Thermal Curtius rearrangement

The pyrrole derivative of carbonyl azide 206 undergoes thermal Curtius rearrangement into isocyanate, which spontaneously cyclizes into pyrrolo-diazepinone 207 (Equation (27) (1991JHC1911)). 

However, Lwowski and co-workers demonstrated that photolysis of 7 in cyclohexane, cyclohexene- or 2-methylbutane does lead to the formation of adducts. Therefore, the acylnitrene 8 is a trappable intermediate. The thermal Curtius rearrangement does not involve free nitrenes and it must be a concerted process. 

The thermal Curtius rearrangement of formyl, acetyl, and benzoyl azides was studied by DFT.78 The authors conclude that the reaction occurs by a concerted mechanism and not by the alternative nitrene intermediate. 

Acyl azides are well-known compounds. Their role in the thermal Curtius rearrangement, a reaction that apparently does not involve a nitrene, will be discussed in subsection 8.3.2. Photochemical decomposition of acyl azides elidts nitrene reactivity. In particular, intramolecular C-H-insertion reactions have been observed, but not usually in high 5deld. ... 

We anticipated that a similar strategy would be applicable to the synthesis of l,3-oxazine-2,4-diones, involving the in situ formation of isocyanates by thermal Curtius rearrangement of acyl azides under the same conditions of the Wolff rearrangement. Indeed, tenperature-regulated microwave irradiation of a 1 1 mixture of a 2-diazo-1,3-diketone and an acyl azide afforded the desired l,3-oxazine-2,4-dione products 33 (Scheme 3.281. This sequence nicely illustrates the utilization of microwave activation for the selective one-pot thermal rearrangements of azido and diazo compounds. 

The thermal Curtius rearrangement of benzoyl azide in the presence of Lewis acids was studied using DFT. ... 

In 2010, Studer et al. reported A -heterocyclic carbene catalyzed oxidative amidations of various aldehydes to the corresponding acyl azides by using the readily available organic oxidant [43]. Acyl azides can readily be converted via thermal Curtius rearrangement to carbamoyl azides as shown for the transformation of benzaldehyde to phenyl carbamoylazide (Scheme 5.7), when r-BuOH was further added to the crude reaction mixture, which was then heated for 1 h to reflux. Therefore, a mild NHC-catalyzed oxidative azidation of aromatic aldehydes to form the corresponding acyl azides which can be rearranged to carbomoylazides in the same pot. 

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... 

Acyl azides can undergo photolytic cleavage and rearrangement upon irradiation at room temperature or below. In that case acyl nitrenes 8 have been identified by trapping reactions and might be reactive intermediates in the photo Curtius rearrangement. However there is also evidence that the formation of isocyanates upon irradiation proceeds by a concerted reaction as in the case of the thermal procedure, and that the acyl nitrenes are formed by an alternative and competing pathway " ... 

The origins of sonochemistry lie in the study of homogeneous systems and among the examples of early synthesis is the Curtius rearrangement which appeared in 1938 [34]. In this example benzazide gives nitrogen and phenyl isocyanate when sonicated in benzene (Eq. 3.1), and the rate is increased in comparison to the normal thermal reaction. This reaction was not fully investigated at the time and the observation that the reaction stopped after rapid initial steps was not explained. 

In a related type of reaction, the styryl isocyanates, readily available by Curtius rearrangement of cinnamoyl azides, undergo thermal cyclization to l-isoquinolones in good yield (equation 34) the reaction can also be carrried out using Friedel-Crafts catalysts. 2,3-Dihydro-4( 1//)- isoquinolones are obtained by Dieckmann cyclization of N- (o- carbalkoxy-benzyl)glycine ester derivatives (equation 35). The same reaction has been used for the synthesis of a range of non-aromatic heterocycles (equations 36 and 37). 

Eloy and Deryckere have applied their synthesis of isocarbostyrils (69HCA1755) to the preparation of thieno[2,3-c]- and thieno[3.2-c]-pyridines (Scheme 65) (70BSB301). Thermal-cyclization of 3-thienylvinyl isocyanate prepared from the corresponding azide (269) by Curtius rearrangement yields thieno[2,3-c]pyridin-7-one (270), which is transformed to (259) following usual methods. 

Compound 610 was transformed thermally to the tetrazolo[5,l-b]purin-7(8//)-one 639 through Curtius rearrangement via the isocyanate intermediate 638 (86H1899). Reaction of 626 (R = Ph) with bromine or sulfuryl chloride led by ring opening and decarboxylation to the halogenated tetra-... 

The Curtius Rearrangement is the thermal decomposition of carboxylic azides to produce an isocyanate. These intermediates may be isolated, or their corresponding reaction or hydrolysis products may be obtained. 

Styryl isocyanates 183, readily available by Curtius rearrangement of cinnamoyl azides, undergo thermal or Friedel Crafts cyclization to 1-isoquinolones 184 (Scheme 110). 

Curtius rearrangement (Section 24.6) the thermal rearrangement of an acyl azide to an isocyanate, followed by hydrolysis to yield an amine. 

The outstanding thermal reaction of the acyl azides is their rearrangement to isocyanates. This Curtius rearrangement occurs also in the photodecomposition of the acyl azides, but the yield of isocyanate is lower than in the corresponding thermal process, and other products appear which can be accounted for by a nitrene precursor. A considerable amount of work has been directed towards an elucidation of the mechanism of the photo-Curtius rearrangement and the bulk of the evidence seems to favour the view that it ° is a concerted process not involving a discrete nitrene intermediate. Nitrenes are nevertheless formed in the photolysis of acyl azides and the products of this alternative route will now be discussed. 

Azides of the type RCON3 are colourless liquids or solids. Alkanoyl and alkenoyl azides are often prepared, but rarely isolated because they undergo Curtius rearrangement already at or below room temperature. Aroyl azides are much more stable thermally. 

There are great differences in the thermal stabilities of acyl azides. While the aromatic derivatives are considered to be comparatively stable, their aliphatic counterparts very often decompose close to room temperature. As in the course of this destruction the preparatively most useful Curtius rearrangement will take place, this instability is not necessarily a disadvantage. 

Perfect retention of the geometrical configurations has been observed in the Curtius rearrangement of cis- and franr-crotonyl azides (10), which respectively give cis- and franr-propenyl isocyanates (11), characterized as their urea derivatives (12 Scheme 8). The cis isomers (10b) and (11b) tend to isomerize thermally to the trans isomers (10a) and (11a) respectively. 

Curtius rearrangement Thermal or photochemical rearrangement of acyl azides to give isocyanates. 116... 

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