Curtius rearrangement Schmidt reactions

Related reactions Curtius rearrangement. Lessen rearrangement, Schmidt reaction ... 

This reaction is related to Curtius Rearrangement, Lossen Rearrangement, Schmidt Reaction, Weerman Degradation, and Wolff Rearrangement. 

By application of the Schmidt reaction, the conversion of a carboxylic acid into an amine that has one carbon atom less than the carboxylic acid, can be achieved in one step. This may be of advantage when compared to the Curtius reaction or the Hofmann rearrangement, however the reaction conditions are more drastic. With long-chain, aliphatic carboxylic acids yields are generally good, while with aryl derivatives yields are often low. 

An extensive review of the chemistry of aliphatic and aromatic azides is given by Boyer and Canter [167] and Gray [168]. Organic azides are subject to various reactions such as the Bergmann degradation and the synthesis of peptides, the well known Curtius rearrangement, the Darapsky synthesis of a-aminoacids [169], for synthesis of triazoles [170], tetrazoles ( Schmidt reaction ) [169] and [171] etc. These reactions lie beyond the scope of the present book. 

A closely related reaction of general applicability is the Curtius rearrangement222 of acyl azides. The rearranging species in the Schmidt reaction (see p. 898) is in fact also a protonated acyl azide these azides are readily prepared by the action of nitrous acid on acyl hydrazides which are themselves formed from esters and hydrazine (Section 9.6.17, p. 1269). On heating in aprotic solvents the acyl azides decompose to yield the corresponding isocyanates in good yield. 

This reaction is related to the Hofmann and Schmidt Reactions and the Curtius Rearrangement, in that an electropositive nitrogen is formed that initiates an alkyl migration. 

Into this group fall the named oxidation rearrangement reactions which proceed with carbon-carbon bond cleavage and 1,2-transfer of an alkyl group to a heteroatom, such as the Baeyer-Villiger reaction (discussed in Chapter 5.1, this volume) and the Beckmann reaction (found in Chapter 5.2, this volume) of ketones, as well as the Hofmann reaction/Schmidt reaction/Curtius reanangement of carboxylic acid derivatives. The two examples discussed here involve related reactions of alcohols. 

The rearrangement has a mechanism similar to those of the Hofmann rearrangement of amides, the Lossen rearrangement of acylhydroxamic esters, the Schmidt rearrangement of carbonyl compounds and the Wolff rearrangement of diazoketones. Evidence concerning the mechanism of one can often be applied to the others, and the whole family has been reviewed briefly . Sometimes the distinction is made that the conversion of an acyl azide into an isocyanate or urethane is the Curtius rearrangement whereas the overall sequence is the Curtius reaction, but usually the former name is used for both processes. 

Although the rearrangement of alkyl and aryl azides on treatment with acids, reaction (39), was discovered by Curtius , it is sometimes called a Schmidt reaction, although Schmidt did not work with these... 

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... 

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). 

Rearrangement of amides. Primary amides undergo oxidative rearrangement to isocyanates when treated with lead tetraacetate. The reaction is generally carried out in an alcohol (t-butanol generally), in which case the product is isolated as the carbamate. Triethylamine or stannic chloride catalyzes this reaction. This oxidation provides a useful alternative to the classical Hofmann, Schmidt, and Curtius rearrangements. 

Schmidt Reactions. This term is used for several transformations, general examples of which are shown in eqs 11 and 12. The former is used infrequently due to the drastic conditions required compared to the analogous Curtius and Hoffmann rearrangements and the discovery that DPPA effects the transformation under mild conditions. TMSN3 has been used frequently. 

The Schmidt reaction of carboxylic acids with hydrazoic acid has the advantage over Curtius rearrangement that it is only one step from the acid to the amine, but the conditions are more drastic (usually sulphuric acid plus sodium azide). Under these harsh conditions, the isocyanate intermediate is rarely isolated. For these reasons, the Curtius rearrangement is frequently employed to convert acids to amines. The Schmidt reaction of ketones with hydrazoic acid is a powerful method for the synthesis of amides and lactams. TTiis process is somewhat related to the Beckmann rearrangement of oximes however, the Schmidt reaction is more succinct, allowing the conversion of ketones to amides in a single operation. Considering its widespread... 

This reaction, known as the Lossen rearrangement, has seen but little synthetic application 2 . ito possesses no distinct advantages over the Curtius, Hofmann, and Schmidt reactions. It appears to be useful when hydroxamic acids result as primary products. The reaction has been reviewed recently. 

The Hofmann rearrangement with hypohalites, the Schmidt reaction, and the Curtius reaction are three closely related methods of preparing amines. While all of them have been used extensively, since they afford a means of converting carboxylic acid derivatives to amines with one less carbon atom, they all are considered somewhat hazardous reactions. Of the three, probably the Hofmann rearrangement is... 

The Hofmatm, Curtius, Schmidt and Lossen rearrangements generally involve nucleophilic migrations from a carbon to an electron deficient nitrogen center (Scheme 5), giving isocyanates as the initial products, which undergo further reactions as already shown in Scheme 2. 

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