Imidates Beckmann rearrangement intermediates

The main intermediate of the rearrangement may be a nitrilium ion (225) in some cases or an imidate (226) in others. The resulting intermediate reacts with water to produce the amide (218) after tautomerization. If other nucleophiles (Nu ) are present, they can intercept the reactive intermediates (both inter- or intra-molecularly) and several different imino-substituted derivatives (227) can be formed. These rearrangement-addition reactions will be analysed later in this chapter as they can effectively broaden the scope of the Beckmann rearrangement reaction (Sections VI.D.2 and VI.E.2). 

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 ... 

Various imidates 227 can be produced by trapping the electrophilic intermediate of the Beckmann rearrangement with a nucleophile (Nu ) other than water (equation 109). 

The significance of imidoyl halides as intermediates in the Beckmann rearrangement has been pointed out earlier (see Chapter 3). In the Chapmann rearrangement imidoyl chlorides are used to prepare imidates, which undergo rearrangement to yield N-aroyl derivatives of diarylamines C ). For example, reaction of the sodium salt of dimethyl 4,6-dihydroxyisophthalate (XIX) with the benzimidoyl chloride XX yields the bis-imidate XXI, which can be thermally rearranged and hydrolyzed to produce the diamines XXII C). 

The mechanism of the Schmidt reaction has not been established with certainty. Schmidt proposed a mechanism in which the hydrazoic acid is cleaved by the strong mineral acid to nitrogen and the imide radical (NH). This radical is supposed to add to the carbonyl group, followed by a rearrangement either directly or by a Beckmann transformation of an intermediate oxime to the amide. ... 

---