Azides, acyl Curtius reaction

The thermal decomposition of an acyl azide 1 to yield an isocyanate 2 by loss of N., is called the Curtius reaction - or Curtius rearrangement. It is closely... 

Tertiary alkyl azides can be prepared by stirring tertiary alkyl chlorides with NaN3 and ZnCl2 in 82 ° or by treating tertiary alcohols with NaN3 and CF3-COOH or with HN3 andTiCl4 or BF3. Acyl azides, which can be used in the Curtius reaction (18-14), can be similarly prepared from acyl halides, anhydrides, " esters, or other acyl derivatives. ° Acyl azides can also be prepared... 

A violent explosion occurred during vacuum distillation of 4-chlorophenyl isocyanate, prepared by Curtius reaction from the azide. It was found by IR spectroscopy that this isocyanate (as well as others prepared analogously) contained some unchanged azide, to which the explosion was attributed. The use of IR spectroscopy to check for absence of azides in isocyanates is recommended before distillation [1], Subsequently, the explosion was attributed to free hydrogen azide, produced by hydrolysis of the unchanged acyl azide [2],... 

The pyrido[l,2-tf][l,3,5]triazine-2,4(3//)-dione derivative 89 was obtained in a cycloaddition reaction of diphenyl-methyl isocyanate 90 with 2-pyridyl isocyanate 91 derived from the corresponding acyl azide via Curtius rearrangement <2002ARK438>. Compound 89 was also synthesized by the reaction of diphenylacetyl chloride 118 and picolinyl azide 116a in the presence of triethylamine (Scheme 11) <2002ARK438>. ... 

Curtius reaction org chem A laboratory method for degrading a carboxylic acid to a primary amine by converting the acid to an acyl azide to give products which can be hydrolyzed to amines. kard e as re,ak-shan cyanaicohoi See cyanohydrin.. sT an al ka.hol ... 

Reactions of pyrimido[4,5-3] [l,4]thiazines were discussed in CHEC-II(1996) <1996CHEC-II(7)737> more recently, reported reactions of this system involve nucleophilic substitution in a number of guises. Hemiaminals at C-3 react with ammonium acetate to form aminals (Equation 166) <1999CHE97>. The formation of acyl hydrazides from pyrimido[4,5-3][l,4]thiazine-2-carboxylic acids, along with their subsequent conversion to acyl azides and Curtius... 

The reaction sequence - including subsequent reaction with water which leads to amines - is named the Curtius Reaction. This reaction is similar to the Schmidt Reaction with acids, differing in that the acyl azide in the present case is prepared from the acyl halide and an azide salt. 

Triazole-based annelations have also been used to prepare 2-phenyl-2/7-[l,2,3]triazolo[4,5-e]py-ridin-4-one (233) from 2-(2-phenyl-l,2,3-triazole)acrylic acid via a thermal Curtius reaction of the a,/J-unsaturated acyl azide (232) (Scheme 43). Further functionalization of the nucleus was accomplished using the intermediate 2//-4-chlorotriazolo[4,5-c]pyridine (234) <85 LA 1922). 

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. 

As described earlier (Section 4.4.1.1), the intermediates of the Curtius reaction are acyl azides, which themially rearrange to isocyanates. One of the classical procedures for the preparation of acyl azides consists of the formation of hydrazides from esters and hydrazine, followed by treatment of the hydrazides with nitrous acid, generated from sodium nitrite and acetic, hydrochloric or sulfuric acid. Acyl azides are commonly used in the crude state or in solution since they are thermally unstable and potentially explosive. 

Alkyl nitrites and nitrogen tetroxide together with acid are also effective for the formation of acyl azides from hydrazides under anhydrous conditions. As shown in Scheme 16, the Curtius reaction is especially suitable for the formation of complex reactive molecules. 

Formation of acyl azides from acyl chlorides is also a classical procedure - for the Curtius reaction. Sodium azide is used for the azidation under aqueous and anhydrous conditions, called the wet and dry methods respectively (equations 27 and 28 ). 

A useful variant of die Curtius reaction comprises the reaction of acyl chlorides in dichloromethane with sodium azide under phase transfer conditions, followed by treatment of the dried dichloromethane solution, which contains acyl azides, with a slight excess of trifluoroacetic acid. The resulting trifluo-roacetamides can easily be cleaved with potassium carbonate in aqueous methanol at room temperature, giving the amines (equation 30). ... 

Acyl azides (61) and (62) ° and isocyanate (63), the reaction intermediates of the modified Curtius reaction using DPPA, have been isolated in a pure state, since they are relatively thermally stable, and further converted to urethanes or ureas by the addition of alcohols or amines respectively. 

It is not always necessary for the Curtius reaction that the azide be isolated in substance. It is often possible to treat an acyl halide with sodium azide and reach the isocyanate in one stage. This variant may be exemplified by the preparation of methyl isocyanate as follows 124... 

The terminal carbon of organic azido compounds is nucleophilic. Most azido compounds are unstable and release N2 gas. Curtius rearrangement has the intermediate acyl azide in the reaction. Some azido compounds are 1,3-dipoles, which are used in cycloaddition. 

During the 1890s, Thomas Curtius studied the preparation and reactions of acyl azides. For example, treatment of benzoylhydrazine (1) with nitrous acid provided benzoyl azide, which Curtius represented as 2, consistent... 

Carbamates are also formed by one-stage, two-step processes involving isocyanates as intermediates and precursors. Thus, carbamates are prepared under modified Hofmann rearrangement conditions, using NBS/NaOMe as the reagent [443], by Curtius rearrangement of the acyl azide if the reaction is carried out in alcohol, by the reaction of amides with lead tetraacetate [444], or by the Lossen rearrangement, if the carbamates are not sensitive to the presence of base. 

Chromone-2-carbonyl azide (67) may be prepared in excellent yield either by adding sodium azide to a solution of the acyl chloride (68) in dioxan, or by the action of nitrous acid on the hydrazide (69) [157], A Curtius reaction on the azide led to an interesting series of derivatives of 2-aminochrbmone (70, R = OEt, Me, CCI3, NEta and similar groups), whose reactions have been studied [175-179]. 

The same reagent allows to convert aldehydes into acyl azides. If the reaction is run in the presence of NaNs, the intermediate acyl azide is converted into a carbamoyl azide via Curtius rearrangement (Scheme 8.20). 

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