Acyl nitrenes, insertion

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

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]... 

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. 

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... 

The azabicyclononane system is a common structural feature in diterpene alkaloids, and the nitrene insertion route to the ring system has been studied in detail in model decalins as well as in steroids (Scheme 11). Thus irradiation of the rrans-acyl azide (21) gave, in addition to isocyanate (30-33%), a mixture of Ae 7- and 8-lactams (22) and (23). The y-lactam (22) predominated, dthough the overall yield was poor. > The corresponding c/s-azide (24), however, gave the 8-lactam (25) as Ae major product, again in low yield. One elegant application of this type of intramolecular nitrene insertion reaction... 

The first step of the mechanism is the deprotonation of the O-acylated ketoxime at its a-position, which gives rise to the corresponding enolate. This enolate then can react via two possible pathways 1) a concerted anionic pathway in which the leaving group is directly displaced to give the isolable 2H-azirine or 2) a nitrene pathway that leads to the same 2/-/-azirine intermediate via nitrene Insertion. The nitrene pathway has not been disproved experimentally. 

Similar to singlet carbenes, singlet nitrenes (R-N ), the nitrogen analog of carbenes, are also known to insert into C-H bonds." Acyl nitrenes and sulfonyl nitrenes insert rather readily in C-H bonds [Eq. (6.133)] ... 

Interest in the photochemical generation and fate of acyl nitrenes has again been expressed. In benzoyl, p-methoxybenzoyl, and m-fluorobenzoyl azides, Curtius rearrangement of the photochemically generated acyl nitrenes to the corresponding isocyanates competes with formation of insertion products in halogen-free solvents.78 In halogenomethanes, the yields of isocyanates are... 

The acyl nitrene generated from compound (I6) led to products (17) and (18) by insertion, and product (19) by a novel intramolecular transacylation (Scheme 4) various similar model systems... 

The dimerization appears to be a characteristic of triplet nitrenes, since it occurs on sensitization by such compounds as benzophe-none. When 2-azidobiphenyl is irradiated in Ihe presence of a triplet quencher (penta-I,3-diene to inhibit the formation o( an azocompound, the main product iscarbazole (5.36), which is formed by internal insertion into an aromatic C—H bond. Other nitrenes may also participate in intramolecular insertion, and this allows lactams to be formed from acyl azides (5.37). 

Irradiation of acyl azides (374) results in the Curtius-type rearrangement1163 to produce the isocyanates 375 and nitrogen in a concerted fashion (no nitrene intermediates are involved) (Scheme 6.176). Esters of azidoformic acid (374, R = alkoxy) produce singlet nitrenes, which can be trapped by alkenes or inserted into a C H bond.1112,1164... 

Benzo derivative (46) was formed in 7% yield through intramolecular Friedel-Crafts acylation of an acyclic sulfamoyl chloride (Equation (15)) <920PP463>, while the benzo-2,1-thiazocine derivative (48) was formed in 30% yield through the intermecliacy of the sulfonyl nitrene, generated thermally from the corresponding sulfonyl azide, which underwent intramolecular insertion into an aromatic C—H bond (Equation (16)) <81CC1087>. 

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. ... 

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