are immensely reactive and electrophilic, and the same Wolff-style migration takes place to give an isocyanate. The substituent R migrates from carbon to the electron-deficient nitrogen atom of the nitrene. Isocyanates are unstable to hydrolysis attack by water on the carbonyl group gives a carbamic acid which decomposes to an amine. 

Methoxy-N-chloromorphinan may be prepared in 78% yield by treatment of 3-methoxymorphinan with sodium hypochlorate.(139) Upon silver ion catalyzed rearrangement, it gave 13% of 146. The alkyl migration to the electron-deficient nitrogen was proposed as being mediated by a nitrenium ion transition state (145). 

Rearrangement of NJV-dilmloanunesf N,N-Dichloro-tri-n-butylcarbinamine (2), prepared by treatment of tri-n-butylcarbinamine (I) with calcium hypochlorite, when treated with aluminum chloride in methylene chloride at - 30°, followed by acid hydrolysis, gives di-n-butyl ketone (3) and n-butylaminc (4) in high yield. The reaction is considered to involve alkyl migration to electron-deficient nitrogen. 

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