Acyl nitrene

Loss of Nt and migration of the group R is likely to be a concerted process, since evidence for a free acyl nitrene RCON in the thermal reaction has not been found " ... 

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 " ... 

Insertion (see 12-12). Nitrenes, especially acyl nitrenes and sulfonyl nitrenes, can insert into C—H and certain other bonds, for example. 

In contrast to the reaction of benzamide and other carboxylic acid amides with lead tetraacetate, which has been said to proceed via an acyl nitrene intermediate 46>, methanesulphonamide and 2-biphenylsulphon-amide are completely inert towards this reagent 42>. 

The photochemistry of aryl azides is quite complex, suggesting that the nitrene 14 may not be the only reactive intermediate and that insertion reactions may not be the only route to form photoconjugates.Although aryl nitrenes are much less susceptible to rearrangements than acyl nitrenes, they may still occur and lead to the formation of reactive intermediates such as azepines, which may go on to react with nucleophiles.[911 141 Addition of nitrenes to double bonds will generate azirines, while dimerization will produce azobenzenesJ11 Aryl azides are stable to most of the procedures used in the course of peptide synthesis except for reduction reactions. Non-photochemical reduction of aryl azides to the primary amines by thiols has been reported by Staros et al.[15]... 

As you might expect from the structure of an acyl nitrene (only six electrons in the valence shell of nitrogen), it is highly unstable but can become stabilized by having the substituent group move as R e from carbon to nitrogen 5... 

A practical example of this reaction is given in Table 23-6 together with examples of related reactions known as the Curtins and Schmidt rearrangements. The latter two probably also involve rearrangement of an acyl nitrene, this time formed by decomposition of an acyl azide ... 

The photochemistry of 3-aryl-substituted l,4,2-dioxazol-2-in-5-ones (79) can be interpreted in terms of ring cleavage and loss of carbon dioxide with the formation of an acyl nitrene (80) such nitrenes are also formed by the photolysis of acid azides. In dimethyl sulfide, therefore, the 3-phenyl derivative itself (79 Ar=Ph) is converted into the photoproduct (81).05 When the phenyl group is substituted in... 

Acyl nitrenes are highly reactive but unfortunately their precursors, acyl azides, are chemically reactive, e.g. they acylate amino groups. While sulfonyl- and alkoxycarbonyl azides are somewhat less reactive than acyla-zides, and the nitrenes derived from them are highly reactive (Lwowski, 1970 and for sulfonyl nitrenes see Abramovitch et al., 1981 and references therein), the azides must be irradiated in the UV, below 300 nm. Phos-phorylnitrenes are the most reactive nitrenes known and will insert into CH bonds in the presence of hydroxyl groups (Breslow et al., 1974). Again, photolysis occurs at ultraviolet wavelengths. 

It is known that singlet acyl nitrenes derived from the acyl azides upon direct irradiation may undergo stereospecific addition to C=C bonds to yield aziridines [97]. 

The lifetime of the nitrene is governed by its stability this in turn governs its reactivity with respect to added substrates. The most reactive nitrenes, i.e., those having electron acceptor substituents such as sulfonyl and acyl nitrenes are extremely short-lived [6b], In contrast, amino nitrenes that are stabilized by resonance can be stored for several hours at —78 °C [11]. 

The nitrene 28 is not produced from the azide precursor, but from heterocycles via photolysis and thermolysis as shown in Sch. 11 [20]. Iminoacyl nitrenes react intramolecularly giving benzimidazoles with good yields (Sch. 11), and, dependending on the precursor used and the reaction conditions, varying amounts of carbodiimides are obtained. The reactivity of the acyl nitrenes is influenced by the substituent connected to the acyl group (see Sch. 10), however all acyl nitrenes are quite reactive and therefore rather unselective. Apart from cycloaddition reactions with Tt-bonds, insertion reactions into a-bonds, additions to lone pair electrons of... 

It is often the case that the oxygen and nitrogen atom of the acyl nitrene are involved in the cycloaddition reaction. In this respect, acyl nitrenes are able to react like a 1,3-dipole [37] however this is different to other species commonly classified as 1,3-dipoles as they are not octet stabilized. 

The triple bond of nitriles is attacked by aroyl nitrenes to give rise to oxadiazoles as illustrated in Sch. 25 [22,41]. However, additions of acyl nitrenes to olefmic double bonds can be carried out in acetonitrile solution because the cycloaddition reaction to the solvent is much slower. 

Some attempts have been made to modify diastereoselectivity by introducing chiral substituents into the azide precursor of the nitrene (see Sch. 28) [22,43,44]. The photocycloaddition of acyl nitrenes bearing chiral substituents to cycloalkenes having enantiotopic faces such as compound 85, or prochiral ketones, can lead to the formation of two diastereomers. However, this chiral induction has not been observed in the reaction of the nitrenes... 

Benzofurazans show greater thermal stability but may be cleaved photochemically. Irradiation of benzofurazan in benzene and in methanol gives the azepine (26) and the urethane (27), respectively in the presence of triethyl phosphite (Z,Z)-l,4-dicyanobuta-1,3-diene is formed. The proposed mechanism (Scheme 3) involves nitrile oxide, oxazirene, acyl nitrene and isocyanate intermediates, and is supported by spectrophotometric studies (76HCA2727) and by trapping of the nitrile oxide as its isoxazole cycloadduct with DMAD (75JOC2880). 

Formation of a Nitrene. The decomposition of acyl azides is one of several ways in which acyl nitrenes 3 are formed (see p. 293). After the migration has taken place, the atom at the migration origin (A) must necessarily have an open sextet. In the third step, this atom acquires an octet. In the case of carbocations, the most common third steps are combinations with a nucleophile (rearrangement with substitution) and loss of H" " (rearrangement with elimination). 

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