Rearrangements Involving Electronically Deficient Nitrogen Atoms

MOLECULAR REARRANGEMENTS INVOLVING ELECTRONICALLY DEFICIENT NITROGEN ATOMS 

Curtius Reaction.49 When acid azides are allowed to decompose, the first product which can be isolated is an isocyanate. It is evident that 

49 For a review of the Curtius reaction, see Smith, in Adams, Organic Reactions Vol. Ill, p. 337, John Wiley and Sons, New York, 1946. 

A mechanism may be written for the reaction similar to the ones just described if we suppose that the driving force for the rearrangement is indeed a nitrogen atom with only six electrons in its valence shell. Separation of a nitrogen molecule from the azide (LXXII) leaves the electronically deficient nitrogen atom shown in (LXXIII). Interchange of the alkyl group and the electron deficiency leaves the completely polarized form of an isocyanate (LXXIV). 

In several ways this reaction resembles the molecular rearrangements involving transitory carbonium ions. The reaction is acid-catalyzed60 (see the Schmidt reaction). In the presence of triphenylmethyl free radicals no mixed products are formed.61 The migrating group never leaves the field of the electronically deficient atoms concerned since the Curtius reaction with (—)o-(2-methyl-6-nitrophenyl)-benzoic acid (LXXV) produces an optically active amine62 (LXXVI). 

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Degradation reactions of carboxylic acids and their derivatives to amines and their derivatives with one less carbon unit involve rearrangements of a carbon to an electron deficient nitrogen atom as key steps, namely the Hofmann Curtius (C), - Schmidt and Lessen rearrangements, as... 

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